High Temperature Corrosion of Ceramics

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HIGH

TEMPERATURE

CORROSION

OF CERAMICS

J.R. Blachere F.S. Pettit Department

of Materials Science and Engineering University of Pittsburgh Pittsburgh. Pennsylvania

NOYES DATA CORPORATION Park Ridge, New Jersey,U.S.A.

Copyright @ 1989 by Noyes Data Corporation Library of Congress Catalog Card Number: 88-38242 ISBN: O-8155-1188-4 Printed in the United States Published in the United States of America Noyes Data Corporation Mill Road, Park Ridge, New Jersey 07656

by

10987654321

Library

of Congress Cataloging-in-Publication

Data

Blachere, J.R. High temperature corrosion of ceramics / by J.R. Blachere and F.S. Pettit. cm. P. Bibliography: p. Includes index. ISBN O-8155-1188-4 : 1. Ceramic materials--Corrosion. I. Pettit, F.S. (Frederick S.), 1930. II. Title. TA455C43B57 1989 620.1’404217--dc19 88-38242 CIP

Other No yes Publications

CERAMIC

RAW MATERIALS Edited by D.J. De Renzo

The broad-based ceramics Industry encompasses all types of glass; refractones; abrasives; whitewares. such as porcelain and pottery, structural clay materials; etc. Increasing use of advanced ceramic materials in the automotive and aerospace Industries, as well as in such diverse areas as electronics and medical devices, IS expected to push the demand for raw materials far beyond that associated with the traditional ceramics industry. Prepared directly from manufacturers’ data sheets and tables at no cost to, nor influence from, the contributing companies, this book

provides chemical and physlcal property data for more than 1000 products supplied by 181 ceramic raw material suppliers in the U.S. and Canada. Part I is an alphabetical listing of 99 raw material categories. Raw material suppliers are then included alphabetically under each category, along with their product information. The categories in Part II include additives; semiprocessed materials, some of which are available as unfinished shapes or substrates; and materials intended for specific end uses. Raw materials categories are:

Parr I Alumina Alumina Chrome Alumina Zirconia Aluminum Nitride Aluminum Titanate Andalusite Antimony Compounds Ball Clays Barium Compounds Bauxite Beryllium Oxide Bismuth Compounds Bone Ash Borates Borax Boric Acid Boron Boron Carbide Boron Nitride Calcium Compounds Carbon Celestite Chlorite Mineral Chrome Ore Chromium Oxide Clays (Miscellaneous) Cobalt Compounds -. . Cofemamte Copper 8 Copper Oxide Cordierite

Potassium Compounds Pyrophyllite Duartz Rutile Sand Silica Silicon Silicon Carbide Silicon Nitride Sillimanite Slags Soapstone Soda Ash Sodium Silicate Sodium Sulfate Spars Spine1 Spodumene Stannates Stannic Oxide Strontium Carbonate Talc Titanates Titanium Boride Titanium Carbide Titanium Dioxide Titanium Nitride Ulexite Vermiculite Whiting Wollastonite

ISBN O-8155-1143-4

Corundum Diatomaceous Earlh Dolomite Feldspar Ferrites Fireclay Flint Clay Fluorspar Fluxes Frits Garnet Graphite Iron Oxide Kaolin Kyanite Lanthanide Compounds Lead Compounds Lime Limestone Lithium Compounds Magnesia Magnesite Manganese Compounds Mica Mullite Nepheline Syenite Nickel Compounds Ochre Periclase Petalite Phosphates

(1987)

Yttrium Oxide Zinc Oxide Zircon Zirconates Zirconia Zirconium Boride Zirconium Nitride

Part II Binders Ceramic Additives Ceramic Adhesives, Potting Materials, and Putty Ceramic Coatings Ceramic Colors Ceramic Fibers and Whiskers Ceramic Materials Ceramic Precursors Dielectric Compositions Dispersing Agents Electronic Ceramics Reagents Glazes Glaze Stains Pie20 Compositions Refractory Materials Sealing and Solder Glasses

8%” x 11”

900

pages

Foreword

This

book

describes

vestigated

in this

carbide.

In addition

neering

materials

conditions

The

This

major

of

tools

electron

single

crystals

purities

corrosion the

research.

A

or CVD

surfaces the

gaseous

corrosion

method

of

was developed

for

and

observed

the

were

at lower

oxide

experiments

by also

tempera-

SO2 and SOs. The

microanalysis of

engi-

corrosion

in the scanning

x-ray

measurement

typical The

was enhanced

containing

related

in-

and silicon

experiments

at IOOO’C

were

materials

materials,

and oxidation

and oxygen

and

The nitride

in the study.

studied

oxygen

silicon

included

in which corrosion was

of pure of

were

of ceramics.

alumina,

electron were

thickness

on silicon

the

in the

nitride

and

carbide.

In the use of materials parent

that,

resistance.

in most While

and

polymers,

this

study,

materials

order

to

corrosion

than

with

keep the

to

materials

used

was developed,

in this

developing were

selected

practice

and their

were

based

of and

by

V

number,

resistance environment

immune

environments of their

to

the used

being

In

the ceramic

being

likelihood

severity.

cer-

environments.

on the basis by which the

the

of cer-

of representative

corrosive

at a reasonable

selected

of corrosive

to the corrosion

a variety

of different

Furthermore, upon

alloys

purpose

of ceramics

for use in a variety

applicable

investigate

in particular,

passivity.

corrosion

to

metallic The

the corrosion

are available a theory

of experiments

study

environments.

it is apcorrosion

to all ceramics.

a number

number

resistant

certain investigate

materials

it was necessary

corrosion by

be more

to develop

exposed

environments, to provide

applicable

of ceramic

in harsh

are the best choice

react

generally

In order

in general,

amic

may can

temperatures

ceramics

was to systematically

A great number amics

ceramics

a theory

environments.

at elevated instances,

ceramics

then,

to develop

or

pure

instrument

microprobe

silicon

silica,

gaseous

in morphology

microscope.

were

corrosion’

hot

corrosion

study

the

Some

in the presence

changes

temperature

various

‘hot

deposits.

performed.

to of

were

Na2S04 tures,

high

particular

to

to

produce

encountered

in

vi

Foreword

The

information

prepared

in the book

by J.R.

Blachere

U.S. Department The

table

of Energy,

of contents

is from

High

and F.S. Pettit December

is organized

Temperature

Corrosion

of the University

of Ceramics,

of Pittsburgh

in such a way

and provides easy access to the information

as to serve as a subject

contained

in the book.

Advanced composition and production methods developed by Noyes Data Corporation are employed to bring this durably bound book to you in a minimum of time. Special techniques are used to close the gap between “manuscript” and “completed book.” In order to keep the price of the book to a reasonable level, it has been partially reproduced by photo-offset directly from the original report and the cost saving passed on to the reader. Due to this method of publishing, certain portions of the book may be less legible than desired.

NOTICE The

materials

of work Neither

in this book

sponsored the

ment

of Energy,

their

contractors,

States

the

Government

sub-contractors, makes

mation,

completeness,

apparatus,

resents that

of Energy.

nor the Depart-

or their

any warranty,

or assumes any legal liability

accuracy,

as an account

nor any of their employees,

nor the Publisher, plied,

were prepared

by the U.S. Department

United

express

not infringe

for

of any infor-

or process disclosed

its use would

or im-

or responsibility

or usefulness

product

nor any of employees,

or rep-

privately-owned

rights. Final tion

determination

and the manner the user. The be exercised atures, before

of the

or procedure

for

of that

reader

suitability

informa-

by any

use, is the sole responsibility

is warned

when

dealing advice

implementation.

of any

use contemplated

and expert

for the

1987.

that caution

with ceramics should

user, of

must always

at high temper-

be sought at all times

index

Contents and Subject Index

INTRODUCTION.........................................l

. . . . . . . . .. . . . . Special Techniques . .. . .. . . . . . . . Measurement of Oxide Thickness . Measurement of Contact Angles. . . . .

EXPERIMENTAL

Experimental

Cross Sections GASEOUS

. . . . .

PROCEDURES.

. . . .

Conditions

and Related

. . . . .

. . . .

. . . . .

.......... .......... .......... .......... .......... ..........

Techniques

....................... .............................

...... ...... ...... ...... ...... ......

CORROSION

Introduction.

Gaseous Corrosion

..........

of Silica and Alumina.

................................ .............................. Alumina Silica.

Gaseous Corrosion

of Silicon

HOT

CORROSION

OF SILICA.

HOT

CORROSION

OF ALUMINA.

HOT

CORROSION

OF SILICON

Nitride

and Silicon

. . . . . .

Carbide

. . . . . .. . . . . .

. . . .

CARBIDE

. . . . . AND

.6 .6 .7 .7 .9 .9

. . . . 11 . . . . 11 ...

. . 13

. . . . 14

. .. . . . . .

. .16

. . . . .. . . . . . . .

SILICON

NITRIDE.

11

. . . 11

.21

. . .26

REFERENCES..........................................30 APPENDIX IN SULFUR

A-GASEOUS OXIDE

CORROSION

OF SILICA

H. R. Kim, J. R. Blachere,

AND

ALUMINA

. . . . . . . . . . . . . . . . . . . . . . . . 31

ENVIRONMENTS F.S. Pettit

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Gaseous Corrosion of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . 33 Introduction.

.

vii

viii

Contents

and Subject

Index

Procedure. .............. ....................... Experiments. .................... Results and Discussion ................ Silica. ......................... Alumina ....................... General Results. ................

Experimental Materials

Thermodynamics Single

of Sulfate

.

Formation

..................

Crystal

Polycrystalline

..........

Aluminas.

.................. Conclusions ....................... References ........................ General

APPENDIX

Discussion.

B-HOT

M. G. Lawson,

CORROSION

H. R. Kim,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

. .35

. . . .

. .35 . .35

. .

. .37

.37 . .39 . .39 . .41

. .43 . .43 : :53 52 . .55 . .5B

.................

OF SILICA.

F.S. Pettit,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

J. R. Blachere

. 58 : :60 59

.................................... ................................... Experimental Procedure ............................. Materials ..................................... Gaseous Corrosion. .............................. ................................. Hot Corrosion. Results and Discussion .............................. Gaseous Corrosion. .............................. Wetting by Sulfates .............................. ................................. Hot Corrosion. ......................... Kinetics of Crystallization. General Discussion. .............................. .................. Discussion of Crystallization Kinetics ..................... Hot Corrosion of Silica Formers. Conclusions ..................................... ..................................... References. Introduction.

Hot Corrosion.

APPENDIX M.G.

C-HOT

Lawson,

CORROSION

F.S. Pettit,

OF ALUMINA

. . . . . . . .

Experimental

Procedure

. . . .. . . . . . Materials . . . . . . . . . . . . . . Hot Corrosion Experiments. . Results and Discussion . . . . . . . Wetting . . . . . . .. . . . Hot Corrosion of Al*Os .. . . Results, Acidic Conditions . . . High

Crystal Purity

Medium Low Results,

Alumina. Polycrystalline

Purity

Purity

. .60 63

. . . :

.63

. . . .

.82

.64 .66 176 72

.85 .86 .87

.

. . . .

. . . . . .

.89

. . . .

. . . . . . . . .

. . . . .

.89 .95 .95 .97 .97 .97

J.R. Blachere

Introduction.

Single

: :60 60

Basic Conditions

. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Alumina.

Polycrystalline

Polycrystalline

. . . . . . .

. . . . . .

. . . . . . . .

Alumina. Alumina

. . . . . . .

. . . . ..

. . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

. . . . .

. . . . . .

. . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . .

102 107

. . . . .

107 109 112 113 115

Contents

and Subject

. .. . . . . Medium Purity Polycrystalline Alumina. . Low Purity Polycrystalline Alumina .. . . Discussion of Results . . . . . . . . . . . . . . . . References. . . . . . . .. . . .. . . . . . . .. . Single Crystal High Purity

APPENDIX SILICON

D-HOT

. . . . . . . . .. . . . .

Polycrystalline

CORROSION

. . . . . . . .

Alumina

OF SILICON

NITRIDE

J. R. Blachere, D. F. Klimovich,

................................ ......................... Results and Discussion .......................... Acidic

Procedure

Hot Corrosion

Discussion Acidic

Model

of Silicon

for the Oxidation

Carbide

Under Acidic

............

Nitride. of Silicon Carbide

Nitride.

..........................

Structure

and Stoichiometry

APPENDIX

of Silicon

of Silica

Carbide

............................. .......................................

E-PUBLICATIONS

References.

.

. . . 137

. . . . . .

. . . . 137 . . 137 . . 138 . . 152 . 157 . . . . 161

of Silicon

..............

.................. Acidic Hot Corrosion. ...................... References. ................................. Oxidation

.....

............

and Hot Corrosion Conditions

Model

Proposed Defect

of Silicon

of the Hot Corrosion

Hot Corrosion

116 116 118 119 122 136

F.S. Pettit

Introduction.

Experimental

ix

AND

...............................

CARBIDE

Index

.......

. . . 168 . . . . . 169 . . . . 170 . . . . 172 . . . 178 . . . . 182 184 185

Introduction In the use of materials is apparent

that in most instances

corrosion metallic

at elevated

resistance.

The purpose ceramics

ceramic

in general,

materials

than

was to systematically

investigate

the corrosion

applicable

materials

are available

In order to develop it is necessary

exposed

resistant

environments.

generally

to a number

order to keep the number of experiments

for use in a variety

a variety

of different

used in this study were selected

corrosion

was developed,

to the corrosion

environments.

number,

on the basis by which resistance

to

by being immune to the environment

Furthermore,

the environments

passivity.

were selected

based upon the likelihood

used to produce

of their being encountered

In

the ceramic

in particular,

by developing

of

of representative

corrosive

at a reasonable

materials

of

to all ceramics.

a theory applicable

to investigate

it

to provide

with certain

A great number of ceramic

of ceramics

are the best choice

can react

a theory

environments.

in harsh environments,

may be more corrosion

ceramics

of this program

and to develop

corrosive

ceramics

While ceramics

alloys and polymers,

temperatures

or

corrosion

in practice

and their severity. A material is in equilibrium

is considered

to be immune to a particular

with that environment.

Total or complete

practice

but in some cases the amount of reaction

achieved

is very small, and consequently

material

are very small.

elevated

temperatures,

For example,

the changes

the oxygen activity

equilibrium

upon the oxygen activities between

immunity

when it

is rare in

for equilibrium

in the properties

when Al303 is heated

to be

of the

in oxygen at

in the Al.303 may not be the same as

that in the gas, and oxygen will be incorporated depending

required

environment

into or removed

from the Al303

in the Al303 and in the gas.

the Al303 and the gas, which may require

Upon obtaining

extremely

long

2

High Temperature

Corrosion

of Ceramics

times depending upon the temperature, the Al303 does not exhibit any significant changes in mechanical properties.

The concentrations of point defects in the

Al303 however will change and there could be significant changes in properties such as electrical and ionic conductivities.

Nevertheless, in terms of a corrosion

reaction the Al303 may be considered to be immune to this gaseous environment. Alumina and silica are two ceramic materials which can be considered to be immune to many environments which are extremely corrosive to metallic systems, and therefore these two materials were studied in the present investigation.

The

purity and structure of these ceramics can also affect their behavior in different corrosive environments and therefore these two parameters were also examined in the present investigation.

Four types of crystalline Al303 were studied of

varying purity as defined in Table I. Only one type of silica was studied. As described in Table I it was of relatively high purity and was vitreous. Passivity to corrosive environments is achieved by the formation of a reaction product barrier through which the reactants involved in the corrosion process must diffuse.

The development of passivity is the principal means by

which metallic alloys achieve resistance to corrosive environments, but passivity is also an important mechanism in certain ceramic materials, depending upon the ceramic and upon the environment causing corrosion.

For example, Si3N4 and

Sic react with most environments encountered at elevated temperatures and resistance to corrosion is achieved via the formation of a passive reaction product barrier. Furthermore, Al303 can react with gaseous environments containing sufficient SO3 to form sulfates and again the properties of the sulfate reaction product play a significant role in the corrosion properties. Si3N4

In the present studies

and SIC specimens with purities as defined in Table I were therefore used.

TABLE I - MATERIALS UNDER STUDY

Designation

Material

Purity (Wt%)

Supplier

S.C.

Sapphire

-99.99

Saphikon

998

poly

99.8(0.1MgO,

995

poly A1203 (a)

99.6 - 99.5(0.17 M@ 0.17 Si02)

AlSi Mag 772 (3M) or ADS 995 (Coors)

975

poly A1203 (a)

97.4 (0.75 MgO, 1.6SiO2 0.1 Na20)

S-697 (Saxonburg)

Silica

fused silica

99.99

Corning

SIN CVD

CVD*Si3N4

99.99

Deposits

SiN H.P.

H.P.** Si 3N4

93(6Y203Fe,

Sic SC.

s.c.***

99.99

W.J. Choyke

Sic CVD

CVE (Sic)

99.98

W. J. Choyke

Sic H.P.

H.P. (aSiC)

94(3Al203,

*

Chemical

**

Hot Pressed

A120; (a)

(a6H S.C.)

Vapor Deposition

***

Single Crystal

+

Polycrystalline

O.llSiO2)

Al, 02)

2.5W, Fe, 02)

Alumina

Lucalox (G.E.)

7940 & Composites

Airesearch

Norton

NC-203

4

High Temperature

Corrosion

of Ceramics

When ceramic materials are used in practical applications, a variety of corrosive environments can be encountered.

Most of these environments will

contain oxygen but other reactants such as sulfur, nitrogen, carbon and chlorine can also be present.

Moreover, deposits such as metallic sulfates, carbonates or

chlorides may also accumulate upon exposed surfaces and substantially affect the corrosion processes.

In the case of studies concerned with the corrosion of

metallic alloys it has been useful to examine corrosion reactions in environments of increasing severity extending from gaseous environments containing oxygen, to mixed gases containing one or more reactants in addition to oxygen, and finally considering the effects of deposits such as Na3S04, Na3C03 or NaCl. The environments used in the present studied are identified in Table II. These environments consisted of gases containing oxygen, and other reactants such as sulfur, carbon and hydrogen at temperatures of 700, 1000 and 14OOOC. The effects of deposits were studies by using Na3S04 deposits in O3-SO3-SO3 gas mixtures at temperatures of 700 and 1000°C. These conditions were selected because they are frequently present in many environments encountered in practice.

Furthermore, as established from studies using metallic alloys, the

principles established from studies using these selections should be generally applicable to other corrosive systems. In the following sections of this report the experimental procedures will be described and then a summary of the results will be presented and discussed. Some of these results have been presented in previous reports for this program.(l)

Other results are included in student theses and are also presented

in drafts of papers about to be submitted for publication.

Consequently, some of

the results will not be repeated in this report but will be referred to by references or by draft papers included in appendices in the present report.

introduction

Results obtained from studies using gaseous environments will be discussed first and then results obtained from studies using deposits on silica, alumina, Sic and Si3N4 will be presented in sequence.

5

Experimental

Procedures

Experimental Conditions The materials that were studied in this program and the environments that were used to produce gaseous corrosion and hot corrosion attack have been briefly discussed previously in this report (Tables I and II). The ceramics that were studied (Table I) consisted of Al3O3, SiO3, Sic and Si3N4. A range of purities were examined in the case of Al3O3, Sic and Si3N4. Single crystals of SIC and Al303 were also studied and compared to polycrystalline specimens. The gas compositions that were used are presented in Table II. Gas mixtures consisting of O3-SO3-SO3 were used in both gaseous corrosion studies and the hot corrosion studies. Hot corrosion experiments were also performed in pure oxygen. The gaseous corrosion studies were performed at temperatures of 700, 1000 and 1400°C whereas the hot corrosion investigations used temperatures of 700 and 1000°C.

When O3-SO3-SO3 mixtures were used at 700°C, this mixture

was passed over a platinum catalyst to ensure that equilibrium was achieved. The catalyst was not required to achieve equilibrium at 1OOOOC.The flow of the gas was 1 cm3/s. The experimental procedures have been discussed in detail in previous reports and are also discussed in Appendices A, 0, C and D. The procedure usually consisted of exposing specimens in a horizontal tube furnace at a fixed temperature to a flowing gas stream of fixed composition.

In the case of the hot

corrosion studies the specimens were coated with deposits of Na3S04 for

6

Experimental Procedures

7

investigations at 1000°C, or a NaSS04-CoSO4 equimolar solution for studies performed at 700C. These deposits were applied on specimens by spraying warm specimens with an aqueous solution saturated with NaSS04 or the sulfate mixture. Most specimens were diamond polished and they were ail cleaned before the experiments. The characterization

techniques consisted of morphological studies in the

light microscope and particularly the Scanning Electron Microscope (SEM) with microanalysis of salient features by EDS (Energy Dispersive X-ray Spectroscopy) and WDS (Wavelength Dispersive X-ray Spectroscopy used particularly for light elements).

These techniques were supplemented by X-ray diffraction, weight

change measurements and in some cases surface analysis techniques (ESCA). A large number of other techniques were used in special cases they are SIMS, ISS, FTIR, Laser beam ellipsometry. Furthermore, since after the corrosion tests some deposited salt often remained on the sample, the Nag.504 was washed off to reveal the sample surface.

The samples were characterized before and after this washing.

Experience has shown that the examination of the sample before washing is extremely fruitful, and the observed morphologies have taken many forms. The washwater was analyzed as described in a previous report (l); this analysis is now a routine semi-quantitative

method which allows the identification of the

elements dissolved in the salt and the stoichiometry of the salt.

Special techniques Measurement of oxide thickness A method using WDS measurements in the electron microprobe (EPMA) was adapted from that described by Yakovitz and Newbury

to estimate the

thickness of coatings from the intensities of emitted characteristic X-rays.

The

8

High Temperature

Corrosion

of Ceramics

intensity of the oxygen Ka line is used in the present research to measure the thickness of oxide layers generated on non-oxide materials. This intensity is measured under constant conditions (say 1OkVaccelerating voltage and 2 x lo8 A beam current) for a scale and a bulk standard of pure fused SiO2. Both samples are coated with 200A of carbon. The ratio k of their intensities corrected for background is correlated to the thickness of the oxide layer. The calibration curve for the relationship between the k ratio and the oxide thickness was calculated with a computer program written for this research following the semiempirical approach of Yakovitz and Newbury(2) to generate the Q (pz) curve which is the intensity of X-ray generated at a weighted PZ depth into the sample. This calculation includes many corrections and correlations used to predict the characteristic X-ray intensity. It must be understood that the data available for these corrections (such as absorption) for light elements is relatively poor and that this calibration can only be approximate. based on light element are usually not used quantitatively.

Measurements

However the method

developed in this research gives good reproducibility ? 2% for thickness measurements on silica layers up to about 0.8pm - lum. It was published recently(3) and more details will be found in Appendix E which contains a copy of the article on this measurement. It must be emphasized that since the characteristic

X-ray intensity for oxygen is used to calculate the oxide

thickness, a stoichiometric silica (SiO2) was assumed in the calculations.

This

may lead to significant error if the composition of the scale deviates markedly from that assumed in the calculations.

However the method is quite

reproducible and has a high spatial resolution. The direct measurement of scale thicknesses on cross sections in the SEM is probably not better than + 10% in accuracy considering magnification

Experimental Procedures

calibration and other sources of error under the best conditions.

9

It is good down

to about lum and depends greatly on the thickness of the layer (constancy and magnitude). In some cases the scales are difficult to separate from the substrates in cross sections in the SEM.

Measurement of contact angles The contact angles of the deposits have been measured in the SEM at room temperature using a method previously described by Murr(4). The wetting angle is particularly important since deposits often break into droplets decreasing the area of interaction between the sample and the melt. After coating with carbon (about ZOOA)the samples are placed in a tilted position in the SEM which allows the location of the drop under the electron beam and then they are tilted further to a vertical position to record the profile of the drop. The edges of the drops are enlarged for the measurement. A number of drops are measured under those conditions on the same sample. Experience with the measurements show that their reproducibility is about f: 2O. Since this reproducibility is very good, it has been possible to establish when two wetting behaviors occurred simultaneously on the same sample.

Cross sections and related techniques The study of cross sections is necessary for a number of measurements. However the samples are often damaged (silica and silica formers) during exposures and processing.

Also, cross sections are often desired with salt on the sample, so

that sawing or polishing in wet media has been avoided to preserve the soluble sodium compounds. or fractured.

Most cross sections were either dry-cut with a diamond saw

Fractured specimens show structural features well as has been

10

High Temperature

established

Corrosion

of Ceramics

in many studies of the microstructure of ceramics.

The multilayer

nature of the scale and sometimes its apparent layered growth is shown clearly in cross sections.

The cross sections can also be used directly for the measurement

of oxide layer thickness (~l~rn). In some cases salt drops have been crossed by the fracture giving valuable information.

However the lack of flat cross sections

has limited the use of the crystal spectrometers of the electron microprobe. In many cases the deposit drops do not adhere welt to the sample surface after cooling so that the underside of the drops and the area of the sample which was under the drops can be examined and analyzed at least qualitatively. Mechanical bursting of bubbles in surface layers and removal of drops are also used to study the underlying microstructure prior to washing of the specimens.

Gaseous

Corrosion

Introduction The results from the gaseous corrosion studies will be discussed by considering first the studies of silica and alumina, and then the studies performed using Sic and Si3N4.

Gaseous Corrosion of Silica and Alumina The gaseous corrosion of silica and alumina were performed at temperatures of 700, 1000 and 14000C in a number of different gas environments which included oxygen, 02-H20-H2,

02-CO2-CO

and O2-SO2-SO3 gas mixtures (Table II). The

results obtained from these studies are discussed in detail in Reference 1 and Appendix A. The specific conclusions developed from these studies are as follows: Silica (1) Devitrification of silica glass to cristobalite took place rapidly under all atmospheres studied at 14OOOC. The rate of crystallization increases with increasing temperature and time. (2) The devitrified layer undergoes a displacive transformation with a large volume change on cooling which causes cracking. Such a transformation in crystalline SiO2 is important with regards to the use of crystalline SiO2 scales on Sic, Si3N4 and metallic alloys as protective barriers for high temperature applications. (3) The silica is significantly affected when exposed to low oxygen pressure at 1400°C. Silica weight losses occurred after exposure to either wet hydrogen or a CO-CO2 gas mixture and are related to the decomposition of silica in the low oxygen pressure and the reaction of either hydrogen or CO with silica to form SiO vapor and either H20 or CO2 gas. Weight losses increase with increasing temperature and decreasing oxygen pressure.

11

12

High Temperature

Corrosion

of Ceramics

Gaseous Corrosion

13

(4) No significant reaction of silica in CO2 or 02 was observed at all temperatures except for devitrification

at 14OOOC.

(5) Silica is very resistant to attack by SO2-SO3-02 gas mixture under the test conditions.

Alumina (1) Alumina is resistant to attack by H2-H20-02

gas mixture but impurities

in the alumina materials such as SiO2 and Na20 can result in volatilization.

The

volatilzation is favored by low oxygen pressures and high temperatures. (2) Alumina is very resistant to attack by CO2 gas. (3) Alumina is resistant to corrosion by SO2-SO3-02 gas mixtures. Some reactions occurred especially at high SO3 pressures (e.g. 7~10~~ atm. SO3) and low temperature (7OOOC). (4) Corrosion of alumina in SO3 containing gas can occur even on the highest purity alumina where reaction products with activities less than unity are formed. It may be due to the formation of a solid solution of Al2(SO4)3 with Al203 or a nonstoichiometric sulfate. The observed sulfur was identified as a ~+6 (as in sulfate) by ESCA. (5) Degradation of alumina in SO3 containing gas becomes more severe when Mg or Ca containing impurities are present, and it increases as impurity content increases. The formation of products occurs preferentailly along the grain boundaries (i.e., on the impurity second phases at the grain boundaries). (6) Corrosion of alumina in SO3 containing gas becomes more favorable at higher SO3 pressures and lower temperature (e.g. at 7000 than 1000°C), since a lower SO3 pressure is necessary to form sulfate at 700°C than at 1000°C. severity of corrosion increases with time.

The

14

High Temperature

Gaseous Corrosion

Corrosion

of Ceramics

of Silicon Nitride

The gas induced corrosion

and Silicon Carbide

of Si3N4 and SIC was studied

oxygen and at 1OOOoC in an 02 - SO2 - SO3 gas mixture pressure

of 0.01 atm.

These limited

studies

devitrification

of the silica scales formed

and to provide

baseline

with an initial SO2

were performed on these

data for comparison

only at 1400°C in

to determine

materials

occurred

with the hot corrosion

if

at 1400°C,

studies

at

1ooooc. The studies

in oxygen at 1400°C showed varying results

form of the substrate crystallization

was observed.

12 hours of oxidation

that were believed

original

In the case of the single crystal

of the silica scales

1 urn thick after patterns

material.

and cracked

to radiate

nuclei of the crystallization

These scales

from nucleation

most cases than those formed

on the single crystals

surface

and contained

crystallization

bubbles resulting

was attributed

glass structure.

The hot pressed

samples

were thicker

in

of Sic being about 10 urn

particularly

exhibited

a glazed The lack of

Al2O3, stabilizing

in these scales

The hot pressed

a 10 urn thick scale which was composed

the

layer into cristobalite.

silicon carbide

was identified

XRD along with a large “glass” peak.

indicating

from CO and CO2 evolution.

to impurities,

Some cristobalite

Sic which were about

centers

of the amorphous

on the polycrystalline

12 hours of oxidation.

upon the

upon cooling to form star

The silica scales which formed

after

depending

however

the by

silicon nitride

also formed

and enstatite

(MgSiO3).

of cristobalite

The CVD silicon nitride,

which was much purer than the other silicon

nitrides

developed

thin silica scale after

There was

virtually

an extremely no change

specimens of oxidation

in surface

prior to oxidation

morphology

12 hours of oxidation.

of the oxidized

specimens

and the weight change of specimens

was below the detection

limits of the techniques

compared

after

to

12 hours

used in this program

Gaseous Corrosion

15

to measure weight change. No impurities were detected in these silica scales on CVD silicon nitride which XRD analyses showed to be a mixture of cristobalite and glass. The sintered silicon nitride developed a scale composed of glass containing some cristobalite.

Yttrium silicate crystals were observed to protrude

out above the surface of the silica scale. Impurities influenced the oxidation of this material substantially. The results obtained from the studies performed at 1400°C show that the oxidation of SIC and SiSN4 is dependent upon the structure and composition of the silica scales that are formed upon these materials. Glassy silica provides a more protective reaction product barrier than crystalline silica, however, the incorporation of impurities into the glassy silica can cause the protectiveness of the glass structure to be decreased very substantially by promoting devitrification. At 1000°C the pressure of SO2 and SO3 in the gas mixture along with oxygen did not significantly affect the oxidation of pure silicon nitride or pure silicon carbide compared to oxidation in pure oxygen. The major effect of SO2 and SOS occurred when the specimens contained impurities. While the effects of impurities were significant but not documented extensively, these effects were not as substantial as those observed at 1400°C, since the impurities cannot concentrate in the oxide scale at 1000°C and thereby affect the protective properties of the glassy silica scales.

Hot Corrosion of Silica The results obtained from the studies on the hot corrosion of silica are presented and discussed in more detail in Appendix B of this report. In the following the important results from these studies are briefly summarized. Specimens of fused silica (Corning 7940) about 1 cm x 1 cm square and 1 mm thick were exposed to a variety of conditions known to cause the hot corrosion of metallic alloys. The experimental procedures have been described in the experimental section of this report. Results from four different sets of experimental conditions will be discussed in this summary. These experiments were performed at 700 and 1000°C. The deposits of Na3S04 which were applied to the specimens’ surfaces were liquid at 1OOOoC. At this temperature two different gas compositions were used. One consisted of an SO3-03 gas mixture and the other was pure oxygen. The SO3 pressure in the gas mixture was 1.5 x 10e3 atm. When Na3S04 is exposed to gases containing SO3 the activity of Na90 in the sulfate is inversely proportional to the SO3 pressure. If the activity of Na30 in the sulfate is taken as a measure of the basicity of the Na3S04, higher SO3 pressures established less basic, or more acidic melts, whereas the gas containing only oxygen causes the more basic liquid sulfate to develop. Similar sets of experiments were performed at 700°C however at this temperature the SO3 pressures in the gas mixtures was 7 x 10m3atm. Furthermore the deposit applied to the specimen surfaces was Na3S04 - 50 mole percent CoSO4 since pure Na$304

melts at about 883OC and the sulfate mixture

is liquid at 700°C.

16

Hot Corrosion

of Silica

17

The liquid sulfate deposits wetted the silica specimens in varying amounts depending upon the experimental conditions.

Time at temperature and the

thickness of the deposit also affected wetting. Generally the liquid deposits wetted the silica more completely under basic conditions.

At 1000°C under

basic conditions the salt wetted most of the coupon after 1 hour with a wetting angle of 20. After 24 hours the wetting was continous. At this temperature and under acidic conditions droplets of liquid were formed with wetting angles Of 24O, and 130 on large droplets (’ 0.2 mm dia), after 24 hours. Wetting was not as complete at 7OOoC. In the case of basic conditions droplets with wetting angles between 13 -240C were observed after 24 hours, whereas under acidic conditions the angles ranged between 36 - 49O. At 7000C under acidic conditions some limited localized attack of the silica was observed under of the salt droplets. No evidence of devitrification

of

the silica was observed. After water washing to remove the salt, small weight losses (0.1 - 1 mg/cm2) were detected and small voids were evident in the silica where salt droplets had been present prior to water washing. The voids were more concentrated near the perimeter of the droplets. Analysis of these results has been complicated by the decomposition of the CoSO4 in the liquid via the reaction, coso4

* coo

+ so3

since the SC3 pressure in the gas was lower than the equilibrium pressure required to maintain the initial liquid deposit. The principal deposit

reaction

of the

with the silica should involve the Na20 component of liquid since

sulfates and sulfides of silicon are not stable under the experimental that were employed.

Hence,

conditions

a reaction of the following type seema plausible,

XSiO2 + YNa2S04 = YNaZO-XSi02

+ ~~03

18

High Temperature

Inspection becomes

less favorable

permit silicate

formation.

silica interface

affect

as the SO3 pressure

of sodium silicate

is increased.

in the melt over most of the specimen

at localized

localized

of Ceramics

of this reaction shows that the formation

SOS pressure

sulfate

Corrosion

However,

It is believed

surfaces

some dissolution

that the

is too high to

of silica occurred

regions under the drops, in particular

is believed to result from impurities

in the

along the sulfate

which caused voids to be evident upon water washing.

dissolution

phases

-fused

This

in the silica which

its stability. Under basic conditions

the CoSO4 component the gas phase.

at 700°C the complication

Cristobalite

some cobalt oxide resulting surfaces.

acidic conditions,

beneath

from decomposition

of CoSO4 remained

The important

however is that crystallization

the salt droplets.

at some specific

upon the under the

the activity

result obtained

was not observed

of NaSO in the liquid deposit

level in order for crystallization cristobalite

but no devitrification

from the droplets.

The weight losses of specimens

was observed

being formed at the higher temperature.

must

to proceed. to form beneath

of the fused silica was evident

away

under acidic conditions

This can be accounted

at

of the fused silica

Since crystallization

all of the droplets,

1000°C were less than at 700°C.

because

is believed to involve NaSO, such a reaction

At 1OOOoCunder acid conditions

silicate

the salt drops.

of the fused silica was observed

in the more basic melt.

at 700°C under acidic conditions, be established

beneath

after water washing were not meaningful

and this process

7000 under basic conditions was observed

were observed

spherulites

Since dissolution

would also be expected

of

of the liquid was more severe since there was no SOS in

Weight change measurements

specimen

from decomposition

for by less sodium

at

The most extensive conditions surface

degradation

of the vitreous

at 1OOOoC. A layered reaction

of the fused silica specimens.

interface

the following sequence

exposures:

sodium silicate,

thickness

of the crystallized

isothermal

conditions.

observed

parabolic

silica occurred

under basic

from the salt-specimen

cristobalite,

silica conformed

after very long

unaffected

to a parabolic

fused silica.

this crystallization

aa a result of thermally

however,

that some sodium silicate

dependent

upon the composition

As discussed significantly

previously

affected

are formed

early in

times when the crystalline

in the section

phases

to form silicates

and the structure

on gaseous corrosion,

by any of the gas environments

degradation

was detected

is

of silica.

by two different,

processes

but related,

of NaSO in the deposit. was devitrification.

the liquid was increased

silica was not

used in these studies.

No

at 700 or 1OOOOC. On the other hand

of fused silica was observed deposits

degradation

products

of the liquid deposit

in oxygen, when liquid sulfate

activity

There is no

becomes richer in silica it is less soluble in water.

nor devitrification

significant

The

Analyses of wash water,

Such results suggest that the reaction

Also as the silicate

corrosion

was formed.

process than after long reaction

have been formed.

products

for by assuming the crystallization

shows that more water soluble corrosion

the corrosion

proceeded

induced stresses.

of the fused silica is caused by sodium from the liquid deposit. question

The

rate law under

to a linear rate law since the crystalline

rate has been accounted

19

was formed over the total

Proceeding

Under cyclic conditions

from the specimen

of Silica

of phases was observed

tridymite,

more rapidly and conformed spa&d

product

Hot Corrosion

in SOS-02 gas mixtures,

were present.

and

This degradation

occurred

both of which were dependent

upon the

The process which caused the most severe This process

and it occurred

increased

as the Na70 activity

at both 700 and 1000°C.

It requires

in a

20

High Temperature

Corrosion

of Ceramics

threshold

Nag0 activity.

thermally

cycled since the crystallized

thermally

induced stresses.

silicate latter

as a reaction process

than vitreous

It was especially

products

The other process

product on the surfaces

is less prevalent silica, nevertheless

Nag0 in the liquid is increased. impurities

severe

when the specimens

spalled under the influence involved the formation

of fused silica.

when the liquid is reacting this reaction This reaction

were

of sodium

It appears

silica

as the activity

also appears to be affected

in the fused silica when its driving force is low.

that this

with crystalline

also increases

of

of by

Hot Corrosion of Alumina The results obtained presented

and discussed

from the studies on the hot corrosion

of alumina are

in detail in Appendix C. In the following

important

results are summarized. The weight changes measured small but not negligible.

after hot corrosion

They were due to offsetting

solution of Al203 into the sulfate

reactions

melt, the silica and silicate

mostly due to impurity phases and the precipitation oxygen.

of the aluminas

were

such as the precipitation

of Co0 at 7OOoC in pure

In general the weight changes appear greater

than for gaseous

corrosion. The sulfate tended to wet the aluminas partially conditions

(~20~ after 24 hours).

a function

of time apparently

reduced the affinity

The wetting

of the sulfate

The wetting

was better

wetting

tended to increase

of the reaction

tendency

results in more contact

area between

a little.

than under acidic tendency.

The wetting

with the substrate.

the sulfate

and the substrate

alumina tended to react very little

in line with the gaseous corrosion

results

than under basic conditions.

The corrosion

and

At higher

improved

the wetting

of the sulfate

as

Good which

the corrosion.

The single crystal

conditions

the wetting

at 1000°C under basic conditions

as the impurities

also promotes

into the sulfate

for the alumina substrates.

(lOOO°C) under similar conditions

is an indication

of the purer aluminas decreased

as some alumina dissolved

temperature

conditions

at 7OOoC under acidic

of the polycrystalline

grains is a function

of orientation,

more reaction

occurred

The single crystal

aluminas indicated with greater

21

attack

with the Na2S04 and under acidic

has basal orientation.

that the corrosion

of the

of planes away from the

22

High Temperature

basal orientation.

Corrosion

of Ceramics

This result is expected

for solution of a single crystal

in a

melt. Under acidic conditions,

at 7OOoC sulfates

were formed and after long exposures polycrystailine silicate

materials.

impurities

silica.

solubility

the substrate corrosion

globular silica was observed

by the melt generating gradient

by acid fluxing and precipitation probably cristobalite,

is concentrated

of silica.

to the microstructure

melt are smooth with no marked preferential

significant

At 1000°C sodium magnesium

attack

with The

at 700°C and occurs on a the regions under the sulfate at the grain boundaries.

are formed

and alumina

At

is

in sodium aluminum silicates.

Under basic conditions

silicates

of alumina

of silica globules,

of the substrate.

attack

and calcium sulfate

the

and precipitating

melt away from the interface

wider scale at 1OOOoC. At the higher temperature

incorporated

on all

dissolution

The precipitation

near the grain boundaries

sulfate

sulfates

is set for continued

occurs in the sulfate

and is not related

1000°C magnesium

and magnesium

This shows that under the most acidic conditions

are attacked

The required

of aluminum

were formed.

corrosion

aluminum silicates

occurred

and sodium calcium

The salt on cooling contained

of the alumina grains

at 1000°C and significant

was limited

intergranular

at 700 and 1000°C. aluminum

Mg, Al and some Ca.

under the more basic conditions corrosion

was evident

The overall

in the

micrographs. The impurities polycrystalline

played a major role on the corrosion

aluminas,

oxygen (basic conditions). single crystal, present

particularly

While there was little evidence

the high Nag0 activities

at the grain boundaries

aluminum silicates

at high temperature

promote

reaction

of the polycrystalline

grew from the melt with transport

behavior

of the

(lOOO°C), in pure of basic fluxing of the with the silicates

materials.

Sodium

of silica and other oxides

Hot Corrosion

along the grain boundaries. grain boundaries silicate

This is illustrated

of high purity polycrystalline

after long term cyclic exposures.

silicates

formed near triple points.

was formed and the crystals

growth of silicate interesting,

crystals

various alkaline

in the materials.

role in the corrosion

dissolve the alumina grains by formation The two reactions, proceed

the grain reaction

cooperatively,

Therefore polycrystalline

most technical coatings

ceramics.

on superalloys

based impurities

but in a general

are not favorable

in the field of stability

gaseous corrosion

experiments.

alumina was observed high temperature

lower the Na30

in the melt by acidic fluxing.

is proposed

will

by the other.

for the hot corrosion

play a major role.

mechanism

of

As shown which means on

tends to dissolve

the alumina even

of aluminum sulfate

by acid fluxing.

The formation

in the

melt a wide range of activities

Under basic conditions

little

or no attack

although basic fluxing should be possible,

(more basic conditions).

at

of

of alumina was already observed

In a sulfate

silica-

applies.

for the formation

The alumina is dissolved

locally.

It is

manner since they do not contain

the sulfate

aluminum sulfate

be established

of the alumina.

This may apply also to alumina scales grown on

Under acidic conditions,

unit activity.

grains is

even on 99.8% purity alumina,

for which the following

though conditions

and

While this fluxed

and the grain boundary reaction

alumina in which the impurities

above they have a major influence

more silicate

This more acidic Na3S04 then can

of sulfate

mechanism

in the

and impurity

as the ion needed for one is produced

a fundamental

of the

was present

which form sodium silicates

and raise the SO3 locally in the melt.

23

of this

earth elements

feeding from grain boundaries

proposed that the impurity reactions activity

alumina with crystals

Some magnesium

as impurities

it plays a fundamental

decoration

With the more impure aluminas

contained

which were present

potassium

by the perfect

of Alumina

can

of single crystal particularly

However this is not promoted

at in

24

High Temperature

presence

of silica-based

Corrosion

of Ceramics

impurities

which are present

as second phases in the polycrystalline conditions

so that intermediate

the dissolution

aluminas.

activities

basic conditions environment,

of this study.

the sulfate

silicates

which are dissolved

is increased.

in the sulfate

can proceed

This promotes thus decreasing

cooperatively.

1000°C, higher Nag0 activities sodium aluminosilicates

as discussed

the acidic impurity

the attack

of the

the Nag0 activity. of Na70

and SiO7 is precipitated.

At

by the same mechanism

phases, thus promoting The two reactions

and

the sulfate

the dissolution

proceed

cooperatively

earlier. just presented

are extremely

of catastrophic

circumstances

attack

without replenishment

it appears

the sulfate

of the experiments,

even after 400 hours exposure.

provide some of the requirements.

aluminas

slow and there was no experimental

one may ask if any of the proposed

attack

sodium sulfate

explain that the polycrystalline

under both acidic and basic conditions

however the processes

factor

and

set by the gaseous

Under basic initial conditions,

alumina by acid fluxing.

were attacked

continuous

of the silicates

At 700°C the activity

are generated

are formed.

The mechanisms

evidence

modify the local

prevail under the acidic and the

was always too low to form sodium silicates

of neighboring

and

tends to dissolve alumina and as this is done the

of Nag0 in the sulfate

melt tends to attack

the attack

Under acidic initial conditions,

activity

The two reactions

The impurities

favoring

of the alumina in the sulfate

at the grain boundaries

mechanisms

of the salt although

Under the would lead to slow acidic conditions

However in view of the rate of attack

that in many industrial

will be replenished

depleted.

Higher temperatures

conditions

might become predominant

before

processes

in which it could be a

it might become

might increase

saturated

the rate of corrosion

under usual (percent

by

or

and basic

or less) sulfur

Hot Corrosion

concentrations

in the atmosphere,

limit the corrosion temperatures.

since sulfate

however deposits

the sulfate

of Alumina

vapor pressure

are no longer formed

then will

at higher

25

Hot Corrosion of Silicon Carbide and Silicon Nitride The results of recent Appendix D. Important

results

The hot corrosion presence

experiments

are summarized

of silicon nitride

of Na9SO4, under acidic,

pure oxygen, conditions corrosion

and silicon carbide

in the temperature

increases

of the oxide layers formed increases

between

the drops) to basic hot corrosion

very limited

clearly different conditions.

the sulfate

melt near the interface the oxide dissolved

During basic hot

activity

and devitrify

devitrification

for the pure materials

by the activity

into the sulfate.

the sulfate

the vitreous

silica and modifies

it.

rapidly

was sparse

In general,

In all cases the materials

droplets.

26

and

under the two

oxidize

the sulfate

of sodium oxide in equilibrium

is set up in between

and the

of sodium oxide in the sulfate

For acidic conditions,

atmosphere

it

than for dry

under acidic conditions.

with the substrate.

wet the oxide, and a surface

and basic,

the rate of oxidation

and both are greater

droplets

are observed

They are controlled

in

markedly from acidic (measured

For the purer materials

behaviors

has been studied

oxygen initially

The oxide layers formed tend to be vitreous

in between

in

the samples while during acidic corrosion

The hot corrosion

under the liquid sulfate.

in detail

range 900-1000°C.

thickness

oxidation.

and discussed

below.

1% SO9-balance

the salt wets completely

breaks up in droplets.

are presented

and

does not

with the

The sodium diffuses

into

Hot Corrosion

Under basic conditions, was consumed

into the sulfate

and a silicate

cristobalite

phase remains

otherst5).

melt.

As the silicate

at the interface.

The melt contains

Nitride

27

and the NagSO4

oxidize at their surfaces

The Nag0 activity

and

builds up at the

enriches

in silica,

After long exposures,

on top of the silica and the sulfate

on top of the silicate. as indicated

The ceramics

layer is formed.

is nucleated

and Silicon

a thick product layer was formed

slowly in the reaction.

the oxide dissolves interface

of Silicon Carbide

the silicate

left is in small isolated

a high concentration

drops

of silica initially

by the wash water analysis and it has also been well documented The observations

Mayer and Riley(S) except

are consistent

with the mechanism

that the reaction

for NagCOS which they studied. product layer since greater

by

is much slower with Nags04

Some protection

degradation

proposed

is offered

was observed

by

than

by this complex

after preoxidation

of the

samples. Under acidic conditions reactivity

with the salt were observed.

even between in detail.

the sulfate

drops.

This oxide growth between

They were thicker

by spherulitic

thicknesses

crystallization

droplets

the drops was studied

into the silica

devitrified

rapidly

or random globular formation.

layer formed

outside the drops except

for which these thicknesses

of oxide formed under the drops tended

(CVD silicon nitride,

was enhanced

(~10 A thick) and sodium diffuses

than the vitreous

the C-side silicon carbide

and little

In these regions a very thin

The silica layers formed under the sulfate

into cristobalite

surfaces

However the oxidation

The oxide formed was mostly vitreous.

layer rich in sodium is detected formed.

(1.5~10-~ atm SOS), poor wetting

C-side silicon carbide

studied but they were still smaller than the product

were similar. to be similar

for

The

for all three

and Si-side silicon carbide) layers formed

under basic

28

High Temperature

conditions.

Corrosion

This is generally

liquid phase and therefore

consistent

was no more differentiation

correlates

with the results

and under the droplets

surface.

for acidic conditions

obtained

of random cristobalite

for the hot corrosion

occurs by transport

there

globules

which

of bulk silica.

Selective

through the liquid phase between

the

fibrils.

enhanced

of this formation

which occurs between

oxidation.

nitride are complicated

The kinetics

apparently

was obtained

silicon carbide has different

of each side.

they oxidize parabolically

In order to interpret

for the oxidation

energies

how this

so strongly.

The

and for the silicon-side. at different

were obtained

these results,

of silicon carbide.

Some evidence

It is not clear presently

slows down the oxidation

is

of silicon

of oxynitride.

behavior for the carbon-side

activation

droplets

of this hot corrosion

in this research.

During the acidic hot corrosion Kinetic data and apparent

the sulfate

by the formation

thin layer formed during the oxidation

oxidation

by the

in the behavior of C-side and Si-side silicon carbide.

The acidic hot corrosion essentially

determined

of the nature of the substrate

for the formation

attack of the substrate spherulite

with a mechanism

independent

Indeed for the basic conditions

A model was proposed

of Ceramics

rates.

for the hot corrosion

it was logical to modify a model

However,

a satisfactory

explanation

of the

of silicon carbide had not been proposed.

Based on the oxidation premise that the parabolic

results of others, oxidation

the oxide layers to the substrate all cases, the different oxygen deficient

is controlled

interface.

vitreous

structures

by diffusion

Although

rates of oxygen transport produced

carbide C-side and silicon carbide Si-side. for the oxidation

a model was developed

vitreous

of oxygen through silica is formed

are associated

in the oxidation

Tentative

of the two sides of silicon carbide

on the

in

with different of silicon,

mechanisms

silicon

were proposed

in which the oxides are formed

Hot Corrosion

under different defect

oxygen pressures

structures.

and therefore

The observed

for the acidic hot corrosion

of Silicon

trends in the variation

qualitatively

and Silicon

are expected

of these materials

that of silicon are predicted

Carbide

by these

constants

to their oxidation

models.

and

High apparent

activation

energies

consistent

with the models although that for the silicon-side

such as those measured

to have different

of the parabolic

compared

29

Nitride

for hot corrosion

are generally of silicon carbide

could not be estimated. The behavior impurities

of engineering

they contain.

materials

is strongly

MgO and Y3O3 tend to segregate

during oxidation

and hot corrosion.

acidic conditions

so that wetting

as the conditions

are more basic than promoted

atmosphere.

Conversely,

these materials between

is improved

the wetting

experiments.

and thicker

The impurities

the oxide layer appears to stabilize

In particular devitrification preferential corrosion

layer formed on oxidation

The corrosion

correlates

attack

occurred

of these materials

mechanisms

proposed

intermediate

selected

in the silicon carbide or hot corrosion.

by the activity

of sodium.

for the oxidation

silicon carbide suggest directions

of silica.

It was shown also that

qualitatively.

and hot corrosion

for further

of

The

of the scales by their

locally under acidic corrosion. is well understood

The

of the hot corrosion

well with the results on the hot corrosion

by these results.

for the

varies with environmental

the major role played in the degradation is emphasized

for

it against devitrification.

has been made in the understanding

and it was shown to be controlled

behavior observed

set conditions

Alumina impurities

alumina entering

conditions

with the

with the atmospheres

to the product

silicon nitride and silicon carbide.

under

oxide layers are formed

by equilibrium

do not segregate

Great progress

toward the surface

is not as good under basic corrosion

by the equilibria

basic and acidic corrosion

by the

They lower the acidity of the sulfate

as for the purer ones.

those promoted

influenced

studies.

Overall the hot The atomic

of silicon nitride

and

1.

J.R. Blachere and P.S. Pettit “High Temperature DOE Report ER45117-2, March 1988 a) DOE Report ER45117-1, June 1985 b) DOE Report ER10915-4, June 1984 C)

2.

a) b)

Corrosion

of Ceramics”

H. Yakovitz and D.E. Newbury, SEM/1976/1, IIT Research Institute, Chicago, IP, p. 151. J.I. Goldstein et al.,References Scanning Electron Microscopy and X-ray Microanalysis, Plenum, No. 4, 1981, p. 354.

3.

J.R. Blachere (1987).

and D.F. Klimovich,

J. Am. Ceram. Sot., 70 [ll] C324-C326

4.

L.E. Murr, Interfacial Phenomena Reading, Mass., 1976, p. 67.

5.

N.S. Jacobson,

6.

M.I. Mayer and F.L. Riley, J. Mat. Sci., l3, (1978) 1319-1328.

in Metals and Alloys, Addison-Wesley,

J. Am. Ceram. Sot., 69 [l] 74-82 (1986).

30

Appendix A-Gaseous Corrosion of Silica and Alumina in Sulfur Oxide Environments H.R.

Kim, J.R. Blachere and F.S. Pettit

Abstract: A fused crystalline SOj-

silica

to corrosion

pressures. intensity

Supported

high

forms with

SO2~02 at

Sulfates

of

different

formed

The reaction increasing

U.S.

under

impurity

Department

less

content,

of

to mixtures

with

low temperature

severe

conditions

particularly

Energy

under

meetings

of

and high and with

Number

DE-ACOZ-8 lER109 15-A000

Presented Society

in

part

to

in Cambridge,

the

1982 Fall

Mass.

31

American

the silica.

Ca and Mg.

Agreement

Ceramic

of

resistant

are very

was observed

at

and poly-

crystal

xiere exposed

and alumina

No reaction

particularly

occurred

in single

levels

The silica

environments.

on the aluminas

by the

and alumina impurity

700°C and 1000°C.

in these

with

purity

SO3

greater

32

High Temperature

Corrosion

I. Ceramics

(

chemical

extensive

stability

research

over

recent

the needed

improvements

peratures coatings

them severe

the

df

in advanced

such

as AlZO3,

of

Na2S04 in

in

this

discussed

here.

(5,6)

in oxidizing

in

salts

been

systems

refractorithrcugh

partially

may be anticipated result

as

from

that the

at high

thermal

from

ceramics

melts

such

melts,

gaseous

such

tem-

barrier

environments

(3)

which

cause

products

of the

of

salt

for

of

great

of

performed

research

importance

of since

on silicon

encountered

that molten

has been

on the

the

components

with

those

on

combustion.

the

ceramics

of

costly

known about

susceptibility

and SiOZ forms

are

shown recently

The corrosion

deposits).

resistant.

The environments

representative

the baseline

environments.

is

ZrO2 can react corrosion

as

on superalloys

has been

little

has been

to establish

are

research but

corrosion

and glasses

atmospheres.

It

atmospheres

materials

totally

as slags

corrosive

corrosion.

only

are

as NaZSO4 mpy form on

The gaseous not

also

These scales

high

may be used

alloys

and yttria-stabilized

by gaseous but

that

may contain

gaseous

‘1 C i n presence

as protective carbide

of

Si3N4

systems

have

will

superalloys,

As a result

in deep

systems

the

research

however by oxide

SO3 gas.(b)

corrosion

and it

components

metallic

industries.

deposits

to enhance

they

power

much improved

Ceramics

rePlacing

materials

energy

tend

crramics(

assume

ceramics

these

In addition,

energy

not

of

processing.

advanced

They have

(*)

of

of

problems.

and glass

corrosion

years.

projects

protecting

refractories

steel

because

properties

reliability

parts

as coatings

of

corrosion

the

in

components

is

twenty

demonstration

corrosion

Corrosion

(1) This

past

on ceramic

One should

to the

the

as monolithic or

for

and mechanical

in

emphasis

actively

temperatures.

successful

present

INTRODUCTION

considered

at high

operating ness

are

of Ceramics

of

and ceramics

deposits studied

ceramics

encountered hot

corrosion

silica they nitride

to in of

and alumina perform and silicon

is

Appendix

II.

Gaseous just

as in

the

corrosion

that

are

metals

the

case

formed

gas.

following

are

with

being

possible

result

in

based

phase

the

being

of

silicon

properties produced

of

the

possibility

duction

of

reactions

SiOz

probably

could

proceed

ation

of

in

the

gas

cationic carbonates

such

of

plays

would

involve

fast

phases

its

the

the

gas

silicon

by

type

the

of

oxygen of

diffusion

surface

of

and react

to

the

compounds

depend

with

oxygen

but

be very

important.

gaseous

ceramics. a significant compounds

such to CrOj

to

on the

ceramic

processes

form

chrome-

cyclings.

will

(s)

the

of

with

of

of

stoichrometry

reacts

species

CrpOj

cations

degradation

volatile

through those

of

can also

oxidation

to

reaction

surface

formation the

of

atmosphere

nitride

and the

Such a process

changing with

species

as an oxidant

may form more stable

CO2 or Hz0 may possess oxide

ceramic.

role

by

states the

oxygen

or merely

upon the

compared on the

the

In

to reducible

First,

of

products

ceramic.

in oxidized

case

the

characterized

electron

and particularly

displaced

or

always

Moreover,

of

gas and the

already

a major

This

the

nature

the

into

and nitrogen.

to SiO (g)

or hydrates.

in

example

formed

by the

can be proposed.

for

products

or

33

mechanisms

and alloys.

of

are

of

metals

almost

new phases

This

ceramic,

as SOj.

component

is

transfer

which

of

or molecules

at rates

condensed

gas

incorporated

involves (s)

between

ceramics

temperature

by the elements

h third

controlled

via

states

formation

dioxide

of

processes

oxidized.

the

corrosion

the

the

being

with

the atoms

cations

form

the

refractories

Second,

of

to higher

could

by a variety

reaction

oxidized

corrosion

in oxygen

can occur

ultimately

reaction

result

the

gaseous

of

would

of

the

The cations

may be oxidized

ceramics

as a result

elements

the

of

Corrosion

GASEOUS CORROSION OF CERAVICS

of

process

and alloys

metallic in

corrosion

A-Gaseous

effects

as

the

re-

(g).

Such

boundary

layers

and

involving

the

Finally,

molecules

affinity such

form-

for

as sulfates

the

34

High Temperature

The rates

of

such

corrosion

products

The gaseous respects in

to

the

the

determine rate

processes

corrosion

of

relative

the

corrosion

the

liquid,

the

density

in

the

liquid,

the

porosity

by

the

liquid,

the boundary

layer

and the

structure

of

size

dissolution.

The liquid solid

for

by the

must be used. wetting play

grain

effect,

a similar There

is

This

is

the role

significant

to be on their

For

SiO (g) instance,

also size,

(*l!

significant

only

decomposition weight

faster

influence

with

pore

of

the

of

reaction the

gradient solid

phases

of

coarse

grain

the

rate

of

the

on the

experiments

geometries

so

that

and microstructures

studies

and the

system.

in engulfment

corrosion

corrosion

the

materials.

the microstructure

of

viscosity

controlling

resulting

structure

often

geometry

in than

simple

gaseous

which

concentration

factors

boundaries

since

grain

except

for

boundaries

should

corrosion. on the

literature

under

major

extensively

process.

phases,

and the

interpretation

the

reduction

temperature

by thermal

grain

literature

the

the

liquid,

dissolve

for

various

interface

are

systems

as in liquid

appears

of

valid

the

some

studied

The corrosion

the wettability

the

.The

the

of

the

solid, at

the

Simple

grain

carbide

at high

the

(9,101

greatly

silicon

Silica

into

studies

in

been

and products

process.

rate

materials

liquid

complicates

fundamental

of

pore

may penetrate

grains

corrosion

Fine

gradient

has

in

as a dissolution

reactants

corrosion

to be similar

which

modelled

through

may prevail.

expected

in melts

of

of

The grain

is

usually

transport

rate

on the

is

upon transport

mechanisms

ceramics ceramics

It

the

the overall

depends

of

(gv 3l

be dependent

complicated

of

corrosion

involve

of Ceramics

will

and rather

literature

The kinetics

Corrosion

oxidation

on gaseous

and volatilization low oxygen and/or losses

pressures by reaction

were

observed

of

silicon

corrosion

of

particulary

in wet

and

simple

oxides

for

volatilizes with

nitride

silica.

forming

hydrogen hydrogen

or

CO (12*13!

(51

at

Appendix

1400oc. to

Alumina

lose

is

impurities

alumina

are

Steam at of

silicate

of

sulfur

such

only

low

much more

as sodium

observed

can

refractories.

with

there

and silica;

this

the

silica

are

is

tvpe

losses

of

19OOOC. (14)

of

resulting

in degradation

on the

discussed

35

and tends

order

no reports

this

Corrosion

Significant of

temperatures,

However,

on alumina III.

(1)

high

of

temperatures.

form hydrates

(15)

gases

to conditions

at high

at very

temperatures

oxide

resistant

A-Gaseous

Influence

below.

EXPERINENTAL PROCEDURE

Materials They include

commercial A brief the

four

suppliers

is

a transparent

in

the

form of

The more

is

of

given

in Table

densities

I.

substrate

(0001)

(S)

with

transparent alumina

and (MI aluminas

alumina

(19Urn).

Their

washers;

(S)

on the

microstructure

will

selected

(M) are

average

be discussed

material

is

of

impurities.

L-kin I’ 1

later

crystal

substrates.

effects size

investigate

The single

white

gross

grain

from

to

corrosion. The (L)

and

obtained

configurations.

were

to show the

a much finer

were

and flat

orientation.

was included

have

which

The aluminas

and impurities

The (S)

(‘2)

high

silica

microstructure

almost

impure

and a fused

relatively

in

description

influence

aluminas

than

(L)

as needed.

Experiments Flat

specimens

(

in either

as received,

they

cleaned

were

oxyeen

atmosphere

z

l~l.cm) polished

thoroughly at

or

IOOOoC for

a platinized

catalyst

with

the

The conditions

operations.

at

The SO3 pressures

decomposition

of

Na2Sfl4

environments.

The initial

exposed

relief

in acetone

over

samples.

were

were

enough

corrosion

SO2 contents

and then

of

the

at

Prior heated

the

experiments

ml

and

in were

flowing passed

before

contact turbine

relevance

to gas

in most

instances

to prevent

gases

performed were

“s~ally

lno~~c

to exposure

to have

exueriments of

eases

The ~02 and 02 mixtures

selected

high

oxide

conditions.

and alcohol

temperature were

in hot

polished

one hour.

the

to sulfur

in

the

the

same

1%. 0.1%

and

TABLE Designation

I. MATERIALS

Materials

UNDER STUDY % Purity

Silica

fused silica

3 2.20

8.C.

sapphire

3.97

99.99

L

a-A1203*

3.97

99.79

M

u-A1203* a-A1203*

S

3.89

99.6

3.74

97.4

* Polycrystalline

(1)

Corning

(2)

Tyco Laboratories

(3) General

Glass

Electric,

(4) 3M, Technical (5) Saxonburg

Lamp Div.

Ceramic

Ceramics.

Prod. Div.

Inc.

99.89

(Imp)

(0.1 H20)

Suppliers Corning

7940(l)

Saphikon(2) (0.1 MSO. 0.11

Lucalox

(3)

SiO2) (0.17 t&O, AlStiS 772 (4) 0.17 SiO2) (0.75 ugo, +l.6SiO2, O.lNa20 S-697 (5)

Appendix

0.01% of

with

about

II.

the balance

times

development

Since

the materials

about

o 200A of

(SEM).

This

of

samples

the

duplicate did

were

Some experiments

product

carbon

exposed

be shown, cm2 of

exposed

product

about

with

little

coverage

I urn thickness.

of

the

before

exposure.

were

coated

Electron

that

with

Microscope exposure

by comparison time

to one

follow

any additional

equipped

with

analysis.

when they

formed

this

of procedure

Energy

X-ray

diffraction

in sufficient

Soectroscopy

specimen

day

Table

experiments.

elemental

the

change

for

was checked

quantities.

Chemical

Analysis

on an analytical

size

of

the

surface,

were

products

they are

and Discussion

was measured

in any of

the

that

samples,

to

the

The corrosion

in no case These

Scanning

same total

SM’s

to

samples

was checked

sequential

the

for

Results

corresponds

lum thick.

and ESCA, but

(1)

the

surface

the

shown in

from one

also

race

the experiments.

weight

considering

were

of

the

in

by Electron

The weight

IV. No significant

the

are

examined

37

and a flow

exposures

in 02 before

the

products

analyzed

and after

of

atmosphere

ware varied

sample,

It

for

(EDS)

the

also

exposure.

before

tools

Spectrometers

were

balance

once

results

investigative

Some specimens

700°C

experiments.

the

to identify

the

Corrosion

SO3 pressures

non-conductive

at

only

one

sequential

to any observation

sequential

(XRD) was used

in of

electrically

with

X-ray

(ESCA) after

same area

prior

of

one week but

performed

was oxidized

in

The major dispersive

were

are

carbon

interfere

pressure equilibrium

usually

on the

samples

not

a total

The corresponding

lcm3/sec.

The exposure

month.

02 for

A-Gaseous

they

of

were

could

equivalent described

a weight

formation

products

the experiments. change

a continuous

usually

be detected

using

the

of

and often

the

layer

can

D.Img/

layer

discrete

to a continuous in

of

It

of

SEM the

,

EDS order

following.

Silica Silica

No visible

is

very

product

resistant formed

to atta’ck

and no sulfur

by SD2-

SO3 atmospheres

was detected

by EDS or

at

700 or

ESCA after

1DDO’C. exposure

38

High Temperature

Tr\BLEII:

Corrosion

of Ceramics

Environmental Conditions A1203 and Silica

in Experiments

on

Initial Pressure (Atm)

Pressure 7oooc

02

Sl.

‘Ll .

SO2

10-2

2.9

x 10-3

8.7

x 1O-3

7.2

x 10-3

1.3

x 10-3

Gas

SO3

(Atm) at 1ooooc

-1.

02

%I.

Sl.

SO2

10-3

2.9

x 10-4

8.7

x 10-4

7.2

x 1O-4

1.3

x 10-4

SO3

%I.

%I.

02

-1.

Sl.

SO2

10-4

2.9

x 10-5

8.7

7.2

x 10-5

1.3 x 10-5

SO3

x 10-5

Appendix

up to one month.

where

dynamic

silicon

high

data

for

the

atmospheres the

as

after

for

were

silicon

performed

above

1000°C

the

evolution

cable

is

samples

of

the

were

exposed

products.

of

types

of

sulfur

small

the

techniques

were

analytical products

(P)

is

two techniques.

If

in

the

in

the

as S-6

sulfur 1 elements

de:ec:cd (L)

conclusion

that were

Table111 sulfates with

only

*o

experiments

thermo-

was

too

formation the

is

less

silica

favorable

glass

was observed

with

increasinR

including

sequential are

results:

amounts

the

close

in

one

the

of

summarized

their

limit

table

only

of

suRRested

to observe III.

products

of

This in

the

sulfates

were

formed

of

detection,

when it

700cC

in Table

observation

products

at

exposures

identification

to

technique

atmospheres

in many cases the

was indicated

the presence

of

and

formation

by at

least

products,

Su

table. that

products

as in

on exposed

sulfates

reported

was established

sulfur

is

in C3-SO3-So3

of

three

by EDS and the

very

of

Sulfate

to all

The results

only

It

the

No experiments

1OOOoC since

experiments

on three

detection

was placed

of

and there

39

1OOOoC.

by ESCA. Since

of

pressure

.No devitrificatioa

700 or

in extensive

based

SEM. the

formation

Corrosion

Results

The alumina and

sulfate

sulfide.

increases. at

of

The oxygen

of

temperature

Alumina General

no evidence sulfate.

formation

the exposures

(2) (a)

is

A-Gaseous

the

the

samples

identified clearly decreasing

products

were

also

sulfate

the was

shows

that

SO3 pressure

in

the

on the

with

combined

of

valency

the

purer

same combination,

is

by the

aluminas

on the

purer

increasing

aluminas,

These

tendency impurity general

of

this

the

leading

on the more impure

an increasing

atmosphere.

thus

observations

Extraction

aluminum and sulfur.

manner

there

temperature,

only

formed

in a similar

sulfates

identification

by ESCA. Moreover,

showed

aluminum

were

by EDS and the

sulfates

in

products

to

the

Mg and Ca samples. to

form

contents trends

and are

replicas

TABLE Results

Tempe'rature (OC) 700

P: N: su: *

of Alumina

Corrosion

Experiment

SO3 pressure (atm.) in approx. 1 atm 02

III in SO3 Gas at 700° add

1000°C for One Week

Single Crystal Alumina

Polycrystalline (L)99.8% M)99.5%

Aluminas S)97.4%"

7.2 x 10-3

P

P

P

P

7.2 x IF4

P

P

P

1'

7.2 x IO-'

N

P

P

P

1.3 x 10-3

N

P

-

P

1.3 x 10-4

N

SU

P

P

1.3 x 10-5

N

N

SU

SU

Sulfur containing products present No sulfur containing products Presence of sulfur suggested but not confirmed Purity

Appendix

consistent

with

(b)

A-Gaseous

Corrosion

41

thermodynamic analysis.

Thermodynamics of sulfate Considering

formation

the reaction: Al*03+ 3SO3(g)= Al*(S04)3

at equilibrium

one has: g_ a Al2 W4)3 a Al203

and the SO3 pressure The stability generated unit

for

the equilibrium

diagrams of

bv considering

activity

boundaries

for for

Figure

between hl2(SO4) 3 can be calculated.

1 for A1303 in SO3-SO3 atmospheres

the appropriate

the solid

(pso3j3

equilibrium

The test

phases.

reactions

conditions

were

assuming

and the relevant

the HgO- Mg SO4 and CaO- CaSO4 equilibria

are shown on the

diagrams.

The oxide-sulfate lower

temperatures.

eqdlibfiumSO3 sulfates

Furthermore,

pressure

increases

have a greater

tendency

of Figures that

equilibrium

la and 16 and observed

the SO3 pressures

required

boundaries

shift

to lower SO3 pressures

for a given initial with decreasing

mixture

temperatures

experimentally:

It is also

for

sulfate

formations

CO ?IgSO4 and finally

co A12(SO4; 3 so thar under similar

CaSC4 and YgSO4 will

form more readily

in product

with

*Preliminary sulfates

formation

results formed

increasing

obtained

on S alumina

than contents

Al?(SO4)3

Therefore, as shown by comparison obvious

increase

from CaSO4

activity this

from Figure

conditions

explains

the increase

of CaO and YgO.

by Raman spectroscopy (16)

of 02- SO3 the

temperature.

to form at lov

ac

also

indicated

that

1

High Temperature

42

a

Corrosion

of Ceramics

0

cao C&m* WN Me

4-

log

POY -lo

-

-1a

-

A12S3 -1(1

b

A12(so4

Al203

4

-10

log

Psoa

log

PSOl

0

Ow 4-

log

Por -10

Figure

*

1.

Stability diagrams for A1203 in SO@03 atmospheres at: (a) 700°C (b) 1OOOoC. The experimental conditions (+) and the boundaries for CaO and MgO are also shown.

I

Appendix

Single

Cc)

Very

about

0.5 pa

gas

of

after

of

should

at

by for

not

detected

the

at

formed

considered

less

in

these

that

The role

of

of

MgD.(L6)

to

the

grain

segregate

aids

its

added

experimental

diagrams

of

the highest

of

exposure

to

the

but

the

dissolved

in

for

crystal

conditions

were

inter-

was was

any of

single

the material most

1 show that Al2(SC4)3

Figure

phases.

sulfur

NO sulfur

nor

the

These

the experiments.

diagrams Solid

Al2O3)

have been

solutions or other

(e.g.

phases not

formed.

less

clear

solubility

boundaries,

lnrterial

at

to the

MgO isilithin was found

is

greatly this

when the

may be

to

700°C

show that

activity

one week

by ESCA analysis.

results

of

of

be found

at

tetragons

Aluminas

spine1

If

could

SD3 pressure

activity

diagrams

the

Sin2

unit

than unit

The sintering

No magnesium

43

during

A few thin

IOCWC.

same time

products

The stability

for

the

indicated

lower

in any of

Polycrystalline

anticipated

the

environment

however

Al2(SO4)3

were

was observed

an exposure

For

no

alumina

700°C and

after

III).

70C°C

crystal

at

IOOOoC. These

reaction.

be

constructed

in

were

to

single

gas mixtures

SC3 at

exposure

compositions

favorable

ion

the

by EDS and sulfates

was affected

(d)

of

7000C (Table

pressure

detected

the

on a side

SO3 pressures

found

reaction

to any of

mediate

Corrosion

Crystal

little

exposure

A-Gaseous

grain is

its

and it

this

very

may have

has not

srr.alL.

degrade limits

metallography

exceeded,

boundaries

material

solubility

by optical

is

but

(L)

but (17)

it been

the

even

but

electron

solubility

be expected

observed.

Ca does

purity

it

is

in A120,. or

enhanced would

its

microprobe.

in presence to segregate

MgO apparently though

its

average

does

not

concentrat_

44

High Temperature

More products crystal the

the

with

observable

to

products

The products same exposure shown

and films. grain strong due

but

sulfates

influence

in part

impurities

to

of

of

concentrated

these

grains

segregation

of

impurity

polycrystalline

content

SO3 pressure

(Table

III)

at

and were

content.

tha

same axpari-nt

the

products

These

than

the

under 1000%.

single

all

crystal.

conditions

In general.

larger

and in greater

The samples

in Figure

(7 x 10-3atm.S03at

form preferentially

at

quantity

4 were 700°C

grain

700°C

or in

for

of

and

as

the

same a

nay be

could the

laths

any products.

aluminas

polycrystalline were at

formed least

formed

and of on all

under

more

4) with

polished

one week).

boundaries.

such

on the

(Figure

relief

cuboids.

This

in no case

products

are

indicating

grain.

Products at

the

faces

all

any

the

small

grain

are

at

produce

which

evident

formed

materials

than

by EDS and ESCA.

detected

surfaces

products

not

are

the

but

on these

corrosion

crystal.

materials

higher

impurity

single

of

also

quantity

no products

impurities

as Ca or Mg be detected

M, S than on the

greater

exhibited

orientation

?iore L.

on certain

of

observed

One is

shapes

all

were

apparently,

were

altiina.

to

products

in less

for

single

amount of

1OOW’C did

vary

other

the

one week

on specimens

did

(L)

also

crystallographic

surface

such

were

but

at

morphologies

as received

many cuboids

faces

of

of

2 the

but

identified

alumina

Two types

2 and 3 for

other

(L)

to

products

III),

SO3 pressure

were

on the

Figure

Corrosion

1OOOnC (Table

intermediate

compared

exposures

As shown in

the

The prodlicts

while

at

alumina

after

SO3 pressure.

formed

contains

(L)

evident

7OO’C.

conditions.

in Figure

The other

at

SO3 pressure

Exposure

7oonc.

on the

were

increasing

higher

of Ceramics

evident

Products

gas mixtures

increased at

were

material.

three

Corrosion

increasing

and exposed

It

the

readily

shows

also

in that

Appendix

Figure

2.

A-Gaseous Corrosion

(L) alumina surfaces (A) as received, B,C,D, exposed for one week at 700oC to 7 x 10-4 atm. 503 and 7 x 10-3 atm. 503 respectively. More products form at higher P503 (5EM)

45

46

High Temperature

Corrosion of Ceramics

A

Figure

3.

L alumina surfaces exposed to 7 x 10-3 atm. SQJ at 700oC for one week. A shows various morphologies of reaction products and 8 only a few discrete ones. (SEM).

Appendix

Figure

4.

A-Gaseous Corrosion

47

Relief polished polycrystalline A1203 surfaces exposed to 7 x 10-3 atm. 803 at 700oC for one week (a) L , (b) M, (c) another M-type (d) 8 aluminas. Products increase in size and quantity with decreasing A1203 purity. They form preferentially along grain boundaries. (8EM).

48

High Temperature

Two types atm SO3 for with or

of

Corrosion

no ca1ci.m

were found

products

one week at

of Ceramics

700°c.

one

it

and the

other

purfty

(S)

in

on the

was

size

after

Ca.

exposure.

They appear

solid

smaller

in size

alumina

had

mosf

EDS analysis

to be mainly

solutions.

The larger

the

magnesium

7 ~10~~

sulfate

was a CaSOh with

products

in

and along

grain

boundaries.

Some smaller

be on

the

grain

boundaries.

(Fi_eure

grains

of

(S)

occur

products

and aluminum present

are

as large

and of included

as silica

grains

formed

little

by reaction

is

probably

dissolved

aluminum

is

expected

to come mostly

as sintering

The evolution in

Figure

of

exposures

phases

6,

coarsening

are

and the

the

mostly

It of

is

from

the

the

In the impurity

Mg,

on impurity

The impurity alumina

grains

CaSO4,

appeared of

the

techniques.

believed

SO3 with

sulfates.

formed

by analysis

by replica

Si,

and their

which

the

and

that

silicon

impurities

alumina,

phases

large

silicon

the

silicate (S)

to

this

Since

talc

and

clay

aids. of

the

from

which

contrast

for

in

of

As indicated

51.

S, Al,

exposure.

size

number

sulfate

MgSDq

products,

products.

Al

used

the

extracted

the

calcium

mostly

of

in both

Indicated

by EDS before

products in

products

were

identified

product

sulfate,

phase( s’)

while

grains

of

the

silicate

is

to

and was a magnesium

larger

magnesium

are

exposed

no magnesium. The lowest

is

(M) alumina,

the

corrosion

same area

three

days

to

darker

on the

relief

polishing.

occurring

a calcium-magnesium

ddring sulfate

the

of

products

as a function

a relief-polished

30 days. initial

The products micrograph

The products latter of

sample

stages.

(S)

nucleate

time

is

after

shown a sequence

on the

impurity

due to a weak acomic

number

grow wirh The product

unknown aluminum

of

content.

coalescence marked

by

and the

arrow

Appendix

Figure

5.

A-Gaseous Corrosion

Relief-polished 5 alumina exposed to 7 x 10-3 atm. 503 700oC for one week. EDS spectra (1) whole micrograph, areas indicated by numbers. (5EM).

at 2,3,4

49

50

High Temperature

Figure

6(a).

Corrosion of Ceramics

Same area of relief-polished (S) alumina exposed to 7 x 10-3 atm. SQ3 at 700oC. (A) before exposure, (6) for 3 days, (C) for 10 days, (D) for 30 days. (SEM).

Appendix

A-Gaseous Corrosion

0.

~ tD... QJ CJ ..:3 ~'C .-£

0... ~ ~

00 .cVI QJ

. VI ~QJ

/;;,~ 0 ~ CJ ~

E

..

'C:3 0 QJ '6' ~ ~ 0. QJ..~ ..~QJ ~ E ..r/)0. ~0.U QJ~r5 ..~ ~.,;< o.~.. ~:3° UVl ~o~ l1Jo.o

<~~..QJ 'C0.~ QJ ~...

~C::

~

~U~ ~ ~bD E

r/) <.0...0 ~~/;;, :3 ~ II .. ~ ~

51

52

High Temperature

Corrosion

of Ceramics

V. GENERAL DISCUSSION ‘Ihe results silica than

that

and alumina metallic

thermodynamic

the

experimental silica

occur

are

alloys.

the

with

have been

considerably

and these

were

such

In the

reactions

were

the

reaction

conditions

conditions possible.

in

present

more resistant

Nevertheless.

and kinetic

were

obtained

of

are that

case

studies

to sulfur-oxygen such

materials

appropriate.

alumina,

significantly

of

do occur

the

however,

affected

gas mixture

In the

no reactions

of

show that

by

present

studies

gas mixtures

reactions

the

when

did

purity

of

the

specimens. On the on the

sample

reactions with

are

alumina

the

other

than at

lower

possible

by solid

in

the

case

although in

K~u(Su4)3

and

the

grain

the

control

role

in all

part

the

the

the

is

formed.

only

increase

nuclei

for

involving

stability,

the

the

the reaction

of

of

the

cannot

alumina

in

lower

PSO3

to

be ruled

above

impure

This in

are

impurities form at

produced

studied.

force

The

form out

at

Ca and Mg. Intermediate

impurities

sulfates.

impurities

sulfate

the more

driving

a monolayer.

the

compounds

boundaries

absorbed

maybe anticipated

were

for

first

probably

A1203 in which

by the

grain

growth

sulfate

known with

K20.3

aluminas

may not

thermodyhamic

of

than

solutions

(lg).Sulfates

of

products

are

is

and with

intermediate

formation

the

the

crystal

as documented

sulfate

giester

of

solid

sulfates

sulfation

sulfate

and magnesium

their

role

provide

single

of

of

thicknesses

sulfate

polycrystalline

prominent

impurities

the

no such the

a layer

solutions

so that

boundaries of

that to

The formation

occur

sulfdte

of

Calcium

sulfate

PSO3.

compounds

appears perhaps

materials.

point

reaction

it

building

aluminum

this

also

aluminas

this for

A major formed, itself

300%

on the

impurity

aluminas;

appears

sulfate point to

becomes

a major

at least

corrosion.

phases

however,

to play

explains

due

a mixed

in

The formation

is

that

their

but

when greater

possible.

Appendix

The observation content with in

is

consistent

oxygen

where

the oxidation

properties

of

In view deposits

of

alumina

(hot

that

below

data

impurities products

the of

the

This

in

such

that

results

are

that

Na7304

is

solution

of

emphasis

on the hot

of

formed

the

and Si3N4

protective

alumina

influence should

liquids

This

Sic

to accumulate

the

with

have

pronounced

the

impurity

of

found

53

materials.

obtained

products

phases. of

reaction

been

to significantly

Na7SOh due to

Corrosion

upon

modify

on such

Moreover,

corrosion

dependent

(19) for

literature

be especially

Na7S04 and impurity

is

significantly

these

liquid.

ooint

the

have been

because will

alumina

as Mg and Ca have

can be expected

effect

the melting

of

and thereby

scales

corrosion)

the

corrosion

with

Na7S04

in NazS.04. such

that

A-Gaseous

the

samples,

corrosion

significant

solubilities

when temperatures

can be expected

formation

of

research

is

are

to be

phases

of

formed

involving

continuing

with

major

ceramics.

IV CONCLUSIONS

(1)

Silica

and alumina

(2)

No interaction

(3)

Small

(4)

The products

and at (5)

was

amounts

conditions

in

of

found

SO3 gases

temperatures.

were

to

purity

formed

of from

the

sapphire.

It

crystal

is

is

sulfate

formation

favorable

suggested

stabilized

aluminas. under

formed

which

are are

at higher

that

silica.

only

They

by SO3 gases.

the

by solid

materials.

that

sulfate with

The severity

of

the

corrosion

increases

with

the

(7)

The severity

of

the

corrosion

increases

with

time.

(8)

In

polycrystalline

aluminas

the

impurities

the

agreement

corrosion

SO3 pressures

(6)

the

favorable quantities

in qualitative

aluminum solution

the more

in greater

polycrystalline

thermodynamics

becomesmore

on the

formed

PSC3 on the more impure trends

corrosion

SO3 and fused

as sulfates

on the high

lower

resistant

between

products

predictions

Single

very

identified

The general with

are

of

and lower formed

on the

alumina.

impurity

dominated

content.

the

corrosion

alumina

54

High Temperature

by formation The

corrosion

Corrosion

of various products

of Ceramics

solid-solutions

of

formed preferentially

CaSO;- MgS04

and

on impurity

Alp

(So,)-,

_

phases and at grain

boundaries.

Acknowledgements: The authors materials, preparation

thank

B. Draskovich, of

the

3M and Saxonburg M. Zedar

the manuscript.

Ceramics

and L. Fisher

for

Companies contributions

for

donating to

the

Appendix

A-Gaseous

Corrosion

55

References: 1. Anon., " Reliability of Ceramics for Heat Engine Applications". National Materials Advisory Board, Commission in Sociotechnical Systems NMAB-357. 2.

A.F

McLean, "Ceramic Technology for Automotive Turbines"

~~11.

AIII.

&ram.

Sot. 61 (8) 861-71 (1982) 3.

A.R. Cooper, "Kinetics of Refractory Corrosion" Ceram. Eng. Sci. Proc. 2. 1963-89 (1981)

4.

R.H. Barkalow and F.S. Pettit, "Mechanisms of Hot Corrosion Attack of Ceramic Coating Materials" Conf. on Advanced Materials for Alternate

Fuel Capable Directly Fired Heat Engines, Castine. Maine, Aug. 1979. 5.

H.R. Kim u Gaseous Corrosion of Oxide Ceramics" M.S. Thesis University of Pittsburgh, 1983.

6.

H.R. Kim, J.R. Blachire and F.S. Pettit "Gaseous Corrosion of Ceramics", paper 11411.164th Meeting Electrochemical Society, Washington, D.C. Oct. 1983.

7.

B. Draskovich, J.R. Blachere and F.S. Pettit, "Hot Corrosion of Silicon Nitride and Silicon Carbide", Basic Sci. Div. Fall Neeting

Am. Gram.

sot., Columbus, Ohio, 1983 8.a W.D. Kingery, H.K. Bowen and D.R. Uhlmann, "Introduction to Ceramics", 2nd edition, Wiley, 1976. b.

A.R. Cooper, Jr and W.D. Kingery, "Dissolution in Ceramic Systems: I Molecular diffusion. Natural Convection and Forced Convection Studies of Sapphire Dissolution in Calcium Aluminum Silicate", J. Am. Ceram. Sot. 41

(1) 37-43 (1964)

56

c.

High Temperature

Corrosion

of Ceramics

L. Reed and L.R. Barrett, "The Slagging

of Refractories

II lhe Kinetics of Corrosion", Trans. Brit.

CSUII. SOC. 63 ~~‘h

534 (1964) 9.

M. Millard. H. Wuttig and H.U. Anderson, "Influence of Grain Boundaries on Liquid Corrosion of Al203 and N&l"

paper No. 76-SII-82, An. Gram.

sot. May 1982 10. K.K. Kappmeier, ItThe Importance of Microstructural Considerations in the Performance of Steel Plant Refractories" in Ceramic Microstructures, R.M. Fulrath and J.A. Pask eds. Wiley, 1968, p.22 lla..D. Cubicciotti and K.H. Lau "Kinetics of Oxidation of Hot-Pressed Silicon Nitride Containing Magnesia", J. Am. Gram.

Sot. 61 (11-12)

512-17 (1978) b.

S.C. Singhal "Oxidation Kinetics of Hot Pressed SIC". J. Mat. Sci l_l, 1246-53 (1976)

12.

R.A. Gardner, "The Kinetics of Silica Reduction in Hydrogen" J. Solid State Cliem9 336-44 (1974)

13.

M.S. Crowley, "Hydrogen-Silica Reactions in Refractories" Bull.

14.

Am. Ceram. Sot. 46 (7) 679-82 (1967)

L.J. Trostel, Jr, "Stability of Alumina and girconia in Hydrogen", Am. Ceram. Sot. Bull 46 (12) 950-52 (1965).

15.

D.E. Day and F.S. Gac. "Stability of Refractory Castables in SteamN2 and Steam-CO Atmospheres at 199OC, "Bull Amer. Ceram. Sot. 5f! (7 1 644-48

(1977)

16.

A. Negelberg, private communication.

17.

I.K. Lloyd and H.K. Bowen "Iron Tracer Diffusion in Aluminum Oxide" J. Amer. Ceram. Sot. 64 (12) 744-47

(1981)

Appendix

A-Gaseous

Corrosion

57

18s. W.C. Johnson "Magnesium Distributions at Grain Boundaries in Sintered A1203 Containing Mg A1204 Precipitates," J. Am. Gram.

Sot. 61

(5-6)

234-237 (1978). b.

P.E.C. Franken and A.P. Cehring 'Grain Boundaries Analysis of ?!gO-Doped A1203", J.

Nat. Sci., l6, 384-88 (1981)

19. N.G. Krlshnan and R.W. Bartlett "Kinetics of Sulfation of Reduced Alunite" Environ. Sci. Tech, ,1,923-929 (1973)

Appendix M.G.

B- Hot Corrosion of Silica

Lawson,

H.R.

Kim,

F.S. Pettit

and J.R. Blachere

INTRODUCTION There is a great emphasis which is dependent science

on advanced

and technology

parts,

temperature-high

device&).

conditions

air, the stability encountered, material

related

e.g.

generated

systems,

in addition

compounds

condense

process

superalloys

superalloys.

Vitreous silica is protective

metals

in oxygen,

Nags04

enhance

This type of degradation

may differ

in properties

and

in a

is well known in

in oxidizing

and on some coatings

in clean oxidizing

from the same materials

to the Journal of the American

Ceramic

on

environments

as it begins to crystallize@).

58

these

in ceramics.

and silicon nitride

it is less protective

a

may be

the gaseous corrosion

with some ability to self heal at higher temperatures.

above llllO-12000C

To be submitted

At high temperatures

scale at high temperatures

on silicon carbide

growing scales

which might be

such as NagC03,

in detail

atmospheres

in

with gases is called gaseous corrosion.

in the engine and often

named ‘hot corrosion’.

under

While

gases t2). Below some temperatures

but has not been studied

it is continuous

resistance

at high temperatures

of some gases, i.e.

various compounds to combustion

serious

important.

must be established.

by reaction

Silica forms as a protective

and other high

their corrosion

oxides are quite stable

in presence

of ceramic

are becoming

under various conditions

of a material

reliability

to these uses becomes

in a heat engine,

may not be stable

In combustion

understanding

for heat engines

has been made in the

in car engines

Since ceramics

in particular

of ceramics

the degradation

complex

leading to greater

for these applications,

compounds,

As progress

to be incorporated

stress

environmental ceramic

materials.

of ceramics

they are beginning

candidates

on higher working temperatures

Society

because However

Although

in bulk, it was

Appendix

believed

that valuable

information

B-Hot

could be obtained

corrosion

of silica as a part of a systematic

ceramics

(4). This paper covers the study performed

gas turbines,

with deposits

Corrosion

of Silica

59

from a study of hot

investigation

of the hot corrosion

under conditions

of

relevant

to

of sodium sulfates.

HOT CORROSION Hot corrosion

is dependent

they affect

the stability

materials.

If the deposit

upon temperature

of the deposit

and the gas atmosphere

and its reactivity

is in equilibrium

since

with high temperature

with the atmosphere,

equation

(1)

applies:

Na2S04 (1) = SO3 (g) + Na20 (1)

(1)

Kl = PSO3. a(Na20)

The activity temperature activity

of Na20 and the pressure

and PSO3 in the atmosphere

as defined

by Kl.

can be taken as unity, and as a(Na20) increases,

viceversa.

An Na20 activity

equilibrium

between

the hot corrosion lower.

of SO3 in the melt are fixed by the

In presence

defined by reaction

of 2~1O-l~ was calculated

the melt and the S03-containing

experiments

PSO3 decreases from equation

atmosphere

at 1OOOoC. The corresponding

of pure oxygen, the activity (l), it is expected

to increase

For pure Na2S04, and

(1) for the

selected

activity

its

for

at 7OOoC is

of Na20 in the melt is not to values of the order of 10-6

to 10s4 since SO3 and SO2 are evolved from the Na2S04 to the gas. These values are high and such Na2S04 melts are considered equilibrium

of the deposit

with the atmosphere

beginning of the hot corrosion specific

reactions

basic.

However

may be established

only at the

process and acid or basic behavior depends on the

that occur between

the sulfate

and the ceramics.

60

High Temperature

Corrosion

of Ceramics

EXPERIMENTAL

PROCEDURE

Materials The experiments

reported

here were all performed

silica (Corning 7940). The major impurities ppm of chlorine. samples

All other impurities

on a high purity fused

are 1OOOppmof ‘water’ and lo-100

were in the ppm or subppm range.

The

were cut to about 1x1 cm in area and polished prior to exposure.

Gaseous Corrosion The samples mixtures

were exposed

in tube furnaces

flowed through a platinized

the samples.

catalyst

The initial SO3 contents

balance oxygen at a total pressure

at 700 and 1000°C.

in the furnace

before passing over

of the gas were usually 1, 0.1 or O.Ol%,

of 1 atmosphere.

The corresponding

PS03 and PSO3 are given in table I. The gas flow rate was 1 cm3/s. time was usually one week but varied from one day to one month. were checked

on an analytical

with X-ray spectrometers WDS) for elemental

balance.

ESCA. (X-ray photoelectron

equilibrium The exposure

Weight changes

The major tool was an SEM equipped

(Energy Dispersive,

microanalysis.

SO3-03 gas

EDS and crystal

These experiments

spectrometers

were supplemented

by

spectroscopy).

Hot Corrosion The polished specimens aqueous solution

of Na3S04 to form a continuous

loading of 5mg/cm2.

The samples

and exposed in tube furnaces catalysts

were placed on a hot plate and sprayed

were as described

layer of sulfate

with an

usually with a

were placed in a boat lined with platinum

for times of 1 to 495 hours. for gaseous corrosion.

Flow conditions

The extreme

conditions

foil

and of

Appendix

B-Hot

Corrosion

of Silica

Table I SO3 Pressures Temp. (OC)

for Corrosion

Experiments

Initial (SO2) (1)

S03(atm.)

(1)

700

1 0.1 0.01

7x10-3 (2) 7x10-4 7x10-5

1000

1 0.1 0.01

1.5x10-3 (2) 1.5x10-4 1.5x10-5

(1)

96 S02-Balance

(2)

Environments

02 - total pressure for Hot corrosion

1 atm.

experiments.

61

62

High Temperature

table

I were selected

conditions.

Corrosion

of Ceramics

since they correspond

acidic and basic

They are the initial mixture of 1% S02-balance

pure oxygen (basic) at 700 and 1000°C. equimolar

to relatively

mixture

experiments

Since Nags04

oxygen (acidic) and

is not molten at 700°C, an

of CoSO4 and Na2SO4 was used at that temperature.

were also performed

with or without

Cycling

water washing and reapplying

the salt usually on a 45 hours cycle. The weight of the coupons was checked sensitivity

of 0.1 mg before

salt application

on an analytical and after

balance

exposure once the salt was

washed off in 10 cm3 of high purity water per cm2 of exposed area. lasted 20 minutes samples

in an ultra sonic cleaner.

was analyzed

dried.

The deposit

processed

under the same conditions

after subtraction

by X-ray diffraction

salt on the substrates

was measured

micrographs

The crystallized

of cross sections

* SSQ software

coupon and

and the spectrum

was obtained

The specimens

and the SEM before and after removal

When possible,

on photographs Contact

for microanalysis crystalline

products

The wetting of the droplet

thicknesses

were measured

of

were were

angle of the profiles

angles have been measured

of the coupons.

by Tracer-Northern

previously

of a water spectrum

with a diffractometer.

in the SEM at room temperature. by this method(7).

adapted

on a beryllium

the SEM with its X-ray spectrometers

the major tools of this investigation.

The washing

of some of the

and scaled to the same background.

in the light microscope

the salt, however

identified

was deposited

left on the coupon was analyzed

semi-quantitatively*

were examined

The washwater

by EDS in the SEM using a procedure

(5~6). A single drop of the washwater

with a

taken

previously

on SEM

Appendix

B-Hot

Corrosion

of Silica

63

RESULTS AND DISCUSSION Gaseous corrosion The gaseous corrosion(*) corrosion(g).

was studied for comparison

No weight changes

SEM and no sulfur was detected

after exposures smallest

were detected,

were visible in the

by EDS or ESCA on the surface

weight change that could be measured

An estimate

of the specimens

given in table I. While the

(0.1mg/cm2)

of the order of 1 urn, the SEM with careful

ESCA are much more sensitive. products

no products

up to one month under the conditions

uniform deposit

with the hot

would require examination

of the detection

a

and

limits of discrete

in the SEM is Under these assumptions*

10m7 to lo-* g/cm2. SEM could be detected microanalysis

sulfur in the products

by EDS. ESCA is a surface

technique,

and its detection

technique,

seen in the

not a

limits were of the order of lo-* to

10mg g/cm2. No evidence

of degradation

was found in these experiments. no reports expected

of the existence

of a silicon sulfate,

to form under the experimental

discussed

recently(lOl.

deposition

devitrification

of the silica was observed

1OOOoCin many experiments reported

less favorable

containing

sulfur

since there are

and no silicon sulfide was

conditions

The formation

No experiments

becomes

of solid products

This might have been expected

was always around one atmosphere.

sulfate

or formation

since the oxygen pressure of silicon sulfide has been

were performed

above 1000°C because

at higher temperatures.

No

after any of these exposures

for times as long as 720 hours.

at 700 and

Others have

that SO2 has a fluxing action on fused silica.

* A 1% coverage with islands 20nm thick containing 20% sulfur was assumed for the calculation of the limits of detection using SEM and EDS. In ESCA, 0.1% of a sampling depth of 5nm was assumed.

64

High Temperature

Corrosion

of Ceramics

Wetting by sulfates The wetting

angles of the salts used in the hot corrosion

with the environments. developed occured

They are given in table II. The general

in a few minutes at the temperature with time with the wetting

the sulfate

wetted

conditions

measured;

after

24 hours the wetting

Some evolution In general

than acidic conditions

After 24 hours under acidic

angle was 24O on the droplets

most of the coupon after

and 13O on an

Under basic conditions

1 hour, and a wetting

at 1000°C

angle of 2O was

was continuous.

In some cases, the salt formed drops of several sizes and different angles were observed measurements

for different

size-range

were very reproducible

are called wetting

angles and not contact was reached.

with time and the influence

of atmosphere

between

The wetting substrate conditions

the sulfate

tends to increase

surface

It was established which were cycled, that the droplets suggested

by careful

that no preferential

This conclusion

on the wetting

wetting

wetting are

even under acidic conditions.

between

the liquid and the

of the silica by the salt under basic

of the basic salt for the acidic silica.

microscopy

preferentially local attack

was substantiated

terminology

that reactions

of the same areas of samples

with washing off the salt and reapplication

did not reform

angle

It is clear from the increased

with the affinity

the higher affinity

wetting

The angles reported

angles since the latter

and the silica surface,

and the greater

reflects

(table II). The wetting

for given size range.

may imply that equilibrium

occuring

varied morphology

from 1 hour to 24 hours.

were formed.

large drop about 0.02cm across.

the salt wetted

wetting

of the experiments.

under basic conditions

small droplets

at 1OOOoCthe wetting

exceptionally

increasing

the silica better

under which discrete

experiments

between

on previous droplet

cycles,

sites.

This

would occur under acidic conditions.

by a long term experiment

reported

below.

Appendix

B-Hot

Corrosion

of Silica

Table II Wetting Angles of Na2 SO4 on Silica (O)

lhr.

7oooc (1) 24hr.

Acidic

52

36-49 t3)

Basic

-

26 t4)

lhr.

1ooooc (2) 24hr. 13-24 t3)

2

0

(1)

(Nag Co) SO4

(2)

Na2 SO4

(3)

Range of angles with drop sizes - smaller angles for larger drops.

(4)

Salt phase separated.

65

66

High Temperature

Corrosion

of Ceramics

Hot Corrosion Under acidic conditions, droplets.

the salt did not wet well the silica and it formed

At 700°C the weight

the deposit mg/cm2

by washing there

after

localized

24 hours.

corrosion

changes

were small, for example,

were losses of 0.1 mg/cmz

No evidence

occurred

of devitrification

under the droplets,

suggested

that this ‘wormy’ void texture

in water.

This corrosion

losses.

The attack

drops suggesting

an interaction

495 hours of cyclic exposure

of the silica. cycles.

exposure

and a region 33 urn thick contained

droplets

did not reform

was devitrifying characteristic

under the drops. patterns

Devitrification was detected

occurred outside

than at 700°C. compared silicate probably established spherulate

offered

of the salt.

had improved

spherulitic

and the silica

formed

and globular (figure 2).

The weight loss of 0.3 mg/cm2, by the greater

stability

some general

The cristobalite protection

attack

fibrils as reported

occured

was smaller

of less sodium

layer which did not spall

to the vitreous

silica but it was not

at the boundaries

for silicon carbide

of attack

of the cristobalite

silica which will result in the formation

in the salt.

if preferential

that the

reapplication

24 hours the cristobalite

between

This can be explained

to dissolve

observation

after

over the whole

also under very small drops and no evidence

the drops.

to the vitreous

with the separate

After

intermediate

had a strong texture This attack

at 1000°C the wetting

after

was washed and salt

sodium.

after

of the

No spalling occurred

The substrate

at the same locations

Under acidic conditions,

of sodium silicate

by the small weight

The sample

(5 mg/cm2)

of the coupons is consistent

Some

under the salt at the perimeter

reapplied

surface

by dissolution

with the atmosphere.

between

1 hour and 1.1

was apparent.

as indicated

preferentially

removing

as shown in figure 1. It is

formed

was very limited

occurred

after

after

between

the

under similar conditions(l2).

Appendix

Figure I.

B-Hot

Corrosion of Silica

Hot corrosion of fused silica under acidic conditions, 24 hoursat 700oC. SEM of sulfate drop area after washing off sulfate showing wormy voids formed preferentially at edge of drop.

67

68

High Temperature

Figure

2.

Corrosion of Ceramics

Fused silica exposed for 24 hours under acidic conditions showing coarsened spherulites under sulfate drops (after washing).

Appendix

B-Hot

Corrosion

of Silica

69

Under basic conditions at 7OOoCthe salt tended to decompose with extensive formation of cobalt oxide which complicated

the interpretation

of the

data, particularly weight change. Under the salt drops there was limited formation of cristobalite spherulites mixed with some random globules after 24 hour exposure and some localized cracking (figure 3) but there was no evidence of spalling or extensive attack of the substrate. The most extensive degradation of vitreous silica occurred under basic conditions at 1OOOoC. After 1 hour the salt wetted the coupon almost completely and a layer of cristobalite spherulites had formed under the salt. As shown in figure 4, the fibrils of the spherulites had already broken down significantly into arrays of globules. The spherulites had grown to impingement with radii of the order of 30 pm, underlining the high velocity of the surface crystallization.

After 24 hours nothing was left of the spherulitic surface

morphology as tridymite had formed at the silica-salt interface. separates the tridymite from the vitreous silica.

Cristobalite

After 212 hours the tridymite

near the surface had coarsened into laths about 15 pm wide and over 60 pm long. The crystalline layer spalled extensively. The washwater analysis indicated that significant silicon was water soluble with the sulfate after 1 hour, however this was no longer found after 10 or 100 hours. The sulfur was never depleted as the Na/S ratios in the water remained of the order of 2 (t 0.4). This suggests that less reaction occurred after a continuous crystalline layer was formed and that at the longer times sodium silicates with low solubility in the sulfate at 1OOOoC formed between the crystalline silica and the sulfate. It became clear, as the previous results were obtained, that the major mode of degradation of vitreous silica under hot corrosion conditions was due to the crystallization and associated spailing. Qualitatively the extent of the degradation at constant time increased in the order:

70

High

Temperature

Figure 3.

Corrosion

of Ceramics

Fused silica exposed under basic conditions at 700oC for 24 hours. Limited nucleation occured in small portions of drop areas.

Appendix

Figure

4.

S-Hot

Corrosion of Silica

Fused silica exposed to basic conditions at lOOOoC for 1 hour. The fibrils of the spherulites have broken up into globular arrays.

71

72

High Temperature

Corrosion

of Ceramics

Acidic 7000 < Basic 700° < Acidic 1000° < Basic 1000°C The degradation

increases

with increasing

equilibrium

PSO3 under these conditions.

emphasized

the degradation

data was obtained

Kinetics

associated

Nag0 activity

Since the hot corrosion with the devitrification,

results quantitative

on the crystallization.

of Crystallization

The thickness

of crystallized

basic hot corrosion measurements isothermal

kinetics

under an hour. 46 hours.

interface.

at 1000°C for times up to 300 hours.

the thickness

region of fractured

The plot is parabolic

A long term experiment cycles,

previously.

were thermally

cycled.

specimens.

and the incubation

was performed

Extensive

spalling of the crystalline

silica glass was measured The plot of the thickness

is given in figure 6. The plot is linear instead

of the

cycles of

experiment

of 100 hours total duration,

structure

after

and had

for 6 cycles

and the total thickness

cycling.

In a separate

a sample was cycled without

reapplication

1 hour and 10 hours of exposure.

as the other samples

(figure 7). One can identify

layer and a cristobalite

layer occured

where the fracture

of parabolic

(about 1000 urn) is over twice that without

of salt by cooling to room temperature

the samples

crystallized

crystallized

had the same layered

The

time is

with exposure

Since this was done at room temperature,

at the interface.

had a thickness

The

the salt was washed off and new salt was reapplied

of the remaining

cycled or uncycled,

on fused silica under

are shown in figure 5. They are for the propagation

In between

as described

propagated

conditions

silica was measured

were made on the unspalled

cristobalite-glass

tridymite

in the salt and decreasing

after

the long exposures,

a thin sodium silicate

layer over the glass.

The crystallized

layer, a layer

of the order of 400 urn which is also about twice the thickness

It

Appendix

I/2 TIME

Figure

5.

Kinetics corrosion

B-Hot

Corrosion

73

of Silica

l/2 (Hours)

of cr,stallization of silica at lODOW in Oxygen.

glau

during

basic

hot

74

High Temperature

Corrosion

of Ceramics

900 800

600

500 400

300 200

IOC

0

I

2

3

Y

5

6

CYCLES (48 Hour, Resa It)

Figure 6.

Kinetics of crystallization of silica glass exposed to basic hot corrosion at 1OOOoC in Oxygen, with 45 hours cycles and reapplication of sulfate. Note linear kinetics.

Appendix

B-Hot

Corrosion of Silica

+"..'0 01 ~ C

~

e

-

M-ln

... In 01

75

1n~0I C tI ~ 0 .-In

.-~ +"

tI

'0>.>. Col'O .0tI .-~

01

~In

e

~

+" .-

C 0

tI

~ .-

In~~.aoIC+"ba .-~ .. -tI C tI .-.c .->.+" '0 tI .;n .c

e

0.

0

~

><..'0... ..~ 0 -.Clnol 0I...~+"

tI 1n~>.C ~ .'0

tI .-.c -+"w

c .-

...

~

~~.Cln In ~ E-o .~O .c IntI

0

OO~:c CO-~ O ba ..+"

...

tI

4i...0InUO~ 1n0 eO Inoln... E o ~ .~

U °.C..

~ GI .. ~ ~

76

High Temperature

crystallization

and the change

influence

the cycling

experiments

in the type of kinetics

is due to the stresses

The larger rates

of

under cycling show the

but the increased generated

damage

under

during the cycling

and

of the salt.

Discussion

The wetting wetting

conditions.

of the salt on the crystallization

not to the replenishing

General

of Ceramics

for the same time under isothermal

crystallized

strong

Corrosion

was greater

angle decreased

under basic than under acidic conditions.

The

in the sequence:

Acidic 700 > Basic 700 > Acidic 1000 > Basic 1000°C The increased

wetting

due to a decreased equation

at higher temperature

acidity

of the salt at higher temperature

(1). The negative

liquid enters the wetting

free energy of a reaction

in the interfacial tendency

(lOOO°C) under acidic conditions

energy balance

between

of the salt and in particular

indicate

a tendency

towards

a substrate

the influence

Therefore

to a reaction

xSi02 + yNa20 = yNa2OxSiO2

Assuming constant

that the silicate

for equilibrium should therefore performed

activity

is greater

by Jacobson

of the form: (2)

asil=l, y=l and x=2, the equilibrium 11. Therefore

than this value.

to form the silicate.

et. al for the corrosion

reaction

2 should proceed

The experimental

with 1.5x lo-5 atm So2 which correspond not be .sufficient

on it

(aSiO2)x. (aNa2O)y

at 1000°C gives aNa20=9.2xlO-

to the right when aNa

and a

the wetting

of the atmosphere

basic fluxing according

I(2 = a&/

from

of a sessile drop so as to increase

of the solid by the liquid(ll).

behavior

as predicted

is

conditions

to aNa20=2xlO-15 A similar analysis

of silicon carbide(13).

was

However

Appendix

the wetting

behavior,

silica indicate

the crystallization

that the equilibrium

that at least locally all conditions basic to permit

for reaction

least partially

and the reaction

is shifted

decrease

basic conditions

less basic experiments

Some limited

its stability.

these conditions,

and has activities

over long term exposures.

hot corrosion

on the basis of reaction

They are the dissolution salt and diffusion

exposure

long time a silicate washing).

(2) was limited effect

in the fused silica.

hot corrosion

interpretation

by the presence

of the crystallization

formed

silica.

of the washwater

of the of cobalt

in

under all

these complications,

via reaction

Accordingly,

increases

It is proposed

this reaction

The OH and Cl impurities

under the more

crystallizes;

after

a

silica (after

is localized

that the reaction

(2) in the

early during the

on top of the crystallized

Under the less basic condition,

clusters

is

the salt and the silica occur at their interface.

layer is detected

‘wormy’ texture.

formed

the fused silica and the sodium sulfate,

later as the silica at the interface

observed

to form under

of the salt, significant

of sodium into the vitreous

but decreases

phase was observed

much less than 1.

in the salt of the silicate

the Si content

phase and the

the silicate

the less basic (acidic 7OOOC). Neglecting between

at

might get close to 1. In the

The quantitative

the melt at 700°C and the overwhelming

the major interactions

so

at 700°C (acidic 700°C in table I.) Under

accumulated

basic conditions,

It is only expected activity

occurs between

with cyclic replenishing

except

silicate

and early in the basic experiments

reaction

with the

This can be explained

A discrete

at 1000°C.

even under the less basic conditions

conditions

of the sulfate

77

to much lower values of aNa

to the right.

where the silicate

in the sulfate

of Silica

for this work might have been sufficiently

2 to proceed

under long term basic exposures

dissolved

Corrosion

by aSiO2 > 1 since the glass is not an equilibrium

OH and Cl impurities

strongly

B-Hot

leading to the

begins at impurity

are connected

only to one

78

High Temperature

Corrosion

silicon and they may generate susceptible

to attack

of Ceramics

microregions

under reaction

products

and it will be suppressed As a result

in the salt.

drops (figure

in the surface

morphology

it occurs

is released

these defects observed

after

in concentration

more readily

the case in any of the present Some of that difference impurities

discussed

above.

expected

to contain

corrosion.

near the edge of the salt

more easily to the atmosphere.

Others

expected

to be more representative

Sodium diffuses coefficients

rapidly

have reported

may be due to the influence in combustion

so that the samples of industrial

studied

In a recent

seem to affect

discussion

it was pointed

of the environment

are

here are

diffusion In type III silica

the OH’s may slow down the out that impurity

DNa( 13). Sodium can be incorporated

through

This was not

applications.

(DNa) of the order of 10s6 cm2/s at 1000°C(15). experiments,

no

lower temperatures.

into fused silica with reported

glass such as used in the present

modifier(17)

which included

Silica scales formed

OH impurities,

of the gaseous

may play a role, but it is not as obvious

experiments

in behavior

(acidic

of these defects

of Na2SOq with fused silica under acidic conditions(14).

interaction

sodium.

is small, the influence

by increases

1) where the SO3 produced

Under more basic conditions

more

(2). Since for the less basic condition

7OOoC), the driving force for the reaction is emphasized

with more open structures

and structure

in the glass as a network

the reaction: Na20 + Si-0-Si

= 2Si-O- + 2Na+

(3)

(CSiO)2. (CNa)2 g3 =_________________ Cst. aNa in which an oxygen bridge (Si-0-Si) oxygens to incorporate network

modifiers,

has been broken into two single bonded

the oxygen into the network.

are located

in holes of the structure

The sodium ions, as near the single bonded

Appendix

oxygens.

The oxygen bridges

the structure(l6), associated

and their concentration

are important

have been used by Schaeffer

oxidation

of silicon(lg).

and probably

Initially

of Nat0

in the melt as it increases

situation

is modified

sodium through channels

the driving force of Na+. Diffusion

3 which could be rate

of the silica glass.

is very difficult

silica or quartz.

between

is probably

protons

However,

Diffusion

since it does not contain

Diffusion

of

diffusion(20).

in the glass and sodium might play a role.

cristobalite

of vitreous

to increase

is slow it is likely that ion exchange

by the crystallization

These

and similar

of a vacancy

by reaction

in

of water on the

of Na+ for the low concentrations

If this reaction present

of silica formers

in terms

79

the sodium ions are

of sodium is expected

glass has been discussed

of the hydroxyls

Although

to discuss the influence

it must enter the glass structure

controlling.

of Silica

they are mobile in the structure.

The penetration

the mobility

sodium in silicate

is Cst.

in the oxidation

concepts

rapidly with the activity

Corrosion

most likely to be broken are the more strained

with single bonded oxygens

considerations

B-Hot

easier

the

of

the

along the grain

boundaries. Since it breaks up the network,

reaction

crystallization

of silica and the required

member

In this research,

rings.

Na30 activity

(acidic,

basic condition.

From reaction (aNaaO)*.

as expected

conditions

The strong influence

the extent

discussion

the to 6-

under the lower

of the crystallization

from the previous

under the more increased

and reaction

of single bonded oxygens of sodium on the crystallization

with (3).

is proportional

to

of silica is well

here it has been shown that under basic hot corrosion

at 1OOOoC the crystallization

that observed

of the network

the silica did not crystallize

3, the concentration

know in ceramics(21),

rearrangement

to promote

700°C) but it did at the same temperature

Qualitatively,

the Nag0 activity

3 is expected

proceeds

above 1400°C on the same materials

at a rate of the same order as in oxygen or an enhancement

80

High Temperature

Corrosion

of Ceramics

of many orders of magnitude

due to the sodium.

results

of sodium silicate

on the crystallization

of crystallization,

of the order of 0.1 mm/min.

soda silica glasses

with Na20 contents

very low concentrations crystallization

glasses(21).

have been observed

observed

to obtain the

here.

at 1000°C, the greater

in the salt as well as sodium penetration

and the resulting

a-g cristobalite

spalling as the specimens

and the a-B-y tridymite

inversions

contraction

of the crystalline

degradation

of fused silica under basic conditions.

under cycling

Na20 activities

in the vitreous

is due to the cracking

scale and

with no significant

In line with this conclusion, increase

for most of the observed

of silica under hot corrosion The crystalline

under cycling for the less

will be discussed

phase formed initially

of tridymite between

conditions

degradation

was globular

of high cristobalite

in morphology

extensively.

was always cristobalite

since a lower energy path is available the structures

the crystallization

which forms

because

of the greater

and vitreous

(figure 2-4).

silica(221.

Under the more basic

conditions

cristobalite

globules were aligned in radial arrays related

spherulitic

formation.

Under less basic conditions

nucleate

little

at 7OOoC (acidic 7OOoC) for which the silica did not crystallize.

Since it was responsible

cristobalite

for most of the

and spalling of the crystalline

was observed

the

as well as the differential

degradation

degradation

similarity

The

The increased

of the salt through the cracks.

basic condition

silica,

are cooled through

phases on cooling are responsible

penetration

instead

at 1000°C in

this is short lived because of the onset of devitrification.

devitrification

rates

of the order of 1%. This shows that only

rates of the order of 0.01 -0.1 um/min.

lead to dissolution

Much greater

of sodium, in the ppm range are necessary

Under the more basic conditions,

however

This is in line with previous

the spherulites

under all the salt drops or over the whole areas covered

to their

did not by the drops

The

Appendix B-Hot

Corrosion of Silica

and for less basic conditions

yet (acidic 700°C) no crystallization

even after

Under the intermediate

nearly 500 hours.

globules

formed

between

spherulites.

different reacts

randomly

mechanisms

depending

on the Na20 activity.

with SO3 pressure

(2). Under the greater glass interface their fibrils,

and formed

must be reached fibrils coarsen arrangements

network,

the previous

(reaction

The spherulites

promote

On the other hand under intermediate reaction

of the sulfate

in the sulfate

diffuse

crystallization silicon,

is initiated.

probably

decreased reaction

as reported proceeds

precipitation cristobalite

down the

rapidly through the liquid phase. basic conditions,

decreases

gradients

the surface

the silicate

ions dissolved

on which no

as the activity

The solubility

of Na20 activity

of the silicate

leading to supersaturation

of silica in the salt which generates (figure 3).

layer in which

of silica by breaking

away from the interface

in the sulfate

at the interface,

away from the interface

giving the radial

by Kim(22) in the same range of activities.

are set up across the salt.

These

At the same time it is likely that the solubility

as silicate,

of

concentration

at the interface.

with the silica is less extensive, into the sulfate

radial morphology

grow in a thin surface

3) and silica is transported

(1) and

at the sulfate-

that a threshold

instability

the crystallization

by two

reactions

nucleated

of the spherulites

and break down due to interfacial of globules.

by combining

suggest

or in

and shown in reaction

with the well-known

results

formed

In both cases the sulfate

the cristobalite

spherulites,

for the nucleation

high Na20 activities

cristobalite

was initiated

earlier

is obtained

Na20 activities

however

conditions,

that the crystallization

with the silica by basic fluxing as discussed

2. The relationship

was observed

under the drops where no spherulites

This suggests

81

of

of Na20 is Thus as the

and SO3 pressure

decreases

as it diffuses

and homogeneous the random globules of

82

High Temperature

Corrosion

of Ceramics

Discussion of Crystallization Kinetics The crystallization of silica which has rapid parabolic kinetics under the more basic isothermal conditions is not controlled by the transport of reactants or products through the crystalline layer, since the crystal has the same composition as the glass. As already discussed qualitatively, the very large rates of crystallization are associated with high Na30 activities.

The sodium breaks

oxygen bridges according to reaction (3). It is proposed that the crystallization begins when a threshold Na30 activity is present at the silica glass-sulfate interface generating sufficient concentrations of oxygen bridges to allow the local rearrangement of the network to the crystalline form. This could occur by a mechanism similar to that proposed by Fratello et. a&18) for the high pressure transformation of fused silica to quartz. The sodium in Si-0-Na groups would play a role similar to that of hydrogen in SiOH bonds, although the ONa bonds are more ionic than the OH bonds and as a result they were considered ionized in reaction (3). Since it is less tightly bound to the oxygen and because of size considerations, the Na+ is more mobile than the protons(24). The active defects sre still the non-bridging oxygens which are very mobile in combination with either thermally created single bonded oxygens, SiOH or other SiO- Na+ groups. The single bonded oxygens can attack oxygen bridges by simultaneous bond formation and breaking, and reshape the network into six member rings. After nucleation the crystallization front propagates at rates many orders of magnitude greater than in the pure system. From the present results this is due to the sodium which catalyses the crystallization.

It has to be present at the

cristobalite-glass interface, although it could not be detected with the electron microprobe, except in special cases. Only very small quantities of sodium are required for the previous mechanism which can rapidly propagate a crystal ledge into the glass.

Appendix

In such an interfacial the controlling kinetic

reaction

step is the break up of a strained

stoichiometry

of the silica and directly

3 to 4 orders of magnitude that most hydroxyls

sessile(z51.

During the induction

cristobalite-glass and essentially

trapped

by the formation

In the proposed

interface

any diffusion

cristobalite, interface

earlier.

with the cristobalite

sodium injected

initially

in the moving interface.

into the glass. of a continuous

crystalline

decrease

the concentration

as a spike of constant

Quantitatively

area, representing as a function

as:

cNa.(Dt)+ = s

decrease

the glass

in the

the

of time

into the glass is

of sodium CNa at the interface

the constant

through

of Na in the glass at the

of the sodium ahead of the crystallization

with time in line with the observed

crystallization.

cristobalite

layer and the solubility

into the glass, which spreads

on this model, the concentration

Soon this

layer has formed.

(figure 8). This spread which is due to the sodium diffusion slowed by the rejection

bound and nearly

model, the sodium at the

through the crystalline

one can approximate

have a

in small concentrations.

is supplied by this original diffusion

once a significant

of

The hydroxyls

silicas are strongly

period some sodium diffuses stopped

of the

than Nalz41. It has been suggested

of sodium ions is not constant

is essentially

layer, as discussed

Neglecting

in the model.

83

this

to the concentration

The sodiums are highly mobile although

The concentration

diffusion

in synthetic

Adapting

(u) should be a function

proportional

smaller

of Silica

many bonds and

Si-0 bond(l@.

of the interface

sodium which take the place of the hydroxyls

previously

Corrosion

a sodium can rapidly rearrange

model the rate of advance

mobility

B-Hot

front.

Based

is expected

to

of the rate of

sodium in the spike can be written

84

High Temperature

Corrosion

of Ceramics

I

idym.

%I,0

0

Figure 8.

n

Proposed mechanism of crystallization of silica during basic hot corrosion. (1) Diffusion of Na into glass, nucleation of cristobalite; (2) A continuous layer of cristobalite forms rejecting Na; (3) with cristobalite growth the Na concentration at the cristobalite-glass interface decreases by diffusion into the glass.

Appendix

in which D is the diffusion

coefficient

And the rate of crystallization

B-Hot

Corrosion

of Silica

85

of Na+ into the glass and t is the time.

u is

u = dx/ dt = A CNa+ = A S / (Dt)*

which integrates independent

to x = B tt / Dt and if D is

of time, x takes the form:

x=kt*

which predicts the thickness constant

the observed

parabolic

of the crystallized

behavior.

In the previous

discussion,

layer while S, A, B, and k are constants

x is at

temperature.

One could still assume that the sodium at the interface be diffusion

through the crystalline

the cristobalite to tridymite parabolic

behavior.

layer by grain boundary diffusion

grain size by over an order of magnitude

at the salt interface

it is rejected catalyzes

changes

Furthermore

side without

Na is not consumed

the crystallization

one cannot expect

such as the observed

parabolic

mechanism

above provides

Hot Corrosion

behavior.

however

and it transforms

departures

from the

in the crystallization interface

reaction,

and since it

a falling rate of crystallization

Therefore

a better

mostly

it is concluded

that the

explanation.

of Silica Pormers

The present

results

of silica scales on ceramics superalloys.

significant

into the glass at the cristobalite-glass

proposed

is supplied

are in general

(silicon carbide

The silica was not affected

of this study, however

agreement

with the protective

and silicon nitride)

and coatings

by the SO3-SO3 containing

sodium plays a dramatic

properties on

environments

role under hot corrosion

conditions.

86

High Temperature

Corrosion

of Ceramics

The vitreous

silica has a tendency

particularly

at low PSO3, but more importantly

crystallization

which promotes

these conditions

to react

to silica formers.

expected

the nature

cycling.

Under

and fundamentally of these conclusions

of the impurities

It is likely that OH impurities Although only indirect

promote

indications

structural

applications

not will

associated are more

the hot corrosion

are presented

from the role of OH in glasses( 18p2@. This is important

temperature

to

and the rate of growth of the scale since thick scales

to spalling.

and crystallization.

affected

The validity

depend on the purity of the silica formed,

sensitive

it is very sensitive

spalling through temperature

silica scales should be strongly

afford good protection

with the sealest

with Na2S04 by basic fluxing,

since water is a major product

here, it is

for high of

combustion. The results than vitreous

suggest

also that some silicate

scales

might be more desirable

silica if they could be used at high temperatures

without

devitrification.

CONCLUSIONS - No evidence of gaseous corrosion was obtained in atmospheres containing initially up to 1% SO2 and balance oxygen at 700 and 1000°C for times up to 720 hours. - No devitrification was observed experiments in pure oxygen.

under these conditions

and similar

- The wetting of the silica by the sulfate varied significantly with the PSO3 and the Nap0 activity. It increased with aNa20. It was greater under basic than under acidic conditions at both temperatures. Under basic conditions at 1000°C the wetting was complete. - In all cases there was a tendency with the activity of Na20.

towards

basic fluxing which increased

- Some limited corrosion was observed under acidic conditions at 7000C. At 1000°C under the same conditions, the silica devitrified and little reaction was observed between the salt and the silica.

Appendix

B-Hot

Corrosion

of Silica

87

- The most extensive degradation of vitreous silica occurs by crystallization and the associated spalling during temperature cycling. In general it increased with the activity of Na20 and was very severe under basic conditions (pure oxygen atmosphere) at 1000°C. - The sodium accelerates (catalyzes) dramatically the devitrification of silica and the rate of crystallization at 1000°C under basic conditions is of the same order as that observed by others at 1400°C in air (without sodium). - The kinetics of crystallization at 1OOOoC under basic conditions were parabolic with a short incubation time. Cycling increased the damage and changed the kinetics to linear. The extra damage was associated with the strains due to cycling not with additional salt applications. - The parabolic behavior observed under isothermal conditions can be explained with a model in which the crystallization is controlled by the sodium at the crystal-glass interface which diffuses into the glass prior to crystallization. - The need for the development of vitreous ceramics and metallic alloys is stressed.

coatings

for the protection

of

Acknowledgements The authors gratefully of the Department

acknowledge

the support

of Energy for their financial

of Basic Energy Science

Division

support.

REFERENCES

1.

a.

M. Taguchi, Adv. Ceram.

b.

Heat Engine Ceramics,

Mat. 2 [4] 754-762 (1987). Bull. Am. Ceram.

Sec. 64 [Z] 268-294 (1985).

2.

N. Birks and G.H. Meier, Introduction Metals, Arnold 1984, Chap. 8.

3.

P.J. Jorgensen

4.

J.R. Blachere and F.S. Pettit, High Temperature Corrosion DOE Basic Energy Sciences DEFG OZ-84-ER45117.

5.

B. Draskovich, Hot Corrosion of Silicon Mitride Thesis, University of Pittsburgh (1984).

6.

J.I. Goldstein et. al, Scanning Electron Plenum, (1981), p. 378.

7.

L.E. Murr, Interfacial (1975), p. 69.

8.

H.R. Kim, Gaseous Corrosion Pittsburgh (1983).

et. al, J. Am. Ceram.

Phenomena

to High Temperature

Oxidation

of

Sot. 42 (12) 613-616 (1959). of Ceramics,

and Silicon Carbide,

Microscopy

M.S.

and X-ray Microanalysis,

in Metals and Alloys, Addison-Wesley,

of Oxide Ceramics

M.S. Thesis, University

of

88

High Temperature

Corrosion

of Ceramics

of Silica and Alumina,

M.S. Thesis,

9.

M.G. Lawson, Hot Corrosion of Pittsburgh (1987).

10.

0.0.

11.

I.A. Aksay et. ai, J. Phys. Chem. 78 [12] 1178-1183 (1978).

12.

J.R. Blachere et. al, Paper #77-BEG-86P, Sot., New Orleans (1986).

13.

N.S. Jacobson,

14.

N.S. Jacobson 1987.

et. al, Workshop on Corrosion

15.

G.H. Prischat,

J. Am. Ceram.

16.

G.H. Prischat

17.

W.D. Kingery et. al, introduction

16.

V.J. FrateIlo

19

H.A. Schaeffer,

20.

Z. Boksay: “Mass Transport in Non-Crystalline Solids”, in The Physics of Non-Crystalline Solids, G.H. Frischat, Ed. Trans. Tech., (1977) p. 428.

21.

H. Rawson, Inorganic

22.

Reference

23.

G.M. Kim, M.S. Thesis,

24.

R.H. Doremus,

25.

R.W. Lee and D.L. Fry, Phys. Chem. Glasses,

26.

Reference

Vander Biest et. al, J. Am. Ceram.

Workshop on Corrosion

University

Sot. 70 [7] 456-59 (1987).

Basic Science

in Ceramics,

Div. Am. Ceram.

Pennstate,

in Ceramics,

Nov. 1987.

Pennstate,

Nov.

Sot 5_1[9] 528-530 (1968).

and W. Beier, J. of Non-CrystaLline to Ceramics,

Solids 7l, 77-85 (1985). Wiley (1976) p. 103.

et. al, J. Appl. Phys. 51 [12] 6160-64 (1980). J. of Non-Crystalline

Solids, 38 and 39 (1980), 545-550.

Glass-Forming

Systems,

Academic

University

of Pittsburgh,

Press (1967), p. 53.

17, p. 314.

Glass Science,

24, p. 229.

(198-).

p. 168. 3 19 (1966).

Appendix M.G.

C-Hot Lawson,

Corrosion of Alumina F.S. Pettit,

and J.R. Blachere

INTRODUCTION Failures due to corrosion limit the selection of materials in countless operations. The corrosion of ceramic materials is a significant factor limiting the design of new systems for coal gasification, coal liquification, energy conversion, thermal storage, and the battery storage of power. (l) Advances in the science and technology of processing, fabricating, and testing of brittle materials have resulted in ceramic parts with mechanical properties acceptable in a broad range of energy related applications. Severe material degradation is common in environments where the combustion of fuel occurs. As superalloys and complex cooling systems have been developed the operating temperatures of turbine engines has been raised to increase their efficiency.

Many of the raw materials used in superalloys are expensive and can

be obtained from a limited number of foreign sources.

Low heat rejection diesel

and gas turbine engines are in development which allow a substantial decrease in the size of power source, primarily due to the elimination of bulky cooling systems.(2T3) The continuing development of more efficient and compact engines as well as the reduction of cost and dependence on strategic materials is dependent on advances in materials technology.

In addition to being inherently refractory

and resistant to corrosion, most ceramic materials are available from domestic sources and are much less expensive than the elements used in superalloys. Although ceramics tend to be refractory, the corrosion of ceramics occurs at an appreciable rate in many systems. This is reflected in cost of refractoris used by the steel and glass industries. The research resulting from the

89

90

High Temperature

widespread

Corrosion

use of ceramics

of Ceramics

as vessels

to hold and direct

and glass has led to an understanding

of the corrosion

the flow of molten

of ceramics

metal

in the presence

of a melt. Alumina

is an excellent

pure forms.

It is also generated

superalloys.

Its corrosion

of deposits corrosion

by SO2-02

mixtures

In a turbine

to turbine

with gas mixtures

A. The conditions

applications

sodium.

Gaseous

of this study

since the gases were generated

were Na2 S04.

of about 10e4 to lo- 5. f4) Deposits

of 0.2 atm or

results

from the

when burning fuels with sodium and sulfur impurities

The composition

will be governed

of condensed

by the decomposition

Na2S04 at equilibrium reaction:

Na2S04 = Na20 + SO3 K1 = [aNa

on

in presence

in this research.

engine at 1000°C, gases have oxygen pressures

of gases produced

in air-containing

was studied

in appendix

and the deposits

and SO3 pressures

condensation

of coatings

to gases at high temperatures,

the corrosion

of alumina was discussed relevant

which is used in impure and fairly

as a scale which is protective

resistance

which may enhance

are particularly

greater

bulk refractory

(11

[PSO3]

or so42assuming

= 02- + so3

that the salt is at unit activity.

melt are the basic and acidic species, During hot corrosion determine

deposit

in an engine can be determined material

and the pressure

the type and extent

with components

The Na20 and SO3 dissolved

of Na20 and pressure of reaction.

of SO3 in the

The composition

of the

by the gas phase or by the reaction in the salt.

If the activity

of SO3 is low or nil, solid oxides exhibit

melt as in the reaction:

in the

respectively.

the activity

deposit

substrate

(21

of the

of Na20 is high

a basic solubilityin

the

Appendix

C-Hot

Corrosion

MO + 02- = M022If the activity

of Na20

an acidic solubility

intermediate

is low and the pressure

of SO3 is high, solid oxides exhibit

in the melt as in the reaction: (4)

of Na20 (i.e. 02-) and pressure range then the oxide is stable

More specifically,

the degradation

molten Na2S04 film may proceed

of SO3 in the deposit

and negligible

solubility

of an Al203 substrate

(3)

Na2S04(1)

(6)

+ Al203 = s NaA102 + SO3(g)

The solubilities have been measured

diagrams

is observed.

by reaction

3 Na2SO4(1) + Al203 = Al2(SO4)3 + Na20

shown on the stability

are within an

with a

by acidic or basic fluxing reactions:

The regions where the hot corrosion

Figure 2.(7-g)

of Al203 occurs by either

mechanism

are

in Figure l.@y6)

of many oxides over a range of compositions experimentally.

Results

Plots of the solubilities

of Na2S04

for Al203 and SiO2 are shown in

of different

left and right of each other depending

oxides are displaced

on the relative

acidity

to the

or basicity

of those

oxides. Alloys resistant by the formation transport

to degradation

of continuous

of metallic

or oxidant

at elevated

compact

is partially

determined

temperatures

are characterized

solid oxide scales with a low rate of

species.

In addition,

the oxides in the scale must be negligible. corrosion

The extent

by the solubility

the volatilization of degradation

rate of by hot

of the oxide scale in the

molten salt. Where the solubility a function

of the acidity

of the oxide in the molten salt deposit or basicity

rapid rate until the liquid is saturated. corrosion

91

(3)

MO = M2+ + 02If the activity

of Alumina

rate decreases.

of the melt, corrosion

is low or is not

may proceed

When the solution reaction

at a

stops the

92

High Temperature

Corrosion

of Ceramics

0

LOG

P

BASIC

O2

FLUXING

-10

AIO;

LOG

Figure

0

-10

-20

l(a).

&03

:Stability diagram showing phases of alumina that can be stable in Na2SO4 at 700°C, and defining regions where acidic or basic fluxing are possible. Dashed lines are SO4 isobars (atm).

Appendix

C-Hot

0

I/ -

/

/

BASIC LOG

FLUXING

P 02

/

AIO;

/

/

/

-a

1 -8

/

/

/

/

/

/

/

/

/

/

of Alumina

93

t 1

-16 IO / /

-10.6, IO / / 1 /

/

/

/

-s ’ IO ’ / / t / I

ACIDIC FLUXING

A13+

c

A’203

AG3 0

-Y

LOG

Figure l(b).

/

/

/

/

Corrosion

Ps,

Y

3

Stability diagram showing phases of alumina that can be stable in NaZSO4 at 1000°C, and defining regions where acidic or basic fluxing are possible. Dashed lines are sulfur isobars. Very high SO3 pressures are required for acidic fluxing. Refractory metal oxides are believed to make acidic fluxing favorable at lower SO3 pressures as indicated by the displaced boundary (arrows).(ll)

94

High Temperature

Figure

2.

Corrosion

Solubilities

of Ceramics

of alumina

in fused Na2S04

at 927OC.

Appendix

In acidic or basic conditions and precipitation a negative

sustained

C-Hot

attack

Corrosion

results

of Alumina

if solution

of the oxide in the salt away from the oxide-salt

gradient

of the solubility

gradient

in the solubility

activity

of Nat0

(i.e. 09-) across the film.

precipitation

of the oxide produces

passivating

oxide scale on a metallic

of oxide

interface

is by

of the oxide across the salt film.(lO-lll

of the oxide is established

by the local variation

The resultant

a porous scale.

solution

95

The in the

and

The growth of a non-

engine component

results

in high corrosion

rates. In acidic conditions is derived

sustained

attack

from the gas phase.(5p12*131

acidic component the deposit.

may arise when the acidic component

Als o, acid fluxing can result

comes from the solid phase and reduces

This causes the melt to become

fluxing results

from the solution

in Na$304.

More specifically,

molybdates

reduces

more acidic.

of refractory

of tungstates,

the oxide ion concentration

manner,

the oxidation

of the vanadium

promote

acidic fluxing.

in

Alloy induced acid of superalloys

vanadates,

in the melt.(121

which is contained

EXPERIMENTAL

the Na90 activity

metal components

the formation

when an

and

In a similar

in many fuels can

PROCEDURE

Materials The materials aluminas

containing

been discussed

chosen are single crystal different

in previous

first part of this report. analysis

levels of impurity

reports.

Their sources

The single crystal

of the polycrystalline

alumina

aluminas

and three poiycrystalline

and microstructures.

They have

were given in Table I of the

is 99.99% pure.

is given in Table I.

The chemical

96

High Temperature

Corrosion

of Ceramics

TABLE I Chemical

Analysis

Aluminas*

of Polycrystalline (units:

wt%)

High PP**

Med PP

Low PP

Fe203

O.Ol^

O.Ol^

0.02

CaO

0.01,.

0.02

0.07

MgO

0.10

0.17

0.75

Ti02

O.Ol^

O.Ol^

0.02

SiO2

0.11

0,17

1.65

K20

O.OOl^

0.001-

O.OOl^

Na20

O.OOl^

0.02

0.09

** High PP =

High Purity Polycrystalline

Alumina

Med PP

=

Medium Purity Polycrystalline

Low PP

=

Low Purity Polycrystalline

Undetected,

Alumina

Alumina

Limits of detection

Appendix

Hot Corrosion

After

and exposed subjected

of Alumina

had areas about 1 x 1 cm which were polished

cleaning,

they were usually coated

to cyclic

exposures

cycles.

for cycle,

Two temperatures

pure 02 and initially

and the SC2 containing

of Na2S04 were also

and washing and

700°C and 1000°C

1% SO2-balance

of 1 atm, were used in the experiments.

basic conditions

Some samples

with 45 hours exposure

of Na2SO4 between

and two atmospheres,

oxygen at a total

The pure 02 tended

atmospheres

mixture

883OC, the sulfate

over a platinized

catalyst

at 700°C.

used at 700°C was an equimolar

to give

set up acidic environments

with pSO3 = 1.5 x 10e3 atm at 1000°C and pSO3 = 7 x 10N3 atm after the initial

97

down to 1 urn

with 5 mg/cm2

for various times from 1 to 100 hours.

reapplication

pressure

Corrosion

Experiments

All specimen diamond.

C-Hot

Since Na2S04

mixture

passage

of

melts at

of Na2S04 and CoSO4

which was molten at that temperature. The changes characterized

in morphology

and products

focused

on the samples

mostly with the SEM, and its X-ray spectroscopy

and WDS) for microanalysis.

After

exposures

they were washed and the weight changes water was analysed When sufficient These procedures

as required

products

were measured

using a semiquantitative

were formed,

were described

and observations

in previous

attachment

(EDS

of the samples,

to iO.l mg. The wash microprobe

they were identified

in detail

were

method.

by X-ray diffraction.

reports.

RESULTS AND DISCUSSION

Wetting The wetting measured

angles for the 1 hour and 24 hours isothermal

exposures

and are given in Tables II, and III. In most cases the wetting

were

morphology

98

High Temperature

Corrosion

of Ceramics

TABLE II WETTING DATA Al203 1 & 24 Hour Isothermal 1 HR

7oooc

ACIDIC 24 HR

Exposures 1000°/24

Low PP

MS04 BM(S) LD18O SD 40°

MS04 140 RP=50°

Na2S04 190

Med PP

MS04 300

MS04 220 RP=48O

Na2S04 130 RP 2O NC

High PP

MS04 8O

MS04 200

Na2SO4 120

sxtaI

MS0 140

MS04 220

Na2S04

KEY:

MS04

180

(Na2,CO)S04 CWContinuous Wetting NCNearly Continuous Wetting PSPhase Separated Drop Wetting Angle (Cobalt Oxide+SaIt) BMBimodal (Two Wetting Angles) BM(S)Angles f(Drop Size) LDLarger Drop Wetting Angle SDSmaller Drop Wetting Angle BM(H)Angles = f(Drop Habit) RFReaction Product Angle

Appendix

C-Hot

Corrosion

of Alumina

TABLE III WETTING DATA Al203 1 & 24 Hour Isothermal 1 HR

1ooooc

Exposures

BASIC 24 HR

700°/24

Low PP

Na2SO4 30 NC

Na2SO4 BM(H1 40/21°

MS04 PS =16O

Med PP

Na2SO4 cw

Na2S04 cw

MS04 PS =14o

High PP

;$2s04

Na2S04 40

MS04 PS =14o

sxtsl

~o~so4

Na2S04 E”

MS04 PS 200

KEY:

MS04 cw NC PS

WwWSO4

Continuous Wetting Nearly Continuous Wetting Phase Separated Drop Wetting Angle (Cobalt Oxide+Salt) BM Bimodal (Two Wetting Angles) BM(S1 Angles - f(Drop Size) LD Larger Drop Wetting Angle Smaller Drop Wetting Angle SD BM(H1 Angles = f(Drop Habit) Reaction Product Angle RP

99

100

High Temperature

which distinguished

of Ceramics

each set of exposure

was nearly developed except

Corrosion

minutes

after

conditions,

atmosphere,

the coupon reached

and temperature,

the furnace

temperatures

in a few eases. The phase separation

only occurred exposure

in the oxygen atmosphere were composed

the deposits

oxide crystals. surface.

of the (Na2,Co)SO4

as predicted

The cobalt oxide formed

from wetting

Bimodal wetting

in contact

wetting

morphologies

the salt or from the formation the difference

by thermodynamics.

After

a discontinuous wetted

cobalt

layer on the substrate

by the molten salt which

the alumina substrate.

In some cases two distinct coupons.

used in the 700°C exposures

of Na2S04 and masses of equiaxed

The cobalt oxide was preferentially

was prevented

deposit

angles were measured resulted

a reaction

from the phase separation

product

angle was a function

on some of the

in the deposit.

of

In other cases

of drop size as reported

in Tables

II and III. The wetting

of all the aluminas

In acidic conditions

the wetting

(High PP and single crystal) 700°C.

The wetting

and MedPP) decreased the wetting

salt on the aluminas reaction

reaction

between

is apparent.

In general,

approaches

at 700°C.

the wetting

with the molten salt results

and the spreading

the equilibrium

of the droplets.

and the wetting wetting

angle.

at

(LowPP No trend in

angle of the

is lower at 1OOOoC than at 700°C.

on the two higher purity substrates

angle of contact

1 and 24 hours of exposure

on the two lower purity aluminas

in acidic conditions

products

was limited.

on the two higher purity aluminas

1 and 24 hours of exposure

of the more impure aluminas

of multiple reaction

increased

between

in acidic conditions

angle observed

angle observed

with composition

exposed

The

in the formation There is less

decreases

as the

Appendix

In basic conditions

the wetting

1000°C than at 700°C.

However,

salt at 700°C affected

the crystallization

the results.

a slight decrease

hours in basic conditions increases

were wetted

purity aluminas. affected

coupons occurred

of impurity

specific

since localized

In those experiments angle was high, relatively thick the effect the vicinity

the area for reaction and the distance salt-gas

when the contact

where partial

layer.

in corrosion

in the molten salt.(141

of the repeated

before

were observed.

on the composition

of the

wetting

each cycle.

of

This is

is

of the salt, especially

in

less than when the salt wet the

and salt-gas

interfaces

must be transported

Hence, the corrosion John observed

contact

When the deposit

On coupons where a low wetting

at both the oxide-salt

angle is low.

wetting

was poor and the resultant

may be significantly

is decreased.

in a decrease

solubility

thick droplets

over which the oxidant

interface

of the

in temperature.

evidence

where the wetting

of the substrate,

is

could be very damaging.

of the atmosphere

coupon in a thin continuous

results

by an increase

substantial

attack

behavior

phases with the basic component

areas of a coupon when salt was reapplied

important

generally

The lower purity aluminas

taken during long term exposures did not provide

1 hour and 24

the wetting

to the fact that the wetting

melt, sodium oxide, which is promoted Photographs

between

by the molten salt than were the higher

This is attributed

by the reaction

angle occured

of the substrate.

of the

than acidic conditions.

In basic conditions

much more extensively

101

is lower at

the wetting

in basic conditions

at 1000°C. content

of Alumina

of cobalt oxide from the

At both temperatures,

in the contact

with the impurity

Corrosion

angle of the salt on the alumina

alumina by the salt is more extensive In generai,

C-Hot

is increased

inward from the

rate may be enhanced

that an increase

rate, for exposures

angle is observed,

in salt film thickness

of alloys with significant

102

High Temperature

Hot Corrosion

Corrosion

of Ceramics

of Al203

Even after

long term exposure

with thermal

of the salt the total weight changes even after

measured

500 hours of cyclic exposure.

405, and 495 hour experiments

The weight changes

product

was observed

coupons from the exposures

made in either

crystalline

products

reaction

coupons from expures or silicate

reaction

concentrated

in either

products

hydrofluoric

were present

quantities

recorded

for 1, 24,

atmosphere

were etched acid.

on the washed polycrystalline

atmosphere

were observed

at 700°C.

at 1000°C.

After

exposure,

the silica

from some of the coupons using are listed in Table VII.

of the coupons indicate

on the washed polycrystalline

which were not unreasonably

Well defined

on the washed polycrystalline

The weight decreases

The weight losses due to the etching products

were very small, less than 1 mg/cm2,

are listed in Tables IV,V, and VI.

A globular silica reaction

silicate

cycling and the reapplication

that these reaction

substrates

large compared

in significant

to the impurity

content

of the samples. The weight changes the coupon surface

must be affected

was not wetted

by the smaller

completely

at both temperatures

all the materials

oxide on the surfaces

of the coupons exposed

the weight of the samples. particularly

more contributions, and hot corrosion

materials,

over fractions

Coupons of the same materials atmospheres

of

0.1 mg.

sign.

The formation

of cobalt

at 700°C in basic conditions recorded

when

In acidic conditions

increased

for any of the exposures,

are probably the result of two or Also, a combination

of the coupon surfaces

were exposed

for 168 hours (appendix

with a sensitivity

lost weight.

possibly of opposite occurred

by the melt.

The weight changes

of the polycrystalline

area of reaction

during exposure.

to gaseous corrosion

A) and no weight changes

of both gaseous

in the same

were detected

Appendix

TABLE

C-Hot

Corrosion

of Alumina

IV

WT CHANGE ALUMINA [mg/cm2] 1 & 24 Hour Isothermal Exposures

BASIC 1OoOoC

1 HR

LOWPP MedPP

0 0 - 0.5 - 0.1

HighPP sxtal

24 HR + -

0.2 0.1 1.1 0.4

BASIC 7OOoC LOWPP MedPP HighPP sxtal ACIDIC

+ 0.4 + 0.1 0 + 0.1 7OOoC

1HR

LOWPP MedPP HighPP sxtal ACIDIC

-

0.1 0.8 0.5 0.1

1OOOoC

24 HR - 0.2 - 0.4 - 0.2 0 24HR

LOWPP MedPP HighPP sxtel

- 0.3 -0.3 - 1.1 - 0.3 LowPP MedPP HighPP sxtal

= = = =

Saxonsburg 3M Lucallox Single crystal

103

104

High Temperature

Corrosion

of Ceramics

TABLE V WT CHANGE 495 HR Cyclic

ALUMINA

[mg/cm2]

Exposure (45 HR Cycles

+ ResaIt)

ACIDIC 700°C

Material Cycle

LOWPP

MedPP

HighPP

SXtal

-0.2

-0.3

-0.3

0

2

-0.1

-0.1

-0.1

0

3

0

0

0

0

4

0

0

0

0 0

5

0

-0.1

6

-0.1

-0.1

-0.1 0

-0.1

7

0

0

0

0

a

0

+0.1

0

0

9

-0.1

-0.1

0

0

10

-0.1

0

0

-0.1

11

0

0

0

0

-0.6

-0.6

-0.5

-0.2

Appendix

C-Hot

Corrosion

of Alumina

105

TABLEVI WTCHANGE

ALUMINA[mg/cm2]

405 HR CyclicExposure(45 HR Cycles+ ResaIt) BASIC 1000°C

Material Cycle

LOWPP

MedPP

HighPP

SXtal

1

-0.2

0

0

0

2

+0.3

-0.1

0

0

3

0

+0.1

0

0

4

0

-0.1

0

0

5

0

0

0

0

6

0

0

0

0

7

0

0

0

-0.1

8

0

0

0

0

9

0

0

0

0

-0.1

0

-0.1

+0.1

106

High Temperature

Corrosion

of Ceramics

TABLE VII RESULTS HP ETCH WT CHANGE ALUMINA [mg/cm2] Exposed Samples Etched in HP BASIC 1000°C LOWPP MedPP HighPP ACIDIC

24 HR 0 - 0.7

700°C

1000°C

LOWPP MedPP

- 0.6 - 0.3 0 495 HR

LOWPP MedPP HighPP ACIDIC

405 HR

- 0.3 - 0.1 - 0.1 24 HR 0 - 1.0

Appendix

C-Hot

The data given in Table VIII were obtained processing

of the EDS spectra

performed

on 1, 10, and 100 hour exposures

Results,

Acidic

is favored

sulfate

Alumina reaction

was detected

after

of exposure

the wetting

angle was 22 o. The EDS spectra

100 microns

in diameter

indicated

Negligible

reaction

exposure.

Aluminum

was detected

A limited

contamination

for the single crystal

present

amount of Si was detected

occurred

24 hours

of Al had occurred. after

100 hours of

in the wash water at 1, 10,

after

10 and 100 hours of exposure.

in the single crystal,

of the samples

it is assumed

that

and that the levels of Si detected

alumina are not significant.

networks

solution

After

of many drops below

on the washed substrate

ions were consistently

were heated

1 hour of exposure.

angle of 14O was measured.

that limited

silicon was present

which the samples

because

for a given SO2 pressure.

was spotty and a wetting

cyclic exposure,

was

at 700°C.

were emphasized

The wetting

limited

This analysis

in acidic conditions

in acidic conditions

At 700°C limited

Since negliglble

107

from the semiquantitative

taken from the wash water.

at the low temperature

Single Crystal

100 hours.

of Alumina

Conditions

The 700°C exposures formation

Corrosion

The alumina furnace

tube in

may have been a source of Si. After 495 hours of

of shallow depressions

were observed

over portions

of

the coupon. The coupon exposed perimeters EDS spectra to several

of drop areas.

at 1000°C for 24 hours exhibited A significant

amount of Al was present

of the salt at the drop edges. millimeters

a faint etching

across with a wetting

The substrate angle of 180.

at the

in some of the

was wetted

by drops up

108

High Temperature

Corrosion

of Ceramics

TABLE VIII Wash Water Analysis, Al203 Exposures 700°C Acidic Conditions

Material

Element

1 Hour

Na S Si Al Mg Ca co 0**

17.0 14.0 0.99 2.0 2.4 0.00 4.1 60.0

18.0 14.0 0.96 1.2 1.3 0.00 4.2 60.0

16.0 14.0 1.4 2.2 1.3 0.00 3.5 61.0

Na S Si

18.0 13.0 0.94

19.0 11.0 1.5

Al

2.2

16.0 13.0 3.6 1.5

10 Hour

100 Hour

LOWPP

MedPP

Mb?

Ci

co 0

1.8 0.00 4.2 59.0

::: 0.00 3.9 58.0

I?“00 3.3 61.0

17.0 15.0 0.46 1.0 1.4 0.00 3.0 62.0

19.0 15.0 0.00 0.52 1.0 0.00 3.9 61.0

17.0 16.0 0.40 0.66 0.43 0.00 3.9 62.0

19.0 15.0 0.00 0.60 0.81 0.00 3.9 61.0

18.0 15.0 0.46 1.3 1.6 0.00 3.7 61.0

16.0 16.0 0.56 0.16 0.00 0.00 4.4 63.0

HighPP Na S Si Al ME Ca co 0

sxtal Na S Si Al I% Ca co 0

**

by stoichiometry

Appendix

High Purity Polycrystalline At 700°C limited substrate area.

was wetted

A wetting After

was detected

by two large patches

patches

the preferential silicate

the wetting

angle was approximately

20°.

The solution

of salt at the edges of drops.

of significant solution

The

of Al These

amounts of Si. Washing the

of alumina grains and the presence

10 to 20 microns across, containing

of alumina grains throughout

100 hours of exposure.

A thin discontinuous

in a drop area is shown in Figure 3 (top). in the wash water after After

A few Na and

of the substrate

and fractions and preferential

The coupon exposed drops about 2 millimeters were detected

of Na and Mg detected

after

layer of silica on the edges of grains Aluminum

ions were consistently

small thin patches of individual solution

of silica covered

grains (Figure 3, bottom).

across with a 12O wetting

angle.

of salt at the drop edges.

of a thin poorly defined in some spectra.

In

of grains was apparent.

at 1000°C for 24 hours was wetted

in EDS spectra

the presence

the drop areas was evident

1, 10, and 100 hour exposures.

495 hours of cyclic exposure

these areas the etching

fraction

about half of the coupon

along the edges of the drop areas.

of cobalt sulfate

The etching

revealed

The

amount of Si were observed.

significant

portions

109

1 hour exposure.

under 1000 microns across.

the presence

of a fine poorly defined

present

after

covering

by some of the EDS spectra

revealed

isolated

reaction

and typically

also indicate

substrate

of Alumina

angle of 8Owas measured.

drops were patchy

spectra

Corrosion

Alumina

24 hours of exposure

was indicated

C-Hot

by several

Traces of Al and Si

Washing the substrate

silica-containing

This discontinuous

of the grains in the drop areas (Figure 4, top).

discrete

layer with traces layer covers

a large

110

High Temperature

Figure 3.

Corrosion

of Ceramics

High purity polycrstalline alumina exposed in acidic conditions at 700oC or 100 hours {top) and 495 hours {bottom). Etched grains in drop areas and thin patches of silica are shown on washed substrates.

Appendix

Figure 4.

C-Hot

Corrosion of Alumina

High, medium, and low purity polycrystalline aluminas (top, middle, and bottom, respectively) exposed in acidic conditions at lOOOoCfor 24 hours. Silicate reaction products on washed substrates are shown.

111

112

High Temperature

Corrosion

of Ceramics

Medium Purity Polycrystalline Alumina At 700°C after 1 hour of exposure a significant amount of Al and a limited amount of Si were indicated by the EDS spectra of drops. Wetting was spotty except for a single large patch of salt covering about a fifth of the coupon area. The droplets were typically 10 to 200 microns across. A wetting angle of 30° was measured. A few isolated patches of cobalt sulfate 10 to 20 microns across and containing Na and a significant amount of Si were observed. After 24 hours exposure the wetting was spotty with some small patches. Substantial solution of Al was indicated by the EDS spectra of salt in drops with a measured wetting angle of 22O, and in droplets containing larger well defined crystals richer in Al with a wetting angle of about 48O. Washing the substrate revealed that fine poorly defined bands a few microns wide, containing Co, Si, and traces of Mg and Ca, skirted large fractions of the perimeters of the large drop areas. In the drop areas, some of the grain boundaries had been etched. The etching of grains throughout the drop areas is evident after 100 hours of exposure. Small globules rich in Si scattered on raised blocky areas 20 to 30 microns across were observed.

Aluminum and Mg ions were consistenly present

in the wash water at 1, 10, and 100 hours in much greater concentrations than for the single crystal or the high purity polycrystalline material. The concentration of Mg detected after 10 hours of exposure was relatively large. The concentration of Si ions in the wash water was also much greater, and increased drastically between 10 and 100 hours. After 495 hours of cyclic exposure patches of a continuous layer of well defined globular silica covered portions of the substrate. The etching of grains was apparent.

Appendix

C-Hot

Corrosion

of Alumina

113

The coupon exposed at 1000°C for 24 hours was wetted nearly continuously over some portions of the surface and by discrete droplets with a 13O wetting angle in other areas. In both areas, substantial amounts of Ca, Mg, Si, Al, a limited amount of Ba, and traces of K were present in EDS spectra of the salt. Washing the substrate revealed the presence of numerous well defined sodium aluminum silicate and sodium magnesium aluminum silicate crystals scattered across the substrate (Figure 4, middle). Also, smaller sodium magnesium aluminum silicate crystals had formed as platelets aligned in parcels on the substrate. Some crystals were present in groups but in most cases they protruded from grain boundaries where discontinuous etching had occurred.

Low Purity Polycrystalline Alumina At 700°C after 1 hour of exposure the wetting was spotty with some larger patches of salt over 500 microns across. Wetting angles of la0 for the larger drops and 40° for the droplets were measured. A significant amount of Al and a limited amount of Si were indicated by the EDS spectra of drops. Blocky crystals about 20 microns across were visible below the salt in several droplets. After 24 hours of exposure patches of salt nearly as large as 2000 microns across wetted the substrate. Substantial solution of Al was indicated by the EDS spectra of salt in drops with a measured wetting angle of 14O. Sulfate drops with a wetting angle of about 500 contained well defined sulfate crystals rich in Al. Some of the crystals contained Al and Mg as well as Si (Figure 5). Washing the substrate revealed that a thin poorly defined layer as large as 100 microns wide containing Co, Si, a significant amount of Mg, and traces of Ca skirted the perimeters of the larger drop areas. In the drop areas a relatively even etching between the grains had occurred.

A few isolated patches of cobalt sulfate 10 to

30 microns across and containing Na and a significant amount of Si were observed.

114

High Temperature

Figure 5.

Corrosion of Ceramics

Low purity polycrystalline alumina exposed in acidic conditions at 700oC for 24 hours. Salt crystals containing Al and Mg are shown (top and bottom).

Appendix

The etching isothermal

of grains throughout

exposure.

Distributions

have formed

on fractions

the globules

coalesced.

C-Hot

Corrosion

the drop areas is evident

of the drop areas. Aluminum

After

between

After covered

was also much greater,

495 hours of cyclic

portions

exposure

of the substrate.

particularly

wetting

present

in EDS spectra

of the salt.

solution

of grains in the drop areas. in limited

observed

for the medium

by drops up to 3000

angle of 19O. Spotty wetting

Washing the substrate Well defined

was also

of the larger drops.

of Ca, Mg, Si, Al, and traces

quantities

layer of silica

of Ba and K were

revealed

the selective

aluminum silicate

(Figure 4, bottom).

In

containing

Minor amounts

of Mg

along with the Ca in a few of the crystals.

Basic Conditions

The 1OOOoC exposures extensive

of Si

of grains was apparent.

in the areas outside the perimeters amounts

Results,

but the increase

a thin discontinuous

The etching

both areas, substantial

were present

than for the

The concentration

at 1000°C for 24 hours was wetted

microns across with a measured

Na and Ca formed

in the

material.

The coupon exposed

observed,

materials.

many of

present

concentrations

10 and 100 hours was not as great as that observed

purity polycrystalline

10 hours of

100 hours of exposure

and Mg ions were consistently

or high purity polycrystalline

ions in the wash waters

after

115

of silica globules about 1 or 2 microns across

wash water at 1, 10, and 100 hours in much greater single crystal

of Alumina

reaction

term exposures

were emphasized

at the higher temperature

were made in oxygen.

salt did not remain the formation

occurred

in basic conditions

in contact

of cobalt oxide.

when preliminary

At 700°C a large fraction

with the substrate

because

throughout

more short

of the molten

the exposure

due to

116

High Temperature

Corrosion

of Ceramics

Single Crystal At 700°C phase separation of the salt occurred within 24 hours of exposure. The wetting was patchy and irregular. A wetting angle of about 20° was measured. The concentration of Al ions was limited to the EDS spectra of a few small drops, where the molten salt had remained in contact with the substrate. Cobalt oxide separated the sulfate from the substrate over most of the area of the larger drops. No reaction was detected after 1 hour of exposure at 1000°C. A wetting angle of loo was measured and broad patchy wetting covered large areas of the coupon. After 24 hours of exposure an So wetting angle was measured. Some sodium aluminum silicate was observed after 405 hours of cyclic testing at 1000°C.

Negligible quantities of Si are present in the as-received

substrate material. The alumina furnace tube is a likely source of the Si.

High Purity Polycrystalline Alumina At 700°C phase separation of the salt occurred within 24 hours of exposure. The wetting was patchy and irregular. A wetting angle of about 14O was measured. Traces of Si and limited concentrations of Al ions were indicated by the EDS spectra of small drops where the molten salt had remained in contact with the substrate. A network of globular silica associated with Co formed on the substrate in the perimeter of the larger drop areas. From the morphology of the washed substrate it appeared that preferential solution of grains and growth of Si rich needles had occurred in the drop areas. The growth direction of the needles was constant across the surface of each grain (Figure 6, top). A few larger needles were observed at or near the triple points between grains and contained significant amounts of Ca.

Appendix

Figure 6.

C-Hot

Corrosion of Alumina

High purity polycrystalline alumina exposed in basic conditions at 700oC for 24 hours (top) and 1000oC for 405 houl'S (bottom). Oriented silica rich needles (top) and silicate reaction products at grain boundaries (bottom) are shown on washed substrates.

117

118

High Temperature

After

1 hour of exposure

hemispherical Crystals

Corrosion

silicates

from beneath

across the substrate silicate

between

the salt near the centers

So was measured. A distribution

magnesium

aluminum

silicate

preferential

of the surfaces

of certain

was measured. remained

in contact

pronounced

etching

a poorly defined associated After

over most of the coupon. in EDS spectra numerous

of the salt.

well defined

the substrate

on

The

within 24 hours of

A wetting

angle of about 14O

of small drops where the molten salt had indicated

that significant

Washing the substrate

layer of silica.

on the substrate

1 hour of exposure

and sodium

(Figure 6, bottom).

of the grains in drop areas accompanied

with Co formed

of

grains is apparent.

and irregular.

in the salt.

discontinuous

pattern

grains was apparent

of the salt occurred

with the substrate

Si, Al, and Mg were present

Some

Alumina

was patchy

The EDS spectra

angle of

angle was measured.

along grain boundaries

At 700°C, phase separation The wetting

A wetting

in the drop areas.

405 hours of cyclic exposure

Medium Purity Polycrystalline

exposure.

5 and 10 microns across protruded

at the triple points between

after

in the drop areas.

Ca and Mg. A consistent

silicate

the washed substrate solution

surface

a 4O wetting

had formed

large globules contained of sodium aluminum

of well defined

of some of the drop.

After 24 hours of exposure

of globular silicate

anomalously formation

at 1000°C a distribution

formed

of sodium aluminum

of Ceramics

A sparse network in the perimeter

amounts

revealed

aluminum silicate

and a random distribution

the

of globular

of silica

of the drop areas. a continuous

film

of Ca, Mg, Si, and Al were present

Washing the substrate

magnesium

revealed

of Ca,

by the deposition

at 1OOOoCthe salt had formed

Substantial

amounts

crystals

the presence

of

scattered

across

of vugs up to 20 microns across

where

Appendix

preferential

solution

continuous partially

wetting covered

of the substrate

occurred.

of the coupon resulted with poorly defined

the one near vugs, contained

C-Hot

in a substrate

of the salt.

Rosettes

on the substrate.

amounts

top).

of grains was apparent

exposure.

etching

Numerous

were scattered

Low Purity Polycrystalline

The wetting

was measured. remained

in contact

a dense network portions

the globules suggesting After

ions.

crystals

within 24 hours of

A wetting

angle of about 16O

indicated

that significant

amounts

A significant

with Co on the substrate

amount of Mg was present

that they were a magnesium

at 1OOOoC the salt had formed A wetting

revealed

revealed over

in many of

a nearly continuous

angle of 3O was measured.

of Ca, Ba, Mg, Si, Al, and K were present

Washing the substrate

of Ca,

silicate.

in EDS spectra

the salt, both in the salt film and in small groups of well defined rich in impurity

7,

405 hours of cyclic

in the salt (Figure 8). Washing the substrate

1 hour of exposure

amounts

after

in

of small drops where the molten salt had

film on over half of the coupon surface. Substantial

in the salt (Figure

of the salt occurred

of globular silica associated

of the drop areas.

in the

Alumina

with the substrate

Al, Mg and Si were present

of

On the unwashed

sodium aluminum silicate

was patchy and irregular.

The EDS spectra

particularly

surface.

At 700°C phase separation exposure.

which was

of K were present

of Be were present

bunches of poorly defined

over the substrate

the

about 10 to 15 microns across were present

some areas where substantial The pronounced

119

of Si and Mg. Small patches

sample large peaks for Ca, Ba, Mg, Al, Si and traces EDS spectra

surface

Some of the crystals,

amounts

a thin layer of silica were also observed

of Alumina

After 24 hours of exposure

crystals.

substantial

Corrosion

salt crystals

a fine globular silicate

of

120

High Temperature

Figure

7.

Corrosion of Ceramics

Medium purity polycrystalline alumina exposed in basic conditions at lOOOoCfor 24 hours. A multi-phase deposit with rosettes containing Ca, Ba, Al, Si, Mg, and K (top) and a typical multi-phase deposit containing Ca, Al, Si, Mg and K (bottom) are shown.

Appendix

Figure 8.

C-Hot

Corrosion of Alumina

Low purity polycrystalline alumina exposed in basic conditions at 700oC for 24 hours. Salt crystals containing Ca, Mg, Al, and Si are shown (top and bottom).

121

122

High Temperature

containing

Corrosion

a significant

of Ceramics

amount of Mg. An intermittent

distribution

of larger

globules up to over 2 microns across contained

Ca.

A random distribution

below 10 microns across where preferential occurred

solution of the substrate

A wetting

a bimodal wetting

angle of 4O was measured

thin discontinuous

of the molten salt.

on the edges of the film. Al ions and traces

On the unwashed

were observed

of poorly defined

of Ca and had become

Discussion

was left

concentration

by the EDS spectra

crystals

grains was apparent

of sodium magnesium

less well defined

(Figure 9, bottom).

the substrate,

tabular

A

angle was measured

sample a substantial

and blocky sodium aluminum silicate

Blocky crystals

exposure

A 21° wetting

across.

of

of the

of sodium magnesium crystals

containing

Ca

(Figure 9, top).

The etching exposure.

had developed.

of holes and branches

of Ca, Si, and Mg were indicated

On the washed sample well defined

aluminum silicate

morphology

on drops as large as 4000 microns

film of salt with a network

behind by the dewetting

probably

silicate

than after

after

405 hours of cyclic

were predominantly

24 hours of isothermal

A phase rich in Al and Mg formed

magnesium

aluminate

free

on some sites on

spineL

of Results

Impurities

increase

the attack

of the polycrystalline

molten sulfate.

This effect

grain boundaries

and at the triple points between

phases.

had

was also observed.

After 24 hours of exposure

salt.

of vugs typically

In each condition

polycrystalline

materials

phases with the melt.

is enhanced

of exposure was dominated

aluminas

by their pronounced

segregation,

grains in impurity

the hot corrosion by the reactions

by the at the

second

of the lower purity of the impurity

silicate

Appendix

Figure 9.

C-Hot

Corrosion of Alumina

Low purity polycrystalline alumina exposed in basic conditions at 1000oC for 24 hours (top) and 405 hours (bottom). Aluminum silicate crystals containing Ca left and Na right (top) and Mg and Na (bottom) are shown on washed substrates.

123

124

High Temperature

The data available the composition Si are present stability

on the solubility

in the melt.

diagrams

Activities

applicable

The formation

occurred

of phases not predicted

at less than unity activity

across the thickness

of alumina the salt tended

by increasing

wetting

polycrystalline

aluminas,

particularly

At 400°C in acidic conditions droplets

of salt.

during the exposures

of the deposits

of the substrate

droplets. increase

and promoting

in the introduction.

all times of exposure insoluble products

Impurities

the extent

of a

This is observed

were covered

for the

below 2.0 for Since no

it is likely that SO3 from the

as the composition

aluminum sulfate

by

from the analysis of the wash water

(Table VIII). This is true for all four aluminas.

being less acidic because

of

the formation

all the coupons exposed

The data obtained

the deposit

In

under basic conditions.

rich in Na were detected,

enriched

and

is not known.

that the atomic sodium to sulfur ratios were significantly

atmosphere

work on

have been reported.

to form discrete

the area of attack

molten salt film as discussed

indicates

of

by thermodynamic

as in the previous

of Na20 in the deposits

which result in more extensive degradation

amounts

of Na20 in the salt as great as 10-6 to 10e4 at temperatures

the actual activity

the exposures

when significant

of

(Appendix A).

the profile of the activity

discrete

of alumina in Na2SO.t as a function

7OOoC and 1000°C in an oxygen atmosphere

However,

thinner

of Ceramics

of the salt is not directly

gaseous corrosion

between

Corrosion

of the melt shifted

and other sulfates

formed

toward in solution

in the melt. The presence all exposures

of Al in the wash waters

in these conditions

The wash water indicates the salt, probably

are

and the weight losses recorded

COnSiStent

that Mg was present

with the

fOrtTIatiOn

as a soluble reaction

MgSO4, since it is stable at lower pressures

of

for

A12(S0.+)3.

product

of SO3 than

in

Appendix

Al3(SO4)3.

The well defined

sulfate

amounts of Al and significant was observed

crystals

but the single crystal

of alumina is the formation (Appendix A). A mechanism term exposures

interface

the Rapp-Gotto

reaction

for the reactions

of Si rich impurity

criteria

interface

for gaseous

for fluxing. (11) Impurity

interface,

they advance

silicates

As the silicate into a negative

during the long

Figure 11. The solution

It

of SO3 at the

resulting

in a

solubility

from the melt results

on fractions

of the substrate

are dissolved

from the

ions are transported

The coalesence,

the size and shape of the patches

The morphology

corrosion

aluminas has been developed.

gradient

growth,

toward

and the

and coarsening

in the formation

surface.

of SiO3 in the melt occurs some distance

by the microstructure

10 (bottom).

for silica across the molten salt layer and satisfying

of nearly pure SiO3 results.

globular silica patches

materials

during the hot corrosion

which occurred

below that at the salt-gas

gradient

of all of the

grains as in Figure

of Al3(SO4)3, as was observed

of the SiO3 which is precipitating

salt interface

of surface

the principal

grain boundary areas on the substrate.

precipitation

on portions

that the acidic solution of alumina lowers the pressure

solubility

precipitation

substantial

of the two lower purity polycrystalline

of the less pure polycrystalline

substrate-salt

the salt-gas

125

of these crystals

(Figure 10, top). The solution

At 700°C in acidic conditions

negative

in Figure 5, contain

silica was present

in a marked lack of connectivity

is proposed

of Alumina

on the two least pure aluminas.

phases at the grain boundaries resulted

Corrosion

amounts of Mg. An abundance

After 495 hour cyclic exposures substrates

C-Hot

Because

of

the

away from the substrate-

of globular silica were not dictated

of the substrate. of the reaction of silicates

grains lowers the local Na30 activity

products present

are shown in the schematic

at the grain boundaries

in

and as impurity

of the melt in a manner analogous

to the

126

High Temperature

Figure 10.

Corrosion

of Ceramics

Medium purity polycrystalline alumina exposed in acidic conditions at 700°C for 495 hours. Globular silica (top) and etched grains (bottom) are shown on washed substrates.

Appendix

C-Hot

Corrosion

of Alumina

127

S/G

Distance

O/S

= Oxide-salt Interface

S/G

= Salt-gas Interface

Q = Impurity Silicate Phases R = Globular Silica Reaction Product S = Sulfate Deposit Containing (Al, Mg, Si) A = Alumina Grains at Substrate Surface Solution Reaction at A: 3NagSO4 + Al303 = Al3(SO4)3 + 3Na30 B = Intergranular Attack Solution Reaction at B: Silicate + Nag0 = Na3O*xSiOg (in sulfate) C = Away from Oxide-Salt Interface where Melt is More Acidic Precipitation Reaction at C: Na3O.xSiOg (in sulfate) = xSiO3 + Nag0 (in sulfate) (Between grains and impurity phases at substrate surface Reaction A + Reaction B: 3NagSOq + Al303 + xSilicate = Al3(SO4)3 + 3NagO-xSi0g) Figure 11.

Schematic diagram of the morphology of reaction products on polycrystalline aluminas exposed in acidic conditions at 7OOoC.

128

High Temperature

Corrosion

way in which the solution

of refractory

sodium

oxide concentration

certain

superalloys.(11~131

concomitant

The formation

and increasing

of the substrate

decrease

surface.

in the connectivity

For the proposed

oxides in molten

of salt deposits

to and in the vicinity

the Nag0 activity vicinity

of Ceramics

Na3S04 lowers the

during the alloy induced fluxing of of Al3(SO4)3 and MgSO4 occurs

of the localized the solubility

solution

of silicates,

of the silicate

Intergranular

corrosion

impurities

results

of the grains of the substrate.

the inward transport

of reactive

across the thickness

of the film or by the transport

species

is controlled

(i.e. SO3) from the salt-gas of soluble reaction

interface.

reactant

species

proceeds

more slowly than the dissolution

alumina grains or of impurity

silicates

at the substrate

that it is correct. detected

to check this statement,

As mentioned

in low temperature on the chemistry

transport

of SO3 to interface

droplets

polycrystalline

of salt, except

of Al were

material

where nearly continuous

losses were recorded

for all the exposures

well defined

with the form and chemistry

crystals

the washed polycrystalline

substrates.

of the samples has greater

the distance

for

is minimized.

all of the coupons exposed

for the exposure

of the

is some evidence

The SO3 atmosphere

or Na30 to surface

of

The kinetics

there

of the salt at the drop edges because

At 1OOOoC in acidic conditions discrete

however,

surface.

above, higher concentrations

acidic conditions.

products

that the transport

in the drop edges than in the bulk of the salt on several

exposed effect

This assumes

by

interface

away from the substrate-salt

could not be measured

in the

in a substantial

model it is likely that the rate of reaction

and product

raising

were wetted

by

of the medium purity wetting

was observed.

in these conditions. of silicates

After

Weight

exposure,

were observed

on

Appendix

The silicates

on the washed substrates

after 24 hours of exposure

crystals

present

of the polycrystalline

materials

crystals

containing

of the medium purity substrates.

containing

substrate.

of Alumina

substantial

amounts

quantities,

of Ca cover portions

the substrates

corrosion

silicate

of

silicate

of the low purity are

more deeply than on

Within the etched

which had been observed

amounts

where the crystals

are etched

similar exposed at 700°C in acidic conditions. intergranular

substantial

Sodium aluminum

In the drop areas on the washed substrates in substantial

129

of the drop areas on the high impurity

Sodium aluminum silicate

Mg cover portions

Corrosion

were shown in Figure 4. A fine poorly defined

covers grains over large portions substrate.

C-Hot

on the samples

drop areas the exposed

at

700°C was absent (Figure 12). The salt deposits

on the two least pure materials

contained

significant

amounts of Al, Si, Mg, and Ca after

exposure.

smaller salt droplets

Al or Si, so the Ca and Mg are in solution

the salt as sulfates atmosphere

do not contain (Figure 13, top).

The EDS spectra

Since the partial

pressure

at 1000°C was less than at 7OOoC, the formation

not have been predicted

by thermodynamics,

corrosion

(Appendix

occurred

at 1000°C but not at 700°C although

This is believed

formation

The multi-phase

interface

The uniform

etching

shown in Figure 13 (bottom)

on the unwashed

is characteristic

blocky phase contains

for gaseous of the CaS04

of impurity

of the substrate,

in areas of copious silicate

salt deposit

of SO3 in the

it is stable at both temperatures.

of Al2(SO4)3, could result from local decreases

melt at the oxide-salt

The lighter

the formation

to be the result of more severe degradation

phases at the higher temperature.

in

of Al2(SO4)3 would

but was observed

A). Under acidic conditions

of most of the

in the basicity crystal

silicate by the of the

growth.

medium purity substrate

of the two least pure materials.

Ca, Si, Al, and Mg, and the remainder

of the

130

High Temperature Corrosion of Ceramics

Figure 12.

Medium purity polycrystalline alumina exposed in acidic conditions at 1OOOoC for 24 hours. Sodium aluminum silicate reaction products and etching of drop area are shown on washed substrates.

Appendix C-Hot

Figure

13.

Corrosion of Alumina

Low and medium purity polycrystalline aluminas (top and bottom, respectively) exposed in acidic conditions at 1000°C for 24 hours. Sulfate crystals containing Ca, Si, Al, and Mg (bottom) are shown.

131

132

High Temperature

salt contains Al.

Corrosion

of Ceramics

The two phase salt droplet

be linked to the growth of aluminum

reaction

silicates

product

at surface

morphology

heterogenieties

must in the

substrate. As a result concomitantly

of the substrate

over different

in the composition aluminum

crystals

The formation

occurred

described

as follows. pressure grains.

earlier

The increased

where it reacts

and grains of silicate

with these silicates,

as well as calcium

of the sulfate transport

forming

mechanism

inter-granular

attack

The results to interpret substrate

the substrates.

of the alumina grains.

in basic conditions formation

that significant

The formation

occur nearly as extensively

the acidity As the

is quite uniform

and a

is slower and the acid and basic leading to strong

at 700°C are more difficult

of cobalt oxide crystals

Weight gains were recorded indicate

and alumina

11).

due to the extensive

Photomicrographs

of Nag0 in the melt

This maintains

mostly near the grain boundaries (Figure

near the alumina

700°C under acidic conditions,

but the transport

of the exposures

surface.

the transport

under the sulfate

similar

cooperate

accordingly

sulfates.

than at ‘i’OO°C. At lower temperatures,

reactions

the local SO3

sodium silicates

deeper

operates

by the sulfate,

the dissolution

is rapid at 1000°C, the corrosion

of the

phases in the low purity aluminas,

and magnesium

melt thus promoting

and

in Figure 14 can be explained

increases

promotes

of well defined

and the morphology

and shown schematically

Nap0 activity

to the grain boundaries

silicates

The results

and the Nag0 activity

local variations

at the grain boundaries

As the alumina grains are dissolved

decreases

occurred

and produced

of a network

preferentially

triple points of the microstructure. products

unique reactions

areas of the substrate

of the melt.

silicate

microstructure

on the

for the exposures.

amounts

of alumina

were etched

of nearly pure silica on the substrate

as at 700°C in acidic conditions.

did not

from

Appendix

C-Hot

Corrosion

r I0

of Alumina

_ _----

I I

1

,s

o

o

133

00

i

I

L-_-_-_-I

Q = Silicate Reaction Products 0 = Original Substrate Surface Multi-phase Deposit R = Bulk of Deposit Sulfate Containing (Al, Mg, Ca, Si) S = Other Phases Complex Reaction Products Containing

+

Transport Na20

of

-+

Transport

of

so;

Silica

A = Alumina Grains at Substrate Surface Solution Reaction at A: 3Na3SO4 + Al303 = Al3(SO4)3 + 3Na30 B = Intergranular Areas at Substrate Surface Reaction at B: Na3SO4 + Mg-, Ca-, Al- silicate = NaSO*silicate + MgSO4 + CaSO4 + Al3604)3

Figure 14.

Schematic diagram of the morphology of reaction products on polycrystalline aluminas exposed in acidic conditions at 1000°C.

I

oI

134

High Temperature

The presence

silicate

of Ceramics

of CaS04 in the salt on the two lower purity aiuminas,

was not detected temperature

Corrosion

on samples

(700°C),

exposed

may be due to a more severe

in the oxygen atmosphere.

not conducive conditions.

in acidic conditions

The conditions

to the type of sustained Significant

amounts

Because of the depletion

at the same

attack

of the impurity

in the oxygen atmosphere

fluxing which occurred

of cobalt oxide, it is likely that the initial dissolution

generated

at the aluminum

interface

in the salt.

of the melt due to the

formation

oxide-salt

are

at 700°C in acidic

of both Si and Al were dissolved

of the basic component

which

products

were silica and aluminum

sulfate. In basic conditions aluminas occurred. emphasizing

The lower purity materials

the influence

triple points between Even after

polycrystalline

of the impurities.

grains on the surfaces

substrate.

materials.

Aluminum

purity substrate.

Sodium calcium

aluminum silicate

formed

exposure

morphologies

at IOOOoC in oxygen.

with various aluminum elements

contained

more extensively

Vugs developed

at some of the

negligible

silicates

reaction

formed

silicates

as impurities

was detected

on the substrates silicates

on

of the

at grain

aluminum

silicates

at triple points on the high

aluminum

silicates

and sodium magnesium

on the lower purity substrate. formed

of all four of the

were wetted

They were sodium aluminum

and sodium magnesium

phase deposit

wetting

of the two least pure substrates.

405 hours of cyclic exposure

the single crystal

boundaries

at 1000°C the most extensive

A variety

on the two least pure materials

These deposits

consisted

of sodium, potassium in the substrates.

of mixtures

of multiafter of sulfate

and the rare earth

Appendix

The formation

C-Hot

of NaA102 as the primary

would be predicted

on the basis of solubility

reaction

formation

materials

of aluminum

molten salt.

cannot silicates

without

which occurred

the degradation

consideration

on the substrates

of the

of the and in the

the multiple

phase morphology

small weight gains and losses were recorded

for the exposures

1000°C in basic conditions.

Intergranular

of the surface

weight changes

corrosion

resulted

and photomicrographs

indicate

that the formation

and the solution of alumina were not as extensive

occurred

in acidic conditions impurities

in a decrease

grains of the two lower purity substrates.

silicates

between

in the deposits

on the two least pure materials.

Relatively

silicate

at the same temperature.

shift the activities

the acidic and basic conditions

promoted

The

of alumina

as that which

It is proposed

in the melt towards

that the

values intermediate

by the atmosphere,

thus

behaviors

at 1000°C under both atmospheres

similar.

However,

acidicity

of the melt under acidic conditions

increases

the greater of alumina.

with the attack

Under basic conditions

of the silicates

sodium alumina silicates of the melt providing formation

is limited

acidic conditions,

impurities

for acidic solution and the silicate

leading

the cooperative

reactions

by the basic sulfate

including those of the rare earths.

forming

mostly to intergranular

is more extensive corrosion.

the

initiate various

This raises the pSO2

of the alumina grains.

formation

at

in the

making the corrosion

dissolution

135

by Rapp (Figure 2).

material,

This is obvious when one considers

of the deposits

connectivity

be analyzed

of Alumina

product

data published

While this may be the case for the single crystal polycrystalline

Corrosion

This sulfate than under

136

High Temperature

Corrosion

of Ceramics

REFERENCES 1.

Wachtman, John B., et. al., “An Evaluation of Needs and Opportunities for Fruitful Fundamental Research in Ceramics,” Ceramic Bulletin, Vol. 57, No. 1 (1978), pp. 19-24.

2.

Johnson, Project,”

3.

T. Yonushonis,

4.

Goebel, J.A. and F.S. Pettit, “Na2SO4 - Induced Accelerated Oxidation (Hot Corrosion) of Nickel,” Metallurgical Transactions, Vol. 1 (1970), pp. 1943-1954.

5.

Giggins, C.S. and F.S. Pettit, Hot Corrosion Degradation of Metals and Alloys - A Unified Theory, Pratt and Whitney Aircraft, Report No. FR11545 (1979).

6.

Goebel, J.A., P.S. Pettit and G.W. Goward, “Mechanisms for the Hot Corrosion of Nickel-Base Alloys,” Metallurgical Transactions, Vol. 4 (1973), pp. 261-278.

7.

Jose, P.D., D.K. Gupta and R.A. Rapp, “Solubility of Alpha - Al203 in Fused Na2S04 at lOOoK,” J. Electrochem. Sot., Vol. 132, No. 3 (1985), pp. 735-737.

8.

Shi, D.Z. and R.A. Rapp, “The Solubility of SiO2 in Fused Na2S04 at 900°C,” J. Electrochem Sot., Vol. 133, No. 4 (1986), pp. 849-850.

9.

D.R., et.al., “Ceramic Technology for Advanced Heat Engines Ceramic Bulletin, Vol. 64, No. 2 (1985), pp. 276-280. Cummins

Engine Company,

Private

Communication.

Kim, G.M., “The Effect of Contaminants and Solubilities of SiO2 In Fused at 1200°K,” Masters Thesis, University of Pittsburgh, 1982.

Na2S04

10.

Rapp, R.A. and K.S. Goto, “The Hot Corrosion of Metals by Molten Salts, “Symposium Fused Salts, Electrochemical Society Meeting (Pittsburgh, 1979).

11.

Rapp, R.A. and K.S. Goto, Fused Salts, (J. Braunstein eds.), Electrochemical Society, 1978.

12.

Birks, N. and G.H. Meier, Introduction to High Temperature Metals (London: Edward Arnold, 1983), pp. 146-158.

13.

Shores, D.A., “New Perspectives on Hot Corrosion Mechanisms,” H&$ Temperature Rapp, R.A. (ed.), (Houston: National Association of Corrosion Eng., 1983), pp. 493-501.

14.

John, R.C., “Corrosion of Metals by Liquid Na2C03,” State University, 1979.

and J.R. Selman,

Oxidation

Doctoral

Thesis,

of

Ohio

Appendix

D-Hot

Corrosion of Silicon Nitride

and Silicon Carbide J.R. Blachere,

D.F.

Klimovich

and F.S. Pettit

INTRODUCTION Silicon nitride and silicon carbide are two ceramics materials considered seriously for structural applications at high temperatures.

They form a protective

silica scale in oxidizing atmosphere which is quite stable thermodynamically.

In

highly corrosive environments such as those prevailing in incinerators or gas turbines burning low grade fuels, SiOz may be attacked due to the pressure of SOS, CO2 as well as oxides of metallic impurities (e.g. NagO, Na$O&

It has

been shown in this program that Nap0 is the corrosive agent in the hot corrosion of silica in contact with NagSO4 deposits (Appendix B). In particular the sodium oxide promote devitrification and the crystalline layer formed tends to spa11 under temperature cycling.

Using the results for silica as foundation, the

mechanisms and the extent of the hot corrosion of silicon nitride and silicon carbide must be established.

EXPERIMENTAL PROCEDURE The general procedures have been discussed in previous report&)

and in

previous parts of this report. The materials are shown in Table I of the main report. High purity materials (single crystal silicon carbide and CVD silicon nitride) were studied in detail. Morphological studies of the corrosion were performed on these materials and representative engineering materials usually characterized by a significant level of impurities added during processing particularly as sintering aids. The two atmospheres used throughout the experiments were SOg-02 mixtures with a total pressure of 1 atmosphere. One contained 1% SO2 initially, which generated a pressure of 1.5 x 1W3 atm of SO3 at 1000°C. The other was pure oxygen. The gases flowed at the rate of 1 cm5/s. 137

Usually the sodium

138

High Temperature

sulfate

Corrosion

was applied with a surface

The surface

loading of 5 mg/cmz

loading as well as the temperature

purer materials. standard

of Ceramics

The times of exposure

were varied

scanning

were used as described electron

microscope

developed

earlier.

They depended

measurements

for this part of the research(2),

particular surface

were supplemented

X-ray diffraction,

before

on the features

by

in the SEM.

with a number of other techniques,

The morphology

measurements,

as described

in

and many types of

will be discussed

was characterized

washing off the soluble materials

wash water was performed

of salient

on cross sections

analysis (ESCA, SIMS, ISS). The methods

and after

strongly

where used for scale of thicknesses

weight change

the text of this appendix.

on the

based on X-ray spectroscopy,

under 1 pm. Above 1 urn they were measured These experiments

in the studies

The usual characterization

(SEM) with microanalysis

EDS and WDS. The thickness

substrates.

varied from 1 hour to 168 hours, with

times of 24 and 168 hours for all materials.

techniques

on polished

in water.

after

as needed

in

exposure

Analysis

of the

earlier.

RESULTS AND DISCUSSION The results morphologies, carbides(1~3~4). conditions

were described

earlier

Scale thickness

are compared

single crystal) corrosion,

of 24 and 168 hours exposures,

after

in particular

the product

for the various silicon nitrides 168 hours under gaseous,

and silicon

acidic and basic

in Fig. 1 and table I. For the purer materials

the thickness

of oxide formed

which was essentially

oxidation,

increased

by the impurities.

the sulfate

was not depleted

Under basic conditions on any samples

in order from gaseous

acidic and then basic corrosion.

trend is no longer clear for the more impure specimens dominated

(CVD and

whose behavior

is

it must be emphasized

which were not preoxidized

This

that

before

Appendix

D-Hot

Corrosion

of Silicon

Nitride

and Silicon

Carbide

139

168 hour:

sic-c

Figure

1.

Sic-Si

Si3

IV,

Thickness of layers formed for the oxidation, corrosion of C-side and S-side single crystal CVD silicon nitride after 168 hours at 1000°C sulfate drops for acidic corrosion). Note that increase as OxA&.

acidic and basic hot silicon carbide and (measured between oxide thicknesses

140

High Temperature

Corrosion

Table I Thickness

SC SC -

si

of Ceramics

of oxide scales after

Gaseous

Acidic

0.11

0.6

168 hours exposures

(urn)

Basic

12-25

c

0.5

1.1

CVD SC

_-

1.2 (1.8)*

7.7

HP SC

0.78

0.61

-- 1.4 (24 hrs)

CVD SN

0.07**

0.3 (1.41)

-6

HPSN

0.9

4.3 - 7.1

1.5 - 2.1

Sin SN

2.5

1.3 (2.1)

2.4 - 3.2

SC SC -

*

Values in parenthesis are for spherulites under sulfate drops. For acidic corrosion the values not parethesis were measured between the droplets.

**

0.045 by WDS, 0.06 by ellipsometry,

SC SC -Si

=

single crystal

Sic, silicon side

SC SC-C

=

single crystal

Sic, carbon side

CVD SC

=

CVD silicon carbide

HP SC

=

Hot pressed

CVD SN

=

CVD silicon nitride

HP SN

=

Hot pressed

Sin SN

=

Sintered

silicon carbide

silicon nitride

silicon nitride

0.09 SEM

Appendix

exposure

D-Hot

Corrosion

to the corrosion.

of Silicon

Experiments

before

of the sulfate

on these

tendency

Nags04 In many cases,

no sulfate

overall stoichiometry gradient

materials

crystalline

silicas

in large amounts preoxidation

+ SiOq = Na silicate

composition

are richer

This is probably

as the silicates

slowed this reaction yttrium

as surface

not preoxidized

oxidized

more as indicated

materials

not preoxidized.

by greater

who reported

through

the

scales

washing) than the same were formed

during cooling before

at 14OOoC,

application

during the hot corosion.

the sulfate

acceleration

that

since the preoxidized

wets completely

(table I). This has been documented

a transient

on

as the scale thickens.

the sample surface

with the scale as it forms leading to very thick silicate

purer silicon nitride

Impurities

it is the rate of oxidation

weight gains (after

extensively

of the salt and were not protective

reacts

with

which was water soluble but they

Since the preoxidation

and cracked

of a

composition.

and the transport

This is indicated

more silicates

Under basic conditions

in equilibrium

the rate of oxidation

the scale is protective.

they had crystallized

formed

layers decreases

not only formed

indicative

it is clear from these results

the amount of silicate

samples

and the silicate

silicates)(1p3).

which controls

Therefore

(table II). This

phases were formed

(1) occurs under basic hot corrosion,

and sulfate

complete

(1)

in the washwater

while reaction

silicate

with thick

by EDS of the surface

+ SO3

in silica than this average

For the purer materials

141

for the reaction

could be detected

(Mg silicates,

as indicated

wash water analysis

was about NagSiO3.

in the silicate

specimens

Carbide

lead to the essentially

washing and by the semiquantitative

shows the fundamental

and Silicon

on preoxidized

scales (>lO um)(3) and on bulk fused silica(s) consumption

Nitride

of oxidation

and

layers even on the

by Mayer and Riley(G)

with injection

of NagCOS.

142

High Temperature

Corrosion

of Ceramics

Table II Wash Water Analysis From Basic Hot Corrosion Material

Si02 At%

(168 Hours, 1000°C)

SO3 At%

Na20 At%

25 52

33 0

42 47

CVD SC CVD SC (PO)

22 50

27 2

48

CVD SN CVD SN (PO)

0 53

62 12

38 35

SN SN SN SN

-_

-_

--

59

13

27

SC SC

SC SC

(PO)(l)

(PO)

SC SC

= Silicon Carbide

CVD SC

= CVD silicon carbide

CVD SN

= CVD silicon nitride

Sin SN

= Sintered

(1)

= Preoxidized

PO

52

single crystal

silicon nitride in 02 for 10 hours at 1400°C

Appendix

D-Hot

Corrosion

With Na2S04 the reaction hours.

Also the extent

materials

formed

carbide

of the reaction

thicker

present

discussed

in a previous

corrosion

report,

magnesium

were similar in morphology Early in the research

conditions. understand and establish crystal

temperature

As

into the MgC in on

the acidic

layers,

these

layers

silicon nitride

and

under basic conditions. that some hot corrosion

to the expected for shorter

corrosion

times

degradation.

occured

under basic

were needed

of hot corrosion

and the CVD silicon nitride

in order to

under acidic conditions Therefore

were exposed

at 1OOOoC. In other experiments

the single for 1 to 24 hours the

and Na2S04 loading were varied as necessary.

Under acidic conditions and silicon carbide both materials. variation

silicate

and sintered

apparent

mechanisms

acidic conditions

tend to segregate

In general

continuous

if it could lead to significant

silicon carbide

under standard

conditions.

The

phase.

basic conditions

it became

the fundamental

or hot corrosion.

the glassy silicate

and yttrium

to those observed

Data on pure materials

silicon

The alumina showed no

on oxidation

silicon nitride

in addition

and the purer

and tends to promote

formed

143

in 168

conditions.

under acidic environmental

were quite thick for hot pressed

was not consumed

also the hot corrosion

silicates

on the impure samples

Carbide

for the hot pressed

to stabilize

They complicate

under acidic conditions

and Silicon

by impurities

aid.

in the scale either

forms magnesium

the surface

possibly

alumina as a sintering

in the scale tended

scale on oxidation. particular

is affected

scales except

for segregation

aluminum

Nitride

is milder(3~4~71 as all sulfate

which contained

preference

of Silicon

completely.

the sodium sulfate The wetting

The drop size distribution

in the values of the wetting

with drop size, exposure

time or nature

does not wet the silicon’nitride

angle was of the order of 40° for was bimodal (fig. 2). There was a

angles measured of the sample.

but it was not consistent However

the values

144

High Temperature

Corrosion

of Ceramics

Appendix

observed

were greater

The evolution crystal

D-Hot

Corrosion

than measured

of the deposit

silicon carbide.

smaller

after

process.

after

are shown in figures

and generally

order to minimize

the influence

data for silicon carbide of time suggesting show clearly

silicon carbide

and CVD

of vitreous droplets.

oxide measured

All the thicknesses

by the same method (X-ray spectroscopy(2)) of systematic

plots as good straight

a parabolic

(fig. 2).

behavior.

this trend and are plotted

errors

in the interpretation.

lines as function

The kinetics as a function

workers

indicate

the formation

and interpolated

that the hot corrosion,

in in The

of the square root

for silicon nitride

do not

of time in figure 3a and as a

of the square root of time in figure 3b. The lines for oxidation

from previous

between

and they are usually

3-5 for times from 1 to 24 hours at 1000°C.

away from the sulfate

were obtained

research

as a result of a coarsening

for single crystal

3 and 4 were for thicknesses

these two figures

function

As shown in the figure the

with time, probably

of acidic corrosion

The plots of figures between,

at 1000°C.

to those in fig. 2 were

10 to 24 hours by bands free of small droplets

The kinetics silicon nitride

as a layer around 120°C

similar

The large drops do not move on the surface

surrounded

145

Carbide

of time is shown in figure 2 for single

melts and morphologies

tend to disappear

and Silicon

which was sprayed

a few minutes of exposure

droplets

Nitride

on fused silica under similar conditions.

as a function

The deposit

breaks up as the sulfate observed

of Silicon

from previous

experiments

in this

even under acidic conditions

of the oxide layers in all cases for the purer materials

data

enhances even

the salt droplets.

The amount of sodium sulfate from 0, 0.1 and 5 mg/cm2, for oxidation, independent

applied per unit area of surface

the standard

the growth of the vitreous of the surface

surface

loading.

scale between

was varied

While higher than that the sulfate

droplets

loading as shown in fig. 4. The data for different

is

146

High Temperature

Corrosion

of Ceramics

1000-

-0

/

CVDSN /

2 J; v)

z

5 i

b

/ /

.

/ /

Hot

Corrosion

/

.

/

500.

/

---Q

/ / /

Oxidation

,

168 TIME(HR)

Figure 3.

Thickness of scale formed in the acidic hot corrosion silicon nitride at 1000°C (5mg/cm2 of NaZSOq). (a) linear plot

of CVD

Appendix

D-Hot

of Silicon

Corrosion

and Silicon Carbide

Nitride

/ / / / / //

/

/’

b

/O

d

01

I

I--_-_-l

I.0

5.0 VT

Fimre

3.

(b) parabolic Figure 3(a).

I

L

13.0

9.0 (h OUTS ) ‘L

plot measured between

the drops of

147

148

High Temperature

Corrosion

of Ceramics

C-side

1.0

5.0

7rme

Figure

4.

‘12 (/+%)

(a) Thickness of scale formed at 1OOOoC in the Acidic hot corrosion of single crystal silicon carbide. Three different loadings of NaZSO4 on the surface 5mg/cm2 (A), less than 0.1mg/cm2 (“) and 0.0mg/cm2 (0) fell on the same parabolic plot. The data for silicon and carbon side fall on two separate lines. (parabolic)

Appendix

D-Hot

Corrosion

of Silicon

Nitride

and Silicon

Carbide

149

H(t) --

---

--

--

--

A--I

0

5.0 TIME

Figure

4.

(b)

“*H

“*I

Parabolic plots comparing of the data of figure 4(a) for single crystal silicon carbide (solid lines) with the oxidation of silicon (D + G )8 and single crystal silicon carbide (for carbon side (“) and silicon side (A) (Harris) 22 at IOOOoC.

150

High Temperature

Corrosion

of Ceramics

0

SPHERULITES UNDER

DROPS

O-SC(C) A-

____I_

SC(Si)

O-CVDSN 0

22

/

0 0

I3

A i-.---.

A

l:o

’

510 I/2

l/2

TIME

Figwe

5.

(HR

)

Thickness of spherulites (oxides) formed under the sulfate droplets for the C-side and Si-side of single crystal silicon carbide and CVD silicon nitride (Acidic hot corrosion at 1000°C). The solid line corresponds to the data of figure 4 for the acidic hot corrosion of C-side silicon carbide.

Appendix

D-Hot

Corrosion

of Silicon

fall on the same lines.

Nitride

and Silicon

Carbide

surface

loadings

samples

with 0 mg/cm2 loading but they were exposed with the other samples,

and they picked salt apparently the data on the surface maintained constant

through aNa

No salt was applied initially

151

through vapor transport.

loading is consistent

on the

The independence

with a constant

of

salt activity

the vapor phase and a fixed pSOS (1.5 x 10e3 atm) setting

for the experiments

at 1000°C according

a

to the reaction

Na2S04 = Na20 + SO3 Kg = aNa20.pS03/aNa$304 No sulfate

could be detected

on the vitreous

was found by ISS in the surface silicate

layers of the scale suggesting

layer about 10 A thick at the surface

the role of sodium in the acidic hot corrosion but it underlines

scale by SIMS and ISS, but sodium

of the scale.

the formation This conclusion

will be discussed

that the role of sodium is different

in detail

of a and

below,

in acidic corrosion

from that

in basic corrosion. The rate of oxide build up under the sulfate corrosion

on the purer silicon nitride

that outside the droplets crystallizes

and silicon carbides

thickness

measured

is close to that calculated at that temperature. fine crystalline

of this spherulitic

after washing,

cristobalite material

follows approximate

during acidic

is much greater

spherulites.

than

at the center parabolic

The evolution of the behavior

from Deal and Grove%(*) data for oxidation

However this is fortuitous

material

formed

for C-side SIC as shown in fig. 5. The silica

rapidly under the salt forming

of the average droplets,

except

droplets

which

of silicon

since this part of the scale is

which grows from the melt.

152

High Temperature

Acidic Hot Corrosion

Corrosion

of Ceramics

of Silicon Nitride

The data of figure 3a, is fairly linear with an intercept axis at about 80 A. This suggests with the present corrosion

experiments.

experiments

although

measurements

The silicon carbide

were cleaned

essentially

in HF prior to the hot

appear responsible

but a systematic

as discussed

Laser ellipsometry

the one for vitreous figure 3 measured conclusive oxynitride.

silica.

This index of refraction

The corresponding

on CVD silicon nitride

the thicknesses

specular

The thickness

were generally

thickness

Tressler reflection

is

on the

of 1.75 and the

is much higher than 1.46, are larger than those of were not

to the presence

et al. recently

of

reached

FTIR spectroscopy@).

of the glassy product

lower when measured

on cross sections

measured

layers formed

did not appear sensitive

extensive

than measured

is that oxynitride

(Oku). IR measurements

and was not pursued further. after

does not

could not reach the major peak for the silicon

The method in general

ESCA data is still expected.

spectrometry

on the product

by X-ray spectrometry

the same conclusion

explanation

with an index of refraction

since the spectrometer

silicon oxynitride

The method of thickness

below.

measurements

given in the table.

like the

(fig. 4). The silicon carbide

error in the measurements

Another

of table III were consistent

thicknesses

in the

is also pushed to its limit with the very

for this intercept.

on oxidation

as established

which were pretreated

y-intercept

through the origin.

by X-ray spectroscopy

thin oxide layers formed,

samples

samples

do not have a significant

data extrapolate

crystal

The samples

so that an oxide layer close to 100 A is not expected

industry.

silicon nitride

formed

an initial oxide layer which is not consistent

some oxygen and water are always readsorbed,

electronic

with the vertical

layers formed

by X-ray

in the SEM. For the Sic single

by X-ray were larger than measured

in the

Appendix

D-Hot

Corrosion

of Silicon

Nitride

and Silicon Carbide

Table III Scale Thickness

of Selected

X-ray (4)

Samples (A)

Ellip (5)

A

A

CVD SN (1)

450

605

CVD SN (2)

120

356

CVD SN (3)

130

326

(1)

CVD silicon nitride exposed to gaseous corrosion balance 02 for 168 hours at 1000°C.

1% SO2-SO3

(21

CVD silicon nitride at 91OOC.

exposed

to acidic hot corrosion

for 24 hours

(3)

CVD silicon nitride at 955OC.

exposed

to acidic hot corrosion

for 24 hours

(4)

X-ray spectrometry

(51

Laser beam ellipsometry R & D.

[2] courtesy

Terry O’Keefe

Westinghouse

153

154

High Temperature

Corrosion

SEM(2). The of thicknesses on the silicon nitride thickness

silicon nitride.

and subject

is consistent

Further

to errors but the trend in the

with an oxynitride

This is in qualitative

ellipsometry. consistent

in the SEM on very thin layers such as those formed

are difficult

measurements

of Ceramics

agreement

with the atomsphere).

graded layers of oxynitride

apparently

results

data indicated

without overlayer

by the surface

reaction

3, the pressure

the interface

favoring

silicon nitride

of nitrogen

of silica which

reaction:

kinetics(l3). thickness

is expected bound.

rate controlling

The sequence

of the three

data due to the formation

the thickness

calculation

of oxynitride

which grows between

to be more difficult

The transport at steady state mechanisms

of oxynitride

from X-ray spectrometry

after

168 hours was measured

urn by imaging of cross section would be required

to establish

became

after

parabolic

at

the

Diffusion than through

the

of oxygen through this giving parabolic

and the distortion

instead

of the

of the silica assumed

are responsible

shape of the plot of figure 3a. The scale grows more slowly after thickness

decreases

silica as proposed previously(ll).

oxynitride

scale becomes

(3)

rises and the oxygen potential

the formation

silica(12) since it is more tightly layered

of silica

for short times in figure 3. As a result of

and the vitreous

through the vitreous

that it was

of silica.

Si3N4 + 3 02 = 3 Si02 + 2 N2 giving very steep linear kinetics

by

Raider et al.(lO) reported

are in line with an initial formation

would be rapid and controlled

on the

obtained

with possibly an overlayer

50-100 A thick ( in contact

The present

with the results

analysis of the ellipsometry

with a graded film of oxynitride

layer formed

for the linear 24 hours.

as 1 pm by X-ray spectroscopy

The

and 1.2

in the SEM. However data for longer times yet if the growth had reached

one week exposure.

steady

in

state

Some data was obtained

and at lower

Appendix

D-Hot

Corrosion

of Silicon

and shorter times in an attempt

temperatures

of the linear process

Nitride

and Silicon

to determinethe

but this was not justified

because

Carbide

activation

155

energy

of the complexity

of the

early kinetics. The formation has been proposed

of silicon oxynitride by previous

oxidation

and concluded

of silicon nitride.

oxidation(l@.

and used as passivating material

formed

indicated

oxidation

that graded oxynitride

conclusions. formed

Tressler

on oxidation

of silicon oxynitride

layers were formed measurements

extensive

for a layered

by the mobility of a specie

through two layers,

order to oxidize the silicon nitride.

is assumed

interface

oxynitride

and

on silicon oxidation(lO).

characterization

of scales methods

that a thin layer

scale growing by transport

layers of oxide scale to oxidize at the gas-scale

oxynitride-silica

by thermal

and concluded

in detail by Yurek et al.( 17) for the diffusion

oxygen must diffuse

deposits

readily

under the silica.

kinetics

the layers are controlled

is prepared

using many different

SIMS, FTIR, ellipsometry

formed

of this

were used to reach these

of silicon and silicon nitride

The steady state

treated

of silicon nitride

of ESCA

during the

of the vapor deposited

et a1.(gy13) performed

such as etch back rates,

was formed

only silica as the product

Comparison

ESCA and index of refraction

a combination

industry silicon oxynitride

layer.

by thermal

(141, others

that an oxynitride

Others report

In the electronic

convincingly

which could not be interpreted

Hench used IR spectroscopy

and depth profiling

of silicon nitride

workers but often not demonstrated

since they usually used one technique unambiguously.

during the oxidation

through the scale.

interface.

In the present

the silica and then the oxynitride

Part of the oxidation

is controlling(g~l*)

This was

of metal ions through

and the other at the silica-silicon

that oxygen transport

through

two case in

occurs at the nitride

as generally

interface.

accepted

for

It

156

High Temperature

dry oxidation difficult

Corrosion

of silicont lg).

of Ceramics

Since diffusion

than through silica, the oxynitride

should be thinner thickness

than the oxide layer.

of the layers under diffusion

through

the oxynitride

layer under steady

state

controlled

steady state

respectively.

will be given by

and the rate constants

of individual

layers of oxynitride

Kp* for the total thickness

y+x can be written (5)

and if y is small Kp* = Kp’sil. Then the overall

distinct

oxide layers.

of vitreous

directly

through the oxynitride

concentration conclusion

distance

from the previous

The detailed

This discussion

the formation

kinetic arguments

assumes

of a graded

except

than a layer of specific

to those

layer

that this layer

composition.

since the mobility

This is

of oxygen

is expected

to increase

as the nitrogen

shape of the nitrogen

distribution

would depend on the specific

dependence is consistent

large oxygen content

of the diffusion

coefficient

of the scale found by X-rays.

The conclusions

the layer.

found a lower index of refraction They find high actication

energies

The

and the

of the scale,

of

this may

yet in the ellipsometry

of this analysis are in agreement

et al.cg) on the oxidation

decreases.

The index of refraction

graded structure

be due to the fact that steady state was not achieved

work of Tressler

through

content

with the graded layer found by ellipsometry

the scale appears high for the proposed

samples.

would be similar

would not change this conclusion

will spread over greater expected

kinetics

on silicon nitride.

To a first approximation

oxynitride

for the

and silica,

Kp* = (1 + Y/x)~ Kp’sil

of a silica layer forming

that the

(4)

in which y, x, Kp’oxy, Kp’sil are the thicknesses on silicon nitride

conditions

Based on(17), it is expected

y/x = Kp’oxy/ Kp’sil

formation

is more

of silicon nitride

with the extensive except

to films grown under different

that they conditions.

for both the linear and parabolic

constants

of

Appendix

D-Hot

Corrosion

a Deal and Grove analysis(*)

of Silicon

for the oxidation

Nitride

and Silicon

of CVD silicon nitride

1100 and 13OOOC. They conclude

that the low rate of oxidation

highactivation

to silicon must be due to transport

energies

compared

through the silicon oxynitride kcal/mole

for the parabolic

between

activation

oxygen molecular

oxygen transport,

and it must be rate-controlling.

molecular,

be slower and with a higher activation analysis based on the formation

between

and of oxygen

The 110-120

energy seem to rule out the rate control

or ionic diffision

particularly

157

Carbide

through vitreous

silica.

through the oxynitride

While

is expected

to

energy than for fused silica the previous

of layered

scale suggests

that it cannot

be rate

controlling.

Discussion

of the Hot Corrosion

The oxidation observed

of silicon nitride

in previous

in this research

of Silicon Nitride

studies

is very slow.

were due to impurities

in the gaseous corrosion

hot pressed silicon nitride

conditions.

added for sintering

as shown

of CVD, sintered

as shown in table I. The oxide layers formed

or CVD silicon carbide This may not establish

silicon carbide

rates(20)

(and hot corrosion)

high purity silicon nitride (CVD) were thinner single crystal

The larger oxidation

in this respect

than observed

but it is certainly

or hot corrosion

is intrinsically

not inferior

on the

on the high purity

under the same oxidation that silicon nitride

and

superior

as stated

to

in older

literature. The oxidation

of silicon nitride

at 1000°C in both cases a vitreous transport

through

the detailed nitride.

the scale controls

mechanisms

The general

is much slower than that of silicon although

oxide scale is formed

in both cases in that temperature

are apparently

features

and it appears

different

of acidic corrosion

and not established already

discussed

oxygen range but for silicon

are similar

158

High Temperature

for silicon nitride was

increased

the sample, behavior

Corrosion

of Ceramics

and silicon carbide

and in particular

under acidic corrosion.

The presence

of Na20 on the surface

supplied as Na2S04 (or Na2C03 etc...)

of the Na20 depends on its activity

by reaction

2. From the equilibrium

the growth of the scale

modifies

in the sulfate

constant

this oxidation.

by the SO3 pressure

acidic conditions

(here at 1000°C, a pSO3 of 1.5~10~~ atm was selected)

is small and the sodium sulfate

oxidation

of the silicon nitride

of the wetting

reacts

in previous

of sodium oxide is

then determined

aNa

little

or silicon carbide

as reported

expected,

loosening

above.

Some penetration

the network

reports.

Under the

with the silica formed

as indicated

angles on bulk silica(5) and on the scales

specimens

The

melt as determined

Kp, the activity

as described

of

by

by the high values

formed

on the purer

of Na20 into the silica scale is

by the reaction

Na20 + -Si-O-Si-

= 2 -Si-O- + 2 Naf

Kg = [Csio12.[CNa+12/

(6)

aNa20.Csiosi

* [Csio14/aNa20 This reaction

should be applicable

also to the oxygen bridges of the oxynitride.

As a result of this opening of the network

molecular

nitrogen

scales,

is increased

oxidation.

through the vitreous

It is likely that the sodium ion diffuses

where it is reduced

as proposed

help in the silicon-oxide may affect

structural

also the stability

sodium has been suggested the evidence

for silicon,

diffusion

thus generating to the reaction

it was proposed

transition

of the oxynitride

phase.

in the network according

diffusion

in silica glass as discussed

to reaction

interface

also that it would

In the present

Only very small amounts

dissolved

a higher rate of

at the interface(21).

in the scale with the electron

is not conclusive.

of oxygen and

microprobe

The sodium experiments and ISS, but

of sodium need to be

6 in order to increase

interstitial

later for SIC. Under acidic conditions

the

Appendix

rate of oxidation

D-Hot

Corrosion

of Silicon

of CVD silicon nitride

probably by the mechanism

discussed

is increased

above.

This adsorbed

transport

159

Carbide

the droplets

by surface

it did not contain diffusion

any

or vapor

would supply the sodium into the glass. under the sulfate

outside and crystallization exposure

starts

droplets

oxidizes

more rapidly

early (in the first hour).

After

than

24 hours of

and washing off the salt the region which were under the droplets The corresponding

droplets.

thicknesses

The crystal-glass

the surface

interfaces

of the sample.

Thus the crystallization

provides

scale under the sulfate

the intercrystalline spherulites

provide high diffusivity

concentrated

fast transport

outside

stood

the

paths normal to

in the intercrystalline

regions

during the crystallization. paths for oxygen leading

drops and greater

penetration

regions such as the interfilamentary

to

of the scale under

regions of the

shown in figure 6.

Under acidic conditions which crystallize in between

in Table I were measured

Some liquid is present

in which sodium and other impurities

thicker

between

the droplets,

layer fed by the droplets

The silicon nitride

out(l).

and Silicon

A very thin (slOA) layer rich in

sodium was found by ISS on the scale between sulfur.

Nitride

for pure materials,

very slowly outside the droplets.

the droplets

exposure.

The difference

nucleation

started

disperse

in growth rates

drops appear slower yet than in this example. A, the crystallization aNa20) in the structure,

of vitreous

silica depends

and the presence

with the network

(reaction

are formed

is extremely

drop.

occurs

slow as

in the glass even after

is illustrated

under the edge of a sulfate

scales

Some crystallization

but it is sparse and the growth

judged by a few very small spherulites

oxide reaction

vitreous

168 hours

in figure 6 where The growth away from the

As discussed

in detail

in appendix

on the Oxygen ion activity

of defects

in the network.

6) provides

those defects

(or

The sodium and tends to

160

~6.

High Temperature

Corrosion of Ceramics

Acidic hot corrosion of CVD silicon nitride at lOOOoC. Under the sulfate drops (washed off in micrographs) the oxide crystallizes as spherulites. After 24 hours the spherulites coarsen to globular arrays.

Appendix

D-Hot

Corrosion

of Silicon

Nitride

and Silicon

Carbide

161

occur preferentially at the interface between the salt and the sulfate where aNa

is highest. Under acidic conditions, in close proximity to the atmosphere

(outside the sulfate droplets) the gaseous potentials remain close to pSO3~1.5 x 1V3 atm and ~02~1 atm and the aNa

is low. Under the sulfate droplets, the

oxygen and SO3 are more difficult to replenish as they are depleted by the oxidation reaction at the silicon nitride-oxide or sulfate interfaces.

Based on

equation (2) and so3 = so2 + l/202 K7 = (p02)li2

(7)

.pSO2/pSO3

The activity of Na20 is raised under the sulfate droplets (it is higher than outside) and the crystallization is promoted under the droplets.

Acidic Hot Corrosion Of Silicon Carbide The morphologies of the samples of single crystal silicon carbide after exposures were described previously (lp3) and for short times are generally similar to those described above for the silicon nitrides (see fig. 2). They are characterized by the formation of sulfate droplets separated by smooth vitreous regions. The spherulitic growth of cristobalite occurs rapidly under the drops and it is generally similar to the behavior observed on the silicon nitride (fig.6). Preferential attack occurs also under regions between the cristobalite crystals as shown in Figure 7 in which the silicon oxide has been etched away. The kinetics of acidic hot corrosion for single crystal silicon carbide at 1OOOoCare plotted as a function of the square root of time in figure 4 and they fall on two distinct lines both higher than the oxidation data of Harris(22) sketched on the figure. The two lines have been associated with the carbon side and the silicon side of the single crystal. In agreement with the oxidation results

162

High Temperature

Corrosion

of Ceramics

of Harris and Tressler et al.(g) the carbon-side is the fast or thick side and the silicon-side is the thin or slow side. Hot corrosion data was obtained also at 955OC and 910 oC. The data at 910°C are plotted in figures 8 and 9 and both sides still show parabolic behavior at that temperature.

Activation energies for

the growth of oxide scale in the temperature range 910-1000°C were determined from the parabolic constants B* calculated from x2 = Bt in which x is the scale thickness at time t. The thickness measurements were all performed by one method (X-ray spectroscopy) in order to minimize the influence of systematic errors. The plots of figures 10 and 11 give activation energies of 34 and 118 kcai/mole for the carbon-side and the silicon-side, respectively. Silicon carbide is a polar crystal and has different basal surfaces (0001) and (0001)(23) which can be differentiated by a number of methods based on different surface morphologies after high temperature wet oxidation or attack by fused salts(24). The present hot corrosion experiments resulted in milder attack but it was clear after extensive experience that slight differences in morphologies existed between the two surfaces which allowed their identification.

Namely

after hot corrosion for long times, the silicon side between the droplets appeared as-polished except for some interference colors on the thicker scales and the carbon-side was rougher, duller in texture.

Different oxidation behaviors have

been reported(g*22) for the two sides. This difference in behavior was no longer apparent at high temperatures (1400-15OOoC). Similar basic hot corrosion behaviors were found for the two sides in this research at 10000C(1~3~4(. Under acidic hot corrosion, the vitreous scale between the droplets grows faster on the

*B was determined also by the Deal Grove formulation but it gave the same results since the data plotted as functions of t1/2 falls on good straight lines extrapolating through the origin (fig. 3,7,8).

Appendix

~7.

D-Hot

Corrosion of Silicon Nitride and Silicon Carbide

163

Acidic Hot CorroSion 0( silicon carbide single crystal (lOOOOC). The substrate was attacked under the spherulites as shown after disSOlution 0( oxide in HF.

164

High Temperature

Corrosion

of Ceramics

31OC KINfllCS

-

C

5000

4000

3000 3 x I? 5 0 5

2000

’ 000

0 0.00

2.00

4.00

6.00

a.00

SQRT(TIME(HR))

Figure 8.

Kinetics of acidic hot corrosion of C-side single crystal carbide at 910°C (parabolic plot). Note reproducibility

silicon of data.

Appendix

D-Hot

Corrosion

of Silicon

Nitride

91OC KINETICS

-

and Silicon

Carbide

‘I65

SI

600

500

4oc

z x Y $ I! F

300

200

100

0 0

Figure 9.

2.00

4.00 SQRT(TIME(HR))

8.00

6.00

Kinetics of acidic hot corrosion of Si-side single crystal carbide at 91OoC (parabolic plot).

silicon

166

High Temperature

Corrosion

of Ceramics

ACTIVATION ENERGY

-

C

1400

i 3.80

13.60

13.40 G 5 1320

13.00

i 2.80

12.60

1

0. 78

0.80

0.82

0.84

1000/T

Figure 10.

Arrhenius plot for the acidic hot corrosion silicon carbide.

of C-side

single crystal

Appendix

D-Hot

Corrosion

of Silicon

ACTIVATION

Nitride

ENERGY

-

and Silicon

Carbide

St

13.0

12.0

1 1 .O

10.0

8.0

3.78

0.82

0.80

0.84

1000/-T

Figure

1.

Arrhenius plot for the acidic hot corrosion silicon carbide.

Si-side single crystal

167

168

High Temperature

Corrosion

of Ceramics

carbon side than on the silicon side as reported on figure 4 both sides show well defined results

the carbon-side

was parabolic

parabolic

energy

behaviors

However

while in Harris’

and the silicon side had linear kinetics For the hot corrosion

the broad time scale of the experiments. activation

by Harris for oxidation.

over

the apparent

of the slow side (Si) is much larger than that of the fast

carbon-side.

Model for the Oxidation Conditions

and Hot Corrosion

In order to discuss the hot corrosion conditions

in between

oxidation.

The formation

dependent

on the transport

The molecular established

the sulfate

diffusion

of silica scales

of oxygen to the substrate

diffusion.

Others

of silicon carbide

about the same it is difficult experiments(g) oxidation

of silicon carbide

work it was concluded temperatures(l*).

that transport

in the oxidation

one with low activation

(216kUmole)

at 1 atm pressure

of oxygen.

recent

at higher

of C-side

silicon

energy (123kJ/mole)

at low

energy at high temperatures

However

associated

the

was controlling

in the oxidation

and the other with high activation

is

(below c 14OOoC). In earlier

of C-products

They found two regimes

had only one regime

oxygen

of oxygen is controlling

at low temperatures

as

that as the

of CO produced

Considering

temperatures

silicon carbide

to occur.

and since the size of the two molecules

that the diffusion

carbide from 1200-1500°C,

is

for the oxidation

are made by the network

to distinguish(z5).

it is concluded

its

the scale might be rate controlling

have argued that the diffusion is rate controlling

under acidic

to understand

It has been proposed(g)

contributions

under Acidic

on silicon and on silica formers

of oxygen through

is increased

of silicon carbide

drops, it is important

for silicon dry oxidation.

temperature

of Silicon Carbide

the oxidation

with a high activation

of silicon-side energy

Appendix

(240kJ/mole controlled several

D-Hot

in 1 atm of oxygen). by oxygen transport

processes

and molecular analysis(g).

contribute

interstitial

temperature

diffusion

to control

Harris reported

reactions,

energy

of the defect

then mechanisms

In the

of the carbon side is energy

and

diffusion

a vacancy

through the scale. for the complete

1200-1350°C

1350-1500°C.

with the same activation

linear behavior

in terms

in the

at low temperatures

At low oxygen pressure

the oxygen transport

must be considered

and the surface

slower

that

in the network

specifically

the oxidation

169

oxidation

diffusion

at high temperatures

experiments

the kinetics.

slower and has a high activation

results

vacancy

Carbide

assuming

of B on the oxygen pressure (gl. The molecular

oxygen is dominating

1200-1500°C(gl.

satisfactorily

dominates

dominates

and Silicon

of the carbon-side

were considered

range of the present

linear dependence

appears

were modelled

except

Nitride

The kinetics

diffusion

diffusion

to that of silicon,

of Silicon

to this transport;

The molecular

while the vacancy

similar

Corrosion

of

mechanism

The silicon side is temperature

range of

at low temperatures. structure

These

of the silica scale

will be proposed

for the oxidation

and then for the hot corrosion.

Proposed

Model

The following around 1000°C.

model is concerned

with the lower temperature

range

It addresses:

il

the different rates of oxidation and C-side silicon carbide

ii)

the enhancement role of Na20)

iii)

the activation

iv)

the magnitude of B, the parabolic for silicon oxidation

of oxidation energies

and hot corrosion

for Si-side

under acidic hot corrosion

(the

for hot corrosion constant,

relative

to that

170

High Temperature

Corrosion

of Ceramics

It is based on:

0

the variation in stoichiometry of vitreous silica with its formation under different ~02 at the reaction interfaces. This variation in stoichiometry and the associated defect structure modify the transport of oxygen through the scale

ii)

the oxidation results of others summarized above

iii)

the oxygen transport model of Tressler and Speartg) applied to lower temperatures

iv)

the surface analysis results of Muehloff et a1.(26)

v)

our acidic hot corrosion results.

Defect Structure and Stoichiometry of Silica Vitreous silica is not often considered non-stoichiometric

in the glass

literature although glassy structures which are great solvents allow greater variation in composition than the corresponding crystals.

It is known that the

silica formed under oxygen deficient environments crystallizes more slowly than similar silica formed under more oxidizing conditions(27).

Silicon rich silica has

been prepared in the electronic industry. Fratello et a1.(28) discussed the influence of OH impurities on the crystallization of silica on the basis of the defects and stoichiometry of vitreous silica.

While the defect structure of

vitreous materials is relatively controversial for strpctural defects and mass transport, the electronic defects have been studied in great detai1(2gy30). In general it is accepted that the SiOg glass structure is built with SiO4 tetrahedra connected through oxygen bridges to form a continuous random network. Thus structural defects may extend to any kind of orderin$31).

The major defects in

silica have been reviewed by Motttao); they are dangling bonds in particular the well known single bonded oxygen, but also 3 bonded silicon. They include also oxygen vacancies in oxygen bridges as well as Si-Si bonds. The single bonded

Appendix

D-Hot

Corrosion

oxygens tend to form in silicate associated expect

with network

glasses

modifying

that oxygen vacancies

conditions.

It is anticipated

thermally.

Reaction

of Silicon

Nitride

and Silicon

to accommodate

It is reasonable

and Si-Si bonds are favored

8 generates

171

extra oxygens

ions in the structure.

also that compensating

Carbide

to

under oxygen deficient

defects

2 single bonded oxygens

can be generated and an oxygen vacancy

2 Si-0-Si = Si- -Si + ZSi-0 nil = Vo + 2SiO

(8)

K8 = [Vo] [SiO12 = Cv [SiO12 in which [V,] = C, is the concentration network.

Under low oxygen pressure

equation

so familiar

for crystalline

of oxygen vacancies oxygen vacancies

Si- -Si in the

could form by the

oxides

00 = Vo + I/2 02 + 2e K = [e12 [Vo] [~02]~/~ = 4Cv3 [~02]l/~ where the vacancies supplied directly assumed

are formed

by the atmosphere

to be supplied

appropriate

glass at the scale-gas

through

in that environment Si-0-Si

[e] is

are dissolved of dissolved

Henry’s law.

inside the scale under pO2 lower than at the interface is that dissolved

concentration

molecules

with the concentration

to the oxygen pressure

and the oxygen is

so that [e] = 2 Cv. It is more

that the oxygen

interface

interface

and the electron

only by the reaction

here to consider

proportional

at the oxide-gas

(concentration

Reaction

into the oxygen 9 occurs

and the molecular

oxygen

Cm) so that

= Vo + l/2 02 dis + 2 Si-

(9)

Kg = 4Cv3 Cm112 in which the electronic

charges

includes the dissolution

of 02 gas into the glass.

equation

9 may have appeared

they exist commonly

in vitreous

are associated

unrealistic

with silicon dangling The formation

bonds and Kg

of electrons

since silica is an insulator.

silica in trapped

form such as dangling

in

However, bonds on

172

High Temperature

Corrosion

silicon shown in equation contains

positive

conduction

of Ceramics

9. It has been shown that thermally

and negative

charge

centers.

grown silica

Silica films support

electronic

during anodization(32).

The vitreous

structure

randomly

distributed

diffusion

of gas molecules

is built up on 5 and 6 member

on 3 dimensions.

These rings provide

and impurity

a little

scale the growth

anisotropy

flux may introduce

of more aligned

than 50A in diameter and reported

channel

Oxidation

over 3A(33).

than the

In the growth

to this structure

of a

by the

Larger channels

in order to explain

silicon oxidation

in the TEM(35).

of silicon

very low oxygen pressures

carbide

which cannot

for the ternary

at the reaction

be calculated

equilibrium

interface

exactly

Sic-SiO2-C.

except

The results

generates in presence of this

calculation

which gives pO2 of the order of 10m30 atm at 1000°C can be

considered

a lower bound for the oxidation

carbon.

less

of Silicon Carbide

The oxidation

of free carbon,

paths for the

ions smaller

in the flux direction(34).

have been postulated

from observations

tortuous

network-modifying

size of the windows in the structure,

formation

rings which are

If active

oxidation

interface

is expected

defects:

vacancies,

of materials

does not occur the silica formed

to be oxygen deficient 3-bonded

of the type of equation

not containing

silicons

8 will suppress

at the reaction

with the formation

and possibly Si pairs. the concentration

excess

of corresponding

Thermal

equilibrium

of single bonded

oxygens CJoxid = mNB/2x

(10)

in which CJoxid is the sum of the oxidant oxidation

and N, B, x are the number

fluxes which contribute

of oxygen molecules

to the

incorporated

in 1 cm3

Appendix

D-Hot

of scale,

the parabolic

constant

m is determined

Corrosion

constant

defects. mechanism

and the scale thickness,

respectively.

are transported

contribution

Assuming

and Silicon

between

are made to the transport

of oxygens

silica and network

Nitride

by the mass balance

Two types of contribution which molecules

of Silicon

through

molecular

channels,

which can occur through

the various network

transport

is by a vacancy

10 becomes (11)

in which Dv, Dm, Cv and Cm are respectively and concentrations,

respectively.

oxidation

of silica formers

transport

paths which are determined

given temperature. result

in different

pressure

tending diffusion.

through

of dissolved

the oxidation

of silicon(g).

and hot corrosion

which oxygen transport

molecular

diffusion

simply DmCm = NB/2. an activation

energy

of the two

of the scale at a of the scale will

8 and 9. Under low oxygen and on the basis

over transport

of a gradual

of silicon carbide

of

(Cv) will be increased

mechanism

Based on equations

change

by molecular

from molecular

at low pressures

for the

8,9 and 11 one can explain in the low temperature

range in

to be rate controlling.

the oxidation

is dominating

equations

to ionic mass transport

is assumed

First let us consider

contribution

on formation

of oxygen vacancies

This in line with the proposal

oxidation

the kinetics

oxygen (Cm) will be depressed

by a vacancy

at high pressures

and molecular

by the stoichiometry

stoichiometries

9 the concentration

mass transport thermal

may depend on the relative

oxygen pressures

to favor transport

the vacancy Therefore

Different

the quantity

of equation

in

in the vitreous

DvCv + DmCm = mNb/2

diffusivities

The

and products(g).

of oxygen,

that the major mode of network

equation

oxidants

173

Carbide

of silicon

the transport@)

The parabolic

Q = 28 kcal/mole

constant

in 1 atm of dry oxygen, and equation

B is proportional

which is similar

11 becomes to pO2 and it has

to that measured

for the

174

High Temperature

permeability

later,

that the silica scale formed

oxygen deficient

then by equation

because

it was formed

9, Cv increased.

of silicon,

DmCm is still greater

dominates

but from equation

by Tressler

dependence

Molecular

network diffusion

controls

activation

energy (55-60 kcal/mole)

with the proposed results

different

conditions

silicon carbide

obtained

which are discussed

As discussed at different

rates

This indicates

by others

of formation

earlier

depressed

So that now DvCv>>DmCm

and the

resulting

in a much lower B, a higher Therefore

picture

for the

for silicon and silicon carbide.

of the scales

on silicon-side

It postulates

and carbon-side

of

below. silicon carbide

the two sides maintain

The two sides maintain

as shown in the present

a1.(261 that the two surfaces

On

Cm is greatly

to derive a consistent

that under those conditions

(sputtering)

and an

was formed

for long periods of time at lower temperature

grinding and polishing

oxidation.

of silicon carbide

the two sides of single crystal

as they oxidize.

bombardment

This was observed

and a low pO2 dependence(gl.

model it is possible

oxidation

character

the oxidation

of oxygen

for silicon oxidation.

by the same reasoning

since Cv * Kg/(Cm) li6.

transport

energy (27kcaVmole)

to that measured

and Cv increased

than for the oxidation

than for silicon.

has activation

the other hand if the scale on the silicon-side

is

under a low pO2 (Cm decreased),

than DvCv.

similar

As discussed

on C-side silicon carbide

it is smaller

11, B is smaller

under very low oxygen pressure,

silica(331.

Although

et al.tgl, this oxidation

oxygen pressure

of Ceramics

oxygen through bulk vitreous

of molecular

if one assumes

slightly

Corrosion

to mix the surface. had different

their separate

experiments

oxidize

below 1300° C. their specific identity

and after

after

ion

It was shown also by Muehloff

stabilities

under low pressure

Under ultra high vacuum the carbon side was observed

carbon above 900 K while the Si-side did not start

et

to graphitize

to cover with until 13000K.

At

Appendix

D-Hot

that temperature

Corrosion

the graphitization

sides of silicon carbide

maintained

of Silicon

of the carbon-side qualitatively

during the early oxidation

for thicknesses

mixing due to sputtering.

This carbonization

explained

the evolution

Nitride

and Silicon

Carbide

was extensive.

their relative

175

The two

rates of oxidation

of silica up to 20A, even after

surface

of the of the carbon side was

of silicon under the very low oxygen pressure

at the site

of the reaction Sic + l/2 02 = CO + Si Under 1 atm of oxygen, sides is maintained however

it is assumed

(12)

that this difference

in stability

and that silicon still tends to volatilize

both carbon and oxygen are oxidized

from the C-side,

in the reaction

SIC + 02 = SiO + CO

(13)

In the pO2 range (s~O-~O atm) which is expected the prominant possible

vapor specie

by this reaction

at the interface silicon carbide oxidized

results

as it oxidizes

interface

As shown in figure

SiO is

12, it is

the order of the Si and carbon atoms

carbon is always at the surface

thus maintaining

the C-side

of the

behavior.

The SiO is

near the interface

but not at

and the silica forms as a rough porous scale and a rough interface

from the active

oxidizes

to always maintain

as it meets with higher oxygen pressure

the interface

at the reacting

in the Si-0 system(36).

so that on the C-side

of the two

oxidation

reaction.

As the oxidation

in the pores so that the scale porosity

under higher pO2 than is found at the interface

is filled.

proceeds

the SiO

The scale is formed

and it will be more

stoichiometric. On the Si-side, oxygen is adsorbed penetration between

the silicons

are at the surface

and it is proposed

that

first on the silicon (figure 13) and the carbon is oxidized

of oxygen through

this oxide layer always maintaining

the oxide and the SIC in line with the Si character

by

silicon

of that side.

This in

176

High Temperature

1)

Figure 12.

Corrosion

of Ceramics

SiC

Model for oxidation of C-side silicon carbide. A rough interface is formed and the C-side is maintained at the interface during oxidation. The SO2 is formed away from the interface.

Appendix

D-Hot

Corrosion

of Silicon

Nitride

and Silicon

Carbide

177

Sic

Sic

3)

Atmosphere

-

I02

I

01 Sib

sic

Figure 13.

Model for oxidation of Si-side silicon carbide. nature of the interface is maintained.

The silicon side

178

High Temperature

Corrosion of Ceramics

line with the results of Harris (linear kinetics of oxidation) as the CO would form under the silica layer and desorption of CO from that position could control the oxidation.

At higher temperatures or in presence of sodium this desorption is no

longer rate controlling

and parabolic kinetics are observed. The scale is formed

under very low oxygen pressure and it is expected to be very oxygen deficient discussed earlier. oxygen deficient

as

The scale formed under these conditions would be more than that formed on the C-side. In the oxidation of SiC the

network transport and the molecular transport add to supply the oxygen. This interplay between the two types of transport processes is underlined as the temperature

of oxidation is increased and their relative contributions

the C-side and the Si-side. As the temperature

is increased, transport by the

vacancy and other network mechanisms with high activation rapidly and becomes a major contributor

change for

energies increases

to the oxidation of the carbon side. The

oxidation of the silicon-side has been increasing also since it was dominated by the network transport processes. As the temperature is increased the mobility increases also in the SiC interfaces and they can no longer be differentiated

by

the mechanisms of fig. 12 and 13 and around 1350oC the two sides oxidize at about the same rate with the same activation

energy as reported by Tressler(9).

Therefore the proposed model is qualitatively

consistent with the data on the

oxidation of silicon and silicon carbide, now we shall apply it to our acidic hot corrosion results.

Acidic Hot Corrosion As discussed earlier acidic hot corrosion is oxidation enhanced by the presence Na2Q at a low activity.

Na2Q enters into the silica structure by

reaction 6 and generates single bonded oxygens thus modifying the defect

Appendix

Corrosion

in the vitreous

concentrations

defects

D-Hot

of Silicon

silica.

and Silicon

of oxygen deficient

defects

as Na20 is introduced

bonded oxygen concentration (eq.

(eq. S).This can be written

into the scale,

it tends to increase

the molecular

the vacancy

as a single equation

showing the Cm is proportional such as those formed

in the glass

which is the algebraic

sum of

to aNa20*.

molecular

(15)

[e12/aNa20

on silicon carbide

and increase

stoichiometric

Therefore

in oxygen deficient

the sodium will decrease

transport

scales

network

as it drives the scales towards

more

compositions.

This result and the previous acidic corrosion the oxidation

oxidation

of C-side silicon carbide.

under 1 atmosphere

controlled

by molecular

paragraph,

in the hot corrosion

corrosion

concentration

6, 14 and 9.

Kl6 = [Na+][02]1/2

molecular

the single

oxygen concentration

Na20 = 2 Na+ + l/2 02 gas + 2e-

transport

through

(14)

(eq. 6), this decreases

14) which inturn increases

equations

179

of the type 2 Si-0-Si = Si- -Si + 2 Si-OK14 = [Vo] [Si-O12 = Cv [Si-012

Therefore

Carbide

Single bonded oxygens are oxygen excess

which lower the concentration

reactions

Nitride

transport

According

to the previous

dry oxygen at low temperatures

diffusion

is enhanced

model will now be applied

as DmCm >> DvCv. Na20 increases giving greater

of C-side silicon carbide

compared

to the discussion,

(1OOOoC) is

As shown in the previous

Cm and decreases

Cv, therefore

parabolic

for the acidic

constant

to that for oxidation

under

*Equation 15 may be considered as the dissolution of sodium oxide in the glass in an interstitial position (network modifier). Interpretation beyond the relationship between aNa and [02dis] could be misleading since defects in the glass structure are required to accommodate the Na+ ions and the electrons. Therefore equations 6, 9 and 14 appear more representative of the reactions occuring in the glass.

180

High Temperature

corresponding

Corrosion

conditions.

increased

by a factor

is similar

to B=3.2xlO-I4

silicon(*).

In our experiments

of about 4 at 1000°C.

This result

stoichiometric

of silicon carbidethan energy

permation

of molecular

catalyst

measured

is expected

mixture

and therefore

relation

with increasing

plotted

apparent

activation

energy greater

diffusion

of oxygen and the oxidation

When the generation energy

of 02 molecules

for diffusion

DmCm and both terms

by reaction

(equation

with aNa20,

7). pSO3 an

for molecular

15 will be dominating

defect

to

It can be seen from reaction have exponential

exponent

in this report.

the operating

the

since B will be proportional

and n is a fractional

15 and in general

over the

of silicon and C-side SIC will be measured.

temperature

reaction

11 and 12

with decreasing

than the 26-28 kcal/mole

will be increased

considered

temperatures

Since B increases

in which both K and aNa

equations

of the

oxygen at one atmosphere

of Na20 increases

15 that Cm o (K aNa20P

from the defect

the

and not constant

After passage

will depend on temperature.

dependences

analysis

in figures

in the atmosphere

pSO3 for increasing

temperature.

by oxidation

to be that of the

of 1% S02-balance

2 the activity

of scales

higher because

was used at all three temperatures. decreasing

being more

it should be 26-28 kcal/mole.

constants

sulfur content

of

silica, the same as for the

Therefore

The parabolic

The initial

From equilibrium

cm2js which

From the previous

is close but significantly

at constant

it generated

activation

of silicon.

and silicon.

nature of the experiment.

total pressure

B was

in dry oxygen oxidation

in the formation

oxygen through vitreous

of silicon carbide

Na20 activity.

expected

for acidic hot corrosion

were determined

of silicon carbide

than those formed

in the oxidation

activation

constant

It was about 3x lo-l4

is in line with the scales

under acidic corrosion

The 34 kcal/mole

the parabolic

cm2js of Deal and Grove for the oxidation

and with lower oxygen pressures

oxidation

of Ceramics

such as 2/9 obtained

However

equations

the enthalpy

of

are not known so that

Appendix

it is doubtful

D-Hot

Corrosion

of Silicon

that an exact temperature

Nitride

dependence

and Silicon

181

Carbide

can be derived.

It is clear

that the activation

energy for hot corrosion

should be significantly

higher than

that for oxidation.

The difference

the measured

activation

energy (34 kcal/mole) representative dominate

and that for molecular

because

the activity

the transport.

of generation magnitude

diffusion

in the present

(27 kcal/mole)

suggest

oxygen molecules

experimental

produced

apparent

may not be

of sodium may be too low to completely

Simple considerations

of the diffusing

Na30 activities

between

that the two mechanisms

were of the same order of

conditions.

by initial gas mixtures

Experiments

with higher

such as 0.1% SOZ-balance

oxygen would answer this question. For the acidic hot corrosion reasoning

is applicable,

dominated

by network

constant

however

the network

contribution reactions

the parabolic

transport

B. As for the C-side,

decreases

of Si-side silicon carbide,

(vacancy

mechanism)

the Na30 introduced

contribution

(raises Cm) according considered

oxidation

Cm increases

was expected

by the hot corrosion

(lowers Cv) and increases to equations

9 and Dm >> Dv by about 4 orders of magnitude.

in defect

concentration

transport

of oxygen although

temperature

the molecular

as observed

experimentally.

B extrapolated activation activity

increases

correlation

between

being considered.

flux became

the results

since Cm = k/Cv6 in Therefore

dramatically

of about 16 compared

et al.tgl.

above.

to the

if and at what

to

The large apparent

as for the carbon-side,

as discussed

the change

contributions

B is increased

by a factor

data of Tressler

is due to the same effect with temperature

to state

dominant.

It is increased

from the oxidation

energy

the relative

it is not possible

the molecular

6, 9, and 14 or 15. In the

rapidly as Cv decreases

dramatically

to be

with a low parabolic

equation

affect

the same general

the Na30

A quantitative

for C-side and Si-side acidic hot corrosion

is

182

High Temperature

Corrosion

of Ceramics

REFERENCES 1.

J.R. Blachere and P.S. Pettit “High Temperature (a) DOE Report ER45117-2, March 1986 (b) DOE Report ER45117-1, June 1985 (c) DOE Report ER10915-4, June 1984

2.

J.R. Blachere and D.F. Klimovich, *J. Am. Ceram. (1987) paper appended in Appendix E.

3.

B.S. Draskovich,

MA

thesis,

University

of Pittsburgh,

1985.

4,

D.F. Klimovich,

M.S. thesis,

University

of Pittsburgh,

in preparation.

5.

M.G. Lawson, M.S. thesis,

6.

M.I. Mayer and F.L. Riley, J. Mat. Sci., 13, (1978) p. 1319-1328.

7.

(a) (b)

8.

B.E. Deal and A.S. Grove, J. Appl. Phys., 36 (1965), p. 3770.

9.

R.E. Tressler

10.

S.I. Raider et. al., J. Electrochem.

11.

S.C. Singhal, Ceramurgia

12.

R.H. Doremus,

13.

R.E. Tressler

14.

L.L. Hench et. al., Ceram.

16.

I. Franz and W. Laugheinrich

17.

G.J. Yurek et. al, Oxid. Metals, 4 265 (1974).

18.

J.A. Costello

and R.E. Tressler,

J. Am. Ceram.

19.

F.P. Fehlner,

Low Temperature

Oxidation,

20.

D.C. Larsen et. al, Ceramic 1985, p. 234.

91

Reference

-*.

University

Corrosion

of Pittsburgh,

of Ceramics”

Sot. 70 [ll] C324-C326

1987, (Appendix

B).

N.S. Jacobson, J. Am. Ceram. Sot., e, [l] 74-82 (1986). N.S. Jacobson and J.L. Smailek, J. Am. Ceram. Sot., a [8] (1985), p_ 432-39.

19

and K.E. Spear, GRI Report

1987.

Sot., 123 [4] (1976), p. 560.

International,

Glass Science,

GRI-87-0088,

2 [3] 123-130 (1976).

Wiley 1973, p. 121.

and K.E. Spear, GRI Report,

GRI-86/0066,

1985.

Eng. and Sci. Proc., 3 [9] (1982), p. 587-595.

Materials

Sot., 64, (1981) p. 327-331 .

Wiley 1986, p. 211.

for Advanced

Heat Engines,

Noyes

Appendix

22.

(a) (b)

D-Hot

Corrosion

of Silicon

Nitride

and Silicon

Carbide

183

R.C.A. Harris and R.L. Call, in Silicon Carbide 1973, R.C. Marshall et. al., eds., University of South Carolina Press, 1974, p. 329. R.C.A. Harris, J. Amer. Ceram. Sot., 68, 1975, p. 7-9.

23.

J.W. Faust Jr., in Silicon Carbide, A High Temperature J.R. O’Connor and J. Smiltens, (Pergamon, 1960).

24.

Ibid, pv 403.

25.

D.M. Mieskowski

26.

(a) (b)

27.

H. Rawson, Inorganic

26..

V.J. Fratello

29.

D.L. Griscom,

30.

N.F. Mott, Adv. Phys., 3,

31.

Reference

19 p. 67

32.

Reference

19 p. 233

33.

Reference

12 p. 133

34.

A.G. Revesz and H.A. Schaeffer,

35.

Reference

36.

U.S. Department of Energy, Thermochemical Stability Diagrams for Condensed Phases and Volatility Diagrams - DOE/FE/13547-01, May 1980.

et. al., J. Am. Ceram.

Semiconductor,

sot., 67, (1984), C17-Cl8

L. Muehloff et.al., J. Appl. Phys., fl [7] (1986), p. 2558-2563. Ibid, fl [8] (1986), p. 2842-2853. Glass Forming

Systems,

Academic

Press (1967), p. 53.

et. al., J. Appl. Phys., 51 [12] (1980), p. 6160-6164. MRS Bulletin,

June 16/ August 15, 1987.

1977, p. 363.

J. Electrochem.

Sot., 129, 1982, p0 357.

19 p. 230

Appendix

Many presentations

E-Publications

have been made on this research.

The most recent

ones

are:

- Hot Corrosion

of Non Oxide Ceramics

- High Temperature

The research

Corrosion

is being written

- Oxide Thickness

of Silicon Nitride

up for publication.

Measurements

in the Electron

of Silica (Appendix

- Hot Corrosion

of Alumina (Appendix

- Hot Corrosion

of Silicon Nitride

- Hot Corrosion

of High Purity Silicon Carbide

of Ceramics

The first paper is appended. work.

The material

be supplied as soon as they become

and Silicon Carbide

These publications

* Hot Corrosion

- Corrosion

need further

[l] [2]

are:

Probe Microanalyzer

[3]

B) [4] C) [5]

and Silicon Carbide

[6]

and Silicon Nitride

[7]

[8]

Appendix in Appendix available.

184

B is ready for submission. C will be submitted

next.

The others Reprints

will

Appendix

E-Publications

185

REFERENCES

1.

J.R. Blachere, D.F. Klimovich and F.S. Pettit, “Hot Corrosion of Non Oxide Ceramics,” paper # , Fail meeting, Basic Science Div. Am. Ceram. Sot., Nov. 5, 1966.

2.

J.R. Blachere, D.F. Klimovich and F.S. Pettit, “High Temperature Corrosion of Silicon Nitride and Silicon Carbide,” Invited paper, Workshop on Corrosion of Ceramics, Penn State, Nov. 12-13, 1967.

3.

J.R. Blachere and D.F. Klimovich, “Oxide Thickness Measurement in the J. Am. Ceram., 70 [ll], C324-C326 (1967). Electron Probe Microanalyzer,”

4.

M.G. Lawson et al., “Hot Corrosion Am. Ceram. Sot.

of Silica,” ready for submission

5.

M.G. Lawson et al., “Hot Corrosion Ceram Sot.

of Alumina,”

6.

B.S. Draskovich in preparation.

7.

D.F. Klimovich et al., “Hot Corrosion Silicon Carbide,” in preparation.

a.

F.S. Pettit

et al., “Hot Corrosion

and J.R. Blachere,

in preparation

of Silicon Nitride

to J.

for J. Am.

and Silicon Carbide,”

of High Purity of Silicpn Nitride

“Corrosion

of Ceramics,”

in preparation.

and

186

High Temperature

Corrosion

of Ceramics

,” wh,ch the/(X)

Oxide Thickness Measurement in the Electron Probe Microanalyzer J. K. BLACHERE*

An X-ray

method/or

Ihe micromobr

for silica

tilms

on silicon

of wpporred nrrride

rhmjilms

and silicon

in

carbide.

inctvhrrirs o/rhe o.&rn Ka linrbre measured on bulk SIO- und on rhe/i/m. The derivation of the colibrarion curve giving the rhicknru of the film from rhe rario of rhesr inrensiries is ourlined. The method has been used /or srl~ca Jilnts rhwwr rhrr~~ I um wirh a lawral resolurion of a few micromrrurs.

0.

thxkness

mcasuremen,s

arc

nctded for mnny applications such as Ihe rczareh on rhc oxidation and hot corroSL”” of uhcon “node and sdxon carbblde. Many methods hzve been used LOassess the ev”luo”n of Ihe oxidaoon process; bowever. they arc often mdlrect or desuuctive. Fur ~“rtimcc. wghc changes arc the nc, rewll of simuluncous rcacu”“~ whxh g,ve “lfvxing comnbuuona. More dwcct mcah”rrmcnts arc oficn dependent on etchmg or frxurc of the specimens Other methud> dppbcrble to rhm films ax very dceur.w bu! do nor have spatial re)oIulion. The ctecrron pr”tw microanalyzer IEPMA) otfcrr a nundcslruct~vc means of mcaurimg ttlc Thickness of suppancd films wnh a high lalcral resoluuon which we have uxd for rihca films a” silicon nilnde and silicon carb,dc.

The menrny of characteristz X-rays, yeneraed by a” ctec~ron barn for an element conlrmcd m a thin titm and not in the bubruale. 15 related 1” the thickness of the thm film and c”mp”w~“n of the sample.

The m~enuty-thickness

rclaoonsh~p is nor

slmplc smcc the Xaya generdlcd are a function of depth I” the sample. and they are abrorbed rccordmg Lo the prrh of their cx~l from rhc samolc The mrcroan~lyrir of dun tilmr has bee” revxwcd by Guldslem.’ In order 1” calculate rhe inw&ry of the X-rays genernled in [hc aample. Yakowtz and Newbury’ “rooaed a” emomeal aooruach bdscd on ii&g Ihe X-ray’depfh ~roducrion ewe +(pzt to rhe combmmoo of a parabola and an ripmrnt,~l. The mdss rhicknrss p: IS ,hc pr”duct “f ,he d,,ww from Ihc rurfacc

oi

: by the drnrlry p “f Ihc sample m Ihrt lhlchne,,. Cornoared t” Ihnr for a bulk a!“&ard of Ihe co&wlion of Ihe tilm. the @(pi) ~“rvc for the lhm tilm IS tnmcrred at [he mas, tblckncs, pz, of dw lilm and it mctudo P mo,hf~ed b;rckscattering y!eld of the cIcc~r”“s wh,ch accoum, fur the sub,,ra,c c”nlr~bu!wn The mwn~~ty of the charnclerwc X-rays emmcd frum the film II compared to thrl en,,rled by a bulk ,tand.ud of Ihe film ma!eriA The in!may r.11,” I, of these I~ncr. which can be mr~wed. IS pr”pon~onrt 1” tbr rat” of Ihc mlcn&llcs 1” the lilm l/L) standard /,:

,cr,s,,c

X-ray,

along

,, 15 dcfmcd

[he”

the cor-

of Ihe chrracprthr

ar I,,/,,)

1” ,hc

csc + I”

I, the mar, abwrpr,“”

cocffi-

event for the hnc measured I; the titm. p is the denrny “f the tilm. and $ IS the

Thr

X,DE

rbaorpo””

wh,ch s/p

“f P,,,,burgh. P,,t,burph.

of the thickness

rhe meosuremenfs

is modified

for dx

delccwr.

AND D. E KL~MOV~CH*

cepamle”~ “f Mawnrtr Sacncr and E.ng,nrennp. V”lw,#ty Pcnniytunlr ,526,

funcoons prwdc

reel,““,

of lhc X-raya gencrafrd 1” [hat generated I” a bulk

L3ke”ff angle. The ex~en~~vc. but slmplc. ealcuta,mns are dr,cr,bcd ,n dclrd ,n Ref, ? and 3. They rcpon ~“4 rcwl,, tar ,mgteelemcnl tilmr For muloclemcnl lilms. It 15 powble I” obtam buth Ihc tilm rhrcknos nod Ihe c”mpo,mon by mcrrur,ng r( ~1,“s for all co,noonems of Ihe film. In txxh CBYI, all Acrclc”lc”lrl effccr, (atomic number. rbrorpo”“. nnd lluoreicencct arc a\rumrd nept~glblc il, mlfhr be cxpccwd I” a first ~ppruxmu,~“” fur lhm f,lm,

The prcw”“, method was adrpled Lo meas~remcnta of the ,hxknos of s,hca films on sd,c”n carb,de md s,hc”” mmde wng lhc oxygen Ko hnc t,, appllulu”” LO light elcmcms tatamlc oumbcr Z< req&cd scvcr.d mod,f,cau”n, ,,ncc the low-energy X-ray, p&wed ,,re wungly absortxd and Ihe ab,“rprw” c”rrec,,“n 13 w_mlarge I” use Ihe approruna~c/(~) ral“es. The mwn,mc,,~ crlsut,md for Eq d,ffercm trum Ihe mc.,rured m,c”,meS 1. becau,e of ~brvp,,“” .l’he X-rry, SC grncrad A! var,““, dcplh, I” ,hr san~ptc and I” calcul~!c I,. lhe,r m,cn,,,y must be mtcgraad over dw depth of prwtucuon:

tI)

CI)

xe

For

un4

ahsorou””

[he ~“lal

mtcn,W

Appendix

r

November\

E-Publications

187

1987

h,k

k

#(PZ)

00

IO

I‘XOO

1000

loo Thickness

,nm,

Parabolic

kh

h

and iir IS calculr,sd from mca,ured ,“,cn,,,,eh as

the prcdlcled

and 0.1425

were calcul.Acd

rer~ctwely

for

bulk SIO,. S,,N,. md SIC The>e value) were “blamed for a pnmrry clec~ron barn ener~v elem;;l,al

of

IO keV

bv m~erwlrtwn

drta of He;“r,ch’;nd

of the

rddmon

of

mars-weighted elcmcmal c”clf~c,en,~. Therefore, the fdm and Ihe wb\,rrtcs .~e so simdar dxa, .I c”“a,a”, brckscartermp cc& fic,en,. Iha, of the ondc. ua, urcd for dll

in which

film thxknesse$ rouhmg I” an enor es,,mr,ed I” br
(p:-h++

X[-@z-t&f

in which If 15 Ihe X-WV m,e”s,,v measured from Ihe f~irn md /Z ;s rhu lot& mcawed fur the bulk aodard. The mwgration for /: ,s over ,hr mas lh,ckness of [he film

The inlegrrl for Ihe parabolic and exponenoal r.eg,on 8” a lhxkness p ,uch as

pi,>p:>l.Sh

and Ihrl for Ihe bulk mtenwy is over Ihe uhole depth of X-ray producd”” p:, g,ven by Ihe X-ray praluc!,“” range of Hemnch.’ Equroon (4) ha, ken wed rucce,rfully by KelM,. Ttw mwosmc, of Eq (4) were integrated I” clozd form u~,“e Ihe “arabolic-exThe arrumrd 4(p:c) cwve 1s sketched I” Fig. I. For [he prrabohc ,eg,on

ponenwdexpresrm-for &p;)

lip.-J=h~‘(~:-h)‘(a-k)+k and for the rrponenwl

”

1s

x

-(pz,?+K(p-_.-I

exp(w

5hl

Q=exP[P&&-X

)

(6)

j]

(for pi = I.Sh)

1

C&Bk. and h are a func-

don of [he backscattering clewon yield. Using values for the bulk matenal, the I”te”,,,y for lhr bulk standard I, crlculaed: ,“.=I,

glass on s,bcon

nitride

,ubw.oes

and slightly over 1% for sd~con crrbrde wbsorlo. For other subsmale-tilm comb,“a,,““,. when ,h,r a,aump,,“” 1s no, \rhd. a film b;rcksca,ten”p cwflicwn, “wt be calculawd as a funcoon “1 p:, d.,. I. aod h are different for lilm .md wb,trr,e.” Cabbra”“” curve, of 1, II film ihlckncss were calculr~ed u,,h a compu,er u,mg Eq. (4) and Ihe mclhod oullmcd above. S,l,ca glr,s u!,h .I den,,,y of 2 2 g/cm’ was ured for the lilm I” ,dI ma rage. ma,, Ih,ckne,,. and ab,“rp,,on COICUIIu”“,. The cahbrat,“” cunc 1, ,houn m Fig. 2 for sdx”” carbldc md ,dxo” mmde ,ubwa,es.

were

The inteosmes of Ihe “x)pc” measured 1” a ,ca”“,“~

Ku line elcc~“”

m,croscope* fmcd wlh IWO ud\slmglh disperwe spec,r”mctcn equ~ptwd for hgh, eleme”, .mrly~r. All wmple, uere coaled w,lb &a~, 20 “m of carbon. Duphcnle mmsurcment~ were made ,,muluncously wth ,hr IWO spec,r”me,ers ill a” .,ccelcrdlion voltage of IO kV and a beam curre”, of 20 rA. The ra,a”s kr tie calculaled from

ifor pr=pz,)

However, for thm films Ihe elrcrron backxa,,er yield mcludes a conrnbution from the subwale. Approx~ma,~“na of Ihe da, of Cusslet were u,rd prev~ou,ly”’ wed m lh,s case Some var,~,,““s

(7)

(8)

1.Shx I;=I,

in which Ihe parameters 6, k. and h are cab culzued followmg Keuwr’ and Goldstem.’ The mlegral for Ihe parabolic reg,““. for a dwknes, ,x
Q

wnh

The panme,er,

x

IShI

(5) rrg,o”

j]

dnd on Ihe

electron ,rrnrm,ra~“n cwffiwn,’ of the ,ubauele were also med. The aweme val“es of Ihe backsca,ter cwfticlen, of the film ,h”uld be thrl of the ,ub,rrate for a film mfmwly lhm md lbsl 01 Ihe bulk film mr,er,;ll for a IhIck film.’ I” tb,, case. bilckscaltrr cocffic~ent?, of 0.1316. 0 I37 I,

Ihe measured intms,l~es corrected for background. The oxide Ihicknes,es CA” be read from Fig. 2: ntedd. a am~ple m,erp&o”n program and E.q. (4) were used LOgel them more exacdy. The meawreme”lS are very reproducible. and a standard error of 1% or less IP c;rlcula!ed’ for Le values corre,pondme 10 th,cknesxs >IW nm for a se, 01 lob-s co,m,~ on pea, md background. for film and srandsrd. I, i, ~mprowd by lhr comblnarlon of several of these measuremen,,. They were uwlly repcrIed at ,hr ,ame we and it, sevrral Icat~ons of

188

High Temperature

Corrosion

of Ceramics

Communicdonr Table I.

Oxide Thickness

of rhe American

Ceramic

better

Typr

2. httle addmonal

SEM

energy beau%

0.21

deep in tbe sample by hght elements such

SIC.SC

0.32

: 4

SIC, SC SIC. Sic. SC

0.31 0.20 0.17

a, “xygro

the sofr X-ray,

Tb,s rc~ults m Ihe shallow

0.30 0.13 0.11

CVD

.sc=nng!x clyrul. WDS..X.R) yzuuu.

similar

momholoev

a” Ihe

sample. It I; esu&red

0.10 0.05

,lopc

Imermedmte as 20 kV

“lr-

Of the

surface

that tix rcprcduc~-

by as much as 30 to 50%.

However.

repmducibday

method 1s ex-

of the X-ray

tbe

procedure was of Ihe order of II.

cellent. and Ihe meahurements are srraightforward. Reuter’ clawned an accuracy of

REIWLTS AND Discuss&

Al. Cu.

+10%

for the thicknesses of lhin films of N,. and Au on various substrates

measured by a slmdill. method

The Ihicknenes of glassy regions of “xzde layers formed by oxidation and hot

of the X-ray

corrouon on GIlcon carbide and silicon ni-

creases the uncenrmry

tnde

quanuties,

were measured

of

ducknose,.

electron beam energies such

did

not

provide

signlftcanl

impmvemcn~~

bdtty of Ihe thicknesses measured by this

al IooO”C

of the

curve for duckcr tilms and Ihe prormuty

0.09 0.12

mlcmuulvrir. ud sEM=lnugw

produced

are almost completely absorbed

rhe two curves for low film SI,N, SI,N..

lhrckness

can bc measured w,th dur higher electron

WDS

I

2

1987

SUN+ Oxan Eq (I) for tits La\k. As

shown in Rg.

Ilwkncncu (PI, SImplc

November

Societv

Comparison*

bv lhls

mothal and by unagrng of cross sec&

in

tbe SEM. The rcsulls are gwe” I” Table 1. The mraruremen~s by SEM Imaging proved difficult for films under I gm duck. The magnX~ar~“n was cahbraled wilh staodud latex spheres. The samples were fractured and brhdv elched I” HBF. to reveal the oxide idye;. They had been carbon coated pnor 1” fracrure to observe tie morphology of the wface. and I, was found that dos coating helped mantain Ihe surIace edge of the oxide lilm during etching. The samples were recoated to avold chargmg I” Ihe electron beam and the coating dwk”ea,es were large compared lo some of the “ride layers of Table 1. Under tbr c”“d#i”ns of Ihe measurements. il is expxred lhar Ihe resolution of the SEM did not reach 10 em. The experimenrai difficulties in the SEM imagmg of these thin Inyen resulled in large experimenlal scaLLer and poor accuracy. II is estimated that the uncexiaintv of tbe thicknesses measured dlrecrly I” Ihe SEM could be as high as 40 to 50% for the dunner oxide films. As shown I” Table I, rhe LW” methods gwc tie same order of magmwde but their results differ

Application

method I” hgh! elements I”smce Ihe physical

such as roass absorpoon coeffi-

cientb. used in Ihe caIcuIa11ons are not as well estabhshed. Also. X-ray absorplnn play, a mqor role for Lhe longer wave,eng,hs. Con~parison wnh Ihe SEM mrasure,,,en,s ruaeerts that the accuracy ,sbe,rer dun 30-g. The shape of the callbrat,“” curve of F,g. 2 s”gge,t, Ihat thick-

nessts inIhc rang~l0nmlo0.8 to I pm

can

be measured: these values correspond ap “roxlmatelv to the linear part of tbe curve. lower se1 by counring statisLies. For the thicker lilms, the sens~liwly decreass as the curve of Fig. 2 levels off. It was clamxd’ lhrl the ongmal method \yas reasonably accumte up 1” 306 of dx bulk inter~cf~on volume, correspondmg in this car to <0.5 ,.un. However. the more extensive absorprion correction performed

ihe

iimir is

here should allow

dir

the range of the “lea-

surements to be extended Increasing the pnmuy electron energy should also increase Ihe film thicknesses which can be measured by generaling Xrays deeper in!” the sample. This WPI shown’ for metal films with Eq. (I). A calibruion cuwe for 30.kV prunary elecmoos was generated wrh Eq. (4). which 16

The X-ray melhod ,u,r dexnbed has bee” used consi~~rntly I” our rebesrch’ for the measoremeot of StOl films wrh thicknesses from producibdtry

IO “m I” 1.0 pm

with a re-

of a few percent.

Thicker

films were imaged m cros) secuon m the SEM.

a procedure whtch is earbe, and more

accurare than dzscussed earher for the thanncr films. do,,uc,,ve

The X-ray method I, nonand ha a lswal rraoluuon oi a

few mrcrolnel~r,;

II ;Ill”WS a correlauon

of

tb~kneas rorasureznen~ with surface fca[ores such a, spheruhlrs or glassy regtom. Other techniques such as chemical ;malysir. opr~cal mwferometry. ad elhp5”meuy. whrle roorc accur.~~c. do no! offer lhls lateral re,o1u~~“” whtch 1s highly dewable in roatmal5 sc~cnce.