The Electrochemistry and Characteristics of Embeddable Reference Electrodes for Concrete Structural Engineering: Efc 43 (EFC)

The electrochemistry and characteristics of embeddable reference electrodes for concrete

European Federation of Corrosion Publications NUMBER 43

The electrochemistry and characteristics of embeddable reference electrodes for concrete Roar Myrdal

Published for the European Federation of Corrosion by Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals & Mining

CRC Press Boca Raton Boston New York Washington, DC

Cambridge England

Woodhead Publishing Limited and Maney Publishing Limited on behalf of The Institute of Materials, Minerals & Mining Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2007 by Woodhead Publishing Limited and CRC Press LLC © 2007, Institute of Materials, Minerals and Mining The author has asserted his moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publishers cannot assume responsibility for the validity of all materials. Neither the author nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN-13: 978-1-84569-234-6 (book) Woodhead Publishing ISBN-10: 1-84569-234-9 (book) Woodhead Publishing ISBN-13: 978-1-84569-255-1 (e-book) Woodhead Publishing ISBN-10: 1-84569-255-1 (e-book) CRC Press ISBN-13: 978-1-4200-5118-6 CRC Press ISBN-10: 1-4200-5118-0 CRC Press order number: WP5118 ISSN 1354-5116 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International Ltd, Padstow, Cornwall, England

Contents

Series introduction

vii

Volumes in the EFC series

ix

1

Introduction

1

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Reference electrodes in general Basic concepts Types of reference electrodes Potential scales and potential conversion Reference electrode design Potential measurement Ideal reference electrode properties Phenomena disturbing reference electrode potentials 2.7.1 Liquid junction potentials 2.7.2 Temperature 2.7.3 Chemical environment pH and oxygen sensitive electrodes Time stability requirements

3 3 3 5 6 7 7 8 8 9 10 10 11

Reference electrodes for concrete Types 3.1.1 Silver/silver chloride (Ag/AgCl/KCl) 3.1.2 Manganese dioxide (MnO2) 3.1.3 Copper/copper sulphate (Cu/CuSO4) and zinc/zinc sulphate (Zn/ZnSO4) 3.1.4 Lead (Pb) and zinc (Zn) 3.1.5 Graphite (C) 3.1.6 Mixed metal oxide (MMO) 3.1.7 Metal–metal oxide

13 13 13 14

2.8 2.9 3 3.1

15 15 16 18 18 v

vi

Contents

3.2

3.3

Installation of reference electrodes in concrete structures 3.2.1 Quality control of embeddable reference electrodes 3.2.2 Location of reference electrodes 3.2.3 Installation procedures Field experiences

19 19 20 20 22

4

References

25

European Federation of Corrosion Publications: Series introduction

The EFC, incorporated in Belgium, was founded in 1955 with the purpose of promoting European co-operation in the fields of research into corrosion and corrosion prevention. Membership of the EFC is based upon participation by corrosion societies and committees in technical Working Parties. Member societies appoint delegates to Working Parties, whose membership is expanded by personal corresponding membership. The activities of the Working Parties cover corrosion topics associated with inhibition, education, reinforcement in concrete, microbial effects, hot gases and combustion productions, environment sensitive fracture, marine environments, refineries, surface science, physico-chemical methods of measurement, the nuclear industry, the automotive industry, computer based information systems, coatings, tribo-corrosion and the oil and gas industry. Working Parties and Task Forces on other topics are established as required. The Working Parties function in various ways, e.g. by preparing reports, organizing symposia, conducting intensive courses and producing instructional material, including films. The activities of the Working Parties are coordinated, through a Science and Technology Advisory Committee, by the Scientific Secretary. The administration of the EFC is handled by three Secretariats: DECHEMA e.V. in Germany, the Société de Chimie Industrielle in France, and The Institute of Materials, Minerals and Mining in the United Kingdom. These three Secretariats meet at the Board of Administrators of the EFC. There is an annual General Assembly at which delegates from all member societies meet to determine and approve EFC policy. News of EFC activities, forthcoming conferences, courses, etc. is published in a range of accredited corrosion and certain other journals throughout Europe. More detailed descriptions of activities are given in a Newsletter prepared by the Scientific Secretary. The output of the EFC takes various forms. Papers on particular topics, for example, reviews or results of experimental work, may be published in vii

viii

Series introduction

scientific and technical journals in one or more countries in Europe. Conference proceedings are often published by the organisation responsible for the conference. In 1987 the, then, Institute of Metals was appointed as the official EFC publisher. Although the arrangement is non-exclusive and other routes for publication are still available, it is expected that the Working Parties of the EFC will use The Institute of Materials, Minerals and Mining for publication of reports, proceedings, etc. wherever possible. The name of The Institute of Metals was changed to The Institute of Materials on 1 January 1992 and to The Institute of Materials, Minerals and Mining with effect from 26 June 2002. The series is now published by Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals and Mining. P. McIntyre EFC Series Editor The Institute of Materials, Minerals and Mining London, UK EFC Secretariats are located at: Dr B. A. Rickinson European Federation of Corrosion, The Institute of Materials, Minerals and Mining, 1 Carlton House Terrace, London SW1Y 5DB, UK Dr J. P. Berge Fédération Européenne de la Corrosion, Société de Chimie Industrielle, 28 rue Saint-Dominique, F-75007 Paris, FRANCE Professor Dr G. Kreysa Europäische Föderation Korrosion, DECHEMA e.V., Theodor-HeussAllee 25, D-60486 Frankfurt, GERMANY

Volumes in the EFC series

1

Corrosion in the nuclear industry Prepared by the Working Party on Nuclear Corrosion

2

Practical corrosion principles Prepared by the Working Party on Corrosion Education (Out of print)

3

General guidelines for corrosion testing of materials for marine applications Prepared by the Working Party on Marine Corrosion

4

Guidelines on electrochemical corrosion measurements Prepared by the Working Party on Physico-Chemical Methods of Corrosion Testing

5

Illustrated case histories of marine corrosion Prepared by the Working Party on Marine Corrosion

6

Corrosion education manual Prepared by the Working Party on Corrosion Education

7

Corrosion problems related to nuclear waste disposal Prepared by the Working Party on Nuclear Corrosion

8

Microbial corrosion Prepared by the Working Party on Microbial Corrosion

9

Microbiological degradation of materials – and methods of protection Prepared by the Working Party on Microbial Corrosion

10

Marine corrosion of stainless steels: chlorination and microbial effects Prepared by the Working Party on Marine Corrosion ix

x

Volumes in the EFC series

11

Corrosion inhibitors Prepared by the Working Party on Inhibitors (Out of print)

12

Modifications of passive films Prepared by the Working Party on Surface Science and Mechanisms of Corrosion and Protection

13

Predicting CO2 corrosion in the oil and gas industry Prepared by the Working Party on Corrosion in Oil and Gas Production (Out of print)

14

Guidelines for methods of testing and research in high temperature corrosion Prepared by the Working Party on Corrosion by Hot Gases and Combustion Products

15

Microbial corrosion (Proc. 3rd int. EFC workshop) Prepared by the Working Party on Microbial Corrosion

16

Guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production Prepared by the Working Party on Corrosion in Oil and Gas Production

17

Corrosion resistant alloys for oil and gas production: guidance on general requirements and test methods for H2S service Prepared by the Working Party on Corrosion in Oil and Gas Production

18

Stainless steel in concrete: state of the art report Prepared by the Working Party on Corrosion of Reinforcement in Concrete

19

Sea water corrosion of stainless steels – mechanisms and experiences Prepared by the Working Parties on Marine Corrosion and Microbial Corrosion

20

Organic and inorganic coatings for corrosion prevention – research and experiences Papers from EUROCORR ’96

21

Corrosion–deformation interactions CDI ’96 in conjunction with EUROCORR ’96

22

Aspects of microbially induced corrosion Papers from EUROCORR ’96 and the EFC Working Party on Microbial Corrosion

Volumes in the EFC series

xi

23

CO2 corrosion control in oil and gas production – design considerations Prepared by the Working Party on Corrosion in Oil and Gas Production

24

Electrochemical rehabilitation methods for reinforced concrete structures – a state of the art report Prepared by the Working Party on Corrosion of Reinforcement in Concrete

25

Corrosion of reinforcement in concrete – monitoring, prevention and rehabilitation Papers from EUROCORR ’97

26

Advances in corrosion control and materials in oil and gas production Papers from EUROCORR ’97 and EUROCORR ’98

27

Cyclic oxidation of high temperature materials Proceedings of an EFC Workshop, Frankfurt/Main, 1999

28

Electrochemical approach to selected corrosion and corrosion control studies Papers from 50th ISE Meeting, Pavia, 1999

29

Microbial corrosion (Proceedings of the 4th international EFC workshop) Prepared by the Working Party on Microbial Corrosion

30

Survey of literature on crevice corrosion (1979–1998): mechanisms, test methods and results, practical experience, protective measures and monitoring Prepared by F. P. IJsseling and the Working Party on Marine Corrosion

31

Corrosion of reinforcement in concrete: corrosion mechanisms and corrosion protection Papers from EUROCORR ’99 and the Working Party on Corrosion of Reinforcement in Concrete

32

Guidelines for the compilation of corrosion cost data and for the calculation of the life cycle cost of corrosion – a working party report Prepared by the Working Party on Corrosion in Oil and Gas Production

33

Marine corrosion of stainless steels: testing, selection, experience, protection and monitoring Edited by D. Féron

xii

Volumes in the EFC series

34

Lifetime modelling of high temperature corrosion processes Proceedings of an EFC Workshop 2001. Edited by M. Schütze, W. J. Quadakkers and J. R. Nicholls

35

Corrosion inhibitors for steel in concrete Prepared by B. Elsener with support from a Task Group of Working Party 11 on Corrosion of Reinforcement in Concrete

36

Prediction of long term corrosion behaviour in nuclear waste systems Edited by D. Féron of Working Party 4 on Nuclear Corrosion

37

Test methods for assessing the susceptibility of prestressing steels to hydrogen induced stress corrosion cracking by B. Isecke of EFC WP11 on Corrosion of Reinforcement in Concrete

38

Corrosion of reinforcement in concrete: mechanisms, monitoring, inhibitors and rehabilitation techniques Edited by M. Raupach, B. Elsener, R. Polder and J. Mietz on behalf of Working Party 11 on Corrosion of Steel in Concrete

39

The use of corrosion inhibitors in oil and gas production Edited by J. W. Palmer, W. Hedges and J. L. Dawson

40

Control of corrosion in cooling waters Edited by J. D. Harston and F. Ropital

41

Corrosion by carbon and nitrogen: metal dusting, carburisation and nitridation M. Schutze and H. Grabke

42

Corrosion in refineries J. Harston

43

The electrochemistry and characteristics of embeddable reference electrodes for concrete Prepared by R. Myrdal on behalf of Working Party 11 on Corrosion of Steel in Concrete

44

The use of electrochemical scanning tunnelling microscopy (EC–STM) in corrosion analysis: reference material and procedural guidelines Prepared by R. Lindström, V. Maurice, L. H. Klein and P. Marcus on behalf of Working Party 6 on Surface Science

1 Introduction

Monitoring of the electrochemical potential of steel reinforcement in concrete is a well established technique for assessing the severity of corrosion and for controlling cathodic protection systems. A reference electrode is the electrochemical device used for measuring these potentials. The reference electrode is either placed on the concrete surface during the measurements or permanently embedded in the concrete in close proximity to the steel. The latter technique enables remote long-term monitoring. The objective of this report is to give a state-of-the-art overview of the electrochemical and physical characteristics and performance of embeddable reference electrodes for concrete and the method used for installing these electrodes in steel-reinforced concrete structures. First, the concepts of electrochemical potential and reference electrodes in general are treated. This is considered important in understanding the performance of reference electrodes and minimising misunderstanding and misinterpretation of potential measurements. Due to an unfortunate lack of published reports on field experiences with reference electrodes in concrete structures, the field experiences referred to in this document are rather scarce and may not necessarily reflect the true essence of reference electrode behaviour in terms of in-situ life time, long-term potential drift and failure rates. Surface potential measurements, like the ASTM C 876 potential mapping technique [1], are not covered. Some of the technical information given in this report has also been treated in a recent National Association of Corrosion Engineers (NACE) Technical Committee Report [2] on reference electrodes for concrete.

1

2 Reference electrodes in general

2.1

Basic concepts

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is ‘volt’ (1 V = 1 JC−1 s−1 in SI units). The metal, or strictly speaking the metal–electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is 0 V, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE.

2.2

Types of reference electrodes

Although it is a highly reproducible, very accurate and stable reference electrode, the SHE is impractical for many applications. Therefore, other reference electrodes have been developed. The most common ones are classified into three main categories [3]: •

Electrodes of the first kind Typically a metal electrode at which thermodynamic equilibrium is established between the metal atoms and the corresponding cations in solution. One example is the copper/copper sulphate reference electrode (Cu/CuSO4). 3

4 •

•

Electrochemistry and characteristics of embeddable electrodes Electrodes of the second kind Typically an electrode consisting of a metal covered by a sparingly soluble salt containing the metal ion and exposed to a solution containing a soluble salt of the same anion as the sparingly soluble salt cover. Two interfaces are set up and equilibrium is established between the metal and its cation in the sparingly soluble salt and between the anion of this salt and the anion in solution (examples: SCE and Ag/AgCl, see below). Oxidation-reduction electrodes An inert metal (usually Pt) immersed in a solution of two soluble oxidation forms of a substance, e.g. Fe3+/Fe2+. The metal merely acts as a medium for the transfer of electrons between the two forms. This type of electrode differs from electrodes of the first kind only in that both oxidation states are present in the solution while, in electrodes of the first kind, one of the oxidation states is the electrode material.

Generally, electrodes of the second kind are more accurate. The most frequently applied reference electrodes fall into this category. Typical examples are the ‘saturated calomel electrode’ (SCE) and the ‘silver/silver chloride electrode’ (Ag/AgCl/KCl) (see below).

Electrode

Metal

Sparingly soluble salt

Solution

SCE Ag/AgCl/KCl

Hg (Mercury) Ag (Silver)

Hg2Cl2 AgCl

KCl (saturated) KCl (various)

As the name suggests the SCE contains a saturated solution of KCl. Therefore, all SCEs have the same electrochemical potential value at the same temperature. Ag/AgCl/KCl electrodes, on the other hand, exist with a variety of KCl concentrations, ranging from 0.1 M solution to saturated solution. Consequently, Ag/AgCl/KCl electrode potentials vary according to type, as shown in Table 2.1. Due to the variety of Ag/AgCl/KCl electrodes, care must be taken when dealing with potentials obtained with this kind of reference electrode. Many ‘reference electrodes’ do not fall into any of the categories described above. For many field applications simple electrode materials are currently used as references. A typical example is simply a piece of metal immersed in the test solution. Reference electrodes of this kind are treated generally in Chapter 2.8. The specially designed versions for concrete applications are described in Chapters 3.1.4 (lead), 3.1.5 (graphite) and 3.1.6 (MMO).

Reference electrodes in general

5

Table 2.1 Reference electrode potentials vs standard hydrogen electrode (SHE) at 25 °C Reference electrode

Reference electrode solution Type

Potential mV vs SHE

Reference

Concentration

Cu/CuSO4

CuSO4

1.27 M (saturated)

+318

4

SCE

KCl

4.16 M (saturated)

+244

5

Ag/AgCl/KCl

KCl

4.16 M (saturated) 4.0 M 1.0 M 0.6 M (seawater) 0.1 M

+199 +200 +235 +250 +288

6 6 7 7 7

Potential vs SHE/mV + 318

Cu/CuSO4

+ 244

SCE

+ 199

Ag/AgCl/KCl (sat)

0

− 40 + 34

Test electrode

SHE

Fig. 2.1 The electrochemical potential scale and electrode potential conversion at 25 °C.

2.3

Potential scales and potential conversion

As each type of reference electrode has its own potential with respect to the SHE, the measured potential values must frequently be converted. The conversion from a value measured with one type of reference to the value for another reference is a simple operation.This is illustrated in the example below and in Fig. 2.1. Example (see Fig. 2.1) The potential of steel in a test solution at 25 °C was measured with a saturated calomel reference electrode (SCE) and found to be +34 mV.

6

Electrochemistry and characteristics of embeddable electrodes What would the steel potential have been if measured by a Cu/CuSO4 reference electrode? Answer: E (test vs Cu/CuSO4) + E (Cu/CuSO4 vs SHE) = E (test vs SCE) + E (SCE vs SHE) calculated

constant

measured

constant

+ 318 mV

+34 mV

+244 mV

Therefore E (test vs Cu/CuSO4) = (+34 + 244 − 318) mV calculated = −40 mV

2.4

Reference electrode design

Reference electrodes are normally constructed in such a way that the reference electrode electrolyte, often called the inner electrolyte (e.g. the KCl solution), is separated from the test solution by a porous plug or a membrane. This is called a double junction electrode design (see Fig. 2.2). Besides being a physical barrier hindering the electrolyte from leaking into the test solution, the plug functions as an ionic conductor. Although extremely low, some ionic current has to pass through the plug to obtain a measurement. The plug must be open enough to let this tiny current pass through, but sufficiently tight to minimise leakage and contamination of the inner electrolyte. The term ‘double junction’ reflects the existence of two

Cable Electrode housing

Metal, e.g. Ag Sparingly soluble salt, e.g. AgCl

Inner electrolyte, e.g. KCl solution Porous plug

Fig. 2.2

Double junction reference electrode design.

Reference electrodes in general +

Voltmeter

7

−

Test solution

Test electrode, metal structure

Fig. 2.3

Reference electrode

The principle of potential measurement.

junctions: (1) the interface between the metal and the inner electrolyte and (2) the interface (junction) between the inner electrolyte and the outer test solution.

2.5

Potential measurement

To carry out electrochemical potential measurements, a voltmeter is needed to which two cables are connected, one from the test electrode (a piece of metal) of unknown potential and one from the reference electrode. Care must be taken as to where to connect the cables. In accordance to the Stockholm sign convention [7] the reference electrode shall be connected to the ground terminal (minus) of the voltmeter, often called ‘COM’ on multimeters, and the test electrode to the positive terminal, often called ‘V,Ω’ on multimeters. The voltmeter must have a high internal impedance, 10 MΩ or higher, so that the current used to measure the potential is very small and does not affect the potential of the reference electrode and/or the test electrode. A schematic set-up of the measurement is shown in Fig. 2.3. The voltage reading is normally given in millivolts (mV). To avoid confusion and misinterpretation both sign and type of reference must be stated.

2.6

Ideal reference electrode properties

To sum up the above, an ideal reference electrode should have the following properties [6]: 1. 2. 3.

have a stable potential with respect to time; obey the Nernst equation for some species in the inner solution, i.e. the potential is defined by a thermodynamically reversible reaction; meet the demands of charge transfer imposed by the measuring instrument without changing its potential (be non-polarisable);

8 4. 5.

Electrochemistry and characteristics of embeddable electrodes return to its fixed reference potential after accidental polarisation; if it is an electrode of the second kind, the solid compound covering the metal must be only sparingly soluble in the electrolyte.

The degree to which these properties must be adhered depends upon the experiment, of course. Both SCE and Ag/AgCl/KCl have all the above properties [6]. Normally, time stability is considered the most important property.

2.7

Phenomena disturbing reference electrode potentials

2.7.1 Liquid junction potentials The porous plug in double junction reference electrodes (see Fig. 2.2) gives rise to a disturbing phenomenon of considerable concern to the designers and manufacturers of reference electrodes. Diffusion occurs when solutions of different composition are brought into contact, e.g. in the porous plug of the reference electrode. The diffusion of ions at different rates and quantities across the interface region or junction results in a slight separation of charges and consequently an electrical potential difference develops across the junction. The potential difference in turn affects the transport of the charged species, i.e. the potential difference opposes the attempt at charge segregation. A steady state is reached when the electrical counteraction balances the difference in diffusion. The basic phenomenon is that the transport of charged species is helped or hindered in accordance with their need to keep the situation as electroneutral as possible. The resulting potential difference between the inner reference electrode solution and the outer test solution is called the liquid junction potential. The magnitude of this potential depends upon the kinds of ions and their concentrations on each side of the junction. During measurement the liquid junction potential adds algebraically to the potential of the test electrode. To minimise the liquid junction potentials the inner reference electrode electrolyte typically is made of a salt with almost identical cation and anion mobilities (drift velocities in an electric field). This is the reason for using KCl, rather than NaCl, in SCE and Ag/AgCl electrodes. The mobilities of K+, Cl− and Na+ are 7.62, 7.91 and 5.19 × 10−4 cm2 s−1 V−1, respectively. In most test solutions the magnitude of the liquid junction potential set up by ‘KCl type’ double junction reference electrodes is seldom more than 2 to 3 mV. In the case of Cu/CuSO4 reference electrodes the corresponding potential is somewhat higher. The largest liquid junction potential error across the porous plug of a double junction reference electrode is obtained in cases

Reference electrodes in general

9

Table 2.2 Temperature coefficients, dE/dT, at room temperature for common reference electrodes [6, 7] Reference electrode

Temperature coefficient dE/dT (mV °C−1)

Cu/CuSO4 (sat) SCE Ag/AgCl/sat KCl Ag/AgCl/1 M KCl

+0.90 +0.22 +0.09 +0.25

where the inner reference electrode solution is highly acidic or highly basic. This is due to the high mobilities of H+ and OH− compared to the mobility of the counter ions, e.g. Na+ in the case of NaOH [8].

2.7.2 Temperature Reference electrode potentials change with temperature. Both electrochemical reactions (Nernstian thermodynamics) and chemical solubilities, e.g. of the inner reference electrode solution, are affected. Accordingly, the temperature coefficient, dE/dT (mV °C−1), varies from one type of reference electrode to another. To minimise errors in potential readings the coefficient should be low and at least known. Examples of temperature coefficients are given in Table 2.2. Table 2.2 shows that the Cu/CuSO4 electrode is 10 times more sensitive to temperature than the Ag/AgCl/satKCl electrode. It is recommended to record the ambient temperature and, if necessary, correct the readings to a fixed temperature. Without temperature compensation, misinterpretation can occur. Example Consider a cathodically protected structure (e.g. a steel pipe) buried under soil with a constant temperature. The electrochemical potential of the structure was determined regularly using a portable Cu/CuSO4(sat) reference electrode placed at the surface of the soil. On a +30 °C day (T1) a potential of −840 mV (E1) against Cu/CuSO4(sat) was recorded, a value not satisfying the common cathodic protection criteria of −850 mV against Cu/CuSO4(sat). The same measurement was later carried out on a +5 °C day (T2) giving −863 mV (E2) against Cu/CuSO4(sat), a value satisfying the protection criteria. Assuming the reference electrode temperature on the two days was +30 °C and +5 °C respectively, the recorded potential

10

Electrochemistry and characteristics of embeddable electrodes difference is entirely caused by the temperature sensitivity of the reference electrode: E2 = E1 + (dE/dT)(T2 − T1) = −840 mV + 0.90(5 − 30) mV = −862.5 mV

2.7.3 Chemical environment The inner active part of double junction reference electrodes does not respond electrochemically to changes in the outer chemical environment. The electrode is ‘protected’ by the porous plug. However, the liquid junction potential error (described in Section 2.7.1) changes with changes in the electrolytical composition of the outer solution. In most cases this error is insignificant but, for reference electrodes with strongly acidic or strongly basic inner solutions, the error might become significant. Placed in an outer test solution subjected to a severe change in pH, the potential error can be more than 100 mV [8]. Example Using KOH or NaOH with pH in the range 13 to 14 as the inner electrolyte gives rise to a change in the liquid junction potential error of about 30 to 35 mV for one pH unit change in the outer test solution. Consequently, when pH in a high pH test solution gradually decreases due to carbonation, a reference electrode of this kind will ‘drift’. Simple reference electrodes, like a piece of metal, provide a ‘reference’ potential that is produced by the electrochemical interaction between the metal and the chemical environment. If the concentration of the species taking part in this interaction changes, the potential of the metal will change.A change in the chemical composition of the environment can cause severe drift in the potential.

2.8

pH and oxygen sensitive electrodes

Metals, oxides and carbon materials are sometimes used as reference electrodes. However, the potentials of these materials are influenced by pH and oxygen concentration in the test solution. Generally, the potential decreases with increasing pH and increases when the oxygen concentration increases. Under ideal thermodynamic reversible conditions metal–metal oxide electrodes respond to pH in the same way as the hydrogen electrode with a dE/dpH slope of −59 mV. A few of them are capable of pH measurements of quite high accuracy. The influence of oxygen is more complicated, and the dE/d log PO2 slope may vary considerably (PO2 is the partial pressure of oxygen). Some metal oxides and activated carbon materials are strongly

Reference electrodes in general

11

influenced by oxygen, while graphite materials are influenced to a lesser extent.

2.9

Time stability requirements

Ideally, a reference electrode should have a stable potential with respect to time. However, time stability requirements depend on the objective of the electrochemical measurement. For example, for long-term monitoring of the corrosion potential a time stable reference electrode is a must. Otherwise, it will be almost impossible to distinguish between drift in reference electrode potential and drift in corrosion state. Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long ‘life’ when exposed to this environment.

3 Reference electrodes for concrete

3.1

Types

Various types of reference electrodes are used for permanent embedment in concrete. Some fall into the category of double junction electrodes of the second kind and are therefore reference electrodes in the true sense. Others are simply a piece of metal or another material put into the concrete. Although stable and accurate, SCE is not used for permanent embedment in concrete, mainly because it contains a liquid metal, which makes it difficult to manufacture in a rugged form. In addition, environmental reasons make it undesirable for permanent use in the field (poisonous mercury and mercury compounds).

3.1.1 Silver/silver chloride (Ag/AgCl/KCl) Ag/AgCl/KCl reference electrodes are widely used as embedded reference electrodes for concrete. Two main types, specially designed for concrete, are commercially available. One design resembles that of the double junction electrode shown in Fig. 2.2 with an inner gel containing approximately the KCl. (The inner electrolyte in embeddable reference electrodes is often made as a gel containing an anti-drying agent to minimise dry-out and contact problems with the electrochemically active metal.) The dimensions of the electrodes vary in length from roughly 50 to 140 mm and in diameter from 10 to 25 mm. Expected or designed life times are in the range of 10 to more than 50 years. Due to the design of some of these electrodes contact problems with the installation mortar have been reported [9, 10]. To overcome this problem, a common procedure is to precast the electrode in a body of mortar (cylinder shaped), which in turn is put into drilled holes in the concrete. The other main Ag/AgCl/KCl type is relatively big (length 160 mm and diameter 40 mm) and has a cotton bag enveloping the reference electrode. The bag is filled with a proprietary compound, a so called ‘backfill’, which 13

14

Electrochemistry and characteristics of embeddable electrodes Concrete surface Installation mortar

Ag/AgCl/KCl Conventional design without ‘backfill’

Ag/AgCl/satKCl ‘Cotton bag’ design containing a ‘backfill’

Fig. 3.1 Two types of Ag/AgCl/KCl reference electrodes embedded in concrete.

provides good ionic contact and mechanical bonding to the mortar/ concrete. The Ag/AgCl element is of the saturated KCl type. The electrode is well documented [11, 12] and the design life is 15 years. Retrofit installation in existing concrete structures differs depending on the type of Ag/AgCl/KCl used, as illustrated in Fig. 3.1.

3.1.2 Manganese dioxide (MnO2) The MnO2 electrode is a commercial reference electrode specially designed for concrete. It does not fall into any of the three main categories described in Section 2.2, but is based on battery technology and the electrochemistry of manganese dioxide in alkaline solution. The electrode is designed as a double junction electrode containing an inner NaOH solution of pH 13.5. A reversible thermodynamic reaction that produces the electrochemical potential obtained has not been found. In other words, the electrode does not, in a strict electrochemical sense, fulfil the requirements set for an ideal reference electrode (see Section 2.6). However, the electrode is used worldwide, has proven long-term stability in the field (more than 10 years), and is well documented [13, 14]. The length of the electrode is 85 mm and the diameter is 16 mm. The double junction design makes use of a porous plug made of a cement based material. This is shaped in such a way that it will have intimate contact with the concrete in which it is placed. The potential of the MnO2 electrode in a saturated Ca(OH)2 solution at 25 °C is +150 mV vs SCE, or +396 mV vs SHE. Compared to SCE and Ag/AgCl/KCl electrodes, the liquid junction potential error is significant. This is due to the composition of the inner electrolyte (NaOH) and the difference in mobility of Na+ and OH−. Fig. 3.2 illustrates the disturbing effect of the liquid junction potential. The difference in potential readings shown in Fig. 3.2 is caused by the liquid junction potential across the porous plug of the MnO2 electrode. The overall potential of this electrode decreases by approxi-

Reference electrodes for concrete Voltage reading: +150 mV MnO2

+

−

15

Voltage reading: +120 mV SCE

MnO2

+

−

SCE

pH 12.5 solution

pH 13.5 solution

E(MnO2) = +150 mV vs SCE

E(MnO2) = +120 mV vs SCE

Fig. 3.2 Schematic illustration of the pH sensitivity of the MnO2 reference electrode using two different test solutions.

mately 30 mV when the pH of the test solution increases by one pH unit. If the pH of the concrete is normal, the problem is insignificant. Only carbonation would really influence the potential of this electrode.

3.1.3 Copper/copper sulphate (Cu/CuSO4) and zinc/zinc sulphate (Zn/ZnSO4) Embeddable double junction Cu/CuSO4 reference electrodes have been widely used in soils. Although embeddable Cu/CuSO4 and Zn/ZnSO4 reference electrodes are commercially available, they are not recommended for permanent installation in concrete. The main concern is the possible chemical reactions that can occur in the porous plug and in the interface between the plug and the concrete. Cu2+ and Zn2+ form insoluble salts with OH− and carbonate (CO32−). Ca2+ from the concrete pore solution forms an insoluble salt with sulphate (SO42−). The precipitation of these compounds in the porous plug will eventually result in a semipermeable membrane, leading to high liquid junction potentials and, after long-term exposure, the pores in the plug become blocked. The expected potential drift is strong for these reference electrodes.

3.1.4 Lead (Pb) and zinc (Zn) Although not in common use today, thin metallic rods of high purity lead (Pb) and, to a lesser extent, zinc (Zn) have been used as ‘references’ in concrete. They are simple and easy to make, but their potentials are produced by corrosion reactions (mixed potentials) as opposed to known reversible

16

Electrochemistry and characteristics of embeddable electrodes

Table 3.1 pH and oxygen sensitivities of graphite and lead electrodes [19] – approximate experimental values obtained in synthetic concrete pore solutions Sensitivity parameter

Graphite (C)

Lead (Pb)

dE/dpH dE/d log PO2

−40 mV +115 mV

−55 mV +25 mV

half-cell reactions in reference electrodes like Ag/AgCl/KCl. Commercial Pb electrodes mounted in epoxy filled PVC bodies are available, but usable electrodes of this kind can easily be manufactured by a corrosion technologist. Zinc electrodes are sometimes used as references in marine environments. In sea water (pH ≈ 8) the rate of corrosion is at a minimum and the presence of chloride ions ensures the continuous dissolution of the zinc surface (ZnCl2 is soluble). This process provides a potential, which is stable within 20 mV [15]. Their use in concrete (alkaline solution, pH ≈ 13) is questionable, and not recommended. At pH about 12.5, zinc reacts rapidly to form soluble zincates and hydrogen gas is liberated [16]: Zn + OH− + H2O → HZnO2− + H2↑ Laboratory tests with zinc embedded in concrete have shown unstable and fluctuating zinc potentials in the range of −470 to −390 mV vs SCE [17] and roughly −0.5 V vs SCE [18]. Lead has a more stable electrochemical behaviour in concrete in comparison with zinc. The reactions producing its electrochemical potential in concrete have not been studied in detail, but corrosion reactions are likely to occur. The electrochemical potential of lead in aqueous solution changes with pH and the amount of dissolved oxygen. The dE/dpH slope in synthetic concrete pore solutions has been reported to be significant, approximately −55 mV [19]. The dE/d log PO2 slope seems to be less significant (see Table 3.1 and the comparison with graphite in Section 3.1.5). When embedded in concrete slabs the electrochemical potential of lead has been reported to be about −640 mV [17], −700 ± 40 mV [18] and −740 to −710 mV [19] vs SCE.

3.1.5 Graphite (C) Graphite electrodes for embedment in concrete have been used worldwide for many years. Like lead electrodes, they are relatively simple and easy to make. A wide range of commercial types exists. Unlike zinc and lead,

Reference electrodes for concrete

17

graphite and other carbon materials do not corrode or dissolve in alkaline solutions. Their electrochemical potentials are caused mainly by the presence of oxygen dissolved in the electrolyte. Carbon materials act as electrocatalysts for oxygen reduction [20, 21]: O2 + 2H2O + 4e− = 4OH−

(direct 4-electron pathway)

O2 + H2O + 2e− = O2H− + OH−

(2-electron pathway, peroxide pathway)

Theoretically, according to Nernstian thermodynamics, these reactions provide the following electrochemical characteristics at 25 °C: 4-electron pathway dE/d log PO2 = +2.3RT/4F = +15 mV and dE/dpH = −2.3RT/F = −59 mV where R = gas constant, T = temperature in Kelvin and, F = Faraday’s constant. 2-electron pathway dE/d log PO2 = +2.3RT/2F = +30 mV and dE/dpH = −2.3RT/2F = −30 mV Accordingly, carbon electrodes are sensitive to changes in oxygen concentration and pH. However, graphite electrodes for concrete, like most carbon materials, are non-reversible in a thermodynamic sense. This implies that the slopes or sensitivity to oxygen and pH may vary from one type of carbon to another. Carbon and graphite of different origin and quality exhibit different potential responses due to the presence or absence of residual groups attached to the carbon surface. It is reported [22] that ‘proprietary modifications’ of graphite can produce more stable and reproducible electrodes compared to unmodified ‘normal’ graphite. Table 3.1 gives an example of pH and oxygen sensitivities obtained in alkaline solutions with commercial graphite (‘normal’ type) and lead electrodes. A dE/d log PO2 value of +115 mV (see Table 3.1) indicates that an ‘oxygen starvation’ in concrete, lowering oxygen partial pressure from 0.2 (air) to 0.1 bar in the concrete pore water, would decrease the graphite potential by approximately 35 mV. The range of graphite potentials in ‘normal’ concrete is reported to be −40 to −130 mV vs SCE [19, 23].

18

Electrochemistry and characteristics of embeddable electrodes

3.1.6 Mixed metal oxide (MMO) MMO is short for an electrode of which the electrochemically active part consists of a titanium metal rod coated with mixed metal oxides, mainly proprietary oxides of platinum metals, ruthenium and cobalt. This is actually the same type of material that is used as the anode for cathodic protection systems (impressed current) in concrete. Since these metal oxides are normally good electrocatalysts for oxygen reduction, one would expect the electrochemical potential of MMOs to be sensitive to oxygen and pH (somewhat similar to carbon materials). However, laboratory tests have shown a lack of oxygen sensitivity, but significant pH dependence, suggesting that the potential of MMO is mainly determined by the electrochemistry of iridium oxides in water (iridium is a platinum metal) [24]. MMO reference electrodes are often precast into a cementitious grout. This ensures a rugged reference electrode with good ionic contact with the surrounding concrete. MMOs are regarded as suitable reference electrodes for short-term potential measurements [23]. Their use for long-term monitoring is questionable.

3.1.7 Metal–metal oxide Metal–metal oxide electrodes, which are strongly influenced by pH, can be regarded as relatives of MMOs. Two types of metal–metal oxide reference electrodes were tested in concrete some years ago [25], but they are scarcely in use today. Molybdenum/molybdenum oxide (Mo/MoO3) The electrochemically active part of the electrode consists of a molybdenum wire or rod that has been oxidised in molten potassium nitrate and soldered to an insulated copper wire [26]. Joints are normally sealed with epoxy or silicone compounds. Mo/MoO3 was suggested as a possible reference electrode in highly corrosive alkaline systems as early as 1967 [27]. For concrete application, two-year stability is reported [25]. The potential of Mo/MoO3 in concrete is approximately −450 mV vs SCE [26]. Field performance documentation is scarce. Mercury/mercury oxide (Hg/HgO) Hg/HgO reference electrodes have been widely applied in liquid alkaline solutions [28]. However, the natural and most appropriate use of the electrode would probably be in a pH-measuring role, namely as an OH− sensor in strongly alkaline solutions [29]. It is a thermodynamically reversible and

Reference electrodes for concrete

19

Table 3.2 Characteristics of embeddable reference electrodes for concrete Type

Inner electrolyte

pH sensitivity dE/dpH

Oxygen sensitivity

Extent of use

Ag/AgCl MnO2 Cu/CuSO4 Lead Graphite MMO Mo/MoO3 Hg/HgO

KCl NaOH CuSO4 None None None None NaOH

No ∼ −30 mV No ∼ −55 mV ∼ −40 mV High High ∼ −59 mV

No No No Low High No (or low) No (or low) No

Very common Very common Very seldom Seldom Common Common Very seldom Very seldom

highly accurate electrode (dE/dpH = −59 mV). In saturated Ca(OH)2 solutions at 25 °C (pH = 12.5) the potential of Hg/HgO is −52 mV vs SCE [28]. The electrochemical part consists of a platinum wire in contact with a paste made of liquid Hg and HgO powder. A ‘double junction’ electrode design (see Fig. 2.2) with an inner high pH electrolyte provides a stable potential. However, only six months stability in concrete has been reported [25], probably due to drying out of the inner electrolyte. An attempt has been made to develop a better ‘double junction’ type specially designed for concrete and with high reproducibility [30], but it has not gained widespread use in concrete. The two major disadvantages connected with these electrodes are probably: (1) high price and (2) the fact that mercury is strongly discouraged for environmental reasons. The main characteristics of embeddable reference electrodes for concrete are summarised in Table 3.2.

3.2

Installation of reference electrodes in concrete structures

3.2.1 Quality control of embeddable reference electrodes It is good practice to check carefully the electrochemical potential of the embeddable reference electrode against an accurate reference (SCE or Ag/AgCl), preferably in a laboratory, before the electrode is embedded in concrete. Normally, a saturated Ca(OH)2 solution is used as a test solution. By prolonging the exposure time in the solution, the magnitude of shortterm potential drift can be detected (be aware of temperature dependence). Potential values should always be compared with data provided by the supplier of the reference electrode. It is recommended that the functional and/or calibration check procedures given by the supplier are followed.

20

Electrochemistry and characteristics of embeddable electrodes

After installation in concrete, it is almost impossible to check the potential accurately. Measurement of the potential of the embedded electrode with a reference electrode placed on the concrete surface may give highly variable results, depending on the condition of the concrete cover (moisture, carbonation depth and chloride content). A better practice is to install two reference electrodes in approximately the same position in the concrete and compare their mutual potential difference.

3.2.2 Location of reference electrodes It is common practice to decide reference electrode locations from the results of a surface potential mapping survey. Electrodes are normally embedded at locations with the most negative surface potentials, i.e. the locations most likely to corrode. In new structures, it is common to install reference electrodes at locations most likely to be exposed to future corrosion problems. The number of reference electrodes installed will mainly depend on the size and complexity of the structure and the cost.

3.2.3 Installation procedures Normally, suppliers of reference electrodes give detailed instructions on how to install their electrodes. In some cases a pretreatment procedure is also suggested, for example that the electrodes should be soaked in an alkaline solution for a certain amount of time prior to installation. It is recommended that the supplier’s specific instructions are followed very carefully. However, some general installation procedures may be outlined. New structures Before casting the concrete, the reference electrode must be firmly fixed at the depth of interest – usually at the depth of the first layer of steel reinforcement. The reference electrode is normally held in place with plastic ties attached to the steel. Depending on the type of reference electrode, care is taken to ensure that the concrete completely envelopes the electrode and that no ionically conductive or metallic part of the electrode comes into direct contact with the steel. The electrical connection to the steel is normally done by using a selftapping screw in a drilled hole in the steel at some distance from the reference electrode location (typically 0.5 metres away). The connection point is carefully sealed off with a moisture-free jointing compound to avoid unwanted electrochemical activity at the connection point. Such activity will disturb the potential measurement.

Reference electrodes for concrete

21

Existing structures In existing structures reference electrodes are placed in drilled holes or cavities in the existing concrete (see Fig. 3.1). It is prudent that the steel to be monitored (adjacent to the reference electrode) is left undisturbed and embedded in the original concrete. If not, the electrochemical behaviour of the steel will change and no longer be representative of the corrosion state at that location. In some cases, there are two main options regarding where to drill the holes, either from the outside of the structure or from the inside, e.g. the hollow section of a bridge. Drilling from inside without disturbing the outer concrete surface is probably the best way of doing it, at least for long-term monitoring of the corrosion state (see Fig. 3.3). One starts by drilling a hole (typically of 20 mm diameter) in the original concrete and then filling the hole with a cement mortar. The mortar should have a consistency which allows the electrode to be squeezed into place. Furthermore, the mortar should have a water/cement ratio and an electrical conductivity in the hardened state that resemble those of the original concrete. The electrical connection is made as for new structures.

Cross section of concrete wall

Inside part of structure

Drilled hole filled with mortar

Cable

Steel

Outer weather exposed concrete surface

Reference electrode

Fig. 3.3 Three alternative ways of installing reference electrodes in drilled holes in a concrete wall. The electrical connection to the steel is not shown in the figure.

22

Electrochemistry and characteristics of embeddable electrodes

3.3

Field experiences

Reported experience with embeddable reference electrodes for concrete deals mostly with tests carried out under controlled laboratory conditions. There is a severe lack of published reports describing long-term field experience. However, some experiences from Norway have been reported. In the atmospherically exposed part of Gimsøystraumen Bridge (in Norway) 64 reference electrodes were installed from 1993 to 1995 for long-term corrosion potential monitoring of the reinforcing steel. Since January 1994 the potentials have been monitored and logged by a data acquisition system every six hours. By the end of 1996, about 50 of the 64 reference electrodes were still performing satisfactorily [9, 10]. In January 2002 the number of electrodes still performing satisfactorily was reduced to 39 [31]. An overview showing types of reference electrodes and failure rates is given in Table 3.3. Table 3.3 shows that the MnO2 reference electrodes had the lowest failure rate while the graphite and the ‘cotton bag’ Ag/AgCl/KCl electrodes had the second lowest failure rates. The four ‘conventional’ Ag/AgCl/KCl electrodes used in this monitoring programme malfunctioned a few months after installation, probably due to loss of contact between the ionically conductive part of the electrode (the porous plug) and the installation mortar. The lead (Pb) electrodes also failed, but more gradually than the ‘conventional’ Ag/Ag/KCl electrodes. After three years eight out of 11 were still performing satisfactorily, but by 2000 only one performed satisfactorily [31]. Due to the low numbers of electrodes in this study ‘statistical conclusions’

Table 3.3 Characteristics of reference electrodes installed in Gimsøystraumen Bridge [31] Type of reference electrode

MnO2 Ag/AgCl/KCl conventional design ‘cotton bag’ design

Number of reference electrodes Installed 1993

Installed 1994

Installed 1995

Total installed

Satisfactory performance January 2002

16

12

4

32

26

–

–

4

4

0

–

6

4 8

–

6

Graphite (C)

11

–

–

11

Lead (Pb)

11

–

–

11

1

Total:

64

39

Reference electrodes for concrete

23

with regard to electrode malfunction should not be drawn. However, the results from Gimsøystraumen Bridge show approximately 20 to 30 % failure rates among MnO2 and graphite reference electrodes eight to nine years after installation. Obviously, very stable and ‘long life’ embeddable reference electrodes are a must for this type of application (long-term corrosion monitoring). However, for cathodic protection control applications the stability requirement is not that important. It is surely desirable for the reference electrode to have a long embedded ‘life’. More reports on field experiences with embeddable reference electrodes for concrete would be highly appreciated.

4 References

1. ASTM C 876–91, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, American Society for Testing and Materials, Philadelphia, PA, USA, 1991, pp 437–442. 2. NACE International Technical Committee Report No. 24204, ‘Use of Reference Electrodes for Atmospherically Exposed Reinforced Concrete Structures’, NACE International, Houston, TX, USA, March 2000, 11 pp. 3. J. Koryta, J. Dvorak and L. Kavan, Principles of Electrochemistry, Chapter 3.2, John Wiley & Sons, Chichester, England, 1993, pp 169–180. 4. D. L. Peron, Electrochemistry of Corrosion, Chapter VI, National Association of Corrosion Engineers (NACE), Houston, TX, USA, 1991, pp 129–131. 5. D. J. G. Ives and G. J. Lanz, Reference Electrodes. Theory and Practice, Chapter 3, Academic Press, New York, USA, 1961, pp 154–162. 6. R. D. Caton, Jr, ‘Reference electrodes’, Journal of Chemical Education, 50(12), 1973, pp A571–A578. 7. ASTM G 3–89, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, American Society for Testing and Materials, Philadelphia, PA, USA, 1989, pp 56–64. 8. R. Myrdal, ‘Phenomena that disturb the measurement of potentials in concrete’, Corrosion’96, Paper No. 339, NACE, Houston, TX, USA, 1996. 9. K. Videm and R. Myrdal, ‘Instrumentation and condition assessment performed on Gimsøystraumen bridge’, Proc. International Conference on Repair of Concrete Structures, Svolvær, Norway, May 28–30, 1997, Norwegian Road Research Laboratory (Ed. A. Blankvoll), pp 375–390. 10. R. Myrdal, K. Videm, Ø. Vennesland and E. J. Sellevold, ‘Sensor technology and electrochemical measurements. What has been learnt on the Gimsøystraumen project?’, Proc. International Conference on Repair of Concrete Structures, Svolvær, Norway, May 28–30, 1997, Norwegian Road Research Laboratory (Ed. A. Blankvoll), pp 419–424. 11. F. J. Ansuini and J. R. Dimond, ‘Long-term stability testing of reference electrodes for reinforced concrete’, Corrosion’94, Paper No. 295, NACE, Houston, TX, USA, 1994. 12. F. J. Ansuini and J. R. Dimond, ‘Factors affecting the accuracy of reference electrodes’, Corrosion’94, Paper No. 323, NACE, Houston, TX, USA, 1994. 13. H. Arup and B. Sørensen. ‘A new embeddable reference electrode for use in concrete’, Corrosion’92, Paper No. 208, NACE, Houston, TX, USA, 1992.

25

26

References

14. H. Arup, O. Klinghoffer and J. Mietz, ‘Long term performance of MnO2 – reference electrodes in concrete’, Corrosion’97, Paper No. 243, NACE, Houston, TX, USA, 1997. 15. K. G. C. Berkeley, C. Eng and S. Pathmanaban, ‘Practical potential monitoring in concrete’, UK Corrosion’87, Brighton, UK, October 26–28, 1987, 2, pp 115–131. 16. ACI 222R-96, Corrosion of Metals in Concrete, American Concrete Institute, Formington Hills, MI, USA, 1997, p 5. 17. Ø. Vennesland, ‘Reference electrodes for monitoring the corrosion of steel embedded in concrete’, Proc. Third International Conference on the Durability of Building Materials and Components, Espoo, Finland, August 12–15, 1984, pp 239–249. 18. P. Pedeferri, G. Mussinelli and M. Tettamanti, ‘Experiences in anode materials and monitoring systems for cathodic protection of steel in concrete’, in Corrosion of Reinforcement in Concrete, edited by C. L. Page, K. W. J. Treadaway and P. B. Bamforth, Elsevier Applied Science, London, UK, 1990, pp 498–506. 19. R. Myrdal and K. Videm, ‘Evaluation of corrosion of steel reinforcement in concrete from potential measurements of embedded reference electrodes’, Corrosion’95, Paper No. 512, NACE, Houston, TX, USA, 1995. 20. E. Yeager, ‘Electrocatalysts for O2 reduction’, Electrochimica Acta, 29, 1984, pp 1527–1537. 21. A. J. Appleby and J. Marie, ‘Kinetics of oxygen reduction on carbon materials in alkaline solution’, Electrochimica Acta, 24, 1979, pp 195–202. 22. A. A. Sohanghpurwala, W. T. Scannell and A. LaConti, ‘Improvement in graphite reference cell for reinforced concrete’, Corrosion’94, Paper No. 307, NACE, Houston, TX, USA, 1994. 23. J. E. Bennett and T. A. Mitchell, ‘Reference electrodes for use with reinforced structures’, Corrosion’92, Paper No. 191, NACE, Houston, TX, USA, 1992. 24. P. Castro, A. A. Sagüés, E. I. Moreno, L. Maldonado and J. Genescá, ‘Characterization of activated titanium solid reference electrodes for corrosion testing of steel in concrete’, Corrosion, 52(8), 1996, pp 609–617. 25. K. N. Gurusamy and M. P. Geoghegan, ‘The long term performance of embedded reference electrodes for cathodic protection and insitu monitoring of steel in concrete’, in Corrosion of Reinforcement in Concrete, edited by C. L. Page, K. W. J. Treadaway and P. B. Bamforth, Elsevier Applied Science, London, UK, 1990, pp 333–347. 26. C. E. Locke and C. Dehganian, ‘Embeddable reference electrodes for chloride contaminated concrete’, Materials Performance, 18(2), 1979, pp 70–73. 27. R. L. Every and W. P. Banks, ‘Reference electrodes in acid and base systems’, Corrosion, 23(6), 1967, pp 151–153. 28. See reference 5, Chapter 7, pp 335–336. 29. P. Longhi, T. Mussini, R. Orsenigo and S. Rondinini, ‘Redetermination of the standard potential of the mercuric oxide electrode at temperatures between 283 and 363 K and the solubility product constant of mercuric hydroxide’, Journal of Applied Electrochemistry, 17, 1987, pp 505–514. 30. G. Gonzales, A. Ocando and S. Montilla, ‘A modified Hg/HgO electrode to be embedded in concrete’, Corrosion Science, 33(6), 1992, pp 959–964. 31. Unpublished data provided by Dr Claus K. Larsen at Norwegian Public Roads Administration, Road Technology Department (NRRL), Oslo, Norway, August 2002.