(efc 4) european federation of corrosion guidelines on electrochemical corrosion measurements -...

European Federation of Corrosion Publications

NUMBER 4

A Working Party Report

Guidelines on Electrochemical Corrosion Measurements

Published for the European Federation of Corrosion by The Institute of Metals

THE INSTITUTE OF METALS

1990

Book Number 497

Published in 1990 by The Institute of Metals 1 Carlton House Terrace, London SWlY 5DB

and The Institute of Metals

North American Publications Center Old Post Road, Brookfield VT 05036

US A

0 1990 The Institute of Metals

All rights reserved

British Libra y Cataloguing in Publication Data

European Federation of Corrosion. Working Party on Physico- Chemical Methods of Corrosion Testing

Guidelines on electrochemical corrosion measurements. 1. Corrosion. Testing.

I. Title 11. Institute of metals 1985- 111. Series

620.11223

Libra y -of-Congress -Cataloging-in-Publication-Data

available on application

I S B N b901462-87-X

Text processing by P i c A Publishing Services from original typescripts and illustrations provided by the authors

Printed in Great Britain

Contents

European Federation of Corrosion ~ Series Introduction

Foreword

Introduction

Chapter 1 Instrumentation, Performance and Calibration I . C. Rowlands

1.1 Introduction

1.2 Performance Limitations

1.2.1 Impedance Matching 1.2.2 Ammeter Loading 1.2.3 Amplifier Bias Current 1.2.4 Bandwidth Limitations

1.3 Error Sources

1.3.1 Resistance Noise 1.3.2 Triboelectric Noise 1.3.3 Piezoelectric Effects 1.3.4 Thermoelectric Effects 1.3.5 Magnetic Field Loops 1.3.6 Earth Loops 1.3.7 Earth Potentials 1.3.8 Capacitive Transformer Coupling 1.3.9 Long Lead Cable Capacitance

1.4 Instrument Calibration

1.5 References

Chapter 2 Design of Electrochemical Cells F. P. IJsseling

2.1 Introduction

2.2 Electrochemical/Electrical Requirements

2.3 Solution Requirements

2.4 Construction Requirements

2.5 References

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11

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16

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19

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21

22

25

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Chapter 3 Electrode Design E . Heitz

27

3.1 Introduction

3.2 Size of Electrodes

3.3 Design with Regard to Various Corrosion Parameters

3.3.1 Current and Potential Distribution 3.3.2 Mass Transfer 3.3.3 Heat Transfer 3.3.4 Mechano-chemical Testing 3.3.5 High Temperature/High Pressure Testing.

3.4 Problems Concerning Application Techniques

3.5 References

Chapter 4 Reference Electrodes E . Eriksrud and E. Heitz

4.1 Introduction

4.2 Choice, Stability, Incompatibility

4.3 Checking of the Reference Electrode, Some Practical Advice

4.4 More Specific Requirements

4.5 Some Common Electrodes for Aqueous Systems

4.5.1 Mercury-mercurous chloride (calomel) 4.5.2 Mercury-mercurous sulphate 4.5.3 Mercury-mercuric oxide 4.5.4 Silver-silver halide 4.5.5 Reference Electrodes in Non Aqueous Systems

4.6 References

Chapter 5 Effect of Specimen Preparation and Surface Condition 1. Simpson

5.1 Introduction

5.2 Choice of Sample

5.3 Sampling and Specimen Preparation

5.4 Surface Preparation Before Immersion in the Test Solution

5.5 Effect of the Conditions Within the Cell Before Commencement of the Measurement

5.6 Changes Caused by the Measurement Itself

5.7 References

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31

32

34

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34

34

35

35

36

37

37

37

37

38

39

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39

6

Chapter 6 Evaluation and Compensation of Ohmic Drop L. Clerbois and F. P . IJsseling

6.1 Introduction

6.2 Examples

6.2.1 Polarisation Resistance 6.2.2 Polarisation Curves in Aqueous Solution 6.2.3 Polarisation Curves in Non Aqueous Solution

6.3 Principal Methods

6.3.1 Minimising Ohmic Drop by Cell and Electrode Design 6.3.1.1 Cell Design 6.3.1.2 Microelectrodes

6.3.2.1 Positive Feedback 6.3.2.2 Interrupt Methods

6.3.3.1 Measurement of Ohmic Drop 6.3.3.2 Direct Calculation 6.3.3.3 Mathematical Methods

6.3.2 Active Methods

6.3.3 Passive Methods

6.4 Conclusions

6.5 Acknowledgements

6.6 References

Chapter 7 Automatic Measurement Systems 0. For& and 1. Aromaa

7.1 Introduction

7.2 Parts of a Measurement System

7.2.1 Computers and Peripherals 7.2.2 Measuring Devices 7.2.3 Data Transfer Busses 7.2.4 Software

7.3 Analysis of Measured Data

7.4 Examples of Automatic Systems

Chapter 8 Field Testing G. Turluer

8.1 Objectives

8.2 Test Methods

40

40

42

44

52

52

52

55

55

56

58

59

61

61

61

8.2.1 Polarisation Resistance Measurements (R,) 8.2.2 Material (or Component) Potential and Process Redox Potential Monitoring 8.2.3 Impedance Measurements 8.2.4 Potentiodynamic Scanning 8.2.5 Galvanic Current Measurements 8.2.6 Spatial Potential Scanning

7

8.3 Specific Requirements and Precautions

8.3.1 General 8.3.2 Probe configuration and location 8.3.3 Reference electrodes

8.3.3.1 Arbitary electrodes 8.3.3.2 In situ reference electrodes 8.3.3.3 External reference electrodes

8.4 Interpretation and Possible Limitations

62

63

8

Introduction

At the present time electrochemical test methods in corrosion are becoming more diverse and, as aconsequence, new techniques have been added to those well established. Each technique has limitations and has requirements for condi tions to be satisfied for practical application. This subject was the topic of an international workshop held at Ferrara, Italy, 11-13 September 1985, organised by the European Federation of Corrosion Working Party on Physico-Chemical Methods of Corrosion Testing [l]. This Workshop outlined the range of practical application of electrochemical corrosion testing as well as reaching conclusions concerning the use, significance and limitations of various methods. However, this meeting, and others, on electrochemical corrosion measurements have generally not provided specific information on how experiments should be performed[2-41. It was the Working Party’s intention to fulfil this requirement with the compilation of this booklet. In undertaking this task, members were confronted with a great variety of techniques to be considered, although, inevitably, the same experimental principles are involved with the various techniques. Various practical aspects in conducting experiments, both in the laboratory and in the field, have been considered in achieving the objective of this work.

There is a notable absence of specifications or codes of practice on the use of electrochemical corrosion testing and it is hoped that these guidelines will fill this gap in corrosion science.

REFERENCES

1. Electrochemical Corrosion Testing. Monograph 101, published by DECHEMA, Frankfurt, FRG, (ed. E.Heitz, J. C. Rowlands and F. Mansfeld) 1986.

2. Electrochemical Techniques for Corrosion. Published by NACE, Houston, USA ( ed. R. Baboian) 1977.

3. Electrochemical Corrosion Testing. Special Technical Publication727. Published by ASTM Philadelphia, USA (ed. F. Mansfeld and V. Berucci) 1981.

4. Electrochemical Methods in Corrosion Research. Published by Trans. Tech. Publications Ltd., Switzerland (ed. M. Duprat), 1986,8.

12

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 is based upon participation by corrosion societies and committees in technical Working Parties. Member societies appoint delegates to Working Parties, whose membership is also expanded by co-option of other individuals.

The activities of the Working Parties cover corrosion topics associated with inhibition, educa- tion, reinforcement in concrete, microbial effects, hot gases and combustion products, environment sensi tive fracture, marineenvironments, surface science, physico-chemical methods of measurement, the nuclear industry, and computer based information systems. Working Parties on other topics are established as required.

The Working Parties function in various ways, e.g. by preparing reports, organising symposia, conducting intensive courses, and producing instructional material, including films. The activities of the Working Parties are co-ordinated, through a Science and Technology Advisory Committee, by the Scientific Secretary.

The administration of the EFC is handled by three Secretariats: DECHEMA in the Federal Republic of Germany, the Soci6t6 de Chimie Industrielle in France, and the Institute of Metals 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 fromall 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 an occasional 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 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 Institute of Metals was appointed as the official EFC publisher. Although the arrangement is nonexclusive and other routes for publication are still available, i t is expected that the Working Parties of the EFC will use the Institute of Metals for publication of reports, proceedings etc. wherever possible.

A D Mercer EFC Scientific Secretary Institute of Metals London, UK

9

EFC Secretariats are located at:

Mr. R. Wood European Federation of Corrosion The Institute of Metals 1 Carlton House Terrace LONDON SWI Y 5DB UK

DrD. Behrens Europaische Foderation Korrosion DECHEMA Theodor-Heuss-Allee 25 D-6000 FRANKFURT (M) FRG

M. R. Mas Federation Europtkne de la Corrosion Societe de Chimie Industrielle 28 Rue Saint-Dominique

FRANCE F-75007 PARIS

10

Foreword

Electrochemical methods have been used in corrosion testing ever since the electrochemical nature of corrosion processes was discovered. In the present age electrochemical measurements involve the use of sophisticated 'black boxes' which are invariably blamed for any 'pitfalls' which occur. Hence the European Federation of Corrosion Working Party on Physicochemical Methods of Corrosion Testing felt it desirable to remind research workers, students and instrument designers of the more fundamental aspects of the measurements.

Chaptersin this book were prepared by the Working Party members named in the Contentslist. Copyright of any particular chapter may be the property of the authors or their employers, but the publication rights havebeengranted to theInstitute of Metals. The Working Party wishes toexpress its appreciation to Mr A D Mercer for the final editing of this booklet on behalf of the European Federation of Corrosion.

Members of the Working Party are as follows:

Belgium L Clerbois

Finland 0 Forsen

France P Lacombe G Turluer

Germany W Fischer E Heitz

Great Britain J W Oldfield J C Rowlands

Italy F Mazza G Rocchini

Netherlands F P IJsseling

Norway E Eriksrud

Spain J M Costa

Switzerland J Simpson

L Clerbois Chairman EFC Working Party Physicochemical Methods of Corrosion Testing

CHAPTER 1

INSTRUMENTATION, PERFORMANCE AND

CALIBRATION J . C. ROWLANDS

ARE Holton Heath, Poole, Dorset, UK

1 .I INTRODUCTION Since most electrochemical measurements relating to corrosion of metals are satisfied with a sensitivity of 1 pV or 1 pA, modern instrumentation usually employs electronic operational amplifiers where the noise limits control the range of measurements. The function of the operational amplifier is to amplify the potential (VJ applied at the input so that it can be displayed on a low impedance analogue or digital meter (V,) as shown in Fig. 1.1. The output potential of the operational amplifier is proportional to the source potential and is required to have sufficient input impedance to avoid polarisation of the potential source.

The basic corrosion instrumentation requirement involves the measurement of potential difference. Currents are measured as the potential across a resistor (R,) as shown in Fig. 1.2, where the potential difference is again determined with an operational amplifier. More sophisticated measurements such as polarisation characteristics and zero resistance ammetry involve the use of potentiostats which again use operational amplifiers in a differential mode. The potentiostat is an instrument for maintaining the potential of an electrode under test at a fixed potential compared witha reference cell, and the basic circuit is similar to that for potential measurement with the earth return circuit broken to an auxiliary electrode in the electrochemical cell. Such a circuit would maintain the potential of the test electrode at the reference cell potential. This potential may be varied by inserting a variable potential source (V,) in the input circuit as shownin Fig. 1.3. The actual cell potential (VJ and the current required to polarise the test electrode to this potential may be measured using the basic circuits shown in Figs. 1.1 andl.2 respectively.

A further modification of the basic voltage measurement, given in Fig. 1.1, is the zero resistance ammeter (ZRA) shown in Fig. 1.4. The measurement of current as the potential drop acrossa resistor (R,) shown in Fig. 1.2 involves an error due to the value of resistor A. This may be overcome with the ZRA in which the current is determined as the potential measured across a feedback resistor (R,) of the high gain operational amplifier. Thus, the current required to maintain electrodes A and B shown in Fig. 1.4 at the same potential can be determined and displayed on a high impedance voltmeter for which most commercial digital multimeters are suitable.

1.2 PERFORMANCE LIMITATIONS The performance characteristics of commercial instrumentation should be supplied by the manufacturers. There are, however, measurement limitations which are controlled by the electrochemical cell under investigation. The major limitations and requirements are listed under their separate headings.

13

Fig.1.1: Voltage measurement

b

n

Fig.l.2: Current measurement

b AUX REF TEST

Fig.l.3: Potentiostat

r------- 1 r------- 1. & I I

1 - 1 I I

I I I I

I I

I I I I

I I

I I

I I I I I I I I

1 - 1 I I

I I I I

Fig.l.5: Impedance matching

14

1.2.1 Impedance Matching The measurement of a potential difference between two metals or a metal with respect to a reference electrode in an electrolyte requires the determination of a source voltage (V,) which has a source resistance ($). This source resistance is the sum of the resistivity of the electrolyte and the polarisation resistance of the electrodes. In order that the measuring instrument does not draw a significant current from the source, thus avoiding polarisation, it is necessary that the instrument or meter impedance must be high compared with the source impedance. Such a circuit of the source instrument impedance is shown in Fig. 1.5. For this circuit the actual potential measured (Vm) is related to the source potential (V,) by equation 1. The resultant error if the amplifier input impedance (R,) is not large is given in equation 2.

Rl vm=vs - Rl + RS

VS

Error = vs- - vm 100%

Although it is most desirable to have R, very large compared with% a very high value of I$ may present problems due to pickup of dc or ac error signals from other instruments and the mains electricity supply. A high source impedance, &, is invariably present in conducting electrochemical measurements in non aqueous media or when investigating organic coatings on metals. Hence it may be necessary to compromise the accuracy of the measurement for the sake of reducing error signals from other sources by limiting the impedance of the measuring instrument. For most corrosion measurements an error of 1% is acceptable.

1.2.2 Ammeter Loading The internal resistance of an ammeter (R,) in Fig. 1 . 2 presents a potential drop in the circuit. This may be overcome using the zero resistance ammeter shown in Fig. 1.4.

In many instruments such as potentiostats the value of resistance (R,) is chosen to provide a limiting current in case the output becomes shorted.

1.2.3 Amplifier Bias Current Electronic amplifiers have a small bias current across the input. In electrochemical corrosion measurements it is a requirement that this bias current is very small, such as less than 1 FA to avoid polarising the electrodes of the corrosion cell.

1.2.4 Bandwidth Limitations When following potential changes, eg electrochemical cell capacitance charging or discharging, ac impedance measurements, or electrochemical noise measurements, the bandwidth response of the measuring instrument may limit the application.

The frequency response of an instrument is usually specified as the 3 dB point (f3 dB) and is determined by the R, and C, values for the input of the circuit shown in Fig. 1.5.

1 - - - 2.rr R, C, ‘3 dB (3)

The rise time (t>, i.e. the time to rise from 10% to 90% of the signal amplitude, is given by the equation:

tr = 2.2 qc, (4)

15

Thus, from equations 3 and 4:

t = - 2 . 2 -r

2T f3dB

(5)

1.3 ERROR SOURCES

1.3.1 Resistance Noise Resistance or Johnson noise in a resistor is caused by the thermal energy produced due to the passage of a current. In a metallic conductor the Johnson voltage noise developed in a resistor is given by the equation:

E = v4kTRAf (6)

where k = Boltzmann’s Constant, T = temperature K, Af = noise bandwidth in Hz.

In high resistance circuits the noise bandwidth is limited by the time constant (t,> of the source resistance (RJ in parallel with the input resistance and the input capacitance (q) as shown in Fig. 1.5. The noise bandwidth is then given by the equation:

7 R + R

A solution to this problem is keep the connecting leads from the source to the instrument as short as possible.

1.3.2 Triboelectric Noise Currents are generated by charges created due to friction between an insulator and a conductor. Such noise is dependent on the length, degree of movement and the materials of the cable, but may give rise to large electrostatic charges which can be minimised using low noise cables.

1.3.3 Piezoelectric Effects Electrostatic charges are generated when a mechanical stress is applied to some materials, and hence the cables coupling the source to the instrument should not be under tension and not be free to vibrate.

1.3.4 Thermoelectric Effects Due to different parts of the circuit being at different temperatures, or at dissimilar metal joints as in thermocouples, an error potential in the millivolt range could be generated. Hence the test cell and the instruments should be maintained at a uniform temperature and equilibrium reached before undertaking measurements.

1.3.5 Magnetic Field Loops A loop of cable in the circuit between the test cell and the measuring instrument may be subject to a change in magnetic field which develops a potential. This potential (EB) is given by the equation:

dB E , a A- dt

where dB/dt is the rate of change of magnetic field intensity and A is the area enclosed by the loop of cable.

This is particularly relevant to remote sensing in field tests where potential errors of the order of millivolts can be generated.

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1.3.6 Earth Loops A potential error may be developed due to the use of a common earth line between the test cell and the measuring instrumentation, due to the current carried in the earth line. This error may be minimised by earthing both the cell and the measuring instrument to the same earthing point using separate cables. Similarly, the use of any current carrying lead should be avoided when making a potential measurement due to the voltage drop in the lead.

1.3.7 Earth Potentials When making measurements in process plant it is not uncommon to have various metallic conductors making the connection to earth, resultingin the presence of electrical earths at different potentials, as shown in Fig. 1.6 where the electrolyte containing the electrodes forms an earth connection with the plant in which the measurement is being undertaken. It is usually not acceptable on safety grounds to remove the mains earth, even when the instrumentation is earthed through the test cell and, in such cases, the problem is best avoided using battery operated instrumentation.

ELECTROLYTE RESISTANCE

TO EARTH

Fig.l.6: Multiple earth ELECTRODE

MAINS EARTH

1.3.8 Capacitive Transformer Coupling Although the primary and secondary windings on a mains transformer are considered as an inductive coupling there is also an intercapacitive effect between the high and low voltage windings. This results in an ac voltage to earth superimposed on the secondary output asillustrated in Fig. 1.7. This error source is usually only applicable when measuring very low potentials and currents and may be reduced by transformer earth screens. The remedy is to use battery powered instrumentation.

I I

I 1 Fig.l.7: Mains supply earth current

1.3.9 Long Lead Cable Capacitance The use of long leads between the potential source and the measuring instrument can result in an effective change of the output capacitance of the measuring instrument, thus altering its frequency response. Typically the capacitance of a twin core cable is of the order of 100 pF/m. The effect on the frequency response can be calculated using equations 3 or 4. The remedy to this problem is to keep the cables as short as possible or, where long cable systems are unavoidable, a driver amplifier at the source may be required.

17

1.4 INSTRUMENT CALIBRATION The instrumentation described in this chapter is very dependent on the maker supplying a reliable specification. Thereafter it is usually only necessary to check the instrument periodically using a commercially available standard potential or current source. Any deviation found should be corrected by following the instrument manufacturer's setting up procedure or by returning to the maker for recalibration.

1.5 REFERENCES

cations. Chapter 13. Published by John Wiley & Sons Inc., 1980. 1. A. J. BARD and L. R. FAULKNER: in Electrochemical Methods - Fundamentals and Appli-

2. Instrumental Methods in Electrochemistry, Southampton Electrochemistry Group. Published by Ellis Horwood, Chichester, U. K., 1985.

3. J. F. KEITHLEY, J. R. YEAGER and R. J. ERDMAN: in Low Level Measurements. Revised 3rd Edition. Published by Keithley Instruments, Ohio, U. S. A., 1984.

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CHAPTER 2

DESIGN OF ELECTROCHEMICAL CELLS

F.P. IJSSELING Corrosion Laboratory of Royal Netherlands Naval College, Den Helder, The Netherlands

2.1 INTRODUCTION The general purpose of an electrochemical cell is to be able to conduct electrochemical measure- mentsona metal sample - the working or test electrode - which should have a well-defined area and surface condition. In the cell the working electrode is brought into contact with the corrosive environment, usually under well defined conditions i.e., with respect to environmental composition, temperature, flow, etc. [l, 21. Apart from the working electrode an auxiliary electrode is required to allow electric current to flow through the cell during polarization; in addition a reference electrode is needed to be used as a zero point for measuring the potential difference between the working electrode and the solution. Most electrochemical cells are based on the above mentioned three electrode system. Only in simple cases, for instance when the measurements are to be made under zero current (e.g. the free corrosion potential) or very low current flow, a two electrode cell may suffice. Then a reference electrode is used which, under low current flow, can also act as the auxiliary electrode. The contents of this chapter are aimed at threeelectrode cells to be used on polarizing the working electrode. In the following a list of requirements is given, which are further divided into the following sections:

1. electrochemical/electrical 2. solution 3. construction.

2.2 ELECTROCHEMICAUELECTRICAL REQUIREMENTS The first and foremost requirement is the need to establish a homogeneous electric field at the working electrode surface and consequently aneven current distribution. In this respect the relative positions of the working-, auxiliary- and reference-electrodes are of paramount importance.

(a). The best way to obtain an even current distribution is to position the auxiliary electrode as symmetrically as possible to the working electrode, the latter preferably having the smaller or at least the same dimensions (Fig.2.1). A large distance between the electrodes will also promote a more even current distribution. However, too large a distance may create an unduly high ohmic resistance in the cell [3,41.

(b). The reference electrode should be positioned in sucha way as not to disturb the even current distribution by shielding effects, at the same time keeping the uncompensated ohmic resistance in the solution at a minimum. The classical solution is to place the reference electrode in a separate compartment and to use a Luggin capillary, the tip of which is to be placed 1-2 outer diameters of the capillary opening from the working surface (Fig. 2.2a) [5 ] . In any case, positioning of the reference electrode should be possible in an exact and reproducible way. However, other satisfactory solutions have been found, including the side channel probe and the back probe (Fig. 2.2b, c). The first consists of a small opening at the side of the working electrode as a connection with the reference electrode, while in the second case the Luggin probe is attached to the back of a narrow hole drilled through the electrode 16-81.

(c). In all cases the cell should provide a well-defined electrical system; in the first place good electrical connections with the electrodes should be ensured, which even at longer exposure times

19

WE

a.

E Fig. 2.1: Simple three-electrode cell containing working electrode (WE), auxiliary electrode (AE) and reference electrode (RE).

rE

b.

I RE

IRE II. r c.

Fig. 2.2: Different methods of minimizing IRdrop: a. Luggin capillary b. side channel c. back channel.

a.

Fig. 2.3: Exampleof cell with separate compartment for AE, connected via a membraneor coarse sintered glass: a. single separation b. double separation by which an extra compartment is created in which the possibly contaminated solution can be flushed regularly by means of a stopcock.

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do not lead to failures. The problem is most urgent in cases where aggressive ions are present and/ or high temperatures that could eventually lead to oxidation of the contacts. The use of a double wiring system and detection of possible contact resistance is recommended. The inner electrical path should also be considered and possible errors resulting from clogging of the Luggin capillary or the formation of poorly conductive areas in the cell should be avoided as these could give rise to unduly high electrical resistances in the power loop or the control loop.

The pick-up of electrical and electromagnetic noise should be avoided, especially in the case of the application of electrochemical pulse methods and sensitive dc measurements involving small potential perturbations and currents.

The factors which should be taken into consideration in such cases, involve:

1. Earthing - in many laboratory applications the working electrode is earthed or kept at virtual earth; in a number of commercial potentiostat designs it is possible to disconnect the working electrode from the instrument zero. All measuring instruments and ancillary equipment should be fed from the same line connection. In addition all instrument houses and bodies of ancillary equipment should be interconnected and earthed at one point for safety reasons [9,101.

2. Short connections - generally the control instrument e.g. potentiostat should be located near the cell to ensure short electrical connections. If this is not feasible the use of a buffer amplifier in the control loop near the test cell is to be considered.

3. Reference electrode - a high resistance which is common in many commercial products may induce noise and even instability in the control loop.

The connection with the potentiostat should be shielded; shielded reference electrodes are commercially available. However, in the case of excessive capacitance to ground instability of the control potentialmay occur,insuchcases the shieldingmaybemaintained at the reference potential via an operational amplifier [5]. Another possibility is to provide a platinum wire to be used as the reference in the control loop, next to the reference electrode proper.

4. Screening - if necessary the cell should be placed in a Faraday cage or screened electrically and electromagnetically by other means as, for instance, by tin plate, copper foil, etc.

(e) Also, in the case of sensitive high-performance measurements the potentiostat and the cell design should be matched to each other. Generally the requirements for rapid and accurate response are: low total cell resistance, low-resistance reference electrode, small-area working electrode, and low stray capacitances [ll, 121. In general it can be said that the faster the response of the potentiostat, the greater its sensitivity to interference. So it is not advisable to use fast response potentiostats for applications where this feature is not required.

2.3 SOLUTION REQUIREMENTS The solution requirements depend to a large extent on the purpose of the measurement. Generally the solution consists of a solvent, the electrochemically active redox system and possibly complexing agents, buffers, a supporting electrolyte to enhance the electrical conductivity, etc. In the case of industrial measurements, e.g. corrosion monitoring, the solution is generally that being used in the process.

In laboratory experiments special precautions are often required. The main points can be summarized as follows:

(a). The solution should be free from contaminants as even small concentrations may exert relatively large effects on the electrochemical reactions by adsorption effects, etc. So in appropriate cases freshly double-distilled water and analytically pure salts should be used.

(b). Moreover, the solution should not be contaminated during the measurements, either by components of the materials to be used for cell construction or by reaction products evolved at the current carrying electrodes. The first possibility should be avoided by proper materials selection. To eliminate the second one the electrochemical reactions at the working as well as the auxiliary electrode should be taken into account. For this reason the auxiliary electrode is often placed in a separate compartment, the contents of which are electrically connected with the test solution via a membrane, glass frit, etc. (Fig. 2.3).

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As it is not feasible to separate the working electrode from the test solution the first thing to be considered is to keep the cell volume large enough to avoid the accumulation of an excess of corrosion products [131. Monitoring the corrosion product concentration is advisable and the cell contents should be refreshed as soon as unduly high concentrations arise. Other possibilities include the use of special cells which permit continuous or intermittent replacement of the test solution and, in the limit, the use of once-through conditions. Apart from the auxiliary electrode the reference electrode is also sometimes placed in a separate compartment, for instance when the release of chloride ions in the test solution from a saturated calomel electrode cannot be tolerated. Even then contamination by release of chloride ions may become a problem, which can be solved by placing an extra compartment between the cell proper and the reference compartment (Fig. 2.4). Reference electrodes with intermediate electrolyte chambers are commercially available.

(c). The test solution should maintain its intended corrosive properties during the experiment, e.g. O,-content, pH etc. If necessary, the concentration of the corrodents should be monitored and kept at a specified level. Special care must be exercised when biologically active solutions are used, in which the bacteriological components interfere with the corrosion process, such solutions often being prone to ageing. Seawater is an example of a solution in which significant changes of the corrosive properties may occur on storing and during exposure.

(d). In a number of cases it might be desirable to provide a constant gas-atmosphere in the cell above the solution. Examples are the removal of oxygen by the addition of nitrogen, hydrogen or mixtures thereof, or the creation of an oxygen-rich environment by the addition of oxygen. For many purposes a simple provision to introduce gas in the cell under a slight overpressure, combined with a device to distribute the gas in the solution in the form of small bubbles, suffices (Fig. 2.5).

However, in separate cases, for instance when poisonous gases are involved, a more elaborate methodology is required.

2.4 CONSTRUCTION REQUIREMENTS The constructional design of a cell also depends to a large extent on the type of measurements to be made. Generally the construction should be rugged and allow for easy assembly and disassem- bly as required, for instance, for cleaning purposes. Moreover the positioning of the electrodes should be accomplished easily and in a reproducible way.

The following points should be kept in mind:

(a). materials - in many cases glass is satisfactory, although sometimes PVC, PTFE or other plastics are to be preferred, keeping the contamination problem in mind. Often it is possible to construct a cell from standard laboratory glass components, although care should be taken in the use of glass at temperatures above about 6OoC because of the possibility of contamination by silica resulting from dissolution of the glass [141.

(b). temperature - in many cases it will be desirable to study the corrosion process at a well- defined temperature, necessitating temperature control of the cell. The following methods can be envisaged:

1. direct introduction of heatingand/or cooling element sinto the test solution (Fig. 2.6a), taking precautions to avoid current leakage from the element into the solution;

2. external circulation of the test solution through a heat exchanger (Fig. 2.6b); 3. positioning of the test cell in a thermostatted bath (Fig. 2.6~); 4. the use of a double walled cell, circulating thermostatted water through the double wall to

ensure temperature constancy of the inner test solution (Fig. 2.6d). It must be remarked that for high-temperature applications the Luggin capillary also should be

similarly equipped with a double wall for circulation of thermostatted water. (c). working pressure - most tests are performed under atmospheric pressure, obviating special

requirements. However, in some cases it isessential to test under increased pressure, so the cell has to be adapted to this purpose. Generally autoclaves have been used for this purpose, one of the problems involved being the gas-tight isolation of the electrode connections 1151.

(d). solution flow - frequently the flow conditions at the working electrode surface are not too well defined. However, if so required this aspect can also be taken into account, for instance, by using rotating disc or cylindrical electrodes.

Another possibility is the use of tubes or plate electrodes in ducts, channels or test loops, e.g. generally in flow through cells (Fig. 2.7 and Chapter 3) [16].

22

Fig. 2.4: Electrochemical cell with separate compartments for the reference and the auxiliary electrodes; to prevent contamination by the solution in the reference electrode the contents of this compartment can be flushed out regularly by means of a stopcock.

Fig. 2.5: Closed cell with inlet and outlet for gas.

1

a. C.

b.

ti l+d

IT0 Fig. 2.6: Different methods of temperature control: a. direct b. via external heat-exchanger c. via temperature bath d. via double walled cell

C. I RE

d Fig. 2.7: Different methods for creating solution flow (see also Chapter 3): a. rotating disk electrode b. rotating cylinder electrode c. pipe system with one test pipe as WE and two as AE d. pipe system with one test pipe as WE and a central rod or wire as AE. In cases c and d the RE can be

1

a. b. connected via a hole through the wall of the WE.

23

Fig. 2.8: Example of electrochemical cell for SCC testing u n c r potentiostatic contrc

WE: working electrode, consisting of notched rod possibly isolated from the solution except at the notched position; AE: auxiliary electrode, consisting of circular platinum gauze around the working electrode; L : Luggin capillary to reference electrode RE; G : inlet for gas; S : seals.

1 2 3 L 5 6

a. b.

Fig. 2.9: Electrochemical cell with separated creviced anode and cathode for crevice corrosion testing [201.

a. General set-up 1. graphite electrode connected to earth for safety precautions; 2. heating element; 3. contact thermometer; 4. cathode (in principle the same material as anode material under test); 5. holder for anode, provided with a large number of microcrevices, obtained by pressing a

6. reference electrode. rubber O-ring on a scratched metal surface;

b. Detail of anode holder: 7. PVC-pressing plug; 8. anode specimen; 9. rubber O-ring; 10. platinum wire making electrical contact with anode.

24

(e). illumination - in some special cases it might be desirable to control the light conditions at the working electrode, for instance varying between exposure under dark conditions or under illumination within a given spectral area.

(f). local corrosion- also dependingon the test involved, it maybe necessary to introduce special conditions, aimed at the creation of local corrosion attack. Well-known examples are stress- corrosion cracking and fatigue corrosion involving test cells in which the working electrode can be exposed under stressed condition (Fig.2.8), and crevice corrosion, in which case the working electrode may be provided with artificial crevices 117, 181. For some applications, e.g. crevice corrosion and bimetallic corrosion testing the corroding metal surface (anode) and the cathode have been separated in space, being electrically connected externally via an appropriate measuring instrument, e.g. a zero resistance ammeter (Fig. 2.9) 119,201.

In a number of cases the requirements as listed above are conflicting and contradictory, necessitating a compromise. However, generally it is not necessary to meet all requirements. The ultimate requirements to be incorporated in a certain cell design are of course strongly related to the purpose of the test and the test method and should be selected accordingly. In the official standards and guidelines only a few starting points can be found [e.g. 211. A number of textbooks and papers contain useful basic information 1e.g. 1,2,5,6,10,221. In the literature a large number of cells have been described. Apart from the references already given - a more of less random selection including some general information in addition is presented in refs. 123-411.

2.5 REFERENCES

1. D.T. Sawyer and J. L. Roberts: in Experimental electrochemistry for chemists, Chapter 3. Published by John Wiley & Sons, Inc. 1974.

2. N.D. Greene: in Experimental electrode kinetics. Published by Rensselaer Polytechnic Institute, Troy, New York, 1965.

3. J.E. Harrar and I. Shain: Anal. Chem., 1966, 38- 1148.

4. W.A. Mueller: Corrosion, 1 9 6 9 , s 473.

5. R. Greef et al.: in Advanced instrumental methods in electrode kinetics, Southampton Electro- chemistry Group. Published by Ellis Horwood Ltd., Chichester, U. K. , 1985.

6. J.A. von Fraunhofer and C.H. Banks: in Potentiostat and itsapplications. Chapter3. Published by Buttenvorths, London, 1972.

7. I. Gillet: Bull. Soc. Chim. France, 1962,377,

8. U. Landau, N.L. Weinberg and E. Gileadi: J. Electrochem. Soc., 1988,135, ( 2), 396.

9. R. Morrison: in Grounding and Shielding Techniques in Instrumentation. Published by John Wiley & Sons, Inc., 1967.

10. Various potentiostat instruction guides and leaflets to be obtained from manufacturers, for instance EG and G Princeton Applied Research Application note G-2: Grounding and Shielding in Electrochemical Instrumentation: Some Basic Considerations.

11, D.D. Macdonald: in Transient techniques in electrochemistry. Chapter 2.2. Published by Plenum Press, New York and London, 1977.

12. B.D. Cahan, Z. Nagy and M.A. Genshaw: J. Electrochem. Soc.,1972,119,64.

13. L. Clerbois, E. Heitz, F. P. IJsseling, J. C. Rowlands and J. P. Simpson: Br. Corros. J., 1985, X!, (3), 107.

14. A. D. Mercer and G. M. Brook: Trib. Cebedeau, 1975, Aug.-Sept., (417/418), 299.

25

15. J. Postlethwaite and R.A. Brierly: Corros. Sci., 1970, 'UJ, 885.

16. Yu.V. Pleskov and V.Yu. Filinovskii, (transl. by H.S. Wrobla): in The rotating disk electrode. Chapter 9 . Published by Consultants Bureau, New York and London,1976.

17. E.A. Lizlovs: J. Electrochem. Soc., 1970, 117,1335.

18. F.P. IJsseling: Br. Corros J., 1980,

19. I.R. Scholes et al.: in Proc. 6th Eur.Congress on Met.Corrosion. Published by the Society of

51.

Chemical Industry, London, 1977,161.

20. J.M. Krougman and F.P. IJsseling: in Proc. Intern. Workshop Electrochemical Corrosion Testing, Ferrara; 1985. Monograph vol. 110. Published by DECHEMA, Frankfurt, FRG, 1985,135.

21. A.S.T.M. G5-82: Standard Practice for Standard Reference Method for making Potentiostatic and Potentiodynamic Polarization Measurements. Published by A. S. T. M., Philadelphia, U. S. A.

22. Techniques in Electrochemistry, Corrosion and Metal Finishing, a Handbook, part A (ed. A. T. Kuhn). Published by John Wiley & Sons, Inc., 1987.

23. R.E. Geisert, N.D. Greene and V.S. Agarwala: Corrosion, 1 9 7 6 , z 407.

24. B. Gerodetti and K.H. Wiedemann: Werkst. und Korr., 1 9 7 7 , a 173.

25. J.R. Scully, H.P. Hack and D.G. Tipton: NACE Intern. Corrosion Forum, Boston, 1985, paper no. 214. Published by NACE, Houston, Tx., U. S. A., 1985.

26. T. Hakkarainen: in Proc. 8th European Corrosion Congress, Nice, 1985. Published by Soc. Chimie Indus., Paris, France 1985,26.

27. P. E. Francis and A.S. Dolphin: Br. Corros. J., 1984 ,B 181.

28. R. Manner and E. Heitz: Werkst. und Korr., 1978, a 559.

29. H. Lajain: Werkst. und Korr., 1972,23- 537.

30. T. Suzuki and Y. Kitamura: Corrosion, 1 9 7 2 , a 1.

31. A. Tamba: Br. Corros. J., 1 9 8 2 , E 29.

32. D. D. Macdonald, B. C. Syrett and S. S. Wing: in NACE Intern. Corrosion Forum, Houston 1978, paper no. 25. Published by NACE, Houston, Tx., U. S. A., 1978.

33. F. Hunkeler and H. Bohni: Werkst. u. Korr., 1981,32- 129.

34. J.R. Scully, H.P. Hack and D.G. Tipton: Corrosion, 1986,42- 462.

35. T. Suzuki, M. Yamabe and Y. Kitamura: Corrosion, 1973, a 70.

36. R. B. Diegle: Materials Performance, 1982,21, (3), 43.

37. E. Bardal, R. Johnsen and P. 0. Gartland: Corrosion, 1984, 628.

38. G. A. Gehring, Jr. and J. R. Maurer: in NACE Intern. Corrosion Conf.,Toronto, Canada, 1981, paper no. 202; Published by NACE, Houston, Tx., U. S. A.

39. D. A. Jones: Corrosion, 1984,40- 181.

40. 0. Varjonen and T. Hakkarainen: in Proc. 8th Europ. Corrosion Congress, Nice, 1885. Published by Soc. Chimie Indus., Paris, France, 1985,44.

41. L.L. Shreir: Werkst. und Korr., 1970,2l- 613.

26

CHAPTER 3

ELECTRODE DESIGN

E. HEITZ Dechema Institute, Frankfurt, FRG

3.1 I NTRO D UCTl ON An electrode may be defined as a solid electron conductor which is in contact with a liquid (solid, gaseous) ion conductor (electrolyte). At the interface charge transfer reactions take place. During corrosion processes this charge transfer involves anodic and cathodic partial reactions of various kinds which are dependent on many parameters and which have to be taken into account when designing an electrode.

Theelectrodemaybea metalor any electron-conductingmaterial, for examplea semi-conductor which acts as a source or sink for electrons.

The electrolyte usually consists of a solution of salts, acids or bases in water or protic solvents, such as alcohols, carboxylic acids, etc. [l l . Pure solvents, too, can act as electrolytes if enough conductivity by autodissociation is produced (water, methanol, ethanol, etc.). Moreover, molten salts constitute electrolytes with sometimes extremely high conductivity. It is important to state that the electrolyte should be free from any electronic conductivity otherwise no electrochemical reaction will occur at the electrode/electrolyte interface.

As discussed in a previous chapter electrodes are used as working electrodes, counter electrodes and reference electrodes. In this chapter emphasis is laid on working and counter electrodes.

Working electrodes are further divided into:

(a) Electrodes for polarisation measurement in electrochemical cells (b) Electrodes for free corrosion experiments.

Designs for working electrodes are diverse. Therefore, in the following sections only the most important design principles will be discussed.

Counter electrodes should be made from a corrosion resistant metal, for example a noble metal, and their design should allow for uniform current (potential) distribution and free convection of aggressive agents to the working electrode.

3.2 SIZE OF ELECTRODES Size and geometry are among the most important aspects to be considered in designing or choosing a particular electrode configuration [21.

In order to study kinetics and mechanisms small plates, foils, spheres, discs, rods or wires are used. They permit high current densities with minimumohmic heating or cases where high-current sources are not available. If localised corrosion has to be studied a certain minimum area must be guaranteed. The size has to be chosen so that the corrosion effect occurs with high probability on the electrode surface. This is especially valid when measuring the potential dependence of pitting, stress corrosion cracking, etc.

27

Another reason for designing electrodes of a certain size is the combination of weight loss with electrochemical measurements. Thus, very small electrodes should not be used when gravimetric measurements are required.

On the other hand, the use of microelectrodes has attracted interest in the study of corrosion effects on a microscopic scale. Microelectrodes the size of a few micrometers have been described [3,41.

Problems associated with size can be assessed by the principles for scaling corrosion tests. A report has been published by the European Federation of Corrosion Working Party on “Physicochemical methods of corrosion testing - fundamentals and application”[51 which includes information on specimen size for testing uniform, bimetallic and pitting corrosion and stress- assisted environmental cracking.

3.3 DESIGN WITH REGARD TO VARIOUS CORROSION PARAMETERS

3.3.1 Current and potential distribution In principle, current measurements only give integral values as current per total electrode surface exposed. On real electrode surfaces the current distribution is more or less non-uniform. This holds particularly for cases of non-uniform corrosion attack.

Since measurement of the current distribution is only possible with considerable effort (segmented electrodes), some general rules for minimizing non-uniformity will be presented (for theoretical background, see refs. [5 - 71).

If a working electrode (length L) with a parallel counter electrode (distance h), as in Fig. 3.1, is considered the current distribution is the more uniform the larger the ratio L/h. From this the following recommendations can be given:

-a design should be chosen with working and counter electrodes as close as possible (taking into account possible interference of anodic and cathodic reaction products)

-the smallest electrodes possible should be selected (without interfering with statistical effects, section3.2).

From the concept of the dimensionless Wagner Number [5, 71 the following conclusions are derived:

- if possible, low conductivity solutions should be avoided - polarisation resistances should be as high as possible.

This last statement means that electrodes producing higher currents during polarisation show a more non-uniform current distribution, since high currents are correlated with low polarisation resistances.

The rulescited give some useful working/counter electrode designs which are shown in Fig. 3.2.

It should also be mentioned that the cylindrical geometry has also been successfully used to minimize ohmic drop in media of low conductivity.

When using plate electrodes it is necessary to consider whether the reverse side should be insulated. This is not necessary in systems with high conductivity and with sufficient distance between the electrode and the cell wall. Insulated plate electrodes produce problems as a result of crevice formation. The best solution is, however, to provide the plate electrode with two counter electrodes situated on both sides of the plate.

Reference electrodes are generally used together with Haber-Luggin capillaries (for details, see Ref. [2]).The design and position of these capillaries pose current and potential distribution problems. In order to minimize ohmic drop they have to be placed as close as possible to the electrode surface. But if the distance is too small they act as a current shield and non-uniform current distribution arises. In practice the tip of the Luggin probe should be at a distance of about 2 d from the working electrode where d is the external diameter of the capillary.

28

.... /..#.. ......... ...

P r t-

Fig. 3.1: Current distribution on parallel plate electrodes of length L and distance h

9

cylindrical geometry

? Q Q

rotating disc parallel plate (parallel plane arrangement)

Fig. 3.2: Electrode design with various geometries to minimize non-uniformity of current distribution

3.3.2 Mass transfer Mass transfer influenced corrosion reactions can only be investigated on electrodes or specimens which are exposed under well-defined hydrodynamic conditions. A number of electrode designs used in the past are shown in Fig. 3.3 [8,91.

For basic studies the most suitable type is the rotating disc which is either placed in the end face of a rotating cylinder, Fig. 3.2, or is used as a free rotating system with an integral shaft, Fig. 3.3 [71. Further models are the rotatingcylinder, free or co-axial [ll, 121, and pipe and channel flow [7,141.

Compared with the situation with the rotating disc channel and pipe flows will generally become turbulent at relatively low flow rates although the extent to which this happens will depend on the dimensions of the cross-section (characteristic length). Their practical advantage is that they affordbetter access to themeasurement section thando rotatingmodels,but the flow isnot soeasily achieved. The test pieces are arranged and the dimensions chosen so that the test pieces are electrically insulated and inserted flush in the flow path. Care must be taken to ensure that the join between the wall of the pipe and the test piece is hydrodynamically smooth, this being particularly necessary with the flow channel. Another possibility is to construct the test piece itself as a rotating entity. This is the case, for instance, with the rotating disc and the rotating cylinder.

A special arrangement is the ring-disc electrode which is currently often used for kinetic and mechanistic studies [13]. In this case the rotating disc is surrounded by a ring electrode which is separated by a thin non-conducting gap.

For measurements in disturbed turbulent flow which is important for many practical applications the segmented pipe technique has been successfully applied [14,15]. The pipe is divided into rings which are arranged in electrical insulation from each other, permitting local mass loss measurements and electrochemical measurements along the pipe axis.

29

I

@- Rotating disc Disc i n casing Rotating Coaxial cylinder Flow in Flow in

cylinder a channel a pipe

Fig. 3.3: Electrode arrangements for investigation of flow dependent corrosion [8,91

Counter electrode Referenrn elnrtrodn- I .

‘R I Cooler

Heater

Holder

Thermosensor

PTFE Cover

. . - . -. -. . - - - . - - _. - - - --..

PTFE Container

working electrode/ specimen

Thermal insulation

Corrosive medium

Conductivity sensor

Fig. 3.4: Electrodes for CERTexperiments in a PTFE cell for aggressive corrosionconditions 1211

3.3.3 Heat transfer The simultaneous measurement of heat transfer and electrochemical quantities has been described in the literature [16-181. Tubular and disc-shaped specimens proved to be suitable although difficulties arose in obtaining uniform heat transfer on the surface. For example, it was found that crevice corrosion can occur in the side insulation of disc electrodes. This effect may be attributed in some cases to the decreased heat flux and therefore higher surface temperature on the outer circumference of the specimen.

30

Electrodes of the "cooling finger" type could also be used for controlled heat flux experiments. Specific experience with a rotating cooling finger has been made available 1191 and a combination of ring-disc with heat transfer measurements has recently been described [201.

3.3.4 Mechano-chemical testing Mechano-chemical testing comprises the suitable application of stress, pressure, bending, thrust and torsion on specimens in specific media either in a time constant or cyclic mode. There are no special problems in applying potentials or currents to such specimens provided suitable provisions for electrical contacts and electrode insulation are made.

As an example, provisions for electrochemical measurements during CERT (Constant Exten- sion Rate Testing) are shown in Fig. 3.4 [21,221.

Special requirements for this set-up are that for standardized specimens for CERT the insulated specimen holder should be of the same material as the test material and there should be low friction sealing of the specimen holder.

Standardized specimens for CERT are conventional cylindrical specimens, notched or smooth, which are loaded at a slow strain or continuous extension rate 1231.

In order to avoid bimetallic corrosion the specimen holder of the same material has to be insulated by a polymer coating. More noble material can be used if bimetallic corrosion is absent, but this has to be proved by separate electrochemical experiments.

During CERT an extremely slowly moving specimen has to be sealed. A design as shown in Fig. 3.4 may be chosen.

3.3.5 High temperature/high pressure testing Testing under high temperature/high pressure conditions is done in normal autoclaves or in refreshed autoclaves. There are a number of problems to be overcome, which include sealing and insulation of the electrodes, the choice of a suitable (pressure and temperature stable) reference electrode and possibly control of the parameters discussed above.

Relevant reviews [24, 251 and special papers on technical solutions to problems exist, as, for example, for high temperature reference electrodes [26,27l, apparatus with exchangeable electrodes for measurements up to 330°C and 300 bar [281, CERT experiments under high pressure and temperature [29] and electrochemical measurements for hydrogen permeation up to 600 bar [30, 311.

3.4 PROBLEMS CONCERNING APPLICATION TECHNIQUES Essential experimental problems to be overcome are [21:

(a) defining the working area (b) making good electrical contact (c) holding the electrode rigidly.

If an electrode is exposed in an electrochemical cell it is necessary to know its working area. Defined working areas may be achieved by theuse of coatings or compression gaskets. A systematic comparative study of a number of systems revealed that hot-cured epoxy resin and compression type gaskets are relatively the best solutions 1321. One of the problems encountered with pas- sivating systems is crevice corrosion at the threephase boundary electrode-coating- electrolyte. Due to the low ratio of volume/surface in the crevice small anodic polarisation or even free corrosion leads to preferential attack in the crevice. Coatings and gaskets free of crevices therefore have to be applied. A survey of the literature on attempts to overcome this problem has been published 121.

In making electrical contact to electrodes the resistance of the contact between the specimen and the current lead should be as low as possible. Pressure contact is often used, but problems may arise as a result of passivation or contamination of the contact if an electrolyte should enter the contact

31

area. Spot welding or soldering are attractive solutions, but care must be taken to avoid preferential attack at the contact. It is useful to protect the contact area by PTFE tape or an electroplater's lacquer[21.

If the current lead is a noble metal, for example platinum, and if the working electrode is a metal in the active state, it is possible to expose the noble metal wire to the electrolyte provided its area is small compared with that of the working electrode and its polarisation resistance larger than that of the working electrode.

Electrode holders need to be held rigidly in place and should be easily removable. Ground glass jointsor screw capadapters withcompression ringsareuseful solutions forsimplelaboratorywork. Descriptions of techniques for cases of extreme conditions of temperature, pressure, flow and mechanical load are described elsewhere in this volume.

3.5 REFERENCES

M. Fontana and R. Staehle), London, 1974, & 149. 1. E. Heitz: in Advances in Corrosion Science and Technology. Published by Plenum Press, (ed.

2. A. T. Kuhn: Techniques in Electrochemistry, Corrosion and Metal Finishing. Published by John Wiley & Sons, Inc., 1987.

3. A. M. Bond, M. Fleischmann and M. Robinson: J. Electroanal. Chem. 1984, 168.299.

4. R. T. Atanasoski, H.S. White, W.H. Smyrl: J. Electrochem. Soc. 1986,133,2435.

5. L. Clerbois, E. Heitz, F.P. IJsseling, J.C. Rowlands and J.P. Simpson: Br. Corr. J. 1985, a 107.

6. N. Ibl: Current Distribution, in Comprehensive Treatise of Electrochemistry (Eds. E. Yeager, J. 0. M. Bockris, B. Conway, S. Sarangapani) Vol. 6, Plenum Press, 1983,b.

7. E. Heitzand G. Kreysa: Principlesof Electrochemical Engineering; Published by VCH-Verlag, Weinheim, 1986.

8. U. Lotz and E. Heitz: Werkst. u. Korr. 1983,_34,454.

9. DIN Specification 50920 Corrosion Testing in Flowing Liquids: Beuth Verlag, Berlin, 1987.

10. N. Ibl and 0. Dossenbach: Convective Mass Transport, see Ref. [61.

11.0. Dossenbach: Ber. Bunsenges. Phys. Chemie 1976, 34.

12. E. Heitz, G. Kreysa and C. Loss: J. Appl. Electrochem. 243 (1979)

13. Instrumental Methods in Electrochemistry; Southampton Electrochemistry Group. Pub- lished by Ellis Horwood Limited, Chichester, U. K. ,1985,113.

14. U. Lotz, M. Schollmaier and E. Heitz: Werkst. u. Korr. 1985, 36- 163.

15. T. Kohley and E. Heitz: in The Use of Synthetic Environments for Corrosion Testing', STF 970. ASTM, Philadelphia, U. S. A., 1988, 235.

16. Corrosion under Heat-Transfer Conditions: MTI Publ. Nr. 17, Materials Technology Insti- tute, Columbus, Ohio, U. S. A., 1985.

17. Ya.M. Kolotyrkin, V.S. Pakhomov, A.G. Parshin and A.V. Checkhovski: in 'Proceedings of the 9th International Congress on Metallic Corrosion', NACE, Houston, Tx, U. S. A., 1984,z 1.

18. M. Yasuda, M. Okada and F. Hine: Corrosion - NACE, 1982,38- 256,

19. M. Shirkhanzadeh, V. Ashworth and G.E. Thompson: Electrochim. A. 1988,33- 265.

32

20. Arbeitsgemeinschaft Korrosion e.V., Arbeitsblatt Wl, Werkst. u. Korr. 1988,B.

21. H.-G. Fellmann, H. Kalfa, U. Schare and E. Heitz: Werkst. u. Korr. 1 9 8 9 , a 34.

22. E. Heitz, R. Henkhaus and A. Rahmel: Korrosionskunde im Experiment, Verlag Chemie, 1983, (English edition in preparation).

23. DIN Specification 50922, Untersuchung der Bestindigkeit von Metallen gegen Span- nungsrisskorrosion, Beuth-Verlag, Berlin.

24. G. Jones, J. Slater and R.W. Staehle (Eds.): HighTemperature/ High Pressure Electrochem- istry in Aqueous Solutions; NACE, Intern. Corrosion Conference Series No.4. NACE, Houston, Tx, U.S.A., 1973.

25. J.V. Dobson: EMFMeasurements at Elevated Temperatures and Pressures; Adv. in Corr. Sci. and Technol. (Eds. M. Fontana, R. Staehle); Plenum Press, 1980,2,177.

26. P.D. Macdonald: Corrosion, 1978,34- 75.

27. M. Hishida, H. Takabayashi, T. Kawakubo and Y. Yamashina: Corrosion, 1985,4l- 570.

28. M.L. Brown and G.N. Walton: J. Appl. Electrochem.l976,6- 551.

29. H. Hurst, D.A. Appleton, P. Banks and A.S. Raffel: Corros. Sci. 1985,26- 651.

30. D. Festy: Proc. Eurocorr. 87, DECHEMA, Frankfurt, 1987,641.

31. G. Schmitt : in ”Wasserstoff und Korrosion” (Ed. D. Kuron), p. 332, Verlag Irene Kuron, Bonn/FRG (English translation in preparation).

32. N. D. Greene, W.D. France Jr. and B.E. Wilde: Corrosion, 1965,2l- 275.

33

CHAPTER 4

REFERENCE ELECTRODES E. ERIKSR UD

Veritas Research, Hovik, Norway

AND

E. HEITZ Dechema Institute, Frankfurt, FRG

4.1 INTRODUCTION Thebookeditedby Ivesand Janz [ll andmorerecently thatbyBard,Parsons,and Jordan[21 contain both theoretical and practical aspects related to reference electrodes. Preparation, application and limitations of various types of reference electrodes such as the hydrogen electrode, the calomel and other mercury-mercurous salt electrodes, the silver-silver halide electrodes, and sulfide and sulfate electrodes are covered and general reference to these excellent critical reviews is recommended.

The role of the reference electrode in electrochemical studies is to provide a fixed potential which does not vary during the experiment. In most cases, the potential of the reference electrode relative to an agreed standard, for example to the normal hydrogen electrode, is required. In other cases it is only necessary for the reference electrode to remain at the same potential during the experiment, for example during a linear polarization resistance or a potential sweep experiment.

4.2 CHOICE, STABILITY, INCOMPATIBILITY In corrosion tests all the types of electrodes listed above may be used depending on the corrosive environment and the aim of the test.

If possible the reference electrode should be designed to be as similar as possible to the system under investigation. This is both to minimize contamination of the reference electrode (e.g. chloride ions diffusing into the sulphate solution of a copper/copper sulphate reference electrode will change the potential of the reference electrode) and to avoid misleading corrosion test results by ions from the reference electrode entering the test solution (e.g. chloride ions in a pitting corrosion test in sulphate solution). With large differences in the ionic concentration of the test and reference electrode solution, a ”liquid junction potential” may make an essential contribution to the potential measured between the working and reference electrodes. However, this is usually not a serious error in corrosion tests.

It should be pointed out, that if a liquid junction potential is minimized by using a common test solution and solution for the reference electrode, e.g. by dipping a Ag/AgCl wire directly into the test solution containing chlorides, the potential of the reference electrode may vary with concentration of the test solution. For the Ag/AgCl electrode the change will be roughly60 mV per decade change in chloride concentration.

4.3 CHECKING OF THE REFERENCE ELECTRODE - SOME PRACTICAL ADVICE It is good practice to have reference electrodes to be used only for checking the reference electrodes used in the tests. Preferably, three saturated calomel reference electrodes should be kept for this

34

purpose. These should be within 2 1-2 mV of each other when checked for example in a chloride solution. These electrodes should be stored in a dry condition for periods of non use, with a rubber sealing ring over the filling hole and a black rubber cap on the immersion tip during storage.

The working reference electrode should be checked both before and after a test. In long term tests, it should also be checked during the experiment.

When using saturated KC1 solution bridged electrodes it is essential that the solution contains undissolved KCl to ensure that it remains saturated. Air bubbles must not be present inside the electrode. If KC1 crystals seem to trap air bubbles in the electrode and these cannot be removed by shaking [31, the working tip should be immersed in warm distilled water, until most of the crystals are dissolved. The air bubbles can now be removed by shaking the electrode. The inside of the electrode should never be flushed with anything but saturated KCl solution.

Recently, for environmental reasons Ag/AgCl reference electrodes are becoming more widely used as they contain no mercury or mercury salts.

4.4 MORE SPECIFIC REQUIREMENTS In practice the main requirement of a reference electrode is that it hasa stable potential and that this is not substantially changed during the experiment. This is the case with the hypothetical, completely non-polarizable electrode, the potential of which is unaffected when electric current flows across the metal-solution interface. For practical conditions this means that the exchange current must be large compared with any net current that it is required to pass in use. Ideally ”no” current flows through the reference electrode (in a three electrode system) if a high impedance (>lOMn) voltmeter is used.

If temperatures and/or pressures other than 25 “C and 1 atmosphere are to be used, the stability of the reference cell with respect to such environments must be ensured.

In line with requirements for the cell in general, the ohmic resistance of the reference electrode should be reduced to a minimum in order to make the best use of the sensitivity of the recording instruments. It may be tempting to use thin capillaries to prevent interdiffusion of the electrode solutions, but thisis often found to result inan intolerable lossof sensitivity. Theconflict of interests may sometimes be resolved by inserting a wide-bore tap, opened only when measurements are to be made, between the electrode vessels. Unobserved gas bubbles trapped in “solution bridges” are sometimes responsible for high cell resistance.

A lowering of the internal resistance of the electrode by eliminating ceramic frits and porous plugs, also has the advantage of a faster response time [4] which is particularly important in the study of fast electrode reactions using transient techniques.

4.5 SOME COMMON ELECTRODES FOR AQUEOUS SYSTEMS [4]

4.5.1 Mercury-mercurous chloride (calomel) This is probably the most widely used reference electrode. It is usually made with a saturated aqueous potassium chloride solution bridge, although 1 mol/dm3 and 0.1 mol/dm3 solutions also are commonly used (see Table 4.1). Calomel electrodes can have very low resistance and good performance. For this reason they are frequently used for checking other types of electrodes.

4.5.2 Mercury-mercurous sulphate This electrode corresponds to the calomel, with sulphate instead of chloride. It is useful for sulphate solutions, but becomes unstable if the sulphate concentration falls below 0.1 mol/dm3.

4.5.3 Mercury-mercuric oxide This electrode is recommended for use in alkaline solutions.

4.5.4 Silver-silver halide These electrodes give very stable potentials in halide solutions provided the halide concentration is not too high, when the increasing solubility of the silver salt causes problems [2]. They are easily prepared in the laboratory, but must be shielded to exclude light and contamination should be avoided.

35

~~ ~~

Table 4.1: Potentials and fields of application of various reference electrodes

Electrode Electrolyte Potential 'Brnpe- Tempe- Field of system v s SHE rature rature application

coefficient

m V O C mV/'C

at 25OC range

Hg/Hg, C1 2 / C 1- KC1. sa t . + 242 0 to 70 -0 .65 general

Hg/Ngz Cl 2/C 1- ICC1 1 M + 280 0 to 7 0 - 0 . 2 4 general

0 to 70 - 0 . 0 6 general HglHg2Cl2/C1- K C 1 0 . 1 M + 334

Hg/Hg2S04/Sd- K2SO4 sa t . + 640 0 to 70 - sulfate con- 4 taining media

Hg / H go / OH- NaOH 1 M + 98 - - alkaline media

AgIAgClIC 1- KC1 0 . 1 M + 288 0 to 95 - 0 . 5 general

AgIAgC1 I C1- KC1 3 M + 207 80 to 130 - 1 . 0 0 hot media

HgTI/TlCl KCl 3.5 M - 507 0 to 150 0 . 1 hot media

In Table 4.1 the temperature range of applicability, the temperature coefficient and the fields of application of various reference electrodes are given.

4.5.5 Reference electrodes in non-aqueous systems All reference electrodes are prepared with aqueous solutions. Their use in non-aqueous systems is possibleaslongasliquid junction potential sat theaqueous/non-aqueoussolutionphaseboundary are small. This is the case for primary alcohols such as methanol, ethanol, isopropanol and for dioxan solutions. The standard electrode potentials of the calomel electrode and the silver/silver chloride electrode have been determined in such solutions. If potentials in aprotic (non water-like) solvents have to be measured, difficulties arise. More detailed discussions are given in refs. [SI and El.

4.6 REFERENCES

Press, 1961. 1. Reference Electrodes, Theory and Practice, Edited by D.J.G. Ives and G.J. Jam, Academic

2. A. Bard, R. Parsons and J. Jordan, Standard potentials in aqueous solutions. Marcel Dekker, New York and Basel, 1985.

3. Fixed Offshore Installations, Monitoring of cathodic protection systems, Technical note TNA 705, Det norske Veritas. April 1984.

4. Instrumental Methods in Electrochemistry, Southampton Electrochemistry Group. Pub- lished by Ellis Horwood, Chichester, U.K., 1985

5. E. Heitz, Corrosion of metals in organic solvents from "Advances in Corrosion Science and Technology" (ed. M. Fontana and R. Staehle), Vol. 4, Plenum Press, 1974.

6. C. T. Mussini and F. Mazza in "Electrochemical Corrosion Testing", Monograph 101. Published by DECHEMA, Frankfurt, FRG (ed. E. Heitz, J. C. Rowlands and F. Mansfeld) 1986, pages 67 and 79.

36

CHAPTER 5

EFFECTS OF SPECIMEN PREPARATION AND SURFACE

CONDITION J . SIMPSON

Sulzer Bros, A. G., Winterthur, Switzerland

5.1 INTRODUCTION The state of the surface at the time of measurement can have a large influence on the values of the electrochemical parameters being measured. The surface state is dependant upon the whole preparation history right up to the moment of measurement.

The stages which must be considered are:

(a) Choice of specimen material (b) Sampling and specimen preparation (c) Surface preparation before immersion in the test solution (d) Effect of the conditions within the cell before commencement of the measurement (e) Changes caused by the measurement itself.

From the nature of the subject, it is not possible to treat this topic exhaustively within these guidelines; this section is intended to increase the reader’s awareness of the many factors in the preparation stage which can influence subsequent electrochemical corrosion measurements.

5.2 CHOICE OF SAMPLE It is important that the metallurgical condition and chemical composition of the sample are well defined. These parameters can vary considerably depending on manufacturing process and heat treatment .

A further problem is that a material is rarely homogeneous. The choice of the source material for the specimen will largely depend upon the aims of the experimental work. When studying corrosion behaviour for practical applications, samples taken from components or material with a similar production history are preferable. On the other hand, if material is simply taken from the next available source of material of nominally the same specification, it may not be truly represen- tative of the material as used in practical applications.

5.3 SAMPLING AND SPECIMEN PREPARATION Metallurgical structure can vary considerably with position and orientation of the sample.

Castings and weldments are obvious examples where structure and chemical composition can vary considerably. Even wrought material, whether drawn, extruded, rolled or forged, is very rarely homogeneous; these materials will commonly have a texture (i.e. a preferential grain orientation) and other directional features such as elongated grains and aligned elongated inclusions. Through section variations must also be considered.

37

The surface composition can differ from the bulk after heat treatment: processes such as decarburization, nitriding, surface oxidation with diffusion of dissolved oxygen into the metal (often observed in titanium alloys), or depletion of an alloying element at the surface, can give the surface very different properties to the bulk material. In such cases it must be decided whether the properties of the modified surface or the bulk material are of interest .

It is important to bear such points in mind before sampling and preparing the specimen for mounting. Specimen sampling procedures, orientation, surface preparation and specimen mounting techniques should be decided before sectioning the material rather than later.

To illustrate the above points, one may consider the pitting potential of stainless steels in chloride solutions. The value of this potential is strongly dependent upon the degree of cold work and the orientation of the sample. A decrease in pitting potential was demonstrated for AISI-316L stainless steel in physiological saline solutions with increasing degree of cold work.The pitting potential was lower in the transverse than in the longitudinal direction. Such anisotropy may also be present in hot worked and annealed samples due to texture effects [ll. Differences in electrochemical behaviour with cold work and specimen orientation were also noted for austenitic stainless steels in acid solutions; the critical current density for passivation in 1M I-$SO,at 25°C was 10 times greater in the transverse than the longitudinal direction at 30% cold work and the passive current density in 0.1M HC1 at 25°C differed by up to 1000 times depending upon specimen orientation [21.

Methods used to prepare the sample should not produce any changes in its condition. The most common problems are heat generation on separation (e.g. from high speed abrasive cutting wheels) and cold work on machining, grinding, shearing or cutting. In general standard metallurgical specimen preparation techniques are adequate for separation from bulk material. Water cooled abrasive high speed wheels for cutting down to the final size are suitable in most cases. For fragile or extremely heat sensitive specimens a cold slow speed diamond wheel is often thebest alternative.

Specimen size and mounting are discussed in Chapters 2 and 3 as they are highly dependent upon cell design and the type of measurement envisaged.

5.4 SURFACE PREPARATION BEFORE IMMERSION IN THE TEST SOLUTION The specimen surface is often prepared before measurement to ensure a reproducible and known surface condition. Metallurgical surface preparation techniques are commonly used, these include: wet grinding on silicon carbide abrasive papers, polishing with diamond or alumina media, electropolishing, chemical polishing, pickling or etching.

Electrochemical corrosion measurement results can be strongly dependent upon surface preparation technique. The causes can range from a simple surface area effect of different surface treatments, through secondary effects of the surface preparation technique on the substrate, to chemicalchanges on the surface duringsurfacepreparationand during the timebetween preparation and immersion in the test medium.

Mechanical preparation techniques such as grinding can introduce significant cold work into the surface layers e.g. the pitting resistance of ground austenitic [31 and ferritic stainless steel surfaces [4] has been shown to be inferior to that of electropolished surfaces. This was attributed to the presence of cold worked surface layers from grinding, although chemical or electrochemical surface treatments can preferentially remove less resistant phases, e.g.inclusions, which would otherwise be responsible for an inferior corrosion performance .

A point often neglected is the handling procedure between preparation of the surface and immersing the specimen in the test medium. For instance, oxide film formation on oxide-passive materials and or tarnishing layers on copper or iron alloys formed at this stage can influence the electrochemical behaviour considerably; these chemical changes on the surface depend on such factors as temperature, humidity and time [5].

38

5.5 EFFECT OF THE CONDITIONS WITHIN THE CELL BEFORE COMMENCEMENT OF THE MEASUREMENT The time between placing the specimen in the corrosion cell and the commencement of the measurement itself must also be counted as part of the surface preparation procedure. The value of the electrochemical parameter sought may vary considerably if insufficient attention is paid to this point [6-81. It is preferable to consider this period as part of the measurement itself, reducing the chances of random variations.

For measurements made at or near the free corrosion potential (e.g. polarization resistance or some impedance measurements), the most important factors to be considered are:

(i) Control of the medium; e.g. temperature, composition and agitation. For instance, the rest potential of mild steel in aerated salt solutions is dependant upon the degree of agitation [61, in this system even convection maybe significant, as would consumption of dissolved oxygen by the steel.

(ii) Time; prepared surfaces are rarely in their stable surface state with respect to the medium when immersed, for instance the zero current potential of a mild steel sample in aerated 0.01 M NaHCO, at 65°C changed from-400mV (S.H.E.) to +240 mV after 100 minutes and 6days immersion respectively due to the formation of surface oxides [71.

The situation becomes more complex when any in situ surface pre-conditioning techniques are employed; often cathodic or anodic pretreatments are used. These should be treated with caution. Cathodic treatments can reduce oxide films and introduce hydrogen into the metal, both of which can effect any subsequent electrochemical measurement [7,8]. Anodic treatment may passivate the surface or roughen it by anodic dissolution.

5.6 CHANGES CAUSED BY THE MEASUREMENT ITSELF It is important to distinguish between changes in the surface condition caused by the corrosion process and those forced on the specimen by the measurement technique. The aim may be to monitor the former, the latter may lead to misleading readings or interpretation problems.

Many electrochemical corrosion techniques are perturbation techniques, i.e. either an external potential or external current is applied. These can modify the surface, be they anodic causing dissolution or passivation, or cathodic causing surface oxide reduction or hydrogen production.

The effect is normally one of degree. The measurement of a corrosion potential does not influence the surface condition. Electrochemical noise and impedance measurements carried out at the corrosion potential also have little effect as does a polarization resistance measurement if the perturbation is small, although rest potential drift may be a problem if potential control techniques are used. Techniques involving large potential differences will in general modify a surface significantly.

It should be borne in mind when repeating an experiment without using a fresh specimen or fully repreparing the surface, that the surface condition is unlikely to be the same as the initial state, it will have been modified either by the exposure to the medium or by the measurement technique.

5.7 REFERENCES 1. A. Cigada, B. Mazza, P. Pedeferri and D. Sinigaglia: J. Biomed. Mater. Res.,1977, ll- 503.

2. B. Mazza, P. Pedeferri, D. Sinigaglia, A. Cigada, G. Fumagalli and G. Re: Corros. Sci., 1979,19,907.

3. R. W. Revie and N. D. Greene: Corros. Sci., 1969,a 763.

4. R. P. Frankenthal: Corros. Sci., 1968,& 491.

5. D. E. Dobb, J. P. Storvick and G. K. Pagenkopf: Corros. Sci., 1986,26- 525.

6. D. M. Brasher: Br. Corros. J., 1967,295.

7. G. K. Glass: Corros. Sci., 1986, & 441.

8. C. D. Kim and B. E. Wilde: Corros. Sci., 1970, 735.

39

CHAPTER 6

EVALUATION AND COMPENSATION OF OHMIC DROP

L. CLERBOIS Solvay et Cie, Brussels, Belgium

AND

F. P. IJSSELING Corrosion Laboratory of Royal Netherlands Naval College,

Den Helder, The Netherlands

6.1 INTRODUCTION Electrochemical systems generally contain a working electrode (WE), an auxiliary or counter- electrode (CE) and a reference electrode (RE).

During current flow the voltage difference between WE and CE consists of two parts : electrode polarization and ohmic drop through the solution (IR).

The potential measured between WE and RE may be substantially influenced by an IR drop, in particular if high current densities are applied or an electrolyte of low conductivity is used. This is due to the fact that the reference electrode is connected with a point in the solution some distance away from the electrochemical double layer. Thus an ohmic resistance - the so-called uncompensated resistance (Run> -is included in the solutionbetween the tip of the reference electrode and the surface of the working electrode (Fig. 6.1). As a result an error will be introduced in the measure- mentof the potentialdifference in sucha way that the potential differencebetween the working and reference electrodes is not as large in an absolute sense as indicated by the potentiostat or an auxiliary voltmeter.

The importance of knowing the exact value of the ohmic drop or uncompensated resistance in an electrochemical system has been pointed out by many workers. In studies of the kinetics of electrode processes by potentiostatic techniques, the ohmic potential drop produces a distortion of the steady state polarization curve which, if uncorrected, will yield erroneous values of the characteristic parameters (Tafel slope, reaction orders) of the electrode reactions (Fig. 6.2).

Also, measurements of the polarization resistance, R might be subject to considerable errors due to the ohmic resistance which may lead to an underestimation of the corrosion rates by up to several hundred percent.

PI

The effect of uncompensated IR drop on corrosion rate determination using polarization resistance measurements was discussed in depth by Mansfeld [1-31. He showed that in electrochemical measurements of the polarization resistance the experimental value R * is the sum of the true value R and the uncompensated ohmic resistance RU,, which is essentially the electrolyte resistance but can also contain the resistance of surface films.

P Po

40

T

m electrolyte

WORKING I ELECTRODE I

I WE

@

CE e+

RE o X

P Fig. 6.1 : (a) 3-terminal cell schematic; (b) detail showing Runt between surface of WE and iso- potential plane of RE; (c) equivalent circuit. Z = impedance of working electrode

Rmc = uncompensated resistance Rs = solution resistance

Fig. 6.2 : Effect of ohmic drop on the shape of polarization curves. The values of parameters are ba = 30 mV, bc = 120 mV, IC = 0.1 mA/cm2 [l, 21.

Since

R * = Rpo + Run, (1) P

the relative experimental error is

Even if Runc is low, the error can be appreciable if R is also low, e.g., in systems with high corrosion rates. The same magnitude of error can be found for systems with low conductivity and low corrosion rates. It is, therefore, not a question whether the absolute value Runc is low, but whether the value Run, / Rpo is low. Similar equations were derived by other authors, e.g. Rocchini MI.

P O

41

Also in crevice and pitting corrosion, the ohmic potential drop may be responsible for the stability of local attack on passive surfaces.

The subject is treated in most textbooks and introductions to electrochemical experimentation (for instance refs.[5-14]) and two literature reviews have been published 115,161.

6.2 EXAMPLES Three examples showing the importance of ohmic drop correction are given below.

6.2.1 Polarization resistance Mansfeld showed that even in highly conducting solutions involving high corrosion rates, the IR drop, although small, cannot be disregarded, since the polarization resistance and RmC are of the same order of magnitude. This is in particular true for the corrosion rate determination of carbon steel in acid solutions [l-31. The dangerous consequence of neglecting Runt is that corrosion rates willbeunderestimated. AnexamplecanbeseeninFig. 6.3: without correction thecorrosioncurrent density was very low, whereas weight loss showed a high corrosion rate. After applying a correction for the ohmic drop a significantly increased corrosion current density was found, which correlated better with the corrosion rate obtained by weight loss.

4

3

2

1 h

Tu E

4 \ O O

E -1 v

H -2

-3

-4

c o r r e c t e d i i a

- #' experimental .................................................. * .................................................

-300 -200 -100 0 100 200 300

Fig. 6.3 : Effect of ohmic drop on the shape of the polarization curve for the system : low alloy steel + EDTA 100 g/l; pH = 6, T = 100 "C, exposed area = 60.5 cm2, flow rate = 1 m/s [171

6.2.2 Polarization curves in aqueous solution Mansfeld [ 1,2] also pointed out that the lack of curvature in experimental polarisation curvescould be a result of a high resistance. Some results of theoretical calculations have been shown in Fig. 6.2.

A practical example is the case of gas bubbles reducing the conductivity of the solution. Gas evolution from vertical or inclined working electrodes produces a varying electrolyte resistance, with the effect being greatest at the top of the electrode. Electrolyte resistance measurements made during gas evolution may therefore have little meaning.

42

A cathodic polarization curve for hydrogen evolution on platinum in 2.8 M sulfuric acid is shown in Fig. 6.4.

1.2

1.0

.8

“E .6

a .-- . 4

: .2

u

- K

3 u 0

-.150 -300 -,450 0.600 9.750 -.900 -1.050

potential, U, volts Fig. 6.4 : Polarization curve during cathodic hydrogen evolution (linear plot without elimination of ohmic drop)

Due to a significant IR-drop, this is nearly a straight line. Elimination of IR-drop yields over a large range of current density a semi-logarithmic straight line, with a Tafel slope of 27 mV. Such a result which gives also a reasonable Tafel slope at high current densities up to 1 A/cm2 may be quoted to demonstrate the reliability of the method (Fig. 6.5).

. 2 0 . 0 -. 2 -. 4

9 - . 6 5 - . e 4- -1.0 0) -1.2 -- c. -1.4 2 -1.6 5 -1.8

-2 .0 -2 .2

.- 0

C

L-

/. - - b = 27 m V

l o g i = -3.’1

/ .- L

-.225 ,240 7255 -270 ,285 -300

potential (U-IR), volts (SCE)

Fig. 6.5 : Tafel plot of IR-drop corrected polarization curve of Fig. 6.4

6.2.3 Polarization curves in non aqueous solution Depending on the conductivity of the solutions the ohmic drop may vary considerably in organic solvents. Whereas in solutions of primary (waterlike) alcohols such as methanol, ethanol (EtOH) and propanol polarization curves with ohmic drop corrections can be obtained it is impossible to make similar measurements in non water-like solvents such as long chain alcohols, halogenated hydrocarbons and other aprotic solvents.

43

Figure 6.6 shows an example for Fe in 0.01N HCl/EtOH with and without IR-drop compensation. Without IR-drop compensation, a straight line is observed over the entire range of polarization. WhentheIR-dropiscompensated,curvature isseenand the shape of thecurveindicates that ba < bc . Without compensation of the ohmic drop Rp* was 357 ohms. On applying a compensation technique Rp” was found to be 74 ohms, R,, being 212 ohms.

212

74 The error e = - = 2,86 or almost a factor of three.

Runc =: 2120 R: = 7 4 0 1-150

Fig. 6.6 : Experimental polarization curve for Fe in 0.01 N HCl/EtOH with and without IR-drop compensation [21

6.3 PRINCIPAL METHODS Although ohmic drop cannot be eliminated completely it can be minimized, while the remaining effect can be taken into account, either by active methods, e.g. by some means of instrumental compensation or passivelv, by calculation and subsequent correction.

In practice a combination of these methods is often used i.e., a good cell design to minimise the ohmic drop, instrumental compensation of the greater part of the remaining error and, finally, removal of the last part by evaluation, calculation and correction of the experimental data.

6.3.1 Minimizing of ohmic drop by cell and electrode design Thus, the first step of any remedial action will usually be to minimize the value of the uncompensated ohmic drop. The design of the cell and the electrodes are the principal means of achieving this objective.

Both subjects have already been treated in these guidelines and reference should be made to Chapters 2 and 3.

6.3.2.1 Cell design Generally cell design is dedicated to obtaining a constant current density over the surface of the working electrode in combination with the use of a Luggin capillary for measuring the potential difference between the surface of the working electrode and the solution. As the distance between the tip of the Luggin capillary and the electrode surface decreases, so will the ohmic drop error. However, the distance cannot be made very short without introducing screening effects on the working electrode surface.

In practice a capillary tip of outer diameter d may be placed as close as 2d from the electrode surface with negligible shielding error [181.

44

Figure 6.7 shows a schematic diagram of the equipotentials in question for the case of capillary of diameter d placed 2d from a planar electrode; it can be seen that in this particular case the potential that is sampled corresponds to anequipotential surface that is positioned 5d/3 away from the electrode [191.

E l .ec tro de

Desired Potential 7

Measured Potential

Capillary

b

f d

Fig 6.7 : Schematic diagram of IR-drop in the electrolyte between capillary tip and electrode

The IR-drop not only depends on the diameter of the Luggin capillary and its distance from the working electrode, but also on the specific conductivity of the solution and the geometry of the working electrode. In Table 6.1 the IR-drop has been calculated for some simple geometries, assuming constant specific conductivity [71.

Table 6.1 Theoretical equations for the IR drop for simple geometry types. Values have been calculated for the probe placed 2d away from the electrode with 6 = 5d/3; K = 0.02lX' cm-'; ro = 4 x 10-3cm, d = 0.02cm and i = 20mA cm-* [71.

Georne t r y Equat ion V I R /mv

Planar 33

From the equations given in the table it can be seen that the ohmic error is linearly dependent on the distance between the RE and WE for a planar electrode. For a cylindrical WE, the error is a logarithmic function of distance, whereas for a sphere of small radius, the error is less dependent on RE position.

Although the applicability of spherical electrodes in practical situations is often limited, this is a definite advantage, the distance between the capillary tip and the electrode being not so critical.

Ahlberg and Parker [20] also warned of the dangers of trying to place the Luggin tip very close to the WE. Small variations in the positioning of the tip will make a large difference to the uncompensated resistance. They advocate the use of small-diameter spherical electrodes and a large WE- RE separation.

45

In all cases the ohmic drop is proportional to the current density. Special care should be given to transients at short times when large currents may flow, possible error sources being current oscillations, double layer charging and stray capacitance to ground.

Apart fromresistance, capacitive effects should also be taken into consideration. Thebest design of a Luggin probe is one with a narrow capillary at its tip with thin walls to prevent shielding, but with thick walls in the main body which widen rapidly away from the tip to reduce resistance in the control loop [211.

A possible solution to avoid ac interferences is the use of a special high-frequency by-pass, for instance a platinum wire coupled to the normal reference electrode by a 0.1 pF capacitor.

Any additional resistance in the working electrode itself, for example due to the formation of resistive films, will be included in the uncompensated resistance and can only be reduced by electronic compensation.

In summary :

1. Large capillaries (diameter > 1 mm) are useful only at relatively low current densities in solutions of high conductivity.

2. Even with the smallest convenient capillary (0,2 mm diameter) placed as close to the electrode as possible without incurring shielding errors, the IRcorrectionsgreatly limit themaximumcurrent density for accurate polarization measurements, especially in solutions of relatively low conductivity.

3. Small cylinders (wires) or spheres if practically feasible may be used advantageously as test electrodes in polarization studies, since the IR correction decreases as the electrode is made smaller.

6.3 .I .2 Microelectrodes The current can be kept small by the use of small electrodes 1221. The advantages of small-diameter cylinders and spheres have been described previously. Although there are some possible disadvan- tages (for instance the small surface available when studying pitting), microelectrodes are becoming popular for use in non-aqueous and aqueous resistive media. For instance, Genders, Hedges and Pletcher [23] described an application of microelectrodes to the study of the Li/Li+ couple in ether solvents. They showed that it is possible to obtain high-quality data for the electrodeposition and anodic dissolution of lithium. The experiments required only a two-electrode cell and simple instrumentation because the experimental currents (not current densities) were very small. The electrodes were constructed from Cu wires (40 and 80 p.m diameter). The electrolyte was always lithium hexafluoroarsonate LiAsF, in tetrahydrofuran (THF). The resistance of the experimental cell filled with THF + LiAsF, (0.6 mol dm3) measured using an a.c. bridge was 5000R. Only cell currents below 3 p.A (current density 60 mA cmS2) were analysed quantitatively so that the maximum IR drop was 15 mV. A test of the absence of significant IR-drops is to repeat the experiment with an electrode of different area when only the current should change and there should be no shift in peak potentials or changes in shape of transients.

6.3.2 Active methods A number of active methods are available in which the compensation of the ohmic drop is necessarily incorporated in the control system of the potentiostat.

One of the methods which has been used is the ac method, based on the application of a high- frequency signal (e.g. 50 kHz) through the cell, using the ac voltage after amplification and rectification as the control signal. The active methods mostly used are :

a. positive feedback b. current interrupt.

46

6.3.2.2 Positive feedback In the positive feedback methods a voltage signal is produced which is proportional to IRand which is added to the control input voltage. The aim is to compensate for IR automatically. Depending on the principleof the potentiostatic circuit several solutions havebeen proposed 1151 (forexampleFig. 6.8).

CE

1 negat ive feedback l o o p

WE

CE : C o u n t e r E l e c t r o d e RE : R e f e r e n c e E l e c t r o d e WE : W o r k i n g E l e c t r o d e VF : V o l t a g e F o l l o w e r CF : C u r r e n t F o l l o w e r C : C o n t r o l A m p l i f i e r -L

R o < 7 R , : M e a s u r i n g R e s i s t o r R x : Feedback R e s i s t o r S : C o n t r o l V o l t a g e

Fig. 6.8 : Basiccircuit forpotentiostat with IRdropcompensationbymeansof positive feedback in the control loop

The requirement is that at all frequencies the applied positive feedback signal is smaller than the negative feedback of the control loop of the potentiostat itself.

Often instability problems will arisebecause the system behaves as a purely capacitive electrode if 100 % compensation is obtained [ 151. Nevertheless the method is used frequently, many commercially available potentiostats being equipped with, or easily adapted to, this form of IR drop compensation 1241.

The value of the resistance in the potentiostat feedback circuit (Rx in Fig. 6.8) may be set on the potentiostat in several ways, the most common being by "trial and errox". For example on applying a square wave signal of small amplitude (e.g. 50 mV, 50 Hz) to the control input the value of the resistance is increased gradually until "ringing" (= overshoot + relaxation) is observed with an oscilloscope connected across the WE and RE. The resistance value is then reduced a few percent until stability is restored and this value is used in the feedback circuit. An alternative method is first to measure the resistance, and then to set this value in the feedback circuit. The uncompensated resistance is assumed constant during the subsequent current-voltage measurements, leading to a constant correction; hence, errors may arise when for some reason the uncompensated resistance changes, e.g. due to film formation.

It should be realized that overcompensation does not always result in "instability" and so great care is needed to avoid it. Positive feedback in IRcompensated potentiostats has been discussed in detail by Britz [15] and by Mc Kubre and MacDonald [131.

An analogue technique of ohmic drop compensation has been proposed by Gabrielli et al. 1251, which avoids some stability problems related with positive feedback.

Many workers subscribe to the view that any compensation isbetter than none and so will use positive feedback.

47

6.3.2.2 Interrupt methods Interrupt methods in particular are becoming popular, being used in conjunction with both potentiostatic and galvanostatic techniques [lo, 16,26-301. The principle is to measure the rapid change in potential of the working electrode as the current is suddenly switched off (interrupted). The ohmic drop is given by the immediate change in potential on switching, the potential change due to polarization decaying relatively slowly and certainly more slowly than the purely resistive ohmic drop component i.e., because of the very short time constant of the latter (Fig. 6.9 a). Hence this method is not applicable when the double layer capacity is very small.

For determining the ohmic drop a transient recorder or an oscilloscope is required. One of the potential problems associated with this method is the possible measurement of artefacts, i.e. the decay characteristics of the measuring circuit overlapping the desired potential decay of the working electrode. Other difficulties are associated with the need for reliable triggering and fast data acquisition. The method is mostly applied by interrupting the current periodically, necessitating fast current switching of possibly large currents.

f- lnterruptlon- tlme

A unc = A U / A I

I

to pulse + t i m e

L

C

m m E n

m Runc = l i m Z K J I d

Real part 2' R unc

Fig. 6.9 Different techniques for the measurement of the ohmic potential drop (schematic) (a) interrupt technique (b) pulse technique (c) high frequency ac technique [311

Some of the more expensive commercially available potentiostats are, or can be, equipped with these forms of ohmic drop compensation technique. In principle, the method can be applied when the values of Run, and the measuring resistance change during the measurements.

Some special designs have been published, in which the potentiostat, the interrupter and the data acquisition apparatus have been combined in a sophisticated computer controlled instrument [28-311. An example of this is the measuring equipment described recently by Heitzet al.(Fig. 6.10) [XI.

48

F' I COMPUTER - - - - - - I

I 1 1 I 1 I 1 I

I I I

I I I I

TRANSIENT RECORDER

POTENTIO-

I 1

INTER- POTENTIO- RUPTER STAT

'

-

I 1 I - - - - - - - - - - -

Fig. 6.10 : Flow diagram of the computer-controlled measuring equipment used to eliminate ohmic potential drop [321

RE WE

The method is based on a setup which has been succesfully used to eliminate ohmic potential drop at gas-evolving electrodes at high current densities. With modem equipment for elimination of ohmic drop as previously described it is possible to make rapid corrections in solutions with conductivities below 1 k s cm-'.

CE I -

6.3.3 Passive methods This heading covers all methods in which the ohmic drop is measured or calculated, followed

by mathematical correction afterwards when the experimental data have been collected [15,16,25, 331.

6.3.3.1 Measurement of ohmic drop The measurement of the ohmic drop can be performed by a number of methods, the most well- known being :

a. Positive feedback method as described above, determining the resistance at which the potentiostat oscillates by manual adjustment of the feedback resistance.

b. Current interrupt method as described above or alternatively, the measurement of the instan- taneous potential change when the current is switched on (pulse methods : Fig. 6.9 b).

c. Alternating current methods, including the use of bridges and four-point methods [MI.

d. An extension of method c is the application of impedance techniques, either the full Nyquist diagram being determined or the impedance at one or more discrete frequencies (Fig. 6.9 c) [331.

e. Extrapolation technique, in which the distance between the tip of the Luggin capillary and the surface of the working electrode is varied; the potential difference between the working electrode and the reference is measured as a function of the distance and extrapolated to zero distance [351.

f . An extension of method e is the multiple potential method, in which the potential of the WE is measured with respect to several RE positioned at fixed, known distances 1331.

An example of the application of impedance techniques (method d) is shown in Fig. 6.11.

49

1.2

-8 - c v

.6 n

N - . 4

-2

0

Fig. 6.11 : Determination of solution resistance for different systems consisting of low alloyed ferritic steels in contact with aqueous solutions [36]:

- - - _ _ _ _ _ _ _ -eo-- -* - -

1 - - _ c _ c _ - c - - aUl a- - * - -m- - - -

-

UAA.& -A- - -A- =- _ _ - - - - - - - I l l I 1 I t 1 1 I 1

(a) 100 g/1 EDTA, T = 100 "C, pH = 6 (0)

(b) 100 g/1 EDTA, T = 100 "C, pH = 7 (B)

(c) 150 g/1 HC1+ 1 g/1 inhibitor, T = 76 "C (A)

The evaluation of ohmic drop with alternating current at high frequencies is based on the electric equivalent of the metal-solution interface of Fig. 6.12, where Run, represents the electric solution resistance between working and reference electrodes.

Fig. 6.12: Equivalent circuit of metal-solution interface

The impedance, Z, between the points A and D, at angular frequency o, is given by

Z = Runc + Rt/Q - j o CR2, /Q, (3)

50

where Q is equal to

From equations (3) and (4) it follows that

From a physical point of view the previous relation is intuitive because a capacitor is a short circuit for alternating current at high frequencies.

Elsener and &hni [371 reported results of investigations that combine measurements of the ohmic resistance by interrupter and ac impedance technique for pitting systems. It was found that the ohmic resistance measured by the interrupter technique always corresponds to the primary current distribution in the cell. In contrast, the ac impedance technique also measures the ohmic resistance in a pit. The results are compared with calculated valuesof R and discussed with respect to the applications and limitations of an automatic IR drop compensation in the case of localized corrosion.

In homogeneous corrosion systems (active dissolution, passive state) where the same electrochemical reactions occur over the whole surface, the interrupter and ac technique can be successfully applied and the same value for theohmic resistance is measured by both techniques. Problems arise in localized corrosion systems, where small active areas coexist with a large passive surface and the impedance of the active areas (pits) is short circuited by the surrounding passive surface.

6.3.3.2 Direct calculation Direct calculation of the theoretical value of the ohmic drop is only possible for a number of well- defined and comparatively simple cases [38].

6.3.3.3 Mathematical mefhods Independent of the method whereby the value of IRis obtained, the experimental polarizationdata will have to be corrected. Apart from some less accurate graphical procedures, mathematical methods such as linear regression analysis and successive approximation have been used [391.

Sophisticated mathematical methods have been reviewed by Britz [15]; they start from the uncorrected experimental data and mathematically extract the desired parameters free from ohmic error.

The advantages of the mathematical methods are their suitability for computerization, while it is also possible to apply statistical methods to gain insight into the accuracy of the calculations.

Some examples are given below :

a - A method using a programmable calculator or a minicomputer has been described by Hayes et al. [161; they showed that the Tafel parameters could be calculated from the experimental q-i data, using regressional analysis for the interpretation of the deviations from the log-rate law.

b - Ahlberg and Parker 1201 eliminated the effect of Runc by performing linear regressional analysis on equations derived from the relation :

E: = EP + IPRunc r

where p refers to values at the current peak (in linear sweep voltammetry) and m and r refer respectively to measured and real values.

51

c - Kajimoto et al. [391 discussed the effect of uncompensated IR drop in their investigation of the corrosion rate of pure iron and carbon steel in acid solutions. IR drop compensation was attempted with thepotentiostat itself, which had a built-in positive feedback IRcompensation unit, but its sensitivity was not sufficient to perform the task.

To take into account the IR drop, it was assumed that the applied AE included, in addition to activation polarization (AEA), an IR drop term :

AE=-AE,+RAi= b)ogi*+b)og )Ai I +RAi ( 7 )

where R is the ohmic resistance. The unknown parameters b , i* and R were obtained by a least squares fit for both carbon steel and commercially pure iron. In all cases a good fitting of the equation to the experimental data was obtained.

Some form of error assessment should be applied to all data, as shown above by Kajimoto et al. WI.

d - Hayes and coworkers [161, in their regressional analysis fit to the Tafel equation, described how to calculate the confidence limits of the Tafel parameters. They found it easy to detect when "non-Tafel" data were being processed because the confidence limi ts were much larger than usual.

6.4 CONCLUSIONS When performing polarization measurements an error due to the ohmic drop over the uncompensated resistance will be included in the potential between the working and the reference electrode. The significance of this error is decided by the ratio between the value of the uncompensated resistance and the polarization resistance of the system. The uncompensated resistance can be minimized by careful design of the cell and the positioning of the electrodes. Several methods of instrumental compensation of the ohmic drop are available, of which the interrupt methodsare the most versatile. Such methods are applied during the polarization measurements.

Finally, it is possible in principle to evaluate and/or calculate the ohmic drop, and subsequently, to correct the experimental data for its influence after the polarization measurements have been performed.

6.5 ACKNOWLEDGEMENTS The authors gratefully acknowledge the valuable criticism of Dr. J.C. Verhoef (DSM Research, Geleen) in compiling this chapter.

6.6 REFERENCES 1. F. Mansfeld: Corrosion, 1976,32- 143.

2. F. Mansfeld: in Adv. in Corr. Science and Technology, vol. VI. Published by Plenum Press, New York, 1976, p. 163.

3. F. Mansfeld: NACE Intern. Corrosion Forum, Boston, U. S. A, 1985, paper No. 70. Published by NACE, Houston, Tx, U. S. A, 1985.

4. G. Rocchini: Corrosion, 1988,44- 158.

5. D.T. Sawyer and J. L. Roberts: Experimental Electrochemistry for Chemists. Published by J. Wiley & Sons, Inc., 1974, ch. 3.

6. N.D. Greene: Experimental Electrode Kinetics. Published by Rensselaer Polytechnic Institute, Troy, New York, 1965.

7. R. Greef et al.: Advanced Instrumental Methods in Electrode Kinetics. Southampton Electro-

52

chemistry Group. Published by Ellis Horwood Ltd, Chichester, U. K., 1985.

8. J.A. von Fraunhofer and C.H. Banks. Potentiostat and its applications. Published by Butter- worths, London, 1972, ch. 3.

9. D.D. MacDonald: Transfer Techniques in Electrochemistry. Published by Plenum Press,New York, 1977, ch. 2.2.

10. M. Hayes: in Techniques in Electrochemistry, Corrosion and Metal Finishing, a Handbook. Published by John Wiley & Sons Inc., (Ed. A. T. Kuhn), 1987, ch. 4.

11. A. J. Bard and L.R. Faulkner: Electrochemical Methods, Fundamentals and Applications. Published by JohnWiley & Sons Inc., 1980, ch. 1.3.4.

12. E. Heitz and G. Kreysa: Principles of Electrochemical Engineering. Published by VCH Ver- lagsgesellschaft mbH, D-6940, Weinheim, 1986, ch. 1.3.

13. M. C. H. McKubre and D. D. MacDonald: in Comprehensive Treatise of Electrochemistry. Published by Plenum Press (New York), ( ed. R. E. White, J. O M Bockris, B. E. Conway and E. Yeager) 1984,l .

14. E. Yeager and J. Kuta: Techniques for the study of electrode processes in physical chemistry : an advanced treatise. Published by Academic Press, New York/London, 1970, u.

15. D. Britz: J. Electroanal. Chem., 1978,88,309.

16. M. Hayes, A.T. Kuhn and W. Patefield: J. of Power Sources, 1977/78,& 121.

17. G. Rocchini, private communication.

18. S. Barnartt: J. Electrochem. Soc., 1952,99- 549.

19. S. Barnartt: J. Electrochem. Soc., 1961,108.102.

20. E. Ahlberg andV. D. Parker: J. Electroanal. Chem., 1980,107.197 and ibid., 1981,121.57.

21. A. Bewick, M. Fleischmann and M. Liler: Electrochimica Acta, 1959,1,83.

22. K. Wikiel and J. Osteryoung: J. Electrochem. Soc., 1988,135.1915.

23. J. D. Genders, W. M. Hedges and D. Pletcher: J. Chem. Soc., Faraday Trans., 1984, @, 3399.

24. P. Doigand P. E. J. Flewitt: Br. Corros. J., 1976, ll- 78.

25. C. Gabrielli, M. Ksouri and R. Wiart: Electrochimica Acta, 1977,22- 255.

26. J. Gsellmann and K. Kordesch: J. Electrochem. Soc., 1985,132.747.

27. W. J. Wruck, R. M. Machado and T.W. Chapman: J. Electrochem. Soc., 1987,134,539.

28. P. J. Moran: Corrosion, 1986,42- 432.

29. P. Cassoux, R. Dartiguepeyron, P. -L. Fabre and D. de Montauzon: Electrochim. Acta, 1985, a 1485.

30. M. Berthold and S. Herrmann: Corrosion, 1982,38- 241.

31. B . Elsener and H . Biihni: Corros. Sci., 1983,23- 341.

32. M. Kuhn, K. -G. Schiitze, G. Kreysa and E. Heitz: Electrochemical Corrosion T e s t i n g u Published by DECHEMA, Frankfurt, FRG, (ed. E. Heitz, J. C. Rowlands and F. Mansfeld) 1986,265.

33. M. Eisenberg, R. E. Kuppinger and K. M. Wong: J. Electrochem. Soc., 1980,117,577.

53

34. G.W. Walter: Corros. Sci., 1978, 927.

35. J. OM. Bockris and A. M. Azzam: Trans. Faraday Soc.,1952, @, 145.

36. G. Rocchini: NACE Intern. Corrosion Forum, Boston, 1985, paper No. 188. Published by NACE, Houston, Tx, U. S. A., 1985.

37. B. Elsener and H. Bijhni: Electrochemical Corrosion Testing. Monograph 101. Published by DECHEMA, Frankfurt, FRG (ed. E. Heitz, J. C. Rowlands and F. Mansfeld), 1986, 279.

38. K. Tokuda, T. Gueshi, K. Aoki and H. Matsuda: J. Electrochem. Soc., 1985,132,2390.

39. Z. P. Kajimoto, S. Wolynecand H. C. Chagas: Corros. Sci., 1985, 35.

Further information can be found in:

L. L. Scribner and S. R. Taylor (eds): The Measurement and Correction of Electrolyte Resistance in Electrochemical Tests. Proc. Symposium held in Baltimore, MD; ASTM Special Technical Publica- tion STP 1056, Philadelphia, 1990.

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CHAPTER 7

AUTOMATIC MEASUREMENT SYSTEMS

0. F O R S ~ N AND J. AROMAA Helsinki University of Technology

Vuorimiehentie 2,02150 Espoo, Finland

7.1 INTRODUCTION The rapid development of inexpensive microcomputers has increased their use in laboratory instrumentation. The function of laboratory instrumentation is usually simultaneous control of experiment and acquisition of measurement data. Processing of measurement data to values, graphs or tables can also be included. The personal computer is a powerful tool with capability for system control, data acquisition and data processing and combined with measuring equipment it can perform all the functions of larger and more expensive systems.

Development of an automatic measurement system is practical, if the same kind of measurements are made frequently. Also, the type of experiment affects the demand to develop an automatic system. Very long or very fast experiments are often impossible to control and follow manually, because a human operator cannot work long periods without pauses and in rapid experiments the sampling rate can exceed that possible by hand. In large experiments the number of controlled and measured quantities restrict operators ability to follow every detail. These cases require automation.

Electrochemistry is a field, where control of experiments and data acquisition methods do not differ greatly, but proper interpretation of measured data gives information on several different phenomena. In electrochemical experiments one controls the system with electrical quantities and also measures electrical quantities, i.e. voltage and current. The electrochemical corrosion process, its probability and rate can be described as a function of three variables; potential,current and time. Other electrical and electrochemical parameters can be derived from these variables. As an electrochemical process, corrosion can also be studied with electrical measurement methods. The measurements are suitable for automation which has the following advantages:

- continuous manual operation is avoided - more data can be obtained - measurements are more accurately controlled and reproducibility is improved, hence

- storing and handling of large quantities of data becomes more efficient thus leading to effective

- electrochemical impedance spectroscopy and other sophisticated measurement methods are

comparability of results from one operator to another is improved

data analysis

more convenient with automatic data processing equipment

The main disadvantage of automatic measurement units is, that if one can get the results ready processed, they can easily be over-processed. Therefore, persons using automatic systems must have good theoretical background to assess the reliability of test results. Blind trust on ready values can lead to wrong conclusions. Potential and current, which are measured from an electrochemical system, describe only the thermodynamical possibility of reactions to proceed and the net rate of these reactions. During an experiment the potential of the working electrode is produced by the complicated interaction between electrode and electrolyte and by an external control signal. The measured current is a sum of all those reactions, which proceed at the working electrode surface.

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In a normal system there are always parallel reaction mechanisms and succeeding reaction steps, and so it may be impossible to interpret measured data.

7.2 PARTS OF A MEASUREMENT SYSTEM An electrochemical process can be considered as a "black box" with electrical response to excitation signals and therefore it is possible to study electrochemical processes with electrical measurement instruments. At the moment there are several commercial systemsavailable, whichcontainboth the equipment and necessary software. If a commercial system meets users, technical and economical needs, it is advisable to choose it. A commercial systemcanbe installed and put to work much faster than a sel -built system. However, if the system is self-tailored, it will usually be more suitable to particular requirements. A self-made system with personal computer,adequate I/Ocards and own software provides flexibility. When developinga selfbuilt system one has to choose the equipment with care and ensure compatibility. When choosing measurement units it is often wise to buy from big manufacturers in order to ensure reliable maintenance, installation and operational instruc- tions.

Anautomaticmeasurement systemconsistsof four different blocks: computer and itsperipherals, measurement devices, data transfer busses and software. It is easy to get hardware devices, but creating software can be difficult and troublesome. Worst problems are usually found when one is trying to connect devices to other devices and software. The users manuals are usually not sufficiently helpful for such purposes and the operation of data transfer busses is not guaranteed.

7.2.1 Computers and peripherals Good and useful computers can be obtained from big manufacturers, whose systems have become de facto standards. Even MS-DOS operating systems, which are being developed for office operational environment, are capable of handling laboratory size measurement operations. In laboratory instrumentation the computer requires at least 256 kB memory and two floppy disk units. Measurement programs do not require very powerful computer and most of the measurement procedures can be done in measuring instruments. Data processing with commercial software requires more memory and often a hard disk unit is also useful. A computer which controls instrumentation must have adequate capacity to handle error situations, even though many errors can be handled in measurement programs.

A dot matrix printer, which is capable of producing graphics, is adequate as a peripheral device. For producing higher quality hardcopies one should consider buying a good digital plotter, which can produce A4- or A3-size printouts.

7.2.2 Measuring devices For simple measurements a potentiostat and possibly separate multimeters for voltage and current measurements are adequate. For complicated experiments a function generator, an oscilloscope and a frequency response analyzer should be added to the system. When choosing an instrument one should check its capacity (operating rate, accuracy etc), maintenance, compatibility with other devices and price.

The first instrument to buy is usually the potentiostat. One must choose between analog and digital devices. Analog instruments are older technology when compared to digital instruments. Analog instruments may be less accurate, but their operation is easier to follow and their output capacity is usually greater. Digital potentiostats are more suitable for automatic systems, but manual operation of these may be almost impossible. When using a digital potentiostat data transfer is usually made via a digital data transfer bus. When using an analog potentiostat one has to use analog/digital and digital/analog converters or digital multimeters with data transfer bus. Controlling the potentiostat can be done via data bus (digital potentiostat) or with an external voltage signal (digital or analogpotentiostat). Production of the external control signal canbe made with a D/A converter or with a function generator.

When choosing a function generator one should check the maximum output level and especially the frequency range. In corrosion experiments potential sweep rates are 1 mV/h - 100 mV/s and output voltages -10 to +10 volts. Most function generators cannot produce potential sweeps slow enough forcorrosionstudies. Forexampleina polarizationexperiment with sweeprate lOmV/min and maximum polarization 2000 mV, the frequency is about 10 mHz. Most function generators are designed for the electronics industry, where high frequencies up to GHz range are used.

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Special equipment for transient and impedance measurements consists of a digital storage oscilloscope and a frequency response analyzer. A suitable oscilloscope should have at least two channels with 1 kB (1024 measurement points) storage capacity. A frequency response analyzer is usually a complete unit which can produce the excitation signal, make measurements, process and analyze the measurement data.

7.2.3 Data transfer busses Data transfer between measuring instruments and computer is necessary but it is often prblematic. The simplest solution is use of analogjdigital and digital/analog converters which convert analog voltage or current signals to bytes or vice versa. Operating rates of converters depend on the conversion algorithm and on the resolution (usually 8,12 or 16 bytes) of the converter. A 12 bit converter is satisfactory and its relative error is 0.025 % of maximum value of the converter range. The resolution can be increased by using signal amplifying.

Manufacturers of measuring instruments use standardized digital data busses, which transfer data as bytes. Best known busses are the RS232C and GPIB busses. GPIB (General Purpose Instrumental Bus) is known also as HPIB or IEEE-488 bus. The serial RS232C bus is a recommended standard for data transfer between a peripheral and a computer. It is built to handle data transfer between two instruments, so using more devices in the same system may be troublesome. GPIB bus is a standard bus and the standard determines mechanical and electrical features but not the commandsused.GPIB busiscapableof controllingasystemwith 10 tol5devicesand so far it seems to be the best bus system for practical purposes. In the bus configuration every instrument has a numerical address. One of the instruments, usually the computer, acts as a bus controller while other instruments are both talkers and listeners. Even though the functions of GPIB bus are standardized it requires some trial and error to get instruments connected to the software.

7.2.4 Software Software makes the computer based data acquisition and control system operational. The software consists of an operating system of the computer, programming language and programs used for measurements and data processing. Operating systems, such as MS-DOS, consist of programs and subroutines, which reside in the memory the whole time that the computer is switched on and deal directly with computer components. Programming languages are used to allow discussion between machine code and human operator. With programming languages the operator writes application programs, which in turn control the computer so that the expected functions are done in the expected order.

Assembly language is close to the instruction code of the machine allowing most efficient memory space utilization and execution speed. A program written in Assembly language is not easy to modify by another person unless it is very clearly documented. By using a high level language the programs can be written more clearly and this allows easy modifications, so that the software can be fitted to individual experiments more easily. There are several high level programming languages e.g. Basic, Fortran, Pascal and C. Structural languages require good programming discipline, which enforces planned work. This provides efficient and easily understandable codes, but for short programs it can be too tedious to program with Pascal or C. Basic is still very useful even though it is described as a 'crude and quick' way to get a job done.

A language such as Basic is easy to learn and use, but modifications can be dangerous due to accumulation of GOT0 and GOSUB statements. Programming is not an extremely difficult task to undertake with high level languages but it is still the hardest part when creating an automatic measurement system. Writing a measurement program requires knowledge both on programming techniques and on the measured phenomena. Measurement programs must be capable to control, data acquisition and data processing without wasting memory space on less important functions. The role of measurement software is simplified in Fig.7.1. Measurement software is usually a precisely timed execution of read and write operations for data acquisition and equipment control. Programs should also output adequate log sheets with experimental details, measured values if required and preferably a graph of measured data points. Experiments are usually made so that measured quantity is a function of time, applied potential or applied current. Applied potential signal can also be time dependent, as in voltammetric experiments. Time dependent current signals should be avoided because the signal is more difficult to produce and data interpretation is ambiguous.

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-LOG SHEETS

Measurement programs are not very large. A Basic program for measurement of polarization curves, including graphics, file maintenance etc, is about 200-250 lines. Before starting measurement operations, the operator must provide experimental parameters, which alter depending on the operating program. Programs need at least the values of rest potential, polarization potentials, polarization rate or polarization time and the area of the working electrode. Input of experimental parameters should be arranged so that inexperienced users know, how to feed the input data correctly. A simple way to arrange this is to inform the user of required parameter and its dimension, e.g. “Enter corrosion potential in mV”, with every input command. Also the validity of input parameter should be checked immediately. Complex data input should be avoided and normally not more than two or three numbers should be requested in one line. After feeding all input data the operator should once more confirm its validity before starting measurements.

In every measurement timing is one of the crucial factors. Electrochemical experiments can be timed either with the internal clock of the computer or with a linearly changing potential signal. In electrochemical experiments one usually measures potential or current at predetermined time intervals or current at predetermined potential intervals. Potentiostatic, galvanostatic and voltammetric programs can be written so that they use two repetitive loops. One loop controls continuously time or potential interval between successive measured points and the other loop controls total time or potential of the experiment. When the interval between two pointsisachieved a new measurement operation is made and when the final time or potential value is achieved the experiment is stopped.

-DATA BASE

7.3 ANALYSIS OF MEASURED DATA When one is conducting an experiment more or less data are collected usually in the form of numbers. These raw data are only a sequence of numbers without any value, which require a proper meaning to become information. A series of potential and current density values is raw data, but can be processed to give polarization resistance values or anodic polarization curves which provide valuable information. When the same type of analysis is used for samples of similar nature, the evaluation and interpretation of a measurement can be totally automated. This situation occurs mostly in corrosion monitoring. In research laboratories a changing variety of samples requires flexible evaluation procedures and a more active role of the human operator.

Data processing can be done at two separate occasions, i.e. during the experiment or after the experiment. During the experiment one must continuously follow the controlling quantity and change its value if necessary. One must also record the values of measured quantities and this is usually done asa function of electrode potential or time. During the measuring operations the data processing must be kept to a minimum, and only those steps that are necessary to control the experiment should be included. Depending on the type of the experiment it is necessary to follow time and potential and sometimes also current density. During the experiment it is not necessary to evaluate and interpret data. However, it is helpful to include some simple function, which allows the operator to check the status of the experiment. A preliminary graph of measured values plotted continuously on the computer screen is reasonable, but printing every measured point would be a waste of paper.

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During the measurement data are collected to mass memory and the most flexible way is to save the data as an ASCII text file. Data processing should be done only after the measurement with appropriate software independently of the experiment. Many commercial spreadsheet programs can read ASCII files, edit the data and finally produce desired numerical values and graphs.

measure A/D

7.4 EXAMPLES OF AUTOMATIC SYSTEMS The first example is a system for voltammetric measurements and it uses an analog potentiostat. Analog potentiostats are usually controlled manually but an external voltage signal can also be used. In this kind of system it is possible to use separate converter cardslocated inside the computer or an expansion chassis architecture. A converter card system is simple and inexpensive. A single circuit board fits into one of the expansion slots of the computer and the board provides rear connector for input and output signals. An expansion chassis system provides more flexibility and greater performance. The expansion chassis holds 1 / 0 channels and it is connected to the computer with an interface card.

ext. control D/A

Figure 7.2 shows a block diagram of an automatic system with analog potentiostat. The potentiostat is controlled with external voltage signal from D/A converter. Potential is measured from the monitoring output of the potentiostat and it is converted to bytes with an A/D converter. Electrode potential can also be measured with an optional voltmeter. The electrode potential must be measured continuously, so the computer can send control commands to measuring instruments in correct time. The cell current is measured with an autoranging multimeter and the current value is sent to the computer using a digital bus. During electrochemical measurements the current can vary several decades and so using a fixed current range of potentiostat leads to less accurate measurements or to overloading. The system in Fig. 7.2 is suitable for voltammetric experiments and it can be built from existing instruments.

I I

interface CONVERTER

CARDS

COMPUTER

/j GPIB bus

PRINTER zd=l

-1 J--l AMPERE-METER

R C

Fig. 7.2 Block diagram of an automatic system with analog potentiostat. The figure also shows some possible data transfer arrangements.

59

The second example is a more sophisticated system with only digital units. A block diagram of the system is shown in Fig. 7.3. The basic idea of the measuring system is that the computer sends all the controlling commands, status signals and measured data via the GPIB bus. This type of system does not require any front panel operations to use the instruments. The cell is polarized with a digital potentiostat, which also measures working electrode potential and current. Changing of electrode potential can be done by using computer controlled internal setting of the potentiostat or by using external voltage signals produced by a function generator. Frequency response analyzer (FRA) and potentiostat are used together in electrochemical impedance spectroscopy. The FR4 produces an alternating excitation signal, which is used as an external control signal of the potentiostat and converts measured electrode potential and cell current to electrode impedance. In these measurements real-time computer control is not necessary. The oscilloscope can be used to measure transient signals and is connected directly to the cell. The computer is used to set, arm and trigger the oscilloscope and to control the potentiostat.

- FREQUENCY RESPONSE ANALYZER

GPIB bus

control I POTENTIOSTAT FUNCTION 1

W

I GENERATOR 1 measure

OSCILLOSCOPE

R C 1 Fig. 7.3 Block diagram of an automatic system with digital equipment. All data transfer is via GPIB bus.

60

CHAPTER 8

FIELD TESTING G .TURLUER

CEA, Fontenay-aux-Roses, France

By means of specially designed single or multi electrode probes many electrochemical test methods can, in principle, be implemented in plant, pilot testing facilities or natural environments.

8.1 OBJECTIVES Theobjectivesof field testingmaydiffer fromthoseinlaboratory testingand may beconcerned with matters such as:

Corrosion monitoring

Detection of process deviations detrimental to the service life of components

Diagnosis of unexpected corrosive conditions

Data collection in complex environments

8.2 TEST METHODS Usually the most popular methods rely upon conventional techniques involving rugged equip ment and preferably using straight forward interpretation of data.

For corrosion monitoringpurposes, or for any other objective, it is advisable to conduct a survey of the corrosion processes over a significant period of time including operation transients, shut downs and startups which to a greater or lesser extent are already documented.

8.2.1 Polarization resistance measurements (Rp) RP measurements, preferably with free corrosion potential monitoring can yield real time corrosion data from which interpretation related to the corrosion regime is possible.

8.2.2 Material (or Component) potential and process redox potential monitoring This monitoring will provide information with respect to the material corrosion regime, i.e., active, passive, transpassive, pitting behaviour etc., and diagnostic possibilities exist only if the corrosion system is well understood particularly as a result of laboratory studies - for example, the significance of the free corrosion potential or the potentials under impressed anodic or cathodic currents.

8.2.3 Impedance measurements Despite some possible implementation problems and the need for sophisticated requirements for interpretation, these measurements can prove particularly valuable in low conductivity media - such as concrete, soils, condensate corrosion, protection by coatings etc. They therefore provide a more accurate determination of Rp.

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8.2.4 Potentiodynamic scanning This method can be recommended as a diagnostic tool when information is needed to determine and interpret conditions responsible for unexpected corrosion behaviour. The technique has to be considered as a destructive test of the working electrode as it can irreversibly influence the subsequent corrosion regime.

8.2.5 Galvanic current measurements These with, or without, potential monitoring and sometimes referred to as zero resistance ammetry find their most common application for the assessment of galvanic coupling between dissimilar materials or possibly between areas of the same material but exposed respectively to the free medium and to occluded areas, as for example, in crevice corrosion and differential aeration conditions.

8.2.6 Spatial potential scanning This technique is mostly applied for the detection of spatial defects in anodic or cathodic protection systems on complex structures or components. Thus, attempts have been made to detect single locations in components that are usually passive but which may undergo some form of active corrosion in particular areas such as welds, crevices, pits, cracks, etc.

8.3 SPECIFIC REQUIREMENTS AND PRECAUTIONS Among the numerous possible recommendations which will depend on the various objectives, corrosion systems and available techniques the following specific requirements and precautions will need to be considered.

8.3.1 General (i) Check that the insulation of probe leads and electrode pins remains satisfactory between

components over a period of time.

(ii) Avoid, if possible, the use of electrochemical equipment (millivoltmeters, potentiostats and ancillary apparatus) that would ground the working electrode thus leading to a possible short circuit with the grounded structure being surveyed.

(iii) Preferably use 4 lead connections to minimize ohmic drop in the external lines.

(iv) Avoid loops in grounding the screening and electrochemical equipment to minimize pick up of electromagnetic noise.

8.3.2. Probe configuration and location The probe configuration, and the quality of the electrode mounting are of primary importance with regard to the quality and reliability of the readings.

The following important features should be mentioned.

(i) The probe design should comply with the hydrodynamic conditions. Extreme care is essential in systems where corrosion is controlled by flow dependent mass transfer, e.g., of oxygen, ferric ions, protective corrosion products etc.

(ii) IR drop in the electrolyte should be minimized by an appropriate disposition of the electrodes (preferably in line for low conductivity media) on the probe.

(iii) Electrode mounting. Crevice formation should be minimized using an appropriate mechanical design and adequate gasket selection. Waterline effects and the cutting and mounting of electrode materials that would expose them to preferential end grain attack should be avoided.

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(iv) Monitoring of crevice or occluded cell corrosion. Considerable care should be taken in geometrical design of the probe, e.g., anode to cathode area relationship and possible plugging by deposits, if significant conclusions are to be drawn from the measurements.

(v) Retrievable probes are often desirable and can be used in on-line designs or in side lines of process streams.

8.3.3. Reference electrodes True reference electrodes to provide thermodynamic information are not always necessary for “in service” applications since their stability with time or with respect to pollution, fouling an and ion concentration are frequently a cause of concern.

(i) Arbitary electrode used as a reference. It is a common practice to use a reference electrode material either similar to that of the working electrode, or more noble (platinum, gold, passive titanium, stainless alloys) or a material whose free corrosion potential is more likely to remain reasonably stable in the process. With the use of a nonreversible reference electrode, thermodynamic or redox references are not obtained but they are not essential for Rp, or galvanic coupling monitoring. However, potentiodynamic scansconducted with respect to anarbitary reference basis can later be approximately relocated on a thermodynamic scale using some other polarisation characteristics, e.g., hydrogen or oxygen evolution or O2 or Fe3’ reduction).

(ii) In situ reference electrodes. It is advisable to use these as retrievable probes since periodic checking and renewal of the electrolyte in the cell is necessary. Unfortunately many process parameters such as pressure, high temperature, temperature fluctuations and thermodynamic incompatibility restrict the use of in situ reference electrodes.

(iii) External reference electrodes. To avoid the problems mentioned above. the reference can be mounted externally to the process stream, Le., in more controlled conditions which exclude the effect of high temperature and aggressive environments. Such electrodes can either be pressurized or at atmospheric pressure. The problems then become associated with the design and implementation of an electrolytic bridge with the following requirements: evaluation of the thermal and junction potentials and their possible variation with time; bridge sealing and the possible interrup- tion of the bridge by plugging or accumulation of gas bubbles.

8.4 INTERPRETATION AND POSSIBLE LIMITATIONS In service the conditions of actual plant processes can vary from those that are relatively straight- forward to situations that are extremely complex from the electrochemical and corrosion stand- points: therefore, measured data and their interpretation can provide information that will fall into the following types:

(i) actual material corrosion rates with complete description of the corrosion regime.

(ii) correlation of fluctuations in process variables with control of these to minimize corrosion.

(iii) obtaining an order of magnitude for the general corrosion rate or the general corrosivity of a process with respect to a given material.

(iv) the means to compare service data with relevant laboratory testing and/or the means to devise laboratory testing conditions that will provide better simulation of the process.

(v) qualitative indications which can be checked with experience, inspection or other process parameters.

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(vi) data that are irrelevant to actual corrosion damage: thus, it should be possible, for example, to recognize when electrochemical testing has become inappropriate - if not totally inaccurate - or when the location, size or material of the probe are not relevant to the most damaging corrosion process. Such situations would be more readily recognized by the simultaneous use of other monitoring or survey methods such as electrical resistance probes, coupons or component inspec- tions.

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