(European Federation of Corrosion Series) Ulf Kivisakk, Bard Espelid, Damien Feron-Methodology of Crevice Corrosion Testing for Stainless Steels in Natural and Treated Seawaters-Maney

Methodology of crevice corrosion testing for

stainless steels in natural and treated seawaters

European Federation of Corrosion Publications NUMBER 60

Methodology of crevice corrosion testing for

stainless steels in natural and treated seawaters

Edited byU. Kiviskk, B. Espelid & D. Fron

Published for the European Federation of Corrosion by Maney Publishing

on behalf of The Institute of Materials, Minerals & Mining

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

Maney Publishing is the trading name of W.S. Maney & Son Ltd.

Maney Publishing, Suite 1C, Josephs Well, Hanover Walk, Leeds LS3 1AB, UK

First published 2010 by Maney Publishing

2010, European Federation of Corrosion

The author has asserted his moral rights.

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the editors, authors and the publishers cannot assume responsibility for the validity of all materials. Neither the editors, authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book.

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Cover photo from Chapter 4: Assembly of the spring loaded PVDF crevice formers.

vContents

Series introduction x

Volumes in the EFC series xii

Preface xvii

1 Crevice corrosion from a historical perspective 1 1.1 Introduction 1 1.2 The mechanism 1 1.3 The ferric chloride test 2 1.4 Field tests 3 1.5 Electrochemical tests 5 1.6 Conclusions 6

2 Objectives and background 8 2.1 Introduction 8 2.2 Establishment of the state-of-the-art 9 2 .3 Formulation of a new synthetic seawater 9 2.4 Electrochemically controlled crevice corrosion test 10 2.5 Inter-comparison testing 10

3 Laboratory calibration 12 3.1 Calibration of participating laboratories in the project 12 3.2 Experimental procedure 12 3.3 Test results 12 3.3.1 Weight loss 12 3.3.2 Number of etchings/attacks 13 3.3.3 Maximum depth of attack 16 3.4 Conclusions from the calibration test 16

4 Crevice formers for specimens of plate material 17 4.1 Optimisation of test parameters of importance for crevice

corrosion testing 17

5 Crevice corrosion testing of tubes 21 5.1 Introduction 21 5.2 Experimental 22 5.2.1 Materials 22 5.2.2 Design of crevice former 22 5.2.3 Finite Element Method Modelling 23 5.2.4 Crevice corrosion testing 23 5.3 Results 24 5.3.1 Finite Element Method modelling 24 5.3.2 Crevice corrosion testing 24

vi Contents

5.4 Discussion 26 5.4.1 Specimen area 27 5.4.2 Crevice former 27 5.4.3 Clamping force 27 5.4.4 Proposed crevice former procedure for tube

specimens 28 5.5 Crevice corrosion testing of stainless steel tubes applied as

umbilicals 28 5.6 Conclusions from crevice corrosion testing of tubes 28

6 Formulation of new synthetic seawater for aerobic environments 30 6.1 Introduction 30 6.2 Experimental 31 6.3 Electrochemical tests 32 6.4 Crevice corrosion experiments 33 6.4.1 After test examination 34 6.4.2 Chemical method versus the biochemical method 34 6.4.3 Infl uence of the tank material 38 6.4.4 Infl uence of the cathodic area 39 6.4.5 Infl uence of stainless steel grades 39 6.4.6 Infl uence of chemicals and biochemicals 40 6.4.7 Infl uence of temperature 41 6.4.8 Infl uence of the crevice holder system 41 6.5 Conclusion 42

7 Simulation of anaerobic environments 44 7.1 Introduction 44 7.2 Experimental 44 7.3 Results and discussion 46 7.3.1 Infl uence of the polarisation scanning rate 46 7.3.2 Breakdown potentials in sterile aerated seawater 48 7.3.3 Breakdown potentials in anaerobic seawater with SRB 50 7.3.4 Breakdown potentials Na2S solution 51 7.4 Conclusion 52

8 Synergy of aerobic and anaerobic conditions 53 8.1 Introduction 53 8.2 Synergy of aerobic and anaerobic biofi lms on EN 1.4404 54 8.3 Synergy of aerobic and anaerobic biofi lms on EN 1.4462

and EN 1.4547 57 8.4 Laboratory simulation of the synergy 58 8.5 Conclusion 58

9 Electrochemical simulation of aerobic environments with or without chlorine 60

9.1 Electrochemical simulation of biofi lm effects and treatment of seawaters 60

9.2 Conclusions from activities related to electrochemical simulation of natural and treated seawaters 66

Contents vii

10 Profi ciency of crevice corrosion methods: inter-comparison tests 67 10.1 Introduction 67 10.2 Experimental 68 10.2.1 Materials 68 10.2.2 Test procedure general 68 10.2.3 Synthetic biochemical seawater tests 69 10.2.4 Natural seawater tests 70 10.3 Results 71 10.3.1 Natural seawater tests 71 10.3.2 Synthetic biochemical seawater tests 78 10.4 Discussion 78 10.4.1 Natural seawater 78 10.4.2 Spring loaded crevice formers 81 10.4.3 Corrosivity of the synthetic biochemical seawater

versus natural seawater 83 10.4.4 Synthetic biochemical seawater and Critical Crevice

Temperature 84 10.5 Conclusion 87

Appendix A Calibration procedures for crevice corrosion tests in 6% FeCl3 88 A.1 Scope 88 A.2 References 88 A.3 Test material 88 A.4 Test specimen 88 A.5 Specimen treatment 88 A.6 Crevice formers 88 A.7 Assembling of crevice formers 89 A.8 Apparatus 90 A.9 Test solution 90 A.10 Test temperatures 90 A.11 Procedure 92 A.12 Evaluation 92 A.13 Report 92

Appendix B Crevice corrosion tests in natural seawater and synthetic biochemical seawater: Test procedures for CREVCORR round robin tests 94

B.1 Scope 94 B.2 References 94 B.3 Test material 94 B.4 Test specimen 94 B.5 Specimen treatment 95 B.6 Crevice assembly 95 B.7 Assembly of crevice formers 95 B.8 Electrical connections and suspension method 97 B.9 Apparatus 97 B.10 Test solution 97 B.11 Test temperatures 97 B.12 Test procedure 98

viii Contents

B.13 Post-cleaning of specimens 98 B.14 Evaluation 98 B.15 Report 98

Appendix C Stainless steel tubes: Procedures for making a crevice with similar crevice geometry as for plate specimens 100

C.1 Scope 100 C.2 References 100 C.3 Test material 100 C.4 Test specimen 100 C.5 Specimen treatment 101 C.6 Crevice assembly 101 C.7 Assembly of crevice formers 102 C.8 Electrical connections and suspension method 103 C.9 Testing 103 C.10 Post-cleaning of specimens 104 C.11 Evaluation 104

Appendix D ISO proposal for synthetic biochemical seawater 105 D.1 Scope 107 D.2 Reference documents 107 D.3 Signifi cance and use 108 D.4 Reagents 108 D.5 Preparation of chemical substitute seawater 108 D.6 Preparation of biochemical substitute seawater 109

Appendix E Electrochemical procedures to simulate aerobic biofi lms for the evaluation of crevice corrosion resistance to various types of seawater 110

E.1 Scope 110 E.2 Test material 110 E.3 Test specimen 110 E.4 Specimen preparation 110 E.5 Crevice assembly 110 E.6 Electrical connections and suspension method 111 E.7 Apparatus 111 E.8 Test solution 112 E.9 Test temperatures 112 E.10 Test procedure 112 E.11 Evaluation 112 E.12 Report 112 E.13 Method A Electrochemical crevice corrosion test simulating

natural seawater 113 E.14 Method B Electrochemical crevice corrosion test simulating

chlorinated natural seawater 114 E.15 Method C Electrochemical crevice corrosion test simulating

high-temperature chloride solution 115

Contents ix

Appendix F Crevice corrosion tests in natural seawater for umbilical applications: Test procedures simulating the crevice corrosion situation inside an umbilical 117

F.1 Scope 117 F.2 References 117 F.3 Test parameters 117 F.4 Test specimens 117 F.5 Specimen treatment 118 F.6 Crevice assembly 118 F.7 Assembling of crevice formers 119 F.8 Electrical connections and suspension method 120 F.9 Apparatus 121 F.10 Test solution 122 F.11 Test temperatures 122 F.12 Test procedure 122 F.13 Post-cleaning of specimens 122 F.14 Evaluation 122

Appendix G General procedure for crevice corrosion tests in natural seawater: Used for the CREVCORR round robin test 124

G.1 Apparatus 124 G.2 Test solution 124 G.3 Test temperatures 124 G.4 Test procedure 124 G.5 Report 125 G.6 Time development for the natural seawater test 126

Appendix H General procedures for crevice corrosion tests in synthetic biochemical seawater: Used for the CREVCORR round robin test 128

H.1 Apparatus 128 H.2 Test solution 128 H.3 Test temperatures 130 H.4 Test procedure 130 H.5 Report 130 H.6 Development of the test for synthetic biochemical seawater 131 H.7 Example of the product document delivered with crude

GLUCOSE OXIDASE 132 H.8 Example of report 133

Appendix I Report format of crevice corrosion test 134

xEuropean Federation of Corrosion (EFC) publications: Series introduction

The European Federation of Corrosion (EFC), incorporated in Belgium, was founded in 1955 with the purpose of promoting European cooperation in the fi elds of research into corrosion and corrosion prevention.

Membership of the EFC is based upon participation by corrosion societies and committees in technical Working Parties. Member societies appoint delegates to Working Parties, whose membership is expanded by personal corresponding membership.

The activities of the Working Parties cover corrosion topics associated with inhibition, cathodic protection, education, reinforcement in concrete, microbial ef-fects, hot gases and combustion products, environment-sensitive fracture, marine environments, refi neries, surface science, physico-chemical methods of measurement, the nuclear industry, the automotive industry, the water industry, coatings, polymer materials, tribo-corrosion archaeological objects, and the oil and gas industry. Working Parties and Task Forces on other topics are established as required.

The Working Parties function in various ways, e.g. by preparing reports, organis-ing symposia, conducting intensive courses and producing instructional material, including fi lms. The activities of Working Parties are coordinated, through a Science and Technology Advisory Committee, by the Scientifi c Secretary. The administration of the EFC is handled by three Secretariats: DECHEMA e.V. in Germany, the Fdration Franaise pour les sciences de la Chimie (formely Socit de Chimie Industrielle) in France, and The Institute of Materials, Minerals and Mining in the UK. These three Secretariats meet at the Board of Administrators of the EFC. There is an annual General Assembly at which delegates from all member societies meet to determine and approve EFC policy. News of EFC activities, forthcoming confer-ences, courses, etc., is published in a range of accredited corrosion and certain other journals throughout Europe. More detailed descriptions of activities are given in a Newsletter prepared by the Scientifi c Secretary.

The output of the EFC takes various forms. Papers on particular topics, e.g. reviews or results of experimental work, may be published in scientifi c and technical journals in one or more countries in Europe. Conference proceedings are often published by the organisation responsible for the conference.

In 1987 the, then, Institute of Metals was appointed as the offi cial EFC publisher. Although the arrangement is non-exclusive and other routes for publication are still available, it is expected that the Working Parties of the EFC will use The Institute of Materials, Minerals and Mining for publication of reports, proceedings, etc., wherever possible.

The name of The Institute of Metals was changed to The Institute of Materials (IoM) on 1 January 1992 and to The Institute of Materials, Minerals and Mining with effect from 26 June 2002. The series is now published by Maney Publishing on behalf of The Institute of Materials, Minerals and Mining.

P. McIntyreEFC Series EditorThe Institute of Materials, Minerals and Mining, London, UK

EFC Secretariats are located at:

Dr B. A. RickinsonEuropean Federation of Corrosion, The Institute of Materials, Minerals and Mining, 1 Carlton House Terrace, London SW1Y 5AF, UK

Mr M. RocheFdration Europenne de la Corrosion, Fdration Franaise pour les sciences de la Chimie, 28 rue Saint-Dominique, F-75007 Paris, France

Dr W. MeierEuropische Fderation Korrosion, DECHEMA e.V., Theodor-Heuss-Allee 25, D-60486 Frankfurt-am-Main, Germany

Series introduction xi

xii

Volumes in the EFC series

* indicates volume out of print

1 Corrosion in the nuclear industry Prepared by Working Party 4 on Nuclear Corrosion*

2 Practical corrosion principles Prepared by Working Party 7 on Corrosion Education*

3 General guidelines for corrosion testing of materials for marine applications Prepared by Working Party 9 on Marine Corrosion*

4 Guidelines on electrochemical corrosion measurements Prepared by Working Party 8 on Physico-Chemical Methods of Corrosion

Testing

5 Illustrated case histories of marine corrosion Prepared by Working Party 9 on Marine Corrosion

6 Corrosion education manual Prepared by Working Party 7 on Corrosion Education

7 Corrosion problems related to nuclear waste disposal Prepared by Working Party 4 on Nuclear Corrosion

8 Microbial corrosion Prepared by Working Party 10 on Microbial Corrosion*

9 Microbiological degradation of materials and methods of protection Prepared by Working Party 10 on Microbial Corrosion

10 Marine corrosion of stainless steels: chlorination and microbial effects Prepared by Working Party 9 on Marine Corrosion

11 Corrosion inhibitors Prepared by the Working Party on Inhibitors*

12 Modifi cations of passive fi lms Prepared by Working Party 6 on Surface Science*

13 Predicting CO2 corrosion in the oil and gas industry Prepared by Working Party 13 on Corrosion in Oil and Gas Production*

14 Guidelines for methods of testing and research in high temperature corrosion Prepared by Working Party 3 on Corrosion by Hot Gases and Combustion

Products

Volumes in the EFC series xiii

15 Microbial corrosion: Proceedings of the 3rd International EFC Workshop Prepared by Working Party 10 on Microbial Corrosion

16 Guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production (3rd Edition)

Prepared by Working Party 13 on Corrosion in Oil and Gas Production

17 Corrosion resistant alloys for oil and gas production: guidance on general requirements and test methods for H2S service (2nd Edition)


18 Stainless steel in concrete: state of the art report Prepared by Working Party 11 on Corrosion of Steel in Concrete

19 Sea water corrosion of stainless steels: mechanisms and experiences Prepared by Working Party 9 on Marine Corrosion and Working Party 10 on

Microbial Corrosion

20 Organic and inorganic coatings for corrosion prevention: research and experiences

Papers from EUROCORR 96

21 Corrosion-deformation interactions CDI 96 in conjunction with EUROCORR 96

22 Aspects of microbially induced corrosion Papers from EUROCORR 96 and EFC Working Party 10 on Microbial

Corrosion

23 CO2 corrosion control in oil and gas production: design considerations Prepared by Working Party 13 on Corrosion in Oil and Gas Production

24 Electrochemical rehabilitation methods for reinforced concrete structures: a state of the art report

Prepared by Working Party 11 on Corrosion of Steel in Concrete

25 Corrosion of reinforcement in concrete: monitoring, prevention and rehabilitation Papers from EUROCORR 97

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

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

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

29 Microbial corrosion: proceedings of the 4th International EFC Workshop Prepared by the Working Party on Microbial Corrosion

30 Survey of literature on crevice corrosion (19791998): mechanisms, test methods and results, practical experience, protective measures and monitoring

Prepared by F. P. Ijsseling and Working Party 9 on Marine Corrosion

xiv Volumes in the EFC series

31 Corrosion of reinforcement in concrete: corrosion mechanisms and corrosion protection

Papers from EUROCORR 99 and Working Party 11 on Corrosion of Steel in Concrete

32 Guidelines for the compilation of corrosion cost data and for the calculation of the life cycle cost of corrosion: a working party report


33 Marine corrosion of stainless steels: testing, selection, experience, protection and monitoring

Edited by D. Fron on behalf of Working Party 9 on Marine Corrosion

34 Lifetime modelling of high temperature corrosion processes Proceedings of an EFC Workshop 2001 Edited by M. Schtze, W. J. Quadakkers

and J. R. Nicholls

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

Corrosion of Steel in Concrete

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

37 Test methods for assessing the susceptibility of prestressing steels to hydrogen induced stress corrosion cracking

By B. Isecke on behalf of Working Party 11 on Corrosion of Steel in Concrete

38 Corrosion of reinforcement in concrete: mechanisms, monitoring, inhibitors and rehabilitation techniques

Edited by M. Raupach, B. Elsener, R. Polder and J.Mietz on behalf of Working Party 11 on Corrosion of Steel in Concrete

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

13 on Corrosion in Oil and Gas Production

40 Control of corrosion in cooling waters Edited by J. D. Harston and F. Ropital on behalf of Working Party 15 on

Corrosion in the Refi nery Industry

41 Metal dusting, carburisation and nitridation Edited by H. Grabke and M. Schtze on behalf of Working Party 3 on Corrosion

by Hot Gases and Combustion Products

42 Corrosion in refi neries Edited by J. D. Harston and F. Ropital on behalf of Working Party 15 on


43 The electrochemistry and characteristics of embeddable reference electrodes for concrete

Prepared by R. Myrdal on behalf of Working Party 11 on Corrosion of Steel in Concrete

Volumes in the EFC series xv

44 The use of electrochemical scanning tunnelling microscopy (EC-STM) in corrosion analysis: reference material and procedural guidelines

Prepared by R. Lindstrm, V. Maurice, L. Klein and P. Marcus on behalf of Working Party 6 on Surface Science

45 Local probe techniques for corrosion research Edited by R. Oltra on behalf of Working Party 8 on Physico-Chemical Methods of

Corrosion Testing

46 Amine unit corrosion survey Edited by J. D. Harston and F. Ropital on behalf of Working Party 15 on


47 Novel approaches to the improvement of high temperature corrosion resistance Edited by M. Schtze and W. Quadakkers on behalf of Working Party 3 on

Corrosion by Hot Gases and Combustion Products

48 Corrosion of metallic heritage artefacts: investigation, conservation and prediction of long term behaviour

Edited by P. Dillmann, G. Branger, P. Piccardo and H. Matthiesen on behalf of Working Party 4 on Nuclear Corrosion

49 Electrochemistry in light water reactors: reference electrodes, measurement, corrosion and tribocorrosion

Edited by R.-W. Bosch, D. Fron and J.-P. Celis on behalf of Working Party 4 on Nuclear Corrosion

50 Corrosion behaviour and protection of copper and aluminium alloys in seawater Edited by D. Fron on behalf of Working Party 9 on Marine Corrosion

51 Corrosion issues in light water reactors: stress corrosion cracking Edited by D. Fron and J-M. Olive on behalf of Working Party 4 on Nuclear

Corrosion

52 Progress in Corrosion The fi rst 50 years of the EFC Edited by P. McIntyre and J. Vogelsang on behalf of the EFC Science and

Technology Advisory Committee

53 Standardisation of thermal cycling exposure testing Edited by M. Schtze and M. Malessa on behalf of Working Party 3 on Corrosion

by Hot Gases and Combustion Products

54 Innovative pre-treatment techniques to prevent corrosion of metallic surfaces Edited by L. Fedrizzi, H. Terryn and A. Simes on behalf of Working Party 14 on

Coatings

55 Corrosion-under-insulation (CUI) guidelines Prepared by S. Winnik on behalf of Working Party 13 on Corrosion in Oil and Gas

Production and Working Party 15 on Corrosion in the Refi nery Industry

56 Corrosion monitoring in nuclear systems Edited by S. Ritter and A. Molander

xvi Volumes in the EFC series

57 Protective systems for high temperature applications: from theory to industrial implementation

Edited by M. Schtze

58 Self-healing properties of new surface treatments Edited by L. Fedrizzi, W. Frbeth and F. Montemor

60 Methodology of crevice corrosion testing for stainless steels in natural and treated seawaters

Edited by U. Kiviskk, B. Espelid and D. Fron

61 Inter-laboratory study on electrochemical methods for the characterisation of CoCrMo biomedical alloys in simulated body fl uids

Edited by A. Igual Munoz and S. Mischler

All volumes are available from Maney Publishing or its North American distributor. See http://maney.co.uk/index.php/series/efc_series/

xvii

Preface

Since the end of the 1980s, many efforts have been made in Europe to understand crevice corrosion of stainless steels in seawater and to give a practical and an industrial response to these phenomena. In the 1990s, the work was mainly carried out as two collaborative research programmes involving a number of European labo-ratories and test sites in eight countries and in the fi ve European seas (Atlantic Ocean, Baltic, Mediterranean and North Seas, English Channel). One programme was organised by the Marine Corrosion Working party of the European Federation of Corrosion and the other by the MArine Science and Technology Directorate (MAST) of the European Union in Brussels. Results of these two programmes have been widely presented during European workshops and at Eurocorr; they have been published in the EFC series (Number 19 and Number 33). At the beginning of the 2000s, the scientifi c knowledge coming from previous programmes was applied to develop and qualify new reliable and reproducible test methods in order to charac-te rise the susceptibility of passive metals to crevice corrosion in marine environments. This project, called CREV CORR, was initiated and funded by the European Community under the Com peti tive and Sustainable Growth Programme. The main objective was to develop a crevice corrosion qualifi cation test for stainless steels to be used in marine environments. Users and manufacturers of stainless steels and other passive metals have a need for a test metho do logy which reliably and reproducibly characterises the crevice corrosion behaviour of these materials in marine environ-ments. For years, many methods have been develop ed to characterise the crevice corrosion behaviour of stain less steels, but the majority of these are of a comparative nature, aimed only at ranking alloys. There is therefore a need to develop a test method for passive metals to refl ect reliably and reproducibly the long-term corrosion behaviour of these materials in natural and treated seawaters.

The objectives of this book are to present the main results obtained which also include the procedures for crevice corrosion tests discussed and used by more than 20 laboratories. The 10 chapters include a historical perspective of crevice corrosion (Chapter 1), an overview of the programme developed (Chapter 2), calibration tests and procedures of the participating laboratories (Chapter 3), and description of the new crevice formers for plates (Chapter 4) and tubes (Chapter 5). Then ways to simulate the corrosivity of aerated natural seawater (Chapter 6), anaerobic seawater (Chapter 7) and mixed conditions (Chapter 8) are proposed and argued. The biological and oxidation capacity of natural and treated seawaters can also be simu-lated electrochemically (Chapter 9). The fi nal chapter (Chapter 10) describes results obtained during large round robin tests performed to compare the developed test methods with standard tests and natural seawater tests to verify the reliability and reproducibility of the new test methods. All of these methods which have been developed and implemented in this programme are described in detail in the nine appendixes of the book which include the calibration procedures (Appendix A), the

xviii Crevice corrosion testing for stainless steels

procedures for crevice corrosion tests in natural and synthetic biochemical seawaters for plates (Appendix B), tubes (Appendix C) and umbilical applications (Appendix F), and the description of the synthetic biochemical seawater (Appendix D). The electrochemical procedure to simulate aerobic biofi lms is also described in Appendix E, while special emphasis is given for crevice test procedures used during the round robin tests in natural seawater (Appendix G) or in biosynthetic seawater (Appendix H). The method of reporting results is given in Appendix I.

These procedures have been successfully applied during this programme. If they are widely used, they may improve the success ful application of the materials and components, thereby minimising replacement costs and costs of claims.

The editors of the volume would like to thank the partners of the CREVCORR-project: DET NORSKE VERITAS (Norway, coordinator), FORCE Institute (Denmark), Commissariat lEnergie Atomique (France), Consiglio Nazionale Del-le Richerche (Italy), SINTEF (Norway), Shell Global Solutions (The Netherlands), Avesta Sheffi eld AB (Sweden later on AvestaPolarit AB and today Outokumpu Stainless AB), TNO (The Netherlands), Statoil (Norway), AB Sandvik Steel (Sweden later on AB Sandvik Materials Technology). In the book, the names of the affi liations that were used when the work was carried out are used.

The editors hope that the book will be useful to scientists and engineers who are involved in the selection of stainless steels and passive materials for seawater applica-tions and will help them to predict reliably and reproducibly the long-term corrosion behaviour of these materials in seawater and in other chlorinated natural environ-ments.

Ulf Kiviskk, Chairman of the EFC WP on Marine Corrosion

Brd Espelid, Past-chairman of the EFC WP on Marine Corrosion

Damien Fron, Chairman of the EFC Science and Technology Advisory Committee (STAC)

11Crevice corrosion from a historical perspective

Bengt WallnAvesta, Sweden

1.1 Introduction

Crevice corrosion is a very important type of corrosion of stainless steels, and in chloride solutions such as seawater, it is often the factor determining the life of a construction. However, crevice corrosion has not always been recognised as a danger. This chapter describes how our understanding of the crevice corrosion mechanism has developed over the years and how different types of tests have come and sometimes gone. All seen from an industrial corrosion laboratorys viewpoint.

1.2 The mechanism

Accelerated corrosion in creviced areas on iron was reported quite early on [1]. Stainless steels were invented in 1912 but it took some time before crevice corrosion was reported for this type of alloy. In a pioneering book from 1926, Monypenny noted that when testing stainless steels in chloride solutions, corrosion almost invari-ably starts at the point of support and is often entirely confi ned to that point [2]. This is still a well known problem to anyone performing crevice corrosion tests.

In the early days, there was great optimism regarding the corrosion resistance of stainless steels. For instance, in a textbook from 1933, Type 304 is considered as suffi ciently resistant to chloride solutions such as seawater although warnings were given for the risk of pitting corrosion associated with surface defects. Only in strong chloride solutions such as brines and hypochlorite solutions were molybdenum alloyed steels considered necessary [3]. Figure 1.1 shows another example of this optimism. The protocol shows the result of an Avesta investigation from 1930. Different steels, among them the very fi rst duplex steels (453E and 453S), were tested in seawater at 50C. The corrosion rate was very low and there were no comments as to localised corrosion!

In Avestas fi rst Corrosion Table published in 1934, crevice corrosion was not mentioned at all. However, in his textbook from 1937, Evans refers to papers pub-lished in the early 1930s describing remarkable examples of pitting of stainless steels by cranny action [4]. Evans introduces the term crevice or cranny corrosion and sug-gests differential aeration as the mechanism. A few years later, Smith [5] and Uhlig [6] described contact corrosion which appears in connection with dirt accumulation on the steel surface or attached barnacles in seawater installations. This is considered as a special case of pitting corrosion caused by differential aeration. Over the years, different mechanisms have been proposed but a unifi ed mechanism was not presented until 1967 in the textbook of Fontana and Greene [7].

2 Crevice corrosion testing for stainless steels

1.3 The ferric chloride test

This is probably one of the most frequently used tests on stainless steels today. However, the test solution is not new; even in the late 1930s, pitting tests using ferric chloride were recommended by MIT [5] in the USA and by Jernkontoret (the Swedish Ironmasters Assoc.).

The latter investigation started in 1938 and was reported after the war [8]. MIT suggested a 6% solution plus 1.8% hydrochloric acid while 30% plus 0.5% hydrochlo-ric acid or 5% plus 10% sodium chloride was the Swedish choice. Ferric chloride slowly became used for crevice corrosion testing too. There was no standard crevice but they were formed by anything from glass pearls or sand to rubber bands.

In many respects, the 1970s was the golden age of crevice corrosion testing achieve-ments and this is true for all kinds of tests. The mechanism was known and the great practical impact of crevice corrosion was recognised. This triggered many improvements. In 1973, Brigham introduced the critical temperature concept [9,10]. He suggested that the ferric chloride solution should be used at successively increas-ing temperatures until crevice corrosion could be seen, that is at the critical crevice corrosion temperature (CCT).

In 1976, the fi rst standardised method appeared [11]. In the well known ASTM G48-B test, the specimens looked like that shown in Figure 1.2. Rubber bands or O-rings were used to press PTFE cylinders against the surface of the specimens. Unfortunately, this was not a very good confi guration because the edges were attacked fi rst and this resulted in uncontrolled cathodic protection of the PTFE crevices which are normally the ones of interest. This inconvenience had been observed almost 40 years earlier by Smith [5] who noted that contact corrosion on the bold surface of a test specimen would be infl uenced by pitting corrosion on the cut edges and that the latter provided protection to the remaining surface. Smith concluded that this may cause the steel to appear better than it is in reality.

1.1 Test protocol from Avesta for testing in seawater at 50C dated 23 February 1930

Crevice corrosion from a historical perspective 3

Around 1980, a great step forward was taken when the MTI-2 procedure was established [12]. Figure 1.3 shows the test specimen. Rubber bands and O-rings were abandoned and the crevice formers were instead bolted to the surface using a specifi c torque. In this way, edge attacks were excluded and the reproducibility of the results increased. Brighams CCT concept was adopted and so was the multicrevice washer invented by Anderson in the mid-1970s [13]. MTI-2 became a defacto standard until ASTM G48 was revised in 1999 [14]. This standard now includes methods D and F, both of which determine CCT using multicrevice washers bolted with a specifi c torque. The ferric chloride test solution now contains hydrochloric acid which brings us back to the 1930s.

1.4 Field tests

From the very beginning, stainless steel specimens seem to have been exposed, typically for several months, in natural or industrial environments which could be anything from seawater to tall oil distillation columns. For a long time, there were no intentional crevices applied on the specimens but they were often simply hanging by a wire or a rope. In the 1930s, the fi rst spool type holders, or test racks, were described in the literature and they were standardised in 1946 [15]. Figure 1.4, taken from Champions book published in 1952, shows the original type [16]. The spacers sepa-rating the specimens were not used to create crevices but only to keep the specimens electrically insulated.

This type of test rack was used for many years and was for instance used in an extensive Swedish fi eld test programme, started in 1970, where the corrosion condi-tions in bleach plant washers were studied [17]. However, in the early 1970s, the test racks were modifi ed. Multicrevice washers started to be used as crevice formers and the torque of the nut was used to ensure a constant contact pressure. A modern,

1.2 The G48-B specimen introduced in 1976


standardised rack is described in Ref. 18. Figure 1.5 shows a test rack frequently used by AvestaPolarit.

Exposure of test racks is an excellent way of comparing the corrosion resistance of different alloys in industrial process solutions. However, like laboratory tests, such tests cannot always be used to predict the corrosion resistance of a practical construction with its variety of crevice geometries. So far, the only reliable way of doing this is to perform tests with prototype systems built from real components taken from ordinary production. Prototype tests, well performed, allow determina-tion of the limits of an alloy, e.g. the maximum temperature or chlorination level of a seawater system. Figure 1.6 shows such a test [19]. A small plate heat exchanger, equipped with plates made from different alloys, is used here to cool hot fresh water with seawater. Unfortunately, prototype tests are expensive so if the CREVCORR project results in a good substitute, life would become simpler for all of us.

1.3 The crevice former in MTI-2

1.4 Test rack from Champion [16]


1.5 Electrochemical tests

Electrochemical tests were rarely used in industrial laboratories in the early days. One exception might be the galvanostatic method, described by Brennert in 1937, in which breakthrough or pitting potentials were determined [20]. In 1959, however, a slight

1.5 Test rack used by AvestaPolarit

1.6 Prototype test in seawater [19]


revolution took place in the corrosion society when electronic potentiostats started to be used generally. The fi rst versions were very simple but they allowed determination of anodic polarisation curves in all kinds of solutions a great step forward.

Although a few electrochemical crevice corrosion tests appeared after the potentio-stat came into use, it was not until 1974, when Crolet introduced the critical pH concept, that they achieved wider use in industrial laboratories [21]. In a modifi ed form, this method is still used for determining critical crevice solutions which are important parameters in the mathematical models used for predicting crevice corrosion resistance.

In the 1990s, two promising electrochemical tests have been presented, which used creviced specimens in contrast to earlier electrochemical tests. In one test, the critical crevice corrosion potential was determined at a constant temperature [22] while the other test determined the critical temperature [23]. In the latter, the potential could be changed with time during the test to simulate the conditions in, for example, natural seawater.

A mathematical crevice corrosion model is not a test in itself but builds on electro-chemical test results and on intricate calculations of chemical equilibria and ion transport in the crevice solution. Pioneering work was carried out by Oldfi eld and Sutton in the 1970s [24]. Their model calculated the time to initiation of crevice corrosion. A very advanced model was presented by Gartland [25, 26]. Besides the time to initiation, it also predicted the propagation rate and time to repassivation, if any. Furthermore, it took the outer potential into account making it possible to observe the differences between corrosion in, for example, natural and chlorinated seawater. Gartlands model is very useful for understanding various aspects of crevice corrosion.

1.6 Conclusions

A literature review regarding crevice corrosion on stainless steel shows:

Accelerated corrosion in creviced areas was reported as early as the 1920s and was then believed to be a special case of pitting corrosion.

Crevice corrosion was identifi ed as a separate type of corrosion in the 1930s when differential aeration was considered the mechanism.

A unifi ed mechanism was not proposed until the 1960s. A great number of crevice corrosion tests have been used over the years but only

in the 1970s did tests with fairly reproducible crevice geometries begin to come into use.

Ferric chloride solution was used quite early on but it was not until the 1970s that the ferric chloride test was standardised.

The introduction of the electronic potentiostat in the late 1950s opened the way for electrochemical crevice corrosion tests, but they only started to be used in the 1970s and were frequently used in the 1990s.

Field tests in industrial environments were performed without creviced specimens for a long time, and it was not until the 1970s that intentional crevice formers became commonly used.

Mathematical models for predicting the crevice corrosion resistance were developed in the 1970s and further refi ned in the 1990s.


References

1. G. T. Moody, J. Chem. Soc., 89 (1906), 723. 2. J. H. G. Monypenny, Stainless Iron and Steel. Chapman and Hall, London, 1926 3. E. E. Thum (ed.), The Book of Stainless Steels. The American Society for Steel Treating,

Cleveland, 1933. 4. U. R. Evans, Metallic Corrosion Passivity and Protection. Edward Arnold & Co, London,

1937. 5. H. A. Smith, Metal Progress, 33 (1938), 596. 6. H. H. Uhlig, Trans. A.I.M.E. 140 (1940), 411 7. M. G. Fontana, N. D. Greene, Corrosion Engineering. McGraw-Hill, New York, 1967. 8. G. Lindh, Jernkontorets Tekniska Rd., 16 (1948), 13 (in Swedish). 9. R. J. Brigham, E. W. Tozer, Corrosion, 29 (1973), 33.10. R. J. Brigham, Corrosion, 30 (1974), 396.11. ASTM G48-76, ASTM, Philadelphia, PA, 1976.12. R. S. Treseder, MTI Manual No. 3 (MTI Project No. 9), MTI, Columbus, Ohio, 1980.13. D. B. Anderson, ASTM STP, 576 (1976), 231.14. ASTM G48-99, ASTM, Philadelphia, PA, 1999.15. ASTM A224-46, ASTM, Philadelphia, PA, 1946.16. F. A. Champion, Corrosion Testing Procedures. Chapman and Hall, London, 1952.17. B. Walln, Proceedings, Pulp & Paper Industry Corrosion Problems, Vol. 2, NACE,

Denver, 1977.18. ASTM G4-95, ASTM, Philadelphia, PA, 1995.19. B. Walln and L. Wegrelius, Proceedings, EUROCORR 2000, Topic No. 4, Paper No. 4,

Sept. 2000, London.20. S. Brennert, J. Iron Steel Inst., 135 (1937), 101.21. J. W. Crolet, et al., Mem. Sci. Rev. Metallurg., 71 (1974), 797.22. S. Huizinga and J. G. De Jong, Proceedings, EUROCORR 1996, part XIV/OR 3, p.1,

Sept. 1996, Nice.23. U. Steinsmo, T. Rogne and J. M. Drugli, Proceedings, EUROCORR 1997, p.773, Sept.

1997, Trondheim.24. J. W. Oldfi eld and W. H. Sutton, Br. Corros. J., 13 (1978), 14.25. P. O. Gartland, Nace Corrosion-88, presentation at Research Symposium, St. Louis, MO,

1988.26. P. O. Gartland, Proceedings, Crevice Corrosion: The Science and its Control in

Engineering Practice, Nace Corrosion-96, Houston, TX, 1996.

82Objectives and background

Brd EspelidDNV, Bergen, Norway

2.1 Introduction

The main objectives of this book are:

to describe the work performed to develop a new test methodology to character-ise the susceptibility of stainless steels to crevice corrosion in natural and treated seawaters;

to describe the experimental procedures to perform crevice corrosion testing according to the proposed new test specifi cations.

For years, many methods have been develop ed to characterise the crevice corrosion behaviour of stain less steels, but the majority of these are of a comparative nature, aimed only at ranking alloys [2]. Until now, no test methods have been recognised for testing passive materials such as stain less steels in marine environments. Many different forms of test set-up have been used, often giving different and varying results. Furthermore, the test procedures may vary somewhat between laboratories, as in-house modifi cations are applied instead of descriptions given in the test specifi cation. This also contributes to the diffi culties in comparing tests between test laboratories. Therefore, there is a need to develop a test method for passive metals to refl ect reliably and reproducibly the long-term corrosion behaviour of these materials. There is a general agreement that the main corrosion risk in practical appli cations is crevice corrosion and that any qualifi cation test should be a crevice corrosion test. The development of a reliable and reproducible test method which provides information on the long-term behaviour of stainless steels will reduce the number of failures related to crevice corrosion of these materials. This will be of signifi cant importance to many industries dependent upon marine technology, as it will have a benefi cial impact on the safety, economics and environmental friendli ness of operations.

During the last decade, the industry has therefore been discussing new test methods suitable for qualifi cation of stainless steels used in seawater applications. Users and manufacturers of stainless steels and other passive metals have expressed a need for a test metho do logy which reliably and reproducibly characterises the crevice corrosion behaviour of these materials in marine environments [3]. Such a test method will contribute to qualifi cation of steel materials for different marine conditions.

In order to develop and qualify a new reliable and reproducible test method to charac te rise the susceptibility of passive metals to crevice corrosion in marine environments, the project CREV CORR [1], which was funded by the European Community under the Com peti tive and Sustainable Growth Programme, has been undertaken (Table 2.1). The main objective of this project has been to develop a crev-ice corrosion qualifi cation test for stainless steels to be used in marine environments. The work of the project comprised four different activities discussed below.

Objectives and background 9

2.2 Establishment of the state-of-the-art

The objective of this activity has been to establish the state-of-the-art on qualifi cation testing of stain less steels to be used in seawater and to defi ne types of natural and treated seawaters to be studied in the testing. The major tasks performed were:

Compilation of relevant and important experience from previous work. Defi ning natural and treated seawaters which were relevant for qualifi cation testing

of stainless steels. Calibrating participating laboratories in the project to examine if in-house

procedures and practices affected test results.

2.3 Formulation of a new synthetic seawater

The objective of this activity was to formulate synthetic seawater which could simulate the corrosivity of natural and treated seawaters. This included the effect of biofi lms present in natural waters. The major tasks within this activity were:

Reproduction of aerobic biofi lms by a chemical method or a biochemical method. Reproduction of anaerobic biofi lms and tests on synergistic effects of different types

of biofi lms. Testing of stainless steels in the developed synthetic seawaters.

Table 2.1 Partners in the CREVCORR project

Project Company Abbreviation Scientifi c offi cer

Coordinator DET NORSKE VERITAS AS, Bergen, Norway

DNV Brd ESPELID

Work package leader

FORCE Technology, Brondby, Denmark

FORCE Ebbe RISLUND

Work package leader

COMMISSARIAT lENERGIE ATOMIQUE, Saclay, France

CEA Damien FRON

Work package leader

The Foundation for Scientifi c and Industrial Research at the Norwegian Institute of Technology, Trondheim, Norway

SINTEF Trond ROGNE

Work package leader

Shell Global Solutions B.V., Amsterdam, The Netherlands

SHELL Sytze HUIZINGA

Partner Consiglio Nazionale Delle Richerche, Genova, Italy

CNR Alfonso MOLLICA

Partner Netherlands Organization for Aplied Scientifi c Research, Den Helder, The Netherlands

TNO Gabrielle FERRARI

Partner AvestaPolarit AB, Avesta, Sweden Avesta Lena WEGRELIUS(Bengt WALLEN)

Partner Den Norske Stats oljeselskap A/S, Stavanger, Norway

STATOIL ystein STRANDMYR

Partner Sandvik Materials Technology, Sandviken, Sweden

SANDVIK Ulf KIVISKK


2.4 Electrochemically controlled crevice corrosion test

The objective of this activity has been to develop a crevice corrosion test procedure where the biological activity and oxidation capacity of natural and treated seawaters were simulated electrochemically. The following tasks have been performed:

Optimisation of test parameters of importance for crevice corrosion testing. Electrochemical control of creviced test specimens to follow initiation and

propagation. Electrochemical simulation of biofi lm effects and treatment of seawaters.

2.5 Inter-comparison testing

The objective of this activity has been to verify the reliability and reproducibility of the test methods developed in paragraphs 2.3 and 2.4 (see above). The following activities have been performed:

Establishing a laboratory group participating in the inter-comparison test programme.

Conduct the inter-comparison test programme in all participating laboratories. Compare the results from the test methods developed within the project with

test results from other standard test methods and longer term exposure to different natural seawaters.

Several stainless steels have been used in the test programme, including austenitic, ferritic and duplex steels. The EN designations have been chosen for identifying the materials. Producers and test materials used are listed in Table 2.2.

The main output of the project has been the development of improved crevice corrosion test methodology for qualifi cation of stainless steels to be used in natural and treated seawaters. This test method will be useful for many industrial sectors using stainless steel in seawater. A major user group will be the offshore oil and gas industry which uses large amounts of stainless steels for seawater handling and equipment exposed to sea water. Another user group will be the shipping industry, which also uses a large amount of stain less steels in piping systems carrying seawater, e.g. for cooling purposes or for ballast water. Also manufacturers that deliver stain-less steels will benefi t from an improved qualifi cation test method. By applying this, inappropriate manufacturing procedures and potentially inadequate materials can be corrected and changed before a complete in stal led component fails. This will improve

Table 2.2 Designations of test materials used in the CREVCORR project

EN Material form UNS Producer Trade name

1.4404 Plate S31603 Outokumpu 44041.4547 Plate S31254 Outokumpu 254 SMOa

1.4462 Plate S31803 Outokumpu 2205Tube S31803/S32205 Sandvik Sandvik SAF 2205b

1.4410 Tube S32750 Sandvik Sandvik SAF 2507b

aTrademark owned by Outokumpu Stainless.bTrademark owned by AB Sandvik Materials Technology.

Objectives and background 11

the success ful application of the materials and components, thereby minimising the replacement costs and costs of claims.

As stated earlier, this book will present both the work performed to develop the new test methodology as well as the test procedures themselves. In addition, it is the intention that the book will contribute to the implementation and use of the test methodology. The use of the methods will hopefully give valuable feedback experi-ence from relevant industries and laboratories, giving the basis for development and improvements in the proposed crevice corrosion test methodology. Possibly, within the not too distant future, the test method could be developed into an ISO standard.

References

1. Development of new method to characterize the durability of stainless steels to crevice attack in natural and treated seawaters (CREVCORR), European Union Fifth Framework Programme (GROWTH type), Ref: G5RD-CT-2000-00139 (start date: 15 April 2000, end date: 15 October 2003).

2. F. P. Ijsseling, Survey of Literature on Crevice Corrosion (1979-1998), European Federation of Corrosion Publications No. 30, ISBN 1-86125-4. IOM Communications Ltd, London, 2000.

3. D. Fron, Marine Corrosion of Stainless Steels, European Federation of Corrosion Publications No. 33, ISBN 1-86125-151-3. IOM Communications Ltd, London, 2001.

12

3Laboratory calibration


3.1 Calibration of participating laboratories in the project

In order to examine if the results from crevice corrosion testing were affected by individual laboratory in-house procedures and practices, a calibration test was carried out. The results from this test also provided guidelines on the most relevant acceptance criteria for a crevice corrosion test.

3.2 Experimental procedure

The test procedure applied was proposed by Avesta and accepted by the project partners and is presented in Appendix A. Avesta have also provided the rectangular coupon test specimens and castellated crevice polytetrafl uoroethylene (PTFE) wash-ers. Specimens were produced from EN 1.4547 super-austenitic steel and from EN 1.4462 duplex steel. The creviced specimens were exposed in a 6% FeCl3 solution.

For EN 1.4547, the temperature for the fi rst test was 32.5C and for EN 1.4462 duplex steel it was 22.5C. Depending on the result of the fi rst test, the second test was performed either 5C lower, namely if attack deeper than or equal to 25 m had occurred in the fi rst test, or 5C higher, if that was not the case. The third test was to be performed either 2.5C lower or 2.5C higher, depending on whether attack deeper than or equal to 25 m was observed in the previous test or not.

The Critical Crevice Temperature (CCT) was determined as the lowest tem perature at which the maximum depth of attack exceeded or was equal to 25 m. Some other criteria were evaluated as well. These included weight loss and number of attacks and etchings observed.

3.3 Test results

Weight loss, number of etchings/attacks and maximum depth of attack are displayed in Figs. 3.13.3, respectively, for EN 1.4547 and Figs. 3.43.6 for EN 1.4462 duplex steel. Note that some of the higher results have not been plotted in order to maintain suffi cient resolution at the lower value end of the y-axis.

3.3.1 Weight loss

Both for EN 1.4547 and EN 1.4462, weight loss does not appear to be a reliable measure of CCT. Only when CCT is clearly exceeded, can a signifi cant increase in weight loss be found (Figs. 3.1 and 3.4).

Laboratory calibration 13

3.3.2 Number of etchings/attacks

The number of attacked or etched areas in itself does not appear to be a reliable measure of crevice attack, with a large variation between laboratories and in some cases also between duplicate tests (Figs. 3.2 and 3.5).

On EN 1.4547, Avesta report the highest number of attacks in the relevant temperature range (up to 35C) and TNO the lowest. Overall, the number of attacks

3.1 EN 1.4547 weight loss results

3.2 EN 1.4547 number of attacks/etching observed


is not high unless at higher temperatures, which is well known for the nobler stainless steels where one attacked area protects the others almost immediately.

On EN 1.4462 duplex steel, Avesta and Sandvik report the highest number of attacks and TNO the lowest. Generally, the number of attacks is higher than for EN 1.4547, which appears to be in agreement with the lower localised corrosion resistance of EN 1.4462.

The absence of any crevice attack, although in theory a good criterion, appears to be too conservative. Requiring all areas to corrode as a criterion is wrong in view of the protection given by one attacked area to the others.

3.3 EN 1.4547 maximum depth of attack

3.4 EN 1.4462 weight loss results

Laboratory calibration 15

It appears that some systematic differences exist between laboratories. This could be due to the surface preparation or to the application of the crevice washer. While the surface preparation is rather straightforward, it may be that the latter is the more likely cause. This could be related to the nature of the PTFE washers, which are readily deformed under pressure. Polyvinylidene fl uoride (PVDF) will work better in this respect. Moreover, the use of a torque wrench does not in itself guarantee a specifi c applied force to the washer.

3.5 EN 1.4462 number of attacks/etching observed

3.6 EN 1.4462 maximum depth of attack


3.3.3 Maximum depth of attack

Again, for both EN 1.4547 and EN 1.4462, there is a large variation in depth of attack, in particular between laboratories, but also in a few cases between duplicates. However, focusing on the deepest attack appears to provide a consistent measure for CCT within the accuracy achievable with the 2.5C test intervals (Figs. 3.3 and 3.6).

Table 3.3 summarises the CCT values based on the criterion of the maximum depth of attack equalling or exceeding 25 m. For EN 1.4547, all laboratories arrive at 30 or 32.5C for CCT. For EN 1.4462, all arrive at 20 or 22.5C (assuming that Sandvik and Sintef would also have obtained less than 25 m attack if they had performed a test at 17.5 or 20C).

In the context of the scatter observed in the number of attacks and depth of attacks, it is clear that some form of statistics needs to be applied, and it appears that simply using the maximum depth serves this purpose.

A basic objection against this approach would be that it relies on crevice pro-pagation, which in turn depends on the available cathodic driving force. Apparently, in the present test with rather concentrated FeCl3 and limited coupon surface area, this driving force does not limit propagation to such a degree that it prevents proper interpretation of the results. However, this may not be the case in all crevice corrosion tests. For instance, it is known that in seawater, a very large cathodic surface area may be needed to prevent cathodic limitation.

It could also be stated that any depth of attack is unacceptable and is in fact crevice corrosion. However, from a practical point of view, 0 (zero) depth of attack is not a manageable criterion.

3.4 Conclusions from the calibration test

Amongst the possible criteria evaluated, it would appear that a maximum depth of attack equal to or exceeding 25 m allows a relatively accurate determination of CCT, even though signifi cant scatter exists between individual depth results from different laboratories and in some cases between duplicate tests.

It cannot be guaranteed that this criterion works as well in other environments, where propagation may be cathodically limited.

Attention should be paid to application of the washer and washer material. Instead of PTFE, PVDF could be used. It would appear advisable to use a well defi ned pressure (force) instead of a torque.

17

4Crevice formers for specimens of plate material


4.1 Optimisation of test parameters of importance for crevice corrosion testing

The selection of a suitable crevice forming method includes choosing a crevice forming polymer material. Based on the literature and experiences during the last 10 years, the materials polytetrafl uoroethylene (PTFE), polyoxy me thylene (POM) and polyvinylidene fl uoride (PVDF) were selected for consideration with PVDF as possi-bly the most versatile material especially when applied at higher temperatures. PTFE has been used in standard ised crevice corrosion testing in ferric chloride solution and could act as a sort of reference material for the two others.

Concerning mounting of crevice formers on test specimens, fi xed mounting has been the most widely used method with tightening to a specifi ed torque in an attempt to increase the accuracy of the crevice geometry thereby increasing the reproducibi-lity. It would appear advisable to apply a well defi ned and constant pressure (force) instead of a torque: this is achievable by using disc springs to apply the load on the crevice formers. It was decided to incorporate disc spring loading of the crevice formers. This could also increase the reproducibility of the crevice geometry.

4.1 Infl uence of crevice former material on the critical crevice corro sion temperature (CCT) in 6% ferric chloride. Material EN 1.4547


A test programme including spring loading of the materials PTFE, POM and PVDF was executed in 6% ferric chloride with fi xed loaded PTFE as a reference and performed by a number of partners. Results of the test are shown in Figs. 4.1 to 4.3. Figure 4.3 shows the infl uence of crevice former material on the determined critical crevice corrosion temperature (CCT) as determined in steps of 2.5C. It is seen that PVDF gives the same CCT as PTFE within one temperature step. The high values reported from TNO are due to unintended variable surface preparation procedures.

4.2 CCT values in ferric chloride with PTFE/MCA crevice formers with (DSMCA) and without (MCA) spring loading. Material EN 1.4547

4.3 Maximum depth values in ferric chloride solution in EN 1.4547 with PTFE/MCA crevice formers with (DSMCA) and without (MCA) spring loading

Crevice formers for specimens of plate material 19

Figure 4.2 shows the infl uence of spring loading on the CCT of EN 1.4547 in 6% ferric chloride. The obtained CCT is consistent within one temperature step. Figure 4.3 shows the obtained maximum depth of the crevice corrosion attack obtained in the tests shown in Fig. 4.2. The scatter of the results is reduced when using spring loading of the crevice formers. Consequently, it is expected that the reproducibility of the test will be better with application of spring loading. The line in the diagram shows the minimum depth of attack accepted for the onset of crevice corrosion.

Based on the results of this test, it was decided to use spring loaded PVDF crevice for mers throughout the CREVCORR project. The assembly is seen in Fig. 4.4. An instruction video [1] for moun ting spring loaded crevice formers was made (Det

4.4 Assembly of the spring loaded PVDF crevice formers


Norske Veritas [DNV]). The video can be downloaded from the webpage for EFC: Working Party Marine Corrosion at www.efcweb.org. The method has been used in the work on development of electrochemical control methods and the execu ted inter-nal round robin test on electrochemical simulation of natural and treated seawaters as well as in the round robin tests in na tu ral and biochemical artifi cial seawaters.

Reference

1. B. Espelid, Development of a new crevice corrosion qualifi cation test for stainless steels, in Stainless Steel World 2003, 1113 November 2003, Maastricht, The Netherlands, paper PO373. KLC Publishing, ISBN 90-73168-20-1, 457462.

21

5Crevice corrosion testing of tubes

Ulf KivisakkSandvik, Sandviken, Sweden

5.1 Introduction

Laboratory crevice corrosion testing of tubes has been performed with several kinds of crevice formers, for instance rubber bands [1], viton rings [1], nylon compressor rings [2], sleeve-type crevice formers [24] and bundles of tubes held together with polyolefi n tubes [4]. Based on general knowledge, there is one standard that covers crevice corrosion test methods for tubular specimens. In ASTM G78, it is stated that a number of crevice formers, that is off-the-shelf devices such as those mentioned above, can be used for cylindrical specimens [5]. Using rubber band and viton rings as crevice formers, EN 1.4410 and EN 1.4547 have been investigated in accordance with ASTM G48 method B. The results showed large scatter and poor correlation to results with the MTI-2 crevice former [6], a PTFE crevice former with 12 equally spaced slots, on fl at material [1]. Klein and Ferrara used sleeve-types of crevice formers of vinyl, nitrile, and nylon compression fi ttings on Alloy 625 tubes [2]. They found crevice gaps varied from several to less than one micron for vinyl sleeves, and the crevice corrosion gap was more uniform than for the nitrile sleeves. For the compressor fi ttings, they report crevice gaps from 1 to 150 m. Further, Klein and Ferrara [2] report that the geometry created by the crevice former had a signifi cant effect on crevice corrosion. According to Kain, the crevice severity depends on the tested tube diameter and the vinyl sleeve diameter [4].

Several models of crevice corrosion have been proposed, for instance in Refs 510. Oldfi eld and Sutton have shown that the crevice gap and crevice depth are of great importance [7]. The same authors have also shown that for AISI 316 specimens with an anode/cathode ratio of 20:1 or less, the crevice corrosion initiation time decreases [8]. Kain et al. proposed that the critical anode/cathode ratio is between 20:1 and 100:1 [9]. Gartland has also shown that a combination of crevice length and gap is needed in his model [10].

For plate material, the crevice former and its application on the specimen do not vary with the dimensions of the plate. In order to perform crevice corrosion testing on tubes with a similar condition as for a plate, several factors must be considered, as listed below:

specimen area dimensions and geometry of the crevice former clamping force.

In the CREVCORR project, the main focus has been to develop a reproducible test method for plate material. This part of the project aimed to transfer the crevice forming technique to tubular specimens as well.


5.2 Experimental

5.2.1 Materials

A standard 22 Cr duplex stainless steel, EN 1.4462, was used in this investigation as commercially produced cold-worked and annealed seamless tubes. The typical chemical composition is shown in Table 5.1. Specimens were cut to lengths giving the same area as for fl at specimens, 100 cm2 each. In the middle of the tube, a hole of 7 mm diameter was drilled and deburred. The tube was ground by hand with 120-grit paper followed by degreasing. After preparation, the specimens were exposed to air for at least 24 h before testing.

5.2.2 Design of crevice former

The same crevice former material used for plate material (PVDF) was used for tubes. Figure 5.1 shows the crevice former. The crevice former was machined to the same curvature as the outer diameter of the tube. The crevice former has the same length as the proposed crevice former for plate material (see Chapter 4). However, it has a 10 mm diameter hole for the clamping bolt, instead of 7 mm for the plate specimens in Chapter 4 for some of the tests in synthetic and natural seawater. A 7 mm hole corresponding to the fi nal crevice former for fl at specimens was used for testing in ferric chloride.

Table 5.1 Nominal chemical composition of the stainless steels (weight %)

Steel C Si Mn P S Cr Ni Mo N

EN 1.4462

Crevice corrosion testing of tubes 23

5.2.3 Finite Element Method Modelling

Finite element analysis was performed in order to investigate the distribution of the clamping force on the surface and the stress distribution in the crevice former. A model was used with the tube below the crevice former and the disc spring device treated as a rigid body. The PVDF was characterized by a Youngs modulus of 2100 N/mm2 and a Poissons ratio of 0.34. The model consisted of 12,206 4-node tetrahedral solid elements with a single integration point for each element. This type of element is well suited to linear elastic analysis for a fi ne mesh size.

The force is applied in 20 unit increments and the resulting data are stored and evaluated. The distribution of normal nodal pressure was studied and compared to the average normal pressure for the case of the crevice former for a fl at specimen with a load of 900 N (or stress of 3.3 N/mm2), the same as used in the CREVCORR programme. The force is then evaluated for a certain outer diameter that gives approximately the same normal pressure as in the case of a crevice former for a fl at specimen.

5.2.4 Crevice corrosion testing

The crevice corrosion testing was carried out at the Det Norske Veritas (DNV) laboratory in Bergen [11] (Norway) and at Sandvik SMTs corrosion laboratory in Sandviken (Sweden). The same bolts, nuts, crevice washers and disc springs were used as for plate material (Appendix B). The crevice formers were mounted on both sides of the tube with the disc spring set-up. An example is shown in Fig. 5.2. Various clamping forces were used in these experiments.

The critical crevice corrosion temperature (CCT) was determined electrochemi-cally at +600 mV vs. SCE for both tubes and plate material. The initial temperature was 20C and the temperature was stepped at 5C/min until the current density increased, indicating crevice corrosion.

The exposure was made in the same way as in the CREVCORR project. A 5-day exposure was made in synthetic biochemical seawater. The criterion for crevice corrosion was a depth of attack greater than 25 m.

5.2 Crevice former set-up for tube specimens


5.3 Results

5.3.1 Finite Element Method modelling

In Fig. 5.3, the minimal clamping load, 3.3 N/mm2, is shown to give the same surface stress in crevice formers as for fl at specimens. The contact surface between the crevice former and the cylindrical tube will always have a distribution of pressure with the highest pressure at the central part of the crevice former near the hole. Furthermore, it is clear how a larger diameter will result in a more even applied stress distribution in the crevice former than in the case of a small diameter tube. The effect of two diameters is shown in Figs. 5.4A and 5.4B, where the distribution of normal pressure is shown for tube diameters of 50 mm and 20 mm. The grey colour nearest the hole represents the same pressure as obtained with fl at crevice formers on plate specimens.

5.3.2 Crevice corrosion testing

Testing in synthetic biochemical seawater (Appendix D) was performed with differ-ent clamping forces on a 25.4 mm tube. The results in Fig. 5.5 show that crevice corrosion was initiated at 30C but not at 27.5C for 2.2 N/mm2 (450 N) and 4.2 N/mm2 (600 N) surface pressures. For higher surface pressure, 5 N/mm2 (900 N), crevice corrosion initiation was observed at temperatures as low as 20C. It should be noted that crevice formers with 10 mm diameter holes were used in both laboratories whereas the model uses a hole of 7 mm diameter. Further testing was performed at CEA in France, where the same torque was used as for fl at specimens (900 N). For a 25.4 mm tube of EN 1.4410, crevice corrosion was detected at 30C on both tested specimens, and for a 25.4 mm tube of EN 1.4462, crevice corrosion was detected on one specimen of two.

Two different tubes of 25.4 mm and 56 mm were tested in Sandviken with a clamping force according to the curve from FEM modelling. For the 25.4 mm and the 56 mm tubes, a load of 780 N was used. For both dimensions, the crevice corrosion temperature was found to be 30C.

5.3 Curve obtained from FEM modelling for recommended clamping force as a function of diameter


5.4 Two-dimensional distribution of normal contact pressure on (A) 50-mm and (B) 20-mm outer diameter tube


Critical crevice corrosion temperature (CCT) was determined electrochemically at +600 mV vs. SCE for both tubes and plate material. Tubes of two dimensions of both EN 1.4462 and EN 1.4410 were tested together with plate specimens for both materials. The results are shown in Table 5.2.

Further fi eld exposure of EN 1.4410 tubes in natural seawater was made in the Adriatic Sea at Dubrovnik, Croatia. The same force (900 N) was used as for plate specimens. No crevice corrosion was detected on the triplicate test specimens after the 6-month exposure [12].

5.4 Discussion

The testing in Bergen and Sandviken, with a clamping force predicted by the curve from the FEM modelling, showed initiation of crevice corrosion at 30C. Higher clamping force showed crevice corrosion initiation while low clamping pressure did not initiate crevice corrosion. In Sandviken, two tube sizes were tested with a clamp-ing force according to the FEM curve. As with the tests in Bergen, initiation was at 30C. In the profi ciency test of plate material in Chapter 10, initiation for EN 1.4462 was at 30C in fi ve laboratories and at 40C in 11 laboratories. In natural seawater,

5.5 Crevice corrosion test for 25.4-mm tube with different surface pressures for the crevice former

Table 5.2 Mean values and standard deviation, S, of CCT measured electrochemically at +600 mV vs. SCE for four tube specimens and two plate specimens

EN 1.4462 CCT (C) S (C) EN 1.4410 CCT (C) S (C)

Plate 36.3 2.5 Plate 60.0 4.1Tube 25.4 mm

46.3 2.5 Tube 25.4 mm

53.8 2.5

Tube 54 mm

42.5 2.9 Tube 57 mm

63.8 4.8


initiation was observed at 2529C for plate specimens of EN 1.4462, see Chapter 10. No corrosion was initiated in the Adriatic Sea for EN 1.4410, but crevice corrosion was found at 30C at CEA in biosynthetic seawater. It should be noted that very high clamping forces were used in these tests. This indicates that the test results for tubes are in the same range as for the plate material.

For testing of crevice corrosion of stainless steel tubes, the following design parameters for the crevice former and clamping force are proposed:

5.4.1 Specimen area

The cathode area is important for the initiation of crevice corrosion [9]. For a given area beneath the crevice former, the anode, it is important to have the same specimen area outside the crevice area since it acts as the cathode. In this work, a specimen area of 100 cm2 of the outer surface is proposed to be used for testing. This is the same area as proposed for fl at specimens in Appendix A. The dimensions of a plate specimen are 10 10 cm. However, for tubes, specimens have to be cut to the right length in order to obtain 100 cm2 area of the outer surface. The inner surface will also act as a cathode area and therefore the total area will be about 200 cm2, the same area as for both sides of fl at specimens. For testing with a potentiostat, no cathode area is required since the potentiostat can simulate an infi nite cathode area.

5.4.2 Crevice former

The crevice gap and depth are of great importance [810]. The crevice former for a fl at specimen is 15 mm high, 20 mm in diameter, with a 7 mm hole in the middle (Appendix B). For tubular specimens, crevice formers as shown in Fig. 5.1 are pro-posed. The side of the crevice former attached to the tube is machined with the same curvature as the tube. The pressure distribution created by the crevice former will become more even as the tolerances of the machined surface are fi ner. Therefore, as for fl at specimens, it is proposed that the crevice formers should be wet ground with a 1200 grit paper to obtain an even surface. In order to attain good contact with the tube, a crevice former can probably only be used once.

Due to the curvature of the tubes and in addition, the curvature of the crevice formers, pressure along the crevice gap is not constant. This may be resolved by using oval-curvature crevice formers. However, the results of FEM modelling showed that, for a small diameter tube, the distribution of the surface pressure is greater than for a larger diameter tube and the distribution is in the radial direction. Furthermore, a smaller diameter tube has a larger crevice gap in the radial direction. For different tube sizes, the crevice gap will be more similar for a round crevice former than for an oval one. This advantage has been assumed to be benefi cial in this work. A 2-dimensional representation of pressure distribution is shown in Fig. 5.4.

5.4.3 Clamping force

When the clamping force for fl at specimens was used, high, unevenly distributed surface pressure was achieved with tube specimens. However, from FEM modelling, the clamping forces needed to produce a more even distribution and similar surface pressure of the crevice former against the tube were obtained. This is important because if the surface pressure deviates greatly, it can create large deviations in


crevice gap and crevice depth. For instance, when using sleeve-type crevice formers, differences in initiation temperature for stainless steels have been found with different set-ups that result in different crevice gaps. This has also been demonstrated by the testing in Bergen [11] and supported by the results obtained at CEA.

5.4.4 Proposed crevice former procedure for tube specimens

The crevice set-up that is described above could be used as a reproducible technique for qualifying materials as in the case of fl at specimens by the procedures in the guidelines derived from the CREVCORR project (Appendix C). The results indicate that this may be achieved even though the hole in the crevice former is 1.5 mm larger than the radius in the model. The CPT measurements indicate that reproducibility can be achieved. However, additional verifi cation testing is necessary.

5.5 Crevice corrosion testing of stainless steel tubes applied as umbilicals

Umbilicals used for control and service purposes of topside and subsea facilities carry electrical services, hydraulic or chemical fl uids or a combination of these. For subsea production systems, umbilicals represent a key part of the total system, and if failures occur, a repair would be very diffi cult or impossible to perform. Therefore, all parts of an umbilical are comprehensively tested to ensure that they are fi t for purpose throughout the lifetime of the installation.

For subsea production systems, the control and service umbilical contain stainless steel pipes. These pipes can be part of a pipe bundle separated by spacers in different types of materials, e.g. PVC. The pipe bundle can be exposed to seawater, which can be stagnant or slowly ex changed. The stainless steel material can be subject to crevice corrosion under these conditions, as it can be exposed to seawater with elevated temperatures and seawater with different levels of oxygen saturation. It is therefore very important to establish the crevice corrosion behaviour of stainless steel tubing exposed to these types of conditions. This includes both the base material tubing and the welds.

A test methodology has been established for this application of stainless steel tubing, and this is given in Appendix F.

5.6 Conclusions from crevice corrosion testing of tubes

In this chapter, an approach has been proposed for a reproducible crevice corrosion testing technique. For crevice formers formed on the outer radius of stainless steel tubes, FEM modelling indicates that similar crevice conditions can be obtained on tubes as for plates. From a calibrating curve for a range of dimensions for crevice formers of stainless steel tubes, the clamping force can be determined. Preliminary crevice corrosion test results in biosynthetic seawater seem to verify the model. However, additional verifi cation testing is necessary.

References

1. M. Nicholls, Internal technical report. AB Sandvik Steel, 1993.2. P. A. Klein and R. J. Ferrara, in Corrosion/1989, Paper No. 112. New Orleans, Louisiana,

1989. NACE International.


3. R. M. Kain and P. A. Klein, Corrosion90, Las Vegas, Nevada, 1990.4. R. M. Kain, Crevice corrosion resistance of duplex stainless steels in chloride containing

waters, in Duplex Stainless Steels 97, paper D97-048, Maastricht, 1997.5. Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless

Alloys in Seawater and other Chloride-Containing Aqueous Environments. ASTM Standard G 78-01, ASTM International, West Conshohocken, PA, 2001.

6. S. Bernhardsson et al., 19th Journers des Aciers Spciaux International Symposium on Stainless Steels, Saint Etienne, France, 1980.

7. J. W. Oldfi eld and W. H Sutton, Br. Corros. J., 13(1) (1978), 14.8. J. W. Oldfi eld and W. H. Sutton, Br. Corros. J., 13(3) (1978), 130.9. R. M. Kain, T. S. Lee and J. W. Oldfi eld, in Electrochemical Techniques for Corrosion

Engineering, 261, ed. R. Baboian. NACE International, Houston, Texas, 1986.10. P. O. Gartland, Corrosion96 Research Topic Symposium. Part II Crevice Corrosion: The

Science and Its Control in Engineering Practice, 1996, 311339.11. O. Tangeland, M.S. Thesis, Hgskolan i Stavanger, Norway, 2003.12. O. Lahodny-Sarc, L. Krstulovic, B. Kulisic, J. Ivic and D. Sambrailo, Stainless steel

crevice corrosion assessed in the fl owing system, in Eurocorr 2003, paper 131, Budapest, Hungary, 2003.

30

6Formulation of new synthetic seawater for

aerobic environment

Damien Fron, Valrie lHostis, Marc RoyCommissariat lEnergie Atomique, Saclay, France

6.1 Introduction

The presently applied tests to examine stainless steel (SS) materials in marine environments are often not representative of reality (such as ferric chloride tests) neither sufficiently corrosive because the tests are performed in synthetic seawater, such as, for instance, ASTM seawater [1] which reproduces the chemical composition of natural seawater, but which does not reproduce the corrosive effect induced by biofilm growth on SS surfaces exposed to natural seawater.

The formulation of new artificial seawaters able to reproduce the corrosive effects of natural aerobic biofilms in the laboratory is the aim of this chapter.

Once SS are exposed to natural aerated seawater, aerobic bacteria first settle on their surfaces. There is now general agreement that the formation of this kind of biofilm increases cathodic reactions rates: one of the consequences is that SS in the passive state reaches free corrosion potentials of about +300 mV/SCE, higher than the maximum potential observed in sterile seawater [25]. This increase is, in itself, a powerful cause of onset of localised corrosion.

Two methods have been investigated to obtain synthetic seawater able to mimic the natural conditions in the case of an aerobic marine environment [46]:

The chemical method which tries to adjust the corrosivity of chemical synthetic seawater by addition of chemical products such as hydrogen peroxide and organic acids (these products have been detected in marine biofilm). This chemi-cal method is based on the addition of hydrogen peroxide (HP) and gluconic acid (GA) to synthetic seawater.

The biochemical method which is based on the enzymatic mechanisms of the biocorrosion processes of stainless steels in natural seawater. This biochemical method is based on the addition of an enzyme (oxidase type) and its substratum to the basic chemical synthetic seawater (NaCl solution 3.5% or ASTM seawater).

Evaluation of the corrosivity of the synthetic seawaters is performed by electro-chemical measurements on stainless steel electrodes and is based on the evolution of free corrosion potential (Ecor) with exposure time, on the evaluation of the cathodic current densitypotential curves at different exposure durations and on the evolution of current densities at some imposed potentials, with exposure time.

Then, crevice corrosion experiments have been performed in the two conditions which have been selected from the electrochemical experiments. After comparisons with previous crevice corrosion results obtained in natural seawater, new synthetic

Formulation of new synthetic seawater for aerobic environment 31

seawater, simulating the aerobic bacteria effect, is proposed for testing the SS resistance to crevice corrosion.

6.2 Experimental

For the evaluation of the corrosivity of the new synthetic seawaters, working electrodes were made with EN 1.4547 stainless steel plates (10 40 3 mm), the chemical composition of which is given in Table 6.1. They were suspended on tita-nium wires and immersed in a glass electrochemical cell together with a reference electrode (saturated calomel electrode, SCE). Unsterilised chemical synthetic seawa-ters (NaCl 3.5% solution or ASTM synthetic seawater) were used. The electrochemi-cal cells were equipped with a thermostatic regulator (5C to 25C). The stirring of each cell was performed by a magnetic stirring system.

Each electrode was linked to a multichannel acquisition system, in order to follow the free corrosion potential. Current densitypotential curves were measured with a three-electrode system: a cylindrical platinum electrode was added to the glass container. The curves were plotted with an EG&G potentiostat, from the free corrosion potential, to cathodic potentials (1.0 V/SCE), with a scan rate of 5 mV/s.

Then, for the crevice corrosion tests (while still awaiting a better design of the crevice former and fixing system; Chapter 4), the crevice corrosion coupons used are schematically shown in Fig. 6.1. They were SS plates (60 60 1 mm or 100 100 1 mm, depending on the test), with a crevice former made of POM (polyoxy-methylene), and fixed with a nut and bolt system at a given torque (6 N m). These devices have been designed and used during the European programme Biofilms on stainless steels of the Mast II project [5].

In a standard crevice corrosion experiment, 10 crevice corrosion coupons, hung with titanium wires, were immersed for 10 days in 10 L of synthetic seawater with the addition of different components, depending on the test. Three saturated calomel

Table 6.1 Chemical composition of tested stainless steels (% weight)

Material Fe Cr Ni Mo Mn Si S C P N

EN 1.4404 Bal. 17.1 11.1 2.03 0.93 0.46 0.001 0.011 0.028 0.040EN 1.4547 Bal. 19.9 17.9 6.05 0.41 0.40 0.001 0.013 0.025 0.196EN 1.4410 Bal. 22.3 5.8 3.24 1.44 0.44 0.001 0.022 0.023 0.170

6.1 Schematic diagram of the crevice corrosion coupons used during the selection tests for biochemical synthetic seawater


reference electrodes (SCE) were used: the evolution of Ecor of each coupon with exposure time was followed with a multichannel acquisition system. A platinum electrode was immersed in the solution to follow the redox potential of the system.

Surface preparation of each electrode or crevice coupon was the same: a chemical pickling (HNO3 20% and HF 2% solution), followed by passivation in air for 5 days. Samples were then weighed before and after the tests. The chemical composition of the SS used is given in Table 6.1.

6.3 Electrochemical tests

The objective of the electrochemical tests is to select chemicals or biochemicals which reproduce the electrochemical behaviour of stainless steels in natural seawater, with mainly the increase in the free corrosion potentials and the cathodic reaction rate.

Hydrogen peroxide and acids are well known chemicals and are used in many corrosion laboratories, but enzymes may be more difficult to handle. The selected enzymes have been chosen because they are claimed [35] to be involved in the SS Ecor increase (like oxidases) but, together with their cofactor, they also need to be not too expensive, easily available, easy to handle, and not toxic. So, the first step in the electrochemical study concerned the choice of an enzyme, which when added to sodium chloride solution, would lead to an increase in the free corrosion potential of the SS, as in natural seawater. Following the claimed mechanisms, the enzyme must catalyse the reaction using oxygen as substratum, and produce hydrogen peroxide, as observed in natural biofilms.

For this purpose, SS electrodes made with EN 1.4547 plates were immersed in an aerated 3.5% sodium chloride solution, with glucose oxidase and glucose, galactose oxidase and galactose, alcohol oxidase and methanol, or diamine oxidase and putrescine. All of these enzymes use oxygen, and produce hydrogen peroxide. Tests were performed at 25C, the free corrosion potential (Ecor) was recorded with the time of immersion (Fig. 6.2). Enzymes (100 U/L) and substrates (20 mmol/L) were added to the solution after 24 h of immersion of the SS coupons in 3.5% NaCl.

All Ecor values were similar before addition of biochemical components, about +0.15 0.05 V/SCE. When enzymes and substrates were added, several Ecor evolu-tions were observed. Additions of glucose oxidas

Documents

(European Federation of Corrosion Series) Ulf Kivisakk, Bard Espelid, Damien Feron-Methodology of Crevice Corrosion Testing for Stainless Steels in Natural and Treated Seawaters-Maney