142,99 €
Mehr erfahren.
- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
LOCALIZED CORROSION IN COMPLEX ENVIRONMENTS A comprehensive exploration of the monitoring, prediction, and prevention of major forms of localized corrosion in complex industrial environments In Localized Corrosion in Complex Environments, distinguished researcher Dr. Mike Yongjun Tan delivers a solution focused approach to localized corrosion issues in complex environments with the potential to affect structural integrity, public safety, environmental protection, or energy and water deliverability. The book focuses on significant civil and industrial infrastructures exposed to complex corrosion environments, like underground and offshore gas, oil, and water pipelines. The author offers information to help ensure the continued safe operation of aging infrastructures and discusses the limitations of current technologies and the need to continuously develop new and more efficient technologies to manage integrity, prevent structural failures, protect the environment, and reduce operational costs. Readers will also find: * A thorough introduction to the major issues relevant to infrastructural corrosion issues * Comprehensive explorations of issues likely to affect future fuel and energy infrastructures, like hydrogen containing pipelines and offshore and onshore wind farms * Practical discussions of recent progress in inspection and monitoring technologies, as well as the protection provided by protective coatings * Fulsome treatments of the use of corrosion inhibitors Perfect for materials and corrosion scientists, physical chemists, engineers, regulators, technologists, and environmentalists, Localized Corrosion in Complex Environments will also earn a place in the libraries of corrosion and materials engineers, maintenance engineers, pipeline engineers, field personnel, and anyone responsible for the integrity of production and transmission of oil, gas, and water.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 621
Veröffentlichungsjahr: 2023
Ähnliche
Table of Contents
Cover
Title Page
Copyright Page
Preface
1 Localized Corrosion in Complex Engineering Environments
1.1 Localized Corrosion Complexity
1.2 Corrosion from Simple to Complex
1.3 Cases of Localized Corrosion in Industry
1.4 Obstacles in Modeling and Managing Complex Localized Corrosion
References
2 Techniques for Localized Corrosion Inspection and Monitoring
2.1 Techniques for Corrosion Detection, Inspection, and Data Acquisition
2.2 Corrosion Monitoring Using Sensors and Probes
2.3 Multi‐Electrode Arrays for Probing Complex Corrosion
References
3 Localized Corrosion in Changing Environments
3.1 Probing Localized Corrosion in Nonuniform and Changing Environments
3.2 Steel Corrosion Behavior in Soil Under Disrupted Cathodic Protection
3.3 Steel Corrosion Behavior in Soil Under Stray Currents
References
4 Localized Corrosion Influenced by Changing Mechanisms
4.1 Localized Corrosion and Materials Degradation with Varying Mechanisms
4.2 Probing Localized Coating Degradation and Disbondment
4.3 Probing Localized Corrosion Under Disbonded Coatings
References
5 Corrosion Affected by Multiple Environments and Mechanisms
5.1 Localized Corrosion Affected by Multiple Environments and Mechanisms
5.2 Probing Multi‐Mechanism Corrosion Across Multiple Environments
5.3 Cases of Probing Localized Corrosion Over Multiple Environments
References
6 Localized Corrosion Impacted by Flow and Erosion
6.1 Localized Corrosion Impacted by Flowing Liquid and Solid Particles
6.2 Cases of Probing Corrosion and Inhibition in Flowing Liquids
6.3 Probing Localized Corrosion Mechanisms Impacted by Flow and Erosion
References
7 Localized Corrosion Induced by Metallurgical Heterogeneities
7.1 Multiscale Corrosion Induced by Metallurgical Heterogeneity
7.2 Various Probes Designed for Probing Multiscale Corrosion
References
8 Challenges and Opportunities in Managing Complex Localized Corrosion
8.1 Future Perspectives in Corrosion Monitoring Tools and Predictive Models
8.2 Opportunities in Developing Smart Anticorrosion Methods and Materials
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Details of the probe’s installation site conditions.
Table 4.2 Probe field installation site CP and open‐circuit potentials reco...
List of Illustrations
Chapter 1
Figure 1.1 Sample localized corrosion‐induced infrastructural failure incide...
Figure 1.2 Illustrating corrosion mechanisms co‐existing on a steel pipe tha...
Figure 1.3 A textbook type galvanic corrosion cell model with uniform anodic...
Figure 1.4 A battery form of galvanic corrosion cell.
Figure 1.5 Galvanic cell starts to become more complex with localized corros...
Figure 1.6 A galvanic corrosion cell formed by joining different metal piece...
Figure 1.7 Micro‐galvanic corrosion cells on a single piece of metal surface...
Figure 1.8 A simplified E‐pH diagram for iron–water–soil systems.
Figure 1.9 An Evan diagram illustrating mixed electrode state of iron corros...
Figure 1.10 Modeling oxygen corrosion of iron or steel in aqueous media.
Figure 1.11 Illustrating localized corrosion on buried pipelines exposed to ...
Figure 1.12 Modeling pitting initiation due to the formation of macroscopic ...
Figure 1.13 (a) Localized corrosion initiates due to erosion induced local a...
Figure 1.14 Steel components corroded due to a rust problem occurred in a de...
Figure 1.15 (a) and (b) showing pitting on H11 steel gear face due to unexpe...
Figure 1.16 Illustrating pitting of cooling tank.
Figure 1.17 Cases of localized corrosion of pipeline at the interface of soi...
Figure 1.18 Some examples of coating disbondment and CUDC.
Chapter 2
Figure 2.1 Illustrating ILI pigging inspection of a buried gas pipeline.
Figure 2.2 Potential survey methods for inspecting unpiggable underground pi...
Figure 2.3 Illustrating corrosion probe monitoring of a buried gas pipeline....
Figure 2.4 Illustrating the concept of electrode array corrosion probe.
Figure 2.5 Illustration of various a typical corrosion probes based on elect...
Figure 2.6 Illustrating the use of PCM probe/sensor for obtaining
in situ
an...
Figure 2.7 Illustration of a typical corrosion under deposit and inhibition ...
Chapter 3
Figure 3.1 A collapsed building due to localized corrosion of a small sectio...
Figure 3.2 Variously designed electrode array probes and measurement systems...
Figure 3.3 The schematic diagram of the test setup for probing localized cor...
Figure 3.4 OCP recordings and the WBE current density distribution maps obta...
Figure 3.5 OCP recordings and the WBE current density distribution maps obta...
Figure 3.6 OCP recordings and the WBE current density distribution maps obta...
Figure 3.7 OCP recordings and the WBE current density distribution maps obta...
Figure 3.8 The surface morphologies of the WBE after rust removal with diffe...
Figure 3.9 The variation of the open‐circuit potential (OCP) of different si...
Figure 3.10 OCP recordings and current density distribution maps obtained wi...
Figure 3.11 Schematic diagram of stray current and the test system (a), and ...
Figure 3.12 The current distribution maps obtained when probe buried in diff...
Figure 3.13 The current distribution maps obtained when probes are buried in...
Figure 3.14 (a) Photos of coupons located at different locations after one h...
Figure 3.15 Current recordings and WBE maps obtained using a WBE probe under...
Figure 3.16 The monitored CP (blue line) and anodic transient (red line) c...
Figure 3.17 The monitored CP (blue line) and anodic transient (red line) c...
Figure 3.18 Pourbaix diagram in the presence of chloride ions to explain the...
Figure 3.19 Coupon electrodes corroded under the cyclic potential fluctuatio...
Figure 3.20 Current recording and WBE current maps measured under the cyclic...
Figure 3.21 Current recording and WBE current maps measured from a WBE under...
Figure 3.22 Current recording and WBE current maps measured from a WBE under...
Figure 3.23 Pourbaix diagram of iron describes the corrosion behavior under ...
Chapter 4
Figure 4.1 Sample field joint coating damage following hydrostatic test and ...
Figure 4.2 Illustrating multiple mechanisms that could occur over a steel su...
Figure 4.3 Cases of coating disbondment and corrosion under disbonded coatin...
Figure 4.4 Schematic diagram of the experimental setup for local EIS mapping...
Figure 4.5 The Nyqusit plots recorded over locations “A”, “B”, and “C” over ...
Figure 4.6 Maps of amplitude of impedance (lZl 300 mHz) over a coated multi‐...
Figure 4.7 Direct current (i) maps obtained on coated multi‐electrode array ...
Figure 4.8 Amplitude of impedance (lZl 300 mHz) maps obtained on coated mult...
Figure 4.9 Disbonded distance against the potential level measured after 600...
Figure 4.10 Direct current (i) maps obtained on coated array electrode sampl...
Figure 4.11 Disbondment behavior of coated multi‐electrode samples with exte...
Figure 4.12 Schematic diagram of the differential aeration corrosion probe w...
Figure 4.13 Schematic diagram of the experimental arrangement for the determ...
Figure 4.14 Current density maps at various immersion times in 0.1 M NaCl so...
Figure 4.15 Current density maps at various immersion times in 0.1 M NaCl so...
Figure 4.16 Comparison of the sensor’s array surface after 23 hours immersio...
Figure 4.17 Comparison of the sensor’s array surface after 23 hours immersio...
Figure 4.18 The total CP current supplied to the 3D probe surfaces with diff...
Figure 4.19 Surface photo of the WBE with gap size of 0.25 mm (a) after expo...
Figure 4.20 Current density distribution maps registered over the array elec...
Figure 4.21 Current density distribution maps registered over the array elec...
Figure 4.22 Current density distribution maps registered over the array elec...
Figure 4.23 Metal loss maps calculated based on Faraday’s law with the crevi...
Figure 4.24 Schematic diagram of the corrosion processes and mechanisms occu...
Figure 4.25 Ilustration of (a) the CUDC probe desings for simulating corrosi...
Figure 4.26 (a) Schematic representation of a probe installation site and ba...
Figure 4.27 A schematic diagram of the experimental arrangement and corrosio...
Figure 4.28 Cumulative corrosion maps for the bare metal probe installed in ...
Figure 4.29 Cumulative corrosion maps for the bare metal probe installed in ...
Figure 4.30 Final cumulative corrosion maps for the disbonded coating probe ...
Figure 4.31 Corrosion under disbondment found on the same gas pipeline about...
Figure 4.32 Evolution of the current density distribution maps on the disbon...
Figure 4.33 Schematic representation of air gap corrosion model under disbon...
Figure 4.34 (a)–(b) Representative current density distribution maps over th...
Figure 4.35 Final cumulative corrosion maps for the disbonded coating probe ...
Figure 4.36 Current density distribution maps for the first wet–dry cycle....
Figure 4.37 Current density distribution maps typical of the second and thir...
Figure 4.38 Typical current density distribution maps from the fourth wet–dr...
Figure 4.39 Appearance of the electrode after testing, profilometry results ...
Chapter 5
Figure 5.1 Illustration of engineering structures crossing several environme...
Figure 5.2 A case of localized corrosion of a lamp post due to multiple mech...
Figure 5.3 A galvanic corrosion cell formed on a steel coupon simulating a l...
Figure 5.4 Illustrating several corrosion control methods for the lamp post....
Figure 5.5 (a) An onshore wind turbine tower that was built on an extremely ...
Figure 5.6 Illustrating a lamp post corrosion monitoring system using variou...
Figure 5.7 Experimental design for measuring electrochemical parameters from...
Figure 5.8 Corrosion potential (in V) and galvanic current (in mA/cm
2
) distr...
Figure 5.9 An aircraft corrosion probe that can be conveniently installed/gl...
Figure 5.10 (a) Shows the corrosion at different regions of a pipeline. The ...
Figure 5.11 A schematic diagram showing corrosion probes designed for simula...
Figure 5.12 The galvanic current distributions (a, b) in the static sate wat...
Figure 5.13 The corrosion depths of WE 1 to WE 10 measured by ER method (a) ...
Figure 5.14 (a) The comparison of the corrosion depths measured by the ER me...
Figure 5.15 Potential and galvanic current distributions over a WBE working ...
Figure 5.16 Various forms of coating damages observed on HDD pipelines.
Figure 5.17 The relationship between defect size (cm
2
) and test coupon surfa...
Figure 5.18 Illustrating multi‐sensors mounted on a HDD pipe, allowing local...
Figure 5.19 Probes that is able to be moved through the HDD borehole for loc...
Figure 5.20 (a) Showing results from a local EIS test using a steel sample c...
Figure 5.21 Local cathodic polarization current measurement through the test...
Figure 5.22 A setup for detecting CP efficiency in a simulated HDD borehole ...
Chapter 6
Figure 6.1 (a) Showing cases of localized corrosion in an oil and gas pipeli...
Figure 6.2 Schematic illustration of flowloop with jet impingement apparatus...
Figure 6.3 A schematic diagram of probing flow influenced corrosion using ro...
Figure 6.4 A schematic diagram of probing flow influenced corrosion using du...
Figure 6.5 Electrochemical cell for simulating multiphase oil flowline corro...
Figure 6.6 Schematic diagram of the setup for probing localized corrosion in...
Figure 6.7 Potentiostatic polarization measurements for SLM‐produced (a) and...
Figure 6.8 Three‐stage potentiostatic polarization measurement for evaluatin...
Figure 6.9 EIS plots before and after rotating the inhibitor filmed electrod...
Figure 6.10 Monitoring dynamic changes in inhibitor film by measuring the el...
Figure 6.11 Corrosion potential, galvanic current, noise resistance, and cor...
Figure 6.12 Corrosion potential, galvanic current, noise resistance, and cor...
Figure 6.13 Observed and calculated corrosion depth maps and values (in μm) ...
Figure 6.14 Corrosion potential, galvanic current, noise resistance, and cor...
Figure 6.15 Observed and calculated corrosion depth maps and values (in μm) ...
Figure 6.16 Photos of the WBEs and coupons after 20 hours exposure to differ...
Figure 6.17 The WBE current distribution maps and the accumulated corrosion ...
Figure 6.18 EIS measurement and fitting results in different test conditions...
Figure 6.19 Photos, 3D profile, and SEM images of the coupon electrodes in d...
Figure 6.20 The photo of the WBEs after 20 hours test in different flowing c...
Figure 6.21 The current distribution maps measured from the WBE exposed to t...
Figure 6.22 The current distributions maps measured from the WBE exposed to ...
Figure 6.23 (a) The max anodic current and (b) the total anodic current obta...
Figure 6.24 The corrosion depth maps measured by 3D profilometry (Map 1), ca...
Figure 6.25 Schematic diagram of the erosion–corrosion propagation and the l...
Chapter 7
Figure 7.1 Some sample images of AA2024 corrosion over different scales afte...
Figure 7.2 Photos of some typical pits on the steel pipe in a water injectio...
Figure 7.3 Illustrating localized corrosion mapping using the scanning refer...
Figure 7.4 Overview diagram of different types of corrosion and hydrogen map...
Figure 7.5 (a) Potentiodynamic polarization curves of the SLM‐produced and c...
Figure 7.6 SEM images from the corroded surface of the (a) P175‐S800, (b) P1...
Figure 7.7 Potentiodynamic polarization curves recorded in 0.6 M NaCl soluti...
Figure 7.8 DCM X‐ray CT volumetric reconstructed images of the lowest‐densit...
Figure 7.9 DCM CT volumetric reconstructions of P125‐S600 specimen (a,c,e) b...
Figure 7.10 DCM CT volumetric reconstructions of P125‐S600 specimen (a,b,c,e...
Figure 7.11 (a) Schematic of the test setup for
in situ
monitoring of corros...
Figure 7.12 (a–f) Galvanic current density distribution maps of the electrod...
Figure 7.13 (i) The schematic setup of experiment using the electrode array ...
Figure 7.14 Illustrating galvanic current measurements over a heterogeneous ...
Figure 7.15 Illustrating direct impingement tests using a 3D electrode array...
Figure 7.16 (a) Shows the corrosion at different regions of a pipeline. The ...
Figure 7.17 Sample measurement data that clearly shows weldment corrosion in...
Figure 7.18 Sample measurement data that clearly shows crevice corrosion in ...
Figure 7.19 (a) Designated different zones Z1 and Z2 in the heat tint area o...
Figure 7.20 (a) CPP test results for selected zones Z1 and Z2 on a typical s...
Chapter 8
Figure 8.1 (i) a setup of the tensile testing rig in combination with an ele...
Figure 8.2 Typical SEM image of (a) the coating surface and (b) the cross‐se...
Figure 8.3 DCM representation for components in high build epoxy coating str...
Figure 8.4 AFM images and models of acrylic‐alkyd composite films formed as ...
Figure 8.5 The long‐term experimental setup in a tank with simulated harsh m...
Guide
Cover Page
Title Page
Copyright Page
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
Pages
iii
iv
ix
x
xi
xii
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
363
364
365
366
367
368
Localized Corrosion in Complex Environments
Mike Yongjun Tan
Deakin University, Australia
This edition first published 2023© 2023 John Wiley & Sons, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Mike Yongjun Tan to be identified as the author of this work has been asserted in accordance with law.
Registered Office(s)John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA
For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.
Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some content that appears in standard print versions of this book may not be available in other formats.
Trademarks: Wiley and the Wiley logo are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries and may not be used without written permission. All other trademarks are the property of their respective owners. John Wiley & Sons, Inc. is not associated with any product or vendor mentioned in this book.
Limit of Liability/Disclaimer of WarrantyIn view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging‐in‐Publication DataNames: Tan, Mike Yongjun, author.Title: Localized corrosion in complex environments / Mike Yongjun Tan.Description: Hoboken, NJ : Wiley, 2023.Identifiers: LCCN 2022039095 (print) | LCCN 2022039096 (ebook) | ISBN 9781119778608 (hardback) | ISBN 9781119778615 (adobe pdf) | ISBN 9781119778622 (epub)Subjects: LCSH: Corrosion and anti‐corrosives.Classification: LCC TA462 .T27 2023 (print) | LCC TA462 (ebook) | DDC 620.1/1223–dc23/eng/20220919LC record available at https://lccn.loc.gov/2022039095LC ebook record available at https://lccn.loc.gov/2022039096
Cover Image: WileyCover Design by Courtesy of Mike Yongjun Tan
Preface
Over the past century, significantly improved understanding of materials behavior and performance, extensively developed materials selection standards and software, and various engineering design tools have facilitated the avoidance of “short‐term” failure of engineering structures. However, “long‐term” materials failure issues, in particular localized forms of corrosion and materials degradation, still remain tenacious threats to the integrity and safety of the huge network of civil and industrial infrastructure assets especially those exposed to complex and varying environmental conditions. Substantial progresses have also been made in corrosion science and engineering, facilitating the effective control of uniform and general corrosion in many industrial structures such as automobile body rusting and radiator corrosion; however, the management and control of localized corrosion in complex engineering environments is still a significant challenge. It is evident by many publically reported catastrophic engineering structure failures and an enormous amount of unreported infrastructure incidents. This problem becomes even more acute when complex forms of localized corrosion occur on “invisible” and highly variable engineering structures such as buried and submerged oil and gas pipelines.
Effective control and management of complex forms of localized corrosion are critical for maintaining the safety and integrity of industrial and civil infrastructures that are vital for the provision of the world’s essential services and the maintenance of its economic activities. Although localized corrosion has been widely studied over the past decades, it should be noted that most studies are typically limited to the investigation of specific forms of localized corrosion such as pitting of stainless steels in defined laboratory environments. Conventionally corrosion science research considers a corrosive environment uniform and stable, a simplification of complex and changing industrial environments. Corrosion prediction models, testing methods, and protection measures are mostly developed under such simplification. Unfortunately, most practical engineering structures can be subjected to highly non‐uniform corrosion under multiple and dynamically changing environmental conditions. An example is localized corrosion of buried steel pipelines that are affected not only by seasonal changes in soil moisture and oxygen levels, inhomogeneous coating defects and coating disbondment but also by fluctuating stray currents and oscillating mechanical stresses. Another example is localized corrosion and materials degradation on offshore structures such as wind turbines and oil platforms that are affected by multizone and dynamically changing marine environmental conditions. Variable and complex environmental conditions can lead to changes not only in corrosion rates but also in corrosion patterns and mechanisms. Unexpected changes in environment and mechanism could also cause suddenly accelerated localized corrosion damages that are not predictable by conventional corrosion models. Currently, corrosion engineering studies have not sufficiently considered these issues, leaving a major knowledge gap in corrosion science and engineering.
The ultimate goal of corrosion engineering would be to prevent the premature failure of engineering materials and to extend the safe operational life of engineering structures through detecting, mitigating, and minimizing corrosion damage. This could be compared with the protection of human health through detecting, diagnosing, and preventing cancer and other diseases. Human body itself has many “sensors,” the eyes, ears, nose, skin, and tongue, that provide disease information through vision, hearing, smell, touch, and taste. Diseases can be further diagnosed and treated through medical testing, doctor’s analysis, and the use of various medical treatment. For engineering structures, unfortunately such “sensors” are not naturally available, and therefore it is essential to have artificial devices installed for sensing structural health issues such as complex forms of localized corrosion – the prime threat to the integrity of metallic structures. An “ideal” corrosion sensing and control system should be one that not only provides in situ and site‐specific corrosion data required to visualize localized corrosion but also uses such data to inform corrosion prediction and control, for instance, to guide local coating repair and to regulate local cathodic protection potential and corrosion inhibitors injection. In this manner, the threat of localized corrosion to the integrity and safety of engineering structures would be minimized and the safe operational life of infrastructure would be maximized. Considering the variable nature of corrosion environments and mechanisms in practical engineering structures, corrosion management and prevention actions may need to be adjusted smartly and dynamically based on the prevailing corrosion condition and mechanism. A prerequisite for effectively doing so is timely knowledge about the initiation, propagation, and seriousness of localized corrosion occurring over an engineering structure.
Currently, knowledge of localized corrosion is generally from time‐based routine inspections using various condition assessment and in‐line inspection tools. Although corrosion data from such inspection are useful for identifying longer‐term corrosion trends, they often do not have sufficient temporal and spatial resolutions required for predicting dynamic changes in complex localized corrosion. Acquiring corrosion data more frequently is necessary not only for early warning of unanticipated structural failure but also for evaluating the efficiency of corrosion control measures such as cathodic protection, corrosion inhibitors, and protective coatings. Over the past decades variously designed corrosion sensors and probes have been developed for in situ and site‐specific corrosion data acquisition. In particular significant progresses have been made in the development and implementation of various forms of electrochemically integrated multi‐electrode arrays, often referred to as the wire beam electrode (WBE), which has already been described in an earlier book by the author (Yongjun Tan (2012). Heterogeneous Electrode Processes and Localized Corrosion. Wiley). The WBE concept has enabled the design of various forms of novel localized corrosion probes for in situ measurement and monitoring of difficult‐to‐measure forms of localized corrosion. I believe that future widespread industrial application of effective corrosion monitoring sensors and probes will lead to wider availability of real‐time corrosion data that, in conjunction with data analytics and artificial intelligence technologies, will enable much more reliable corrosion prediction, control, and management systems.
This present book aims to document new developments and latest knowledge on localized corrosion in selected industrial environments, with particular focus on those obtained from soil and marine environments through the use of variously designed electrode array corrosion probes. Presentation of new knowledge in this book is mostly based on a range of practical engineering cases associated mostly with important civil and industrial infrastructures, with particular focus on underground and offshore oil and gas pipelines. Case studies frequently include systematic comparison and analysis of results and findings contained in a long list of published articles by coworkers and the author, leading to a clearer view on a range of complex localized corrosion events occurring on buried and submerged steel surfaces exposed to dynamically changing environmental conditions. This new knowledge would assist engineers to enhance infrastructure durability and enable life extension of complex engineering structures through the improved modeling, prediction, and management of complex localized corrosion. Although cases discussed in this book are mainly from steel pipelines, they are believed to be useful references to many other engineering structures although their actual corrosion behavior may vary to some extent with the environmental condition and materials in each engineering structure.
I would like to express my sincere thanks to my colleagues and coworkers who have collaborated with me over the past decade especially, from early to late, Bruce Hinton, Maria Forsyth, Tony Hughes, Facundo (Bob) Varela, Ying Huo, Jianyu Xiong, Fariba Mahdavi, Shyama Ranade, Reza Parvizi, Ke Wang, Aifang Amy Wei, Mauricio (Max) Leonel Latino, Yunze Xu, Majid Laleh, and Sha Ji. I thank them and my many other colleagues at Deakin University, Australia, who have taught me patiently over the years. I would also thank industry advisers, in particular Alan Bryson, Bruce Ackland, Klaas van Alphen, Craig Bonar, Alan Creffield, Geoff Cope, Brian Martin, Ashley Fletcher, and Alireza Kouklan, for their advice and comments on our applied research work. I also would like to acknowledge that many practical cases discussed in this book are from published work that was funded by the Energy Pipelines Cooperative Research Centre and Future Fuels Cooperative Research Centre, supported through the Australian Government’s Cooperative Research Centers Program. Last but not the least, I would like to thank my amazing family for giving me the encouragement, time, and support that anyone could ever wish for throughout these years.
1Localized Corrosion in Complex Engineering Environments
Although substantial progresses made over the past century in corrosion science and engineering have significantly improved the understanding and control of materials corrosion and degradation of engineering structures, localized corrosion remains a tenacious structural health concern for a wide range of industrial and civil infrastructures such as oil and gas pipelines, piers and ports, water, wastewater and sewer systems, renewal energy production, and transportation facilities. In particular, localized corrosion in complex and variable industrial environments remains a persistent issue that has not been sufficiently managed and controlled. This chapter discusses the characteristics and sources of localized corrosion complexity in practical engineering conditions through the analysis of corrosion cases, from simple to more complex forms. It is shown that localized corrosion complexity is often associated with dynamic changes in corrosion mechanisms and kinetics over an extended period of exposure to variable environments. Localized corrosion complexity makes the prediction, prevention, and management of industrial structural failure a significant challenge.
1.1 Localized Corrosion Complexity
1.1.1 Localized Corrosion and Its Complexity in Engineering Systems
Localized corrosion is usually referred to as corrosion in which there is an intense attack at local sites on a metal surface. A typical example of localized corrosion is pitting corrosion that rapidly attacks a tiny area of an engineering structure such as an oil and gas pipeline, leading to rapid penetration and failure of the whole structure. Localized corrosion is often linked to the local breakdown of a passive or protective film on the surface of a metallic structure, causing accelerated dissolution at localized sites on the structure [1]. In theory, localized corrosion shares the same electrochemical principles that are developed to explain uniform or general corrosion by pioneers of corrosion science, among them Evans [2], Fontana [3], Pourbaix [4], and Tomashov [5]. However, localized corrosion has some characteristics that make it significantly more complex to understand and much more difficult to control than uniform or general corrosion. Over the past decades extensive research has been carried out to understand and model localized corrosion, in particular pitting, by many corrosion scientists and engineers. Among them, Frankel [6] provided an overview of pitting processes including the breakdown of passive films, metastable pitting and pit growth, as well as critical factors influencing pitting corrosion such as alloy composition, environment, potential, and temperature. Macdonald [7] presented the point defect model to explain the growth and breakdown of passive films on metal and alloy surfaces in contact with aqueous solutions, and for the development of a deterministic method for predicting localized corrosion damage. Marcus et al. [8] considered diverse mechanisms of passive film breakdown at the oxide grain boundaries. Soltis [9] highlighted that there is a clear separation of the passivity breakdown/pit initiation process from the pit propagation, which can be considered in terms of the well‐known pitting localized acidification model [3, 10]. Newman [11] reviewed the use of statistical methods and semi‐empirical models, and the fundamental deterministic processes that occur during localized corrosion.
In numerous literatures, pitting of stainless steels is frequently taken as a typical example to describe the characteristics of localized corrosion, probably because of their wide and countless engineering applications. Susceptibility to pitting is well known to be a major weakness of passive alloys including stainless steels and aluminum alloys when they are exposed to some environmental conditions. In order to explain and predict pitting corrosion of stainless steels, many contested pitting models have been proposed over the past decades [6–11], although currently there are still diverse views on pitting initiation and propagation processes. Nevertheless there are some undisputed general observations regarding pitting corrosion characteristics that are often also applicable to other forms of localized corrosion:
The initiation of pitting corrosion on stainless steels involves a very small pit nucleus that grows over periods of the order of seconds. The cause of the initiation of pitting corrosion is still not entirely clear; however, it is clear that manganese sulfide inclusions play a critical role for stainless steel type 316
[12]
. The initiation of pitting is often described as a random or stochastic process with respect to time and location. However, this author has a view that pitting of a specific metal in a particular environment is not an accident, it is a deterministic event determined by the thermodynamic instability of the metal in the environment, regardless of the size and the shape of a metal specimen or an engineering structure
[13]
. This explains the fact that a small corrosion probe can simulate and detect pitting on a much larger structure surface exposed to the same environment.
Pits become more stable as they become larger and a local acidic environment is developed in the local cavity area. This is because for very small pits the local acidity in small cavity can be easily neutralized by diffusion into the bulk solution. As the pits grow larger the diffusion distances increase and the cavity become isolated, and it gets harder for the acidity in pits to change through diffusing away. Therefore when pits are established, they are difficult to be stopped; however, solution movements and the introduction of inhibitors could have an effect on this. This fact also suggests that pitting growth would be a three‐dimensional event, three‐dimensional corrosion models and probes could be necessarily for modelling, simulating and probing such three‐dimensional corrosion behavior.
It is generally believed that pitting requires a passive external surface that can provide a high potential to cause the current to flow into the pit. If the external surface is active, this driving force is not available and therefore pitting would not grow. For this reason, passive metals such as stainless steels are susceptible to pitting because of the passive film of the external surface. Metals such as carbon steel only pit if the solution (e.g. alkaline solutions) passivates it, it would not pit if the solution (e.g. neutral salt solutions) only corrode it generally. Therefore variability in environment (e.g. pH changes) could change carbon steel behavior completely, creating a major uncertainty in corrosion behavior. This characteristic suggests that allowing the formation of local passive and active environments is important for localized corrosion probes. However, it should be noted that if there is an external current source, such as a piece of coupled noble metal, that can behave as cathode to cause the current to flow into the pit, a passive external surface might not be necessary for pitting to occur.
There is a critical pitting temperature that is considered to provide a robust predictor of the pitting potential
[11]
. Below the pitting temperature, metal would be unable to maintain active pitting corrosion dissolution at anodes because passivation intervenes, even in the most aggressive possible microenvironment. This characteristic suggests the importance of environmental condition simulation, especially temperature, in localized corrosion testing and monitoring.
Awareness of these characteristics is important not only for predicting and managing pitting but also for understanding many other forms of localized corrosion that have somewhat similar processes and mechanisms as pitting. For instance, crevice corrosion is known to have a similar mechanism as pitting, although it is easier to initiate than pitting because of the pre‐existing local environment at the crevice [3, 10]. More extensive discussion on pitting and crevice corrosion characteristics and mechanisms can be found in prime corrosion science and engineering textbooks such as those in references [2, 3, 10]. It should be noted however that although extensive knowledge about localized corrosion has been acquired over the past decades, there is a still major knowledge gap in the field: there is insufficient knowledge of complex localized corrosion in variable engineering environments. Theories and methods discussed in the historical literature are generally limited to specific forms of localized corrosion (e.g. pitting of a stainless steel) in a defined corrosion environment (e.g. a sodium chloride solution). This is because localized corrosion knowledge reported in the historical literature is often developed based on observations and data from simplified and accelerated laboratory experiments where corrosion occurs under controlled environmental and electrochemical polarization conditions. Observations of localized corrosion are usually in relatively small dimensional and short time scales using a range of conventional visual, physical, and electrochemical techniques. Electrochemical methods used for localized corrosion studies are generally designed under steady‐state condition hypothesis, which are often ineffective, if not incapable, for probing dynamic and localized interplay between corrosion mechanisms and changing environmental conditions. For instance, most of conventional laboratory corrosion measurement methods such as electrochemical polarization measurement and scanning probe techniques have limitations when applied to practical engineering structures where major variabilities exist in environmental conditions, materials heterogeneities, and local electrochemistry [13].
For these reasons, currently complex forms of localized corrosion, especially those exposed to complicated and variable environmental conditions, are still not well comprehended nor sufficiently characterized and controlled. Although engineers strive to select suitable materials based on available materials property data to design engineering structures that are strong and durable enough to tolerate the service environment, they are usually unable to predict dynamic and localized environment changes and their effects on localized corrosion over long periods of service. If we examine engineering failure records, many major corrosion‐induced incidents of engineering structures are due to localized corrosion on invisible structural components exposed to complex environments such as buried, submerged pipelines, and other types of hidden infrastructure. The management and control of such localized corrosion, when it is initiated, remains a very difficult issue although corrosion control technologies such as cathodic protection, coatings, and inhibitors have been developed to mitigate corrosion [14]. The threat of localized corrosion to the safety of engineering structures is evident by widely reported catastrophic infrastructure failures and an enormous amount of unreported infrastructure incidents [15, 16]. Figure 1.1 illustrates localized corrosion‐induced major oil and gas pipeline failure incidences reported in the public media.
Figure 1.1 Sample localized corrosion‐induced infrastructural failure incidences reported in public media. (a) Corrosion induced gas pipeline explosion; (b) Localized corrosion induced pipeline leaking.
Source: (a) aapsky/Adobe Stock; (b) chitsanupong/Adobe Stock.
1.1.2 Sources of Localized Corrosion Complexity
In practical industrial environments, localized corrosion‐induced engineering structural failure may not be due to a single form of localized corrosion, it could be due to multiple and changing forms of localized corrosion whose processes and mechanisms can vary significantly over time and location, leading to localized corrosion complexity. An example is localized corrosion of buried steel pipelines that are affected not only by seasonal changes in soil moisture and oxygen levels but also by fluctuating stray currents and oscillating mechanical stresses. Another example is localized corrosion and materials degradation on offshore structures such as wind turbines and oil platforms that are affected by multi‐zone and dynamically changing marine environmental conditions. Such complex environmental conditions can not only lead to changes in corrosion rates but also in corrosion characteristics, mechanisms, and patterns. In some cases other factors such as microbiological activities in soil and ocean could add further complexity to corrosion processes and mechanisms.
One source of localized corrosion complexity in practical engineering structures is major variabilities in environmental conditions. Engineering products and infrastructure often need to function in hostile and changing environments such as corrosive soil and sand, damp air and vapor, flowing water and slurry. For instance, stray currents were found to cause dynamic and rapid localized corrosion namely stray current corrosion of buried and submerged metal structures. The fluctuating stray currents can cause dynamic change in electrochemical corrosion reactions and their thermodynamics and kinetics, adding significant complexity to the protection of underground and marine infrastructures. Over long periods of exposure to service environments, engineering infrastructures can also experience changes in localized corrosion mechanisms. A typical example is a problematic form of localized corrosion on buried and submerged steel pipelines namely corrosion under disbonded coatings (CUDCs). Disbondment of organic coatings can occur on coated pipelines under excessive cathodic protection over extended periods of exposure of a metallic pipeline to soil and seawater. Coating disbondment forms crevices between the pipeline and the disbonded coating layer, leading to changes of local corrosion environmental conditions and changes in the corrosion mechanism from general soil or marine corrosion to highly localized CUDCs. According to a recent survey of the energy pipeline industry, corrosion under coating disbondment is responsible for almost 90% of corrosion‐induced damages of buried gas pipelines [17].
Another source of localized corrosion complexity is from the co‐existence of many forms of localized corrosion. Examples of corrosion mechanisms that commonly coexist on underground and marine structures include,
Differential electrochemical potential caused localized corrosion: These include galvanic corrosion (dissimilar‐metal corrosion) in which one metal with less noble electrode potential corrodes preferentially when it is in electrical contact with a more noble metal in an electrolyte. Localized corrosion with similar mechanisms include thermogalvanic corrosion, selective leaching, and intergranular corrosion. Stray current corrosion could also be considered to be due to differential potential over different areas of a metal structure.
Differential aeration cell corrosion (oxygen concentration cell): Localized corrosion are often caused by local environment differences at lap joints, crevices, insulation, as well as debris. Under such conditions, metal areas with less oxygen serves as the anode while areas that are exposed to oxygen usually behave as cathodes of localized corrosion cells. Common examples of this corrosion mechanism include waterline corrosion and filiform corrosion.
Pitting and crevice corrosion: Pitting and crevice corrosion are considered to have similar mechanisms and characteristics (see descriptions in
Section 1.1.1
). The main difference is in the initiation and the geometry of the corrosion anodic site. Whereas pitting corrosion occurs on ‘weaker' areas over a metal surface, crevice corrosion occurs within a crevice that forms under a fastener, washer or joint, under deposits or under the bottom plate of a storage tank. Pitting and crevice mechanisms could occur simultaneously under some practical engineering conditions. For instance, extended pitting corrosion could generate lots of corrosion products that cover the pits and form crevice, leading to accelerated corrosion as the conditions within the pit become more aggressive.
Mechanical and velocity‐induced localized corrosion occur due to a combination of mechanical factors (e.g. applied and/or residual stresses, cyclic loading, wear) and electrochemical corrosion factors. These usually include stress corrosion cracking, erosion–corrosion, fretting corrosion, corrosion fatigue, abrasion–corrosion, and cavitation corrosion.
These corrosion mechanisms can co‐exist and interact with each other, leading to complex forms of localized corrosion to occur on engineering structures. This can be illustrated in Figure 1.2 where a steel pipe passes through four environmental zones: atmosphere, the upper loose gravel soil, dense clay soil and concrete. In this structure, there are three areas that can be locally attacked by corrosion. The higher concentration of dissolved oxygen in the loose gravely soil (better aeration) would make the steel surface cathodic to the steel in the dense clay soil, leading to localized corrosion just below the loose and dense soil interface. Dissolved oxygen concentration differences can also be established by crevices existed at the soil and atmosphere interface, leading to corrosion just below the soil surface. Corrosion also occurs in soil at the soil and concrete interface due to a different mechanism: Because of the high pH of concrete, steel is passivated and behaves as cathode because of a higher electrochemical potential. Steel in soil is not passivated and therefore galvanic corrosion would occur to steel in soil. It is similar to dissimilar‐metal galvanic corrosion in which one metal with less noble electrode potential corrodes preferentially when it is in electrical contact with a more noble metal in an electrolyte.
Figure 1.2 Illustrating corrosion mechanisms co‐existing on a steel pipe that passes through four environmental zones: atmosphere, loose gravel soil, dense clay soil, and concrete.
Overall, major factors that could contribute to the complexity of localized corrosion include:
Invisibility (e.g. buried pipes and submerged marine structures)
Dynamically changing environmental conditions (e.g. soil moisture fluctuations due to wet‐dry seasonal changes, stray current‐induced potential differences and fluctuations)
Dynamically changing mechanism (e.g. from general soil corrosion to corrosion under coating disbondment)
Multi‐time and length scale corrosion initiation and propagation (e.g. growth of pitting and crevice corrosion of heterogeneous alloys over extended exposure to corrosion environments)
Other factors can also be added to the list include mechanical stress‐induced coating damage and corrosion cracking, hydrogen damage (e.g. hydrogen embrittlement) and microbiological corrosion.
1.2 Corrosion from Simple to Complex
In order to better understand the nature, sources, and characteristics of localized corrosion complexity in engineering structures, let’s consider different ways that localized corrosion could become more complex under the effects of various environmental and electrochemical factors.
1.2.1 Corrosion from Uniform to Localized in Engineering Systems
It is well known that aqueous corrosion is the result of a series of electrochemical reactions occurring in electrochemical cells, often referred to as galvanic cells. Galvanic cell is often used as the simplest model for explaining corrosion electrochemical processes. Figure 1.3 shows an ideal textbook galvanic cell where corrosion anodic and cathodic reactions occur in two completely separated half cells. In the galvanic cell formed by dissimilar metals, corrosion concentrates on the anode, leading to metal dissolution “uniformly” on the anode. In this galvanic cell (deoxygenated), an anodic oxidation reaction (Fe → Fe2+ + 2e−) occurs at the iron–solution interface across which iron loses electrons and is corroded, while a cathodic reduction reaction (Cu2+ + 2e− → Cu) occurs “uniformly” at the cathode–solution interface across which cupric ions gain electrons and is deposited as solids. In Figure 1.3, anodic and cathodic reactions can be considered to occur homogeneously over the anode and cathode, respectively, leading to “uniform” corrosion on the anodic iron electrode surface. In this conventional electrochemical model, the mass transfer, chemical reaction, and physical state change are the sequential steps of corrosion processes occurring over two separated anodic and cathodic half cells. Electrochemical corrosion reactions occurring at the electrode–solution interface in the galvanic cell can be described by the most fundamental relationships such as Faraday’s law of electrolysis, Nernst equation, and Bulter–Volmer formulation. Through an electrochemical mechanism, corrosion avoids a large activation energy barrier for both oxidation and reduction reactions, leading to much easier corrosion dissolution to occur at the anode. Electrochemical corrosion reactions occurring in the cell are chemical in nature, therefore all factors affecting normal chemical reactions such as the chemical nature of reactants, the concentrations of the reactants, temperature, the ability of reactants to come in contact with each other, and the availability of rate‐accelerating or decelerating agents would affect corrosion in the galvanic cell.
Figure 1.3 A textbook type galvanic corrosion cell model with uniform anodic and cathodic electrochemical reactions occurring on separated electrodes.
A more practical corrosion cell is shown in Figure 1.4 where a porous membrane replaces the salt bridge in Figure 1.3. The cathodic cell in Figure 1.4 is acidified and contains not only cupric ion (Cu2+) but also oxygen (open to the air), creating a more complex corrosion environment. This is a galvanic cell with competing reactions. The oxidation of iron has the lowest reduction potential and is therefore forced to undergo oxidation, Fe → Fe2+ + 2e− (E° = −0.44 V vs SHE, iron corrosion). The reduction in the cathodic half‐cell would include three competing cathodic reactions: Cu2+ + 2e− → Cu (E° = +0.34, copper plating), O2 + 4H+ + 4e− → 2H2O (E° = +1.23, oxygen reduction), and 2H+ + 2e− → H2 (E° = 0, hydrogen evolution). In the cathodic half‐cell more than one reduction reactions could occur simultaneously over the cathode surface. The major cathodic reaction is the one with the highest reduction potential, i.e. the one with the greater tendency to undergo reduction. This corrosion system is much more complex than that in Figure 1.3, and the application of the fundamental relationships such as the Nernst equation and Bulter–Volmer formulation needs to take multiple cathodic reactions into consideration. Nevertheless anodic and cathodic reactions can still be considered to occur “uniformly” over the anode and cathode, respectively, and therefore corrosion occurring on the iron electrode surface can still be “uniform.”
Figure 1.4 A battery form of galvanic corrosion cell.
The most common corrosion cell that could exist in many engineering structures is shown in Figure 1.5 where a galvanic cell is simply made up of electrodes of two dissimilar metals that are electrically connected and exposed to an aqueous solution containing salts and oxygen. This cell simulates galvanic corrosion environments in engineering devices such as a car radiator. In this galvanic cell, iron is forced to undergo oxidation, Fe → Fe2+ + 2e− (E° = −0.44 V vs SHE, iron corrosion), while oxygen is the dominating reduction cathodic reaction, O2 + 2H2O + 4e− → 4OH− (E° = +1.23, oxygen reduction). This corrosion system is more complex than that in both Figures 1.3 and 1.4 since oxygen reduction could occur on both cathode and the anode surfaces, leading to local changes in pH on both surfaces. As shown in Figure 1.5, the iron anode in the cell would not be a uniformly corroding electrode anymore because of the nonuniform distribution of iron oxidation and oxygen reduction reactions, especially at the waterline area where more oxygen exists. Oxygen reduction at the waterline area would lead to a local high pH condition at cathodic areas, leading to local passivity of iron [14, 18]. This nonuniform electrode makes the application of the fundamental relationships such as Faraday equation, Nernst equation, and Bulter–Volmer formulation difficult because these relationships are designed for uniform electrodes. The continued operation of the cell would cause significant electrochemical heterogeneity and more complex localized corrosion to occur on the iron electrode as the result of the localization of chemical and electrochemical reactions over the electrode surface.
Figure 1.5 Galvanic cell starts to become more complex with localized corrosion starting to appear on the anode.
Corrosion galvanic cells shown in Figures 1.3–1.5 are composed of two dissimilar metals that form separated anode and cathode in the cell; however in practical engineering systems, corrosion usually occurs on the surface of a single metal structure that may be made of the same metallic material or formed by galvanically coupling different metals or alloys. Figure 1.6a shows a simple metallic structure made by joining two copper pieces with a steel bolt exposed to a conductive environment. Galvanic corrosion would occur on the structure under the effect of differential aeration cell corrosion that is often observed on marine structures such as oil and gas production platforms and wind farm tower structures. In the case of Figure 1.6a, iron is the less noble metal and would be the corrosion anode and would be corroded preferentially and nonuniformly because of the existence of the differential aeration cell on its surface. Galvanic corrosion would be concentrated over the small steel bolt, while cathodic reactions occur over the large copper pieces, leading to focused corrosion attack on the iron bolt. This illustrates corrosion due to a dissimilar metal corrosion mechanism in combination with a differential aeration cell corrosion mechanism, leading to more complex corrosion. This structure shown in Figure 1.6a is a poor engineering design since focused corrosion reaction would cause accelerated dissolution of the small iron bolt and rapid failure of the structure. Unfortunately, the structure of the type shown in Figure 1.6a is a rather common engineering design problem that can occur in various engineering systems exposed to fresh and salt water (e.g. desalination plant environments); soil, sand, and concrete (e.g. buried pipeline and metal structures); acids and alkali (e.g. pipeline internal environments). A better design is shown in Figure 1.6b that avoids the situation of a small anode and large cathode and therefore avoids rapid failure of the structure. A simple solution to this galvanic corrosion issue is to use bolts made of copper to avoid the dissimilar metal corrosion situation.
Figure 1.6 A galvanic corrosion cell formed by joining different metal pieces on the same structure. Figure 1.6a illustrates a large cathode and small anode structural design, while figure 1.6b illustrates a small cathode and large anode design.
Automobile cooling water system (radiator) is a practical case where multiple dissimilar metals are coupled and exposed to an electrolytic solution, creating a multi‐metallic corrosion cell. Cathodic reactions on the more noble metal of the cell (copper) would enhance corrosion of the less noble metal in iron engine, aluminum radiator body and in the radiator’ solder etc. To make things more complex, in some cases dissolved noble metals could re‐deposit on less noble metal surface, creating a complex galvanic corrosion network. Galvanic corrosion in a radiator is often controlled by maintaining a high resistivity of the coolant and the efficiency of corrosion inhibitors. Please note that although in these cases, copper is nobler than steel and behaves as corrosion cathode, opposite cases can occur in engineering practice. For instance, a copper underground gas pipeline failed due to local attack by galvanic connection with steel in concrete. This was because in a high pH concrete environment, an oxide passive layer existed on steel surface, making steel passive and more noble than copper and causing galvanic corrosion to occur on copper.
1.2.2 General Corrosion in Micro‐Electrochemical Cells
In most practical cases, aqueous corrosion reactions occurs over a single piece of metal surface, as shown in Figure 1.7, rather than over separated or jointed pieces of metals or alloys (e.g. in Figures 1.3–1.6). Corrosion on a single piece of metal surface can be in uniform or localized corrosion form depending upon if corrosion cell is a “micro‐” or a “macro‐” electrochemical cell. The simplest form of corrosion on a metal surface, i.e. uniform or general corrosion, is explained by a “micro‐” electrochemical cell model that is based on the following assumptions:
Uniform and general corrosion occurs when microscopic local anodic and cathodic sites are sufficiently small and are randomly distributed over a single piece of corroding metal surface. Anodes and cathodes distribute randomly over areas of the metal surface with different electrochemical potentials.
Individual half‐reactions occur in these microscopically separated half‐cells, causing anode to corrode and electron transfer to cathode through an internal electrical circuit. Ions flow through a conducting solution on metal surface.
Anode and cathode locations change dynamically and a given area on a metal surface could act as both an anode and as a cathode over extended period of time. The averaging effect of these shifting local cells results in a rather uniform attack and general loss of metal and roughening of the surface, usually with the appearance of rusting, as shown in the photo of a corroded pipe of
Figure 1.7
.
Such uniformly or generally corroding metal surface, as shown in Figure 1.7, is often referred to as a mixed electrode [19] since several different redox reactions with different reduction potentials and kinetics occur simultaneously over the same electrode surface. Wagner–Truad proposed the mixed potential theory to explain the operation of mixed electrode cells operating at a mixed potential [19]. Corrosion is a typical mixed electrode process operating at a mixed potential namely “corrosion potential.” Corrosion potential is commonly used in conjunction with the E‐pH diagram (often referred to as Pourbaix diagram) [4] that is used as an indicator of corrosion thermodynamic status for predicting if corrosion will occur, for estimating the composition of corrosion products and for predicting environmental changes that would prevent or reduce corrosion attack. The E‐pH diagram is a graphical representations of the thermodynamics of common electrochemical and chemical equilibria between metal and water, indicating thermodynamically stable phases as a function of electrode potential and pH. The E‐pH diagram visualizes the thermodynamics of corrosion processes and gives information about a metal surface whether it is in a region of immunity where the tendency for corrosion is nil, in a region where the tendency for corrosion is high, or in a region where the tendency for corrosion may still exist but where there is also a tendency and possibility for a protective or passive film to exist. Many E‐pH diagrams have been constructed for common materials–environment systems by corrosion scientists including those in Pourbaix’s laboratory [2–4]. In many cases, an E‐pH diagram can be found from the literature for a particular material–environment combination, although we may need to construct E‐pH diagrams for less common systems. As an example, Figure 1.8 shows a typical illustration of the E‐pH diagram for the iron–water–soil system at ambient temperature. However, it should be noted that the thermodynamically derived Pourbaix diagrams of the type shown in Figure 1.8 only provides information on corrosion tendency. Like any thermodynamic quantity, the corrosion potential value on its own does not provide information on the rate of corrosion.
Figure 1.7 Micro‐galvanic corrosion cells on a single piece of metal surface.
The determination of corrosion rates requires the measurement of the kinetics of the corrosion electrochemical process. The rate of a corrosion reaction, for instance Fe → Fe2+ + 2e−, could be determined if we are able to measure the flows of electrons in the metallic phase or ions in the aqueous phase because the corrosion current, icorr, should be the sum of electron flows. However, unfortunately corrosion electrons flow could not be easily measured from corroding surfaces because we are unable to directly measure electrons flowing between numerous mini‐anodes and mini‐cathodes located on the same electrode surface, as illustrated in Figure 1.7. We need to find an indirect way to determine icorr from a corroding electrode surface. A method of analyzing the kinetics of a mixed electrode under dynamic corrosion is a graphical representation of the kinetics of a mixed electrode (often referred to as Evans diagram) [2, 3, 10]. Figure 1.9 illustrates an Evan diagram for iron electrode corrosion in soil. A mixed potential (corrosion potential Ecorr) is achieved through shifting the potentials of both anodic and cathodic reactions by a corrosion current, icorr,Fe
