Localized Corrosion in Complex Environments
Mike Yongjun Tan
Deakin University, Australia
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Library of Congress Cataloging-in-Publication Data
Names: 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/20220919
LC record available at https://lccn.loc.gov/2022039095
LC ebook record available at https://lccn.loc.gov/2022039096
Cover Image: Wiley
Cover Design by Courtesy of Mike Yongjun Tan
Set in 9.5/12.5pt STIXTwoText by Straive, Pondicherry, India
Contents
Preface ix
1 Localized Corrosion in Complex Engineering Environments 1
1.1 Localized Corrosion Complexity 1
1.1.1 Localized Corrosion and Its Complexity in Engineering Systems 1
1.1.2 Sources of Localized Corrosion Complexity 5
1.2 Corrosion from Simple to Complex 8
1.2.1 Corrosion from Uniform to Localized in Engineering Systems 8
1.2.2 General Corrosion in Micro-Electrochemical Cells 13
1.2.3 Localized Corrosion in Macro-Electrochemical Cells 17
1.3 Cases of Localized Corrosion in Industry 20
1.3.1 Corrosion Due to Unexpected Environmental Changes 20
1.3.2 Pitting Due to Unanticipated Corrosion Mechanism Changes 23
1.3.3 Multifaceted Corrosion of Underground Pipelines 26
1.4 Obstacles in Modeling and Managing Complex Localized Corrosion 30
1.4.1 Challenges in Managing Localized Corrosion 30
1.4.2 Issues in Modeling and Predicting Complex Localized Corrosion 33 References 36
2 Techniques for Localized Corrosion Inspection and Monitoring 41
2.1 Techniques for Corrosion Detection, Inspection, and Data Acquisition 41
2.1.1 Issues in Detecting and Predicting Complex Localized Corrosion 41
2.1.2 Conventional Techniques and Tools for Acquiring Corrosion Data 43
2.1.3 Pipeline Industry Concerns About Integrity Assessment 48
2.1.4 Progress in Techniques for Unpiggable Pipelines Corrosion Assessment 49
2.2 Corrosion Monitoring Using Sensors and Probes 53
2.2.1 Overview of Corrosion Sensors and Probes 54
2.2.2 Electrochemical Probes for Monitoring Localized Corrosion 58
2.3 Multi-Electrode Arrays for Probing Complex Corrosion 61
2.3.1 Localized Corrosion Probes Design Based on Electrode Arrays 61
2.3.2 Various Electrode Array Probe Designs for Examining Crevice Corrosion 67
2.3.3 Further Issues and Challenges in Corrosion Monitoring Probes 71
2.3.3.1 Design Electrode Array Probes for Measuring Electrochemical Processes Over Wide Ranges of Length and Time Scales 72
2.3.3.2 Design Electrode Array Probes for Probing Multiple Corrosion Mechanisms Occurring Simultaneously 73
2.3.3.3 Data Acquisition Options Through Combined Use with Various Electrochemical and Surface Analytical Techniques 73
2.3.3.4 Electrode Array Manufacturing by Means of Various Fabrication Methods 75
2.3.3.5 Design Electrode Array Probes for Industry Application Conditions 76
2.3.3.6 Development of Data Systems and IT Platforms 76 References 77
3 Localized Corrosion in Changing Environments 89
3.1 Probing Localized Corrosion in Nonuniform and Changing Environments 90
3.1.1 Challenges of Localized Corrosion in Diverse and Changing Environments 90
3.1.2 Probing Localized Corrosion in Diverse and Changing Environments 94
3.2 Steel Corrosion Behavior in Soil Under Disrupted Cathodic Protection 102
3.2.1 Localized Corrosion on Steel in Soils Under Varying CP and Moisture Levels 102
3.2.2 Localized Corrosion on Steel with Varying Coating Defect Sizes 110
3.3 Steel Corrosion Behavior in Soil Under Stray Currents 114
3.3.1 Locating Stray Current Corrosion in Soil 115
3.3.2 Dynamic Stray Current Corrosion Under Anodic Potential Transients 122
3.3.3 Stray Current Corrosion Under the Effects of Cyclic Potential Transients 129 References 137
4 Localized Corrosion Influenced by Changing Mechanisms 145
4.1 Localized Corrosion and Materials Degradation with Varying Mechanisms 146
4.1.1 Issues with Changing Corrosion Mechanisms on Buried Pipelines 146
4.1.2 Assessing Coating Disbondment and Corrosion Under Disbonded Coatings 149
4.2 Probing Localized Coating Degradation and Disbondment 153
4.2.1 Probes for Monitoring Cathodic Disbondment 153
4.2.2 Examining Factors Affecting the Rate of Coating Cathodic Disbondment 155
4.3 Probing Localized Corrosion Under Disbonded Coatings 161
4.3.1 Integrated 3D Probes for Probing Corrosion Under Disbonded Coatings 161
4.3.2 Probing CUDC of Varying Disbondment Geometries in Soil 169
4.3.3 Field Monitoring of Localized Corrosion on Buried Pipelines 180
References 197
5 Corrosion Affected by Multiple Environments and Mechanisms 205
5.1 Localized Corrosion Affected by Multiple Environments and Mechanisms 206
5.2 Probing Multi-Mechanism Corrosion Across Multiple Environments 214
5.2.1 Probing Localized Corrosion Across Soil and Air Interface 215
5.2.2 Probing Localized Corrosion Across Water/Air and Soil/Water Interfaces 218
5.2.3 Probing Localized Corrosion of Steel in Concrete 219
5.2.4 Probing Localized Corrosion in Atmospheric Conditions 221
5.3 Cases of Probing Localized Corrosion Over Multiple Environments 227
5.3.1 Probing Localized Corrosion in Marine Environments 227
5.3.2 Probing Localized Corrosion on Shore- Crossing Pipeline by Moving Probes 237
References 244
6 Localized Corrosion Impacted by Flow and Erosion 249
6.1 Localized Corrosion Impacted by Flowing Liquid and Solid Particles 249
6.1.1 Issues with Localized Corrosion Affected by Flow and Erosion 250
6.1.2 Acquiring Corrosion Data from Flowing and Erosion Systems 252
6.2 Cases of Probing Corrosion and Inhibition in Flowing Liquids 259
6.2.1 Probing Erosion–Corrosion Behavior of Stainless Steels 259
6.2.2 Monitoring Corrosion Inhibitor Film Damage by Flowing Liquid 262
6.2.3 Probing Localized Damage of Inhibitor Film and Corrosion 264
6.3 Probing Localized Corrosion Mechanisms Impacted by Flow and Erosion 271
6.3.1 Assessing Factors Affecting FAC and Erosion Corrosion 271
6.3.2 Exploring FAC and Erosion–Corrosion Mechanisms 277 References 292
7 Localized Corrosion Induced by Metallurgical Heterogeneities 299
7.1 Multiscale Corrosion Induced by Metallurgical Heterogeneity 299
7.1.1 Localized Corrosion Over Multi- Time and Length Scales 299
7.1.2 Techniques for Probing Localized Corrosion Over Different Scales 303
7.1.3 Three-Dimensional Techniques for Characterizing Multiscale Localized Corrosion 308
7.2 Various Probes Designed for Probing Multiscale Corrosion 317
7.2.1 2D and 3D Electrode Arrays for Probing Multiscale Corrosion 317
7.2.2 Probing Corrosion of Multi-Metal Structures and Weldments 325
7.2.3 Simplified Method for Probing Multi-Scale Weldment Corrosion 329 References 333
8 Challenges and Opportunities in Managing Complex Localized Corrosion 343
8.1 Future Perspectives in Corrosion Monitoring Tools and Predictive Models 344
8.1.1 Opportunities in Developing More Reliable Monitoring Tools and Models 345
8.1.2 Obstacles in Probing Multiscale Corrosion 348
8.1.3 Challenges in Quantifying Mechanical Impact on Material Behavior 349
8.2 Opportunities in Developing Smart Anticorrosion Methods and Materials 353
8.2.1 Design and Select Materials for Severe Industrial Conditions 353
8.2.2 Design and Apply Environmentally Friendly Anticorrosion Materials 355
References 360
Index 363
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
Preface xii
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.
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 wellknown 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
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.
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
1 Localized Corrosion in Complex Engineering Environments
● 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,
Figure 1.4 A battery form of galvanic corrosion cell.
Electrons
Fe Fe2++ 2e–
Anodic reaction: Membrane separator
Cu2++ 2e–
Three cathodic reaction: Cu 4OH– O2 + 2H2O + 4e–2H+ + 2e– → H2
be considered to occur “uniformly” over the anode and cathode, respectively, and therefore corrosion occurring on the iron electrode surface can still be “uniform.”
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
Anode (–)
Electron flow Ammeter (+) Cathode –
1 Localized Corrosion in Complex Engineering Environments
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
O2 + 2H2O + 4e– 4OH
Copper 1
Fe2+ + 2e– Fe
Possible media:
NaCl/Seawater
Soil/Sand / Concrete Acids/Alkali
Iron piece 1
O2 + 2H2O + 4e– 4OH–
Steel or iron plate
Iron piece 2
Waterline area
Copper bolt
Fe2+ + 2e– Fe
Possible media:
NaCl/Seawater
Soil/Sand/Concrete Acids/Alkali
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.
Copper 2
(a)
(b)
Steel bolt
