Technology & Engineering
Galvanic Corrosion
Galvanic corrosion occurs when two different metals are in contact with each other in the presence of an electrolyte, leading to accelerated corrosion of the less noble metal. This process is driven by the flow of electrons between the two metals, causing the less noble metal to corrode while the more noble metal remains protected. Preventative measures include using compatible metals or insulating the metals from each other.
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12 Key excerpts on "Galvanic Corrosion"
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. Galvanic Corrosion Galvanic Corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. This is the type of corrosion or dissolution that was described in Section 16.2. The less noble or more reactive metal in the particular environment experiences corrosion; the more inert metal, the cathode, is protected from corrosion. As examples, steel screws corrode when in contact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. Depending on the nature of the solution, one or more of the reduction reactions, Equations 16.3 through 16.7, occurs at the surface of the cathode material. Figure 16.14 shows Galvanic Corrosion. The galvanic series in Table 16.2 indicates the relative reactivities in seawater of a number of metals and alloys. When two alloys are coupled in seawater, the one lower in the series experiences corrosion. Some of the alloys in the table are grouped in brackets. Generally the base metal is the same for these bracketed alloys, and there is little danger of corrosion if alloys within a single bracket are coupled. It is also worth noting from this series that some alloys are listed twice (e.g., nickel and the stainless steels), in both active and passive states. The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte, and the rate is related directly to the cathode–anode area ratio; that is, for a given cathode area, a smaller anode corrodes more rapidly than a larger one because corrosion rate depends on current density (Equation 16.24)—the current per unit area of corroding surface—and not simply the current.
eBook - ePub- R. Winston Revie(Author)
- 2011(Publication Date)
- Wiley(Publisher)
Chapter 10 Galvanic Corrosion X. G.Zhang Teck Metals Ltd., Mississauga, Ontario, Canada A. IntroductionGalvanic Corrosion, resulting from a metal contacting another conducting material in a corrosive medium, is one of the most common types of corrosion. It may be found at the junction of a water main, where a copper pipe meets a steel pipe, or in a microelectronic device, where different metals and semiconductors are placed together, or in a metal matrix composite material in which reinforcing materials, such as graphite, are dispersed in a metal, or on a ship, where the various components immersed in water are made of different metal alloys. In many cases, Galvanic Corrosion may result in quick deterioration of the metals but, in other cases, the Galvanic Corrosion of one metal may result in the corrosion protection of an attached metal, which is the basis of cathodic protection by sacrificial anodes.Galvanic Corrosion is an extensively investigated subject, as shown in Table 10.1 , and is qualitatively well understood but, due to its highly complex nature, it has been difficult to deal with in a quantitative way until recently. The widespread use of computers and the development of software have made great advances in understanding and predicting Galvanic Corrosion.Table 10.1 Studies on Galvanic Actions of Miscellaneous Alloys In Various Environments.B. DefinitionWhen two dissimilar conducting materials in electrical contact with each other are exposed to an electrolyte, a current, called the galvanic current, flows from one to the other. Galvanic Corrosion is that part of the corrosion that occurs at the anodic member of such a couple and is directly related to the galvanic current by Faraday's law.Under a coupling condition, the simultaneous additional corrosion taking place on the anode of the couple is called the local corrosion. The local corrosion may or may not equal the corrosion, called the normal corrosion, taking place when the two metals are not electrically connected. The difference between the local and the normal corrosion is called the difference effect, which may be positive or negative. A galvanic current generally causes a reduction in the total corrosion rate of the cathodic member of the couple. In this case, the cathodic member is cathodically protected.
eBook - PDF- Nicholas P. Cheremisinoff(Author)
- 1996(Publication Date)
- William Andrew(Publisher)
Galvanic Corrosion results when two dissimilar metals are in contact, thus forming a path for the transfer of electrons. The contact may be in the form of a direct connection (e.g., a steel union joining two lengths of copper 13 14 Materials Selection Deskbook piping), or the dissimilar iiietals may be immersed in an electrically con- ducting medium (e.g., an elcctrolytic solution). One metal acts as an anode and, consequently, suffers more corrosion than the other metal, which acts as the cathode. The driving force for this type of corrosion is the electro- chemical potential existing between two metals. This potential difference represents an approximate indication of the rate at which corrosion will take place. That is, corrosion rates will be faster in service environments where electrochemical potential differences between dissimilar metals are high. ThermoGalvanic Corrosion is promoted by an electrical potential caused by temperature gradients and can occur on the same material. The region of the metal higher in temperature acts as an anode and thus undergoes a high rate of corrosion. The cooler region of the metal serves as the cathode. Hence, large temperature gradients on process equipment surfaces exposed to service environments will undergo rapid deterioration. Erosion corrosion occurs in an environment where there is flow of the corrosive medium over the apparatus surface. This type of corrosion is greatly accelerated when the flowing medium contains solid particles. The corrosion rate increases with velocity. Erosion corrosion generally manifests as a localized problem due to maldistributions of flow in the apparatus. Corroded regions are often clean, due to the abrasive action of moving par- ticulates, and occur in patterns or waves in the direction of flow. Concentration cell corrosion occurs in an environment in which an electro- chemical cell is affected by a difference in concentrations in the aqueous medium.
eBook - ePub- Roger Francis(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
8Galvanic Corrosion
Roger FrancisRolled Alloys, Unit 16, Walker Industrial Park, Walker Road, Blackburn BB1 2QE, UK[email protected]In the past, Galvanic Corrosion has caused numerous failures of marine systems, some of them very costly. Galvanic Corrosion is briefly mentioned in the previous chapters in this guide but this chapter gives more details on the causes of Galvanic Corrosion, the main factors affecting its severity, and some preventative measures that can be adopted for marine service conditions. Additional information can be found in reference 1 .8.1IntroductionWhen a metal is immersed in an electrically conducting liquid, it takes up an electrochemical potential (also known as the corrosion potential). This is determined by the equilibrium between the anodic and cathodic reactions occurring on the surface and it is usually measured with reference to a standard electrode, such as the saturated calomel electrode (SCE) or silver/silver chloride (Ag/AgCl2 ).Galvanic Corrosion occurs when two metals with different corrosion potentials are in electrical contact while immersed in an electrically conducting, corrosive liquid. Because the metals have different corrosion potentials in the liquid, electrons flow from the anode (more electronegative) metal to the cathode (more electropositive) to equalise the electrochemical potentials. This acts to increase the corrosion on the anode. The additional corrosion is called Galvanic Corrosion, also referred to as bimetallic corrosion, dissimilar metal corrosion, or contact corrosion.The effect of coupling two metals increases the corrosion rate of the anode and reduces, or even suppresses, corrosion of the cathode. Hence, coupling a component to a sacrificial anode can prevent corrosion and this is the principle of cathodic protection (CP).
eBook - PDF- David A. Shifler(Author)
- 2022(Publication Date)
- Wiley(Publisher)
123 LaQue’s Handbook of Marine Corrosion, Second Edition. Edited by David A. Shifler. © 2022 John Wiley & Sons, Inc. Published 2022 by John Wiley & Sons, Inc. In the past Galvanic Corrosion has caused numerous failures of marine systems, some of them very costly. This chapter describes the causes of Galvanic Corrosion, the main factors affecting its sever- ity and some preventative measures that can be adopted. For further information, the reader is referred to Francis (2001). 5.1 Introduction When a metal is immersed in a conducting liquid, it takes up an electrode potential (also known as the corrosion potential). This is determined by the equilibrium between the anodic and cathodic reactions occurring on the surface, and it is usually measured with reference to a standard elec- trode, such as the saturated calomel electrode (SCE). Galvanic Corrosion occurs when two metals with different potentials are in electrical contact, while immersed in an electrically conducting, corrosive liquid. Because the metals have different natural potentials in the liquid, a current will flow from the anode (more electronegative) metal to the cathode (more electropositive) in order to equalize the potentials. This will increase the corro- sion on the anode (Figure 5.1). This additional corrosion is Galvanic Corrosion, which is also known as bimetallic corrosion, dissimilar metal corrosion, or contact corrosion. These terms are used interchangeably throughout this chapter. In general, the reactions that occur under bimetallic corrosion are similar to those that would occur on a single, uncoupled metal, but the rate of attack is increased, sometimes dramatically. With some metal combinations, the change in the electrode potential in the couple compared with the uncoupled potential can induce corrosion that would not have occurred in the uncoupled state, e.g.
eBook - PDFMechanotechnics N4 Student's Book
TVET FIRST
- R Cameron(Author)
- 2021(Publication Date)
- Macmillan(Publisher)
● Stress corrosion develops if the metal is subjected to internal stress when cold worked. Cracks form in the grain structure of the metal. ● Galvanic Corrosion (bimetallic corrosion) occurs when a corrosive medium penetrates at the point of contact between two metals. It is an electrochemical process: two metals with different electrode potentials come into contact in an electrolyte. ● Inter-crystalline corrosion occurs in alloys such as stainless steel which have an irregular internal structure. It is caused by cooling or heating during manufacture. It cannot be seen from the outside and serious defects can develop. ● Pitting corrosion appears as isolated areas of corrosion. It is caused by malformed strips or uneven surfaces in the metal which cause a potential difference, resulting in corroded areas (pitting) on the metal surface. Forms of corrosion UNIT 2.1 UNIT 2.2 UNIT 2.3 51 Metal protection TVET FIRST ● Cathodic protection is a process in which a metal is protected from corrosion by connecting it to a more reactive metal. It is also called the sacrificial anode method. ● Electroplating is the process of coating an object with metal using an electric current. ● Anodising is an electrolytic process for producing a thick oxide coating, usually on aluminium and its alloys. The oxide layer protects the surface from corrosion. ● Galvanising involves the distribution of a thin layer of zinc over the surface of mild steel. Three galvanising processes are hot dipping, sheradising, electrolytic galvanising. ● Phosphating is the process of converting a metal surface to iron phosphate. It is mostly used as a pretreatment and as a method of corrosion protection. It gives a strong adhesive base for painting. Protective processes for metals ● Corrosion testing determines the resistance of metals to corrosion under environmental conditions such as the presence of salt, humidity and temperature.
- S Borenstein(Author)
- 1994(Publication Date)
- Woodhead Publishing(Publisher)
The battery stops producing power when the zinc is depleted and completely ~ o r r o d e d . ~ Similarly, Galvanic Corrosion occurs when one metal is in contact with another metal and the two are in a conductive solution, thus producing a galvanic couple. The corrosion of a more active metal such as aluminium will be accelerated, while the corrosion of the other more noble metal, such as steel, will be r e d ~ c e d . ' ~ Which one will act as the anode and which as the cathode is determined by the direction the electric current flows as a result of the galvanic effect.15 The most direct way to determine current direction is by placing two metals in solution, providing an electrical path, and measuring the current flow. Arranging this information would produce a galvanic series, which could be repeated for many metals in many solutions. If the experiments were repeated for different solutions, values and the order of the relation- ships would change. One commonly used galvanic series is for seawater. Table 4.3 shows the galvanic series in seawater. Note how an anode in one situation may be a cathode in another. For an example of how the galvanic series can be used, consider a stainless steel nut and bolt attached to a carbon steel frame immersed in seawater. There is an electrochemical cell made up of the steel (anode), the stainless steel (cathode), the threaded connection (making a circuit) and the seawater (the electrolyte). Corrosion will occur at the anode. The amount of corrosion and the rate will depend on several factors, such as the size and geometry of the items. The magnitude of the galvanic effects is a separate issue.
eBook - ePub- J. O. Robertson, G. V. Chilingar(Authors)
- 2017(Publication Date)
- Wiley-Scrivener(Publisher)
An illustration of Galvanic Corrosion is presented in Figure 6.4 .Corrosion cell showing flow of electrons and electrical current. Corrosion occurs at the anode. (After Chilingar et al., 2008, figure 1.4, p. 8.)Figure 6.4Thermodynamic data indicate that the corrosion process in many environments of interest should proceed at very high rates of reaction. Fortunately, experience shows that the corrosion process behaves differently. Studies have shown that as the process proceeds, an increase in concentration of the corrosion products develops rapidly at the cathodic and anodic areas. These products at metal surfaces serve as barriers that tend to retard the corrosion rate. The reacting components of environment may be depleted locally, which further tends to reduce the total corrosion rate.The potential differences between cathodic and anodic areas decrease as corrosion proceeds. This reduction in potential difference between electrodes upon current flow is termed polarization. The potential of anodic reaction approaches that of the cathode and potential of the cathodic reaction approaches that of the anode. Electrode polarization by corrosion is caused by:- changing the surface concentration of metal ions,
- adsorption of hydrogen gas at cathodic areas,
- discharge of hydroxyl ions at cathodes, and
- increasing resistance of electrolyte and film of metal-reaction products on the metal surface.
One practical method to control corrosion is through cathodic protection, whereby polarization of the structure to be protected is accomplished by supplying an external current to the corroding metal. Polarization of the cathode is forced beyond the corrosion potential. The effect of the external current is to eliminate the potential differences between the anodic and cathodic areas on the corroding metal. Removal of the potential differences stops local corrosion action. Cathodic protection operates most efficiently in the systems under cathodic control, i.e., where cathodic reactions control the corrosion rate.
- Barbara Renner(Author)
- 2017(Publication Date)
- Routledge(Publisher)
CHAPTER 9 Corrosion Control 9.01 Corrosion occurs constantly in every water/ wastewater treatment plant, as well as throughout the pipe line system. In fact, corrosion occurs in one form or another on everything that is manufactured. Although it is impossible to escape from corrosion, methods are available that can help reduce the effects of corrosion, thereby extending the operating life of all equipment. 9.02 Understanding the basics of corrosion makes it easier to take the necessary corrective action to control corrosion and extend equipment service life. Some of the methods that are used to control corrosion are performed on a routine basis in many plants, and are not referred to as corrosion control, but as “maintenance.” Other methods are more positively directed as being a way of controlling corrosion. BASICS OF CORROSION 9.03 There are many different types of corrosion that occur. However, to simplify the discussion, this text will classify corrosion as two general types— galvanic and chemical. Each of these two types will be further subdivided into other types, but all have some relationship to the two general types. Some of the methods that are used to control corrosion will be discussed later on in the chapter. Galvanic Corrosion 9.04 Galvanic Corrosion occurs between two dissimilar metals, or when a galvanic cell occurs. A galvanic cell is composed of a cathode (positive [+] terminal), anode (negative [-] terminal), and an electrolyte (some type of media that permits electron flow), as shown in Figure 9.1. In treatment plants, the electrolyte is water, and the galvanic action occurs between the different components of metallic materials. 9.05 Before discussing any actual situations, let’s first look at the basic galvanic cell and its interactions. As was stated, the cell consists of a cathode and an anode of two dissimilar metals, suspended in salt water and connected by a wire
eBook - PDF- Yongchang Huang, Jianqi Zhang, Yongchang Huang, Jianqi Zhang(Authors)
- 2018(Publication Date)
- De Gruyter(Publisher)
Galvanic Corrosion indicates sacrificing the metallic anode but proba- bly making the metallic cathode transfer from the passive region to the active region and thus inducing corrosion. However, the galvanic couple can stimulate anodic passivation to alleviate corrosion. Being the cathode or anode in the Galvanic Corrosion is determi- ned by the galvanic series of the stable electrochemical potentials for different metals in the electrolyte. Table 6.2 shows the galvanic series of typical metallic materials in seawater. Although the galvanic series have provided the sequence of the occurrence of Galvanic Corrosion in metallic materials, galvanic corrosive rate and its affecting factors have to be quantified to study the galvanic corrosive dynamics. Polarization dynamics are the basic techniques used to judge corrosive rate and affecting factors. 228 6 Localized corrosion in metallic materials Tab. 6.2: The galvanic series of typical metallic materials in seawater [2, 8]. Galvanic series Materials Electrode potentials (V vs. SCE) Noble metals Graphite +0.19 to +0.30 Pt +0.18 to +0.25 NiCrMo alloy −0.03 to +0.10 Ti −0.05 to +0.05 NiFeCr alloy −0.03 to +0.04 316 and 317 stainless steel −0.10 to −0.01 (Activated: −0.48 to −0.37) 302,304,321 and 317 stainless steel −0.10 to −0.05 (Activated: −0.58 to −0.48) 430 stainless steel −0.28 to −0.21 (Activated: −0.58 to −0.48) CuNi alloy (80/20) −0.29 to −0.21 410 and 416 stainless steel −0.35 to −0.25 (Activated: −0.57 to −0.46) Brass, red copper −0.40 to −0.29 Low alloy steel −0.64 to −0.56 Low carbon steel, cast iron −0.55 to −0.44 Cd −0.74 to −0.69 Al alloy −1.00 to −0.77 Zn −1.04 to −0.98 Base metals Mg −1.62 to −1.59 Figure 6.7(a) exhibits the galvanic coupling polarization diagram for Zn electrode and Au electrode in acidic electrolyte.
- David Bodle, Axel Ghazi, Moninuddin Syed, Ralph Woodside(Authors)
- 2019(Publication Date)
- University of Toronto Press(Publisher)
Variations in the types of metal, electrolyte, or electric circuit will influence 295 the rate of corrosion. When the metals are some distance apart they form a typical galvanic cell. If the metals are in close contact then localized corrosion (natural cell corrosion) will occur. The Natural Corrosion Cell is essentially a short-circuited cell. It may be set up by: a) electrodes of two different metals in metallic contact and joined by solution covering them both; b) electrodes consisting of the same metal surrounded by solutions of different concentrations; or c) electrodes in the same solution with better access for oxygen to one electrode than to the other. The resistance of the metallic path joining the two areas that constitute the cathode and anode respectively is negligible. If the electrolyte connecting the two areas is a concentrated salt solution, the electrolytic path may also have negligible resistance. However, in such cases the current is limited by the degree of pol-arization, which is usually controlled by the limited rate of replenishment of oxygen to the cathode. Crevice corrosion and corrosion of soldered joints are common forms of such localized corrosion. 6.3.3 The pH of the Environment The Pourbaix diagram is based on the principle that al-though a metal is likely to corrode over a wide range of pH, immunity or passivation might be attained by controlling the potential at the interface between the metal and its environment (The Pourbaix diagram for lead is given in Figure 6-1). Each line in a Pourbaix diagram represents some electrochemical equilibrium condition. For example, a horizontal line represents equilibrium for a reaction involving electrons but not H or OH ions. Con-versely, a vertical line represents an equilibrium involving H or or OH ions but not electrons. Normal reactions are represented by lines falling between these extremes.
eBook - PDFMetals and Materials
Science, Processes, Applications
- R. E. Smallman, R J Bishop(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 12 Corrosion and surface engineering 12.1 The engineering importance of surfaces The general truth of the engineering maxim that 'most problems are surface problems' is immedi-ately apparent when one considers the nature of metallic corrosion and wear, the fatigue-cracking of metals and the effect of catalysts on chemical reactions. For instance, with regard to corrosion, metal surfaces commonly oxidize in air at ambient temperatures to form a very thin oxide film (tarnish). This 'dry' corrosion is limited, destroys little of the metallic substrate and is not normally a serious problem. However, at elevated tempera-tures, nearly all metals and alloys react with their environment at an appreciable rate to form a thick non-protective oxide layer (scale). Molten phases may form in the scale layer, being particularly dangerous because they allow rapid two-way diffu-sion of reacting species between the gas phase and the metallic substrate. 'Wet' or aqueous corrosion, in which electrochemical attack proceeds in the presence of water, can also destroy metallic surfaces and is responsible for a wide variety of difficult problems throughout all branches of industry. The principles and some examples of 'dry' and 'wet' corrosion will be discussed in Section 12.2. Conventionally, the surface properties of steels are improved by machining to produce a smooth surface texture (superfinishing), mechanically working (shot-peening), introducing small atoms of carbon and/or nitrogen by thermochemical means (carburizing, nitriding, carbo-nitriding), applying protective coatings (galvanizing, electroplating), chemically converting (anodizing), etc. Many of these traditional methods employ a liquid phase (melt, electrolyte). In contrast, many of the latest generation of advanced methods for either coating or modifying material surfaces use vapours or high-energy beams of atoms/ions as the active media.
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