Technology & Engineering
Wet Corrosion
Wet corrosion refers to the deterioration of materials due to chemical reactions with water and other liquids. This type of corrosion occurs when a metal is exposed to a moist environment, leading to the formation of rust, tarnish, or other forms of degradation. Wet corrosion can significantly impact the structural integrity and longevity of various engineering materials and components.
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11 Key excerpts on "Wet Corrosion"
eBook - PDFSurface Modification and Mechanisms
Friction, Stress, and Reaction Engineering
- George E. Totten, Hong Liang, George E. Totten, Hong Liang(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
13 Corrosion and Its Impact on Wear Processes Einar Bardal Norwegian University of Science and Technology, Trondheim, Norway Asgeir Bardal Hydro Aluminium Technology Center Årdal, Årdal, Norway I. INTRODUCTION Corrosion has been defined in different ways, but the usual understanding of the term is “chemical attack on a metallic material by reaction with the environment.” This definition is also used in the present chapter. Corrosion reactions sometimes strongly interact with mechanically induced local deformation of material, and combinations of various forms of wear and corrosion are among the most important and widespread such chemical-mechanical interactions. The chapter consists of three main parts: General corrosion theories and examples; interaction between wear and corrosion; and characterization of surface topography and structure. The first part is divided in aqueous (wet) corrosion and corrosion (oxidation) in dry gases. Both forms of deterioration are electrochemical in nature, but in somewhat different ways. Aqueous corrosion is characterized by anodic (oxidation) and cathodic (reduction) reaction(s), both taking place at the interface between a metallic material and an electrolyte. On the other hand, when corrosion occur in a dry gas, oxidation of metal is localized at the interface between the metal and a film of oxidation (corrosion) product, while reduction of an oxidant (e.g., oxygen) occurs at the interface between the oxidation product film and the gas. Deviation from this pattern may occur under extremely efficient removal of corrosion product formed in dry gas. Aqueous corrosion is limited to a temperature range where water is in liquid form, while corrosion (oxidation) in dry gases primarily occurs at high temperature. Under certain special circumstances, significant oxidation can also take place at ambient temperature of the gas.
- George Murray(Author)
- 1997(Publication Date)
- CRC Press(Publisher)
Environmental Degradation of Engineering Materials 2.1 Forms of Corrosion The several forms of corrosion to which a metal may be subjected are I. Electrochemical corrosion 2. Uniform corrosion 3. Intergranular corrosion 4. Galvanic corrosion 5. Crevice corrosion 6. Pitting 7. Erosion corrosion 8. Stress corrosion cracking (SCC) 9. Biological corrosion 10. Dezincification (dealloying) II. Concentration cell 12. Embrittlement 13. Filiform 14. Corrosion fatigue 15. Fretting 16. Graphitization 357 All of these forms of corrosion are not present in all applications, but it is possible to have more than one form present. Understanding when each of these forms of corrosion could be potentially present will permit the designer to take steps to eliminate the condition or to limit the corrosion to acceptable limits. Electrochemical Corrosion Corrosion of metals is caused by the flow of energy (electricity). This flow may be from one metal to another, or from one part of the surface of one metal to another part of the surface of the same metal, or from one metal to a recipient of some kind. This flow of electricity can take place in the atmosphere, underwater, or underground as long as a moist conductor or electrolyte, such as water, especially saltwater, is present. The differences in potential that causes the electric currents is mainly due to contact between dissimilar metallic conductors, or differences in concentration of the solution, generally related to dissolved oxygen in natural waters. Any lack of homogeneity on the metal surface may initiate attack by causing a difference in potentials that results in localized corrosion. The flow of electricity (energy) may also be from a metal to a metal recipient of some kind, such as soil. Soils frequently contain dispersed metallic particles or bacterial pockets that provide a natural electrical pathway with buried metal. The electrical path will be from the metal to the soil, with corrosion resulting.
eBook - PDF- Sulzer Pumps(Author)
- 2013(Publication Date)
- Elsevier Science(Publisher)
8. Materials and corrosion 8.1 Introduction DIN 50900 describes corrosion as the reaction of a material with its environ-ment causing a measurable change in the material and possibly leading to corrosive damage. Furthermore, corrosive damage is defined as an Impair-ment of the function of a component or an entire system through corrosion. This definition clearly distinguishes between the corrosion process and its results, namely the corrosive damage; two terms which are frequently incorrectly equated with each other. Corrosive damage is described as damage to a metallic component originating at its surface through chemical reactions of the metal with constituent elements of the environment. By contrast to mechanical wear, corrosion is therefore fundamentally a chemical process, which can be presented by the following reaction equation: Me «± M e z + + Ζ · e~ (1) A metal Me enters into a chemical reaction, as a result of which positively charged metal ions M e z+ and Ζ electrons e~ are released. The metal ions so produced can remain in a dissolved state in the corrosion medium or settle on the corroding surface as insoluble corrosion products or accumulate as sus-pended matter in the corrosion medium. If in addition to the chemical corrosion reaction the metal surface is stressed by mechanical wear or by some other mechanical effect the corrosion reaction may be accelerated consider-ably. 8.2 Factors affecting corrosion The cause of all corrosion reactions is the thermodynamic instability of metals in relation to air, water or other oxidizing agents. They tend to revert to a compounded state, corrosion products being formed from the pure metals and energy being released in the process. In the case of corrosion a material conversion takes place, the speed of which represents the corrosion rate.
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.
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Intergranular corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress corrosion—the formation and propagation of cracks (and possible failure) re- sulting from the combined effects of corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, ap- plication of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 17.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 17.32 and 17.33) is near unity. • The kinetics of film formation may follow parabolic (Equation 17.34), linear (Equation 17.35), or logarithmic (Equation 17.36) rate laws. • Ceramic materials, being inherently corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments. Passivity Forms of Corrosion Corrosion Prevention Oxidation Corrosion of Ceramic Materials 684 • Chapter 17 / Corrosion and Degradation of Materials • Polymeric materials deteriorate by noncorrosive processes. Upon exposure to liquids, they may experience degradation by swelling or dissolution.
eBook - PDF- Nicholas P. Cheremisinoff(Author)
- 1996(Publication Date)
- William Andrew(Publisher)
2. DESIGN AND CORROSION 2.1 INTRODUCTION Corrosion occurs in various forms and is promoted by a variety of causes, all related to process operating conditions. It is a continuous problem that can lead to contaminated process streams which leads to poor product quality and unscheduled equipment shutdowns, which leads to reduced production, and high maintenance and equipment replacement costs. Min- imizing corrosion is a key consideration for the designer and can be accom- plished in two ways: (1) proper material selection for apparatus, and (2) preventive maintenance practices. Both these approaches must be ex- amined by the designer. This chapter reviews principles of corrosion causes and control. It is important to recognize conditions that promote rapid material degradation to compensate for corrosion in designing. 2.2 TYPES OF CORROSION Corrosion is characterized by the controlling chemi-physical reaction that promotes each type. Each of the major types is described below. Uniform corrosion is the deterioration of a metal surface that occurs uni- formly across the material. It occurs primarily when the surface is in contact with an aqueous environment, which results in a chemical reaction between the metal and the service environment. Since this form of corrosion results in a relatively uniform degradation of apparatus material, it can be accounted for most readily at the time the equipment is designed, either by proper material selection, special coatings or linings, or increased wall thicknesses. 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).
eBook - PDFMaterials Science and Engineering
An Introduction
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
Intergranular corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress corrosion—the formation and propagation of cracks (and possible failure) re- sulting from the combined effects of corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, ap- plication of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 17.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 17.32 and 17.33) is near unity. • The kinetics of film formation may follow parabolic (Equation 17.34), linear (Equation 17.35), or logarithmic (Equation 17.36) rate laws. • Ceramic materials, being inherently corrosion resistant, are frequently used at elevated temperatures and/or in extremely corrosive environments. Passivity Forms of Corrosion Corrosion Prevention Oxidation Corrosion of Ceramic Materials 646 • Chapter 17 / Corrosion and Degradation of Materials • Polymeric materials deteriorate by noncorrosive processes. Upon exposure to liquids, they may experience degradation by swelling or dissolution.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
• Metallic corrosion is sometimes classified into several different forms: Uniform attack—degree of corrosion is approximately uniform over the entire exposed surface. Galvanic corrosion—occurs when two different metals or alloys are electrically coupled while exposed to an electrolyte solution. Corrosion Rates Prediction of Corrosion Rates Passivity Forms of Corrosion Summary • 687 Crevice corrosion—the situation when corrosion occurs under crevices or other areas where there is localized depletion of oxygen. Pitting—a type of localized corrosion in which pits or holes form from the top of horizontal surfaces. Intergranular corrosion—occurs preferentially along grain boundaries for specific metals/alloys (e.g., some stainless steels). Selective leaching—the case in which one element/constituent of an alloy is removed selectively by corrosive action. Erosion–corrosion—the combined action of chemical attack and mechanical wear as a consequence of fluid motion. Stress corrosion—the formation and propagation of cracks (and possible failure) resulting from the combined effects of corrosion and the application of a tensile stress. Hydrogen embrittlement—a significant reduction in ductility that accompanies the penetration of atomic hydrogen into a metal/alloy. • Several measures may be taken to prevent, or at least reduce, corrosion. These include material selection, environmental alteration, the use of inhibitors, design changes, application of coatings, and cathodic protection. • With cathodic protection, the metal to be protected is made a cathode by supplying electrons from an external source. • Oxidation of metallic materials by electrochemical action is also possible in dry, gase- ous atmospheres (Figure 16.25). • An oxide film forms on the surface that may act as a barrier to further oxidation if the volumes of metal and oxide film are similar, that is, if the Pilling–Bedworth ratio (Equations 16.32 and 16.33) is near unity.
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 - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
16.7 | | FORMS OF CORROSION It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion–corrosion, and stress corrosion. The causes and means of prevention of each of these forms are discussed briefly. In addition, we have elected to discuss the topic of hydrogen embrittlement in this section. Hydrogen embrittlement is, in a strict sense, a type of failure rather than a form of corrosion; however, it is often produced by hydrogen that is generated from corrosion reactions. Uniform Attack Uniform attack is a form of electrochemical corrosion that occurs with equivalent in- tensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. 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 con- tact 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.
eBook - PDFMaterials Science and Engineering, P-eBK
An Introduction
- William D. Callister, Jr., David G. Rethwisch, Aaron Blicblau, Kiara Bruggeman, Michael Cortie, John Long, Judy Hart, Ross Marceau, Ryan Mitchell, Reza Parvizi, David Rubin De Celis Leal, Steven Babaniaris, Subrat Das, Thomas Dorin, Ajay Mahato, Julius Orwa(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Photograph courtesy of LaQue Center for Corrosion Technology, Inc. Electrochemical corrosion may also occur as a con- sequence of concentration differences of ions or dis- solved gases in the electrolyte solution and between two regions of the same metal piece. For such a concentration cell, corrosion occurs in the locale that has the lower concentration. A good example of this type of corrosion occurs in crevices and recesses or under deposits of dirt or corrosion prod- ucts where the solution becomes stagnant and there is localised depletion of dissolved oxygen. Corrosion preferentially occurring at these positions is called crevice corrosion (figure 17.15). The crevice must be wide enough for the solution to penetrate yet narrow enough for stagnancy; usually the width is several thousandths of an inch. The proposed mechanism for crevice corrosion is illustrated in figure 17.16. After oxygen has been depleted within the crevice, oxidation of the metal occurs at this position according to equation 17.1. Electrons from this electrochemical reaction are conducted through the metal to adjacent external regions, where they are consumed by reduction — most probably reaction 17.5. In many aqueous environments, the solution within the crevice has been found to develop high concentrations of H + and Cl − ions, which are especially corrosive. Many alloys that passivate are susceptible to crevice corrosion because protective films are often destroyed by the H + and Cl − ions. FIGURE 17.16 Schematic illustration of the mechanism of crevice corrosion between two riveted sheets. O 2 O 2 O 2 OH - OH - Cl - M + M + H + e - e - Crevice corrosion may be prevented by using welded instead of riveted or bolted joints, using non- absorbing gaskets when possible, removing accumulated deposits frequently, and designing containment vessels to avoid stagnant areas and ensure complete drainage.
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