{"id":30607,"date":"2021-07-12T11:17:43","date_gmt":"2021-07-12T11:17:43","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30607"},"modified":"2023-09-20T13:11:40","modified_gmt":"2023-09-20T13:11:40","slug":"forms-of-corrosion","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/failure-modes-of-materials\/corrosion\/forms-of-corrosion\/","title":{"rendered":"Forms of Corrosion"},"content":{"rendered":"
Corrosion<\/strong> is the deterioration of a material due to chemical interaction with its environment. It is a natural process<\/strong> in which metals convert their structure into a more chemically-stable form, such as oxides, hydroxides, or sulfides. The consequences of corrosion are all too common. Familiar examples include the rusting of automotive body panels and pipings and many tools. Corrosion<\/strong> is usually a negative phenomenon since it is associated with the mechanical failure of an object. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even noble metals, such as gold, are subject to corrosive attack in some environments.<\/p>\n The areas over which the anodic and cathodic reactions occur can vary greatly during metal corrosion. Corrosion<\/strong> can come in different forms and grow at different rates. This results in various forms of corrosion, such as a uniform attack, pitting, and crevice corrosion. The problem is that many forms of corrosion exist, and each is caused by different reasons and undergoes different mechanisms. Moreover, each form of corrosion has its special mechanism, which can be quite complex in some cases. This is especially problematic when two or more corrosion types occur simultaneously.<\/p>\n In the following section, we will briefly describe the most common forms of corrosion<\/strong>. They are divided into two subcategories: general (uniform) and localized form of corrosion.<\/p>\n <\/p>\n General corrosion<\/strong>, also known as uniform corrosion, is a form of corrosion that affects the entire surface of the metal, whereas other forms affect a specific spot or portion. It is the most common form of corrosion. This type of corrosion is commonly observed in pure metals, which are metallurgical and compositionally uniform. Weathering steels<\/a>, magnesium alloys<\/a>, zinc alloys, and copper alloys are materials typically exhibiting general corrosion. Passive materials, such as stainless steel<\/a>, aluminum alloys, or nickel-chromium alloys, are generally subject to localized corrosion.<\/p>\n It is a very slow reaction that is fairly evenly distributed over the entire metal surface exposed to any circulating water. The fact that it affects a fairly large area of the metal makes it much easier to detect and hence much less severe than localized corrosion. The problem with general corrosion is that it results in a large volume of oxides that tend to attach themselves to the heat transfer surfaces and affect the system’s efficiency.<\/p>\n General Corrosion – Protection<\/strong><\/p>\n Some standard methods associated with material selection that protect against general corrosion include:<\/p>\n In localized corrosion,<\/strong> there is an intense attack at localized sites on the surface of a component while the rest of the surface is corroding at a much lower rate. Localized corrosion occurs when corrosion works with other destructive processes such as stress, fatigue, erosion, and other forms of a chemical attack. Localized corrosion mechanisms can cause more damage than any one of those destructive processes individually. Localized corrosion can further be classified as pitting corrosion, galvanic corrosion, crevice corrosion, selective corrosion, erosion corrosion, intergranular corrosion, chloride stress corrosion, and stress corrosion cracking<\/a> (SCC). Passive materials, such as stainless steel, aluminum alloys, or nickel-chromium alloys, are generally subject to localized corrosion.<\/p>\n Pitting, characterized by sharply defined holes, is one of the most insidious forms of corrosion. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction, and it is supposed that gravity causes the pits to grow downward. Pitting corrosion can cause failure by perforation while producing only a small weight loss on the metal. This perforation can be difficult to detect, and its growth is rapid, leading to unexpected loss of function of the component. Pitting corrosion has also been associated with both crevice and galvanic corrosion. Metal deposition (copper ions plated on a steel surface) can also create sites for pitting attacks.<\/p>\n Causes of pitting corrosion include:<\/p>\n With corrosion-resistant alloys, such as stainless steel, the most common cause of pitting corrosion is the highly localized destruction of passivity by contact with moisture that contains halide ions, particularly chlorides. However, alloying with about 2% molybdenum enhances their resistance significantly. Chloride-induced pitting of stainless steels usually results in undercutting, producing enlarged subsurface cavities or caverns.<\/p>\n Pitting Corrosion – Protection<\/strong><\/p>\n Pitting corrosion is a hazard due to the possible rapid penetration of the metal with little overall loss of mass. Pitting corrosion is minimized by:<\/p>\n Crevice corrosion<\/strong> refers to the localized corrosion at the crevice or gap between two or more joining metals. Crevice corrosion is a pitting corrosion that occurs specifically within the low flow region of a crevice. This type of attack is usually associated with small volumes of stagnant solution caused by holes, gaskets surface, lap joints, surface deposits, and crevices under bolt and rivets heads. The damage occurs due to the difference in constituents\u2019 concentration, mainly oxygen, on the surfaces involved. Crevice corrosion can progress rapidly (tens to hundreds of times faster than the normal rate of general corrosion in the same given solution).<\/p>\n Crevice corrosion is a hazard due to the possible rapid penetration of the metal with little overall loss of mass. Crevice corrosion is minimized by:<\/p>\n Galvanic corrosion<\/strong> occurs when two dissimilar metals are immersed in a conductive solution in presence of some potential difference, and there is a flow of electrons between the metals. It may also occur with one metal with heterogeneities (dissimilarities) (for example, impurity inclusions, grains of different sizes, differences in composition of grains, or differences in mechanical stress). The metal which is less corrosive resistant becomes anode, and the metal with more corrosive resistance becomes a cathode. The corrosion of the less corrosive resistance is usually increased, and attack on more resistant material is decreased. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corroded or electrolyte.<\/p>\n Galvanic corrosion occurs only if the following conditions are met:<\/p>\n If any of these conditions are not satisfied, galvanic corrosion will not likely occur.<\/p>\n Galvanic corrosion only causes the deterioration of one of the metals. The stronger, more noble one is cathodic (positive) and protected. This is the mechanism of galvanic anodes, which are the main component of a galvanic cathodic protection (CP) system used to protect buried or submerged metal structures from corrosion. In some instances, galvanic corrosion can be helpful.<\/p>\n Selective leaching<\/strong> or selective corrosion<\/strong> removes one element from a solid alloy by the corrosion process. The most common example is the dezincification of brass<\/strong>, in which zinc is selectively leached from a copper-zinc brass alloy, and this process produces a weakened porous copper structure. The selective removal of zinc can be in a uniform manner or localized scale.<\/p>\n However, many alloys are subject to selective leaching under certain conditions. A similar process occurs in other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are removed. Elements in an alloy that are more resistant to the environment remain behind. Two mechanisms are involved:<\/p>\n The first system involves the dezincification of brasses, and the second system is involved when molybdenum is removed from nickel alloys in molten sodium hydroxide.<\/p>\n Erosion corrosion is the cumulative damage induced by electrochemical corrosion reactions and mechanical effects from relative motion between the electrolyte and the corroding surface. Erosion can also occur in combination with other forms of degradation, such as corrosion, and this is referred to as erosion-corrosion. Erosion corrosion is a material degradation process due to the combined effect of corrosion and wear. Nearly all flowing or turbulent corrosive media can cause erosion corrosion. The mechanism can be described as follows:<\/p>\n Erosion corrosion is found in the systems such as piping, valves, pumps, nozzles, heat exchangers, and turbines. Wear is a mechanical material degradation process occurring on rubbing or impacting surfaces, while corrosion involves chemical or electrochemical reactions of the material. Corrosion may accelerate wear, and wear may accelerate corrosion.<\/p>\n Intergranular corrosion<\/strong> (IGC) is preferential corrosion along the grain boundaries of material. For some alloys and in specific environments. This type of corrosion is especially prevalent in some stainless steel. In stainless steel, intergranular corrosion may occur due to the precipitation of chromium carbides (Cr23<\/sub>C6<\/sub>) or intermetallic phases.<\/p>\n The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation<\/strong>. For passivation to occur and remain stable, the Fe-Cr alloy<\/strong> must have a minimum chromium content of about 10.5% by weight<\/strong>, above which passivity can occur and below is impossible. But the chromium carbides may precipitate in the grain boundaries, which results in depletion of chromium in the zones close to the grain boundaries due to the diffusion rate of chromium that is slow. The chromium-depleted zones become less corrosion-resistant than the rest of the matrix. In a corrosive environment, the depleted areas may be activated, and corrosion will occur in narrow areas between the grains.<\/p>\n Intergranular corrosion is an especially severe problem in the welding of stainless steel. When it is often termed weld decay<\/strong>. Also, stainless steel, which has been heat-treated in a way that produces grain boundary precipitates and adjacent chromium depleted zones, is sensitized. Stainless steels can be stabilized against this behavior by the addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide, and tantalum carbide preferentially to chromium carbide, by lowering the content of carbon in the steel and case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000 \u00b0C and quenching it in water, leading to the dissolution of the chromium carbide in the grains and then preventing its precipitation.<\/p>\n There are two special cases of intergranular corrosion, but these mechanisms are treated separately:<\/p>\n One of the most serious metallurgical problems and a major concern in the nuclear industry is stress-corrosion cracking<\/strong>\u00a0(SCC).\u00a0Stress-corrosion cracking<\/strong>\u00a0results from the\u00a0combined action<\/strong> of applied\u00a0tensile stress<\/strong><\/a>\u00a0and a\u00a0corrosive environment<\/strong>. Both influences are necessary. SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks subject to corrosion, which is then corroded further, weakening the metal by further cracking. The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching. Failure behavior is characteristic of that brittle material, even though the metal alloy is intrinsically ductile. SCC can lead to unexpected sudden failure of normally ductile metal alloys subjected to tensile stress, especially at elevated temperatures. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments.<\/p>\nForms of Corrosion<\/h2>\n
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General Corrosion<\/h3>\n
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Localized Corrosion<\/h3>\n
Pitting Corrosion<\/h3>\n
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Crevice Corrosion<\/h3>\n
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Galvanic Corrosion<\/h3>\n
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Selective Leaching \u2013 Selective Corrosion<\/h3>\n
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Erosion Corrosion<\/h3>\n
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Intergranular Corrosion – Weld Decay<\/h3>\n
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Stress Corrosion Cracking \u2013 SCC<\/h3>\n