{"id":30594,"date":"2021-07-12T08:52:31","date_gmt":"2021-07-12T08:52:31","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30594"},"modified":"2023-09-20T14:26:46","modified_gmt":"2023-09-20T14:26:46","slug":"corrosion","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/failure-modes-of-materials\/corrosion\/","title":{"rendered":"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, and even noble metals, such as gold, are subject to corrosive attack in some environments.<\/p>\n Most metals<\/a> are not thermodynamically stable in the metallic form; they want to corrode and revert to the more stable forms normally found in ores, such as oxides. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although \u201cdegradation\u201d is more common in this context. Ceramic materials are relatively resistant to deterioration, usually at elevated temperatures or in extreme environments. The process is frequently also called corrosion. For polymers, mechanisms and consequences differ from those for metals and ceramics, and the term degradation is most frequently used. Corrosion degrades the useful properties of materials and structures, including strength, appearance, and permeability to liquids and gases.<\/p>\n Corrosion<\/strong> is electrochemical in nature because corrosive chemical reactions involve a transfer of charge. The chemistry of corrosion is quite complex, but it may be considered an electrochemical phenomenon essentially. The metal ions go into the solution, causing the metal to become negatively charged with respect to the electrolyte. The difference in the charge causes a potential to develop and produces a voltage between the electrolyte and the metal.<\/p>\n Corrosion<\/strong>, as a natural and persistent process, also involves the unintended deterioration of metals, sometimes with disastrous outcomes. How big is the corrosion problem? The problem of metallic corrosion is significant. In economic terms<\/strong>, it has been estimated that approximately 5% of an industrialized nation\u2019s income<\/strong> is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated due to corrosion reactions.<\/p>\n Corrosion is of primary concern in nuclear reactor plants. Corrosion occurs continuously throughout the reactor plant, and every metal is subject to it. Even though this corrosion cannot be eliminated, it can be controlled.<\/p>\n <\/a>Some metals exhibit a passivity to corrosion<\/strong>. Passivity is the characteristic of a metal exhibited when that metal does not become active in the corrosion reaction. Passivation<\/strong> is a natural process of the buildup of a stable, tenacious layer of metal oxide or protective barrier<\/strong> on the surface of the metal that acts as a barrier separating the metal surface from the environment. Passivity decreases or stops the corrosion process because of the formation of the layer. Fortunately, from an engineering standpoint, the metals most susceptible to this kind of behavior are the common engineering and structural materials, including iron, nickel, silicon, chromium, titanium, and alloys containing these metals.<\/p>\n For example, stainless steel<\/a> owes its corrosion-resistant properties to forming a self-healing passive surface film. 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. Once the surface is cleaned and the bulk composition of the stainless steel is exposed to air, the passive film forms immediately.<\/p>\n Aluminium<\/strong> is highly corrosion resistant in many environments because it also passivates. If damaged, the protective film normally re-forms very rapidly. However, a change in the character of the environment (e.g., alteration in the concentration of the active corrosive species) may cause a passivated material to revert to an active state. Generally, at high temperatures (in water, corrosion limits the use of aluminium to temperatures near 100\u00b0C), the relatively low strength and poor corrosion properties of aluminium make it unsuitable as a structural material.<\/p>\n The areas where 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 basically 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. It affects a fairly large area of the metal, making 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\u2019s 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 type of 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 the 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 the composition of grains, or differences in mechanical stress). The metal which is less corrosive resistant becomes an anode, and the metal with more corrosive resistance becomes a cathode. The corrosion of the less corrosive resistance is usually increased, and the attack on the 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 corrodant 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, producing 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 where 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 is 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 with other forms of degradation, such as corrosion, which is called 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 the 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, resulting in depletion of chromium in the zones close to the grain boundaries due to the slow diffusion rate of chromium. The chromium-depleted zones become less corrosion-resistant than the rest of the matrix. Depleted areas may be activated in a corrosive environment, 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 to produce 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>\n See also: Stress Corrosion Cracking<\/a>.<\/p>\n The most effective means of preventing SCC in reactor systems are:<\/p>\n Chloride stress corrosion<\/strong> occurs in austenitic stainless steels<\/a> under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is one of the most important forms of stress corrosion that concerns the nuclear industry. Austenitic stainless steels contain between 16 and 25% Cr and can also contain nitrogen in solution, which contributes to their relatively high uniform corrosion resistance. One type of corrosion which can attack austenitic stainless steel is chloride stress corrosion.<\/p>\n The three conditions that must be present for chloride stress corrosion to occur are as follows:<\/p>\n Chloride stress corrosion involves the selective attack of the metal along grain boundaries. 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, resulting in depletion of chromium in the zones close to the grain boundaries due to the slow diffusion rate of chromium. The chromium-depleted zones become less corrosion-resistant than the rest of the matrix. Depleted areas may be activated in a corrosive environment, and corrosion will occur in narrow areas between the grains.<\/p>\n It has been found that this is closely associated with certain heat treatments resulting from welding. This can be minimized considerably by proper annealing processes. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and using low-carbon steels. Ferritic stainless steels are chosen for their resistance to stress corrosion cracking, which makes them an attractive alternative to austenitic stainless steels in applications where chloride-induced SCC is prevalent.<\/p>\n As was written, the problem of metallic corrosion is significant. In economic terms, it has been estimated that approximately 5% of an industrialized nation\u2019s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated due to corrosion reactions. Therefore, various treatments slow corrosion damage to metallic objects exposed to the weather, salt water, acids, or other hostile environments. Since there are many forms of corrosion, there are many ways to stop or mitigate corrosion. In every case, it depends on the material to be protected and the environment in which the material is used. Metals may be protected from corrosion by using metal in an environment in which it is immune, making a physical barrier between the metal and its environment, using an electric current, or changing the environment.<\/p>\n Corrosion-resistant alloys<\/strong><\/a>, as their name indicates, are alloys with\u00a0enhanced corrosion resistance<\/strong>. Some ferrous and many non-ferrous metals and alloys are widely used in corrosive environments. In all cases, it strongly depends on certain environments and other conditions. Corrosion-resistant alloys<\/strong> are used for water piping and many chemical and industrial applications. In the case of ferrous alloys, we are talking about stainless steel and, to some extent about, cast irons. But some non-ferrous corrosion-resistant alloys exhibit remarkable corrosion resistance; therefore, they may be used for many special purposes. There are two main reasons why non-ferrous materials are preferred over steel and stainless steel for many applications. For example, many non-ferrous metals<\/strong>\u00a0and alloys possess\u00a0much higher resistance to corrosion<\/strong>\u00a0than available alloy steels and stainless steel grades. Second, a high strength-to-weight ratio or high thermal and electrical conductivity may provide a distinct advantage over a ferrous alloy.<\/p>\n <\/a><\/p>\n In nuclear engineering, \u201cCRUD<\/strong>\u201d is a technical term for corrosion and wear products (rust particles, etc.) in the coolant that becomes radioactive<\/a> when exposed to radiation. The term is an acronym for Chalk River Unidentified Deposits, originally found on the cladding, or outer coating, of fuel rods in the Canadian reactor for which it was named. CRUD may be defined as deposited or suspended circulating corrosion products, principally metal oxides, formed by the reaction of water with piping materials. According to the ICRP, CRUD formed in the power plants is the major source of operator radiation exposure.<\/p>\n Besides these radiological aspects, CRUDs can adversely affect the plant and its components. These can include the following:<\/p>\n The power plant must be designed to minimize corrosion and deposition. This design includes efficient removal of corrosion products, the purification system, design and arrange equipment to minimize crud deposition, and select coolant chemistry to reduce corrosion.<\/p>\n<\/div><\/div>\nPassivation<\/h2>\n
Forms of Corrosion<\/h2>\n
\n
General Corrosion<\/h3>\n
\n
Localized Corrosion<\/h3>\n
Pitting Corrosion<\/h3>\n
\n
\n
Crevice Corrosion<\/h3>\n
\n
Galvanic Corrosion<\/h3>\n
\n
Selective Leaching \u2013 Selective Corrosion<\/h3>\n
\n
Erosion Corrosion<\/h3>\n
\n
Intergranular Corrosion – Weld Decay<\/h3>\n
\n
Stress Corrosion Cracking \u2013 SCC<\/h3>\n
\n
Chloride Stress Corrosion Cracking<\/h3>\n
\n
Protection from Corrosion<\/h2>\n
\n
Corrosion-resistant Alloys<\/h2>\n
CRUDs in Power Plants<\/h2>\n
\n