{"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":"
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\"CorrosionCorrosion<\/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

Passivation<\/h2>\n

\"passivation<\/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

Forms of Corrosion<\/h2>\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