Corrosion is the chemical transformation of metal due to chemical reactions. The most common form of corrosion is oxidation, where metal atoms combine with oxygen atoms to form oxides. Iron rust is the most recognizable form of corrosion, and appears when iron oxide forms on iron or steel components that are exposed to air or water, however, virtually all metals and alloys are susceptible to corrosion.
Technically, corrosion can occur in other types of materials, such as ceramics or polymers, but the process is either rare or different enough that the term “corrosion” is generally not used.
Types of Corrosion
There are several types of corrosion, depending on the metal, corrosive agent, geometry, and environment:
- Passivation is actually considered a protective version of corrosion. In certain metals, such as aluminum, a very thin layer of oxide forms when the metal is exposed to air. This layer, which can be as thin as a micron, provides a protective layer to the metal from further oxidizing effects.
- Crevice Corrosion occurs when water or other fluid is concentrated in a crack or crevice in the material. For example, aircraft fuselages are susceptible to water seeping into lap joints and rivet holes and staying there, causing corrosion and weakening the material, leading to crack growth.
- Intergranular Corrosion occurs within the grain structure of a metal, particularly in metal alloys. Stainless steel is susceptible to intergranular corrosion if heat treating is not performed correctly or under the intense heat of welding operations. This effect is similar in many ways to galvanic corrosion.
- Pitting Corrosion often occurs in metals that have a passivation layer. If the passivation is prevented in a local location, such as a crevice, corrosion can occur.
- Galvanic Corrosion is the corrosion of a metal that is in electrical contact with another metal and immersed in an electrolyte. One of the most common locations for galvanic corrosion is in structures immersed in sea water. However, galvanization is also a process widely used to purposely protect metals from corrosion.
- Microbial Corrosion, as its name implies, is caused by microorganisms, which generate byproducts, such as acids, as part of their metabolic processes, and cause corrosion of the base metals.
Engineers often take their specific environment into effect and try to understand the types of possible corrosion when designing metal components and structures.
Engineers Concerned with Corrosion
While corrosion is considered a purely chemical process, it is of critical importance to any engineer designing components or structures that use metals. Mechanical engineers have to protect metal structures from the exposure to air, water, and other environmental effects.
Coatings are often used to protect metals from corrosion and to extend their working life. Electrical engineers are concerned with corrosion affecting electrical contacts. Because gold is naturally averse to corrosion, gold plating is commonly used for electrical contacts.
Sources
Jones, Denny Principles and Prevention of Corrosion (2nd edition ed.). Upper Saddle River, New Jersey: Prentice Hall, (1996).
NACE International website
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