Electro chemical decomposition
Metal surfaces undergo an electrochemical reaction (corrosion) when they come into contact with electrolytes (corrosive agents). Corrosive agents may be atmospheric gases, such as sea, urban and industrial air (e.g. sulfur di- and trioxide, hydrogen chloride and hydrogen sulfide) or liquid corrosives such as salts, bases, acids and seawater or the same dissolved in water (e.g. sweaty hand marks).
When metal and corrosive agent react electrochemically, the metal loses its electrons to the corrosive agent. The metal is abraded, so to speak, i.e. it corrodes. If two different metals are connected via an electrolyte, electrons are exchanged between the metals. This form of corrosion is known as bimetallic corrosion. Fretting corrosion occurs when the surface of a metal is roughened by friction from foreign materials; these metal particles react with the environment. A distinction may be drawn between various Types of corrosion.
The various types of corrosion are listed in the following table:
When metal and corrosive agent react electrochemically, the metal loses its electrons to the corrosive agent. The metal is abraded, so to speak, i.e. it corrodes. If two different metals are connected via an electrolyte, electrons are exchanged between the metals. This form of corrosion is known as bimetallic corrosion. Fretting corrosion occurs when the surface of a metal is roughened by friction from foreign materials; these metal particles react with the environment. A distinction may be drawn between various Types of corrosion.
The various types of corrosion are listed in the following table:
 Uniform corrosion
The reaction starts at the surface and proceeds uniformly.
|  Localized corrosion (pitting corrosion)
The basis metal is eaten away and perforated in places in the manner of holes, the rest of the surface being affected only slightly or not at all.
|  Wide pitting corrosion
The corrosion causes localized scarring.
|
 Intergranular corrosion
Imperceptible or barely perceptible from outside, since the corrosion proceeds at the grain boundaries.
|  Transgranular or intragranular corrosion
The grain boundary material is retained, since the corrosion proceeds preferentially within the grain.
|  Galvanic corrosion
Increased corrosion in crevices or cracks or at contact surfaces between two metal articles.
|
 Selective corrosion
Corrosive attack on structural constituents
|  Exfoliation corrosion
Occurs in deformed articles. Corrosion follows "fiber orientation".
|  Interfacial corrosion
Frequently observed at water-air interfaces.
|
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