Types of Corrosion Damage
Corrosion that has spread over the entire surface of a metal is called continuous corrosion. It may be even or uneven, depending on the uniformity of the depth of the corrosion damage in various areas. In local corrosion, the damage is localized and a significant part of the surface (often most of it) remains virtually unaffected. A distinction is made among corrosion spots, lesions, and pits, depending on the degree of localization. Pits may give rise to subsurface corrosion, which propagates in all directions under a very thin (for example, cold-worked) layer of the metal, which then blows up into bubbles or peels. The most dangerous forms of local corrosion are intergranular corrosion, which penetrates deeply along the less resistant grain boundaries without destroying the metal grains, and transcrystalline corrosion, which forms fissures directly through the grains. These types of damage may lead to complete loss of strength and the destruction of parts or structures without visible marks on the surface. Knife corrosion, which cuts through the metal like a knife along a weld seam during the use of certain alloys in particularly corrosive solutions, is similar to intercrystalline and transcrystalline corrosion. Sometimes a distinction is made between surface filament corrosion, which develops, for example, under nonmetallic coatings, and laminar corrosion, which proceeds primarily in the direction of plastic deformation. Selective corrosion, in which even individual components of solid solutions may selectively dissolve in an alloy (for example, the dezincing of brass), is specific.