Corrosion is the natural degradation of a material due to atmospheric conditions or other factors. Corrosion is generally defined as dissolution of a material due to a reaction with the surrounding environment. Only metals such as gold and platinum are found in a pure form, normal metals only exist combined with other elements. Corrosion is therefore a natural phenomenon, as nature seeks to combine elements which man has produced in a pure form. More specific: iron occurs naturally as iron ore, so pure iron is unstable, as it wants to "rust", that is, to combine with oxygen in the presence of water. Carbon steels without any protection will therefore form a coating of rust, which will in a sense protect the rest of the steel and constantly removing the rust exposes a new fresh layer of steel to be attacked. The corrosion resistance of a stainless steel is dependent on a thin invisible film on the steel surface, the passive film. There are, however, environments that cause permanent breakdown of the passive layer. Under circumstances where the passive layer cannot be rebuilt, corrosion occurs on the unprotected surface. Different media can cause different types of corrosion attack that may vary in nature and appearance, and several forms of corrosion can occur on stainless steels.
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