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corrosion
The insidious destruction of metals and alloys by chemical reaction (mainly oxidation) with the environment. In moist air most metals form a surface layer of oxide, which, if it is coherent, may slow down or prevent further corrosion.
Tarnishing is the formation of such a discolored layer, mainly on copper or silver. (Rust – hydrated iron (III) oxide, FeO(OH) – offers little protection, so that iron corrodes rapidly.) Industrial air pollution greatly speeds up corrosion, oxidizing and acid gases (especially sulfur dioxide) being the worst culprit. Corrosion is usually an electrochemical process: small cells are set up in the corroding metal, the potential difference being due to the different metals present or to different concentrations of oxygen or electrolyte; corrosion takes place at the anode. It also occurs preferentially at grain boundaries and where the metal is stressed.
Prevention methods include bonderizing; a protective layer of paint, varnish, or electroplate; or the use of a "sacrificial anode" of zinc or aluminum in electrical contact with the metal, that is preferentially corroded. Galvanizing – coating iron objects with zinc – works on the same principle.
Related category
• INORGANIC CHEMISTRY
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