CORROSION

"A chemical (often electrochemical) process that destroys structural materials. Typically it refers to corrosion of metals, but any other material (e.g., plastic or semiconductor) will also corrode. The simplest example of metallic corrosion is the rusting of iron in air. Iron is spontaneously oxidized by the oxygen in air to iron oxides (while the oxygen is being reduced). Metallic corrosion is very often an electrochemical process. It is always electrochemical when the metal is immersed in a solution, but even in atmospheric corrosion a thin film of condensed moisture often covers the surface. The metal in the corrosive solution essentially acts as a short-circuited galvanic cell. Different areas of the surface act as anode and cathode, at the anodic areas the metal is oxidized to an oxide while at the cathodic areas the dissolved oxygen is being reduced. The spontaneous complementary oxidation/reduction processes of ""rusting"" are spatially separated while an electrical current is flowing ""internally"" from one part of the corroding metal to another; the current is totally ""wasted"" as it produces no useful work but only generates heat. (A cell arrangement like this is often called a ""local cell."") See corrosion current and corrosion potential. See also an Encyclopedia Article. "