Corrosion:

Destructive oxidation, usually by air in the presence of water; most marked for metals, especially iron. It is an electrochemical process, occurring most rapidly when two different metals are in contact with one another and with air and water, the more reactive metal being oxidized while the other provides a surface for the reduction of O2. Corrosion prevention is best carried out by isolating the reactive metal surface for air and water. Some metals, including aluminum and zinc (passive metals), form adherent oxide coatings which serve this purpose.