The chemical deterioration of a material, usually a metal or metal alloy, is called corrosion. The most common causes of corrosion are contact with water and oxygen, though other substances in the earth and in the atmosphere can also cause corrosion. The material with the greatest economic importance that is most affected by corrosion is iron. The corrosion of iron is called rusting.
The corrosion of metals such as aluminum, tin, copper, and zinc generally stops after a thin layer of metal oxide forms on the exposed surface of the metal. This layer serves as a barrier to further contact with oxygen. Even when iron combines with oxygen, a thin, almost invisible coating of iron oxide forms that prevents further rusting when no water molecules are present. When water is present, however, the oxide that forms is bulky and porous, allowing oxygen continued access to the iron below.
Other metals also corrode slightly under normal atmospheric conditions. Copper and its alloys brass and bronze are protected from continuous and penetrating corrosion by the formation of a green patina, or film, called verdigris, which is composed of copper carbonate. In many instances buildings with copper-clad roofs and trim are deliberately allowed to develop patinas because the color is considered attractive.
Corrosion takes place at a much faster rate in heavily industrialized areas that have high levels of sulfur and nitrogen pollutants in the atmosphere. These compounds combine with moisture in the air to produce extremely corrosive acids.
Metals may be protected from corrosion by coating them. A variety of coating processes are used, including painting, electroplating with chromium, or plating with zinc, which is called galvanizing. Alloying steel with chromium or chromium and nickel produces stainless steel, which is resistant to rusting. Plastics, ceramics, and certain rubber compounds are also used to coat metals.