Corrosion
. Corrosion, partial or complete wearing away, dissolving, or softening of any substance by chemical or electrochemical reaction with its environment. The term corrosion specifically applies to the gradual action of natural agents, such as air or salt water, on metals. The most familiar example of corrosion is the rusting of iron, a complex chemical reaction in which the iron combines with both oxygen and water to form hydrated iron oxide. The oxide is a solid that retains the same general form as the metal from which it is formed but, porous and somewhat bulkier, is relatively weak and brittle. Three methods may be used to prevent the rusting of iron: (1) alloying the iron so that it will be chemically resistant to corrosion; (2) coating it with a material that will react with the corroding substances more readily than the iron does and thus, while being consumed, protect the iron; and (3) covering it with an impermeable surface coating so that air and water cannot reach it. The alloy...