Corrosion is 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 besides being porous and somewhat bulkier, it 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 alloying method is the most satisfactory but the most expensive. A good example is stainless steel, in which chromium or chromium and nickel are alloyed with the iron; this alloy is not only absolutely rustproof but will even resist the action of such corrosive chemicals as hot, concentrated nitric acid. The second method, protection with an active metal, is also satisfactory, but expensive. The most common example of this method is galvanizing, in which iron is covered with zinc.

In the presence of corrosive solutions, an electric potential is set up between the iron and the zinc, causing the zinc to dissolve but protecting the iron as long as any zinc remains. The third method, protection by coating the surface with an impermeable layer, is the least expensive and therefore the most common. It is satisfactory as long as no crack appears in the coating. Once the coating cracks, however, rusting proceeds at least as fast as it would have with no protection. If the protective layer is an inactive metal, such as tin or chromium, an electric potential is set up, protecting the layer but acting on the iron and causing the rusting to proceed at an accelerated rate. The most satisfactory coatings are baked enamels; the least expensive are such paints as red lead.

Some metals, such as aluminum, although very active chemically, appear not to corrode under normal atmospheric conditions. Actually, aluminum corrodes rapidly, and a thin, continuous, transparent layer of oxide forms on the surface of the metal, protecting it from further rapid corrosion. Lead and zinc, although less active than aluminum, are protected by similar oxide films. Copper, a comparatively inactive metal, is slowly corroded by air and water in the presence of such weak acids as carbonic acid, producing a green, porous, basic carbonate of copper. Green corrosion products, called verdigris or patina, appear on such copper alloys as brass and bronze, as well as on pure copper.

Some metals, called noble metals, are so inactive chemically that they do not suffer corrosion from the atmosphere; important ones among them are silver, gold, and platinum. A combination of air, water, and hydrogen sulfide will act on silver, but the amount of hydrogen sulfide normally present in the atmosphere is so small that the degree of corrosion is negligible except for the black discoloration, called tarnishing, produced by the formation of silver sulfide. The corrosion of metals is more of a problem than that of other materials. Glass is corroded by strongly alkaline solutions and concrete by sulfate-bearing waters. The corrosion resistance of glass and concrete can be greatly increased by changes in their composition.