Anodizing, << AN uh `dyz` ihng, >> is a process used to thicken the natural coatings on metal surfaces, called oxides. During the process, the metal acts as a positive pole, called an anode, for electric current. The metal is submerged in a liquid that conducts electric current. As current runs through the liquid, negatively charged oxygen ions collect on the surface of the metal. There, the oxygen ions react with the metal to create the oxide coating. The thickness of the coating depends on the amount of electric current passed through the liquid. Thicknesses usually range from 0.0001 to 0.0008 inch (0.003 to 0.02 millimeter).
Anodizing serves several purposes. For example, it forms a hard coating on the metal that resists corrosion. Special anodizing treatments create a porous oxide layer that can absorb colors from dyes that do not rub or scratch off. Anodized aluminum or magnesium can be used in airplanes and on the outsides of buildings. Aluminum and magnesium are often anodized. However, the process is not used on steel because the oxide (commonly called rust) flakes off.