Passivation 

Passivation, in the physical chemistry and engineering, refers to a material becoming "passive," that is, being less affected by environmental factors such as air and water. Passivation involves a shielding outer-layer of corrosion, which can be applied as a micro coating, or which occurs spontaneously in nature. As a technique of passivation is used to strengthen and preserve the appearance of metallics.

When exposed to air, many metals naturally form a hard, relatively inert surface such as the tarnishing of silver. In metals such as steel, uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. The reduction of the rate of corrosion will vary, depending on the metal and its environment, and is notably slowed at room-temperature air for aluminium, chromium, zinc, titanium, and silicon (a metalloid); the shell inhibits deeper corrosion, and so is the key factor of passivation. The inert surface layer, termed the "native oxide layer", is usually an oxide or a nitride.