Passivation, in physical chemistry and engineering, refers to a material becoming “passive,” that is, being less affected by environmental factors such as air or water. It involves a shielding outer layer of corrosion which can be applied as a microcoating or found occurring spontaneously in nature. Passivation is useful in strengthening and preserving the appearance of metallics.
As a technique, passivating is the use of light coat of material such as metal oxide to create a shell against corrosion. Passivation can occur only in certain conditions, and is used in microelectronics to enhance silicon.
In air, many metals form a hard, relatively inert surface naturally. The reduction of the corrosive rate will vary depending on the metal and its environment. It is notably slowed at room temperature in air for aluminium, chromium, zinc, Titanium, and silicon (a metalloid). The shell inhibits deeper corrosion, and that is the key factor.