Here is your answer When exposed to air, many metals naturally form a hard, relatively inert surface, as in the tarnish of silver. In the case of other metals, such as iron, a somewhat rough porous coating is formed from loosely adherent corrosion products. In this case, a substantial amount of metal is removed, which is either deposited or dissolved in the environment. Corrosion coating reduces the rate of corrosion by varying degrees, depending on the kind of base metal and its environment, and is notably slower in room-temperature air for aluminum, chromium, zinc, titanium, and silicon (a metalloid); the shell of corrosion inhibits deeper corrosion and operates as one form passivation. The inert surface layer termed the ‘’native oxide layer‘’, is usually an oxide or a nitride, with a thickness of a monolayer of a noble metal such as platinum, about 15 Å for silicon, and nearer to 50 Å for aluminum after several years.[
