Iron reacts with oxygen, when exposed to moisture, it forms a chemical substance that is brown in colour and is referred to as rust.
Rusting is a corrosive process that results in the development of red or orange covering on the outer layer of metals. Rusting mainly happens on surfaces of iron and its alloys.
Corrosion results in the breakdown of materials due to chemical, electrochemical or some other type of reactions.
Chemical formula for rust is Fe2O3.xH2O. So rust is hydrated iron (III) oxide.