Rusting is an oxidation reaction, that occurs when some Metals, like Iron are in contact with Water and Oxygen

http://www.bbc.co.uk/education/guides/zqjsgk7/revision/5

Aluminium does not rust (corrode) because its surface is protected by a natural layer of aluminium oxide. This prevents the metal below from coming into contact with air (containing oxygen).

Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminium oxide does not flake off.

Preventing rusting