Rust is caused when iron comes into direct contact with water and oxygen. Technically, rust comes from a chemical reaction between carbon dioxide from the air, water and the iron. Rusting is an oxidation reaction.
The chemical component of rust is iron oxide. Its chemical formula is Fe2O3. Rust is nothing more than simply corroded iron. Because of this reaction, the iron is being dissolved by an electrolyte called carbonic acid. The oxygen that is released by the water combines with the dissolved iron. The combination of those two is called rust. Iron and steel rust more easily than aluminium.