Many metals oxidize and corrode, but rust, or iron oxide, is specific to ferrous metals such as steel. Rust is formed when oxygen bonds with an atom of iron to form an iron oxide molecule. This molecule is considerably larger than the surrounding iron molecules, so it quickly becomes dislodged and flakes off the surface. This exposes fresh metal that can also rust.
Oxidation of iron is a reaction that requires three components: an electrolyte, an anode and a cathode. An anode is an atom that gives up an electron during the process, a cathode is another atom that takes up the electron and an electrolyte is the medium through which the electron can travel. Iron is highly conductive and can act as its own anode as well as cathode. The electrolyte is often carbonic acid.
Carbonic acid is formed when a drop of water, usually rain, falls through the air and takes up a quantity of carbon dioxide. This carbon dioxide combines with the water molecule to form a somewhat acidic compound that rapidly degrades any potential anode it meets. Resting on an iron surface, the carbonic acid permits the flow of electrons from some iron atoms, acting as anodes, to other iron atoms acting as cathodes. The process rapidly eats through metal surfaces and leaves them pitted and red or brown.