Rust is the generic name for iron oxide, Fe2O3 (subscripts). Iron, in the presence of oxygen, using water as a catalyst, will form rust. Rust is the corrosion of iron. It typically occurs at surfaces exposed to the elements, where oxygen and water are readily available. Depending on the availability of of oxygen and water, it makes use of two chemical formulas:
(Fe2+) + 2(H2O) ---> Fe(OH)2 + 2H+
(Fe3+) + 3(H2O) ---> Fe(OH)3 + 3H+
Rust can have a red physical color, although in marine environments, in the absence of oxygen, a green color is manufactured. It should be noted rust does not insure any protection to underlying layers of iron.
To protect iron from rusting, protective layers of coatings must be applied. Stainless steel, or chromium (III) oxide, provides such a coating. The process of galvanization, which coats the iron with a protective layer of zinc, is another process which slows the corrosive process. In harsher, more corrosive, marine environments, cadmium is a more suitable protective coating.