Rust forms only on metals that contain iron. It is the result of a chemical reaction between the iron and the moisture and oxygen in the air. Rust can be prevented by keeping oxygen and moisture from the surface of the iron-containing metal. In nature, few metals are found in a pure form. Iron, for example, is mined as iron ore - a mixture of iron compounds.
Pure iron is extracted from the ore by melting and purifying it.
Rusting reverses this process, turning the pure iron back into an iron compound. Rust can be thought of as a more natural and chemically stable form of iron. The process of rusting actually takes place in two steps. First the iron is dissolved. It is dissolved by a weak acid that is formed when water sitting on the metal combines with carbon dioxide gas from the air.