Rust, the most familiar example of corrosion, on an old train. It becomes brown or red.

Rust is a type of corrosion. It happens to iron and its alloys when it is exposed to air or water for a long time. Rust slowly decomposes iron into other chemicals, because of a reaction with the oxygen molecules. Both air and water are needed for rust. Almost all metals corrode but they can be protected by covering with paint, a coating of oil or grease or even plastic. Sacrificial protection or industrial galvanizing can also work. Alloys (mixes of metals) such as stainless steel, rust more slowly than pure iron. When iron rusts, it becomes red or brown, and the metal eventually decays.