Rust is a form of corrosion that occurs when iron or an alloy that contains iron, such as steel, is exposed to oxygen and moisture over a long period of time. The oxygen in the air reacts with the iron to form iron oxide, which is commonly known as rust. The process of rusting is an electrochemical reaction that involves the transfer of electrons from the iron to the oxygen. Rust weakens the metal and can cause it to disintegrate over time.