Rust forms on iron due to an oxidation reaction that occurs when it comes into contact with oxygen and water. The reaction of iron, oxygen and water forms hydrated iron(III) oxide, which is known as rust. Acid rain or saltwater can cause iron to rust more quickly.
Rust is the common name given to the product of the corrosion that occurs in iron and its alloys. The rust that appears is typically a reddish color. In time, an iron mass will become entirely rust and disintegrate. To prevent rust from occurring, several rust-resistant alloys have been created, such as stainless steel. Other metals that do not rust are aluminum, zinc and titanium.