Become rusty is a term used to describe the process of corrosion, oxidation, and gostaling. Corrosion is the gradual destruction of materials such as metals by chemical reaction with its environment. Oxidation is a chemical reaction that occurs when oxygen combines with other elements in a compound. Gostaling is a form of corrosion caused by exposure to salt water or salty air. All three processes can cause metal to become rusty, which is a reddish-brown color due to the presence of iron oxide.