Answer:
iron oxide. aka. oxygen
Explanation:
Rust is the result of a chemical reaction between iron and oxygen, otherwise known as iron oxide. Since steel wool -- and steel in general -- is largely made of iron, steel wool is prone to rust if it doesn't have a rustproof coating on it. The actual reaction that creates rust happens when two iron atoms mix with three oxygen atoms in water; the oxygen bonds to the metal, and a new compound is formed.
