some metals, such as aluminium, titanium, and stainless steel form a very thin coating of tough corrosion on the metal. the metal cannot continue corroding because the coating isolates the rest of the metal from the source of oxygen. this is why aluminium keeps its shine. it also makes aluminium seem very unreactive, even though it can react with water. rusting iron makes iron oxides that are not tough, so air and water continue to get in and the iron continues to rust.