Corrosion is the process of chemical or electrochemical attack of materials by their external environment.
The tendency/resistance of different metals to corrosion largely depends on the composition of the metal. Iron and steel corrode in the presence of oxygen due to oxidation, while some metals such as chromium, nickel and molybdenum provide a protective layer of oxide film on the surface of the metal which is then resistant to corrosion. Other metals such as aluminum and zinc form sacrificial oxide layers which corrode and protect the metal from becoming completely damaged.