The corrosion of metals is caused by chemical reaction. Water, salts, acids and gases — all can react with metals and over time corrode it. Iron, for example, combines readily with the oxygen in air in the presence of moisture to form red iron oxide, we call rust. Copper as bronze corrodes in presence of acids or sulphur dioxide in air forming a greenish coating of copper salts called patina, Metals can also corrode due to electrochemical reaction if two dissimilar metals such as steel and copper are Joined together and exposed to water. Here one of the metals gives up electrons and gets corroded in the process.