Corrosion

Corrosion is the breaking down of a metal by oxygen in the air. Most metals are reducing agents and react with oxygen to make metal oxides. Iron is the most common metal and it corrodes easily to form reddish-brown iron(III) oxide. Eventually, iron completely breaks down into iron(III) oxide (hematite) again. Other metals, like aluminium, form a thin oxide layer on their surface that protects the rest of the metal from corroding. Precious metals like gold and platinum do not corrode in air. Silver only corrodes in air when a sulfide is present.