Oxidation of metals
Most metals are oxidized by the oxygen in air. This process is called corrosion. Electrons leap from the metal to the oxygen molecules. The negative oxygen ions which are thus formed penetrate into the metal, causing the growth of an oxide surface. As the oxide layer grows the rate of electron transfer decreases. The corrosion stops and the metal is made passive. The oxidation process may continue, however, if the electrons succeed in entering the metal through cracks or impurities in the metal or if the oxide layer is dissolved.