The natural corrosion of steel involves the formation of an electrochemical corrosion cell. This cell is made up of an anode, a cathode, a cathode reactant, an electrolyte and an electrical connection between the anode and cathode. The chemical reaction at the anode site is the oxidation of the metal, followed generally by the oxide or hydroxide formation:                                   
Fe → Fe²⁺ + 2e^-                              (1)

                                 
Fe²⁺  + 2OH^- → Fe(OH)₂        (2)

At the same time an electrochemical reaction, generally the reduction of atmospheric oxygen, occurs at the cathode site: 

½O₂  + H₂O + 2e^- → 2OH^-       (3)