According to this theory, the impure iron surface behaves like a small electrochemical cell presence of water containing dissolved oxygen or carbon dioxide. Such a cell is also called corrosion cell or corrosion couple. In these miniature corrosion cells, pure iron acts as anode and impure surfaces acts as cathode. Moisture dissolved oxygen or carbon dioxide in it comprises electrolytic solution.
At anode, oxidization of Fe atoms takes place. Thus, Fe atoms pass into solution as Fe2+ ions leaving behind electrons in the metal which are pushed into cathodic area. Fe Fe+2 + 2e-; = - 0.44 V (i)
At cathode, the electrons are picked up by the H+ ions which are produced either from H2O or from H2CO3 (formed by dissolution of CO2in moisture). CO2 + H2O H+ + HCO3- 2H+(aq) + 2e- 2H The H atoms, thus, formed, reduce the dissolved oxygen as 2H + ½ O2 H2O The net reduction process at the cathodic area is 2H + ½ O2 + 2e- H2O; = 1.23 V (ii)
The net reaction of the corrosion cell can be obtained by adding equations (i) and (ii) F2 + 2H+ ½O2 2 Fe2+ + H2O; = 1.67 V The ferrous ions so formed move through water and come at the surface of iron object where these are further oxidized to ferric state by atmospheric oxygen and constitute rust which is hydrated iron (III) oxide. 2Fe2+ + ½ O2 + 2H2O Fe2O3 + 4H+ Fe2+ + xH2O Fe2O3xH2O (rust)