During the rusting of iron 

During rusting of iron, metal acts as anode, hydrogen ion as cathode. The overall reaction is $2\mathrm{Fe}+{\mathrm{O}}_2+4{\mathrm{H}}^{+}\to 2{\mathrm{Fe}}^{2+}+2{\mathrm{H}}_2\mathrm{O}$ Formula of rust is ${\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}$.