The following reactions take place at the surface of iron metal whichacts as an electrochemical cell. $\mathrm{Fe}_{(s)} \rightarrow \mathrm{F}^{2+}_{(aq)} + 2 e^{-}$ Oxidation $\mathrm{O}_{2(g)} + 4 \mathrm{H}^{+}_{(aq)} + 4 e^{-} \rightarrow 2 \mathrm{H}_{2}\mathrm{O}_{(l)}$ Reduction $––––––––––––––––––––––––––––––––––––––––––––––––––––––––$ $2\mathrm{F}^{2+}_{(aq)} + 2\mathrm{H}_{2}\mathrm{O}_{(l)} + \frac{1}{2}\mathrm{O}_{2(g)} \rightarrow \mathrm{Fe}_{2}\mathrm{O}_{3(s)} + 4 \mathrm{H}^{+}_{(aq)} + 4 \mathrm{H}^{+}_{(aq)}$ $\text{,,,,,,,,,,,,,,,,,,,,,,,,Atmospheric oxidation}$ $––––––––––––––––––––––––––––––––––––––––––––––––––––––––$The water layer present on the surface of iron dissolves acidic oxides ofair like $\mathrm{CO}_2$ to form acids which dissociate to give H$^{+}$ions. Fe starts losingelectrons in presence of H$^{+}$ions.$\mathrm{H}_{2}\mathrm{O} + \mathrm{CO}_2 \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3} \rightleftharpoons 2\mathrm{H}^{+} + \mathrm{CO}_{3}^{2-}$