The water layer present on the surface of iron `(` especially in the rainy season `)` dissolves acidic oxides of the air suCHM as `CO_(2),SO_(2), ` etc. to form acids whiCHM dissociates to give `H^(o+)` ions.
`H_(2)O+CO_(2) rarr H_(2)CO_(3) hArr 2H^(o+)+CO_(3)^(2-)`
In the presence of `H^(o+)` ions, iron starts losing electrons at some spot to form ferrous ions, i.e., its oxidation takes place. Hence, this spot acts as the anode `:`
`Fe(s) rarr Fe^(2+)(aq)+2e^(-)`
The electrons thus released move through the metal to reaCHM another spot where `H^(o+)` ions and the dissolved oxygen takes up these electrons and reduction reaction takes place. Hence, this spot acts as the cathode `:`
`O_(2)(g)+4H^(o+)(aq)+4e^(-) rarr 2H_(2)O(l)`
The overall reaction is `:`
`2Fe(s)+O_(2)(g)+4H^(o+)(aq) rarr 2Fe^(2+)(aq)+2H_(2)o(l)`
Thus, an electroCHMemical cell is set up on the surface`:`
Ferrous ions are further oxidized by the atmospheric oxygen to ferric ions whiCHM combine with water molecules to form hydrated ferric oxide,`Fe_(2)O_(3).xH_(2)O`, whiCHM is rust.
