In rainy season and in highly humid atmosphere the iron surface has a layer of water. This water layer dissolves acidic oxides of the air like CO2, SO2 etc. to form acids which dissociate to give H+ ions.
H2O + CO2 → H2CO3 ⇌ 2H+ + CO32+
In the presence of H+ ions, iron starts losing electrons at some spot to form ferrous ions, i.e., its oxidiation takes place. Here it is anode.
Fe() → Fe2+(aq) + 2e–(anode)
Now the released electrons move through the metal and reach to another spot where H+ ions and the dissolved oxygen take up these electrons and reduction reaction takes place. Here it is cathode.
O2(g) + 4H+(aq) + 4e– + 2H2O(l)(cathode)
The overall reaction is
2Fe() + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l)
Thus in this way an electrochemical cell is set up on the surface. This ferrous ion further oxidised by the atmospheric oxygen to ferric ions which combine with water molecules to from hydrated ferric oxide,
Fe2O3+ H2O, which is rust.
