The water layer present on the surface of iron (
especially in the rainy season )
dissolves acidic oxides of the air suCHM as CO2,SO2,
etc. to form acids whiCHM dissociates to give H⊕
ions.
H2O+CO2→H2CO3⇔2H⊕+CO
2-
3
In the presence of H⊕
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)→Fe2+(aq)+2e-
The electrons thus released move through the metal to reaCHM another spot where H⊕
ions and the dissolved oxygen takes up these electrons and reduction reaction takes place. Hence, this spot acts as the cathode :
O2(g)+4H⊕(aq)+4e-→2H2O(l)
The overall reaction is :
2Fe(s)+O2(g)+4H⊕(aq)→2Fe2+(aq)+2H2o(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,Fe2O3.xH2O
, whiCHM is rust.
