How rusting of iron is envisaged as setting up of an electrochemical cell?

Solution)

Rusting of Iron is due to corossion.

Rust is an Fe₂O₃.xH₂O

Theory of Rusting :

1. Water present on the metal surface dissolves CO2 and O2 from the air.

       H₂O + CO₂ --------->     H₂CO₃

       O₂ + 4H⁺ + 4e^- -------> 2H₂O

2. Fe in contact with dissolved CO2 and O2 undergoes oxidation.

    Fe ------------> Fe²⁺ + 2e^-  Anode (E_Fe⁺²/Fe = -0.44V)

3. Electrons lost by Fe are taken by H+

      H⁺ + e^-  ------------>  H

      4H+O₂ --------> 2H₂O

The dissolved O₂ can be take electrons directly also

  O2 + 2H₂O + 4e^-  ----------->  3OH^-   cathode

  Overall 2Fe_(s) + O_2(g)  +4H⁺ _(aq)   ------------->    2Fe⁺²  + 2H₂O

4. Fe⁺² react with dissolved O₂ and water

Fe⁺² + 2OH^-  ----------->  Fe(OH)₂

4Fe(OH)₂ + O₂ +2H₂O ----------> 4Fe(OH)₃

2Fe(OH)₃  --------->  Fe₂O₃ + 3H₂O

Fe₂O₃ + XH2O  ------------>  Fe₂O₃.XH₂O

Rust (Hydrated ferric oxide)