how rusting of iron is envisaged as setting up of an electrochemical cell

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)

In the process of corrosion, due to the presence of air and moisture, oxidation takes place at a particular spot of an object made of iron. That spot behaves as the anode. The reaction at the anode is given by,

Electrons released at the anodic spot move through the metallic object and go to another spot of the object.

There, in the presence of H⁺ ions, the electrons reduce oxygen. This spot behaves as the cathode. These H⁺ ions come either from H₂CO_3, which are formed due to the dissolution of carbon dioxide from air into water or from the dissolution of other acidic oxides from the atmosphere in water.

The reaction corresponding at the cathode is given by,

The overall reaction is:

Also, ferrous ions are further oxidized by atmospheric oxygen to ferric ions. These ferric ions combine with moisture, present in the surroundings, to form hydrated ferric oxidei.e., rust.

Hence, the rusting of iron is envisaged as the setting up of an electrochemical cell.