Question

Explain that corrosion of iron is due to formation of an electrochemical cell.

Accepted Answer

The phenomenon of corrosion can be explained with the help of electrochemical theory which involves oxidation and reduction reactions. According to this theory of corrosion, it is believed that non-uniform surface of metal or impurities present in iron behave like small electric cells in the presence of water containing dissolved oxygen or carbon dioxide. A film of moisture with dissolved ${CO}_{2}$ constitutes electrolytic solution covering the metal surface at various places. Oxidation occurs at the anodes of each electrochemical cell.

Therefore, at each anode neutral iron atoms are oxidized to ferrous ions:

At anode: $2 Fe (s) \to 2 {Fe}^{2 +} + 4 e^{-}$

Thus, the metal atoms in the lattice pass into the solution as ions, leaving behind electrons on metal surface. These electrons move towards the cathode region through the metal. At the cathode of each cell, these electrons are captured by hydrogen ions i.e. reduction takes place. The $H^{+}$ ions are obtained either from water or from acidic substances like ${CO}_{2}$ etc in water.

At cathode: $O_{2} (g) + 4 H^{+} (aq) + 4 e^{-} \to 2 H_{2} O (I)$

The overall reaction is:

$2 Fe (s) + O_{2} (g) + 4 H^{+} (aq .) \to 2 {Fe}^{2 +} (aq .) + 2 H_{2} O (l)$

After that the atmosphere oxygen oxidizes the ${Fe}^{+ 2}$ into ${Fe}^{+ 3}$ and then hydrated ferric oxide ( ${Fe}_{2} O_{3} . {XH}_{2} O$ ) which is seen as corrosion on the surfaces of iron materials

Group-A	Group-B
(i) Corrosion	(a) Lanthanides
(ii) Gold sol	(b) Used in treatment of cancer
(iii) Radon	(c) Purple of cassius
(iv) f-block elements	(d) Barbituric acid
(v) Tranquilizers	(e) Hydrated ferric oxide.

Explain that corrosion of iron is due to formation of an electrochemical cell.

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