Mechanism of rusting of iron : Electrochemical theory of rusting.

The overall rusting involves the following steps,

(i) Oxidation occurs at the anodes of each electrochemical cell. Therefore, at each anode neutral iron atoms are oxidised to ferrous ions.

At anode : 

Thus, the metal atoms in the lattice pass into the solution as ions, leaving electrons on the metal itself. These electrons move towards the cathode region through the metal.

(ii) At the cathodes of each cell, electrons are taken up bythe  hydrogen ions (reduction occurs). The H⁺ ions are obtained either from water or from acidic substances (e.g. CO₂ in water)

At cathode : 

The hydrogen atoms on the iron surface reduce dissolved oxygen. 

Therefore, the overall reaction at cathode of different electrochemical cells may be written as,

(iii) The overall redox reaction may be written by multiplying reaction at anode by 2 and adding reaction at cathode to equalise number of electrons lost and gained i.e.

Oxi. half reaction :     

Red. half reaction :           

Overall cell reaction :       

The ferrous ions are oxidised further by atmospheric oxygen to form rust.

It may be noted that salt water accelerates corrosion. This is majorly because of the fact that salt water increases the electrical conduction of electrolyte solution formed on the metal surface. Thus, rusting becomes more serious problem where the salt water is present. 

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