Corrosion: What is Corrosion

 

Corrosion is defined as the process of depositing fine layer of reddish brown colour on the surface of iron. Corrosion is also known as rusting. The reddish brown colour is chemically known as iron oxide. It contains hydrated iron oxide. Fe₂O₃ . xH₂O

The process of corrosion involves following steps:

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

 

=At anode :  Fe (s)              Fe ³⁺ (ag) + 2e^-

Thus the metal atoms in the lattice pass into solution as ion, leaving electron on the metal itself. These electron moves towards cathode region through the metal. Electrons are taken up by Hydrogen ions (reduction) takes place. The H⁺ ions are obtained either from water or acidic substances in water.

 

    =At Cathod :  H⁺  +   e^-            H

 

The hydrogen atoms on the surface reduce dissolved oxygen

 

=4H  +   O₂                                           2H₂O

Therefore overall reaction can be written as,

=4H⁺ + O₂ +4e^-                                             2H₂O

 

ii)                  Overall cell redox reaction may be written as

          =2Fe (s) + 4H⁺  + O₂                                            2Fe ²⁺ (ag) + 2H₂O

The ferrous ions are further oxidized by atmosphere oxygen to form rust,

 

  =4 Fe ³⁺ (ag)  + O₂ {g) + 4H₂O                                       2Fe₂O₃ +8 H⁺  and

=Fe₂ O₃   +  x H₂O                                              Fe₂O₃ . xH₂O   (Rust)