3.A] Define metallic corrosion? Describe the electrochemical theory of corrosion taking iron as an example.
Answer:-
According to this theory corrosion of the metals takes place through the formation of anodic or cathodic regions within the same metal surface in the presence of conducting medium.
Corrosion reactions:
a. Anodic reaction (i.e., at anodic area)
Oxidation of metal takes place resulting in the corrosion of iron.
Fe > Fe2++ 2e-.
b.Cathodic reaction
Cathodic reaction consumes electrons with either by
(a) Evolution of Hydrogen
(b) Absorption of oxygen depending on the nature of environment.
Evolution of Hydrogen: Evolution of Hydrogen takes place in the absence of oxygen as shown below
i. In acidic medium and in the absence of oxygen the reaction is,
2H+ + 2e- > H2
ii. In neutral or alkaline medium and in the absence of oxygen, hydroxide ions are formed with simultaneous liberation of hydrogen.
2H2O + 2e- > 2OH- + H2
Absorption of Oxygen: absorption of oxygen takes place in the presence of oxygen.
i. In acidic medium and in the presence of oxygen.
2H+ + 1/2O2 + 2e- > H2O
ii. In a neutral or alkaline medium and in the presence of oxygen. Hydroxide ions are formed by the following reaction.
H2O +1/2O2+ 2e- > 2OH
Corrosion of iron produces Fe2+ ions and OH- ions at the anode and cathode sites respectively these ions diffuse towards each other, since smaller Fe2+ ions diffuse more rapidly than OH- ions their combination forms insoluble Fe (OH) 2
Fe2+ + 2OH- > Fe (OH) 2
In oxidizing environment, it is oxidized to ferric oxide and yellow rust is hydrated ferric oxide.
2Fe (OH) 2 + ½ O2 +H2O > Fe2O3 3H2O