What is Corrosion?

Corrosion is defined as “the degradation of materials by chemical reaction with the
environment in which the material resides.” This is because of metal oxidation. As
metals have a tendency to return to their natural state, it is a natural process which
produces either salt or oxides. It requires four elements – anode, cathode, an electrolyte,
and a metallic path.
Corrosion of Metals
Metal corrosion is the main cause of metal destruction, like steel rusts due to immersion
in seawater. Similarly iron reacts with oxygen to form rust by exposure to moist air.
It is a very complex process which is completed in the following steps.
 Oxidation of iron- First the iron gets oxidized into ferrous ions [Fe (II)] with the
loss of two electrons.

Fe → Fe +2 + 2 e –

 The ferrous ions again get oxidized into ferric ions [Fe(III)] in the presence of
water and oxygen.

Fe +2 → Fe +3 + e –

 These electrons from the above reactions are used to reduce oxygen.

O 2 (g) + 2 H 2 O + 4e – → 4 OH –

 The ferric ions combine with oxygen and form ferric oxide [iron (III) oxide]. This
ferric oxide gets hydrated with water.

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