Rusting of Iron

Rust is the hydrated ferric oxide with molecular Fe2O3.XH2O. When iron is exposed to moist air, then it is covered with amorphous brown scale which is known as rust and this phemomena is known as rusting of iron. .Various theories have been proposed to explain the phenomena of rusting of iron. The widely accepted theories are given below:

1. Electrochemical Theory: According to this theory,  Rust is hydrated ferric oxide. The rust is formed by the action of water on iron in presence of dissolved oxygen and carbon dioxide. It has been observed that impure iron is more prolonged towards rusting. The following are the favorable conditions for rusting of iron.

  • Presence of moisture
  • Presence of weekly acidic atmosphere
  • Presence of impurities


2. Modern Electrochemical Theory: Among all the theories of rusting, the most accepted theory is modern electrochemical theory. According to it, when impure iron comes in contact with water containing dissolved carbon dioxide, a voltaic cell is set up. The iron and other impurities act as electrode while water having dissolved oxygen and carbon dioxide acts as electrolyte.

Rusting of iron

When iron atom passes into the solution as ferrous ion, then:
$$\ce{Fe->Fe^++ + 2e^-}$$
Thus iron acts as anode and impurities acts as cathode. This makes a complete electrolytic cell.
The Fe++ ions in aqueous solution get oxidized in the presence of air to Fe+++ ions generating hydroxyl ions and precipitating out ferric hydroxide.
$$\ce{4Fe^++ + 2H2O + O2->4Fe^+++ + 4OH^-}$$
$$\ce{Fe^+++ + 2OH^->$\underset{\text{Rust}}{\ce{Fe(OH)3}}$}$$

3. Chemical Theory: This theory is based on the principle that the ferric bicarbonate is formed in the presence of water and carbon dioxide. This theory however fails to explain rusting in the absence of carbon dioxide.
$$\ce{CO2 + H2O->H2CO3}$$
$$\ce{Fe + 2H2CO3->$\underset{\text{Ferrous bicarbonate}}{\ce{Fe(HCO3)2}}$ + H2}$$ Ferrous carbonate is then converted to ferric hydroxide in the presence of water and oxygen.
$$\ce{4Fe(HCO3)2 + 2H2O + O2->4Fe(OH)3 + 8CO2}$$ This theory is supported by the fact that alkalis prevent the rusting of iron because they neutralize carbon dioxide and they do not allow the formation of bicarbonate.
$$\ce{2NaOH + CO2->$\underset{\text{Sodium carbonate}}{\ce{Na2CO3 }}$ + H2O}$$

Prevention of rusting: Rusting can be prevented by applying the following ways:

  • By applying paint or enamel on the surface of iron.
  • By forming protective coating of ferrosoferic oxide on the surface of iron by passing steam.
  • By coating a thin film of zinc, tin, nickel, aluminum etc.


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