Question

Aluminium\(\left( {{\bf{E}}_{{\bf{A}}{{\bf{l}}^{{\bf{3 + }}}}{\bf{/Al}}}^{\bf{^\circ }}{\bf{ = - 2}}{\bf{.07\;V}}} \right)\) is more easily oxidized than iron \(\left( {{\bf{E}}_{{\bf{F}}{{\bf{e}}^{\bf{3}}}}^{\bf{^\circ }}{\bf{/F}}{{\bf{e}}^{\bf{ - }}}{\bf{ = - 0}}{\bf{.477\;V}}} \right){\bf{,}}\) and yet when both are exposed to the environment, untreated aluminium has very good corrosion resistance while the corrosion resistance of untreated iron is poor. Explain this observation.

Short answer

• Aluminium reacts with atmospheric oxygen to form a stable aluminium oxide layer which is very tightly bound to the surface of the aluminium.
• Iron on the other hand in reaction with atmospheric oxygen forms rust which can flake off the surface of iron, therefore, exposing the inner metal to further oxidation and corrosion.

Step-by-step solution

Describe Aluminum is more corrosion-resistant than iron:

• Unlike iron and steel, aluminium does not rust or corrode under damp environments.
• Its surface is protected by a natural coating of aluminium oxide.
• This keeps the metal underneath from coming into touch with air and oxygen.

Find the observation of aluminium:

• Aluminium is indeed a more reactive element than iron, but unline iron aluminium gives a stable corrosion product that protects the aluminium metal from further corrosion.
• Aluminium reacts with atmospheric oxygen to form a stable aluminium oxide layer which is very tightly bound on the surface of the aluminium.
• The oxide itself is not very reactive so it protects the aluminium metal underneath it, it is said that the oxide layer pacifies aluminium.
• Iron on the other hand in reaction with atmospheric oxygen forms rust which can flake off the surface of iron, therefore, exposing the inner metal for further oxidation and corrosion.