Question

Aluminium is much more reactive than iron because its standard reduction potential is higher than iron. Still, aluminium is less easily corroded than iron because

• A. Aluminium reacts with atmospheric carbon dioxide to form a self-protective layer of ${\mathrm{Al}}_2{\mathrm{O}}_3$
• B. It has higher reducing power and forms a self-protective layer of ${\mathrm{Al}}_2{\mathrm{O}}_3$
• C. It has higher reducing power and does not react with oxygen so easily
• D. Both A and B

Answer 1

The correct option is B

It has higher reducing power and forms a self-protective layer of ${\mathrm{Al}}_2{\mathrm{O}}_3$

• The standard reduction potential of aluminium $\left(\mathrm{Al}\right)$ is higher than iron $\left(\mathrm{Fe}\right)$. Hence, it is much more reactive than iron. However, aluminium is still less easily corroded than iron because oxygen forms a protective oxide layer of aluminium oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ on its surface.

$4\mathrm{Al}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{}{\to }2{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)$

• Aluminium has high reducing power than iron, hence it readily loses electrons and gets oxidized to form a protective layer of ${\mathrm{Al}}_2{\mathrm{O}}_3$ which protects it from corrosion.

The explanation for the incorrect option:

Option A:

• Every metal will form an oxide layer on its metal surface when it is exposed to air.
• Aluminium also reacts with oxygen to give aluminium oxide but other metal oxides will be easily hydrated and will detach from the metal surface, while aluminium oxide cannot be hydrated easily, thus it sticks with the metal surface and forms a preventive coating.

Option C:

• Aluminium is a reducing agent because of its low standard reduction potential.
• Pitting corrosion is known as an undesired reaction destroying aluminium and is enhanced by anions such as iron chloride and bromine.
• The aluminium outer surface layer is oxidised whenever it comes in contact with air and results in an aluminium oxide layer which is resistive in nature and remains attached to it

Hence, the correct option is (B).