Question

Aluminum is more reactive than iron. But aluminum is less easily corroded then iron because:
(A) Aluminum is a noble metal.
(B) Oxygen forms a protective oxide layer on an aluminum surface.
(C) Iron undergoes reaction easily with water.
(D) Iron forms both mono and divalent.

Answer 1

Metallic aluminum is very reactive with atmospheric oxygen. It forms Aluminum oxide $(A{l_2}{O_3})$ is responsible for the resistance of metallic aluminum to weathering. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodizing.

Complete step by step solution:
When iron corrodes , it form red-brown hydrated metal oxide, $F{e_2}{O_3}.x{H_2}O{\text{ }}\left( {rust} \right).$ Rust is incapable of providing tight protective oxide film, instead this product formed flake off very easily hence exposing fresh metal to the corrosion medium (Oxygen and water).The formation of rust requires iron, water and oxygen. Although it's a complex process, the chemical equation is -
$4Fe{\text{ }} + {\text{ }}3{O_2}\; + {\text{ }}6{H_2}O{\text{ }} \to {\text{ }}4Fe{\left( {OH} \right)_3}.$
In case of aluminum, as soon as the metal is exposed to the atmosphere a non porous adherent layer of Al oxide is formed over the surface of metal preventing any contact of fresh metal with corrosion medium.Its surface is covered with a thin layer of oxide that helps protect the metal from attack by air. However, in case the oxide layer gets damaged and the Aluminum metal gets exposed, it reacts again with oxygen forming amphoteric oxide (Aluminum (lll) Oxide), $A{l_2}{O_{3.}}$
$4Al\left( s \right){\text{ }} + {\text{ }}3{O_{2\;}}\left( l \right){\text{ }} \to 2A{l_2}{O_{3\;}}\left( s \right)\;\;$
This protective oxide layer on aluminum is very thin. It protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced by the process called anodising. It has high corrosion endurance and high temperature stability, low thermal expansion.

**Thus, the correct option is B. Oxygen forms a protective oxide layer on an aluminum surface.

Note:**
1. Aluminum oxide $(A{l_2}{O_3})$ also acts as an amphoteric substance, meaning it can react with both acids and bases.
2. It is used in various chemical, industrial and commercial applications. It is also known as alumina and present in some minerals like corundum and bauxite.