Question

Chemical passivity is possible with:
A. conc. ${\rm{HN}}{{\rm{O}}_{\rm{3}}}$
B.air
C.both A and B
D.None of the above

Answer 1

Hint: We know that the passivity is the term that refers to loss of chemical reactivity of some metals and alloys under specific environmental conditions. Some examples of metals that can be passive in specific conditions are iron, chromium, aluminium etc.

Complete step by step answer:
Let’s discuss how passivation of metals is done. In this process, the surface of metals is made inactive by forming an oxide layer on the surface. The oxide layer forms due to the oxidation reaction of metal.
Let’s discuss one example of passivation. Aluminium is the metal which produces a layer of aluminium oxide due to its reaction with oxygen. This oxide layer prevents the corrosion of the aluminium.
We know that, nitric acid $\left( {{\rm{HN}}{{\rm{O}}_{\rm{3}}}} \right)$ and oxygen both the oxidizing agents. So, passivation of metals is possible by both concentrated nitric acid and oxygen.
Hence, the correct answer is option C.

Additional Information:
Depending on the situation, the same metal can act as active as well as passive. When a metal is immersed in an air free solution of acid, its corrosion takes place. When oxidizing agents (nitric acid, oxygen) are added to the solution, corrosion rate increases. This indicates the active state of the metal. In addition to more oxidizing agents to the solution , corrosion rate decreases. This indicates the passive state of the metal.

Note:
Due to passivity, some metals and alloys become inert and act as that of noble metals such as gold and platinum. Some metals which possess passivity effects are iron, Nickel etc. Zinc, tin, Cadmium, etc also show passivity effect under limited conditions.