Question

Why is iron rendered passive in nitric acid?

Answer 1

We need to understand the reaction between iron and nitric acid and
accordingly understand what happened in the reaction which leaves the iron to act in a passive manner after the reaction. Nitric acid is an oxidising acid capable of dissolving most metals and converting them to soluble metal nitrates. It has a modest oxidising strength below $2M$ , but when concentrated, it is a potent oxidising acid. Nitric acid oxidises most metals and alloys; however, gold and platinum are not oxidised, and certain metals are passivated when exposed to concentrated nitric acid.

Complete answer:
When treated with pure concentrated nitric acid, metals such as iron, aluminium, cobalt, and nickel become inert or passive owing to the creation of a very thin coating of insoluble metallic oxide on the surface. This layer that has developed prevents the bulk from reacting further. Concentrated nitric acid, being a powerful oxidising agent, converts the metal to ferric oxide, which creates an insoluble protective coating over it. The process comes to a halt when the metal loses contact with the acid. A chemically active metal becomes non-reactive and inactive. With fuming nitric acid, iron becomes passive due to the production of a reddish-brown coating of iron oxide that inhibits it from reacting further with the acid.

Note:
We must be noted that the process of iron being rendered passive with the action of nitric acid is called passivation. Passivation is the process of creating an impenetrable outer layer that is formed by a chemical interaction with the base metal or by spontaneous oxidation in the air.Passivation is critical in a variety of technologies and sectors that rely on the usage of high-corrosion-resistant metals and alloys.