Why is iron rendered passive in nitric acid?

Iron becomes passive in nitric acid due to the formation of a protective oxide layer on its surface. This process occurs when iron reacts with nitric acid, leading to the creation of iron oxides and nitrates.

The Role of Nitric Acid

Nitric acid is a strong oxidizing agent. When iron is exposed to it, the acid oxidizes the iron, which results in the formation of iron(III) oxide (Fe2O3) or iron(II) nitrate (Fe(NO3)2). This layer of oxide acts as a barrier, preventing further reaction between the iron and the acid.

Key Factors in Passivation

• Oxidation: The initial reaction with nitric acid oxidizes the iron.
• Layer Formation: The oxide layer forms quickly, inhibiting further corrosion.
• Concentration: Higher concentrations of nitric acid enhance the passivation effect.

As a result, once the protective layer is established, the iron becomes less reactive and is said to be in a passive state, making it resistant to further corrosion in nitric acid.