passivation

Question

Passivation of iron in concentrated HNO₃ is due to formation of:

RankGuru · Accepted Answer

**Passivity** is the state where a metal becomes chemically unreactive despite its position in the activity series.
- **Reaction with Iron**: When Iron ($Fe$) is dipped in **concentrated Nitric acid ($HNO_3$)**, which is a powerful oxidizing agent, it reacts briefly but then stops.
- **Cause**: The surface of the iron is oxidized to form an extremely thin, impervious, and protective **oxide layer** (specifically **$Fe_3O_4$**).
- This layer acts as a physical barrier, preventing the acid from reaching the underlying bulk metal. In contrast, dilute $HNO_3$ dissolves iron normally as it doesn't form this protective passive layer.

Rank Guru

passivation

Select the correct option: