Which of the following elements shows passivity when treated with concentrated nitric acid (HNO₃)?

Passivity is a condition in which a normally active metal becomes resistant to further corrosion or chemical attack due to the formation of a thin, protective oxide film on its surface. This oxide layer prevents direct contact between the metal and the corrosive environment. When metals like iron (Fe), aluminum (Al), and chromium (Cr) are treated with concentrated nitric acid (HNO₃), they exhibit passivity.

In the case of iron, concentrated nitric acid first reacts with the metal but quickly forms a layer of ferric oxide (Fe₂O₃) on its surface. This oxide film is stable and prevents further reaction, making the iron surface passive.

Aluminum reacts with nitric acid to produce a thin coating of aluminum oxide (Al₂O₃). This film is strongly adherent and protects the underlying metal from further attack, giving aluminum excellent resistance to corrosion.

Chromium forms a layer of chromium(III) oxide (Cr₂O₃) when exposed to concentrated nitric acid. This protective layer is the main reason chromium is used in stainless steel, which remains shiny and rust-resistant even in moist or acidic environments.

The passive behavior depends on the acid concentration and temperature. Concentrated nitric acid is a strong oxidizing agent that promotes oxide formation, while dilute acid may dissolve the protective layer and allow corrosion to continue.

All three metals—iron, aluminum, and chromium—show passivity because each forms a stable, non-porous oxide film that blocks further metal-acid reaction. This property is essential in metallurgy, corrosion prevention, and materials science, where passivation enhances durability and chemical resistance.