Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation
3)Nitric acid reacts with most metals. This characteristic has made it a common agent to be used in acid tests.
Strongly electropositive metals, such as magnesium react with nitric acid as with other acids, reducing the hydrogen ion.
Mg + 2 H+ → Mg2+ + H2
With less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry.
3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3-
The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide. With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry.
Cu + 4 H+ + 2 NO3− → Cu2+ + 2 NO2 + 2 H2O.
PassivationAlthough chromium (Cr), iron (Fe) and aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the metal from further oxidation, which is called passivation. Typical passivation concentrations range from 18% to 22% by weight.