What passivation is
Passivation is the state in which a thin protective film makes a metal surface chemically unreactive. Aluminum is highly reactive, yet the oxide formed by 4Al + 3O₂ → 2Al₂O₃ covers the surface without gaps and stops corrosion, unlike iron rust, which swells and flakes so oxidation continues.
Concentrated nitric acid
Aluminum, iron and chromium become passive in concentrated nitric acid, which is why it can be shipped in aluminum containers.
Anodizing
Making aluminum the anode in an electrolyte such as sulfuric acid thickens the oxide to several to tens of micrometers. The porous layer can be dyed and then sealed for corrosion resistance.
When the film fails
- Chloride ions: cause pitting corrosion in seawater.
- Strong acids and bases: Al₂O₃ is amphoteric and dissolves in sodium hydroxide.
- Gallium and mercury: liquid metals penetrate beneath the oxide and weaken grain boundaries (liquid metal embrittlement); mercury amalgam prevents the film from re-forming, so aluminum oxidizes rapidly.
References
- Vargel, C. (2004). Corrosion of Aluminium. Elsevier.
- Davis, J. R. (Ed.). (1999). Corrosion of Aluminum and Aluminum Alloys. ASM International.
- Greenwood, N. N., & Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann.
- Diggle, J. W., Downie, T. C., & Goulding, C. W. (1969). Anodic oxide films on aluminum. Chemical Reviews, 69(3), 365-405.