The corrosion of aluminium

Corrosion is a slow, progressive, or rapid deterioration of a metal’s properties such as its appearance, surface aspect, or mechanical properties under the influence of the surrounding environment. Note that the corrosion of metals is an irreversible phenomenon. In practice, therefore, corrosion is irreparable. Corrosion is an electrochemical reaction, the result of two simultaneous reactions that are in electrical equilibrium: oxidation of the metal, resulting in a loss of electrons, and the reduction of an ion present in the aqueous solution, in general the proton H+. The global reaction of the corrosion of aluminium in aqueous media is Al + 3H2O ® Al(OH)3 + H2. Aluminium is a naturally passive metal because it is permanently coated with a natural oxide film that governs its corrosion resistance. Depending on the pH that characterizes an aqueous medium, two aspects must be considered: stability of the oxide film and aluminium dissolution rate. The passivity of aluminium is linked to the stability of the natural oxide. It is most stable when the pH is between 5 and 8. However, pH is not the only parameter to be taken into consideration when predicting the corrosion resistance of aluminium in aqueous media. The nature of the anions associated with acids or their salts or cations associated with bases is also a very important parameter.