General corrosion resistance of aluminium (Topic 11003)

The corrosion resistance of aluminium varies widely depending on alloy, environment, design and protective measures taken. However, it is possible to give some general guide-lines.

A clean aluminium surface is reactive and will react spontaneously with water or air and form aluminium oxide. This oxide is very stable and has in addition a very good adhesion to the metal surface and thus protects aluminium from corrosion or further oxidation. This means that aluminium has good corrosion resistance in environments where the oxide layer is stable.

The oxide layer will deteriorate in environments with high or low pH or where aggressive ions are present. Below a pH of 4, acidic conditions, and above a pH of 8,5, alkaline conditions, there will normally be an increase in the corrosion rate of aluminium, but this also depends on which ions that are present in the environment.

Aggressive ions will break down the oxide layer locally and start local corrosion attacks. Among the aggressive ions, chloride (Cl-), is the one with the most practical importance, because it is present in large amounts in both sea-water, road salts and some soils and in lower concentrations in other natural sources.

A so called general corrosion attack proceeds at about the same rate on the entire metal surface. Because of the stability of the oxide layer, general corrosion will rarely be a problem on aluminium, except in very alkaline or acidic environments. Aluminium may however experience local attacks in connection with formation of small anodic areas as a result of a local breakdown of the oxide layer. Some of the most typical local attacks on aluminium are pitting corrosion, crevice corrosion, intergranular corrosion and galvanic corrosion.

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