Aluminum Corrosion

Corrosion Doctors Web pages related to aluminum

Aluminum is the second most plentiful metallic element on earth. It has been estimated that 8% of the earth crust is composed of aluminum, usually found in the oxide form known as bauxite. Aluminum has become the most widely used non-ferrous metal on a volume basis. Whilst more expensive on a tonnage basis, it is the least expensive of metals other than steel on the basis of volume or area. Aluminum and its alloys are divided into two broad classes, castings and wrought or mechanically worked products. The latter is subdivided into heat-treatable and non-heat-treatable alloys, and into various forms produced by mechanical working.

The corrosion resistance of aluminum is dependent upon a protective oxide film. This film is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it.

The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode but by no means always does so. For example, aluminum is quite resistant to concentrated nitric acid. When aluminum is exposed to alkaline conditions corrosion may occur, and when the oxide film is perforated locally, accelerated attack occurs because aluminum is attacked more rapidly than its oxide under alkaline conditions. The result is pitting. In acidic conditions, the oxide is more rapidly attacked than aluminum, and more general attack should result.

Many pages of the Corrosion Doctors site discuss specific issues related to the corrosion behavior of steels. The following are references to some of these pages: