Corrosion Properties of Aluminum

General Surface Corrosion

• When a piece of aluminum corrodes uniformly over the entire surface, this is called general corrosion. Aluminum typically exhibits extremely strong resistance to this corrosive mechanism, largely as a result of a protective barrier that forms on the surface. This protective barrier is called a "passive oxide film" and forms when the aluminum reacts with oxygen in the external environment, creating a thin layer of aluminum oxide (alumina) on the surface of the metal. A physical, very resistive barrier then exists between the surface and the metal and any corrosive environment.

Localized Corrosion

• Certain corrosive environments can reduce the effect of, and compromise the passive oxide film of aluminum. Chloride-rich environments in particular can cause the localized breakdown of this alumina protective film, resulting in corrosion that occurs locally to the breakdown, as opposed to uniformly over the surface as is the case for "general corrosion." This creates small pits and crevices on the surface of the aluminum that can penetrate deep into the material.

Bimetallic Corrosion

• When two different metals come into contact, they can experience bimetallic (or "galvanic") corrosion. Generally the more reactive metal will see a reduced rate in corrosion, whereas the more noble metal will see an increased rate in corrosion. Aluminum, being a very unreactive, noble metal, is extremely susceptible to bimetallic corrosion. Particularly in outdoor, salty environments it can be subject to high corrosion rates when in contact with most common engineering materials.

Effects of Alloying

• Mixing aluminum with other elements is called alloying, and depending on which element is used to create the alloy, different properties can be enhanced to produce a material that has particular strengths. To improve the corrosion resistance properties of aluminum, magnesium, chromium and silicon can be added. Other common alloying elements include copper and iron, which can drastically reduce the corrosion properties of an aluminum alloy but are normally added to improve strength or ductility in circumstances where corrosion is of little concern.