Aluminum and Its Alloys

In general, aluminum and its alloys are more resistant to corrosion than mild steel. They are known to have a very good resistance to corrosion in a variety of environments and chemical compounds, even though aluminum is a relatively reactive metal. Aluminum and its alloys also have a good resistance to various forms of corrosive attack. For the most part, the lower temperature corrosion resistance of aluminum is virtually equivalent to that of stainless steel, and it provides reasonable protection at elevated temperatures. Pure aluminum, however, tends to have a greater corrosion resistance than its alloys, and impurities in the aluminum only act to increase the metal’s susceptibility to corrosion. This is true especially for surface impurities; clean surfaces are much more effective at resisting corrosion than are surfaces with deposits.

Aluminum’s excellent resistance to corrosion can usually be attributed to the rapid formation of an oxide film on the metal’s surface, which acts as a barrier to corrosive environments. For instance, the film inhibits corrosion very effectively in lower temperature, atmospheric and aqueous corrosive environments. An important aspect of the film is that it forms quickly in many environments, but can also be produced artificially by sending an electric current through the metal. This is called anodizing. The tough, virtually transparent, non-flaking, aluminum oxide film is capable quickly of repairing itself when it is scratched or abraded. Therefore, in order to defeat the protective film, continuous mechanical abrasion or chemical degradation in an oxygen deficient atmosphere is required. A further benefit is that the surface oxide film can be modified or thickened to enhance its corrosion protection.