The natural formed oxide layer on aluminum

According to Wernick, Pinner and Sheasby´s book The Surface Treatment And Finishing of Aluminum And Its Alloys, a freshly formed aluminum metal surface, which is exposed to the atmosphere, will immediately be covered with a thin film of oxide. This formation is corrosion, just as the rusting of iron, but fortunately this thin aluminum oxide layer tends to self passivate.

Another important feature of this natural oxide film formed on an aluminum surface is its volume which is 1.5 times that of the aluminum metal. This voluminous, protective surface layer not only covers the aluminum continuously but can also handle a certain amount of substrate deformation without rupturing.

Pure aluminum shows the highest corrosion resistance by forming the most uniform natural oxide layer. As alloying elements are added this resistance decreases. The drawing below shows a natural formed oxide layer on a 6000 aluminum alloy.

Using copper as a alloying element lowers the corrosion resistance the most, whereas magnesium has the lowest influence on the corrosion resistance.

The impact of the various alloying elements and the various mechanical processes on the corrosion resistance is of great importance when using aluminum in all kinds of applications.

Anodizing is a artificial way to produce an aluminum oxide layer by applying a current in an electrolyte but it is still a corrosion process. As for the natural formed oxide layer, anodizing, as well, is very dependent on the alloying elements and the quality of the mechanical processes performed before the anodizing.

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