The electrochemical reaction causes pores to form on the surface of the aluminum as excess positive ions escape. These pores form a geometrically regular pattern and begin to erode down into the substrate. The aluminum at the surface combines with the negatively charged O2 ions to create aluminum oxide. This is called a barrier layer, a defense against further chemical reactions at those spots.
As current continues to be applied, the relatively weak and reactive areas of the pores will continue to penetrate deeper into the substrate, forming a series of column-like hollow structures.
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The longer the current is applied the greater the penetration of these columns. For typical non-hard coatings, the depth can be up to 10 microns. Once this level is reached, and if no color is needed, the process is stopped and the surface can be sealed simply by rinsing in water. That will leave you with a hard, natural aluminum oxide coating, able to withstand chemical attack and very scratch resistant. Aluminum oxide is rated 9 out of 10 on the Mohs hardness scale, meaning second only to