Anodized aluminium has been treated to resist corrosion.

In contact with air, aluminum combines with oxygen to form a thin oxide layer on its surface that prevents further oxidation. This natural oxide, while protective, is extremely thin and is easily worn or damaged, leaving the underlying metal once again exposed and vulnerable to oxidation.

In the anodizing process, a direct electrical current (DC) is passed through a solution of sulfuric acid in which the anode is the piece of aluminium being treated. Anodizing results in a very hard, porous, controlled-thickness layer of aluminum oxide on the aluminum section. This layer is similar in hardness to a ruby or sapphire and protects the underlying surface from further oxidation. The layer can also be electrolytically dyed or colored by various methods.