Anodising is a process used to protect aluminium from corrosion. It is so named as it's an electrolytic process where the part to be treated forms the anode of the electrical circuit.
The thin, natural aluminium oxide layer on the surface of the aluminium is turned into a thicker oxide layer, which increases its hardness and also makes it resistant to corrosion. The oxide forms as microscopic hexagonal "pipe" crystals of corundum, with each having a central hexagonal pore. After anodising, the surface is usually sealed by using hot water or steam to convert the oxide into its hydrated form, so reducing the porosity of the surface as the oxide swells.
The aluminium oxide coating is integrated into the surface of the aluminium so, unlike other coatings, is not prone to peeling or cracking. It is also an excellent thermal and electrical insulator.
Where appearance is important, the oxide surface can be dyed before the sealing stage as the dye enters the pores in the oxide surface. Alternatively, metal (usually tin) can be electrolitically deposited in the pores of the anodic coating to provide colors that are more light fast (resistant to fading). Colors range from pale champagne to black. Bronzes are preferred for architectural use.
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