Anodising
Anodizing or Anodising is a process used to protect aluminium from abrasion, corrosion and allows it to be dyed in a wide range of colors . The process derived the name from the fact that the part to be treated forms the anode portion of a electrical circuit in this electrolytic process.
A layer of aluminium oxide is created on the surface of the aluminium from the action of the current being passing through the part which is bathed in an acid solution. This aides in the formation of the oxide layer. This oxide layer increases both the hardness and the corrosion resistance of the aluminium. The oxide forms as microscopic hexagonal "pipe" crystals of corundum, with each having a central hexagonal pore, which is also the reason that an anodized part can take on color in the dying process.
The aluminium oxide coating is grown from and into the surface of the aluminium. Because of this, unlike coatings, is not prone to peeling or cracking. It also possesses excellent thermal and electrical insulation qualities.
Where appearance is important, the oxide surface can be dyed before the sealing stage, as the dye enters the pores in the oxide surface. The number of dye colors is almost endless, however the lighter colors tend to look better than darker ones.
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). Metal dye colors range from pale champagne to black. Bronzes are preferred for architectural use.
After dying, 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.
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