Anodising

From Academic Kids

Missing image
Ipodmini2.JPG
The iPod mini has an anodised aluminium surface, and comes in many colours

Anodizing or Anodising is a process used to protect aluminium from abrasion and corrosion and to allow it to be dyed in a wide range of colors. The process derives its 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 passed through the part which is bathed in an acid solution. This aids 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), it 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 resistant to fading (light fast). Metal dye colors range from pale champagne to black. Bronze shades 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, reducing the porosity of the surface as the oxide swells.

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