Anodizing
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Wikipedia-Article "Anodizing"
Anodising, or anodizing, is a process used to protect aluminum 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 an electrical circuit in this electrolytic process.
Anodized aluminum can be found on paintball guns, carabiners, and other extreme sports equipment due to its aesthetic properties but also the protection it offers to aluminum.
The aluminum oxide coating is grown from and into the surface of the aluminum. Because of this (unlike coatings), it is not prone to peeling or cracking. It also possesses excellent thermal and electrical insulation qualities.
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Chemistry
Aluminum naturally forms a passivating oxide layer which provides moderate protection against corrosion. The layer is strongly adherent to the metal surface (as compared to corrosion in steel, where rust puffs up and flakes off, exposing new metal to corrosion), and it will regrow quickly if scratched off. In anodization, this layer of aluminum oxide is grown on the surface of the aluminum from the action of the current being passed through the part, which is bathed in an acid solution. The conditions are controlled to prevent the phenomenon of passivation from occurring, allowing the formation of an oxide layer many times thicker than would otherwise form. This oxide layer increases both the hardness and the corrosion resistance of the aluminum. The oxide forms as microscopic hexagonal "pipe" crystals of corundum, each having a central hexagonal pore (which is also the reason that an anodized part can take on color in the dyeing process).
Dyeing
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, lighter colors tend to look better than darker ones.
Alternatively, metal (usually tin) can be electrolytically 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. Bronze shades are preferred for architectural use.
After dyeing, 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.
Related processes
Aluminum and its alloys are not the only materials that can be anodized. Titanium can be anodized and the coating can create a blue color without dyes. The color formed is dependent on the thickness of the oxide; it is caused by the interference of light reflecting off the oxide surface with light traveling through it and reflecting off the underlying metal surface. Titanium nitride coatings can also be formed, which have a brown or golden color and have the same wear and corrosion benefits as anodization.
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