Aluminum anodizing is the process of generating a controlled
oxide film on the surface of aluminum.
The purpose of this film may be to improve the appearance of the
aluminum, protect the aluminum against corrosion, or reduce wear. Aluminum
automatically forms a thin oxide layer in the presence of air, but this
naturally occurring film does not enhance the aluminum except to slightly
reduce its susceptibility to corrosion. The process of anodizing removes the
natural film from the aluminum and electrochemically forms a film of known
thickness and hardness. This film can be dyed different colors or have its own
coloration.
The anodized film is not "deposited" but rather is
"grown." It penetrates into the aluminum as well as grows outward. It
is part of the metal.
The film as formed has pores that reach all the way to the
base aluminum. It is because of these pores that the film can be dyed, as the
dye molecules "fill" the pores. To achieve the best corrosion
protection properties, the anodized film must be sealed as a final step. This
is accomplished either by swelling shut or plugging the pores via thermal or
chemical treatment.
The actual anodizing process is performed in a bath composed
of water and mineral acids. This acids serve as electrolytes. They carry the
current from the cathodes to the anode, which is the aluminum being processed.
