Anodizing

Nature forms a tenacious oxide film on aluminum, when the metal is exposed to the atmosphere. Although thin, this film provides increased resistance to corrosion. A group of electrochemical processes have been developed to take advantage of this natural phenomena in order to achieve a thicker film. These processes have gained much commercial significance and are generally known as anodic oxidation, anodic treatments or anodizing.

A number of dilute electrolytes can be used to form anodic oxide coatings on aluminum. Electrolytes of sulfuric, chromic and oxalic acids are considered the most important. The dilute sulfuric acid electrolytes have gained the widest commercial usage and seem to provide the thickest coatings, the best corrosion and wear resistance, and the basis for most of the decorative coloring processes on aluminum in use today.

The maximum film thickness is controlled by the anodizing operating conditions. For example, increasing the electrolyte temperature or concentration will increase the oxide dissolution rate. On the other hand, increasing the anodizing voltage or current density will increase the oxide growth.
After the aluminum is anodized it must be sealed. This is done by immersing the anodized metal in a hot water bath wherein the aluminum oxide coating is converted to the monohydrate. Thus the microscopic pores in the coating are closed and the coating becomes resistant to further staining.
In order to obtain reproducible results from batch to batch, a large number of variables must be kept under close control. The type of aluminum alloy is the first variable to be considered as the alloy has a pronounced effect on shade. A general rule is, the purer the alloy the clearer the film produced. The 2000 series aluminum is the least pure and produces less clear films.

Other variables that must be considered are the anodizing conditions and the surface preparation techniques employed. If the variables change from batch to batch, the results will be inconsistent.