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The Anodizing Process
Anodizing is an electrochemical process like silver plating, but differs in that the actual surface of the aluminum is actually being changed, not merely coated. Because the anodic oxide structure combines with the aluminum substrate, it cannot be easily removed or worn away.
Steps in the Basic Anodizing Process
Before the actual anodizing begins, the aluminum is cleaned and given either a reflective surface or a matte finish. Next, an acid electrolyte bath (typically containing sulphuric acid) is prepared with a cathode in it. The aluminum objects to be anodized are placed in the bath and act as anodes. Electric current is then passed through the acid electrolyte solution going from the cathode to the anodes -- the aluminum objects. Oxygen ions are released by the current running through the solution; the oxygen ions combine with the aluminum atoms on the surface of the objects to be anodized. In this way, the surfaces are put through controlled oxidation enhancing the otherwise naturally occurring process of oxidation. Finally, as the oxidized surface is porous, it gets sealed to ensure the long life of the finish.
Adding Color
Color can be added to the surface before it is sealed. In one process, the color is incorporated into the surface during the anodizing step; this is called "Integral coloring." Adsorption and electrolytic coloring, on the other hand, are done after the anodizing step, while interference coloring involves an additional step between the anodizing and the introduction of the coloring agents.
Benefits of Anodizing Aluminum
The anodizing process strengthens the aluminum and protects it from natural oxidation. The entire anodizing process is nonpolluting, and all aluminum -- anodized or not -- is 100% recyclable.