The Difference Between Anodizing & Billet Aluminum

Aluminum Billet

• Aluminum billet, also called aluminum "bar stock," is one of the raw products from an aluminum mill. Aluminum billet indicates a bar or rod of aluminum or aluminum alloy produced for shaping into other products. Making parts from billet gives manufacturers more control over the shape and finish of the product than does casting, a process that involves pouring molten metal into a hollow cast.

Anodized Aluminum

• Anodized aluminum goes through a process that gives it a hard, corrosion-resistant surface and seals in dyes used to color the metal. The surface of anodized aluminum is harder than the underlying metal, and brightly colored electronics often feature anodized aluminum shells. Anodizing gets its name from the fact that the treated aluminum piece becomes the anode -- the electrode where oxidization occurs -- of an electrochemical cell. The oxidation of the aluminum in this process creates crystallized aluminum oxide.

The Anodizing Process

• The anodizing process starts with aluminum placed in a bath of water and connected to an electric power source in a circuit with a nonreactive conductive material. As electricity runs through the circuit, electrons are drawn from the aluminum, causing the aluminum to react with the surrounding water. Oxygen from the water combines with the aluminum surface, forming a layer of aluminum oxide crystals. As the process continues, the centers of the crystals dissolve and become hollow. At this point, the piece is removed from the water bath and dyed -- something that's possible because the hollow aluminum oxide crystals absorb the dye. Putting the piece in a hot water bath causes the crystals to swell shut, sealing in the dye.

Advantages and Disadvantages

• Aluminum oxide ranks as one of the hardest minerals, second only to the diamond. Anodized aluminum is, therefore, much more durable than untreated aluminum. Because the crystalline layer of aluminum oxide forms vertically from the underlying aluminum, the dyed surfaced doesn't peel off like paint does. Because of these properties, anodized aluminum is often used for small, portable electronics and for building materials that need to be resistant to environmental wear. It also serves as a protective coating on satellites.