More about Aluminum Anodizing

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Aluminum is anodized in order to protect it from corrosion and wear. The process also allows manufacturers to alter the color of the surface of the aluminum, thereby enhancing its aesthetic value even as it protects the metal. Anodized aluminum is an important product for a number of industries, including aerospace and automotive manufacturers, as well as various sporting activities, such as rock climbing and hiking.

In aluminum anodizing, a layer of aluminum oxide forms around the aluminum being anodized. This oxide is a passive substance that would normally form around any exposed piece of aluminum. However, the anodizing process generates a much thicker and protective later of aluminum oxide than would naturally occur. In the United States, the main form of aluminum anodizing is phosphoric acid anodizing. It is used because of the acid’s comparatively low toxicity relative to materials used in other possible methods.

Under a controlled temperature, aluminum is charged with an electric current and dipped in the acid solution. The reaction of the negatively charged acid and the positively charged aluminum causes a thick layer of corundum crystals to form around the part, which enhances the aluminum’s hardness and corrosion-resistance. Corundum is a special form of aluminum oxide that is often used as an abrasive, but here acts as a protective coating. The corundum crystals that form around aluminum during anodization also contain pores that allow the aluminum to accept dye treatments (until the material is sealed by steam or heated water). Boric acids produce a highly non-porous coating, so consider this when selecting what anodizing process to choose.

Aluminum to be anodized must be completely clear of dirt, grime, grease and debris. Any smudge, even that of a finger print, can diminish the effectiveness of the coating. Anodized aluminum, although generally corrosion resistant, is nonetheless subject to galvanic corrosion. This means that contact with many other metals, including copper and steel, will cause corrosion. On the other hand, rain actually removes harmful and unsightly deposits from anodized aluminum rather than causing part degradation. The effectiveness of the anodic film is dependant on its thickness, which should correspond to the conditions it will encounter. Thicker coatings are typically required for more severe conditions.