Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 402 of 707

surface treatments ANODIZING OF ALUMINUM BY CHARLES A. GRUBBS CHARLIE GRUBBS CONSULTING, LAKELAND, FLA. An aluminum part, when made the anode in an electrolytic cell, forms an anod- ic oxide on the surface of the aluminum part. By utilizing this process, known as anodizing, the aluminum metal can be used in many applications for which it might not otherwise be suitable. The anodizing process forms an oxide film, which grows from the base metal as an integral part of the metal and when properly applied imparts to the aluminum a hard, corrosion- and abrasion-resistant coating with excellent wear properties. This porous coating may also be colored using a number of methods. Many acidic solutions can be used for anodizing, but sulfuric acid solutions are by far the most common. Chromic, oxalic, and phosphoric acids are also used in certain applications. The morphology of the oxide formed is controlled by the electrolyte and anodizing conditions used. If the oxide is not soluble in the electrolyte, it will grow only as long as the resistance of the oxide allows current to flow. The resultant oxide is very thin, nonporous, and nonconductive. This particular property of the anodic oxide is useful in the production of electrolytic capacitors using boric and/or tartaric acids. If the anodic oxide is slightly soluble in the electrolyte, then porous oxides are formed. As the oxide grows under the influence of the applied DC current, it also dissolves, and pores develop. It is this property that allows us to color the oxide using organic dyes, pigment impregnation, or electrolytic deposition of various metals into the pores of the coating. By balancing the conditions used in the anodizing process, one can produce oxides with almost any desired properties, from the thin oxides used in decora- tive applications to the extremely hard, wear-resistant oxides used in engineering applications (hardcoating). Colored anodized aluminum is used in a wide variety of applications ranging from giftware and novelties through automotive trim and bumper systems. Such demanding situations as exterior architectural applications or wear-resis- tant, abrasive conditions, such as landing gears on airplanes, are not beyond the scope of anodized aluminum. Semiprecious and precious metals can be duplicated using anodized aluminum. Gold, silver, copper, and brass imitations are regularly fabricated. New and interesting finishes are constantly being developed, which gain wide appeal across the spectrum of purchasers. The utilization of electropolishing or chemical bright dipping in conjunction with a thin anodic oxide produces a finish whose appeal cannot be duplicated by other means. Matte finishes produced by etching the aluminum surface, affords the "pewter" look, which is oftentimes desired. Matte finishes are also the finish of choice of most architects. EQUIPMENT Tanks A wide variety of materials can and have been used to build anodizing tanks. Lead- 401

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