ANODIZING OF ALUMINUM

摘要

When an aluminum part is made the anode in an electrolytic cell an oxide film is formed on the aluminum. By utilizing this process, known as anodizing, aluminum 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 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. Almost any acid solution can be used, but sulfuric acid is by far the most common. Chromic, oxalic, and phosphoric acids are also used in certain applications. The nature of the film 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 coating allows current to flow. The resultant coating is very thin, nonporous, and nonconductive. This property of the anodic coating is useful in the production of electrolytic capacitors using boric and/or tartaric acids. If the coating is slightly soluble in the electrolyte, then porous films are formed. As the coating grows under the influence of the applied DC current, it also dissolves, and pores develop. It is this property that allows one to color the oxide using either 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 coatings with almost any desired properties, from the thin films used in decorative applications to the extremely hard, wear-resistant films 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-resistant, 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 film produces a finish whose appeal cannot be duplicated by other means. Matte finishes produced by etching the aluminum surface afford the "pewter" look, which is oftentimes desired.