Clear Anodizing
Clear Anodizing
On many aluminum alloys a thick, transparent oxide layer can be obtained by anodizing in a sulfuric acid solution—this is called clear anodizing. The thickness of the layer depends on the current density and the time in solution and is usually between 0.1 and 1 mil in thickness.
Color Anodizing
Color can be added to the film simply by immersing the metal immediately after anodizing and before sealing into a vat containing a dye or metallic coloring agents and then sealing the film. A wide range of colors have been imparted to aluminum in this fashion for many years. However, the colors imparted in this manner tend to fade from prolonged exposure to sunlight.
Integral Color Anodizing
More lightfast colors for outdoor use are achieved through integral color anodizing. These are proprietary processes utilizing electrolytes containing organic acids and, in some cases, small amounts of impurities are added to the metal itself to bring about the desired colors. This is usually a onstage process and the color forms as an integral part of the anodization. Colors are for the most part limited to golds, bronzes, grays, and blacks.
Electrolytically Deposited Coloring
Electrolytically deposited coloring is another means of imparting lightfast colors. Following sulfuric acid anodizing, the parts are transferred to a second solution containing metallic pigments that are driven into the coating by an electric current.
Hard Anodizing
Hard anodizing, or hardcoating as it is sometimes called, usually involves anodizing in a combination of acids and produces a very dense coating, often 1–5 mils thick. It is very resistant to wear and is normally intended for engineering applications rather than appearance.
Electroplating
In electroplating, a metal such as chromium or nickel is deposited on the aluminum surface from a solution containing that metal. This usually is done for appearance or to improve the hardness or abrasion resistance of the surface. Electroplating has a ‘‘smoothing-out’’ effect, whereas anodized coatings follow the contours of the base-metal surface, thus preserving a matte or a polished surface as well as any other patterns applied prior to the anodizing.
Applied Coatings
Applied coatings include porcelain enamel, paints and organic coatings, and laminates such as plastic, paper, or wood veneers. Probably as much aluminum produced today is painted as is anodized. Adhesion can be excellent when the surface has been prepared properly. For best results paint should be applied over a clean conversion-coated or anodized surface.
J. G. Kaufman
Kaufman Associates
Columbus, Ohio
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
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