Metal Finishing Guide Book

2012-2013

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Ferric sulfate with potassium ferrocyanide���blue Cobalt acetate with ammonium sulfide���black Ferric oxalates (ferric ammonium oxalate or ferric sodium oxalate) applied to conventional anodic oxides in the same manner as organic dyes are, under proper conditions, hydrolyzed to deposit ferric hydroxide within the coating pores, imparting a gold to orange color of outstanding resistance to fading. Special proprietary chemicals are available for this treatment. The deposit of ferric oxide produced in the above manner may, in addition, be converted to ferric sulfide, the resultant shade of which is black. Alternatively, a bronze shade may be formed by reduction of the ferric oxide with pyrogallic acid. Cobalt acetate reduction, although commercially used in Europe, is not well known in the U.S. It consists of saturating a conventional anodic oxide with the cobalt solution and then reacting this with potassium permanganate to produce a cobalt-manganese dioxide complex. The resultant bronze shade has excellent lightfastness and offers some potential for architectural applications. MULTICOLOR ANODIZING The application of two or more colors for the production of nameplates, instrument panels, automotive and appliance trim, etc. has now achieved sufficient commercial importance that a number of large firms deal exclusively with such items. The following methods of multicolor anodizing are possible: The multiple anodizing process, which entails a complete cycle of anodizing, dyeing, and sealing; application of a resist to selected areas; stripping of the entire anodic oxide from the remaining unprotected surfaces; and repetition of this entire procedure for each color. The single anodizing method, wherein an anodic oxide of sufficient thickness and porosity to absorb the dye required for the darkest shade is first applied. This oxide is then dyed and left unsealed, a resist applied, and the dye alone discharged or bleached out with a solution that leaves the anodic oxide intact. The operation is then repeated for each successive shade. Finally, the resist is removed with a suitable solvent, and the entire surface sealed. In certain cases, where a dark shade is to be applied after a pastel shade, a modification of this technique omits the bleaching step with the supplementary dye being applied directly over the preceding color. The use of a specialized combination ink-and-resist enables information or designs to be printed directly on the previously formed anodic oxide in several colors. The background color may then be applied by conventional dyeing methods, while the ink serves as a stop-off for the printed areas. Preanodized, photo-sensitized aluminum alloy material is available, wherein the image, in black, may be produced by photographic methods, and the background colored by the conventional dye immersion method. SEALING OF ANODIC COATINGS Hydrothermal Sealing (200-212��F) To achieve the maximum protective qualities and corrosion resistance required for finished articles, the anodic oxide must be sealed after it is formed and/or colored. The sealing process consists of immersing the anodized parts in a solution of boiling water or other solution such as nickel acetate, wherein the aluminum oxide is hydrated. The hydrated form of the oxide has greater volume than the 476

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