Metal Finishing Guide Book

2013

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Proprietary baths containing bath stabilizers, color enhancers, and other additives are being marketed and used throughout the finishing industry. PIGMENTATION BY PRECIPITATION OF INSOLUBLE COMPOUNDS Before the development of special organic dyes for coloring anodized aluminum, the precipitation of various insoluble metal compounds within the anodic oxide was used commercially. The treatment consisted of alternatively immersing the anodized surface in concentrated solutions of suitable metal salts until a sufficient amount of the pigment was precipitated to produce the desired color. Although seldom used in today's state of the art, a number of these reactions are listed below: Lead nitrate (or acetate) with potassium dichromate—yellow Lead nitrate (or acetate) with potassium permanganate—red Copper sulfate with ammonium sulfide—green 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. 422

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