Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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bath usually require this treatment for optimum performance. It is reported that certain selected dyestuffs benefit from after-treatment with other heavy metals; for example, lead, copper, zinc, or chromium. Generally, such treatments are not utilized because of the requirement of an individual sealing tank for each dye. In the case of extremely porous anodic oxides, for example, those formed on alloys of high copper content, effective sealing is particularly important with cer- tain dyes to prevent color loss from sublimation of the dye or by chemical reac- tion in oxidizing or reducing environments. ELECTROLYTIC COLORING (2-STEP) This electrolytic coloring process consists of conventional sulfuric acid anodiz- ing followed by an AC treatment in a bath containing tin, nickel, cobalt, or oth- er metal salts to produce a series of bronze to black colors as well as blues, greens, burgundies, and golds. The most common bath is one containing tin. The colors produced are not alloy or thickness dependent and are easier to con- trol. The process is not as energy intensive as the integral color process. It is for this reason that this process has almost entirely replaced the integral color process in recent years. Unlike sulfuric acid anodizing, the coloring process is controlled by voltage and time, rather than by current density. Depending upon the bath used, the coloring time can range from 20 sec for champagne to 10 min for black. The use of specially built AC power supplies, using electronic timing and voltage control, helps produce a finish that is reproducible time after time. Proprietary baths containing bath stabilizers, color enhancers, and oth- er 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 suffi- cient 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 prop- er conditions, hydrolyzed to deposit ferric hydroxide within the coating pores, imparting a gold to orange color of outstanding resistance to fading. Special pro- prietary 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 413

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