Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 415 of 707

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, instru- ment panels, automotive and appliance trim, etc. has now achieved sufficient com- mercial 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 suit- able 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 pre- ceding 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 sever- al 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 back- ground colored by the conventional dye immersion method. SEALING OF ANODIC COATINGS Hydrothermal Sealing (200-212OF) 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 col- ored. 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 unhydrated form and thus the pores of the coating are filled or plugged and the coating becomes resistant to further staining and corrosion. The use of nickel con- taining seals will, in most cases, prevent leaching of dyes during the sealing operation. When sealing with the nickel acetate bath, a smutty deposit may form on the work. This can be minimized by the addition of 0.5% boric acid to the bath or by the use of acetic acid to lower the pH of the solution to 5.3 to 5.5. Too low a pH, however, causes leaching out of the dye. Use of 0.1% wetting agent in this bath also aids in preventing formation of the smut. Proprietary sealing materials designed to completely eliminate this smut are now available from chemical suppliers. The sealing tank should be of stainless steel or other inert material and must be maintained at 200O F. Use of a filter enables a number of colors to be sealed in 414

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