Metal Finishing Guide Book

2013

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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 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 containing 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 200OF. Use of a filter enables a number of colors to be sealed in the same bath without danger of contamination. Mid-Temperature Sealing (160-190°F) Due to the higher energy costs inherent in hydrothermal sealing, chemical manufacturers have developed "mid-temperature" seals (160-190OF). These seals, which contain metal salts such as nickel, magnesium, lithium, and others, have become very popular due to the lower energy costs and their ease of operation. One disadvantage of the lower temperature is the tendency of organically dyed parts to leach during sealing. This can be compensated for by a slight increase in the bath concentration and by operating the solution at the upper temperature limits (190OF). "Nickel-free" seals (or more "environmentally friendly" seals, as they are called) are fast becoming the seal of choice where clear or electrolytically colored parts are concerned. Because there is nothing to leach, these midtemperature seals accomplish hydration of the oxide without the use of the heavy metal ions. When the seals become contaminated or are no longer effective, they can be discharged to the sewer without subsequent treatment (except possible pH adjustment). This offers the finisher a safer alternative to the effluent treating necessary with heavy metal containing seals. 423

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