Metal Finishing Guide Book


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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 mid-temperature 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. Room Temperature (Cold) Seals (70-90��F) A significant modification in the sealing of anodized aluminum was the development of ���room temperature sealing��� (70-90OF). Unlike the high temperature and mid-temperature seals, which depend on hydration for sealing, the cold seals rely on a chemical reaction between the aluminum oxide and the nickel fluoride contained in the seal solution. Unfortunately, this reaction is slow at ambient temperatures and the sealing process can proceed up to 24 hours; however, it has been found that a warm water rinse (160OF) after the cold seal immersion will accelerate the sealing process, allowing for handling and packing of the sealed parts. The sealing of organically dyed parts in cold seals has been found to be advantageous. Light stability testing (fade resistance) has shown that parts sealed in cold seals gain additional lightfastness. OTHER ELECTROLYTES A number of other electrolytes are used for specialized applications. Chromic acid is used in marine environments, on aircraft as a prepaint treatment, and in some cases when finishing assemblies where acid may be entrapped. Although the film produced is extremely thin, it has excellent corrosion resistance and can be colored if desired. 477

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