Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 185 of 707

Chromic acid, g/L Fluosilicic acid, g/L Acetic acid, g/L Barium acetate, g/L Temperature, oF Current density, A/ft2 Bath A 250 - 216 7.6 100 40 – 90 Table III. Decorative Black Chromium Formulations Chromic acid, g/L Sulfate, g/L Fluorsilicate, g/L Temperature, oF Voltage Time of plating (minutes to produce 10 millionths) SOLUTION MAINTENANCE The best waste treatment method is to minimize the amount of solution dragged out of the plating tank. Secondly, return as much dragged-out material as pos- sible back to the tank. Any plating solution that cannot be recovered must be waste treated. Even though this reduces plating solution waste, reduced drag-out keeps contaminants in the plating solution, thus complicating the requirement to maintain a pure enough plating solution to obtain the required deposit properties. Due to hexavalent chromium's poor draining characteristics and its misting, a large amount of solution is unavoidably removed from a decorative chromium plating tank. Vacuum evaporators and ion exchange are examples of methods used to return dragged-out hexavalent chromium back to the plating tank in order to save chemical and waste treatment expenses; however, due to the difficulty of removing metallic impurities from hexavalent chromium plating solutions, it is common to send a solution containing an excess of metallic impurities out for recovery. Trivalent chromium is much more sensitive to metallic impurities than hexa- valent chromium; however, the chemistry of most trivalent chromium process- es makes it easy to remove metallic impurities. This eliminates the need to ever discard the solution due to normal metallic contamination. Atmospheric evap- orators can be used to reduce solution volume so all the trivalent chromium that can be captured is returned to its plating tank. In most trivalent chromium processes, metallic impurities can be removed quickly by chemical precipita- tion or slowly by dummying. The most effective way is to use a resin treatment 184 Bath A 525 1 7 90 – 105 6 – 18 8 Table IV. Hexavalent Chromium Barrel Formulations Bath B 340 0.98 6 Room - 95 6 - 18 10 Bath B 340 0.34 - 11 70 200 Bath C 250 0.25 - - 90 150 - 450

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