Metal Finishing Guide Book


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Trivalent Chromium Mixed Chromium, g/L pH Sulfate Hexavalent Chromium 15 - 25 10 - 20 100 - 200 2���3 3.2 ��� 3.8 <1 Temperature, oF 70 ��� 120 120 - 140 90 - 120 Cathode Current Density, A/ft2 70 - 150 70 - 150 175 - 300 Anode-Cathode Ratio 2:1 2:1 1:1 ��� 3:1 Anode material Carbon Precious metal coated titanium Lead-Tin (7%) Rectifier voltage Up to 12 Up to 12 4 - 12 Agitation Mild air Mild air Optional >1 ��� 0.3 >5 0.15 ��� 0.25 0.02 ��� 0.03 0.1 ��� 0.18 Maximum deposit thickness, ��m Deposition rate, ��m/min Table I. Chromium Plating - Typical Operating Conditions el tanks. Tank linings must be made from suitable synthetic material such as PVC, plastisol or polypropylene. Air agitation design can be identical. The sulfate/chloride mixed trivalent process uses graphite insoluble anodes that only need to be replaced when mechanically damaged. The sulfate process uses insoluble anodes with a recoatable catalytic coating. Titanium or Teflon spaghetti coils are used for heating and cooling in both trivalent processes. When converting from a hexavalent to a trivalent chromium process it is almost always better to reline or replace the tank and remove the old ventilation equipment. Even a small amount of residual lead can cause plating problems. An operational hexavalent chromium rectifier usually can be used. The current carrying capacity of the plating racks must be designed for the amperage they will carry. They should also be designed so that the parts on the racks will utilize the bath���s plating benefits and minimize the bath���s negative plating characteristics. The plating amps for trivalent chromium processes are at least one half those used for hexavalent processes so the racks can be designed for the lower current. In general, racks designed for hexavalent chromium processes can be used in trivalent processes, but the reverse is not true. Since trivalent chromium processes will not ���burn��� and they have greater covering and throwing powers than hexavalent processes (see Table II), parts many times can be placed closer together on the racks and high current density areas can face the anodes. The racks can be designed for optimum nickel plating. Racks used with hexavalent chromium are designed to accommodate the deficiencies of the chromium process. This increases productivity and makes shielding and robbing of the part���s high current density areas, as is required for hexavalent chromium processes, unnecessary. Auxiliary anodes are sometimes necessary with hexavalent processes to obtain coverage in the recesses but might 298

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