Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 183 of 707

Trivalent Chromium Chromium, g/L pH Temperature, oF Cathode Current Density, A/ft2 Anode-Cathode Ratio Anode material Rectifier voltage Agitation Maximum deposit thickness, µm Deposition rate, µm/min Mixed 15 - 25 2 – 3 70 – 120 70 - 150 2:1 Carbon Up to 12 Mild air > 1 0.15 – 0.25 Sulfate 10 - 20 3.2 – 3.8 120 - 140 70 - 150 2:1 Precious metal coated titanium Up to 12 Mild air ≈ 0.3 0.02 – 0.03 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/chlo- ride 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 hexa- valent 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 resid- ual lead can cause plating problems. An operational hexavalent chromium rec- tifier 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 plat- ing 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 defi- ciencies of the chromium process. This increases productivity and makes shield- ing and robbing of the part's high current density areas, as is required for hexa- valent chromium processes, unnecessary. Auxiliary anodes are sometimes necessary with hexavalent processes to obtain coverage in the recesses but might 182 Hexavalent Chromium 100 - 200 <1 90 - 120 175 - 300 1:1 – 3:1 Lead-Tin (7%) 4 - 12 Optional > 5 0.1 – 0.18

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