Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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tion (8) – single and double cell processes. The original single-cell process uses graphite anodes immersed directly into the plating solution. An interaction between the anodes and the chemistry of the process eliminated the formation of hexavalent chromium. A secondary chemical mechanism converts hexavalent to trivalent if any does appear in the solution. The anodes, which are destroyed only by mechanical means, are placed below solution level to eliminate misting. Since this process utilizes both sulfate and chloride (and boric acid), as in nick- el baths, it is now commonly referred to as a mixed salt trivalent process. Just as with nickel electrolytes, the mixed chloride-sulfate formulation enhances the oper- ation of the process. This is most noticeable in plating rate and deposit thickness (Table I). The chloride helps to make it easier for the mixed process to meet the automotive company's specification for 0.25 to 0.5 microns of chromium. Other factors are listed in Table II. The double-cell process originally reduced the side reaction (8) by isolating the chromium containing solution from the anode through a membrane box. Because of maintenance problems and the amount of space that the anode boxes took from the plating area inside the tank, they have been almost completely replaced by insoluble metallic catalytic composite anodes with a projected life of 3 to 5 years. With the elimination of the need for an isolated anode, today this process is com- monly referred to as a sulfate process. The electrolyte contains no chloride ions. Once through the learning curve, control of trivalent chromium plating processes is typically easier than for hexavalent chromium processes. The liter- ature says that an operator should "think nickel plating not chromium plating" when controlling a trivalent chromium process. The troubleshooting guides for trivalent chromium processes are a few lines long as compared to several pages for hexavalent chromium. The additives are added based upon amp-hours, specific gravity, and pH. In addition, chemical analysis on a monthly basis appears to be sufficient for con- trol. All trivalent chromium processes are far more sensitive to metallic conta- mination than hexavalent processes. Metallic impurities darken the deposit and alter the throwing and covering powers. However, most trivalent processes utilize a regenerateable resin to remove all common metallic contaminates directly from the working solution. Less desirable, but a quick chemical purifi- cation method or a slow dummying method can also be utilized. These methods eliminate the problems attributed to metallic contamination. Today, most industries using decorative chromium deposits, such as the automotive/truck industry, approve the use of trivalent chromium for both interior and exterior parts. The almost complete elimination of the color dif- ference between hexavalent and trivalent chromium deposits and the demon- strated corrosion resistance is greatly responsible for this wide acceptance. Some trivalent chromium deposits have also been found to be much more resistant to calcium chloride corrosion (Russian Mud) than hexavalent chromium deposits. OPERATIONS The typical operating conditions for trivalent compared to hexavalent chromi- um electroplating processes are shown in Table I. EQUIPMENT Trivalent chromium tanks and equipment are very similar to the design of nick- 181

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