Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 196 of 707

In addition to the natural formation of Cr (III), any organic materials will be rapidly attacked by the chrome bath reacting to form Cr (III). The source can come from oils, grease or other dirt from parts, especially the unclean ID of tubular work, or the use of compressed air that forces oils into the bath, or normal shop dirt that is drawn into the system from the pull of the ventilation system. Trivalent is fortunately one contaminant that can be reduced by taking the advantage of the reaction at the anodes in a process called dummying. A larger anode-to-cathode area between10-to-30:1 will reduce Cr (III) back to Cr (VI) state. Your chromium supplier, especially those that supply proprietary types, will be able to help. Other metals can also accumulate in the bath. If reverse etching is done in the same plating bath, significant concentrations of iron (Fe) can build. If copper (Cu) is exposed, it, too, can build in the bath. Nickel (Ni), while more of a contaminant for the decorative process, it can also develop in functional chrome bath if stainless steel is used in various areas of construction. All of these metals, when combined, will lower the conductivity of the solution (see chart 1), resulting in poor quality deposits and higher energy cost. If allowed to increase, the eventu- al result is the need to replace the bath with new chemistry and waste disposal of the old solution. Several systems are available for impurity removal and all have had some degree of success. Perhaps the least expensive is the porous pot system. This sys- tem utilizes a cathode inside a ceramic pot. The system may use its own anodes or use tank anodes already in the tank. As the cathode compartment is elec- trolyzed, it releases hydrogen gases that raise the pH of the solution in the pot until impurities precipitate. The solution inside the pot must be pumped out typ- ically daily or per shift and replaced with solution from the tank. The one con- tamination that does reduce is Cr (III) since the anode area is typically larger than the cathode in the pot. There has been mixed results as to the effectiveness with other impurities and also the cost of operating since energy from a rectifier has to be used. Electro-dialysis has been of interest for many years. It consists of using a resin membrane that will allow only positive ions to flow through. Similar to the porous pot, a cathode is used in a compartment and the metallic cations are deposited onto the cathode. As the cathode builds with impurities resistance builds and the effectiveness is reduced. The cathodes must then be removed and scraped clean in a typical operation. There have been many attempts to design a system based on this basic principle and such systems are available commercially. There are, however, mixed reactions as to the effectiveness of the devices. Ion exchange (IX) units have found success in many applications. In this type of system, a portion of the bath is diluted and cooled. It is then directed into the IX unit which is filled with a cationic resin bed. Metallic (cationic) impurities are exchanged on the bed for hydrogen ions. The bed is then purged of the residual chrome solution and rinsed. The bed is reactivated by pumping an acid which dis- places the metal ion with hydrogen ions from the acid. It is rinsed and the process is restarted on the chrome bath. The major limitations are: 1) a full concentrated bath cannot be sent through the bed—it must be diluted first. 2) The volume of wastewater generated is typically high. The system can, however, be combined with evaporation devices, and waste and chrome can be recovered. 195

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