What are some of the ways to maintain the purity of a hexavalent chromium plating bath? We are constantly battling the buildup of trivalent chromium, copper, iron and nickel in our chromium plating bath. C.P.
This question comes up a lot, and the answer is always worth repeating. But before we look at these methods, it is worthwhile to look at how these impurities get into the plating bath. The usual causes for buildup of trivalent chromium in a plating bath are a large imbalance of the cathode to anode areas with the anode area being less than the cathode area or organic impurities in the plating bath. The first cause is the most common, and the second usually happens by accident (for example, paint dripping into the tank). The heavy metals that show up in the tank can be due to the substrate itself, reverse etching of the part in the chromium tank or dragin in from contaminated rinses.
The following are a number of different methods that are suggested:
- Dilute and reconstitute
- Electrolyzing (dummying)
- Ion exchange
- Porous pot
The first approach is classic but is not acceptable today. The idea behind it is to remove a percentage (usually 50%) of the solution, put it aside and then reconstitute the remaining solution. This approach does reduce the concentration of the unwanted trivalent chromium and other heavy metals. But what do you do with the solution that has been removed from the plating bath? You can't just let the excess solution sit around the shop. Environmental regulations will not allow it! So this method has to be scratched.
The next listed method, electrolyzing or dummying, is useful in reducing the amount of trivalent chromium in a chromium plating bath but doesn't effect the heavy metal concentrations. In order to have success with this method, a large anode to cathode area must be used. The best ratio is approximately 27 to 1. In many plating baths this is very difficult to achieve. Using a lower ratio will work, but a longer period of time is required.
Electrodialysis and ion exchange processes are newer methods for removal of these materials from chromium plating baths. They are proven techniques and work well. You can obtain information on these processes by going to the Suppliers page at www.pfonline.com or looking in the Products Finishing Directory and Technology Guide under Pollution-Control Equipment, water: electrodialysis and ion exchange.
The porous pot method is an older method that uses a porous ceramic barrier placed between a lead anode and cathode. In fact, the method is almost as old as the chromium plating process itself and, if used properly, can be effective in removing trivalent chromium as well as other heavy metals from the bath. The equipment is available commercially. An excellent discussion on the use of porous pots in chromium plating is given in Mandich, Li & Seiman, Plating and Surface Finishing, 84, 82-90 (December, 1997).