Q. We are having a problem with our hexavalent chromium bath in that we are getting very little plating buildup on high-current-density areas. It looks like burning (dull gray) in those areas, and the burnt area shows a lack of deposit buildup. We have had the bath tested and followed the recommendations to no avail. I have never seen anything like this in the 40-plus years I've been plating.—N.E.
A. What you are describing is a classic case of a burnt deposit. Assuming the chemistry of your plating bath is correct, the first thing you should do is look at the temperature/current density balance. As you are aware, the hex chrome plating bath is not the easiest bath to work with. Small changes in the temperature of the bath can cause significant changes in the plated deposit. In simple terms, the current density is usually higher on the corners, edges and high points on a plated surface. To minimize burning, the ideal solution is to get more chromium ions to these areas. This can be done by raising the temperature or increasing the agitation of the bath. An alternate solution is to reduce the current density in these areas. Sometimes this can be done by adjusting the anodes in your bath. If this fails to solve the problem, fixturing may be required. A simple example of fixturing is using current thieves in the areas that experience high current density.
If this doesn’t solve or at least reduce the problem of burning there are some other things you might investigate:
1. Your anode length should be shorter than the length of your part. If the anode is too long, you will end up with heavier deposits on the end of your part or burning.
2. Metallic impurities can also cause problems. Trivalent chromium, iron, nickel, copper and zinc all can affect your chromium deposit. Have you tested your bath for “tramp” metal ions?
3. Parts being plated should be placed at least 3 inches below the surface of the plating solution. Why? The hydrogen gas that is a byproduct of the hexavalent chromium plating bubble to the surface and tends to dilute the plating solution around the work. This will cause a change in the current density in the area.