Products Finishing

JUN 2014

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pfonline.com/experts Phosphate Sludge Q. What is the composition of phosphate sludge from a zinc- phosphate process bath? What are the precipitates that are formed?—K.P. A. The composition of the sludges generated in a phos- phate bath varies with the type of bath in which they are produced. The composition of the sludge found in a zinc- phosphate bath is as follows: Component Amount Iron 20% Zinc 8% phosphate as po 4 45% Water 27% These percentages can vary depending on the oper- ating temperature of the bath. The precipitates are primarily metal phosphates. Search pfonline.com to find a number of excellent articles on phosphating. Gold Pitting Problem Q. How can we solve problems with pitting and haze on our hard gold deposit?—J.R. A. Pitting can be caused by a number of different factors. Hydrogen gassing, particulate matter suspended in the plating solution and inadequate cleaning of the base material are three common causes. Hydrogen gas formed during the plating process can stick to the surface of the parts being plated. If the bubbles that stick to the surface do not slide off rapidly, the plate will form pits. Typically, a pit due to hydrogen has a "small" tail. In most plating baths, this problem can be minimized by adding wetting agents/surfactants. The problem also can be minimized by increasing the agita- tion of the bath. Depending on the type of agitation, different wetting agents may be required. Particulate matter and organic matter can also cause pits. For example, if your plating operation is set up in a dusty area, some of that dust will settle into your plating bath and must be removed by filtration. Also, organic impurities may be introduced into the plating bath by the decomposition of wetting agents or other organic addi- tives in the bath. This is why filtration is a critical part of the plating process. Lastly, cleaning of the parts prior to the plating step is critical. In my opinion, more than 60 percent of plating failures have to do with the cleanliness of the surface prior to plating. This means removing not only things you can see on the surface but also things that you cannot see, like oxides. Hazy gold deposits can be due to insufficient bright- eners in the plating bath or a pH that is too high. Carbon filtration is called for to remove breakdown products from brighteners and wetting agents, but you may be carbon treating your bath too frequently. I have no problems with this, but some operators believe that if a little is good, a lot has to be better. Just remember that, in many cases, after the plating bath is treated with carbon you must rebuild the organic portion of the bath, because carbon filtra- tion will remove both good and bad products. I am not suggesting that you give up on carbon treatment, but that you use a polypropylene-type filter that is suitably sized for your plating bath. You also might try reducing your pH by a couple of tenths and see if that makes a differ- ence. If you are not using a Hull cell to monitor the day- to-day health of your plating bath, you should. Hex Chrome Problem 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: 34 JUNE 2014 — pfonline.com ARTHUR S. KUSHNER Consultant platingclinic@pfonline.com PL ATING C L I N I C 0614_PF_PlatingClinic.indd 34 5/16/2014 1:41:25 PM

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