Products Finishing

SEP 2015

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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pfonline.com/experts Converting a Brass Plating Bath Q. Can a brass plating bath be converted into a bronze plating bath? A. Conceptually, converting a brass plating bath into a bronze plating bath is possible, though the conversion depends on the type of brass plating bath you are using. Brass Plating Bath COMPONENT CONCENTRATION Copper cyanide 4 oz/gal Zinc cyanide 1.25 oz/gal Sodium cyanide 7.5 oz/gal Sodium carbonate 4 oz/gal OPERATING CONDITIONS RANGE Metallic copper 2.8 oz/gal Metallic zinc 0.7 oz/gal Free cyanide 2 – 2.5 oz/gal Temperature 75 – 100°F Current density 3 – 5 ASF Voltage 2 – 3 volts Anodes Brass 80% copper / 20% zinc Bronze Plating Bath COMPONENT CONCENTRATION Copper cyanide 4.0 oz/gal Zinc cyanide 0.3 oz/gal Sodium cyanide 5.0 oz/gal Rochelle salt 2.0 oz/gal OPERATING CONDITIONS RANGE Metallic copper 2.8 oz/gal Metallic zinc 0.17 oz/gal Free cyanide 0.3 oz/gal Temperature 75 – 100°F Current density 2 – 4 ASF Anodes Brass 92% copper / 8% zinc The two plating baths are quite similar. By slightly altering the composition of the bath, you can change the color of the deposit, resulting in a brass or bronze finish. But then is the bronze bath a true bronze? This is where you have to take into account the parts that you are plating and their use. A true bronze has different physical proper- ties compared with that of the bronze that consists of only copper and zinc. Typically, a bronze consists of copper and tin. While the copper zinc bath will give you a bronze-like color, is not a true bronze. My advice is to not convert your brass bath, particularly if it is functioning well. It would be better to prepare a separate bath that is a true bronze plating bath using copper and tin. Where Does My Silver Go? Q. We specialize in plating silver on electrical components. We have been monitoring the number of anodes consumed during a six-month period, and have discovered we are losing 10 percent of the silver when compared with the amount depos- ited. Over a six-month period, this is a large dollar amount. Where is this silver going? A. In many old-time plating shops, silver and gold seemed to evaporate (particu- larly when the boss wasn't around). This problem is now history because of improved plating methodologies and better tracking of silver and gold plating bath components. Assuming this is not the issue, where may this silver be going? Start by looking at the dragout in your plating processes. Each time a rack or barrel is removed from the silver plating tank, some of the plating solution adheres to the surface of the parts, the racks or the barrels. This solution must be returned to the plating tanks. Are you allowing a long enough drain time before going to the rinsing step? Look at how the parts are racked. Parts should be racked in such a way that the dragout can consolidate and drain from a single point on the part. Make sure your barrels are not overloaded. Overloaded barrels tend to drain slowly. Also look at the rinsing part of your plating process. Some of the plating solution will end up in the rinse water. This silver should be recovered. A silver-specific ion exchange system can also help you recover this material. Don't forget that silver is lost by plating on racks and danglers. Consistent stripping is essential. Filters also collect small amounts of bath components. Used filter media should be returned to your precious metal refiner for silver recovery. Can Electroless Nickel Tarnish? Q. What can we do to minimize tarnishing electroless nickel deposits? While this does not happen every day, it occurs often enough to be a problem. A. Electroless nickel tarnishing is not a common phenomenon. Start troubleshooting by reviewing your rinsing process, paying attention to the final rinse. The nickel concentration in the final rinse should be less than 20 ppm, and the pH must be around seven. EN is susceptible to tarnishing in an environment containing sulfur. Look to your chemical vendor for help in passivation of the EN deposit. Search PFOnline. com for current tarnish preventive treatments. 36 SEPTEMBER 2015 — pfonline.com p f o n l i n e . c o m / e x p e r t s ARTHUR S. KUSHNER Consultant platingclinic@pfonline.com PL ATING C L I N I C

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