Metal Finishing Guide Book


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troubleshooting, testing & analysis ZINC & ZINC ALLOY PLATING; PROBLEMS WITH EN BATHS AND ACIDĀ COPPER BY MATT STAUFFER, PAVCO, CHARLOTTE, N.C. Q: Our nickel metal is high in our bright nickel bath. [My vendor] tells us the only way to bring it down is to decant. Is there a way to bring it down slowly without wasting solution? A: The increase in nickel metal growth in most acidic plating processes is caused by the difference in anode and cathode plating efficiencies. In the case of Watts nickel baths, you have 100% anode efficiency and approximately 93% cathode efficiency. The remaining 7% is directed towards the reduction of hydrogen into hydrogen gas. This hydrogen is a common source of gas pitting. So basically, you are dissolving more metal than you are plating out. Most platers dilute their baths to correct them. Some platers look towards insoluble anodes. Care must be taken to avoid the creation of harmful oxidation products when using insolubles. There are products on the market that utilize a membrane system to prevent this reaction. These can be expensive and do require maintenance. The other options to consider involve nickel recycling. This can be done by working with a company that recycles nickel solutions (plating or stripping) into nickel metal or by finding another plater, usually a barrel plater, who has a regular need for nickel salts in his process. Usually, an arrangement can be made to be beneficial to both parties. The final option would be what I would call "home-grown" recycling. Set up a small plate out tank with insoluble carbon anodes. A good alternative for cathodes is nickel anode chips, which can be readily barrel plated. Occasional adjustment of the pH may be required to raise the pH due to the use of insoluble anodes. Fresh solution can be added as the nickel is depleted from the plate out cell. The plated anodes can now be reused in the anode baskets. Q: I am getting excess sulphate in my chrome tank. Due to this excess, I'm getting dull plating. Can you please tell me how to resolve this problem? Also, in my cyanide copper tank the solution becomes dark after 4-5 plating rounds. Can you offer some advice? A: You mention a chrome bath and a copper bath, so I'll go out on a limb and assume your plating copper-nickel-chrome. If so, the usual source for excess sulfate is insufficient rinsing after nickel plating. A typical Watts nickel bath will contain upwards of 35 opg (263 g/l) of nickel sulfate. Even a simple sulfate catalyzed hex chrome has only 0.32 opg (2.4 g/l) sulfate. Based on those relative quantities, it is pretty easy to get a fair amount of sulfate from a nickel bath into your chrome if rinsing is insufficient. Use barium carbonate to remove excess sulfate and consider additional rinse tanks (counterflowed) after nickel plating, and/or use of spray rinsing for blind holes, etc. As far as the dark copper, we need a little more information to go on, but I will throw out the general advice that more cleaning is always good. You may be contaminating your copper with oils or buffing compound due to insufficient cleaning. Carbon treatment of your cyanide copper is an effective way to remove contaminants. Make sure your parts have a water-break-free surface before you attempt to plate them. 573

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