Silver Pitting and Black Spots

Q. We use a silver plating solution that contains 16 oz/gal of potassium cyanide, 9 oz/gal silver and 24 oz/gal of carbonates. The deposit is approximately 0.005–0.008 inches thick. After plating, our parts have pits and black spots. What can be done to prevent this problem? C.S

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A. The first thing you should investigate is your filtration on the plating bath. You are filtering the bath, aren’t you? If your plating solution has particulate matter in it, your silver deposit will have pores. This will cause the “spotting out” pitting that you have observed.

You do not mention whether you are using a brightener in your bath. If you aren’t using one, you might consider it. You should also consider using an anti-pitting agent in the bath. Talk to your chemical vendor about these two items.

You should get your carbonate concentration under control. The potassium cyanide silver bath should contain no more than 14–16 oz/gal of carbonates. It is not uncommon when carbonates are above this value to have pitting problems.

You can use barium or calcium hydroxide to bring down the carbonate concentration. When using barium or calcium carbonate to reduce the carbonate concentration you must thoroughly filter the plating solution to remove the precipitated carbonates. If you do not do a good job filtering, you will end up with rough deposits and more porosity.