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Gold necklaces turn black after electrodeposition

We are a supplier of gold jewelry, our customers are complaining because one month after they receive their product the gold turns black, the electroplating technician does not know why, but I suspect that the bath is contaminated, would you please help me to find out how to use the hull cell to verify the bath or another way to find out if the bath is contaminated?

Bertha Bosch
Quality Manager - Naucalpan Mexico, Mexico
March 16, 2010

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First of two simultaneous responses -- March 18, 2010

Bertha,
The best way to tell if the bath is contaminated is to have the bath run for metallic contaminants. If you can't do it in your shop, your supplier should be equipped to do it. If they won't, an independent lab can. Normally they check for copper, nickel, zinc, iron, and lead. You also want to make sure the gold is at the correct concentration and the right deposit thickness.
Another option is to set up a fresh liter gold bath in the lab with the exact make-up (scaled down of course) as the bath on the wet floor. Plate a few small pieces, send them to your customer for approval. Hull cells are good, but leave too many problem possibilities (mainly for gold baths). Good Luck!

Mark Baker
process engineer - Malone, New York

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Second of two simultaneous responses --

What is under the gold? If silver, brass, or copper, then it migrates right thru the gold and looks black on top. You must have a diffusion barrier of either nickel or palladium.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

March 18, 2010