Solving Discolored Gold Plate Problems
My gold plating is discoloring. Do you have any suggestions as to what might be causing this problem?
Q. My gold plating is discoloring. Sometimes the discoloration shows up immediately after plating; in other cases, it appears after the parts are packaged and shipped to my customer. Do you have any suggestions as to what might be causing this problem?
A. There are a number of possible reasons why your gold plate discolors. For example, if gold is applied directly to copper, the copper will discolor the gold layer when it migrates through the gold layer. This type of problem is easily solved by plating a barrier layer on the copper prior to plating with gold. Nickel is often used as a barrier layer.
Gold may also become discolored if the plating solution is contaminated with other metals. Sometimes this is a desired effect, particularly in jewelry and other decorative applications.
Another cause of discoloration is if the gold layer is too thin and porous. A thin, porous layer of gold plated directly over silver can discolor over time. The silver will migrate through the pores in the gold and form silver sulfide on the surface.
Q. We plate a thin layer of gold over silver parts. To prevent migration of the silver into the gold plate, we plate the silver with a nickel strike before gold plating. We are finding that the gold plate tends to stain after a month or so of use. What is causing this problem?
A. There are a number of possible causes for the discoloration. I would start by looking at the gold plating bath. Is it clean and free of tramp metals? If your chemistry is OK (cyanide concentration, gold concentration and so forth) look for tramp metals such as copper, silver, and cadmium. The next step is to look at your nickel strike bath. Tramp metal contamination of the strike bath might also cause this problem. Lastly, investigate the thickness of the nickel strike. If it is not heavy enough, it may be porous and will allow the silver to bleed through and cause the gold to stain.
Originally published in the July 2015 issue.
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