What are some of the various causes for discoloring in gold plate?

What are some of the various causes for discoloring in gold plate?


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Q. What causes gold plate to discolor? –V.S.

A. There are a number of possible causes for gold plate to discolor. I can give you some general information, but since you did not specify the type of plating bath or the base materials, I cannot give you specific recommendations.

For classic gold cyanide type plating bath some possible causes of discoloration are as follows:

Symptom

Probable Cause

Deposit has a reddish or brown hue

Gold content is low

Cyanide content is low

Temperature is low

Current density is not correct

Deposit is reddish in color

Copper present in plating bath

Deposit is greenish in color

Silver or cadmium present in plating bath

Deposit is blackish in color

Lead or arsenic present in plating bath

 

Another possible cause of discoloration is that the deposit is too thin and porous. For example, if gold is plated over silver and the gold layer is porous, some of the silver can migrate through the pores to the surface and form silver sulfide.

Another cause of discoloration can be the gold is plated directly over a base material that can migrate through the gold layer. Gold applied directly over copper is one such example. To prevent this, a barrier layer is plated on the base layer and gold is then plated over the barrier layer. Classically, nickel was used as a barrier layer. Another material used as a barrier layer in decorative gold plating is white bronze.

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