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

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.