We have recently obtained a contract to plate zinc diecast parts with zinc. Our customer notified us a few days ago that some of the parts that have been stored for a number of months are turning sort of a “blotchy blue.” Needless to say, my customer is somewhat upset. What is going on? How can we prevent this? M. J.
Most likely, you are facing a problem with diffusion of the zinc deposit into the diecast. Based on the questions I receive on this, it appears that Zamac 3 is a particularly bad actor. This is a slow process and does not show up immediately.
There is no simple solution that I am aware of. I do not think that the blue color will adversely affect the quality of the deposit except for appearance. If your customer cannot live with this then you have to consider using a barrier layer. Copper is the material of choice and because of the zinc diecast substrate, you must use either an alkaline non-cyanide or a cyanide-based strike. I recommend a thickness of 0.00025 –0.00030 inch.
Vendors of alkaline non-cyanide copper plating baths can be found at www.pfonline.com.