We plate zinc diecasts with zinc. The parts look great after plating but turn blue after being stored for a period of time. What is going on, and how can it be prevented? J.B.
What you most likely have is a case of diffusion. The zinc electroplate diffuses into the surface of the zinc diecast. When this happens you get a bluish color. This usually happens months after the plating has been completed.
The best solution is to use a barrier layer to prevent this diffusion. A copper cyanide based strike bath classically has been used to deposit this barrier layer. The newer non-cyanide alkaline copper plating baths can also be used. A minimum thickness of 0.3 mil is recommended for the copper layer.
These non-cyanide plating baths are available by going to the Suppliers page at www.pfonline.com or looking in the Products Finishing Directory and Technology Guide under Plating Processes, copper.