Plating Zinc on a Zinc Substrate

Q. Can you plate zinc on top of the zinc diecast? A customer recently has asked us if we are able to do this. We have sent a few samples and the customer seems to be satisfied with the quality, but before we take on this job I would like to know if there are any potential pitfalls. One of the “old-time” platers I know recently told me that the zinc plate may turn colors after a period of time. Can this happen?  M.S.

A. As you most likely know, plating of zinc diecasts does have a potential pitfall. If the casting is porous or if in the process of cleaning the pores are opened, the plated casting may have problems with staining and spotting.
Another problem that is more subtle is this: The chemical makeup of your zinc plate is somewhat different from the chemical makeup of the zinc diecast. This means the zinc electroplate can diffuse into the zinc diecast, usually resulting in a color change, which may or may not be acceptable to your customer. This process does not happen overnight and you may not get a call from your customer until six months later.
Technically, the solution to this problem is straightforward. You must plate a barrier layer on the zinc diecast prior to plating the zinc. Many plating shops use a cyanide-based copper strike for this step. Typically, the copper coating is at least 0.5 mil thick. This will add additional cost that you must take into consideration when quoting for this job.