ZINC ON ZINC DIE CASTINGS
We plated a large number of zinc diecasts with zinc and a few months after the plating the parts changed to a bluish color. What is going on?
Q. We plated a large number of zinc diecasts with zinc. The process used is straightforward but we now know that a few months after the plating the parts changed color—usually to a bluish color. What is going on? A.R.K.
A. I had a similar question a number of years ago. Here is how I answered then:
What you have is a diffusion problem. This is well known when one metal is plated over a different base metal, but is rarely thought to be a problem when plating zinc over zinc diecast.
But the consistency of the zinc diecast is different from the zinc electroplate, and as a result the plated layer will diffuse into the zinc diecast. The result of this diffusion process is a color change. The process requires time.
Fortunately, the solution to this problem is easy. A diffusion barrier layer must be used on the zinc diecast parts. Most shops used a cyanide-based copper strike for this step. The copper coating should be at least 0.3–0.5 mil thick.
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