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.
Why is it important for you to know this?
The processes, chemicals and equipment, plus control and troubleshooting.
A review of available test methods, common applications and innovative instrumentation...