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Zinc Plate on Zinc Die Castings

How can we prevent zinc die cast parts from changing color after zinc plating?
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Q. We have a customer that requires zinc plate on a zinc die cast. The process seems simple and straightforward, but six to 12 months after the plating is completed, the parts change color, usually to a bluish color. Our customer finds this unacceptable and is demanding that we replace all of these defective parts. What can we do so this problem won’t happen again?— M.L.

A. I addressed this problem in one of my earliest columns. Here is essentially what I wrote at that time:

What you have is a diffusion problem, which can be common when one metal is plated over a different base metal but is rarely thought to be a problem when plating zinc over zinc die cast. The consistency of the zinc die cast is different from the zinc electroplate, and, as a result, the plated layer will diffuse into the die cast and produce a change in color.

The solution to this problem is easy. A diffusion barrier layer must be used on the zinc die cast parts. Most shops used a cyanide-based copper strike for this step, producing a copper coating that is least 0.3–0.5 mil thick. 

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