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Tarnishing of zinc plate under lacquer

We zinc plate steel & copper tube, (alkaline non-cyanide zinc solution), then passivate with a trivalent chrome solution (blue). Finally the parts are coated in a clear acrylic lacquer and cured.

We manufacture c25,000 of these per week, and every so often we get 5-10 returns where the zinc has tarnished underneath the lacquer. The colour is black, as if someone has sprayed it with a light coat of black paint before lacquering. The main point to note is that this only happens where the substrate is copper - never on the steel. Any ideas?

Paul Beel
- Durham, UK

The copper is migrating into the zinc.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan

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