Powder Coating Clinic: Adhesion Problems on Chromated Zinc Plating
Q. We have had adhesion issues when powder coating over zinc-plated aluminum. The aluminum had been nickel-flashed and zinc-plated with a hex-free chromate to meet RoHS compliance. Do you have any suggestions as to why the powder will not adhere properly to this surface?
A. This is something that has come up before. I think that the “hex-free chromate” is the culprit. That material is used to passivate the zinc plate so that it won’t white rust. The problem is that it does such a good job that it also passivates the zinc with respect to paint adhesion. The passivated zinc will not chemically react and corrode (as intended), nor will it chemically react with the molten powder and provide a bond. The solution to the problem is to specify that the zinc-plated surface be passivated with a rust-preventative oil that can then be washed off in the first step of pretreatment before powder coating. It may be possible to create some surface roughness (sanding) for a mechanical bond but that is not ideal. Eliminate the chromate step and you should be all right.
Originally published in the January 2017 issue.
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