Carol,

We also powder coat over zinc with yellow chromate, and have problems with Blistering and out-gassing. The blistering in most cases seems to relate to a problem with the plating, poor preclean, or plating too thick. and can be seen easily by removing the powder coat and checking to see if the plating is still intact. The out-gassing is different, the powder coating has what appears to be tiny eruptions in the surface from the gas exiting the the part and bubbling in the surface of the powdercoat while it is in it's liquid state. When the powder coat is stripped on these parts the plating still looks good. Although I do not have anything conclusive, we have gone as far as have a nickel strike (2-3 microns) put on the base metal and then put the zinc over top of it. This is just my opinion but I think this levels the surface of the base metal filling in any tiny voids that may trap moisture that would be released when the part is heated. Another cause of this out-gassing can be not enough time between plating and powder coating, causing the moisture in the chromate to pass through the powder as it is in the liquid state. We have had some success with running the parts through the cure oven before we powder coat, thus dehydrating the chromate.

Hope this helps.

Regards,

Kevin Davis
- Versailles, KY, USA