“We are presently applying a hybrid powder to a clear zinc (electroplated) cold roll steel. We’re experiencing a problem with paint adhesion on the finished product. Our zinc plating sub contractor, provides a bright clear (slightly blue) zinc finish and seals with a thin hexavalent chrome dip.(6 sec.) Our facility then applies powder to specification, bake @ 350 deg. F for > 15 min. Bake temperatures of 375 to 400 deg. F have a higher failure rate. Note: per customer specification, most product is powder coated on one side only! Opposite side must be conductive, offer salt spray protection >10 hours, and be cosmetically appealing. Recent efforts: zinc sub contractor, switched the chrome bath from hexavalent to trivalent and the powder supplier is presently working on a low bake ( 300 deg. F @ >10 min.) Questions: 1. What is the cause, is it the zinc, chrome, powder ! 2.Is there a solution, please note customer spec. 3.Are our “Recent change and projects” a step in the right direction.4. Is there a test method and/or equipment available, which can measure a surface potential adhesion prior to powder application.”
“Adhesion problems exist for two primary reasons; one is cleanliness of the surface prior to powder coating. Two is proper cure. If the part is not clean after the zinc plating or the powder is under cured after powder coating, then the film will not adhere properly to the substrate. People powder coat over zinc plated parts all the time, therefore, the problem is not related to the zinc, chrome, or the powder.
The solution is to properly handle the parts after zinc plating to reduce soils (hand prints, etc.) and/or fully clean the parts prior to powder coating them. Secondarily, all parts must be fully cured to adhere the substrate. I can’t tell you if these “recent” efforts make sense without seeing the parts. There is an ASTM test method that will check adhesion after the coating has been cured. Prior to powder coating the only thing you can check is the surface cleanliness (water break free & white towel testing).”