Q: We are a manufacturer of industrial hardware that is typically used outside. Currently, we plate our steel parts (stampings and castings) with yellow zinc dichromate and then send them for powder coating.
We have had some adhesion and rust problems and wonder if we are doing the right thing here. The plating process seems to clean the products better than the powder coater can and provides a secondary barrier to seal the part from the elements. Do you think we are doing the right thing to best protect our steel hardware items?
A: Powder coatings generally have sporadic problems adhering to zinc dichromate. I’m not sure if it is related to the varying thickness of the zinc dichromate, but it is a recurring problem. The zinc plating process cleans the metal really well but may not be the best surface for the adhesion of a powder coating. In addition, part handling after plating and before powder coating may introduce soils and oils onto the surface.
The ideal means to achieve excellent powder coating adhesion to steel is to skip the zinc chromate and find a powder coater with a high-quality pretreatment system that is used just prior to powder coating. A multi-stage iron phosphate system is good, while a multi-stage zinc phosphate is so good that the automotive and appliance industries use it.
Another piece to the puzzle is keeping the parts from corroding between the time they are fabricated and when they see the pretreatment system. This is a matter of logistics, environment, and possibly temporary protection.
This probably makes you think about investing in the installation of your own pretreatment and powder coating system. I would give it some serious thought during your long-term strategic planning.