What is the proper powder coating material to go over zinc dichromate?
Q. We currently have steel parts plated with zinc dichromate by a plating house. We then powder coat them with a polyester powder, but, upon inspection, typically reject the parts due to small pinholes in the powder-coated surface (+90-percent rejection rate). We believe that these small pinholes are the result of outgassing from the zinc dichromate plating.
Our plating source has not been able to fix the issue on the plating end, although when we asked the powder supplier to reformulate the powder there was a slight improvement. What do you recommend? Are there any other plating materials that could provide the same amount of protection as zinc dichromate? Are we using the proper powder coating material to go over zinc dichromate? K.H.
A. We have received two questions this month that express the same problem—outgassing of zinc plating when applying powder. The answers to the previous question apply to you as well, but I will add some additional comments since this is a common problem.
Zinc dichromate is not a great undercoat for an organic topcoat that requires a thermal cure. The chemicals used in the process can evolve when subjected to high temperature. You could do some preheating of the part to relieve some of the gas, and the powder formula can be engineered to improve the gassing issue, but it is not likely that this will fully resolve the issue.
I assume you are having the surface undercoated for corrosion protection. There are several treatments that will give you excellent corrosion resistance and work well with the powder topcoat. Electrocoat is one good option. The surface is zinc phosphated and then electrocoated, and the corrosion resistance is excellent. The powder can be readily applied over the e-coat. Zinc phosphate alone is another option. Not as robust as e-coat, but a very good corrosion inhibitor. A chrome conversion coating is also an option, but you have the same risks with chrome that you have with zinc plating. You have to have a surface that is treated correctly for the thermal cure process. A powder primer is another good option. This will add to the thickness, but it will hold up very well in the field.
If you stay with the zinc you will need to test different suppliers to find one that works with your powder.
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