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Powder on Zinc-Plated Steel

What is the correct process for powder coating after zinc plating on steel?

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Q. We are struggling to establish the correct process for powder paint on top of zinc-plated steel. We subcontract CR4 for zinc plating and passivation. We tried to powder paint the steel immediately after plating and passivation, but observed a gassing effect after painting. What is the correct process for powder coating after plating? C.C.

A. It is possible to powder coat over zinc-plated steel, but it can be challenging. The plating and passivation process creates good corrosion resistance, but the surface structure is not very compatible with the powder coating process.

The particular zinc solution used is one critical factor. The zinc may be an alkaline product, or it may have additives that leave some trapped gases. The zinc concentration also may be a factor; a higher zinc concentration can be more problematic. The passivation is likely to be a chrome solution that will never work with powder. It is not likely that you will be successful with this particular plated surface.

You could use a different preparation method such as zinc phosphate or powder primer to get the corrosion resistance. If you must use the zinc plating, you should shop around for a different source for the plating. In general, I would look for an alkaline zinc plate instead of an acid-based zinc plate. There are no guarantees, but zinc plating can be successfully powder coated if you have the right plating surface. Sample a few different zinc plating sources and you may solve the problem. 

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