Powder Coatings vs. Liquids

Q. We are an OEM for the mining and oil field industries. Our steel fabricated equipment sees salt water exposure and other wet and dry corrosive environments. We offer two basic paint systems. Both use a two-part zinc-rich primer, which is then topped with either a two-part epoxy for severe corrosive environments or a two-part urethane enamel. We blast to a near white metal finish using steel shot. Of course the epoxies fade and the urethanes tend to fail quicker due to corrosion. We also offer a three-part coating system—zinc, epoxy and urethane on top—but it’s very expensive.

We are considering adding a small parts coating line due to cost and volume, and would like to consider a powder epoxy system. How do the powder epoxies compare to high-end wet epoxies? Also, how do the zinc phosphate treatments for powder coating compare to a zinc primer? Although we doubt we can beat the zinc/epoxy system we currently have, is it possible properly pretreated steel with a powder epoxy may outperform a zinc/urethane system? Is washing as good as blasting? T. S.

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A. Having formulated and tested both powder and liquid coatings and recommended both to our operating divisions when I had a real job, I feel qualified to answer your questions. Epoxy powder coatings compare favorably to high-end liquid epoxy coatings. However, zinc phosphate pretreatment systems do not provide the same degree of corrosion protection to steel as zinc-rich primer systems. An epoxy powder coating over zinc phosphate will not provide the same corrosion protection to steel as a liquid polyurethane over a zinc-rich primer. About the last question, “Is washing as good as blasting?” The answer is that it depends on the materials used in the finish system.

For the type of product you manufacture, an excellent source for information is the Steel Structures Painting Council (SSPC). They publish Painting Manuals in a couple volumes, Good Painting Practice and Systems and Specifications. These manuals outline materials and application procedures for coatings used in corrosive environments. I recommend you contact SSPC and get the benefit of their latest information. The organization may have a finishing system specifically designed for your products. For more information, contact the Steel Structures Painting Council at the Mellon Institute. Their address is SSPC, Mellon Institute, 4400 Fifth Ave., Pittsburgh, PA 15213, phone 412-281-2331.

The alternative would be to test the corrosion resistance of the candidate pretreatments and coatings on your steel. If you can’t do this in-house, testing services are available from coating suppliers and testing laboratories.