Powder Coating Q&A: Proper Coating for Steel Tubing

Q. I currently have a client who manufactures trampolines for backyard use. The structure is made of steel pipe and coated with black powder, which is applied for corrosion protection and appearance. The steel is a Chinese grade DC51D+Z, and the composition is an equivalent ASTM grade (the +Z denotes a zinc-plated coating). The current material is a smooth black polyester triglycidyl isocyanurate (TGIC) powder coating. Currently, a zinc-rich epoxy primer is used with a half cure, followed by the polyester top coat with a full cure. Do you have a recommendation for a powder coating/possible vendor?

A. TGIC polyester is a good idea for the topcoat material. I would use a high-grade epoxy primer without the zinc, unless you are blasting the steel before coating. Zinc in the resin is most effective when it can make intimate contact with the steel to provide the needed cathodic protection. On a smooth surface, not much of the zinc will contact the steel, so the benefit is marginal. The zinc will also make the coating more brittle in areas of stress, so a standard, high-grade epoxy primer may make it easier to avoid cracking. The supplier should be able to help you with the cracking problem.

 

Originally published in the April 2016 issue.