Powder Coating Hot-Dipped Galvanize

Q. I need some help.  Do you know the procedure for powder coating galvanized steel? C.L.

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A. First, it’s important to make sure that the galvanized surface was not quenched by the galvanizer. Quenching is a commonly used process in the galvanizing industry where either water or chromate solution is sprayed on the parts after they exit the molten zinc to accelerate cooling and slow the reactivity of the zinc to prevent excess oxidation. Quenching can cause massive adhesion failure or craters in the coating. Galvanized materials should be coated as soon as possible to limit oxidation, ideally within 12 – 24 hours of being galvanized, but often this is not possible.

Like all metals to be coated, the galvanized surface must be free of oil, grease and dirt. Surfaces can be cleaned with chemicals or with abrasion. Mild cleaners can be effective at removing soils without creating unwanted smut from reaction with the zinc. Heat is helpful (120°–140°F) and time of exposure should be long enough to give the cleaner a chance to loosen soils from the porous zinc surface. Abrasion is desirable because it can remove soil and also remove oxidation. A light blasting step that is referred to as a sweep blast or brush blast is often used to provide superior adhesion to the galvanized surface to SSPC-SP7 standard. Care must be taken when choosing what type of media to use. The blast process requires a trained technician familiar with the media and pressure device and who knows how to remove unwanted soils and oxidation without removing too much of the zinc coating.

An epoxy powder coating is often used to enhance corrosion performance. Primer coats should be sprayed within 12 hours of brush blasting the surface. Primer should be applied at the manufacturer’s recommended film thickness. The primer coating is usually cured for a brief time before the topcoat is applied and then fully cured after the topcoat is applied. The precise process can be worked out with the supplier of the coating to ensure that the topcoat is compatible with the primer and the right cure is used for each coating material.