Chemical Reaction Between Powder and Galvanizing

Q. We have specified powder coating over galvanized metal surfaces for a gate. The contractor recommends not galvanize the gate because the final product will have dots due to chemical reaction. A.S.

 

A. The “dots” that your contractor is referring to are not caused by a chemical reaction between the galvanizing and the powder coating. They are, however, pinholes in the coating that are caused by the zinc galvanizing out-gassing when the powder coating is cured in an oven. What happens is the galvanizing emits a gas when heated to the cure temperature of the powder coating. This gas has no problem passing through the coating while it is still in its liquid stage. However, once the coating starts to solidify the gas pressure builds to a point where it will eventually pierce the coating causing the pinhole. Purchasing an “anti-out-gassing” powder coating formula or purchasing a low-temperature cure powder formula can reduce the effects of this problem to a point they become acceptable. Good luck.