Phosphating Dust and Overspray

Q. I am the president of a company that produces motorcycles in Asia. We pretreat our steel parts, frames and sheet metal with a zinc phosphate. Our paint is applied with electrostatic spray equipment.

After the zinc phosphate and passivating stages, we are getting a white, loosely adherent layer of a powdery substance on the phosphate surface, which we call “phosphating dust.” This dust seems to give us adhesion problems in subsequent coats and appearance and performance problems in the finish coat. Apart from sludging, high iron content in the previous de-rusting bath and poor rinsing, we want to know what other factors can cause this “phosphating dust.”

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We are also concerned about out transfer efficiency. We have weighed the overspray collected in our water wash spray booth and found, by our calculations, it contained 20 – 30 percent of the volume of paint that we spray. We are using an electrostatic spray gun and would like to know what percent the overspray loss should be under normal conditions using our type of spray equipment.

Do you have any suggestions on how we can reduce our overspray losses? B.M.

A. In addition the aforementioned causes in your question, a powdery residue on zinc phosphate surfaces can be caused by a chemical imbalance in the phosphate stage, a low processing temperature, a high process solution concentration as well as long processing times.

About your transfer efficiency: According to your measurements and calculations, you are getting a 70 – 80 percent transfer efficiency. That is very good for the electrostatic paint application method.

A higher transfer efficiency can be obtained using dip coating, flow coating, electrocoat or powder coating methods.

That would mean modification of your painting facility.