White Powdery Residue on Phosphated Steel

Question:

I run a sheet metal shop. We make steel enclosures for equipment manufacturers. After the enclosures are built, we apply an iron phosphate. After this phosphating process, prior to painting, some white residue forms after drying. I had it analyzed and it turns out to be phosphorus, iron and oxygen. What process would you suggest to prevent such white residue formation? We are concerned about quality issues. Thank you. R.D.

Answer:

Congratulations, you have just described the analysis of the iron phosphate crystal! You have every reason to be concerned about quality issues, because too much of the powdery residue could interfere with paint adhesion. Rinsing usually removes the residue before it dries. Perhaps you are not rinsing thoroughly enough after the iron phosphate stage. I don’t want to get into the chemistry of your phosphate bath. However, you should check its parameters including concentrations, total and free acid, as well as bath temperature. They could be out of spec. Chromic acid final rinses were used to remove this residue, but this material has fallen into disfavor, and we don’t like to use them anymore. Perhaps you should try using a non-chromate final rinse.