Conversion from Iron to Zinc Phosphate

Q. I was curious about the hazards of welding steel that is pre-treated with zinc phosphate. I currently run a powder coating line that pretreats with iron phosphate. We assemble and paint DOT air tanks, but due to the requirements of an interior wet paint on the inside of the tanks, I am required to pretreat all of the blanks and heads prior to assembly. I am currently interested in converting my washer from iron phosphate to zinc phosphate but I was concerned with the hazards of welding over the pretreated steel. After the tank is assembled, I still have to wash it again before we can powder coat it. I was also wondering if you can “double treat” with zinc phosphate, since the product will still have a layer of phosphate on it prior to wash. Thanks. —R.W.

A. I cannot comment on the potential emissions from welding the zinc phospating surface, but would recommend industrial hygiene sampling and testing. However, before converting, you should realize that you may not be able to simply change over to zinc phosphate in a line designed for iron phosphate. The zinc phosphate process typically requires additional pretreatment steps such as activator and rinse steps prior to the zinc phosphate, necessitating additional tanks in your system. You should also be aware that the zinc phosphate system requires more maintenance of the chemistry and sludge generated in the phosphate tank. 

You can run parts through the system a second time, but the phosphate build-up is self-limiting, so you will not add significant coating weight.