Iron Phosphate Over Zinc


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Q. This is a comment on the July column about applying iron phosphate over zinc. I agree with your answer. A normal iron phosphate with a little added fluoride will etch the zinc surface a bit, but will not apply any coating that has a “morphology” unless the zinc content of the bath reaches 300– 400 ppm of zinc. At those levels of zinc, you may start seeing small nodules or flowers of zinc phosphate appearing on the panels. They will be few and far between. If one processes enough galvanized through an iron phosphate bath, zinc will build up in the bath and this will become a waste disposal problem. At least this was the case when I was working on this about 20+ years ago. As for iron phosphate being the cause of the extinction of zinc phosphates, I don’t think that will happen. However, the new zirconium-based pretreatments may well be the death knell for zinc phosphate pretreatments used in paint base applications.
As always, it’s been a pleasure reading your column. N.F.


A. I thank you not only for your kind comments but also for sharing your expertise with me and the readers. I always welcome this type of technical input because it has enriched Painting Clinic over the years. Furthermore, it enables me to keep learning. If I don’t learn something new every day, I feel cheated.