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"Low iron content in Zinc Phosphating"


Q. I process metal screw through a zinc phosphate immersion bath followed by an oil dip but lately some of the screws are coming out with a white powder on them which if rubbed off shows a beautiful part underneath. This is normally the case when the tank is in poor condition but lately, it can be when the tank is brand new. Could there be too much phosphate present?

Thank you,

Carrie Habinski
Environmental Technician - Beaverton, Ontario, Canada


A. The white powder is nothing but oxide products of Zinc phosphate. You may require to increase the iron content in the bath, so that the oxidation will be retarded. In a fresh bath you will have almost no iron. You may add iron powder or iron wool to increase the iron content to solve your problem.

v g rajendran
V. G. Rajendran
- Trichy, Tamilnadu, India


A. It sounds as if this is sludge and hard scale from the Zinc Phosphating bath, if the bath is fairly new the sludge /scale could be coming from the tank sides, pipework, heater tubes etc, it is quite normal to descale every one or two years to control this build up,rinses esp those after the phosphate tank also get this build up. I would not do anything to the bath itself, as this would again tend to cause more sludge to form .

A Wagstaff
- Lincs, UK

March 30, 2013

Q. We are struggling for an answer to a problem with Zinc Phosphate. We are a coating company that specializes in Zinc Phosphate coating. It is an alkaline bath. We cleaned the tank which hold 600 gal of water. We built a new zinc bath and we cannot get the iron levels to come up. We have added over 300 steel wool pads to bring the iron up and yet the levels are below the range that the Zinc should operate at. That range is 1200-1800 mg/sq ft.

[name & town rescinded at writer's request]

April 8, 2013

A. Hello,

What do you mean when you say you have an alkaline zinc phosphate bath? I believe all phosphating baths are acid, alkaline zinc baths are zinc plating baths, not phosphating ones. Alkaline may be the degreasing tank, but I think not the zinc phosphate.

Before immersing the steel wool, try immersing them in the pickling bath a few seconds. Then rinse very well in tap water and put them in the phosphating bath. The wool may be activated by the first acid immersion.

Hope this could help!


Daniel Montañés
- Cañuelas, Buenos Aires, Argentina

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