Letter 36052

Zinc Phosphating [Ontario] 

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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

The white powder is nothing but oxide products of Zincphosphate. 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, Team Leader Manufacturing,
- Surface Treatment specialist - Trichy, Tamilnadu, India

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

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