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What is Zinc phosphating?

Dear All,

It's all very confusing. Our client wanted Zinc Phosphated studs and nuts for his project. We told the same to our supplier, but now the client is saying that he has electroplated them and not done zinc phosphating. We are in a fix as we are not getting any info on this thing. Could you please help in enlightening me what is Zn-phosphating and how it is done. and what color would be studs after they have been coated.

Nitin Dutt
- Delhi, India
2007

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Hello, Nitin. In brief, phosphating is a conversion coating that provides some key for adhesion of paints or lubricants, and delivers other beneficial properties to the surface as well. It will be a matte grey to charcoal color probably, but could be slightly pinkish or greenish -- but definitely not metallic looking.

If you need a deeper insight, there are dozens of letters on line here that address your question, and almost every metal finishing book covers it as well.

But this doesn't seem to be the heart of the problem. Your customer must issue specs to you for what he wants, and if you farm the work out you must issue the specs to your supplier. Providing zinc plating in lieu of phosphating could be catastrophic, and if those fasteners are high strength they must be discarded.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007