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Sludge problem in phosphating

Dear sir,

Problem is that I do iron phosphate and zinc phosphate also. Problem is that when we make up new one, after a few days sludge develops in the bath in the zinc--much more than the iron bath . How do we remove it? Why is the sludge produced? Is it a pH problem? What is facts of pH & how we reduce pH? Some times bath colour turns black why I don't know. In the zinc phosphate we also use catalyst. Please tell me good suggestions.

Your sincerely,

Shaikh Rashid Mehboob
- Karachi, Pakistan

Zinc phosphating does tend to produce a lot of troublesome sludge, Shaikh. Many installations use a hopper-bottom tank to decant and thicken it. You can try the phosphating chapter of the Metal Finishing Guidebook. But there are whole books about phosphatizing, and you may wish to get hold of one:

Samuel Spring's Preparation of Metal for Painting [affil link on Amazon ] is a good book that I can recommend.
Another book that I've heard good things about, but which I don't personally have, is "Phosphating of Metals" [on Amazon or AbeBooks]by Rausch.

Good luck.

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

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