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How often should you change a Zinc phosphate bath

Dear people, we have a supplier who makes the metal treatment (zinc phosphate coating) for us, we bond rubber to metal; since we were having problems with the bonding (rubber and adhesive do not stick on the metal), I started to visit our supplier on a weekly basis. Every two weeks, I found them cleaning and changing the zinc phosphate bath and they use to throw away all the sludge; I used to run a metal treatment bath some years ago and what I remember is that you don't change your bath that often and actually when you do it you keep some of the sludge to help balancing the bath.
How can we define a frequency of bath changing more accurately?

David Valdez
rubber to metal bonding - Reynosa Tamaulipas Mexico
April 12, 2011

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You don't keep any portion of the sludge and not more than 5% of the clear old phosphate bath back when you make up a new bath.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

April 29, 2011

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Hello,
Besides removing the sludge, it is necessary to do analysis for free acidity, total acidity and Fe and adjust the concentrations according to the phosphating chemical manufacturer's recommendations.

Ramajayam

Subramanian Ramajayam
consultant - Bangalore, India
December 31, 2011