Zinc Phosphating: Ideal Process Sequence?

A. Hi, Sathesh. One variation of the 7-step process is: Alkaline clean, rinse, rinse, zinc phosphatize, rinse, rinse, chrome-free post-dip rust preventative. But the reason there are whole books on the subject is that there are countless variations on this process and things to consider. The fastest path to a solution is probably to contact a vendor of zinc phosphating processes or a consultant, who can sit down with you and develop a good solution to your situation. Good luck.

Regards,

A. I foresee a problem with quality work. The secret is to have the phosphate tank loaded whenever it is at working temp. A few 5 kg pieces will be a tiny load for the tank and the 1500 kg load will be a problem to keep the tank at operating temperature. A load that large will require a massive heating system or very large quantity of liquid or both. The large quantity of liquid is a problem with tiny loads trying to keep the acid ratio in balance.
It is difficult to overwork a phosphate tank if you can maintain the temperature and have a reasonable amount of liquid per part surface. Conversely, when you under load a tank, it tends to get sick in a short period of time.

A. Go for a proper 7-stage Zinc phosphating process. You shall never find a better process than this.

This process is claimed to be the most suitable one, with a history of success!

The process is:

Tank 1) Degreasing
Tank 2) Water wash
Tank 3) Derusting
Tank 4) Water wash
Tank 5) Surface Activation
Tank 6) Cold/hot Zinc Phosphating
Tank 7) Passivation.

There are parameters for all tanks. For phosphating in a cold process the parameter is that the total acid should be 36 points for a 10 ml solution titrated with n/10 NaOH, with phenolphthalein as indicator until a Pale permanent pink colour is achieved, and free acid should be between 1.5 to 2, titrated with bromocresol green [on eBay & Amazon] as the indicator.