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topic 31949

Zinc Phosphating process control

A discussion started in 2004 & continuing through 2017


Q. Hi,

In a flow line process involving Phosphating, I understand that the concentration of Zinc Phosphate will decrease constantly as production progresses.

To Achieve the correct protective layer Thickness of Zinc, the concentration of zinc phosphate should be maintained in a certain range. This will have to be obviously measured. How can I conduct this measurement on line. Is there any measuring meter that can be purchased from the Market for this purpose.

Farrukh Aamir
buying house - New Delhi, New Delhi, India

Phosphating Metal Pretreatment
by Freeman

Phosphating of Metals
by Rausch

1st of two simultaneous responses--(2004)

A. There are basically four process parameters you need to monitor in a Zinc Phosphate coating operation. They are: Temperature, Accelerator, Free Acid and Total Acid.

These process parameters are specified by the manufacturer of the chemical you are using, along with instructions on how to measure these parameters. There are several Zinc Phosphate coating products out in the market and they all have different formulations specific to the kind of Zinc Phosphate coating desired and they all run at different process parameters.

So the first step I recommend is to contact your chemicals supplier and request this information, and if they do not have it, I recommend switching chemical supplier companies until you find one that provides the customer support you deserve.

Jesus Quinonez
- Riverbank, California, USA

2nd of two simultaneous responses-- (2004)

A. Dear Friend,

There is no such instrument available in the market.

Titration is the only method to check the concentration. But there is a method to keep your concentration constant throughout your production hours: Please calculate the area you are processing in a day. Take before production the pointage of the bath and after the production. See the difference, get the calculated corresponding replenishment quantity from your chemical supplier and that much daily you have to add.

There is a method to add the required quantity:

1. If your line is small,put the required quantity in a suitable plastic can and add the chemical like Doctors give saline to patient with calculated flow per min. Keep the flow rate in a such fashion that the required quantity for the day will drain in the tank till the production is on.

2. If your line is big, go for the auto dosing tanks mainly for degreasing and phosphating tank.

Hope this will help in your purpose.



Suhas S. Mahajan
- Pune, India

August 14, 2014

Q. Kindly advise me for this:
My company's zinc phosphate coating is not perfect. So many places are not covered with zinc phosphate coating.

Phosphate tank is like a mess: it is a color like dark green and so much sludge is there -- please advice me to solve this.
I would very much appreciate a person who concentrates on my problem! As soon as possible I want to solve this.

Madhushanka nihathamanee
- galle, sri lanka

September 11, 2014

A. Hi Madhushanka,

I think this question has been many days on line and no answer; maybe you have solved your issue. If not, please view some of the answers above first. To solve your problem you should give us:

- Phosphate total acid points: 0,1 N sodium hydroxide milliliters per 10 ml sample with phenolphthalein as indicator.
- Phosphate free acid points: 0,1 N sodium hydroxide milliliters per 10 ml sample with bromophenol blue as indicator.
- Iron content: in g/l sample by permanganate method.
- Temperature.

These are your process parameters IN the phosphate tank, you should maintain your degreasing and pickling process tanks in the correct temperature and concentration just as your phosphate process tank.

Hope this helps, best regards!

Daniel Montanes
- Cañuelas, Buenos Aires, Argentina

October 5, 2014

Q. Hi, Sirs:
I work a zinc phosphate bath in the iron side, 75 °C, 50 TA points, 7 FA points, Iron 01-0.2% for fasteners, but they leave the bath (15 minutes) with good coating but also with with a lot of white powder. Can you please tell me how to avoid such white powder?
Thank you.

victor delgado
- bogota, colombia

April 7, 2017

A. White powder formation is due to excess sludge accumulated at the bottom of tank. Filter the solution and remove sludge,there should be no problem.

Mitesh Bhayani
- mumbai, maharashtra, india

February 25, 2015

Q. Please tell me about free acid to total acid ratio. I know how to calculate TA/FA by titration, but how do I control it in a Bath?

Irshad Kabir
- Lahore , Punjab , Pakistan

How to Maintain Phosphate Accelerator Pointage during Continuous Running of Circulation Pumps

September 25, 2017

Q. Dear Sir,

We have to Run our Phosphate Chemical circulation continuously 24*7,but during continuous circulation our phosphate accelerator pointage goes down and effecting problem on car body. Please provide the solution for this.Is really pointage goes down after continuously running.

We have to Continuous run our phosphate chemical to prevent slurry chemical choking in heat exchanger (PHE).


kunal soni
- greater noida,india

September 27, 2017

A. Hi Kunal!

Do you adjust your accelerator pointage by calculating the total volume of your phosphate bath? If you don't add the proportion you need in all the bath, then you won't get the result you want.

Please verify if you are taking all the volume in account. If you do, then you have something in there bringing you accelerator down (I think THAT is much less likely).

Tell us more about your process and parameters, and maybe we can help you!


Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

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