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Improving the Composition of our Zinc phosphating process


Q. To whom it may concern, hello,

We would like to introduce ourselves as a chemical company. We are in the process of zinc phosphating with Nitric acid, phosphoric acid and zinc oxide and the major problem we face is due to the low salt spray resistance. We think the non-uniform coating causes this problem. How we can increase the time of salt spray resistance?

Best regards,

Sara Abiri
- Tehran, Iran

Phosphating Metal Pretreatment
by Freeman


A. Dear inquirer,

Your problem may be because of improper ratio of basic three chemicals and it may further require some special additive like nickel, etc.


Navin Maheshwari
- New Delhi, India


A. Hi,

Give a few more details. Are you processing by dip and is this prepaint or an oiled finish you are referring to? The formulation is far too simple for high performance.

Roger Bridger
- Croydon, U.K.

March 3, 2009

A. Dear Ms. Abiri,

How do you do "Salt-Spray" test? with or without primer (E-Coat, powder,...)?
As you know doing "Salt-Spray" test only on phosphated plates is meaningless!

You should do another test methods such as: "Cyclic Corrosion" test, "VDA",...

Best Regards,

Saman Saatchi
- Tehran, Iran


Q. Dear Mr Maheshwari and Mr Bridger,

Thank you for answering. Our formulation consists of : Phosphoric acid : 48% Nitric acid : 11% Nickel Carbonate : 1.01 Copper Carbonate : 0.01 Zinc Oxide : 14.70 and the accelerator is NaNO2. It is processed by dip and for prepaint. With above formulation with 16 g/L Zinc Phosphate the results were: Free acid : 2 Total acid : 12 We need to decrease the amount of free acid for less than 1 point , but whenever the amount of FA comes down (less than 1 ) precipitation occur in concentrated phosphate. So how can we decrease the amount of free acid without precipitation and having a good salt spray after phosphating? Our actual salt spray with above formulation is: 23 hours.


Sara Abiri, [returning]
- Tehran, Iran


A. Dear Mr Sara Abiri,

From your reply I must assume you are sealing the phosphate with an oil. If you not using any seal at all then salt spray resistance will be minimal. The ratio of free acid to total acid will be temperature dependent. The higher the bath temperature the higher the ratio. Thus at 50 deg or less a 10:1 ratio is possible but on heating to 70+ the ratio will drop to 6:1.

If you want a heavier coating you could try operating on the iron-side (no accelerator. Temp will need to be above 75 deg C. If you require a finer crystalline coating you will require a Ti activated predip or 2 g/l oxalic acid predip.

Roger Bridger
- Croydon, U.K.


Q. Dear Mr Bridger,

Thanks for your information. Would you please let us know your opinion about the mole ratio of Nitric acid to Phosphoric acid ? Any advice that could be given would be much appreciated.

Best regards,

Sara Abiri, [returning]
- Tehran, Iran


A. Dear Ms. Abiri:

a. If you do not use grain refiner before the phosphating stage, you won't succeed to overcome the 24h Oily phosphating requirements.
b. 24 Hours salt spray is the requirement of heavy zinc phosphate coating Military standard, the coating weight of the Zinc Phosphate might exceed 10 grams per sq meter. You must check phosphating coating weight to see if you fulfill the requirement.

yehuda blau
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel


A. Sorry for the delay. If iron containing a 1:1 ratio of phosphate:nitrate is OK. For accelerated baths raise 1:2 to encourage autocatalysis of the nitrite accelerator. Your nitrate content is far too low. Careful with Cu as over-addition will adversely affect corrosion resistance. Just stick with nickel!

Roger Bridger
- Croydon, U.K.


Q. Dear Mr Bridger,

Many thanks for your answering. unfortunately we cannot understand the meaning of 1:1 and 1:2 each ratio refers to which component in the phosphate solution? Which section is in the phase and which one is in denominator? Any information on this topic will be greatly appreciated.

Thanks and best regards,

Sara Abiri, [returning]
- Tehran, Iran


A. Hi again!

The nitrate content is usually higher than the phosphate content. At 70 °C some nitrate will produce nitrite autocatalytically and prevent the process building up in ferrous iron. If the process switches over from nitrite containing to iron, higher temperatures and longer process times will be needed to produce complete coatings, which will be of a higher weight.

As an example of ion contents for a 20 pt bath:
Zinc 6 g/l phosphate as P2O5 8 g/l Nitrate as NO3 15 g/l


Roger Bridger
- Croydon, U.K.

March 15, 2012

A. Dear Ms. Sara. You had better use low zinc phosphating solution.

Reza Moumivand
- Tehran, Iran

July 13, 2016

A. To respond to the first question, your formula is missing a calcium additive. read up on the advantages of Calcium modified Zinc Phosphate processes.

Andre De Graaff
- South Africa

Free acid vs. total acid


Q. dear sir,


I would like someone to describe to me what does it mean the free acid and total acid, and how can I adjust the ratio of free acid to total acid at particular point. ALSO IN THE PHOSPHATING PROCESS WHAT HAPPENS WHEN FREE ACID:TOTAL ACID RATIO CHANGES.

Best of all,



A. Dear,

FA, TA are important factors in zinc phosphating. FA is 0.6 to 1 point, TA is from 14 to 22

Hatem Shaker
- 10th Ramadan, Egypt

Cold Phosphating Formulation

June 12, 2009



September 11, 2010

A. Dear Mr. Nitin Patel,

There are two problems is in the formulation:

1. The basic percentages in the formula are imbalanced.

2. For cold phosphating additional chemicals are required to be used to give the bite in the absence of the heat.


Vijay Anand
- Chennai, Tamil Nadu, India

December 13, 2010



June 26, 2011

A. Dear Mr. Praveen,

The problem is in the ratio of the basic ingredients. Please inform what is the percentages you are working with.

Vijay Anand
- Chennai, India

June 3, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I want to know the composition of zinc phosphating material used for pre treatment in powder coating.

Vijaya Sounthar
Powder Coating - Coimbatore, Tamil Nadu, India

June 6, 2011

A. Hi, Vijaya

As you can see, we appended your inquiry to a thread which may already answer it for you. But please follow up if you have remaining questions. Thanks.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

November 8, 2011

A. Dear Mr. Saunther,

The composition includes Phosphoric Acid and Zinc.

Vijay Anand
- Chennai, India

December 13, 2011

Q. Hi, I just want to know how I can make fine grain phosphate formulation, because the coating presents white powder after finish, and the customers they prefer dark finishing.

Francisco Capacho
- Montreal, Canada

December 14, 2011

A. Hi, Francisco

I don't think people will be able to guess the details of your formulation solely from the fact that the result is white. If you want suggestions for improving it to a darker color, I think you'll need to say what the formulation currently is. Thanks.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Rust on inner surface

January 6, 2013


I'm recently finding red rust in the inner surface of upper part of iron metal after zinc phosphating. We are using hot phosphating manually. Can anybody help me to sort out this problem?

Chikki H
- Bangalore, Karnataka, India

January 7, 2013

A. Hi Chikki. I'm not understanding what you mean by the inner surface of the upper part, but is trapped air preventing the phosphatizing and other solutions from reaching that area? If you are seeing any rust shortly after phosphatizing, that area has not been phosphatized.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

January 10, 2013

A. Hi Chikki,

Like Ted, I am not certain exactly what you are doing, but you do need to beware that zinc phosphating on its own will not prevent corrosion, the coating is a discontinuous crystalline coating and tends to be used for a base to paint, bond or put on some other type of supplementary coating.

If you are seeing immediate flash rusting then Ted may well be right and you are not getting complete coverage, possibly due to air entrapment. It may also be the fact that the surface is remaining wet for too long, so you aren't getting things dried before flash rusting starts, also if the rinsing is not sufficient the cold phosphate solution will also promote corrosion.

If you are seeing the rusting occur after a period of time it will be because the supplementary coating is not being applied in time or no supplementary coating is being applied at all.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

Phosphating of Metals
by Rausch

April 19, 2013

Q. Can anyone help with the Cold Phosphating process, I need to know what chemicals are needed and the application.

Bosco Arokiaraj
- Trichy, Tamil Nadu, India

August 13, 2013

Q. Could you give me the permissible limits of the following parameters with respect to Zinc phosphating bath:
1) percentage of oil content in Acid Degreasing bath
2) percentage of iron content in Phosphating Bath
3) free Acidity of a phosphating bath operating at 28 ml pointage.
4) iron content in Derusting Bath

- Mumbai, Maharashtra, INDIA

July 2013

A. Hi cousins. Please try your best to present YOUR OWN SITUATION so the readers can attempt to help us through our highly specific problems from their experience! Thanks.

If we cast questions in the abstract, the readers almost never help us for a bunch of reasons: first, abstract questions usually can't be answered without a dozen ifs, ands, and buts that probably don't apply in our situation but do apply in other situations; second, flashcard style quizzes convey the impression that we want someone to rifle through textbooks for us because we can't be bothered doing it ourself; finally, although readers can help us resolve a particular situation, they can't possibly in a paragraph or two elevate us to where we become experts, able to independently solve generalized problems, and they don't want that inference drawn from their response. Please try to take this charitably as my intention is to help you get the assistance you seek by telling you what I've learned from curating hundreds of thousands of questions posted on this site over 20 years now.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Standard phosphate coating

September 13, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. In standard phosphate coating after HCl pickling and washing, what should the concentration of phosphating solution be?

Indu Indukumar
quality - Bazpur, Uttrakhand

February 23, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

My name is Hassan. I was searching for how to make a solution for hot and cold phosphate coating for pre-paint treatment for iron and I came across this forum. Can someone help me regarding this.?

Hassan zulfiqar
Student - Lahore, Punjab, Pakistan

June 4, 2014

Q. In zinc phosphate coating when I use zinc nitrate and phosphoric acid it works well; but when I use zinc oxide and nitric acid instead of zinc nitrate it doesn't work. I don't understand why anyone says use zinc oxide and nitric acid.

ali shafizadeh
- iran esfahan

October 29, 2015

A. Dear Shafi:
Zinc oxide is used in the formulation. It reacts with phosphoric acid to make zinc phosphate. Zinc nitrate is not required.

vijay anand
- chennai india

Phosphating bath Free Acid is dropping after filtration

March 23, 2015

Q. Hi, I am umesh. We are powder coaters. We are having seven tank pre treatment line. Recently we installed a online sludge separation filtration system. It is working fine. It is separating sludge and keeping the bath clean. But we are facing a different problem. We are noticing that, after filtration, the Free acid level is dropping drastically. Bath temp 45-50, dip phosphating, 6000 ltr bath size, 5000 sq.m/day. Could anybody tell me why this is happening. What is to be done to avoid this.

umesh kulkarni
buyer - pune, maharashtra, India

April 2, 2015

A. If you are maintaining tank level control, then the reason would be that water is being added to make up for the volume of sludge that has been removed. This will dilute the bath and cause FA to drop.

Dan Mack
- Horicon, Wisconsin

Will nitric acid addition correct low free acid in zinc phosphate bath?

May 8, 2015

Q. Dear sir,

What happens if i will add 500 ml of nitric acid to a 12 kl zinc phosphate bath, as we facing some issues due to low F.A of our bath, and I think with nitric acid F.A will increase? I want to know if there is some issue with it or not.

sumit thakur
tube draw process - chandigarh, india

Surface finish

September 19, 2015

Q. I have a very crucial information about this.
But I want to be more clarify .
So my parts are getting pinhole defect
And obviously they are found at unloading station.
My process parameters all are in medium range.
So it could be in phosphate process will be developed.
I want to trouble shoot this very immediately

Revan Rustampure
- Pune India

September 2015

A. Hi Revan. Sorry, I've never heard of pinhole defects in zinc phosphating, but I'm not sure if I am understanding you. Unfortunately, telling us that everything is in mid range, i.e., that there is nothing wrong, isn't really too helpful because obviously something IS wrong. If you have additional detail, or pictures, perhaps a reader can help. Good luck.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

September 20, 2015

Q. Sir I had used it to coat galvanised iron straps, so this problem occurred on this part only ...

Revan Rustampure [returning]
- Pune India

A. Hi again, Revan. Zinc dissolves extremely rapidly in strong acids like hydrochloric. Are you putting these galvanized straps through the same acid dip which you use to activate steel parts? It will certainly cause pinholes in the galvanizing if not completely destroy it.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

How to decrease zinc phosphate coating weight on steel

May 26, 2016

Q. We are using zinc phosphate for coating in our pickling process.

The issue is that we get a coating weight about 150 mg /ft^2 higher than the standard specs. Could you suggest a method so that we can decrease this wastage ?

Ojas Gupta
student - Ranchi, Jharkhand, India

May 2016

? Hi Ojas. What is your zinc phosphate formulation? Are you dipping or spraying? What coating weight are you shooting for? Thanks.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 26, 2016

Q. Hi Ted
We are willing to achieve a coating weight of 600 mg/ft^2 but are getting a weight of around 730 on an average
We dip the steel into phosphate solution
The formulation of solution is as follows -
Zinc content 9-10
Phosphate content 20-24
Total Acidity 40-48

Kindly suggest a method to reduce this coating weight to desired around 600

We want to reduce this large wastage so please guide us to what can we do regarding the operating temperature of dipping tank, time of dipping, concentration, etc.

Ojas Gupta [returning]
- Ranchi, Jharkhand, India

May 2016

A. Hi Ojas. Thank you for the data.

I was asking for it as the site curator because I felt it would be hard to go further without it. I am not actually a zinc phosphating expert, but my limited understanding of zinc phosphating (pending a better answer from another reader) tells me that efforts toward reducing grain size will reduce coating weight -- so please see Yehuda's suggestion above. For further detail on activator/grain refiner please see letter 14221, "Surface Activation for Phosphating. Grain Size Issues", especially P.M.Saravanan and TSN Sankara's outstandingly informative entries. Good luck.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 28, 2016

A. Hi Ojas!

The Zinc Phosphate solution to be used should be 3 % in water and maintain a temperature of 70 °C.
If you are coming with your own formulation then share it so that it can be corrected.

Also add Titanium based salt bath before phosphating to give fine etching and hence finer and low weight coating. The adhesion also will be excellent.

Please try this and share the results.

vijay anand
- chennai, tamil nadu, india

August 24, 2016

A. Hello,

My advice would be to use a grain refiner; this may be enough to get you to within your limits alone if that is not enough reduce the time in the phosphate bath until you get within your range required. i have just been doing a project on some small base plates and managed to achieve 3-5g M2 using a grain refine and only 1 minute in the zinc phosphate after 2 1/2 they were fully phosphated and i was achieving 17g M2 !!

Paul Rennison
Surface technology - Leeds United Kingdom

October 6, 2016

Q. How to improve quality of Zinc Phosphating? Can anyone sort out my problem ?

Currently we are using Hot Phosphating process (75-80 °C). Mainly we are facing problem for Mild steel, EN 8, EN9, EN 19 & EN 24 material.

After phosphating & oil sealing, around the surface bottom side we are getting very rough coating while at upper side we are getting smooth coating. Also after phosphating, approx. 6-8 months we observed rrddish colour and are required to rephosphate.

I want to update preservation methods of phosphatised components and formulation of phosphating solution to achieve smooth and linear coating.

Pradeep Tanksale
- Pune, Maharashtra, India

October 31, 2016

A. Hello Pradeep!

Please tell us more about your process:

- Is it a barrel process?
- How do you process the parts? (degreasing, how many rinses, pickling, how many rinses, phosphating, how many rinses, sealing?)
- What shape are the parts you process?

I think we can help you if you tell us more.



Daniel Montanes
N FERRARIS S.A. - Canuelas, Buenos Aires, Argentina

How to maintain the Total acid and Free acid ratio in Phosphating process?

October 24, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sir,

How to maintain Total acid and Free acid ratio in case of Zinc Phosphating process .
To lower the FA , caustic may be added but in that case TA will be decreased also. So please suggest how to maintain the ratio.

Thanks and regards.

Rathin Mukhopadhyay
- Kolkata, West Bengal, India

October 31, 2016

A. Hello Rathin!

Do you formulate your own bath? If not, you should ask your vendor, as the products are balanced to mantain the relation between FA and TA in the operational values.

If you are formulating your own bath, my advice is to add one formula to replenish the drag-out and one formula to mantain the TA in process (adding product by parts/barrels/racks processed). The second formula should have more FA than the first one to mantain the relation. There is no magical formula, you should try and adjust it until you have the one that mantains the relation in your optimum range.

Hope you can solve your problem! Regards,


Daniel Montanes
N FERRARIS S.A. - Canuelas, Buenos Aires, Argentina

February 20, 2017

Q. What ratio will have to take zinc nitrate and phosphoric acid? Other question: in phosphating we are using zinc nitrate and phosphoric acid; does it need further coating for sodium nitrite as accelerator? What composition do we have to make it for good coating?

suganya sambath
- chennai, tamilnadu and india

March 17, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi. Can you please tell me formulation of zinc phosphate?
How can I make one Zinc phosphate Drawing liquid?

ali hoseiny
student - TEH, IRAN

March 2017

A. Hi Ali. Although the page may be confusing, the answers are here, very insightful answers.

Because the thing is, it's easy enough to make a zinc phosphating solution with nitric & phosphoric acid and zinc oxide, but what most users are actually seeking is the composition which will work best for them in terms of corrosion resistance, salt spray, grain size, undercutting prevention, paint adhesion, minimal sludging, robustness against small process variations, freedom from toxic ingredients ... and which will work on their steel, cast iron, zinc, and aluminum products ... and will be inexpensive.

Thus the page introduces the ideas of controlling total acid vs. free acid, employing additional ions like calcium and nickel, and grain refiners like titanium, and accelerators, etc. In the West these days, shops tend to not try to formulate their own zinc phosphating solutions, but purchase proprietary formulations which are the result of years or decades of research into all those factors. Good luck.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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