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Excess Sludge Buildup in Zinc Phosphating Bath

A discussion started in 2005 & continuing through 2017


Q. Dear Sir

We are facing problem of excessive sludge buildup and the bath does not remain green even after continuous filtration. What can we add in the bath so that the sludge goes down and the color of the bath remains green.

Can some one help me?


Amitabh Kohi
Paint Shop - Noida, India


A. Dear Amitabh Kohi,

Phosphating is a conversion coating process in which metal dissolution by the attack of free acid (phosphoric acid) is the basis of coating formation. The ferrous ions formed at the metal/solution interface gets oxidized to ferric ions which in turn combines with the phosphate and gets precipitated as ferric phosphate sludge.

If your bath is producing more sludge, then check the free acid value. If the free acid content is higher than the recommended level, then it will cause the formation of excessive sludge.

I am not sure about the green colouration of the bath solution.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at


Phosphating Metal Pretreatment
by Freeman

A. Hi
You have given only basic info, but simply there is nothing you can add to reduce the amount of sludge being produced,however you can look into reducing the amount of sludge produced initially.

1 Is your present filtration/sludge removal system efficient,are pipes becoming blocked,are pumps working correctly.
2 Are you maintaining the correct pointage for total acid and accelerator? High accelerator causes higher sludge levels.
3 Are you getting carryover from the cleaners? Any alkali going into the phosphate tank will cause sludge to be produced.

This is just the start, there is no magic additive.

Tony Wagstaff
- Lincoln, UK


Q. Sir

There is no carry over and we are maintaining 2-3 ml of toner in the bath, T.A. 24-26, F.A. - 0.6 still the sludge is more. I want to know what is the usual amount of sludge generated in a Zinc Phosphating bath may be per square meter. We have a problem in our sludge filter pump which picks up chemicals from the tank it is only 1.5 inches in diameter is this creating a problem.

Please help.


Amitabh Kohli [returning]
- Noida, U.P.


A. Without further information I agree with Tony Wagstaff.
In principle the loss of "green colour" is due to the loss of nickel content of the bath. This will be due to mechanical effects or chemical instability resulting from the wrong TA/FA ratios or the wrong Zinc Phosphate replenisher.
The excessive sludge will be controlled by correct TA/FA ratios, accelerator content and no excessive metal loss, in principle the metal loss should be 50% of the coating weight.
Tank turn-over should be between 4-7 per hour and avoid high temperature above 60° C.
Good filtration is always important using filter press or paper.

Martin Ings
- Bletchley, Milton Keynes, UK


A. Mr. Kohli,

If the green colouration of your bath is due to the nickel salt then I agree with Martin Ings that the loss in green colour is due to the loss in nickel content.

What is the nickel content of your bath?

If you are losing nickel (loss in green colour) then is it also precipitating along with the sludge with ferric phosphate?

Ferric phosphate is white colour. What is the colour of your sludge?

It is surprising to note that your bath is producing heavy sludge for a free acid pointage of just 0.6.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India


Q. Dear Sir

You are very right the colour of the sludge is white.

Nickel concentration is about 20% in the pure phosphating chemicals.


Amitabh Kohli [returning]
- Noida, India


A. Phosphating baths are formulated with a zinc salt, phosphoric acid and an accelerator. The amount of zinc salt and phosphoric acid in a bath are chosen in such a way that it contains soluble zinc phosphate with a small excess of free phosphoric acid (Free Acid). The free acid causes dissolution of the metal and enables a raise in interfacial pH (due to the consumption of acid at the metal-solution interface). When the raise in interfacial pH reaches a particular level (called as the point of incipient precipitation), the soluble primary phosphate gets converted to insoluble tertiary phosphate and deposit on the metal substrate.

The equilibrium between soluble primary phosphate and insoluble tertiary phosphate will be altered by a change in pH and temperature of the bath.

If you operate your phosphating bath at a pH or temperature higher than the recommended level, then the conversion of the soluble primary phosphate and insoluble tertiary phosphate will occur throughout the bath and will get precipitated. The tertiary zinc phosphate is also white in colour.

This is the reason why Martin Ings has suggested you to avoid high temperature above 60 °C. Also as suggested by him, check the metal loss and coating weight. The ratio between the metal loss and coating weight (called as conversion ratio) should be around 0.4-0.6.

Check the free acid, total acid, free acid to total acid ratio, pH and temperature of the bath as to meet the recommended level. Also check the conversion ratio. This will give an idea about the amount of metal loss and the quantity of sludge that will be formed.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India


Q. Sir

I can very well understand the parameters, etc. but when you say ratio between coatings and loss of metal I am not able to understand; can you please let me know the method for the same?

The filtration should be slow and continuous as in this case our outlet pipe from the to the filter is small only 1.5 inch in diameter or it should be big our bath size is 6000 Ltrs., temp is 50 °C.

All the other parameters are within control.


Amitabh Kohli [returning]
- Noida, UP, India

August 13, 2010

A. Dear Sir,

The green color of the bath is because the ferrous ions of the material coated in it. First, the material dissolves in the bath, it causes a raise of pH and that results in the precipitation of a tri-basic phosphate layer of zinc and nickel (and iron) over the surface.

The sludge is produced (I think) because of the low free acid in the bath, if the sludge is white it is probably zinc phosphate or iron (ferric) phosphate. They precipitate if the pH is higher than 3 or so.

If it is an accelerated (hydroxilamine/chlorate if it is an internal accelerant or nitrite if it is external) phosphate bath, the sludge is almost only ferric phosphate, you can avoid it only changing the product or asking for a new formulation.

I hope that you can find a solution to your problem, the buildup of sludge is unavoidable in phosphate baths, but they can be minimized maintaining the parameters fixed during the process.

Best regards,

Daniel Montanes
- Lanus, Buenos Aires, Argentina

April 7, 2011

Q. Sir,

We are planning to install a continuous filtration system for our zinc phosphating bath. How effective is a filter press? Are there any other options like bag filters for this application available. Sludge build up would be around 5 kgs per day.

Thanks in advance.

Jothi Ramalingam
- Chennai, India

February 6, 2012

Q. 1. What should be the quantity of sludge content in phosphating bath?
2. Which elements are present in Accelerator and in Neutraliser?

Tahir Ali
- Gurgaon, Haryana, India

February 7, 2012

thumbs up signHi, Tahir.

It isn't clear to me in what way the thread isn't answering your questions. Can you please post with a lot more words? For example, it's not clear whether you are asking for clarification of previous answers about how to calculate sludge generation rates, or whether you are asking whether you are supposed to leave a certain amount of sludge unfiltered for best operation. It's also not clear if you are asking for formulation information on how to concoct a home-brew solution (some readers are associated with suppliers, and are not at liberty to share their company's trade secrets), or for confirmation of unexpected titration results.

Please try your very best to explain your own particular situation in detail rather than casting questions in the abstract. When you ask abstract questions, the first thought of many readers is probably "Read a book", and only rarely does anyone respond :-(



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

September 25, 2012

Phosphating of Metals
by Rausch

Q. How many (in grams) sludge will be formed per square meter of steel when treated with zinc phosphate. Thanks! Kindly post the reference if there is any.

Cris Jhon G. Caipang
- Manila, Taguig City, Philippines

November 25, 2012

Q. Dear all,

Can anyone help me to find out how much amount of accelerator that we have to add into phosphating bath per Liter volume?

Thanks for your help :)

Mutiara Pangestika G
- Bogor, Indonesia

October 7, 2014

Q. Please tell me on what basis addition of sludge modifier is to be done in phosphate bath. Is there any criteria for addition?

Kedar patil
- pune ,maharashtra

October 9, 2014

A. Hello dear,

Sludge content in Phosphate bath depends on following some factors as :

1. Activation tank condition. How old is activation bath? Do you have any overflowing/refreshing mechanism of activation tank in regular production?
2. For activation bath should maintain TA<6 & pH<9
3. Keep Phosphate TA, FA & Toner value at lower side of Specification.
4. Keep Phosphate bath temperature at middle of spec. (<55°C)
5. Filtration pump capacity should be such as it will filtrate 1/3rd of phosphate chemical per hour.
6. last but imp. point that Main chemical & toner of phosphate should not be added at one location of phosphate tank to avoid its faster reaction. You can keep main chemical addition at tank entry & Toner addition at tank exit.

Hope this will help you to resolve your problem.

Thank you.

Sachin Gite
- Pune, Maharashtra, India

November 10, 2014

Q. Dear All:
How may I reduce the sludge formation in a non-accelerated phosphating bath working at 75 °C, TA 50, FA 5-7, Iron 0.1% to 0.5%? Is there a chemical additive to reduce the sludge formation?
Thank you

victor delgado
- bogota, colombia

July 5, 2016

Q. Respected sir ... please tell me function of fa, ta, accelerator in phosphating process.

Rajesh Kumar
sswl - Dapper,punjab,india

July 2016

Hi Rajesh. We appended your inquiry to a thread which generally addresses your questions.

If you are having a specific phosphating problem I am confident that the readers can help you. Please tell us what items you are phosphating, what metal the substrate is, what type of phosphate you are dealing with, and why you are phosphating (as a break-in coating, as a pretreatment for powder coating, etc.)

Are you using a proprietary phosphating process from a supplier, or attempting to do a home-brew? Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

July 31, 2016

Q. Respected sir, we are using tri-cation phosphate of 40 kl. Parameters are following:

ta 20-26
fa 1-2
accelerator 3-4.5

We are phosphating wheels of mostly car & tractors.
Problem is that my components show yellowness after water rinse, and coating is not observed on components.

rajesh kumar [returning]
sswl,dapper,punjab - hisar,haryana,india

Sludge build-up in Phosphating

August 1, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello,

I am trying to find different ways in which sludge explosion can be avoided in a phosphating bath (zinc phosphating process). I know that this could be avoided via the addition of old sludge or tertiary zinc phosphate crystals. However, I cannot find theoretical resources on the topic.

Any other suggestions of how sludge explosion could be avoided are also highly appreciated.


R Sharq
- Worth am Rhein, Germany

August 2016

Electrolytic and Chemical Conversion Coatings

A. Hi Razan. This thread offers some good insights. Biestek and Weber's "Conversion Coatings" =>
suggests that heavy sludging is due to incorrect coefficient "f", or presence of Ca and Al. If you explain your own situation in detail you will usually get more & better response than to abstract questions. Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Zinc phosphate sludge formation rate

October 24, 2016

Q. My coil finishing department has 2 zinc phosphate tanks to provide coating to carbon steel coils (Weight of each coil=1.4 MT). One tank has a dewatering filter attached to it to filter out the phosphate sludge. We want to incorporate the existing filter for both the tanks. How should i go about this problem? is there a way to determine the sludge formation rate based on bath parameters and total coils coated each day?

Zain Ansari
production engineer - Mumbai, Maharashtra, India

October 31, 2016

A. Hello Zain!

Getting rid of the sludge is a batch process, so if now you are pumping the sludge two times per week, if you use the same filter/tank for both processes, you may be using it four times per week instead of the old two.

We have a phosphating line for 1,4 MT coils with one phosphating bath, and two times per week for a 7000 liter bath is enough to clean it up.

Hope that information can help you decide how you make this work!

Best regards,


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

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