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Surface Activation for Phosphating. Grain Size Issues

A discussion started in 2002 and continuing through 2017 . . .


Q. In zinc phosphating what is importance of activation ? What is chemical composition of activators ? How can it maintained ?

Vishwas M.
painting shop - Indore, India

Phosphating of Metals
by Rausch


A. Surface Activation chemicals activate the metal to obtain fine crystalline structure during phosphate coating which will increase corrosion resistance and adhesion properties. Crystalline structure less than 10 Microns always preferable.If we are not using activation rinse,the phosphate crystal size may be large and results in voids in Phosphate coating (Gap between structure will be large). Corrosion will take place at the voids.

Basically it is a Titanium based weak alkali chemical.

pH and Alkalinity are the parameters to be maintained daily.

   paint shop manager
Chennai, Tamilnadu, India

To minimize searching and thrashing, and to provide multiple points of view, combined formerly separate threads into the single dialog you are now viewing. Please forgive any resultant repetition.

Necessity of Grain refiner for phosphating


Q. In the process of looking for alternative suppliers for grain refiner for zinc phosphating I realized that I do not even know what the grain refiner is. Our MSDS say nothing about ingredients because apparently none of them are "hazardous". I have looked in reference materials such as Metal Finishing Guide Book, Electroplating Engineering Handbook, Chemical Surface Treatments of Metals (book), and Conversion Coatings (book) and have not seen any discussion of what the refiner is and how it works. Are you aware of resources that discuss this?

Tim Denhof
Job Shop Engineer - Chicago, Illinois, USA


Phosphating Metal Pretreatment
by Freeman

A. Grain refiners are chemical compounds usually added in activation (pre-dip) bath and/or the phosphating bath to reduce the crystal size in manganese and zinc phosphating processes.

Grain refinement in phosphate coating could be achieved by activating the surface using colloidal titanium phosphate prior to phosphating. During activation, the colloidal titanium phosphate particles are physically adsorbed on the metal surface and when the activated substrate comes in contact with a zinc phosphating bath, an ion exchange between the sodium ions on the surface of the titanium phosphate particles and the zinc ions of the phosphating solution takes place. The ion-exchanged particles act as nucleation agents for the zinc phosphate crystals because they have nearly the same stoichiometry and offer a crystallographic plane for an epitaxial growth.

The process for activating steel surface prior to phosphating was given in many patents. Use of 1-2% disodium phosphate solution containing 0.01% of titanium compounds have been extensively used in industries. Dilute solutions of cupric or nickel sulphates, oxalic acid and polyphosphonates also help in increasing the number of initial nuclei formed during phosphating and their subsequent growth, to yield thin and compact coatings of fine-grained nature.

The incorporation of calcium ions in a zinc phosphating enables a reduction in the grain size of the phosphating coating. The reduction in grain-size (25 micrometer to 4 micrometer) and the improvement in the compactness of the coating and corrosion resistance make this kind of modification of zinc phosphating baths as an important type of phosphating and it has been classified as calcium-modified zinc phosphating.

Similarly, addition of manganese and nickel ions in the zinc phosphating bath proves to be useful in refining the crystal size and improving the corrosion resistance of the resultant phosphate coatings.

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

Nickel in zinc phosphate solution


Q. Sir,

we supply zinc phosphate solutions which are nickel free to customers who use for paint-base applications on MS. however I would like to know if its alright to add nickel if we want to broaden our range of applications.

Sonali Kokane
electroplating chemicals - Thane, Maharashtra and India


A. Modification of the phosphating baths with nickel salts assumes significance with the introduction of pre-coated steels such as zinc, zinc alloy coated steels, etc., for automotive body panels which present challenges regarding the stability of phosphate coatings. These pre-coated steels show good performance with regard to formability and weldability. The extent of perforation and cosmetic corrosion is also less with these materials and they also posses a good surface appearance. When zinc and zinc alloy coated steels is phosphated using a nickel-free zinc phosphating bath the resultant coating will comprise only the hopeite (zinc phosphate) phase. Phosphate coating on these substrates having only the hopeite phase is not desirable for subsequent cathodic electrophoretic painting.

Nickel modified low-zinc phosphating formulations are the most significant modification proposed to cope up with the cathodic coating, particularly in the automobile industry. The presence of Ni2+ ions in these phosphating baths has several advantages. Although there is a decrease in coating weight, crystal formation occurs at an early stage and the crystal size is fine. The Ni2+ ions participate in the formation of phosphate crystal and enables crystal refinement. In this perspective, nickel addition in zinc phosphating baths is beneficial. However, the metal finishing industries are facing stringent regulations in effluent disposal and there is a transition from Cr(VI) free, Cd free and Pb free processes. Nickel is also carcinogenic. So you might consider other alternatives.

One possible alternative:

Addition of manganese salts in zinc phosphating bath could also perform a similar role as that of nickel in phosphating of zinc and zinc alloy coated steels. The addition of Mn2+ ions in the phosphating bath helps to increase the rate of formation of coating and also the working width of the phosphating bath. Also, the presence of manganese in the zinc phosphate coating resists the formation of white spots on galvanized steel. Manganese modified zinc phosphate coating possesses better corrosion resistance.

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


thumbs up signShri Narayanan,

Thank you for the reply. Our inclination is to go Ni-free.

Sonali Kokane
electroplating Chemicals - Thane, Maharashtra and India

To minimize searching and thrashing, and to provide multiple points of view, combined formerly separate threads into the single dialog you are now viewing. Please forgive any resultant repetition.

Building small zinc phosphate crystals


Q. I am zinc phosphating using an immersion system, soap,rinse,pretreatment,zinc phosphate, rinse sealer, then RO rinse. The problem I am having is I am getting a few large crystals growing with many voids in between them, I am getting 400 mg coating weight which is ok but I want more and smaller crystal growth to give me a better feel for not having adhesions issues in the field.

Todd Dirkes
Production Manager - Longmont, Colorado, USA

simultaneous (2007)

A. Though it has been some years since I was involved with phosphating, I recall that one of the important factors in regulating the property you speak of is the type, and concentration of catalyst present in the solution. I don't know that this is something that the user controls directly. When I did process control analysis on these baths, the only things that I tested for were total and free acid, and iron.

Talk to your vendor. They may have a "solution B" or something of that nature that will boost the catalyst and give you finer grain structure.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


A. For a really nice zinc phosphate surface, consider using a calcium modified zinc phosphate from one of the advertisers at this site. I had great luck with it.

James Watts
- Navarre, Florida


A. Dear Sir

I think you need to check your cleaner alkalinity and rinsing. Also sludge in the phosphating. I feel that your cleaner is not getting washed off properly and thats the reason for a big size crystal deposit.


Anurag Kanaujia
- New Delhi, INDIA

Zinc Phosphate grain size problem

April 28, 2010

Q. Dear friends.

I been having trouble with my zinc phosphate process
1.- degrease
2.- rinse
3.- rinse
4.- rinse
5.- acid (HCl)
6.- Phosphate
7.- rinse

The appearance of phosphate looks with big grains and some parts of the pieces with out phosphate, we can see the substrate.

Thanks for your comments I hope you can help us.

Alexandro Ramirez
plating shop employee - Saltillo Coahuila Mexico

Preparation of Metals for Painting

May 3, 2010

A. Dear Alexandro Ramirez,
For the bigger grain's causes
1) More sludge deposition on your phosphating bath
2) If you have the agitation tank, air pressure may be high or rpm of the stirrer is high
3) Or do the continuous filtration.

or may be the chemical is suitable for only making the Big Grains, better check with the chemical supplier.
Because for the powder coating or paint for the under coats , big grains are advisable for higher finishing thickness.

Kannan Boopathi
- Salem, Tanil Nadu, India

May 7, 2010

A. Hi Alexandro,

Without having seen how your operation is set up, I have to ask some questions and this may lead to a solution. I wonder why you have an HCl stage right before the phosphating stage. The drag-in from the HCl stage will contaminate the phosphating stage and lead to problems like an uneven coating and will make it difficult to control the bath. The extra acid will mess up your titrations.

If it is feasible for your operation, I would do the following. Switch the HCl stage with one of the three preceding rinse stages. Ideally, I would put the HCl stage in Stage 3; this way, you have a rinse between each chemical stage.

I would also ask your chemical supplier about a grain refiner and put it in the rinse stage prior to phosphating. This will decrease crystal size and help keep coating deposition uniform.

George Gorecki
- Naperville, Illinois

April 22, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am a job coater.My process flow is KOD-water rinsing(2 times)-surface conditioning-Phosphating-E.D.coating.
My customer's requirement is --Phosphating grain size less than 10 microns.
How to achieve it?
Parameters are as follows-
T.A.-20 to 22
Temp. 43 °C.

Avinash vidhate
Plating shop employee - Nasik, Maharashtra, INDIA

March 13, 2013

Q. How can I prepare a surface activation chemical and which raw materials are used to prepare it and in what quantity?

dpak kumar
- Balasore, Orissa, India

March 13, 2013

A. Hi Dpak. If you don't want to buy a proprietary activator, then Saravanan's information that it is a titanium based weak alkali should be a good basis for your in-house development efforts. Also, it is very easy today to do a search for expired patents and get some further ideas there.

But obviously nobody can openly publish the trade secrets of other companies, and that's what these formulations are. Good luck.


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

Zinc Phosphate Crystals Too Big for Wire Drawing

February 5, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Members,

I am using zinc phosphate on Carbon Wire Rods using the following parameters:

1) Acid cleaning (HCL at room temperature). Normally for 30 mins
2) Jet Wash
3) Rinse 1 (Ph above 3)
4) Rinse 2 (PH above 4)
5) Phosphate Bath ( 10-12 mins at 70-75 Degrees, TA 40-50, FA 7-8, TA/FA = 5-7), Accelerator 2-4
6) Rinse
7) Borax

Problem is the phosphate bath starts forming large crystals too soon which hinders the drawing process as they break too quickly in drawing. Another indication of disturbed chemistry of the bath is that its color changes to BLACK.

I have the following questions:

1) How do I prevent the bath from making large crystal or turning black?
2) If large crystals are being formed, how do we bring the bath back to its original form making smaller fine crystals?
3) How do I check the Fe content of the phosphate bath? Is it similar to testing the Fe content of Acid?

Thanking the members in advance for their anticipated valued input.

Saud Mahmood
- Karachi, Pakistan

February 13, 2016

A. You can test iron in the bath as the following: Iron points --

   0.1 N potassium permanganate solution, sulfuric acid (50 %), Urea.
   1. Pipette 100 ml bath sample into 250 ml conical flask.
   2. Add 2 ml sulfuric acid (50 %).
   3. Add 2 g urea.
   4. Stir the solution until the urea has dissolved and wait for approx. 3 min.
   5. Titrate with 0.1 N potassium permanganate solution to a stable pink color (stable for at least 15 s).

Consumption in ml = iron points
Fe (II) = iron points x 0.5585 g/l

Mahmoud Farahat
- Egypt , Cairo

March 29, 2016

A. Hello Mr Saud,

Crystals turning black can mean you have iron in your bath. You can easily know if there is with filter paper strips (qualitative grade, any brand) imbibed in 1% alcoholic orthophenanthroline. You let the papers dry, and dip them in the phosphate. If they turn reddish, you have iron in solution. Then, you must add accelerant until your phosphate bath is white in color and there is no reaction in the paper test strips.

You can measure as Mahmoud said the iron content, it must not rise from 0.3 g/l.

Then, you have EVERYTHING in the right level, and you still have the big-crystal-problem: I advise you talk with your vendor, there are many solutions but every one depends on the phosphate bath formulation. I cannot advise any of them without knowing the bath formula...

So, I hope you can solve your problem! Regards,


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

Hero Motorcycles
August 11, 2016

Q. My process is surface conditioning/Phosphating
My customer's requirement is gray coating on Hero motorcycle fuel tanks
How to achieve it?

Parameters are as follows-
T.A. -- 18 to 20
F.A. -- 0.3-0.6
Accelerator -- 3-5
Temp. 41 °C
T.C -- 1-2
pH -- 9-11

But F.A is not controlled down to 0.0; even T.A is 20.3; and I get blue coating in Hero Splender+.

Abhishek tiwari
- Delhi India

August 20, 2016

Q. Is it really required to check the alkalinity in activation process?

R Ramesh
- Chennai, tamil nadu and India

August 2016

thumbs up sign Hi Ramesh. Saravanan said it was necessary in the first answer on the page, and I'm not competent to challenge him on it :-)
Please explain the full details of your situation whereby we might question his recommendation for your particular case. Thanks!


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

January 8, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear sir,
I am working on preparation of zinc phosphating bath. Earlier problem I was getting was of low coating weight. Now coating weight has improved but grain size and grain distribution is not uniform. What are the possible ways to improve grain distribution with high coating weight.

Rahul Jangid
Calligraphy - India

January 2017

Hi Rahul. You had an original formulation and operating conditions which you haven't disclosed but the coating weight was too low ... then you modified the formulation and/or the operating conditions ... and those modifications, which you haven't disclosed, have improved the coating weight but the grain size is inconsistent?

We've appended your posting to one of many threads on the subject, which may offer food for thought, and the opportunity to ask for clarification. The basic problem is that there are countless books of hundreds of pages each about phosphatizing, and there are file cabinets full of expired patents about modifications; I hope someone is able to help you, but I doubt that they can help with your formulation and operating conditions until you describe them. Thanks and good luck!


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

Closely related threads include:

  - Letter 10242 "Zinc Phosphating Problems & Issues"
  - Letter     874 "Zinc phosphate coatings to deter galvanic corrosion"
  - Letter 51772 "What is Tri-cationic phosphating?"

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