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topic 34052

Manganese phosphate coating on steel

A discussion started in 2005 but continuing through 2019


Q. I would like to get a recipe for a manganese phosphate coating for steel. If someone perhaps can explain to me the actual chemical actions taking place on the surface too?

I've learned that there are many applications such as:
1 high wear resistance
2 better resistance against corrosion and a nice finish and 3 as a pre-treatment for painting etc.

Is there a big difference in the recipe and chemicals? The one I'm most interested in is protection against corrosion and finish.


Jan Van Velde
hobbyist - Kesteren, Netherlands

Manganese Phosphate


A. You can use next solution:
25 gm phosphoric acid
1,5 gm manganese dioxide
1 lit water
Boil your objects 2-4 hours.
you must oil,lacquer or wax it!
(according to Angier: "Firearm Blueing and Browning" [link is to info about book on Amazon])

Goran Budija
- Zagreb, Croatia


A. Jan Van Velde

Manganese phosphating formulations contains basically a manganese salt, phosphoric acid and an accelerator. Deposition of manganese phosphate coating is normally performed at very high temperatures of the order of 95 °C for a period of 30-60 minutes.

It is a conversion coating process and involves the dissolution of the base metal which subsequently forms part of the coating. In manganese phosphate coating the species that deposits on steel is manganese iron phosphate and hence requires a large amount of metal dissolution so that the formation of manganese iron phosphate is possible. This is the reason for the requirement of high operating temperature and higher processing time.

Manganese phosphate coating offers good wear resistance. It can retain oil and lubricants and improves the scuffing resistance. Manganese phosphate coating with higher coating weight offers good corrosion resistance. Impregnation with oil further improves the corrosion resistance.

Zinc phosphate coating is mainly recommended as pretreatment coating for painting.

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

Heaters get sludged, then damaged


My company conducts manganese phosphate coating on carbon steel pipe. We use heating elements to heat the bath where the pipe coating is carried out. The problem is after the coating maybe for a week the heating element is covered by sludge. The workers usually remove the sludge from the heating element by hammer (mechanical). It doesn't only remove sludge but also cause damage to the heating element (made from stainless steel). So: how to remove the sludge without causing damage to the heating element ?

Dodi Syafardi
- Cilegon, Banten, Indonesia

A. Hi Dodi. Two quick notes: heaters should be electropolished stainless steel because this surface has no "tooth" at all and will minimize build-up and make cleaning easier. Oversized (derated) heaters or coils should be used because the smaller the heater, the higher the required surface temperature, which accelerates the sludging.

Letters 11465 "Sludge buildup on heating coils in phosphating tank" and 5142 "Materials for zinc phosphating tank" address this issue in some depth. Patient use of our search engine will uncover still more tips on this subject. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

December 6, 2011

A. Dodi Syafardi
for exterior coating & if the ends of pipe are open then for interior coating, you may use glass heaters of required wattage and in required number.

Vijay Yadav
- Bhiwadi, Alwar, Rajasthan, India

How do I formulate a manganese phosphate bath?

October 1, 2010 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello,
I am looking after a Manganese Phosphating line in our company where cast iron components are coated. The phosphating bath concentrate is currently being outsourced and I am not aware of the contents of it.
I am trying to prepare a bath myself using MnO2 [1.5g/L] and H3PO4 [25g/L] and topping the rest with tap water. The problem is I'm not sure what other chemicals to be added (like accelerator, stabilizers etc) and also in what ratio. I also tried measuring the TA & FA values of the above mentioned bath and found them to be 35 & 17.4 respectively. Thus the TA/FA ration is close to 2. But I have read that it should be between 4 ~ 6. Kindly let me know how to improve the ratio.
Alternatively, It would be a great help if anyone can suggest me the method of preparing a Manganese Phosphating Bath and the chemicals required to prepare it in proper ratios.

Savyasachi Hulikal
Plating shop - Shimoga, Karnataka, India

Manganese Phosphating Without Shot Blasting

November 19, 2013

Q. Hello,

Currently we are using Shot blasting as a surface preparation prior to Manganese Phosphating. Would the phosphating coating characteristics and adherence properties change if we change the preceding process to Vibratory finishing?

We have issues with dimensional distortion and surface roughness so we would like to ideally switch over to a milder form of deburring/pre treatment.

Any issues been foreseen with the change?

Abhishek Patel
- Ahmedabad, Gujarat, India

November 19, 2013

A. Hi Abhishek,

I must admit that I don't have experience with using vibratory methods of preparation before manganese phosphate, I have always used an abrasive blasting technique.

Before you go and start looking at new ways of finishing can you describe the exact blasting process you are using (blast media type, mesh size, blast pressure, stand off distance)? It may be with some modification of your technique blasting may still be a viable option for you.

For example I use 180-220 mesh alumina grit at a pressure of 40 psi nominal (some jobs have higher pressure needs, some lower) and a stand off distance of about 6-9". This works for about 80% of jobs I do and I suffer little to no distortion.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

November 21, 2013

Q. Hi Brian

Being a mass production process, we are confined to use wheel blasting & not air....besides the issue is rather the geometry of the parts.

We use cut wire steel shots for blasting with tumblast wheel blasting machines.

Am of the understanding that phosphating happens at a molecular level so only surface cleanliness & uniformity along with proper surface conditioning should do the job.
Though, yes, higher roughness caused by blasting would give more surface area to the phosphate for better coating & adherence.

Just wanted to confirm from all the experts here.

Abhishek Patel [returning]
- Ahmedabad, Gujarat, India

November 29, 2013

A. Hi Abhishek,

Probably going to show a bit of my ignorance of wheel blasting machines now, can you not reduce the rate of feed of blasting media or change the speed of the wheel blasters so that you reduce impact speed? Alternatively can you change your media without modifications to your cabinet, you may be able to use a less aggressive cutting media?

You are perfectly correct that the reaction happens at the molecular level and that the increase in surface area by blasting increases the number of reaction sites.

If you wanted to go to a vibratory method then you need to talk to your local supplier to see what they have to offer in terms of media that will cut sufficiently to give your increase in reaction sites and still avoid distortion. I get a feeling you are going to have to try a bit of trial and error to see if the process is a viable method of manufacture.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

January 23, 2014

Q. We have a situation where two pieces of gun steel were Ion Nitrided in a ammonia rich environment. Sample I was Nitrided at a High Temp (still 100 degrees lower than tempering temp) for a short time and Sample II was nitrided at a low temp for a long time. Both samples were subsequently sandblasted with 140 grit alumina then Mn Phosphated (using a conditioner and hex chrome rinse solutions). Sample I has very little Mn Phosphate, very gray in color, and looks terrible in general (in the nitrided and un nitrided sections of the sample) where Sample II looks very typical black coating in nitrided and un-nitrided sections of the sample.

I have been told that this will always happen but not given a good reason as to why (from my experience the only thing that would impact the ability to produce a good coating would be a layer of non-ferrous material/contaminant). Both samples were examined under a 1000X magnification (we hope to view it under SEM technology soon) and can not see any visible layers/differences/heat affected zones/etc. in the base metals. Does anyone out there have a "oh yeah knucklehead it's this" explanation?

John Cannon
Engineer - Watervliet, New York

January 24, 2014

A. The 2nd most frequent problem with Mn Phos is the absence of a "grain refiner" which I guess you covered with what you call a "conditioner". The first most frequent problem is the total/free ratio. You must adjust this ratio, preferably at the tank side, immediately before inserting the parts. If you check the T/F ratio on Monday and phosphate on Tuesday the ratio is too high,add manganous phosphate until you bring it into proper limits, then immediately phosphate.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

January 27, 2014

Q. Hello sir, we are doing manganese phosphating on 4130, 4140, 4330 steel parts. It's a new plant we have installed, am not able to standardize the process; I face problems like uneven coating surface, carbon coming out of the surface after phosphating, uncoated areas.

I have failed to do the pre cleaning properly; please suggest a best process for precleaning, and my chemical supplier only supplies the phosphating chemical; please suggest how to overcome these problems? My process is degreasing, water rinse, 5% H2SO4 rinse, water rinse, hot water rinse and phosphating; free acid 6, total acid 31, ferrous ion 1gm/ltr, temp. 90 to 95.,

Prakash Raj
oil and gas equipment manufacturers - Dubai, UAE

January 28, 2014

A. You must adjust the T/F immediately prior to putting the parts into the solution. For the 4000's series steels you must use a grain refiner. IF you supplier dos not have one, then change suppliers.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

January 30, 2014

A. Hi Prakash,

After degreasing, whether solvent based or aqueous based I would have an aqueous cleaning step (may sound odd, but the second stage cleaning really does help to get parts really clean). Do a water break check after rinsing to ensure you have an unbroken film of water on the parts, if not then repeat the cleaning process until you get a water break free surface.

Make sure you control the etch, don't over-etch, ensure you chemical analysis is OK and use as short a time as possible to obtain the etch required.

Grain refiners are a good idea to obtain even consistent phosphate crystal formation on your parts. As Robert says, your phosphate solution supplier should have a refiner available.

And I can't emphasise strongly enough that you need to keep the phosphate chemistry under control. Dependent on the level of use I would expect a minimum of twice a day analysis for free acid, total acid and iron, if this doesn't keep the solution under control then increase the frequency. If may be as Robert pointed out that you may need to do the analysis for each bath load, although that would indicate to me you have something else in your phosphate system upsetting the chemistry.

Brian Terry
Aerospace - Yeovil, Somerset, UK

February 1, 2014

thumbs up signThank you very much sir, I am getting good results, maintaining the free and total acid in correct ratio, and improved water rinses, and getting good results.

prakash rajagopal [returning]
- Dubai , uae

How to computer control Manganese Phosphate process

March 7, 2014

Q. Hello to all!

We are zinc and manganese phosphate plating company. Demand on Mn Phosphate coatings are rising, therefore also the demand for quality of our service. My question is: how should we make the process more computer controlled to eliminate the human error factor?

Currently we are controlling the temperature automatically with thermocouples, but pH, concentration and acid radio is measured by hand and with analysis.
What are the options for computer control of Mn Phosphate bath?

Second thing, how would you guys hang bush of dimensions 30 mm dia. x20mm for Mn Phosphating, to minimize contact marks? Both sides of bush are smooth finished.

Thanks in advance

Dobnik Doga
- Krsko, Slovenia

How to Manganese Phosphate the component without any dimensional reduction

"Phosphating & Metal Pretreatment"
by Freeman
from Abe Books
info on Amazon

"Phosphating of Metals"
by Werner Rausch
from Abe Books
info on Amazon

July 20, 2014

Q. How to Manganese Phosphate the component without any dimensional reduction to the base metal of grinding surface?

Required coating thickness is only 5 to 7 microns.

Ajay Pawar
- Pune, Maharashtra, INDIA

July 2014

A. Hi cousin Ajay. It is not possible to do phosphating without any dimensional reduction because, per John Donofrio in the Metal Finishing Guidebook, "The first reaction that occurs ... is the pickling reaction, which will dissolve some metal from the surface. On steel ... the pickling rate is in the range of 1-3 g/m2 of surface area treated ... This pickling reaction is essential for the coating formation ...". But if you run the numbers, this is very little metal -- well under a micron. Are you implying that your present manganese phosphating process is attacking the base metal? If so, there is surely something wrong.

Please do your best to describe your situation and your own findings from it so we can have an enthusiastic discussion of it here. Abstract questions can rarely garner anything but general guidelines; and there's little real value in our extracting these general guidelines from phosphating texts for you, when the first thing we're certainly going to say is that you should have the texts on hand yourself :-)

Luck and Regards,

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

Manganese phosphate is not adhering to steel -- it rubs off

July 28, 2014

I was doing manganese phosphate on AISI4145 steel metal. After some pieces it is failing adhesion test (phosphate layer is removed). Can anyone tell why this happens?
The bath preparation is underground tank and heating coil is used to reach temperature required of 90 °C.

Suresh Thankappan
- Mussafah, Abudhabi, U A E

Must we check Ra BEFORE manganese-iron phosphating or AFTER?

August 7, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread


Engg. Works - karad, maharastra, India

August 2014

A. Hi Anil. Based on the context, I assume MIP means manganese-iron phosphating, so we assigned a title and appended your inquiry to this related thread. Phosphatization is a soft coating (compared to iron), and manganese phosphate is often used as a "break-in" coating ... so I would assume that the surface finish before phosphatizing is far more important than the finish after it.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

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