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"Manganese phosphate coating: issues and answers"

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Current question:

MIM = Metal Injection Moulding
MnP = manganese phosphating

December 10, 2021

Q. Hi,
we make MIM parts in our plant and also do MnP for Medium Carbon low alloy steels (4605 & 4340) grades. recently we have got the issue of surface finish after the MnP process. Initially, we use to get a black color finish after MnP but now the surface looks brownish and our customers are not showing interest to accept the parts because of the finish & Brownish appearance.

Please let me know whether there will be an issue in bath concentrations or due to improper surface preparation (we do grit blasting as surface preparation).

Advice accepted.


Chanikya Polidas
R&D Specialist - Bangalore, India

Closely related historical postings, oldest first:


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

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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" [affil. link to 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 https://advancementinscience.wordpress.com)

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, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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

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

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
supporting advertiser
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 of Metals"
by Werner Rausch
from Abe Books

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"Phosphating & Metal Pretreatment"
by D.B. Freeman
from Abe Books

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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, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

April 28, 2015

Q. Dear Sir, I am a custom cutlery manufacturer and have been using Manganese Phosphate impregnated with oil as a corrosion inhibitor on knife blades for some time with good success. I would like to treat kitchen knives the same way, but have concerns about food safety. Is there any reason to suspect that manganese phosphate treatment of carbon steel might be non food safe?

Adam DesRosiers
- Juneau Alaska USA

June 2015

A. Hi Adam. First, I don't want to toss around the term "food safe" without agreeing what it means :-(

Food contact materials

Many materials that are not "poison" or "toxic materials" are not "food safe" either. For example, zinc is an essential micro-nutrient rather than a poison, but it can dissolve in acids or alkalis, possibly even resulting in an overdose, and it's not food safe by anyone's measure, even if you find no regulation forbidding it. Some of the laws & guidelines set limits on "dissolution" and manganese phosphate is probably not food safe simply by this definition.

Further, if you want to sell such a product there probably are FDA regulations that you can only use GRAS ("generally recognized as safe") materials; there might be NSF or some other applicable guidelines on appropriate materials as well. There are a boatload of regulations in the world today, and they are expanding rapidly; I would not be willing to say that anyone can offer anything for sale without a careful study of the laws regulating that trade.

Then too, what kind of "oil" are you speaking of? A few, like olive oil and mineral oil, are non-poisonous, but petroleum oils are poison.

Interesting question. I hope someone with specific tableware or kitchenware experience will respond.


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

June 20, 2015

Q. Hi,
Good times.
I am Hamidreza from Iran. Sorry that my English is not good.
I have read your article about manganese phosphate and the questions you had.
I do not know during tests of the mixing materials.
Also, after removing the piece of soluble phosphate, phosphate coating on the surface is easily separated.
My solution is mixed as follows:
Manganese carbonate 15 grams per liter
25 ml phosphoric acid per liter
2 grams per liter of nickel nitrate
3 grams per liter of iron sulfate
Sodium fluoride 0.7 grams per liter
Rochelle Salt 2 grams per liter
5 ml of nitric acid per liter
pH = 1.5
Temperature 95 °C
Time: 30 minutes

If possible, please help.

ali ahmad
- iran, tehran

Manganese Phosphating: Lost power for 5 minutes

October 30, 2015

Q. Hello Sir,
During manganese phosphating process if the power goes for 5 minutes and no DG, what are the quality issues can be raised.

S Raghavi
- bangalore,India

Different colour tones on finished Manganese phosphating product

November 12, 2015

Q. Dear Sirs, I am involved on the inspections of the manganese phosphating process. I would like to know if the difference in colour tones of the final coated surface be cause for rejection. Other parameters were within acceptable limits. In the project specs, it is stated that uniformity in appearance as acceptance criteria. Does it include colour tone for uniformity? What is the cause of this defect? And how to rectify?

Ng twuan Huat
Quality Inspections - Singapore

November 13, 2015

A. Good day Ng Tuan Huat.

Surely you are working toward a specification.
I work with a minimum/maximum coating weight requirement.
I would think the variations in colour tones is a process control issue.
You state all other parameters are within acceptable limits.
What are your parameters? Do you have a water-break free surface on the substrate before phosphating? Are you using an acid dip before phosphate? I have found dilute HCl to produce denser crystals.
Are you using a dilute chrome solution after phosphate, and if so, what is the concentration?
What is your free/total acid ratio? What is your iron concentration? What is the temperature?
All of these parameters affect the crystal structure of the deposit.
What is the substrate, and does it contain chromium, and if so,how much?" Max 2% will prove difficult.
Do you examine the deposit under a microscope to determine the density of the crystals? If so, can you see the substrate in between the crystals?
Are you conducting salt spray testing, and if so, what is the outcome?
There were only three questions in your posting, and I have posted you a total of eleven.
There are many variables involved to produce a uniform/acceptable deposit.
Hope this helps.


Eric Bogner, Lab Tech.
Aerotek Mfg. Ltd. - Whitby, Ont., Canada

November 30, 2015

Q. I am facing a problem in distinguishing two washers having almost the same dimension and both are Manganese Phosphate coated. How can I ask the two source to make some visual difference? Please answer, it's urgent.


Manganese Phosphate color variation within the same lot of parts

March 22, 2016

Q. I would like to know if anyone has experience with some steel parts that are manganese phosphate all together as part of a big lot show some brown/pinkish appearance

Rodrigo Ramos
- Salisbury, Maryland, USA

March 23, 2016

A. Good day Rodrigo.

Good Info @ letter #26694. This web site has a plethora of info!


Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada

Three Amigos -- What is a plethora?
March 2016

thumbs up sign Oh yeah, we have a plethora -->


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

March 9, 2017

Q. Respected Sir,

We have a new coating plant and are going to perform Manganese Phosphate coating. Customer specifications are written as: Zn Manganese / Nickel Manganese / Manganese Coating for different parts. What is the difference in all above three or are all above same?

Are there different types of Manganese Phosphating?

Nitin Gunjal
- Pune, India

March 2017

A. Hi Nitin. Unfortunately you can't ask a third party to put a fine point on someone else's slang and casual phrasing. There is no way for me to know whether "Zn Manganese" is intended to mean "Zinc Phosphating with additional dicationic ions of manganese" vs. "Manganese Phosphating", or what exactly "Nickel Manganese" was intended to mean, or whether "Manganese Coating" was intended to mean "Manganese Phosphating". Guessing is not the way to proceed; you must get clarification and hopefully mutually agree to a more carefully worded specification.

Normally there will be additional criteria like salt spray hours for testing, coating weight, compliance with a specific standard number from a generally recognized organization like ASTM or ISO, etc. Good luck.


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

June 26, 2017

Q. Dear readers,

I have set up zinc phosphate and manganese phosphate coating plant in our company.

I wanted to know if we can distinguish parts which are zinc phosphate coated from those coated with manganese phosphate.

Karthik Padmashali
- Pune, Maharashtra, INDIA

June 2017

A. Hi Kathik. Tracking the parts seems a lot more practical than mixing them and then chemically testing every individual piece to re-separate them.

Although I personally have very low confidence in visual identification of finishes, I think only you can answer whether your people can reliably visually sort your specific zinc phosphate from your specific manganese phosphate. Sorry to be of no help :-(


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

July 20, 2017

A. Regarding visual identification of phosphate coatings:
Zinc phosphate coatings will typically exhibit a dark gray color. Manganese phosphate coatings will typically exhibit a dark black color.

Daniel Englebert
Imagineering Finishing Technologies - South Bend Indiana

Glittery Manganese Phosphating

August 20, 2017

Q. Hi,
I am Jerin. I am from U.A.E. I happen to do manganese phosphating on a small component but ends up with glittery look. Can anyone help to know the reason why?


Jerin James
- United Arab Emirates

simultaneous August 21, 2017

Hello Jerin, are you using a surface conditioner before the phosphate?

Yohands Rey
KAT - Chihuahua, Chih., Mexico

August 22, 2017

A. Hi Jerin!

You seem to have big crystals in your part, you can use a grain refiner (your phosphate vendor may have one) or you can manage the crystal size with your Total Acid/Free Acid relation.

Big crystals for a big relation (TA/FA), small crystals for a small relation. Your vendor may have an operative range for this parameters, you can work to manage this value low and get less crystal size and less "glitter".

Hope it works! Best regards!

Daniel Montañés
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina

August 23, 2017

thumbs up sign Hi Yohands, Thanks for your reply. In order to remove oil- film/grease from surface, we cleaned the surface with normal detergent, then had a rinse.

Hi Daniel, Thanks for your reply. Perhaps that would be the problem. I would try as you suggest :)

Jerin James [returning]
- United Arab Emirates

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