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topic 34052p2

Manganese phosphate coating on steel

1       2

A discussion started in 2005 but continuing through 2018

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
Pine Beach, New Jersey
Striving to live "Aloha"

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

March 2016

thumbs up sign Yes indeed, this site has a plethora.


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

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
Pine Beach, New Jersey
Striving to live "Aloha"

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
Pine Beach, New Jersey
Striving to live "Aloha"

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 Montanes
TEL - N FERRARIS - Canuelas, 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

October 5, 2017

Q. Reading this thread brought a bit more clarity for me regarding phosphating process. Thanks to everyone here.

My customer sent me some phosphated samples which had no oil coating, grayish tone and very smooth surface, kind of self lubricated finish.

I am told there's a lacquer available for such applications. Anyone have any experience in this?


Sukhjot Singh
ASCO - Edmonton, Alberta, Canada

October 2017

A. Hi Sukhjot. I think it is important to find out from your supplier what kind of phosphating process and sealer they applied if you don't know yet. I would not try to guess what a finish is from its appearance.

Although there are variations, phosphates are usually one of three general types:
1. Iron phosphates, which are thinnest, cheapest, least corrosion resistant and always painted.
2. Zinc phosphates, which are of middle weight, and usually painted, although occasionally oiled.
3. Manganese phosphates, which are heaviest, and designed to be oiled rather than lacquered or painted. Managanese phosphating is a well known "break in" finish for moving parts.

Unfortunately, 'lacquer' is slightly vague, it is often used to mean a water soluble final dip, or a clear 'paint' among other meanings.


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

October 31, 2017

Q. Dear professionals.

I need to know the actual reason and or need for the iron concentration test when testing the parameters of a manganese phosphate tank.
I have several vendors that all use different chemicals worldwide, some technical data sheets say nothing of iron concentration, however, the written coating specifications do have titration instructions for the analysis of the above.
Is it the variation of the different chemicals?
Is it an essential test?
What do the results tell us?

Thank you.

Steve Page
- Leeds. West Yorkshire. UK.

November 1, 2017

A. Hi Steve,

Manganese phosphate, in general, works with accelerant addition. This accelerant works by oxidizing all iron in solution, but iron dissolves from the parts you process, and this iron deposits with manganese in the phosphate crystals. Actual manganese phosphate coating has iron in its composition, but the actual solution has not.

If you mean the iron test strips for "iron concentration test", as I believe you do, I'll tell you: You don't want iron in solution because you want more manganese in your phosphate, you want to control the deposit and you don't want green if you are doing black phosphate. Manganese phosphate works without iron, and you test Fe(II) presence with the strips. You add accelerant in order to control iron, and control the reaction and grain size, and it works really well.

I don't know if I actually answered your doubt, so if I didn't, feel free to ask again maybe with an example.

Best regards!

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

December 22, 2017

Q. Hello Sir
We are doing Mn phosphating on AISI1208. After some components phosphate white and black spots observed on phosphate material. Due to this we do lots of quantity rework.

Dhanjeet singh
- Ahmedabad Gujrat

December 26, 2017

A. Hi Dhanjeet,

Please tell us how you clean your parts (alkaline degreasing? acid pickling? how many rinses?), parameters in phosphate coating (temperature, free and total acid) and sludge removal system you have so we can help you with some recommendation to improve your situation.

Best regards :)

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

Manganese phosphating coating failure on rub test what is the reason?

April 7, 2018

Q. We are doing manganese phosphate coating. Failure in quality during rub test sometimes; not forming the layer sometimes; failure with the black coat being removed. What are the reasons and how to maintain the quality? Please advise.

Rama Panfian
oilfield services - Dubai UAE

April 2018

A. Hi cousin Rama. Your phosphatizing is not being done properly if it is removed by the rub test or if it doesn't form. Rarely can anyone suggest one simple change to what you are doing when we don't know what you are doing.

But are you sure the parts are spotlessly clean before they get to the phosphatization tank? Are you carefully maintaining your free acid and total acid? Are you using a grain refiner as suggested on this page? Are you operating at 95 °C or at the proprietary vendor's recommended temperature? What spec. no. is the rub test, or what are its details? Thanks.


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

Manganese Phosphate Coating Visual Issue

July 10, 2018

Q. I'm having an issue with the visual appearance of my product. One of the specifications from the customer is a uniform black coating, my company has two other sites around the world with identical lines to mine and are able to achieve this specification for the same product. The issue we are facing is black/grey dots. We control everything in the line, the concentrations of every tank, free/total acids, everything right down to the pH and conductivity of the rinse water. So my question is, what causes these visual defects and what would be the best option to fix them? Thanks

Samuel Hunt
- North Vernon, Indiana, US

July 16, 2018

? Hello Samuel!

Can you describe the part you are coating (the material, if it is machined or forged, oiled or else…) and how you clean the parts? If you are controlling all your phosphate parameters, maybe the issue is where you are not looking…

If you can send some photos of the parts (after-before), maybe we can have an idea of what can be happening.

Best regards!

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

July 17, 2018

Q. Hello Daniel, thank you for responding; it is a pin end con-rod that is being coated, it is a machined surface with no oils. For the phosphate parameters, we monitor free/total acids, iron, and copper concentrations. Due to low surface area being run through the tank we do have an issue with iron concentration being low, we also use a pump to recirculate the fluid which introduces more air into the line, however this design works in two other locations without issue.

In the attached pictures you can see the black speckling and blotching on the part. We use an alkaline soak cleaner and an acid etch in our process line, both of which have are monitored for concentration and temperature. Thank you again.

Samuel Hunt
- North Vernon, Indiana, US

Ed. note: Unfortunately the pics came through only as unreadable files from some application which we don't have and probably can't use on the WWW. Please re-send them in .gif, .jpg, .jpeg, .pdf, .png, or .tiff format. Thanks.

July 24, 2018

A. Hello Samuel, 'please send the images in another format because we are not seeing them...'

I'm really interested in what you call "blotching", because that would be entirely a pretreatment issue, but dots or "speckles" could be a phosphate issue because of sludge agitation, for example.

Best regards!

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

July 24, 2018

Q. Sorry about that, didn't realize they didn't go through. Part not run yet:


Parts with black speckling, gray dots, and streaks:

34052-2blackspeckling  34052-2graydots  34052-2streaks 

I'm not sure if it could be the sludge or not. We use a pump to agitate the process, but it's actually tilted to go towards the parts and don't face the sludge at all. It is possible to see to the bottom of the tank, even in full production. We have never seen or been able to detect any floating sludge either. Thanks

Samuel Hunt [returning]
- North Vernon, Indiana, US

August 21, 2018

A. Hello Samuel, sorry but I had some issues to resolve and could not answer sooner.

I can assure that if you can see the bottom of the tank while you are in production, sludge is not the problem. Speckling, grey dots, and streaks are usually symptoms of poor cleaning (or too much sludge, but we have just ruled it out). I see it is a machined cast iron part, so I would seek:

1) What do the machining process to lubricate, or if it is a dry machining process. Is it the same design as the other locations?

2) Alkaline soak cleaner: Is it the same as the other two locations? See if it could be contaminated by silicones. We had some issue regarding foam production in this tank and silicone de-foamers used in wastewater, so the phosphated parts had marks on them. You can't see whether the alkaline cleaner has or not any silicone, but you could see if the addition of some product of the sort could end in this tank.

3) Grain refiner: Do you use any grain refiner between pickling and the phosphate tank?

Best of luck!

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

August 21, 2018

A. Two problems: (1) You cannot acid etch ahead of manganese phosphate. (2) You must use a titanium salt grain refiner.
Blast the part and use grain refiner.

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

August 22, 2018

Q. Thank you both for the response,

To answer you Mr. Montanes:
1. All chemicals and designs of the lines are identical and purchased from the same manufacturers.
2. As stated above it is the same, I checked with process up stream and none of them use an silicone based chemicals. I don't see any other possible entry point for that either.
3. Yes after the acid there is a rinse and then it goes to the grain refiner,

To answer you Mr. Probert:
1. You can acid etch before, we've done it, and the coating is good. The main issue is it requires high temperature and causes rust on the rest of the part, even with RP.
2. Due to dimensional constraints, blasting is not an option, we've tried this as well already and seen the good results.

Again thank you for your feed back, I look forward to hearing from you.

Samuel Hunt [returning]
- North Vernon, Indiana, US

August 24, 2018

A. Hi Samuel,

To me, this looks like leaching of fluids, post phosphating, from inherent porosity in the casting. Always looks more pronounced on machined surfaces.

If it is indeed porosity you could try alternate hot and cold rinses to try and remove residual acid in the pores; alternatively, prior to phosphating you could try vacuum impregnation, using sodium metasilicate sealer, which could seal any porosity before you phosphate.

Brian Terry
Aerospace - Yeovil, Somerset, UK

November 28, 2018

Q. Dear all,
I would like to thank you all for your enthusiastic contributions on every problem stated here in this thread.
May I submit this one:
We're Mn phosphating maraging forged steel in our company in order to get that specific deep black coating as we all know it.
Not so long ago we've noticed grey-white rather linear indications on the treated surface. We don't know where they come from. The flow of these parts upwards is made of roughly machining - heat treatment - polishing - Nittal etch - sandblasting (120/150 grit Aluminum oxide, 58 psi,12 inches) - water rinsing - degreasing - demineralized water rinsing (+ water break free) - Mn phosphating (Mn carbonate, phosphoric acid, Ti based grain refiner, acid ratio around 5) - heat treatment (de-embrittlment).
We have spotted these indications right after de-embrittlement, but also right after phosphating.

We've performed several analysis such as microhardness and SEM thinking that the problem was related to forged steel itself. Now, we're after contaminations in the process.
Can anyone help to orient our investigation? Any documentation on typical defects with phosphating maraging steel.

Best regards

Noah Nzuamo
- Zaventem, Belgium

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