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topic 34052p3

Manganese phosphate coating on steel

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A discussion started in 2005 but continuing through 2020

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
Aloha -- an idea worth spreading

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
Aloha -- an idea worth spreading

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

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-embrittlement).
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

January 31, 2019

A. Dear sir, to all,
Please don't use alloy sandblast media for cleaning the parts. Please use only sand which is from river or sea. Alloy media streaks or affects MS which is not good for MNP coating.

MNP chemicals react only on MS well. MNP is amalgamation of MS & Manganese.
If there's any other metal in between while process of MNP coating, we can't get good results.


Patwardhan Anant
- Pune Maharashtra India

August 16, 2019

A. In the firearm business, Aluminum Oxide is the preferred choice in blasting material. As is well known 90% of the problems with the bath occur in preparation so if cleaned correctly following the blasting you shouldn't have any problems with the Aluminum.

If you are really concerned, glass bead blasting is an excellent solution, although some results may vary depending on the shape of the material your'e blasting. With rounded objects the glass does not perform near as well as the Aluminum.

If your main problem is a lighter coating I sincerely doubt Aluminum is your problem. In MNP normally the coating is lighter with initial use and darkens over time.

If it is a fresh bath, that is your primary problem, it simply needs to be seeded first with Iron or even Titanium.

You are correct in your assumption as well. If the alloy itself has contaminants it can also lead to such a coating. If using Nitric Acid, you may want to check the content of that as well.


A. JUST A FYI -- Please Stop Using MnO2

While you will get a "coating" using MnO2. This compound is not a very good compound to use to get MNP. MnO2 does not readily dissolve in phosphoric acid. MnO2 is an old wives tale recipe as it is easily obtained from a D cell battery.

With MnO2 you are actually creating more of an Iron Phosphate coat then a Manganese coat which is more desirable.

A much better compound of Manganese to use is Manganese Carbonate which is just as cheap, readily available, and converts almost instantly and completely to Manganese Phosphate in an MNP bath.


Q. I am not quite sure which form of Titanium is best for the phosphate bath, activation or passivization.

I have plenty of Titanium Dioxide, but is this compound useful at all in anything regarding the Manganese Phosphating process?

Perhaps a conversion/ transformation into a Titanium Phosphate would be more useful, but what would be the best way to facilitate that? I have attempted with Phosphoric Acid, but as expected it did not seem to react, even boiling for hours.

Would a Sodium titanite be more useful? or a Titanium Salt (no chloride)

On a different note, would ZrSiO4 be useful in a post passivation process? If so else would possibly be required?

As I can create a Chromic Acid Rinse, should I stick with that?

From research the Chromic Acid on the Manganese Coating should be kept at a pH of 4-6. Is this true?

Brandon Blair
OuterGravity - Dallas, Texas, United States

September 9, 2019

Q. Hello,

We have installed a new manganese phosphating line in our plant with the intention of obsoleting an old line. In our new line we are using the same chemicals (cleaner, grain refiner, manganese) the same time and temperatures and are coating the same parts as in our old line. The problem is on some areas of some parts the coating appearance from the new line abruptly changes from black to gray in random patterns. SEM analysis shows the lighter colored areas have smaller and more rounded crystals. This color variation causes issues with our inspection camera systems at a subsequent operation.

34052-3a   34052-3b

The old and the new line are both functional, so it sets up a great opportunity for testing, and we have done a LOT of testing! From this testing I feel confident that I can rule out a lot of possible causes like; chemistry, cleanliness of parts, tank size and agitation, and part spacing. At this point I have no idea what else the issue could be. I have had many experts come in and review our situation with no luck. Even though I don't think chemistry is the 'problem', I have tested a few different chemicals in case that could be the 'solution' to the unknown problem.

Does anyone have any ideas what the cause could be? Is there any situation where some kind of electrical charge could influence the crystal growth?

Thank You.

Jeff Thelen
shop employee - Saint Johns, Michigan, USA

September 13, 2019

A. Regarding the black and gray. Go back and study the agitation of the grain refiner to be sure the grain refiner is hitting all surfaces.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

September 16, 2019

Q. Thank you Robert for the suggestion. We have a propeller mixer in the old line and the new line. I have tried varying the speed of the mixer and using intervals of ON/OFF within the cycle but did not notice a change to the coating.

It won't be too easy to do but I could perform a test where I add part oscillation to the Grain refiner step.

Generally speaking is it a good idea to have the parts oscillating in the grain refiner tank?

Jeff Thelen [returning]
- Saint Johns, Michigan, USA

Double dipping in Manganese Phosphate

September 12, 2019

Q. I have a question: would there be any issues with double dipping a part in the Manganese phosphate solution that is too long for your tank.

Chuck Hawkins
- Long Beach, California

Adhesion Loss in Manganese Phosphating

September 19, 2019

Q. Hello Everyone, we are having a Nihon Parkerizing Manganese Phosphate bath. We are suppose to phosphate AISI 4130 [affil. link to spec at Techstreet] carbon steel material. We are experiencing loss of adhesion after phosphating. The phosphating layer comes off while rubber test and even when we rub by hand or cotton cloth. Please advise where we are missing the plot. Thank you in advance

subodh sawant
Shop Employee - India

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