Manganese phosphate coating: issues and answers

A. Hi Walker.
Of the general metal finishing texts The Canning Handbook [on eBay, Amazon, AbeBooks affil links] has quite good coverage of manganese phosphatizing, as does ASM Metals Handbook Vol. 5 ⇦ this on eBay, Amazon, AbeBooks [affil links] .

If you search the site and can't find the specific links you think you saw, or if what you see isn't fully applicable, you are always welcome to post your own questions -- that's the principal thing that this site is about.
It is best to explain what you are doing, what kind of parts are involved, and what the problem issues are. Photos can be e-mailed to mooney@finishing.com for posting with your questions.

Finishing.com doesn't offer private consulting services, but our Consultants Directory does list some people who may be able to help you privately. Best of luck! Regards,


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

A. Hi Cameron,

It is quite common for phosphated parts to go through a chromic acid rinse post phosphating. Might there be a typo in your process description regarding "prior to"?

Is this a new issue that has arisen in a process that was formerly quite successful, or is there no real history of satisfactory processing to this sequence? ... phosphating of stainless steel is a problem requiring special formulations because the phosphatization reaction occurs in response to a reaction of the metal to the solution and stainless responds differently than steel, so the reaction works differently. Hopefully a reader with actual experience in manganese phosphatization of chrome plated or chrome rinsed HNV3 parts will respond because it is hard to extrapolate from theory whether the chrome is affecting the phosphate, and it's difficult to demonstrate/prove that it is the problem. But lacking that for now:

Are the chrome plated internal bores masked/capped for phosphatization?

If you are equipped to do so, it probably would be quite informative to analyze your phosphatizing solution for chrome content -- chrome is often troublesome to other processes. If there is no chrome in the solution, and the components are not super expensive, you could take a few and either not chrome plate them or strip the chromium in hydrochloric acid to test the result.

Luck & Regards,


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

Manganese Phosphating Solutions

on eBay or Amazon

(affil link)

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 [on eBay , Amazon, AbeBooks affil links] )

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.

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.

Topics 114/65 "Sludge buildup on heating coils in phosphating tank" and 51/42 "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.

Regards,

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.

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.

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.

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.

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,

A. The part cleaning to be rechecked. During the set temperature either oil or other solvent start releasing. Or, check the oil content in your coolant, if there is any leakage of lubrication oil during machining also will effect. So clean the part in a better way.

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.

Regards,

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

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.

Regards,

A. All depends on your solution temperature. Study the temperature specification given by solution provider follow it. There won't be any reduction of temperature with in 5 or 10 minutes.

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.

Regards,

A. Good day Rodrigo.
Good Info @ thread 266/94. This web site has a plethora of info!
Regards,
Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada

 Oh yes, we have a plethora, guapo ⇨
Regards,

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

A. I had experienced it with different grade of materials. If you have facility to provide bead blasting surface it can be controlled to a certain extent.

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.

Regards,

A. Ji, it all has its own different structures as well as application. Chemistry itself has differences. It is better to take advice of your solution provider for the right process.

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 :-(

Regards,

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.

A. Dear Jerin,
Glittery appearance is mostly related to Iron concentration. Check solution parameters. Modify solution to the optimum. Give chance for acidic formation on the surfaces.