Manganese phosphate coating on steel

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

Regards,

 Yes indeed, this site has a plethora.

Regards,

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. 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. 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.

Regards,

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!

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 :)

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.

Regards,

? 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!

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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.

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!

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!

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.

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.