Controlling Manganese Phosphate Coating Thickness

"Phosphating of Metals"
by Werner Rausch

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"Preparation of Metals for painting"
by Samuel Spring

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A. The bath chemistry. Most contain both accelerators and retardants which not only affect the rate of formation, they affect the structure. MnPhosphate is like a stack of needles. Some brands lay relatively horizontal in formation and some are nearly vertical. Dimensional measurement is significantly affected. Weight loss is the only true measure of the amount of phosphate in the coating, and measuring tools are the practical means of measuring coating "thickness".

A. It is necessary that you titrate for the total/free ratio just before you put the work in the tank, then if the free is too high, knock it down with manganous carbonate. When thru, let the solution cool as fast as possible, and never leave it sitting hot without a work load.

A. Dear Mr.Michael,
I have worked in Industries as a Research scientist depositing films on Automotive components and later on worked as a Lead Aerospace coating Engineer on Aerospace and Aeronautic Space Craft components. Your question is rather interesting provoking me to respond to you. Thickness of deposits is a function of time, current density, temperature and the concentration of the depositing species. pH of bath is one of the most sensitive variable factors determining thickness of electrodeposits that most people tend to neglect.

A. Michael, you do not have to control "current density" in an immersion process like manganese phosphate, there is no electrolysis to measure. Also, pH of a 200° F high acid manganese phosphate solution is not a factor. Start titrating the total and free when you get to 185° F; do it right at the tank side, by the time you have the T/F ratio you will be at 195° (which is hot enough). Knock out the free acid down to ratio with manganous carbonate, put the parts in, coat them. The industry talks in "coating weight" not in "thickness"; when finished, cool it down as fast as possible. Some of the compound suppliers say to use sodium carbonate ⇦ this on eBay or Amazon] to reduce the free acid; that will work, but makes more sludge which settles out on the shelves. By using manganous carbonate you are at least replenishing the manganese and making less sludge!

A. Bassey,

Manganese phosphate coatings are not electrolytic, they are immersion processes. pH is indeed critical in the reaction mechanisms of phosphate baths, but they are controlled normally by the free and total acid measurements and possibly by the iron content.

Thickness can be time dependent where a reaction does not go to completion (this is normally 5-15 minutes for most standard steels), otherwise the limiting factor is in the growth during the reaction phase.


Micheal,

There are so many factors that may affect your coating thickness as mentioned already; may I add just a couple of others:

Surface pre-treatment - if you use a process that roughens and increases the surface area this will increase your coating weight.

Grain refiners - this will create smaller crystals that compact more densely, increasing coating weight per unit area but reducing thickness.

Just thought I'd add those into the mix.

The usual way to check that you have a sufficient coating is to measure the coating weight. I would suggest you read MIL-DTL-16232 [from DLA] ; it'll indicate how Military suppliers control their coatings.

A. Conversion parameter is 0.7
x gr/m^2 * 0,7 = y micron
Example 6 gr/m^2 weight is equal about = 4.2 micron thickness

Regards,

Ilker,

Q. Dear Mr Ilker,

Can you help me, where do you get this calculation - reference specification.

Thank you

---

 Hi cousin Soul. The MIL specs don't discuss this conversion factor. Certainly it's just an empirical estimate derived from trying to measure the approximate thickness of these spikey, granular coatings, then stripping and re-weighing and comparing the weight loss to the estimated thickness. The more interesting question is why do you would want to know!

Are you trying for a precision fit and thus concerned about thickness, or has someone erroneously specified coating thickness instead of weight, or do your empirical results vary too far from this for comfort? People are drawn to answering questions only when they grab their interest, and an abstract discussion of conversion factors probably won't, but you may be confronting a potentially interesting issue that people would love to discuss.

Regards,

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

A. Hi Nora. As you see, we appended your inquiry to a thread which answers it. You are correct that "thickness" is not the appropriate way to express the quantity of coating, for the reason you mentioned, so it is expressed in coating weight.

Regards,

A. Carefully check the bath ratio. The balance of concentration (total acid) to activity (free acid) is critical to controlling the crystal size and maintaining uniformity. Consult your data sheets to get your optimum range and make adds accordingly.

A. Hello Kamrun,

Manganese phosphate tends to deposit more than 1 micron thickness (it's 1.3 g/m2). We could help you maybe if you give us more data:

- Which kind of materials you process.
- How is your process composed.
- Working parameters: TA, FA, accelerant, temperature, time...

Maybe with more data we can assist you out of this problem.

Regards!!

A. Hi Kamrul,

You could try lowering a bit your concentration to lower both FA and TA, and trying to keep your FA high (a little higher is possible, as you can go as far as your TA/FA ratio is OK in your technical bulletin).

Thickness is given by three parameters:

Total Acid, Temperature and Time. You go higher in one of these three parameters, and you get more thickness. Lower one of these, and you get less thickness. You can play with these three and see if you get some results.

In the middle, you can expect: rough parts, uneven phosphate coating (if time is not enough to react evenly with the part or if TA/FA is too high), and not coated parts (if you went too far lowering something). I can't predict how it can go but the path is clearer now than before!

Hope you get to some good result! Keep us posted!

Best regards,