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Manganese Phosphate Coating Thickness

January 21, 2009

I just became the engineer for our company's plating lines. We apply manganese phosphate coatings to a variety of parts, and I would like to gain better control of our coating thickness. We monitor total acid, free acid, acid ratio, and iron percentage in our coating baths. I see little to know correlation between any of these variables and our coating thickness. Aside from time and temperature in the baths, what other variables can be used to predict/control coating thickness? Thanks in advance for your help.

Michael Costello
Plating Engineer - Grand Junction, CO, USA

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".

James Watts
- FL

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.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

Dear Mr.Michael,
I have worked in Industries as a Reasearch scientist depositing films on Automotive components and later on worked as a Lead Aerospace coating Engineer on Aerospace and Aeronautic Space Craft componenents. Your quetion is rather interesting provoking me to response 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.

Bassey J. Udofot Knoxville, Tennessee

Thank you all for your responses to my question. As a follow up for Mr. Probert, how cool is cool enough when the bath is not in use?

Mr. Udofot, for a manganese phosphate chemical conversion bath, what is the optimal operating range for the pH?

Thank you all again.

Michael Costello
- Grand Junction, CO, USA

Michael, you do not have to contrrol "current density" in an immersion process like manganese phosphate, there is no electreoluysis 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 185F, 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 supppliers say to use Sodium Carbonate to reducde 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!!!

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

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 added 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, it'll indicate how Military suppliers control their coatings.

Brian Terry
Aerospace - Yeovil, Somersset, UK

April 18, 2009

Dear All,

Please help, i need to know how to convert the following unit of measurement :

Zinc Phosphate 5 g/m2 is equal to how many micron ?

Manganese Phosphate 1500 mg/ftsq is equal to how many micron ?

Thank you in advance for your kind attention.

Rgds,
Hary

Hary Gho
Manufacturer - Indonesia