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Manganese Phosphate coating presence

Manganese Phosphate coating presence

Manganese Phosphate coating presence

Hello all,

I am looking for a chemical test which will help me to detect the presence or absence of Manganese phosphate coating on steel part.

Your feedback please.



RE: Manganese Phosphate coating presence

A simple non-chemical test is visual observation.  Steel is light gray or steel, while manganese phosphate is dark gray to black.

Concentrated hydrochloric acid will remove manganese phosphate.  So, put a drop of HCl on your unknown piece.  If there is no change, it was uncoated steel.  If you wipe away the drop area and it is lighter than it was, you removed the phosphate coating.

RE: Manganese Phosphate coating presence

Wouldn't conc HCL instantly react with steel- it would form Fe3+. I haven't such an experiment since university or high school but I bet the heat evolved would evaporate the water in the HCL.   

RE: Manganese Phosphate coating presence

If you put a drop of concentrated HCl on a CLEAN steel surface, not much happens within 5 seconds.  On a contaminated surface, reactions, bubbling, etc. can happen.

RE: Manganese Phosphate coating presence

Do you need a test to verify that the part has been coated vs. non-coated, or a test to verify the coating on the part is manganese phosphate as opposed to some other coating (such as zinc phosphate)?

If it is the first case, then the visual process or the chemical process CoryPad mentioned will work.

The second case is a bit more complex.  While I am sure there are spot tests for manganese phosphate, I am not aware of any specific ones.  

If I were given the problem, I'd control the process and verify coating immediately after it was applied.  It the part comes out of a manganese phosphating bath, and is covered with an acceptable coating, you can be pretty sure is has a manganese phosphate coating on it.


RE: Manganese Phosphate coating presence

Hello redpicker and corypad,

Thanks for the feedback.

The case is of first type i.e. to check presence of Manganese Phosphate or no coat.

Let me explain some more background:
We assemble Manganese Phosphate coated part with slight interference on to a shaft. We see some powders (presumably Mn Phosphate) coming out during assembly process.
So, we dis-assembled one pc to check if all of the Mn phosphate has been removed (bare steel is exposed) or still some amount of coating is remaining on the part.



RE: Manganese Phosphate coating presence

Another option is using SEM + EDS.

RE: Manganese Phosphate coating presence

With an interference fit you can bet that at least some of the manganese phosphate is removed during assembly.  In my experience iron and manganese phosphates are good for preventing rust in storage, and as a carrier for a rust preventative oil, or as a base for paint.  But for long term corrosion resistance, especially in situations where the coating is scratched/rubbed/abraded, you're better off with another means to prevent corrosion.

David Benson
Benson's Mobile Welding & Fabrication

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