recumbentbiker

Re: Class 4 Phosphate

We have a requirement for a MIL-DTL-16232 (formerly MIL-P-16232) Type M Class 4 phosphate coating on steel.  According to the specification, this is a phosphate coating that is chemically converted by reacting with an inorganic salt.  The only specification difference between Class 4 and Classes 1, 2, and 3 is corrosion resistance.  Class 4 requires 24 hours in salt spray whereas the other classes require on 1.5 hours (before application of any supplementary coatings).  I have not been able to find any information on what the process is to convert the phosphate coating to Class 4.  Is anyone familiar with this process?

Labrat

Re: Class 4 Phosphate

I am researching this question to get more information, but here are my initial thoughts:
Since the salt spray must exceed 24 hours, I would assume a heavy zinc phosphate layer (400 microinches), then the "inorganic chemical conversion" could refer to a chromate seal rinse. This would maximize the corrosion resistance, but not sure if it would pass 24 hours salt spray. Do you have these materials in-house to try a sample part and salt spray test?

recumbentbiker

Re: Class 4 Phosphate

We do not have the zinc phosphate.  All of our phosphate requirements, including this one, call for manganese phosphate.  The spec requires 24 hours with the Class 4 manganese phosphate.  We were to have some tests ran yesterday with the manganese phosphate and the chromic acid seal to see how long it would last but I don not have the results yet.  I did find a paper that talks to increasing corrosion resistance by the use of the chromic acid seal and work that had been done to find a non-chromated substitute.  Also I believe processing that will result in a finer grain structure will increase the corrosion resistance.  Thanks for you input.

Labrat

Re: Class 4 Phosphate

I apologize. I did not see the "type M" after the mil spec. I am interested in the results from your test. Please reply with your findings.

Ira Donovan

Re: Class 4 Phosphate

Recument Biker,

I apologize for my delay in responding, but I have been bidding on a large project.  There are two things that influence the corrosion resistance of manganese phosphate.

First, the grain structure of your phospate.  Finer grain structure, in my experience, will yield higher corrosion resistance.

Second, an appropriate passivation step, i.e. chromic acid dip.  This dip first stops the phospating reaction and then passivates the exposed free iron.

Some of the new treatments, includes the use of water soluble oils, to further enhance corrosion resitance.

I hope this helps you in your troubles.

Ira Donovan, M.S.F

Ira Donovan's Website