qcboy

Topic: requirements for black oxide mil-std-171 3.3.4?

Anyone know if there are appearance requirements (explicitly stated or implied) for black oxide as required by mil-std-171 3.3.4? we're a machine shop doing a subcontract for a military part that requires black oxide. The parts came back from our outside vendor after black oxide treatment, and they have little or no difference in appearance from before they were sent out. On a previous job, they used an aggressive acid bath that destroyed the tight tolerance on an O.D., so this time around they omitted the acid pre-bath. I flagged the parts, since the quality of the black oxide was in question, but the boss/owner shipped them anyway, saying as long as the outside vendor supplied a cert that says they did the black oxide process it would suffice...that there was no appearance or cosmetic requirement.  I would think that the cert would be in question if on visual inspection there was little or no evidence of the black oxide and especially if they omitted a step in their process. I dont' think an acid bath is required, but it looks to me that the process was no sufficient. Any opinions from people who know this process? or anyone who does QC? thanks.

jimtrottier

Re: requirements for black oxide mil-std-171 3.3.4?

I do not know about the military standards but black oxide does usually have a rich black finish and and the application of oil  helps keep it that way.  As for the pickle (acid treatment) is to remove heat treat scale and it should be done before the machining process so all spec's can be met. Black oxide does not apply anything to the metal like plating does but oxidizes the outer layer of the steel itself for corrosion resistance. I hope this helps.

qcboy

Re: requirements for black oxide mil-std-171 3.3.4?

Thanks, Jim. that jibes with other info. i've found. Strangely, the requirements that are not specific to any military application seem more comprehensive and specific than what I can find in mil-std-171 3.3.4. But, I guess the name of the game is to have a piece of paper that says it was done right, as opposed to actually doing it right. I guess that's probably how 99% of the world actually works or doesn't work.