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QQP-35C vs. ASTM A967

 

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I am a mechanical engineer trying to address a surface treatment problem. We are currently using the mil spec QQ-P-35 [link is to spec at Defense Logistics Agency, dla.mil] C type II to passivate our 416 SS. However, I have heard from one cleaning place that the ASTM standard ASTM A967 [link is to spec at TechStreet] has replaced the mil spec. Can anyone give me some insight as to what the differences between them are and how it affects me? Also, if the spec we are calling out is not compatible with our material, can someone suggested one to replace it?

Thank you.

Pam

Pamela Kong
Santa Clara, California


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Dear Pam,

Unless you would like to possibly destroy parts you must investigate the ASTM spec. Look at finishing.com's FAQ on passivation of stainless steel and you will learn from the experts in the field...suffice to say that I saved myself a lot of grief by going with the ASTM spec.

I don't know if you are a job shop, or a captive shop, and as you will see from the write-ins on the FAQ...it makes a lot of difference. I don't know where your alloys are purchased from, and that makes a huge difference too. Please pay particular attention to the section on tooling changes (frequency) and how they can affect the surface composition of the alloy, and the necessity for sufficient cleaning time (up to 40-60 minutes) to remove all possible oil contamination from your parts. The very characteristics that make for free-machining stainless alloys (is it 416se?) also tends to trap oils. If you have the opportunity to extend your "process improvement technology a.k.a. Deming" to your supplier, or to the manufacturing side of your operation, you may be able to minimize the possible negative effects of overheating due to infrequent tooling changes. Selecting the best possible lubricating solution (that won't impregnate ! itself into the alloying constit uents of your alloy) will help. Ensuring that what you bought (in terms of alloy) is what you really got, is also important. If I were you, I would do some very rigorous multiple-factor variance analysis coupled with copper indicator solution tests, as well as appropriate salt spray and other testing (re: appropriate ASTM specs).

Going from nitric to either citric-based or phosphoric acid solutions would be real advantageous. In fact, if your parts are expensive...investigate electropolishing in phosphoric acid solution. I spoke to ASTM people who were responsible for drafting the spec. Though they wouldn't directly admit it, (or accept any liability for it) they were in effect saying that the reason they drafted the spec. was to encourage and in effect justify what med. eqpmnt and other manufacturers had effectively discovered for themselves...nitric is way too dicey, QC of stainless alloys too problematic, and consistent QA of machining processes (re: bit changes and lubrication) not possible in the job-shop world. Again, I refer you to the excellent compilation of passivation problems covered by finishing.com's FAQ on the subject at their web-site. Good luck, and here's hoping you can get the nitric out of your shop!

Mike Solvie
Spokane, Washington

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