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Dulling/Greying experienced after QQ-P-35 TY VII Passivation of 304 Stainless Steel

I am a production engineer at manufacturer of military equipment. We recently had our first experience passivating per Type VII of QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil] (20-25% nitric acid @ 120-140 °F for 20 minutes)(we typically do Type II). The parts of interest are made of 304 Annealed (ASTM A2760). After passivated the parts had a gray matte finish (they were nice and shiny to begin with). Is this something to be concerned with? I have read numerous other postings of individuals seeking to avoid this outcome, but none clearly stating if this is a BAD (impermissible)thing. Per the spec a slight discoloration is acceptable, but that is subjective. Any incite is greatly appreciated.

Thanks,

Martin Sylvester
production engineer - Philadelphia, Pennsylvania
2007

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First of two simultaneous responses --

I think (reasonably sure) that you are seeing a fairly considerable etching of your parts. It could be from a build up of chlorides from the water or drag in from somewhere else.

The state of the alloy and the heat treat state may make it more vulnerable than other parts in the same tank. If the parts are small, I would set up a lab or pilot plant tank made up with reagent grade nitric. If your parts are fine in that, save it and make up a small tank with nitric from another vendor and use DI or distilled water. If parts are fine in that, make up another tank with your present acid and DI water. If the parts are fine there, dump the original tank and remake it up. (use DI water) At least, you will have a very good idea where your problem is.

James Watts
- Navarre, Florida
2007

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Second of two simultaneous responses -- 2007

My insight: Definitely a concern. If an outside vendor is involved, I'd incite you to require a Corrective Action Report. Possible causes: The cleaning is inadequate, the passivation solution is contaminated, or the material isn't 304.

A water-break test after cleaning is an easy check. Also, verify the material. 304 SS, especially annealed (solutionized & quenched), is easy to bright passivate. And, why bother with Type II or VII passivation when Type VI works?

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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+++++++ Photo from Martin Sylvester

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Great! Thanks for the information and the grammar lesson.

Best regards,

Martin Sylvester
- Philadelphia, Pennsylvania, USA
2007

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It does sound like etching, something that will happen with nitric acid if parts are left in too long, but will never happen when using a citric acid based passivation bath. Let us know if we can help.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2007