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Passivating a brazed 440C assembly [California] 

July 10, 2008

I am a mfg engineer at my company who manufactures a selection of valves & actuators for the aircraft industry. On one item, we use an assembly consisting of (2) 440C components that are copper brazed together (Mil-B-7883).

The components are machined, passivated, brazed, heat treated with a case nitride to HRC 58 to 65, and then simply cleaned and degreased...in that order. I was called to investigate why the assemblies in stock are "rusting". There is no doubt in my mind as to why.

I want to move the passivation to the final step in the manufacturing of these items, but I am unsure what will happen to the braze filler material when exposed to the passivation solution. Will the assembly be unharmed by the presence of copper filler in the passivate solution? Any insight you can provide will be really appreciated.

David Vranson
Finish Specifier - Valencia, CA , USA

The nitric based passivation solution will aggressively attack the braze. Worse than that, I think that after a few runs you will get a copper immersion plate on some parts of the valves.
Very faint but there.
Talk to the folks at this site that sell citric based solutions. It might be less aggressive.
440 is not rust proof, even when passivated. It is rust resistant. I assume that you are using a flux for the braze. I think that the flux will degrade the passivation.
My guess is that you will go to using a light oil and package each valve in an appropriate plastic container( baggie) Note that some of these will also cause rust.

James Watts
- FL

David

ASTM A-967 [link is to spec at TechStreet] advises against passivating nitrided parts. See appendix X1.8

Willie Alexander
- Colorado Springs, CO

Good afternoon:

Just wanted to add a comment to what Mr. Watts has posted. Nitriding stainless steels further reduces their corrosion resistance. The nitrides formed at the surface will be chromium rich. For more details, you may want to consult the ASM Heat Treaters Guide for irons and steels.

Steve Bizub
- St Louis, MO

Nitriding issues aside, I can back up what James Watts said. Citric acid does attack copper, but only slowly over a long-term exposure. For the amount of time of a passivation process, a copper braze would not be harmed.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA