Hydrogen Embrittlement Relief Required After Passivation?

A. Hydrogen embrittlement is known to happen with titanium, although tests with citric acid products formulated correctly have not been know to cause this. We do not have any data with nitric acid, but you could search the internet for data on this.

A. In the aerospace industry it is highly unusual to bake for hydrogen embrittlement relief after passivation. I won't say it's never done, because I don't know what everybody does, but we don't, nor do my colleagues at Boeing, Lockheed, United Technologies, Rolls-Royce, etc. So I would say no, baking after CitriSurf is unnecessary.

If the chromium plating covers all the surfaces, I would not bother to passivate beforehand. If the chromium plating is partial, then I would passivate the entire part before plating. The plating shop will activate the surface appropriately, allowing the chromium to stick to the stainless steel.

Chromium plating is not considered a passivation.

A. We haven't seen hydrogen embrittlement problems with stainless passivated with CitriSurf. It seems to be an issue only when passivating with nitric.

I agree with Lee on not bothering to passivate if the entire surface is getting plated. For that matter, if the entire surface is getting plated, I wouldn't even bother using stainless.

A. This discussion is going on and on. For medical parts the best option is electropolishing, which provides optimal passivation and appearance. Even for surgical instruments, the passivation layer is so strong, that strong acids do not destroy it for an extended period of time.

adv.
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A. That's an interesting question to me, Mike. Electropolishing per se wouldn't seem to me to be capable of creating hydrogen embrittlement since the parts are strongly anodic and drowning in oxygen. And passivation in oxidizing acids like nitric and chromic would seem somewhat similar. But hopefully someone with actual experience in implementing or not implementing embrittlement relief after passivation will chime in.

Regards,

A. Hi Mike!
Generally, the efforts to insert a hydrogen embrittlement (HE) bake requirement into AMS2700 for passivation get defeated in committee because of the lack of evidence that passivation causes an HE problem. Remember that passivation is done only for stainless steels, so the ones that are at risk are 440C, 420, BG42, other martensitic stainless steels, and the precipitation hardening grades like 17-4 and 13-8. These generally aren't as susceptible to HE as are the high carbon (acicular) martensite alloy steels like 4340 and AerMet 100.

Electropolishing is a different animal. HE is bad in electroplating as you are making your part a cathode, sucking in positively charged hydrogen ions. But in electropolishing, you're making the part an anode. You're not going to drive positively charged hydrogen ions into a positively charged part. So no, we'll never require an HE relief bake for electropolishing.

If you need to discuss this further, Ted has my contact information. Or just look- I'm not hard to find.

Good luck!

Ted and Lee, thanks for your replies!

My takeaway is that it isn't required for electropolish at all, and that since there are contrary opinions on the passivation process and since 90% of the parts that I process are aerospace parts which may or may not be flight critical, I will err on the conservative side and bake out only the 440C, 420, BG42, other martensitic stainless steels, and the precipitation hardening grades like 17-4 and 13-8. Thanks again.