titanium and stainless steel galvanic corrosion

[Mark Scatoloni]

I have a 21-6-9 stainless steel (CRES) hydraulic tube connected to a 6al-4v titanium fitting that is subjected to a 56-day salt spray test per RTCA/DO-160_Sect 14. Should the CRES tube connection be showing "rust" at the contact/interface (for CRES and titan)? Also, the connection has apparently weakened since following the exposure period, is failing a minimum burst-to-failure test. The CRES tube is "bare" and the titanium fitting has solid film lubricant in the bore between the tube OD and fitting ID. no other coatings are present. Any feedback?

 

[TugboatEng]

Even without the aspect of galvanic corrosion you mention a contact point which could create conditions for crevice corrosion. Nitronic 40 advertises corrosion resistance similar to 304SS which is very sensitive to crevice corrosion.

 

[EdStainless]

Are you really sure that you have a reduction in strength?
You built and tested control samples, right?
There is no way that you will get SCC in near ambient temperatures.
As for rusting, how wast he 21-6-9 tube sample prepared? Was it acid passivated after forming?
Even with a Ti fitting people usually use a SS ferrel and nut on the SS side of the system.
I would suspect iron on the surface from your tools initiated the corrosion.
You should only see galvanic corrosion effects in a wet system. There shouldn't be water pooled on the surface of a sample in salt spray testing. Or was the sample oriented in a way to trap salt water in the fitting?

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P.E. Metallurgy, Plymouth Tube

 

[Mark Scatoloni]

To Edstainless:
I believe the 21-6-9 CRES tubes are passivated. These are aerospace quality tubing.
The same "unaged" tube assembly components and tubing were burst tested to failure and passed min required psi.
The "aged salt-spray-tested" tube assembly failed aprox 33% short of the required min burst psi.
There is a very small gap between where the tube enters the fitting and the fitting swages/crimps down on the tube for sealing.
The salt spray test is sever with 48 hr cycles in environment and 48 hrs outside (ambient) drying for a cycling period of 56 days total in-and-out.
The post-burst test failed at the swaged-joint (released grip).
I'm leaning more towards crevice-corrosion possibility?

 

[TugboatEng]

22-13-5 (Nitronic 50) will certainly resist crevice corrosion better if you do determine that to be the cause of the weakening.

 

[EdStainless]

I would lean toward Crevice corrosion, but if that is the case you should see a clear difference between ones run joint up and joint down.
You also should be able to take one apart and see the pitting in the joint.
And they should be bleeding lots of red-brown fluid.
Are these Allen style fittings with flare, collar, and nut?
What material are the collar and nut?
I find it hard to see what would be loosening the joint.

Did you controls see the same temp cycles?

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P.E. Metallurgy, Plymouth Tube