SS Sheet Permeation

[kcme2005]

Does anyone know where I can find information on the properties of SS sheet as it pertains to permeation at elevated temperatures?

We have .001" 316L stainless steel sheet used as a pressure diaphragm with approximately 200 psi of Nitrogen behind it.

We have run numerous units though aging tests (one week at 400°F) and every sample has shown a reduction in pressure, though these reductions have not been consistant. Some sample units will decrease 5 psi and others up to 50 psi.

We have ruled out all possible causes of this - except permeation throught the diaphragm. Has anyone seen this phenomenon before, or know where I can find information concerning this?

 

[musashi99]

We make pressure transducers. Sounds like a leak. Any way you can perform a helium leak check?

 

[kcme2005]

We did helium leak checks on a few samples - but found no leak.
Our only idea was that the gas was permeating at the elevated temperature, but showed no leak at room temperature.

 

[btrueblood]

Is the diaphragm sealed by welding, or by a polymeric (O-ring) seal?

 

[btrueblood]

You say you did a helium leak check, and found "no leak" -- you really mean, no leak above the detector's "background" leak rate - what is that value (what reading does the detector stabilize at with the inlet port plugged?). Use that background value as a leak rate, and calculate what change in pressure the trapped volume would undergo as a result.

If the background rate is still too small to be causing the observed pressure drop, then you may be seeing the N2 becoming chemically absorbed (diffused) into the metal; you could look into having a chemical analysis done of the sheet material before/after testing...

 

[kcme2005]

This is a welded diaphragm, and our pass/fail value for performing helium leak checks is 1E-07 sccs.

We perform helium leak check on all units before any testing begins - no leaks were detected. After aging the units and observing the decrease in pressure, we re-checked the helium leak - still no detectable leaks found.

Diaphragm is extremely thin - .001". Anyone know where I can find permeation information for 316L SS?

 

[EdStainless]

There is no permiation of any gas through rolled SS sheet.
You can get gas leakage if there are defect paths. I have seen this in bar products along the length of the bar.

How are you He leak testing? You need to make you test more sensitive.

My guess is that you are welding through inclusions in the SS and creating leak paths at the weld joint.

Are you using special grades of SS? If you have leaks you need to be using ELI or high purity materials. The remelted grades will have fewer inclusions.

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