Water vapour migration through stainless steel

[ramseng]

Hello all,

I read a line a technical manual recently which stated that water vapour can migrate through stainless steel piping. The context is a high pressure (300 bar), very dry (dew point is <-50 deg C), air system. It stated that it is possible for water vapour to migrate through the pipe wall and to therefore be wary of the dew point increasing over time. It did not seem to be talking about migrating through loose/leaking fittings, but through the pipe wall itself.

I have never heard of this before - and could not find any references to it in some quick research.

Is stainless (or any other steel, or metal for that matter) permeable to water vapour? How do coatings (paint, linings etc) affect it?

 

[metengr]

Never heard of this nor would it happen through stainless steel or other metal wall because of the size of a water molecule. Nascent hydrogen because of it's size can diffuse through metals based on pressure and temperature.

 

[TVP]

I agree with metengr on all points-- water vapor should not migrate through wrought stainless steel in the same way that a small molecule like hydrogen can.

 

[EdStainless]

Not through the wall. Not any gas at any pressure in an austenitic stainless at room temp.
There can be a lot of moisture absorbed onto the inside surface of a pipe that has been exposed to the air.
On thing that people don't appreciate though is that a very tiny leak in a high pressure line will let impurities IN. the result is that very small leaks can be the source of significant system contamination.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Doubt the stainless steel is passing the moisture, but soft goods (gaskets etc.) can be fairly permeable.

Ditto what EdStainless said: if there's a concentration gradient, you'd be surprised how large an advective gradient (pressure difference) molecules can climb up to get into your clean medium. Think salmon swimming up a waterfall.

Smaller molecules like hydrogen and helium can make their way through metals and glass, but water is pretty big.

 

[ramseng]

Thanks very much for the replies.