Aluminum and titanium are definitely
out - they ignite easily and burn violently in the presence of HP O2. Steel is
not a good choice either - it burns, as anyone who has ever used a cutting torch
knows. But the big problem with steel is that it sparks so readily, making it a
prime potential source of ignition. SS falls somewhere in between. While it
doesn't spark or burn nearly as easily as steel, it is not completely immune.
The catch is, even good brass fittings are often rated only for only 1500-2000
psi, (100-133 bar), and rarely for more than 3000 psi or so (200 bar).
For higher pressure systems, then, there really isn't any affordable
choice but stainless steel, and just about everyone uses it.
It's hard to get a definitive answer
on how suitable SS really is for HP O2. The first editions of this book
cautioned against it, based on recommendations in several NASA publications, but
dig a little deeper and you find that for every one that says not to use it
there seems to be another that says it's OK. While the NASA Glenn Safety Manual
says that only iconel or monel are suitable for high pressure O2, the NASA
Safety Standard For Oxygen Systems NSS 1740-15 says that "Stainless steels are
used extensively in HP O2 systems" and that "few problems are experienced with
the use of SS storage tanks and lines" but that "ignitions have occurred in SS
components such as valves at high pressures and high flow rates", though they
are probably talking about pressures much greater than the average nitrox whip
sees (both documents are available on the web). And the CGA specifies that
valves for O2 tanks should be made of either brass or SS.