There's a couple of references I used to decide how I approached this:
The
Titanium Metals Corporation has a page that talks about titanium corrosion. They also have a
chart that compares the ignition point of titanium when graphed against the oxygen percentage and the pressure flowing across the metal.
There's also a really good article from Aviation, Space, and Environmental Medicine,
Link
I won't post the article or even large pieces (that's what the link is for) but here are a few snippets:
When a test was performed under dynamic flow conditions with pure oxygen streaming past the fresh titanium surface, an oxygen partial pressure of 4.4 ATA (446 kPa) was sufficient to ignite and propagate the reaction.
...since the studies by Jackson et al. (1) have shown that titanium would not be expected to burn under any pressure when the oxygen concentration is less than 35%.
The NASA Safety Standard for oxygen and oxygen systems (2) states that titanium must not be used with gaseous oxygen at oxygen pressures above 2 ATA (207 kPa).
I haven't seen any serious commentary regarding the 40% rule for titanium and enhanced air. It's always been comments like "they told us this in a class". If a US manufacturer is willing to back up the 40% with assumed liability, that's a strong argument that something does exist. I would be leery about using it with anything over 36% though, especially considering some of the higher pressure tanks that may be used.
1) Jackson JD, Boyd WK, Miller PD. Reactivity of metals with liquid and gaseous oxygen. Columbus, OH, Battelle Memorial Institute, 1963, 1-26.
2) National Aeronautics and Space Administration. Safety standard for oxygen and oxygen systems; chapter 3. Washington, DC, NASA, 1996, 12.
