You will read a lot of wrong things online about this subject. For example that:
The aerospace industry using titanium in it's engines, so it can't be bad in a high O2 environment!
This is simply put wrong and somehow disingenuous, as titanium can and is used on engine parts that are not in contact with an oxidizing environment. The aerospace industry likes titanium for it's superior strength to weight ratio. Where weight is a consideration in an Oxygen system, aluminium is usually used.
...aluminium is not ignited as easily as titanium and has been used extensively in aerospace oxygen systems, where weight is of paramount importance.(1)
Another classic that pops up frequently is:
The partial pressure of O2 in the second stage is way too low, just 10bar if used with 100% O2 at the surface. If our cylinder is charged to 207bar with air, the partial pressure of O2 in that cylinder would be ~43bar! That is way higher and more dangerous!
This statement makes the wrong assumption that all that matters is the partial pressure of oxygen and the oxygen percentage of the gas plays no role. This is wrong.
With oxygen concentrations of less than 35%, titanium would not be expected to burn under any pressure.(2)
I believe it is important to understand why Titanium in an oxidizing environment is a bad idea. Titanium will ignite at room temperature with a pressure of oxygen of around 24bar.
(3) Lucky for us we don't usually encounter a fresh layer of titanium. What is found on the outside of the metal is a protective layer of titanium-oxide, very similar to what you'd find with aluminium and aluminium-oxide.
This layer of titanium-oxide prevents the titanium underneath it from reacting with the free oxygen from the gas.
Trouble starts to arise when this layer of titanium-oxide is removed quickly, as the fresh layer of titanium underneath will start to react with the oxygen to form titanium-oxide. This is an exothermic reaction, meaning it releases heat. This heat can then accumulate if conditions are right, melting part of the titanium-oxide, exposing another fresh layer of titanium. This fresh layer then reacts with the oxygen, releases heat and so on. Once started, a titanium fire will burn until the whole system is consumed.
Exposing a fresh layer of titanium could happen in a few ways:
- Fatigue cracking: This is rather unlikely in SCUBA gear, as the parts are not flexing.
- Electric discharge: I don't see how this could happen in SCUBA gear, although there is always the risk of a static charge building up somewhere. A discharge as low as 1 to 10 Joule is already enough though!
- Impact damage: This is the most likely scenario that would apply to SCUBA gear.
There are other scenarios that I will overlook here, because they are not really applicable in the above scenario of removing a layer of titanium-oxide: (Adiabatic compression, flow friction, friction, chemical reactions, resonance, external heat,...)
One could envision a lose particle, for example a piece of steel from a rusty steel cylinder, that made it somehow into the regulator system. Due to the high pressure differentials between stages, the gas and thus any particle traveling inside that gas are subject to high velocities.
This high velocity particle can scratch off the surface layer of titanium oxide and it would transfer a lot of heat to the metal upon impact when it loses it's kinetic energy.
The above is true for titanium and of course different if you look at real world scenarios. In the real world alloys are being used. Some titanium alloys, e.g. Ti25-15V-15Cr, a titanium, vanadium, chrome alloy, have a higher ignition point than others. However, higher in this regard is still not good enough for oxidization environments. Brass and other copper alloys are still far superior in this regard.
On the extreme ends of titanium alloys lie Ti-Al-Mn which ignites at 7bar up to titanium iodide which ignites at 75bar.
There are several techniques to make titanium more ignition resistant, like nitriding, teflon coating, flame spraying another alloy, ceramic coatings, etc. All of these have in common that they try to reduce the chance of a fresh layer of titanium being exposed. This is flawed in my opinion, as it just reduces the risk, but does not eliminate it.
On another note, oxygen systems have to endure many more micro ignitions than a lot of people realize. Over the years I have torn down countless of first stage oxygen systems which have been abused wildly. On more than a dozen occasions, there were clear signs of ignition, usually around the seat/orifice of the first stage.
The reason that these small ignitions have not led to a catastrophic outcome are, besides other factors, due to the excellent characteristics of brass for this job. Brass has far superior ignition characteristics when compared to steel, aluminium or titanium (Which are getting worse in exactly that order). A material which comes close to brass characteristics would be nickel.
I have tried for years to get the "fear" out of divers from oxygen systems. These systems are so more resilient than divers have been conditioned to believe. We treat our oxygen systems so different from other industries like medical, or welding for example. There is a certain mystique around this subject and I still believe that some dive instructors like it that way. It gives them a superior feeling of knowing something dangerous or being able to handle something dangerous that would kill you, the uneducated! It is kind of an exclusive club that one can belong to, at least that is the feeling if you talk to some of these people.
I have tried to educate people, by showing how resilient most systems actually are. A smudge of oil on a seat of a high pressure O2 system will for example not result in ignition unless also heat is added. I have done this "presentation" countless of times to frightened bystanders.
In the end it comes down to your personal choice. Are you likely to get away with using titanium in your second stage? Most assuredly the answer is a yes, despite all that I have written above.
The problem with oxygen fires is always cumulative. It's not one thing going wrong, but a whole chain of things going sideways. Adiabatic heating by turning on the gas supply quickly, hydrocarbons to lower the point of ignition significantly, impact damage from a lose particle, etc. If they all come together, things go wrong. And when they do eventually go wrong, oxygen fires are an most unpleasant thing to witness.
TL: DR: If I was in your shoes, I would not add one thing to the chain of things that could go wrong, but just stick to brass second stages.
You may find some of the attached documents interesting to read.
(1) NASA - Safety standard for Oxygen and Oxygen systems (1996) - Page 44
(2) Reactivity Of Metals With Liquid And Gaseous Oxygen (1963) - Page 15
(3) Reactivity Of Metals With Liquid And Gaseous Oxygen (1963) - Page 13
