I am using a laser to melt and re-fuse a titanium-aluminum alloy in an inert argon environment. The argon is recirculated through a filter via a pump. Slag from the laser melting/fusing process gets caught in the filter. My problem is that the "slag" - tiny particles of aluminum and titanium - is not passivated with oxygen, so when I open the filter to change the element it reacts, rather dramatically. It catches fire, basically - lots of heat and smoke, etc.
In the past, the system had a low oxygen level, a few hundred PPM. Now I have improved the system design to get the O2 level down near 1 or 2 PPM. I was surprised by the fire problem - didn’t have the fire problem before. I think this is because the Ti and Al were steadily passivated by the low O2 level in the old system. I would like to find a way to passivate while the system is running, but without having a high O2 level. One suggestion was to expose the argon/slag flow to iron oxide. The Al and Ti will react and steal the oxygen, leaving the iron behind. In other words, your basic thermite reaction. I recall that the classic thermite reaction required a lot of energy to get it started - burning magnesium, if I recall a long-ago chem class correctly.
So, my questions:
1. If an argon stream containing tiny particles of Al or Ti is passed over/through powdered iron oxide, will it react at room temperature?
2. If it won’t, is there some other oxide we could use that would react at room temp, or at least close to it?
3. In general, how can I compute the energy required to get this sort of reaction started?
Well, your solution is there - the old way. The incidence of fire only happens because there is excess oxygen available when you open the system. You mentioned that previously you had low levels of oxygen in the system. It is not too hard to hook up a system that will introduce only a very small amount of oxygen - in the 500-1000ppm range, say - and let that sit or recirculate to oxidize the reactive metals before opening the system up to ambient air. That should eliminate any fuel source for combustion. At worst you might get a few hot spots. It's all about the "fire triangle". The thermodyamics just show how easily some reactions may proceed. But if it is already spontaneous at NTP then the control is really about reducing the rate of reaction, not increasing it.
Humidified argon purging might also produced the desired passivation by reaction without excessive heat generation.