Magnesium parts are already in widespread use. The covers you see on the sides of motorcycle engines/transmissions are often magnesium.
In solid form the risk is mitigated because there isn't enough surface area for the reaction with oxygen, it's the powder/shavings that are a concern. You can actually weld magnesium parts without it igniting.
Reading up on it a little more - it seems with enough heat solid magnesium can start a self-sustaining burn without oxygen. I wish I could find a laymans explanation of the difference between oxygen fueled magnesium powder fire and self sustaining. Must take an awful lot of heat if a welding arc isn't hot enough to cause this.
Edit: Also think of steel wool and how well it burns, but a block of steel not so much.
Welding arcs definitely can cause magnesium fires. Generally the shielding gas used to protect from oxidation prevents it, but it's easy to screw up.
It's a tricky metal to weld because of it. Generally you also need to preheat it so you don't get cracking, which makes it even more of an issue.
There is a passivated oxide layer that forms (similar to aluminum) which generally reduces the risk, but if it's compromised (like from cleaning the weld area)....
I used to have a flowerpot furnace and cast small objects. I once stumbled upon a car that had been burnt out. The engine block had dripped down the road and I was able to collect some of the melted pieces. It would pop and spit when poked once cast, very different to the aluminium I often used, and I wondered at the time whether it was the magnesium in the block that was doing it.
"This is so NeXT," I told Sally. "Everything works great in the tests, then when you try to make it work for real, in the field, nothing works. They build a computer out of magnesium, and it doesn't even burn!"
Wow, that story is a great example of "it's better to ask for forgiveness than permission". Just drive to the desert and burn it, don't waste 100 hours trying to get permission from the state of California to burn it.
Is it though? They got a bunch of assistance from the folks at Lawrence Livermore, and the photographer was happy to not take pictures of fire in direct sun. Asking permission likely got a much better outcome; the author sounded pretty unprepared to ignite the thing without the help he got.
Magnesium metal will burn very violently once it gets going. Among the biggest trouble I got into in high school was when my chem lab partner and I decided it would be entertaining to burn a small piece of magnesium metal. It was rather spectacular, and indeed entertaining. Well worth the Very Stern Lecture.
Some aircraft historian will have to fill in the gaps in this story (what aircraft?), but back during the Korean War era, the USAF had a multi-engine piston-driven plane that was either a transport or a cargo aircraft -- not sure which, but the engine blocks were magnesium to save weight. One of the biggest brown-factor events that you could have was an engine fire, because once it got started, your day was going in a bad direction very fast. A friend's dad was pilot-in-command of a plane fresh out of maintenance. An engine caught fire on climb out. He ordered the rest of the crew to hit the silk and he tried to get back to the field. He did, but the landing was not pretty, and he suffered a nasty leg injury. No more combat rating for him, and he finished is USAF career flying transports, and later had a career as a commercial airline pilot. He was luck to survive that engine fire event.
I've seen this assurance before that magnesium flammability isn't a practical concern, usually just saying fires are "rare", but never any real information as to why. From that link it sounds like there are alloys that maybe preserve the desirable properties of the metal but aren't as flammable as elemental magnesium. Anybody know more about that? Why it works? How well?
> From that link it sounds like there are alloys that maybe preserve the desirable properties of the metal but aren't as flammable as elemental magnesium. Anybody know more about that? Why it works? How well?
It's been a long time since my metallurgy courses, but in general there are three major ways that properties are altered at the mesoscale due to alloying:
1) a different solid phase of the dominant metal is formed due to the solid solution of the minor alloy components
2) covalently bonded compounds are formed between the base metal and minor components ("intermetallics")
3) the microstructure is changed (think e.g. alternating layers of intermetallics and metal grains)
Because the thermodynamic driving forces are basically identical regardless of the solid phase, #1 is probably not helpful here (also, it's flammable in liquid form -- I checked). But the intermetallics may not be flammable. So then your fire resistance comes from a combination of #2 and #3: if you form a lamellar structure of the non-flammable intermetallics separating the regions of the flammable majority phase, then that may produce macroscopic fire resistance.
I can't comment on how well it works, of course. But if you have some "secret sauce", maybe a sintering process or special heat treatment, you might be able to manipulate the lamellar structure in a desirable way and end up with a very fire-resistant alloy. The catch is that this might be pretty expensive.
ETA: because this fire resistance is dependent on the microstructure and presence of intermetallics, if the alloy is heated back to a temperature where those intermetallics dissolve into solid solution (or, obviously, if it melts), then it's going to be flammable again. So my educated guess is that while you can't actually light these special alloys on fire, if you were to throw some into an ongoing inferno, it would heat up and then combust. So in an otherwise flammable environment cough hydrogen airship cough, maybe not the greatest idea.
For an airship of the size proposed in that post, I don't think I agree. Presumably there would be some measure of compartmentalization, ventilation, and fire suppression so that even if a portion of the craft was stricken, the entire vessel would not be doomed.
But even so, there's going to be a point where there's just too much fuel to keep the fire under control despite best efforts, and the fire-resistant magnesium could certainly contribute to reaching that threshold once you cross a certain temperature. And you cannot simply vent combusting magnesium the way you can vent combusting hydrogen (hypothetically, anyway).
Oddly, tiny chunks/slivers of magnesium are very flammable, but a big chunk of it is pretty much impossible to set fire to.
Source: I was disappointed to buy a few lbs of magnesium to burn on the bonfire, only to find that chucking a lump of it on the bonfire doesn't even burn. Shavings did though.
In solid form the risk is mitigated because there isn't enough surface area for the reaction with oxygen, it's the powder/shavings that are a concern. You can actually weld magnesium parts without it igniting.
Reading up on it a little more - it seems with enough heat solid magnesium can start a self-sustaining burn without oxygen. I wish I could find a laymans explanation of the difference between oxygen fueled magnesium powder fire and self sustaining. Must take an awful lot of heat if a welding arc isn't hot enough to cause this.
Edit: Also think of steel wool and how well it burns, but a block of steel not so much.