[83]

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.

[lazide]

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)....

[BurningFrog]

If the magnesium is not reacting with oxygen, it must react with something else to produce more heat than is put in.

I discount the possibility of nuclear reactions :)

[GravitasFailure]

Magnesium can't burn without oxygen, but it will happily rip the oxygen out of water and sand, making it exceptionally annoying to put out.

[forgotusername6]

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.

[elihu]

Magnesium isn't entirely without risk. Here's some firefighters spraying water on a Jeep Liberty (see 1:11): https://www.youtube.com/watch?v=KY9ri-UOoLo