[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)....