Magnesium weld stress-corrosion cracking from humidity

Fusion-welded magnesium without stress relief will SCC from atmospheric humidity within two days. Reason most magnesium parts are castings (no residual stress) — auto wheels, dashboard structures.

You also develop residual stresses, so when you try to weld it, the humidity in the air will almost crack it right behind the weld. Not quite that fast — you come back two days later after fusion welding magnesium alloys, if you haven't stress-relieved it, just the humidity in the air will cause stress corrosion cracking. That's why most things are castings — castings don't have residual stresses. Most of the magnesium that went on automobiles used to be in the wheels, and they would just paint those for corrosion resistance. Now it's going underneath the dashboard, in very specialized applications.