[burfog]

Hydrogen is mighty weird too.

For leak testing, it can be better than helium because the molecule is lighter. This raises the speed for any given temperature, and thus increases the rates of diffusion and effusion.

A downside is that hydrogen can burrow right into solid metal, one atom (proton) at a time, and then merge back into diatomic molecules. This creates a sort of internal pressure in the material, making it brittle. Hydrogen embrittlement is a significant problem, particularly with titanium.