Helium is applied as a shielding gas in arc welding and plasma arc welding due to its highest ionization potential of any atom. The protective atmosphere of helium around the welding site prevents that the metal oxidizes in the molten state.
The high ionization potential of helium enables the plasma arc welding of exotic metals such as titanium, zirconium, magnesium and aluminum alloys used in construction, shipbuilding and aerospace.
As shielding gas helium can be substituted by argon or hydrogen, however for plasma arc welding of certain materials like titanium helium cannot substituted, because helium is the only gas with high enough thermal activity to be safe.
Inner atmosphere operations
Helium is applied in pressure purging operations where liquid hydrogen or another gas is replaced by helium as liquid hydrogen or another gas is used, because helium is the only gas with a lower boiling point than hydrogen and helium is inert. Nitrogen is not used as purging gas for liquid hydrogen systems, because in these conditions nitrogen would solidify.
Helium as purging gas is used in the aerospace industry for space agencies like Arianespace and NASA. For example, in Delta IV rockets helium was used to maintain pressure in liquid oxygen fuel tanks. As liquid oxygen is burned as rocket fuel, helium is injected into the fuel tanks to keep them from collapsing. Helium as purging gas can be substituted by nitrogen, except for liquid hydrogen systems.