Shielding gases are gases used in gas arc welding. Shielding gases serve to insulate the weld are from gases such as oxygen and water vapour. This is necessary to prevent oxidation at the weld area at high area and porosity(formation of bubbles in the weld).
Inert and semi-inert gases are used for shielding. Examples are Helium, Argon, Carbondioxide, etc. Shielding gases are generally denser than air (an exception is helium). They also have good thermal conductivity.
