The shielding gas often plays an important role in the productivity and quality of welding. As its name suggests, the shielding gas shields the solidifying molten weld from oxygenation as well as impurities and moisture in the air, which may weaken the corrosion-tolerance of the weld, generate porous results and weaken the durability of the weld by changing the geometrical features of the joint. The shielding gas also cools down the welding gun. The most common shielding gas components are argon, helium, carbon dioxide and oxygen.

The shielding gas can be inert or active. An inert gas does not react with the molten weld at all while an active gas participates in the welding process by stabilising the arc and securing the smooth transfer of material to the weld. Inert gas is used in MIG welding (metal-arc inert gas welding) while active gas is used in MAG welding (metal-arc active gas welding).

An example of an inert gas is argon, which does not react with the molten weld. It is the most commonly used shielding gas in TIG welding. Carbon dioxide and oxygen, however, do react with the molten weld as does a mixture of carbon dioxide and argon.

Argon (Ar) is an inert shielding gas, which does not react with the work piece. It does not cause oxygenation or affect the chemical composition of the weld. This is the most commonly used shielding gas in TIG welding.

Helium (He) is also an inert shielding gas. Helium and helium-argon mixtures are used in TIG and MIG welding. Helium provides better side penetration and greater welding speed compared to argon.

Carbon dioxide (CO2) and oxygen (O2) are active gases used as the so-called oxygenating component to stabilise the arc and to ensure smooth transmission of material in MAG welding. The proportion of these gas components in the shielding gas is determined by the steel type.