Which gas is not commonly used as TIG shielding gas?

• A. Argon
• B. Helium
• C. Nitrogen
• D. Carbon Dioxide

Answer: (C)

TIG welding relies on a protective, inert atmosphere to keep the molten weld metal from reacting with air. Argon is the standard shielding gas because it is chemically inert with most weld metals, providing a stable, clean shield and good bead quality. Helium serves a similar purpose but adds more heat input, which can be useful for certain metals or thicker sections.

Nitrogen stands out as not commonly used for shielding in TIG because it can be reactive at welding temperatures. It can form nitrides with many alloying elements in the weld and heat-affected zone, leading to embrittlement, porosity, and degraded mechanical properties. This makes nitrogen an unreliable shield compared with argon or helium, which remain inert and protect the weld without introducing those issues.

Carbon dioxide is not typically used in TIG shielding for the same reason—it promotes oxidation and poorer bead quality compared with inert gases like argon or helium. However, the main reason nitrogen is avoided is its tendency to react with the weld metal rather than simply shielding it.