Which gas is not a primary shielding gas but can improve weld surface cleanliness when deep penetration is needed?

• A. Hydrogen
• B. Argon
• C. Helium
• D. Nitrogen

Answer: (A)

Hydrogen is the additive gas used to boost cleaning action and arc temperature in GTAW when deep penetration is needed. It isn’t used by itself as the shielding gas because it can cause porosity and embrittlement and poses safety risks. When small amounts of hydrogen are mixed with a primary shielding gas like argon, the arc becomes hotter and more chemically active, helping to break up surface oxides and improve weld surface cleanliness while promoting deeper penetration. The other gases listed are typical shielding gases on their own (argon and helium) or are generally not used for this purpose (nitrogen), which can introduce unwanted nitrides and defects.