What gases are used as backing gases?

• A. Hydrogen and Helium
• B. Oxygen and Argon
• C. Nitrogen and CO2 are often acceptable - but Argon can be used - but expensive
• D. Neon and Krypton

Answer: (C)

Backing gas is used on the backside of the weld to keep a protective atmosphere as the molten metal solidifies, helping prevent oxidation and porosity in the weld root.

Nitrogen and CO2 are often acceptable as backing gases because they provide a protective environment at a lower cost than pure Argon. Argon can be used as a backing gas too, but it’s more expensive, so many setups opt for the cheaper nitrogen or CO2 when suitable for the material and weld type.

Oxygen would cause unwanted oxidation on the backside, making it a poor choice. Neon and Krypton are costly and not commonly used for backing in GTAW. Hydrogen can cause embrittlement or porosity in many metals, and helium, while it can alter heat input, is expensive and not typical for backing gas.