Which gas is sometimes used as a backing gas but is expensive?

• A. Argon
• B. Nitrogen
• C. CO2
• D. Helium

Answer: (A)

When welding with GTAW, a backing gas is applied to the backside of the joint to protect the root and prevent oxidation after the weld pool forms. Argon is a common choice for this role because it is inert and provides a stable, non-reactive shield that helps prevent porosity and oxidation on the weld’s root side.

Argon is often used as backing gas despite its cost because it reliably protects the root without reacting with the molten metal, which makes it suitable for many metals welded by TIG. It is more expensive than some other options like CO2 or nitrogen, which is why its use is sometimes limited to situations where a clean, oxide-free root is crucial. Helium would offer different heat characteristics but is even more expensive, so argon sits in the middle as a practical choice for backing gas in many TIG applications.