What is back purging in GMAW of stainless steel and when is it used?

• A. Back purging introduces inert gas to backside of closed weld to prevent oxidation; used for stainless steels needing clean backside zones.
• B. Back purging is a post-weld cooling method.
• C. Back purging uses oxygen to improve weld appearance.
• D. Back purging is only used for aluminum.

Answer: (C)

Back purging involves directing shielding gas to the backside of the weld while you weld, so the root side is protected from air. For stainless steel, this prevents oxidation that would form chromium oxides and discoloration, which can compromise corrosion resistance and leave inclusions at the root. The back side is kept clean and inert by a purge gas such as argon (often with a purge cap or tubing) to exclude air as the weld root solidifies.

This technique is used when welding stainless steel (and other reactive alloys) where the root or backside needs to stay oxide-free and visually clean, especially for root passes or closed-root welds. It’s not a post-weld cooling method and it does not involve oxygen; oxygen would promote unwanted oxidation.