Which gas should be used to purge copper tubing during brazing?

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

Answer: (B)

Keeping oxygen out of the joint area during brazing is essential because copper oxide can form at high temperatures and block capillary flow, weakening the joint. The purge gas should displace air and remain relatively inert with both copper and the brazing filler metal. Nitrogen fits well here: it’s nonreactive enough at brazing temperatures, effectively pushes air out of the tube, and is inexpensive and readily available. This combination prevents oxide formation without introducing reactive species or costly setup. Argon is inert but costly, and helium is less practical due to higher expense and other handling issues, while oxygen would promote oxidation rather than prevent it.