Plumbers use this gas to prevent oxidation from forming inside the tubing while brazing.

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

Answer: (B)

When brazing inside tubing, you want to keep the inside of the metal free from air and moisture so the filler metal can wet and bond properly. Purging with a shielding gas creates an inert environment that prevents oxidation of the hot metal walls, which would otherwise form oxides that disrupt the joint.

Nitrogen is a practical choice because it’s inert, inexpensive, and readily available. It effectively displaces oxygen inside the tube without reacting with copper or brass at brazing temperatures, helping to keep the interior clean and free of oxide films during heating.

Argon could also serve as a shielding gas, but it’s more costly for internal purging. Helium is less commonly used for this purpose due to cost and diffusion characteristics, and oxygen would promote oxidation rather than prevent it.