Why is nitrogen gas used inside copper tubing when brazing?

Using nitrogen gas inside copper tubing during the brazing process serves a critical purpose in preventing the formation of oxides. When copper is heated during brazing, it has a tendency to oxidize, which can lead to a weak and flawed joint. The introduction of nitrogen displaces the air within the tubing, creating an inert atmosphere that inhibits the reaction between the copper and oxygen. This helps maintain the integrity of the copper and ensures that the brazing material adheres properly, resulting in a stronger and more reliable joint.

The other choices do not accurately describe the primary benefit of using nitrogen gas in this context. Improving thermal conductivity is important, but it is not the main reason nitrogen is used. Similarly, while stronger joints are a desired outcome of proper brazing techniques, the role of nitrogen is specifically centered on preventing oxide formation, rather than mechanically enhancing joint strength. Lastly, reducing costs is not a purpose linked to the chemical properties of nitrogen or the brazing process itself, making that choice less relevant.