Final answer:
The reason for purging a joint with dry nitrogen during brazing is to prevent the formation of copper oxide, which can weaken the joint by reacting with the copper metal when exposed to high temperatures in the presence of air.
Step-by-step explanation:
When brazing, it is essential to purge the joint with oil free, dry nitrogen N.F. to prevent the formation of copper oxide on the inside surfaces of the joint. This procedure is crucial because when copper metal is exposed to high temperatures in the presence of air, it can react with oxygen to form copper oxide. The use of nitrogen acts as a barrier to oxygen and other reactive gases, thereby maintaining the integrity of the joint and ensuring its quality and strength.
As demonstrated in various chemical reactions, copper easily reacts with oxygen to form copper oxide. For instance, the air can convert Cu2S into Cu2O, which is then reduced back to metallic copper by the remaining Cu2S. This conversion and reduction process highlights why it is important to use a protective atmosphere of nitrogen when brazing to avoid undesirable oxidation that weakens the joint.