Which flame has slightly more oxygen than the 1:1 ratio and promotes the oxidation of copper and should not be used for copper brazing?

• A. Neutral flame
• B. Carburizing flame
• C. Reducing flame
• D. Oxidizing flame

Answer: (D)

In copper brazing, the surface condition of copper is crucial for a strong joint, and flame chemistry directly affects that surface. An oxidizing flame has excess oxygen beyond the neutral 1:1 ratio, so it promotes formation of copper oxide on the surface. Copper oxide is difficult to wet and removes the clean metal surface the filler metal needs to flow and capillary into the joint, leading to poor bonding. That’s why this flame type should not be used for copper brazing. In contrast, a neutral flame is balanced and less likely to oxidize the surface; a carburizing flame is rich in fuel and can deposit carbon contaminants; a reducing flame is fuel-rich enough to help keep surfaces oxide-free but can create conditions not ideal for copper brazing. The key idea is that extra oxygen above the neutral ratio oxidizes copper, hindering brazing.