Why is flux required when brazing brass and not required with copper to copper?

Harris Products Group

A fluxing agent (or a controlled atmosphere as found in furnace brazing) is required for all brazing and soldering applications. The purpose of the flux is to remove oxides from the base material and to prevent oxidation during the heating process, thus promoting the free flow of the brazing filler metal.

However, in copper to copper applications, the phosphorus in BCuP alloys acts as the fluxing agent. No additional flux is needed, although a small amount of flux may be used for applications with prolonged heating times to improve the wetting action of the filler metal.