Before picking a flux, it's useful to have a good definition of flux: Flux is a chemical composition critical to the heat joining methods of soldering, brazing and welding that: 1  Chemically: • Cleans metal surfaces to assist the flow of filler metals over base metals.• 2  Provides a protective barrier against re-oxidation and heat scale.efore picking a flux, it's useful to have a good definition of flux: Flux is a chemical composition critical to the heat joining methods of soldering, brazing and welding that: 1  Chemically: • Cleans metal surfaces to assist the flow of filler metals over base metals.