Rosin flux is a chemical used to facilitate soldering. In electronics soldering, solder with a rosin flux core is used, eliminating the need for separate application. In metal brazing and plumbing, rosin flux is applied with a brush before the application of heat and solder.
Bare metal oxidises when expose to air and other contaminates, which prevents solder from adhering. Rosin flux cleans the metal, allowing the solder to form a strong mechanical and electrical bond.
Rosin flux forms an air barrier, preventing further metal oxidation which would weaken solder joints.
Through amalgamation, the chemical process of joining two substances together, rosin flux increases the efficiency of molten solder flow by acting as a wetting agent.
Rosin flux can be highly toxic, particularly the acid fluxes used in plumbing and metal brazing. Gloves are necessary to prevent skin contamination, and soldering should always be performed in a ventilated area to avoid breathing of fumes.
Acid-based flux is corrosive and new exposed solder joints should be cleaned with a damp sponge to remove excess flux.