In metallurgy, a<unk> is a chemical cleaning agent which facilitates soldering, brazing, and welding by removing oxidation from the metals to be joined. Common<unk> s are: ammonium chloride or rosin for soldering tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing or braze-welding ferrous metals. Different<unk> s, mostly based on sodium chloride, potassium chloride, and a fluoride such as sodium fluoride, are used in foundries for removing impurities from molten nonferrous metals such as aluminum, or for adding desirable trace elements such as titanium.
In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of<unk> is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures.<unk> is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally,<unk> allows solder to flow easily on the working piece rather than forming beads as it would otherwise.
Flux 
Excretion
Heat transfer
Adsorption
Oxygenation
Fluency
Secretion
Electrostatics