In metallurgy, flux is an aid to melting, a material which by its chemical
action facilitates soldering or brazing of metals. Some common fluxes are
ammonium chloride or rosin, for soldering tin; hydrochloric acid and zinc
chloride for galvanized iron and other zinc surfaces; and borax, for brazing.
soldering of metals, flux has a double purpose: it removes oxidation from the
surfaces to be soldered, and by facilitating amalgamation improves wetting
characteristics of the liquid solder. Flux is corrosive, so the parts have to be
cleaned after soldering to prevent damage. Several types of flux are in use in
Rosin activated (RA): aggressive action, must be removed
Water soluble (WS): aggressive action, must be removed
A related use of the term flux is to designate the material added to the
contents of a smelting furnace or a cupola for the purpose of purging the metal
of impurities, and of rendering the slag more liquid. The flux most commonly
used in iron and steel furnaces is limestone, which is charged in the proper
proportions with the iron and fuel. The slag is a liquid mixture of ash, flux,
and other impurities.