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.

In 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 electronics:

• No Clean (NC): mild action, hardened residue
• Rosin mildly activated (RMA): mild action, soft residue
• 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.