Corrosivity, which is mostly due to the aggressive compounds of the activators hygroscopic properties of the flux residues may aggravate the effects.The flux most commonly used in iron and steel furnaces is limestone, which is charged in the proper proportions with the iron and fuel. This reduction of slag viscosity with temperature, increasing the flow of slag in smelting, is the origin of the word flux in metallurgy. Slag is a liquid mixture of ash, flux, and other impurities.

In the process of smelting, inorganic chlorides, fluorides (see fluorite), limestone and other materials are designated as 'fluxes' when added to the contents of a smelting furnace or a cupola for the purpose of purging the metal of chemical impurities such as phosphorus, and of rendering slag more liquid at the smelting temperature. Many of these chemicals are toxic and due care should be taken during their use. Traditionally borax was used as a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides as well as wetting agents. This means that fluxes need to be more aggressive and to provide a physical barrier. As well as removing existing oxides, rapid oxidation of the metal at the elevated temperatures has to be avoided. Various tests, including the ROSE test, may be used after soldering to check for the presence of ionic or other contaminants that could cause short circuits or other problems.īrazing (sometimes known as silver soldering or hard soldering) requires a much higher temperature than soft soldering, sometimes over 850 ☌. Ī number of standards exist to define the various flux types. Several types of flux are used in electronics. Some fluxes are corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage. In soldering metals, flux serves a threefold purpose: it removes any oxidized metal from the surfaces to be soldered, seals out air thus preventing further oxidation, and by facilitating amalgamation, improves wetting characteristics of the liquid solder. By preventing the formation of metal oxides, flux enables the solder to adhere to the clean metal surface, rather than forming beads, as it would on an oxidized surface. The role of flux is typically dual: dissolving the oxides already present on the metal surface, which facilitates wetting by molten metal, and acting as an oxygen barrier by coating the hot surface, preventing its oxidation.įor example, tin-lead solder attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures.

In high-temperature metal joining processes ( welding, brazing and soldering), flux is a substance that is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing oxidation of the base and filler materials.

In some applications molten flux also serves as a heat-transfer medium, facilitating heating of the joint by the soldering tool or molten solder. įluxes are also used in foundries for removing impurities from molten nonferrous metals such as aluminium, or for adding desirable trace elements such as titanium.Īs cleaning agents, fluxes facilitate soldering, brazing, and welding by removing oxidation from the metals to be joined. These agents served various functions, the simplest being a reducing agent, which prevented oxides from forming on the surface of the molten metal, while others absorbed impurities into slag, which could be scraped off molten metal. Iron ore was also used as a flux in the smelting of copper. Some of the earliest known fluxes were sodium carbonate, potash, charcoal, coke, borax, lime, lead sulfide and certain minerals containing phosphorus. They are used in both extractive metallurgy and metal joining. Fluxes may have more than one function at a time. In metallurgy, a flux (from Latin fluxus 'flow') is a chemical cleaning agent, flowing agent, or purifying agent. Wire freshly coated with solder, held above molten rosin flux

Rosin soldering flux