Flux (metallurgy)

A substance added during metal processing or joining to promote melting, limit oxidation, or help remove impurities. Its composition and action depend on the materials and process involved.

Oxide reduction: A chemical reaction that removes oxygen from a compound, often converting metal oxides into metal. Some fluxes dissolve or chemically reduce surface oxides that would obstruct molten-metal contact.
Soldering: A joining process that melts a filler metal below 450 °C without melting the base materials. Soldering flux clears oxide films so molten solder can bond to the joint surfaces.
Metal oxide: A chemical compound consisting of a metal and oxygen. Oxide films are common barriers that flux must disrupt or remove.
Mechanical oxide removal: The removal of oxide layers by abrasion, scraping, or other physical means. Mechanical cleaning removes films before heating rather than relying on flux chemistry.
Flux residue: Material left on a workpiece after flux has served its purpose. Residues can cause corrosion, electrical leakage, or contamination unless suitably removed or controlled.
Wetting: The spreading of a liquid across a solid surface, governed by interfacial forces. Removing oxides lets molten solder or brazing alloy spread across the workpiece.
Brazing: A joining process that melts filler metal above 450 °C while keeping the base materials solid. Brazing flux protects hot surfaces and helps filler metal flow into the joint.
Surface tension: The tendency of a liquid surface to resist expansion because of cohesive molecular forces. Flux can alter interfacial conditions that determine whether molten filler spreads.
Pickling (metal): The chemical removal of scale or corrosion products from metal, commonly using an acidic solution. Pickling cleans metal in a bath, whereas flux acts during a joining or melt process.
Rosin: A brittle resin obtained from coniferous trees, used in some soldering fluxes. Rosin-based flux residues are common in electronics and vary in cleaning requirements.

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