Flux — What It Is and Why It Matters
Flux is a chemical cleaning agent used in welding, brazing, and soldering to remove oxides from metal surfaces, prevent new oxidation during heating, and improve the flow and wetting of filler metals. In welding, flux also provides shielding gas and slag that protect the weld from atmospheric contamination.
Flux appears in different forms depending on the process. In stick welding, the flux is the coating on the electrode. In flux-core welding, it is packed inside a tubular wire. In submerged arc welding, granular flux is poured over the joint ahead of the arc. In brazing and soldering, flux is applied as a paste or liquid to the joint surfaces before heating.
The chemical composition of the flux determines its behavior — cellulosic fluxes produce a forceful, penetrating arc; basic (low-hydrogen) fluxes produce smooth, clean deposits with superior mechanical properties; rutile fluxes offer the easiest arc characteristics for beginners. Flux chemistry is a science unto itself and is one of the most important variables in weld quality.
Frequently Asked Questions
Why is flux necessary in welding?
Molten metal reacts aggressively with nitrogen and oxygen in the air, causing porosity, brittleness, and poor mechanical properties. Flux produces a gas shield and a slag blanket that exclude the atmosphere from the weld zone, allowing the molten metal to solidify in a clean, protected environment.
Do all welding processes use flux?
No. MIG and TIG welding use externally supplied shielding gas instead of flux. Stick welding, flux-core welding, and submerged arc welding rely on flux (in different forms) for shielding. Gas-shielded flux-core (FCAW-G) uses both — flux inside the wire plus external gas.