Brazing Flux FAQs
Straight answers to the questions people ask most when choosing and using brazing and soldering flux.
What does brazing flux do and why do I need it?
When metal is heated in air, it forms a surface oxide layer that stops the molten filler from wetting and bonding to it. Flux dissolves and keeps out these oxides while you heat the joint, so the filler metal can flow smoothly into the gap and make a sound bond.
Without flux, the filler tends to ball up and run off instead of flowing into the joint. Flux does not replace cleaning, but it is what lets the filler wet the metal.
How do I apply brazing flux?
Clean the joint first, then coat both surfaces with flux. Paste fluxes are brushed on, and powder fluxes are mixed with clean water into a paste first. You can also dip a heated rod into the flux so some sticks to it. Coat the area generously, covering more than the final joint, because flux also protects the metal around the joint as it heats.
Apply it before heating and keep the joint covered while you heat. If the flux burns away or turns black, there was too little or the joint got too hot.
How do I know when the joint is hot enough to add the filler?
Watch the flux. As the joint heats, most brazing fluxes bubble, dry out and turn into a clear, watery liquid when they reach working temperature. That is the signal that the metal is ready for the filler.
Heat the parent metal, not the rod, and let the heat from the joint melt the filler as you touch it on. If the filler does not flow, the joint is probably not hot enough, or the surface is not clean.
Do I still have to clean the metal before using flux?
Yes. Flux deals with the thin oxide film that forms on clean metal as it heats, but it cannot cope with oil, grease, paint, thick rust or dirt. Those need to be removed first by degreasing, and by brushing or abrading the surface until it is bright.
A clean, close-fitting joint plus the right flux gives the best chance of a strong, neat result. Many failed joints come from poor cleaning rather than from the flux or filler.