What is Flux?

In HVAC, you will hear the word “flux” used in two totally different parts of the trade.

Magnetic flux is the lines of force that emanate from a magnet, and you often hear about magnetic flux lines in discussions of motor theory.

This type of flux has nothing to do with that…

Flux in soldering, brazing, and welding is a powder, paste, or liquid added to the base material being bonded either through direct application or as part of the rod or wire being applied.

Flux can do a few different things to improve the bond, but the main thing it does is bond with metal oxides and keep them out of the work area.

“What are oxides?” you may ask.

Oxides are compounds that result from one material (in this case, metal) reacting with oxygen to create a new substance.

Some common oxides are rust in steel or iron, and that black scale that shows up on copper when you heat it above 600° is called “cupric oxide,” which likes to block up screens and valves.

In the case of soldering, brazing, and welding, we are heating base metals to the point where they generally will easily break down and start to bond with oxygen to form these nasty oxides.

The oxides build up a layer that the alloy just won't want to bond to, resulting in anything from a poor bond to no bond at all.

The most common purpose for flux is to keep those oxides out of the way so that the bond can be made.

Flux isn't a replacement for proper cleaning and heat control, but in many cases, it is a necessity to deal with oxides.

Now, flux isn't always required.

In TIG welding, a shielding gas like argon keeps the oxygen away from the work area rather than using flux.

In silver/phosphorus brazing, the phosphorus acts as the fluxing agent, limiting what metals it can be used with because phosphorus-bearing alloys do not work with steel.

Fluxes must be used according to manufacturer specs. Some are toxic and should not be breathed, and most fluxes should be cleaned off after the bond is made.

—Bryan

P.S. – The flux shown above is for use with Solderweld Multi-Sol: a low-temp multi-metal solder and flux. You can find out more about getting Solderweld products from suppliers near you by visiting productsbypros.com.

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