What's the Purpose of Welding Flux?

Oxygen

• As you know, oxygen combines with iron to form rust, but this reaction is very slow. Chemical reactions proceed more rapidly at high temperatures, however, and oxygen can react with the hot metal in a weld to form weak oxides. A higher oxide content means a weaker weld that will break more readily. Some welding techniques use inert gases to exclude air, while others use flux to help reduce and avoid oxidation instead.

Role

• Just like the metal, at room temperature the flux is not very reactive. At high temperatures, however, the flux is actually more easily oxidized with the metal, so it's preferentially oxidized and is in a sense sacrificed to save the iron in the weld. The flux also serves to help dissolve oxides on the metal surface, where it forms a molten pool welders call slag. The slag helps protect the metal beneath.

Effects

• One of the biggest advantages of flux-cored welding and similar techniques is that you can weld outside, even under windy conditions. You would be unable to do the same with gas-shielded welding, because the wind would disrupt the inert gas shield. Flux-cored welding is also very high-volume, although it will not work for aluminum and other more unusual metals. The combination of high volume and the ability to weld outdoors has made it popular in the ship-building industry.

Considerations

• Unlike the analogous compounds in soldering, fluxes in welding are typically mixtures of inorganic compounds like calcium fluoride. Often these fluxes are sold in the form of a flux core electrode, which has a core of flux surrounded by electrode metal. Fluxes can be potentially hazardous if you don't observe the proper precautions. Dust, fumes and gases from the flux are irritants and may in some circumstances be toxic, especially those that contain fluoride compounds.