News & Views

Basicity Index of Welding Fluxes

By Wesley Wang on Thursday, March 19th, 2015

By definition, the basicity index (B.I.) of a flux or slag is calculated from the following equation:

B.I. equation

For welding consumables with a flux component, it is important to clarify the concepts of flux and slag. A flux can be a raw material that contains different types of oxides, fluorides, etc.; it also can be a flux mix blended or agglomerated by different types of raw fluxes. After the blending of various fluxes per a flux formula, the blended flux can be used to make the core material of a flux-cored wire, the coat of a stick electrode, a  welding flux for submerged arc welding, or a flux for tungsten arc welding. To clarify, flux is the powdered material used before a welding. Its product after the welding is called slag. Due to the complex of chemical reactions during a welding, the chemical composition of a slag is much different from that of flux. As a result, the B.I.s of a flux mix and its resultant slag are quite different.

When a welding consumable is initially designed, the B.I. concept can be used to design and adjust the flux formulas. Usually, the higher the B.I. of a flux mix, the less the residual oxygen is in a weld, and the better the mechanical properties are; the welding performance, however, will be worse. Keeping the B.I. concept in mind, a welding consumable designer must carefully consider de-oxidation, alloying strategies, de-nitridization, hydrogen removal, surface tension control of the resultant weld pool, and so on.

For example, a flux formula normally contains a network builder (SiO2) and a viscosity controller (CaF2). To ensure that the molten slag will cover the weld bead entirely, a proper amount of viscosity controller must be used to break down the slag network to make the liquid slag flow well. If the viscosity controller is excessive, the slag’s B.I. will be high, thus making the slag too fluid to build up at toes. If slags at toes are not thoroughly clean, possible slag inclusions in a multi-pass weld may be incurred. On the other hand, if the amount of network builder, i.e. SiO2, is not balanced by CaF2 to have a low B.I. of slag, an excessive amount of oxygen in the slag will be transferred into the weld to degrade the mechanical properties of the weld.

Knowing the effect of B.I. on the welding, a welding engineer can purposely choose and manipulate welding consumables to design and qualify welding procedures to achieve satisfactory welding results in the field.

To learn more about effect of B.I., contact Wesley Wang, 614.688.5173 or


Related posts:

Share this: