Welding flux basicity index

Basicity is commonly used to describe the metallurgical behaviour of a welding flux. The basicity index is a ratio between basic and acid compounds (oxides and flourides) of which the flux is composed.

There are several ways of calculating basicity and in welding Boniszewski’s formula has become the predominant way of calculating basicity.

Welding fluxes can be divided into three groups:


Type of welding flux Basicity
Acid fluxes <0.9
Neutral fluxes 0.9 – 1.2
Basic fluxes >1.2


Basicity has great influence on impact toughness of the weld metal. Increasing basicity brings down the oxygen content and hence the inclusion level in the weld metal. Consequently, the impact toughness will increase and also, to a limited extent, the ductility of the weld metal.

The relation between basicity and impact toughness is particularly important for high alloyed grades, such as duplex steels.