Effect of NaF and CaO/SiO₂ Mass pct. on the Hydrogen Dissolution Behavior in Calcium-Silicate-Based Molten Fluxes

The effect of NaF and CaO/SiO₂ mass pct. ratio (C/S) on the hydrogen solubility in terms of the hydroxyl capacity of the CaO–SiO₂–NaF welding flux system was investigated at 1823 K. As a cost-effective alternative to CaF₂ in welding fluxes, NaF can potentially increase the fluidity, electrical conductivity, and hydroxyl capacity of welding fluxes in advanced high-strength steels. Additions of 1.9 to 5.9 mass pct. of NaF was found to depolymerize the structural units in the flux system. Depending on the flux composition, the dominant hydrogen dissolution mechanism varied. At C/S values less than 1.06, the incorporated-hydroxyl dissolution mechanism was dominant in the present flux system, in which SiO₂-based complex structural units were pronounced. At C/S values above 1.06, the free-hydroxyl dissolution mechanism was pronounced due to the strong network-modifying CaO content supplemented by NaF. A combination of Fourier transformed infrared, Raman, X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry analysis was used to determine the structural implications of NaF additions and C/S, which were correlated to the hydroxyl capacity of the flux. High NaF contents and C/S values were found to depolymerize the complex structural units of the molten flux by supplying greater amounts of network-modifying free oxygen (O²⁻) and fluorine (F⁻) anions to the flux system in the compositional range of this work.