High Temperature Corrosion in Alkali Fluoride Salts Containing Oxygen

Licentiatavhandling, 2025

Alkali fluoride salts, particularly sodium fluoride (NaF) and lithium fluoride (LiF), play a crucial role in diverse industrial processes, including nuclear reactors, thermal energy storage, battery production and recycling. In their molten state, these salts are valued for their low vapor pressure, high thermal stability, and excellent fluxing properties. However, high-temperature corrosion of containment materials remains a significant challenge in both oxygen-rich and oxygen-depleted environments. Such corrosion can lead to equipment failure, plant shutdowns, premature decommissioning, and considerable financial losses.

This thesis investigates the mechanisms of high-temperature corrosion in contact with alkali fluoride salts, with a focus on NaF and LiF. The primary objective is to elucidate the corrosion behavior of alloying elements, particularly within Inconel 625. Thermodynamic calculations were employed to predict the stability and dissolution tendencies of various alloying elements, and these predictions were validated through experimental analyses.

Fluoride salts degrade protective oxide layers on metals, promoting the formation of transition metal fluorides and subsequent dissolution of alloy components. A detailed comparison between NaF and LiF exposures revealed a distinct influence of the salt type on the corrosion mechanism. No transition metal fluorides were formed during NaF exposure, while LiF exposure resulted in the formation of FeF₂, CrF₂, CrF₃, and NbF₅. Notably, NaF caused significant oxide spallation, indicating a more aggressive attack on the alloy surface.

Temperature gradients, commonly encountered in industries utilizing molten salts, were also examined as a driving factor for corrosion. A comparative study between isothermal and thermal-gradient exposures demonstrated that thermal gradients exacerbate intergranular attack due to galvanic effects, highlighting the importance of temperature control in mitigating corrosion.

Overall, this work provides new insights into the corrosion mechanisms of Inconel 625 in alkali fluoride salts, with emphasis on the role of salt composition and thermal conditions. These findings contribute to the development of more corrosion-resistant materials and optimized operational strategies for high-temperature salt-based applications.