Redox mechanisms and metal fluoride stability in alkali fluoride corrosion -confirmed by experiment

The corrosion behavior of Inconel 625 in contact with LiF was compared with its behavior in NaF at 600°C in air. Thermodynamic calculations were carried out to determine the stability of metal fluorides in both conditions, confirming salt specific element depletion. TOF-SIMS was conducted to examine the lithium-ion behavior in the oxide and metal over short-term and long-term exposures. A redox mechanism is suggested where fluoride ion A redox mechanism is suggested where fluoride ion ingress into the alloy towards an inner anode is discussed coupled with an outer anode at the alloy surface. The diffusion of lithium ions is considered a passive process along cation diffusion paths. The suggested mechanism discusses also why the presence of fluorine gas in the corrosion process is not viable. The presented mechanism and energetic pathway are in full agreement with the experimental observations and can be easily transferred to other alkali fluoride experiments in literature. • A redox mechanism is explaining the element selective intergranular corrosion attack of alloy 625 by LiF in air. • The same redox mechanism also predicted the absence of intergranular attack in NaF, which was experimentally verified. • This mechanism excludes molecular fluorine as reactant and deems "active oxidation" theory unsuitable in alkali fluorides. • No metal fluorides were thermodynamically stable in contact with NaF, which was in line with experimental results. • Ionic flux transport paths are expressed.