Complementary Raman and ToF-SIMS analysis of oxide and fluoride networks in LiF corrosion of Inconel 625

This study investigates the corrosion scale formed on Inconel 625 exposed to LiF in air at 600°C for 168 h and 1000 h, using confocal Raman spectroscopy in combination with SEM-EDS, ToF-SIMS, and XRD. Raman imaging enabled spatially resolved identification of the oxide scale, revealing a multilayered structure consisting of outer lithiated (Li,Fe-doped) NiO, Ni(Fe,Cr)2O4 spinels, CrNbO4, Cr2O3 and an innermost MoO2. After longer exposures, LiCrO2 formed at the interface between the spinel zone and chromia, indicating progressive lithiation and partial transformation of the protective oxide. ToF-SIMS complemented the Raman data by unravelling the intergranular metal fluoride activation, transportation towards the metal oxide interface and incorporation into the oxide scales. Raman and ToF-SIMS prove to be complementary, the former resolving the oxide and lithiated phases while the latter is better for detecting the ionic metal fluorides and lithium ion.