Elucidating the Corrosion Mechanism of Graphite Anodes in Fluoride Molten Salt for Rare Earth Electrolysis: A Multiscale Structure-Property Investigation

Graphite anodes are widely used as consumable electrodes in the high-temperature electrolytic production of rare earth metals within fluoride molten salts. However, their rapid and complex corrosion presents significant economic and operational challenges, including high consumption costs, process instability, greenhouse gas emissions, and product contamination. While the corrosion morphology of specific graphite types has been studied, a systematic investigation linking the intrinsic properties of diverse graphite materials to their microstructural and chemical evolution during corrosion is lacking. This study elucidates the corrosion mechanisms of three distinct graphite anodes—fine-grained, isostatically pressed graphite anodes (#1), medium-coarse-grained, extruded graphite anodes (#2), and recycled, extruded graphite anodes (#3) in industrial PrNdF3–LiF molten salt electrolytes at 1050 °C. Through a multifaceted analytical approach encompassing SEM, EDS, XRD, Raman, and FT-IR, we investigated the macro- and microscale corrosion behaviors across multiple scales. The results revealed markedly different degradation patterns: the #1 anode exhibited intergranular corrosion with granular exfoliation; the #2 anode developed a protective but cracked resolidified salt layer; and the #3 anode suffered the most severe uniform and pitting corrosion. Postcorrosion analysis confirmed surface enrichment with fluorine, praseodymium, and neodymium, the formation of PrF3 and NdF3 phases, and substantial degradation of the graphitic structure. Raman spectroscopy specifically revealed a reduction in the crystallite size, introduction of in-plane point defects, and disruption of the interlayer stacking order. On the base of infrared spectroscopy analysis, all key characteristic absorption peaks of the graphite anodes undergo consistent attenuation after corrosion. This work provides critical insights for the informed selection and optimization of graphite anodes to increase the efficiency and sustainability of rare earth electrolysis.