Raman Spectroscopic Identification of Oxide Phases in a Corroded MgO–Steel Composite Anode Used in Aluminum Molten Salt Electrolysis

This study presents the postmortem phase characterization of a metal–ceramic composite anode composed of 316L stainless steel and recycled MgO, sourced from spent refractory lining bricks, employed in aluminum molten salt electrolysis. The analysis focused on the immersed section of the anode, where direct exposure to the molten Na‐cryolite melt promotes the formation of corrosion products. Raman spectroscopy was applied as a structural characterization technique, providing phase information that complements the morphological and elemental analyses obtained from SEM–EDX measurements. Due to its high spatial resolution, micro‐Raman spectroscopy enabled the identification of local phases within the corrosion layer and the determination of their depth‐dependent distribution. The local chemical analysis revealed an outer Fe–O‐rich layer penetrating several tens of micrometers into the material, followed by a Fe–Al–O‐containing zone. Raman spectroscopy identified the Fe–O layer as magnetite (Fe O) and the inner layer as hercynite (FeAlO), with a transition region consisting of Al‐doped FeO. The results demonstrate the applicability of Raman spectroscopy for identifying corrosion products to provide contributions to the corrosion mechanisms of MgO–steel anodes under electrolytic conditions.