Chemical Degradation of Recycled MgO–316L Cermets in Na-Cryolite: Effects of Processing on Dissolution and Molten-Salt Infiltration

MgO–316L cermets derived from end-of-life MgO–C refractories were evaluated for chemical compatibility with a conventional Na-cryolite melt. Specimens with identical nominal compositions were produced by spark plasma sintering (SPS) and granule-assisted cold isostatic pressing (CIP), resulting in markedly different processing-induced microstructures and open porosities. Current-free immersion tests were conducted at 1000°C for up to 24 h. Thermodynamic calculations indicated favorable MgO fluorination by AlF 3 , while ICP-OES identified Mg as the main released element. SEM-EDX revealed Mg-depleted regions, an Al–O-rich reaction zone, and preserved MgO in the bulk. After 24 h, normalized Mg loss reached 50.46% for SPS and 88.00% for CIP. The apparent Mg-depletion depth reached 1.82 mm for SPS, whereas no Mg recovery was detected within the full 10 mm specimen height of CIP from 16 h onward. Processing-induced microstructure strongly influenced degradation progression, but densification did not overcome the chemical instability of MgO in AlF 3 -containing cryolite.