Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (～15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10^?10 atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.