Activity-mediated suppression of Cr₂O₃ breakdown by Si substitution NiCoCrAlY coatings under NaCl-induced hot corrosion

Marine gas turbines commonly suffer from NaCl-induced hot corrosion at intermediate temperatures, where the degradation of Cr-containing MCrAlY coatings is closely associated with the chlorination-assisted breakdown of Cr₂O₃ scales. However, the mechanistic role of Cr substitution in suppressing this degradation pathway remains unclear. In this study, a series of NiCoCr _20-x Si _x AlY ( x = 0, 1, 3, 5, 10, in wt.%) coatings were deposited on Inconel X750 alloys by magnetron sputtering to investigate the effect of partial Si substitution for Cr on NaCl-induced hot corrosion behavior at 750 °C. The results demonstrate that a pronounced transition in corrosion mode with increasing Si content. Low-Si coatings ( x = 1, 3) underwent severe internal oxidation accompanied by oxide cracking and spallation, whereas high-Si coatings ( x = 5, 10) developed continuous and dense multilayered oxide scales composed of Al₂O₃/Cr₂O₃/SiO₂, resulting in significantly enhanced corrosion resistance. Thermodynamic calculations reveal that increasing Si content reduced Cr activity from 0.95 to 0.22, consequently raising the Gibbs free energy for Cr₂O₃ from -851.67 to -835.09 kJ/mol. The reduced thermodynamic driving force for Cr₂O₃ formation suppressed the chlorination–volatilization cycle associated with NaCl attack and promoted the preferential formation of thermodynamically stable Al₂O₃ and SiO₂ composite scales. A critical transition from internal oxidation to external scale protection occurred at approximately 5 wt.% Si. The improved hot corrosion resistance is therefore attributed to an activity-mediated suppression of Cr₂O₃ breakdown and the formation of a continuous SiO₂-containing diffusion barrier that inhibits chlorine penetration and volatile chloride formation. These findings provide a mechanistic strategy for designing chloride-resistant MCrAlY coatings for marine environments.