Long-Term Environmental Stability of Cr-xMo-8Si Alloys in Air at 800 and 1200 °C: Evolution and Breakdown of Mo-Induced Nitridation Resistance

High Temperature Corrosion of Materials 103 (2026), 95, DOI: 10.1007/s11085-026-10459-2

Cr-Mo-Si alloys are promising high-temperature structural materials due to their low density, high melting point, and excellent strength. Their long-term environmental stability, however, remains insufficiently understood. This study investigated the quasi-isothermal oxidation and nitridation behavior of Cr-xMo-Si alloys (x = 0–40at.%) during long-term exposure in air at 800 °C (up to 700 h) and 1200 °C (up to 1000 h). At 800 °C, all alloys showed slow oxidation without catastrophic pesting. Mo additions reduced mass gain and improved nitridation resistance. Complete suppression of nitridation required at least 20–25at.%Mo, while ≥ 30at.%Mo promoted increased oxide growth and MoO₃ volatilization. At 1200 °C, Mo additions up to 25at.% improved oxidation resistance, whereas ≥ 30at.%Mo caused severe degradation due to accelerated oxide growth, cracking, and volatilization. Long-term exposure revealed additional degradation mechanisms not observed in short-term studies, such as oxide-scale cracking and depletion of the protective Mo-enriched subsurface zone due to the intermediate stops, and thus, thermal cycling. The intermediate stops from the quasi-isothermal exposure resulted in localized nitridation along A2/A15 phase boundaries. In contrast, isothermal exposure largely preserved the Mo-enriched region and maintained nitridation resistance. Amongst the compositions investigated, Cr-25Mo-8at.%Si showed the best overall long-term performance.