High-Temperature Oxidation Behavior of a Novel Compositionally Complex Stainless Steel at 800 and 1000 °C

Compositionally complex alloys, derived from high-entropy alloys, are promising new materials that often present synergetic effects due to their complex compositions that give them properties surpassing conventional alloys. In this study, a medium-Cr compositionally complex stainless steel (CCSS) containing Ni, Co, Si, and Mo was evaluated under isothermal oxidation at 800 and 1000 °C in dry air. At 800 °C, the alloy exhibited protective behavior with a parabolic mass-gain rate (kp = 1.002 × 10 −4 mg 2 cm −4 h −1 ), forming a predominantly chromia-based scale accompanied by discontinuous spinel and mixed M 2 O 3 oxides. A silica subscale was detected at the metal/oxide interface, and Laves-phase precipitates influenced oxidation by acting as diffusion barriers and local Si reservoirs. At 1000 °C, however, extensive scale cracking and spallation occurred after the shortest exposure time used in this study (24 h), hindering the establishment of a stable protective layer. Although chromia remained the main oxide, volatilization and mechanical degradation led to breakaway oxidation. The presence, distribution, and evolution of Laves-phase precipitates were found to strongly affect both diffusion processes and scale integrity. Overall, the alloy demonstrated promising high-temperature performance at 800 °C due to its lower mass variation and simple oxide scale formed during exposure compared to conventional stainless steels but insufficient resistance at 1000 °C, especially considering its higher cost, indicating potential applicability at intermediate temperatures.