Synergistic Roles of Nb and Mo in the Formation of Oxides on Fe-20Cr-25Ni-Nb Stainless Steels in High-Temperature CO2

Fe-20Cr-25Ni-Nb steel is an important material for developing highly compact and efficient nuclear power systems by using the supercritical CO2 Brayton cycle. The in-core materials should possess excellent oxidation resistance in a high-temperature CO2 environment. In this work, a new 20Cr-25Ni-Nb steel with a minor Mo addition was developed, and its oxidation behavior was investigated in a pure CO2 environment at 650 °C under 3.5 MPa. The experimental results show that the oxidation kinetics of the steels followed the parabolic law with the test time, and the oxidation process was controlled by diffusion. The 20Cr-25Ni-Nb steel showed better oxidation resistance after Mo addition, which was attributed to the synergistic effects of Nb and Mo during the oxide scale formation process. In a high-temperature environment, the volatilization of Mo promoted the outward diffusion of Nb, resulting in the formation of an Nb-rich layer at the oxide/metal interface, which slowed the outward diffusion of Fe for the formation of the outermost Fe-oxide layer. Although the volatile nature of Mo at high temperatures promoted the outward diffusion of Nb, the addition of Mo had no significant influence on the overall structure of the oxide scales, which consisted of an outer Cr-rich oxide layer and an inner spinel oxide layer.