The Effect of Molybdenum on High Temperature Oxidation and the Oxide Scale of Fe-22Cr-0.5Mn Ferritic Stainless Steels for SOFC interconnects

摘要

Mo is a common alloying element for high-temperature alloys to improve the high temperaturestrength and creep resistance. However, the effect of Mo addition on high temperature oxidation,electric conductivity of oxide scale and Cr evaporation is not well-understood. Although Mo oxideshave high vapor pressure so that it can be detrimental to the oxidation resistance, Mo showed littleeffect on oxidation resistance and electric conductivity of Ni-base alloy in some studies. In thisstudy, the effect of Mo addition to Fe22Cr0.5Mn ferritic stainless steel on the properties of theoxidation, electric conductivity of oxide scale and Cr evaporation is studied. The effect of Moaddition on the defect concentration of the oxide scale was measured with electrochemicalexperiment. The potential problem of Mo evaporation is also investigated.Three different levels of Mo content were chosen for modification of the base steel ofFe22Cr0.5Mn: 0.1 wt% (level of the reactive elements), 2 wt% (typical addition level in commercialstainless steels containing Mo) and 4 wt% (the amount that forms second phase at 800 °C). Eachspecimen was prepared by vacuum induction melting. To prevent the oxidation of the elements,the chamber was evacuated under 10~5 torr, and then filled with pure argon. Melted specimens werehomogenized at 1200 °C for 24 h in pure argon atmosphere, and then quenched by water. After thehomogenization, all the specimens had a ferrite single phase. The oxidation test was conducted inambient air at 800 °C up to 500 h. After the test, the oxide scale was analyzed by SEM, TEM andGDS. The oxygen vacancy concentration of the oxide scale was measured with an electrochemicalmethod.The addition of 0.1 and 2 wt% Mo to Fe-22Cr-0.5Mn improved the oxidation resistance by reducingthe amount of oxygen vacancy. The electric conductivity of oxide scale was also increased byalloying Mo. However, Mo evaporation and increase in oxidation rate were observed in 4 wt% Moadded specimen. The addition of Mo more than 4 wt% can be detrimental to oxidation resistance,especially in long-term operation. The effect of Mo on the Cr evaporation was negligible.