Effect of gas composition on the oxide scale growth mechanisms in a ferritic steel for solid oxide cell interconnects

The oxidation behavior of a ferritic steel Fe-23Cr-0.5Mn-0.6 Nb-0.1Ti (at%) considered for application in solid oxide cell (SOC) stack interconnects was studied at 800 °C. The oxidation kinetics and oxide scale microstructure formed in Ar-20%O2, Ar-4%H2-4%H2O and Ar-1%CO-1%CO2 atmospheres, simulating the SOC operation environments, were investigated by thermogravimetry (TG) in conjunction with electron microscopy (SEM/TEM) and atom probe tomography (APT). In all three environments multilayered oxide scales formed, consisting of Mn-Cr spinel on top of Cr2O3 and an additional Nb-rich oxide layer at the chromia-alloy interface. The initially faster oxidation in the low pO2 gases was attributed to formation of porous chromia scales compared to a dense scale formed in the high pO2 (Ar-O2) atmosphere. APT revealed segregation of minor alloying elements (Mn, Nb and Ti) to chromia grain boundaries in all three simulated SOC environments in quantitatively similar amounts, suggesting their similar effect on the ionic transport through the oxide scale. The findings indicate that oxygen activity in the test gas plays a dominating role in governing the oxidation kinetics and the oxide scale microstructure of the studied ferritic steel.