Optimization of the electrical properties of Ti–Nb stabilized ferritic stainless steel SOFC interconnect alloy upon high-temperature oxidation: The role of excess Nb on the interfacial oxidation at the oxide–metal interface

Interfacial oxidation of Nb and Si at 650 °C on Laves phase forming Ti–Nb stabilized ferritic stainless steel (Fe–19Cr–0.9Si–0.2Nb–0.1Ti (at.%), grade EN 1.4509) was studied by electrochemical impedance spectroscopy and photoelectron spectroscopy. It was found that excess Nb efficiently hinders the formation of electrically resistive SiO2 layer at the oxide–metal interface. The beneficial role of Nb was attributed to its high segregation rate and the formation of conductive oxides at the interface. However, the oxidation was strongly influenced by age-precipitation of the Laves (FeNbSi)-type intermetallic phase, which removed free Nb from the alloy solution and thus allowed SiO2 layer to form more easily. These results can be applied to optimize the oxide scale composition by Nb alloying of the ferritic stainless steel to maintain high performance under various operation conditions, particularly in solid oxide fuel cell applications.

► Excess Nb in the alloy strongly hinders the formation of resistive SiO2 at the oxide–metal interface.
► Oxidation of Si is promoted by the age-precipitation of the Laves (FeNbSi)-type intermetallic phase.
► Electrical properties of the oxide scale can be controlled by the Nb/Si alloy concentration.
► SiO2 rich oxide–metal interface favors formation of oxide scale with increased Mn/Cr ratio.