Oxidation of ferritic stainless steel interconnects: Thermodynamic and kinetic assessment

Abstract

Planar solid oxide fuel cells with yttria-stabilized zirconia electrolytes typically operate at temperatures in the range of 700–850 °C. The maximum temperature is limited by the use of ferritic stainless steel interconnects which offer significant advantages such as low cost and high thermal and electronic conductivity over traditional ceramic interconnects. However, these alloys rely on the formation of a protective chromia scale for oxidation resistance that results in an increase in ohmic resistance and can volatilize leading to a loss of cathode catalytic activity. To better understand the oxidation behavior of chromia forming ferritic stainless steels, thermodynamic modeling was performed in conjunction with experimental oxidation testing and empirical kinetic evaluation. The phase stability and oxidation behavior of the Fe–Cr–O ternary system were assessed using thermodynamic calculations. Calculated ternary phase diagrams were validated against the experimental oxidation data of Fe–20Cr and Fe–18Cr ferritic stainless steels, GE-13L and AL-441HP, respectively. Results indicate that the use of accelerated testing, such as exposing the system to higher temperatures, can lead to changes in phase equilibria and the oxidation kinetics of the alloys. Through combined thermodynamic assessment and controlled oxidation experiments, the oxidation behavior of high-chromium ferritic stainless steels is presented and discussed.