Analysis of High Temperature Degradation of Alloys in Solid Oxide Fuel Cell

Solid oxide fuel cells (SOFCs) are approaching commercialization to improve power production efficiency. Currently, cost and lifetime reliability limit their spread in the market. The SOFC is ceramic-based, but refractory metal alloys are in fact the majority materials present in SOFC systems. The lifetime behaviour of SOFC stacks, and of the metallic components in particular, remain in need of further investigation. The degradation processes behind the possible failure of selected metal interconnect (MIC) and balance of plant (BoP) alloys were investigated. Samples were extracted from stacks operated for periods up to 18000 hours and analysed with scanning electron microscopy coupled with X-ray energy dispersion spectroscopy (SEM/EDS). Scanning transmission electron microscopy (STEM) coupled with EDS and electron diffraction was applied to identify corrosion products. Electrochemical impedance spectroscopy (EIS) analysis completed the characterization of the stacks. Degradation processes of steam generator BoP alloy were investigated with SEM/EDS as well. Various protective coating solutions to limit interconnect and related cathode degradation were tested as small coupon samples. Area specific resistance (ASR) monitoring and evaluation of Cr evaporation blocking verified the effectiveness of these solutions. A special set-up able to test 8 small MIC samples simultaneously in different gas atmospheres was built and used to correlate operating variables to material degradation through a design of experiment (DoE) approach. The results indicate that in commercial ferritic stainless steel, corrosion kinetics are fast in the first few thousand hours of operation and then tend to stabilize. The main influencing parameter is the initial density of the protective coating, independently of the steel and coating compositions. However, even porous coatings demonstrated to be viable for periods at least up to 2 years. The strong chemical interaction with the cell materials is their main drawback, but they present a competitive advantage in manufacturing cost. The steam generator alloy used in the BoP suffered stress corrosion cracking induced by the strong thermal gradient between hot and cold gas and the aggressive hot outlet gas atmosphere. Aluminizing treatment of the alloy seems to solve the issue.