Long-term stability of ferritic alloys for APU application

SOFC stacks for auxiliary power units (APU) should be light, cheap and have a high power density. The stack power output should be constant (degradation rate <2 % / 1000 hours) for more than 5000 operating hours. To fulfil these criteria Staxera and Fraunhofer IKTS are developing solid oxide fuel cells (SOFC) stacks made of 0.5 mm thick metal sheets of commercial ferritic alloys with and without protective coatings. The metal sheets are stamped and laser welded to cassettes. Breakaway oxidation of ferritic alloys as a function of the thickness and operating temperature is widely discussed in the literature. Forecasts for the breakaway oxidation of different uncoated steels have been made on basis of annealing experiments for 1000 hours. To prove the suitability of commercial steels with a thickness of 0.5 mm for use in SOFC stacks, annealing experiments for more than 6000 hours at 850°C have been made. Cassettes made of ZMG232, CroFer22APU and Plansee ITM14 were annealed at 850°C in air for more than 6000 hours to analyze the long term stability of the chosen alloys. Cross sections of the welding joint, the uncoated oxide scale and the oxide scale of perovskite coated samples were analyzed at the initial state and after annealing for 800, 1600, 3200 and 6400 hours. The oxide scales of ZMG232 material after 6400 hours of annealing were compared with the oxide scales of the ZMG232 cassettes from a SOFC stack operated for about 6400 hours. Obtained results show that the ferritic steels with thickness of 0.5 mm can be used in SOFC stacks for APU application.