Oxidation of potential SOFC interconnect materials, Crofer 22 APU and Avesta 353 MA, in dry and humid air studied in situ by X-ray diffraction

Several stainless steels have been developed for the use as interconnect materials in solid oxide fuel cells in order to reduce costs, while maintaining the required performance. The materials however, are subjected to humid air, at high temperatures up to 800 degrees C leading to enhanced oxidation. High temperature X-ray diffraction, combined with field emission - scanning electron microscopy, has been applied to study in situ the influence of water vapour on the chromia scale formation on the ferritic Crofer 22 APU and the austenitic Avesta 353 MA alloys in comparison to their dry air oxidation behaviour. Both materials form at 800 degrees C, during the first 100 h exposure; Cr2O3 and MnCr2O4, the latter mostly in the surface region of the oxide scale. Additionally, Crofer 22 APU forms internal Al2O3 precipitates, while on Avesta 353 MA a SiO2 layer is found beneath the outer oxide scale. High temperature XRD indicates stress formation and relaxation in the Cr2O3 scale formed in humid air, especially for Crofer 22 APU.