Microstructural analysis of oxide layer formation in ferritic stainless steel interconnects

Abstract

The oxide layer formation of ferritic stainless steel (Crofer22APU) oxidized in an air furnace at 800 degrees C was analysed by transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). EBSD analysis revealed that crystallographic orientation affects the oxidation behaviour of Crofer22APU. On {110} grains, most oxide granules had the same orientation but on {111} grains, they had random orientation. TEM analysis revealed that the oxide layer consisted of spinel-chromia-spinel multi layered structure on the {110} matrix grain but no sub spinel oxide layer on the {111} matrix grain. On the {110} closed packed plane of matrix, the chromia oxide formation with the < 110 > direction of rhombohedral structure on the {001} closed packed plane parallels the < 001 > direction on the {110} plane of BCC matrix was observed, which is known as Pitsch-Schrader orientation relationship. On the {111} plane grain, there was no specific orientation relationship. Energy dispersive spectroscopy (EDS) analysis in TEM showed that an Mn rich region was developed on the {110} grain beneath chromia oxide and the conversion of this region to (Cr,Mn)(3)O(4) spinel oxide layer causes multi layered oxide structures on the {110} matrix grain.