A Nanoscale Study of Thermally Grown Chromia on High-Cr Ferritic Steels and Associated Oxidation Mechanisms

Abstract

Fe-22Cr-0.5Mn based ferritic steels are used as interconnect materials for solid oxide fuel/electrolysis cells. Four steel samples, including the commercial steel Crofer 22 H, were oxidized at 800 °C in a model Ar-4%H2−4%H2O atmosphere simulating the fuel side of the cells and investigated by atom probe tomography (APT) in conjunction with electron microscopy and thermogravimetry. All steels form an oxide scale mainly consisting of MnCr2O4 spinel on top of Cr2O3. APT revealed segregation of minor alloying constituents (Nb and Ti) to chromia grain boundaries and highlighted their effect on mass transport through the chromia scale. Relationships between segregation activity of individual elements (in terms of Gibbsian interfacial excess), oxide scale microstructure and alloy oxidation rate have been established based on the APT results. Comparison of segregation activities revealed that vacancies formation due to Wagner-Hauffe doping with aliovalent Ti and Nb impurities cannot be solely responsible for faster oxidation, assuming alteration of the grain boundary structure and associated changes of their mass transport properties. Controlled Si addition to the alloy (about 0.4 at%) suppresses the detrimental effect of Nb on the oxidation resistance but results in formation of a thin, although still discontinuous, SiO2 layer at the metal-oxide interface.