| Summary: | Research on oxidation kinetics of stainless steel traditionally focuses on flat sheet material. Little is known about the oxidation of steel within porous structures or particles of different sizes. In cases where oxidation of porous materials is reported, the data are seldom related to the actual surface area of the material. Instead, the mass change is often reported as a percentage mass gain only. In some literature references, the oxidation mass gain is assumed to increase with increasing porosity, often without information of the surface area of the pores. If an area-normalized oxidation mass gain is calculated, it is often normalized to the outside dimensions of the investigated specimens, making comparisons between different microstructures difficult. In this work, oxidation of spherical stainless steel powders with different powder particle sizes and of porous sintered stainless steel specimens is analyzed. Oxidation kinetics are correlated to the powder particle size and initial metal surface area of spherical stainless steel powders, addressing this knowledge gap. For oxidation kinetics of spherical steel powders, the dynamic change in metallic surface area over time is taken into account in the model developed in this work. Maximum oxidation mass gain of stainless steel powder based on composition and changes in phase structure, microstructure, and composition of oxides growing under the influence of prolonged exposure to solid oxide fuel cell (SOFC) operating temperatures is analyzed.
The oxidation mass gain of sintered porous stainless steel is influenced by microstructure. The oxidation mass gain correlated to the entire surface area of the 3-D structure of the sintered porous specimens indicates slightly lower oxidation rate kinetics per unit surface area at 1073 K than published kinetics of similar materials in dense form.
Additionally, the chromium diffusion through four spinel coatings that have been proposed as protective coatings for stainless steels used in SOFCs is analyzed in this work. Al-Mg-type spinels have the lowest Cr-diffusion rate at the investigated conditions and among the investigated materials.
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