Microstructure of Oxidised 304L Steel and the Effects of Surface Roughness on Oxidation Behaviour

Abstract

This report deals with the microstructure of the oxide formed on 304L steel exposed to dry oxygen at 600°C. The specimen surfaces were pre-treated by either mechanical grinding or polishing and oxidised for 24, 168 or 672 hours. It is observed using SEM and TEM that the oxide is composed of two types of oxide species, i.e. the sub-micron spinel particles on the outer regions and the finer corundum-type base oxide grains that are formed first on the steel surfaces. The spinel particles contain roughly 40 cation % Mn, 45 cation % Cr, 8 cation % Cu and 7 cation% Fe. Such a result shows a significant Mn, Cr and Cu enrichment compared to the composition of the steel. Individual particles are homogeneous while the composition varies substantially from one particle to another. Also, more spinel particles form on specimens with ground surfaces. The base oxide grains, measuring ≃ 10 nm, are of the corundum-type oxides (Cr,Fe) 2 O 3 with varying Cr/Fe ratios. EDX line scans results from the specimen cross-section indicate that the oxide closest to the steel is most often Cr-rich and that Cr and Mn are depleted from the steel immediately below the oxide.