Jun Eu Tang

Investigation of FIB-thinned TEM cross-sections of oxide scales formed on type 310 steel at 600°C in water vapour-containing oxygen atmospheres

AbstractThe effect of water vapour on the oxidation of the type 310 (25Cr19Ni) austenitic stainless steel was studied through the detailed microstructure characterisation of the oxide scales formed at 600°C in dry O2, O2 containing 40% H2O at 0.5 cm/s, and O2 containing 40% H2O at 5 cm/s. FIB equipment was utilised for imaging and for producing thin foils containing the cross-section of oxide features for detailed analysis using TEM/EDS. Thin protective Cr-rich (Cr,Fe)2O3 was maintained in dry O2 atmosphere. Cr-loss through CrO2(OH)2 evaporation in H2O-containing O2 atmosphere from the oxide scale could, at sufficiently high flow rate, trigger local breakaway oxidation after 168 hours of exposure. These breakaway sites featured large oxide nodules, each consisting of an outward growing oxide island and an inward growing oxide crater. The local post-breakaway oxidation behaviour was discussed based on the composition and distribution of the various phases in these oxide nodules.

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

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.