The oxidation behaviour of a series of iron-chromium binary alloys containing 5%, 9%, 12%, 15% and 20% chromium at 400-600°C has been studied in CO2 -1% gas mixtures. The experimental approach has involved kinetic studies using a microgravimetric method together with detailed characterisation of the oxidation products using optical, X-ray and electron optical techniques. Several oxides are formed during the initial stages of oxidation. The nature of the oxidation product is shown to depend on both the crystallographic orientation and the initial composition of the substrate. These effects are explained by considering the maximum solubility of chromium in different oxide phases together with interfacial and strain energy factors. The kinetics of oxidation together with micrographic observations clearly indicate that as the oxidation proceeds spinel oxide, M3O4 at various sites on the substrate surface. Such sites are associated with asperities on the surface. The spinel nuclei grow both laterally and vertically until they impinge and coalesce. The scale subsequently thickens according to a parabolic rate law. Examination of scales reveals a duplex struc-ture. This is interpreted in terms of an outward diffusion of cations together with simultaneous growth of an inner layer in the space created by the outward movement of metal. Both layers are porous and hence provide a route for gas phase transport of oxidant to support the growth of the inner layer. A series, regularly spaced, of lamellar voids form in the inner layer under certain conditions. This is believed to be associated with a cyclic vacancy condensation process and it is shown that the spacing between lamellar voids is consistent with such considerations. Enrichment of the inner layer in chromium also occurs and a model is proposed to explain this segregation effect based on an analysis of the possible diffusion path networks in close packed oxides.
|Date of Award||1975|