The microstructure of the scale forming on dilute iron-silicon alloys in carbon dioxide

Abstract

The information available relating to the microstructure of scale forming on dilute ironsilicon alloys ( [Si] = 0–1 w/ o ) in CO 2(99%)/CO(1%) at 500°C has been supplemented by new analytical investigations involving X-ray diffraction, electron microscopy and X-ray photo-electron spectroscopy (XPS). X-ray diffraction from the underside of the scale has provided lattice parameter, strain and grain size measurements of the magnetite in the inner part of the duplex scale. Of these different sets of information the grain size shows by far the most dramatic dependence on alloy composition. The presence of 1.0 w/ o of silicon in the alloy brings a reduction in the grain size of the inner scale of more than an order of magnitude when compared with the scale on pure iron. A dry stripping technique has been developed which allows a thorough examination of the boundaries marking the inner and outer limits of the inner part of the scale. For the case of scales on Fe-1.0 Si it is found that the midscale interface, like that at the base of the scale, is marked by a local accumulation of silicon. The two silicon accumulations appear to be of sizes which are independent of time. These sizes are determined in the early stages of oxidation and do not change thereafter. The use of an Auger Parameter (AP) in XPS has allowed demonstration of the difference between the chemical form of the silicon in the scale and that in the outer regions of the alloy. The silicon in the metal near the metal/scale interface shows AP values intermediate between those of Fe 2SiO 4 and SiO 2 while that in the scale has AP values close to that of Fe 2SiO 4. These observations are in close agreement with the theoretical analysis of Atkinson. The assembled microstructural data suggest that the main effects of the presence of the silicon are to hold down the grain size of the inner scale magnetite and to suppress the growth of the outer part of the scale. The observations are consistent with an oxidation mechanism in which the rate controlling step is outward migration of iron along grain boundaries and that in the inner scale this is impeded by the presence of a grain boundary phase containing silicon. On the basis of the XPS and theoretical analyses it is likely that this phase is Fe 2SiO 4.