The role of silicon in the oxidation of iron-chromium-silicon alloys at 500-600°C in CO2-based gas

• Main Author: Hawes, P. M.
• Published: University of Bath 1980
• Subjects: 669; Metallurgy & metallography
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257317

The oxidation behaviour of a binary Fe-9Cr alloy and two ternary Fe - 9Cr - Si alloys containing 0.75wt% silicon has been studied at 500°C-600°C in CO2 based gas. 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. Initial experiments were to determine experimental conditions which allowed the early onset of breakaway oxidation on the Fe - 9Cr alloy. However, breakaway oxidation kinetics were not observed and protective duplex scales were formed under all the conditions investigated. Later experiments investigated the oxidation of Fe - 9Cr - Si alloys in CO2 - 13% CO gas. The oxidation process followed approximately a parabolic rate law for all the alloys, the oxidation resistance increasing with increase in silicon concentration. The lowest oxidation rates were associated with the presence of an internally oxidised zone beneath the duplex scale. Internal oxidation was favoured by an increase in either the oxidation temperature or the silicon concentration of the alloy. A model for the dependence of internal oxidation on the silicon concentration of the alloy has been proposed. Calculations showed that diffusion of oxygen was the dominant transport mode in internal oxidation and oxygen permeability increased with increase in temperature or silicon level. Outward diffusion of iron cations is the rate controlling process in scale growth, which is the dominant factor in the overall oxidation process. The reduction in oxidation rate associated with internal oxidation may be explained by an increase in the diffusion path length of iron cations outwards and a reduction of the iron cation concentration at the scale-metal interface. The model proposed for the internal oxidation of these alloys is also able to account for the differences observed in the structure of the initial thin oxide films.