The effect of rare earth additions on the oxidation behaviour of Fe-20Cr alloys

HIGH-TEMPERATURE oxidation resistance of iron-chromium alloys is due to the formation of a protective layer of chromium dioxide. The addition of small quantities (<1%) of certain reactive elements to these alloys improves their overall oxidation resistance. A number of mechanisms have been proposed to explain the reactive element effect, without consensus as yet. This paper attempts a better understanding of this effect and reports the influence of three different rare earth (RE) elements - Pr, Dy, and Y - on the oxidation behavior of Fe-20Cr alloy at 1000 degrees and 1100 degrees C. Specimens were oxidized for prolonged periods in a muffle furnace, and for shorter durations in a thermobalance. The oxidized specimens were examined in a scanning-electron microscope coupled to an energy-dispersive spectrometer, and the oxide scales were analysed by x-ray diffraction. A thin layer of fine-grained chromium dioxide formed on the RE-containing alloy surface and the higher oxidation resistance of this alloy attributed to stifling of chromium ion diffusion in the oxide layer by the rare-earth ions. The extent to which the RE elements influenced scale growth decreased in the following order: Pr, Dy, and Y. Direct correlation between RE ion radius and extent of influence on chromium dioxide growth rate has been observed.