Breakdown of protective scales during the oxidation of thin foils of Fe–20Cr–5Al alloys at high temperatures

Abstract

The oxidation behaviour of alumina-forming Fe–20Cr–5Al and similar alloys containing small concentrations of lanthanum or lanthanum plus molybdenum in air at 1,150°C has been studied, with emphasis on thin (0.05 mm) specimens, where the aluminium reservoir in the substrate is soon depleted to a very low value. Oxidation of these alloys involves establishment and growth of protective alumina scales. However, once the residual aluminium concentration in the alloy drops below a critical level, a layer of chromia is able to develop and grow at the alumina–alloy substrate interface. Eventually, breakaway oxidation occurs and iron-rich oxides form and engulf the specimen.This paper presents some kinetics of oxidation of these alloys and discusses the growth and breakdown of the protective scales, drawing on the results of detailed examinations of the oxidized specimens using analytical scanning and transmission electron microscopy in cross section. It has been shown that lanthanum increases the time to the onset of breakaway oxidation, probably due to beneficial effects on the mechanical integrity of the scale. Molybdenum additions have been found to decrease significantly the rate at which breakaway oxides are able to penetrate and engulf the alloy substrate. Such additions stabilize the ferrite phase in the substrate at the alloy–scale interface, thereby maintaining a high rate of diffusion of chromium to the interface and facilitating establishment of a healing and partially protective chromium-rich oxide layer at the base of the breakaway oxide scale. In the absence of such additions, depletions of chromium in the substrate adjacent to the alloy/scale interface, arising from oxidation of chromium, enable the austenite phase to be stabilized. The relatively low rate of diffusion of chromium in this phase allows chromium-rich oxide to form as internal precipitates in the alloy rather than as a continuous, healing layer; hence, the breakaway oxide scale is able to penetrate and consume the substrate more rapidly than in the presence of molybdenum additions.