COMPOSITION AND GROWTH MECHANISMS OF ALUMINA SCALES ON FECRAL-BASED ALLOYS DETERMINED BY SNMS

The composition and growth phenomena of aluminia scales on a conventional wrought alloy Fe-20Cr-5Al and on an yttria containing ODS alloy of similar base composition were investigated. The oxide scales were formed during two-stage oxidation at temperatures between 900 and 1100-degrees-C in air and in air containing O-18 tracer. The in-depth concentration profiles of the various elements as well as the oxygen isotope distributions in the oxide scales were determined by secondary neutrals mass spectrometry (SNMS). It was found that detailed knowledge about the time and temperature dependence of scale composition is necessary for deriving unequivocal information on the scale growth process from the oxygen isotope distributions. It appeared that the growth of the oxide scales mainly occurs on the scale/alloy and the scale/gas interface. The existence of growth within the scale could not clearly be confirmed although some results indicated that this effect might have occurred. The scales on the ODS alloy and the conventional alloy differed mainly in the relative contributions of cation diffusion to the overall growth process.