An XPS/SEM/EDX study of the early oxidation stages of the Fe19Cr5Al(+Y) alumina-forming alloys

The scale development on the unmodified and yttrium-implanted Fe19Cr5Al and Fe19Cr5Al0.27Y alloys at 1173 K during oxidation exposures up to 15 min was studied by means of XPS, SEM and EDX. The commercial material Kanthal APM was used as reference. A particular advantage of the XPS method relied on the possibility to distinguish between metallic and oxidized states of the elements. The scale evolution occurred in all cases via the same stages. At the beginning three-layered scales appeared, consisting of the outermost iron oxide, intermediate chromia and innermost alumina. The alumina, being thermodynamically the most stable oxide in this system, successively grew towards the scale/gas interface. Although alumina constituted the majority of scales formed during 15 min exposures, the other oxides were still present in the outermost parts of the scales. The fraction of the iron oxide and chromia in the scale was higher on the yttrium-implanted material than on the other alloys. No remarkable differences were observed regarding this fraction between the yttrium-free unimplanted and yttrium-bearing alloys. Metallic iron and chromium were found for shorter oxidation periods below their oxides. Subsequently, after 15 min exposure, the uniform distribution of the metallic iron and chromium in the scales were observed in all alloys except for the yttrium-implanted one.