Diffusion and Partitioning of Elements in Oxide Scales on Alloys

Abstract

FORMATION of duplex scales on oxidized ferrous alloys and segregation of the alloying elements, are well known. For example, Pfeil1 showed that most of the transition elements, with the exception of Mn, were concentrated within the inner layer of oxide formed on alloy steels in air at 1,000° C and these findings have since been substantiated and extended2–4. It seems that under oxidizing conditions, the alloying elements V, Cr, Mo, Ni, W and Si are concentrated in the oxide layer adjacent to the metal, while Fe and Mn are distributed throughout the double layer. Typical partitioning behaviour is shown in a scanning electron micrograph of a section through a Fe–Cr alloy oxidized at 600° C in CO2-based gas (Fig. 1). The Cr concentration determined from using electron probe analysis is superimposed; it shows that Cr is virtually absent (< 0.2 wt. %) from the outer oxide layer. X-ray diffraction studies established that both layers possess a spinel structure.