COMPOSITION AND STRUCTURE OF SCALE ON EI981A STEEL

The chemical composition of the scale formed on EI981A austenitic stee l before and after heat treatment is investigated by X-ray photoelectr on spectroscopy. The composition and quantity of solid phases and thei r structure are established. The principal phases, which determine the oxidation mechanism, are spinel (a solid solution of variable composi tion based on the ferrites FeCr2O4, FeV2O4, Fe3O4, and NiFe2O4) and mo lybdenum oxides (MoO3, MoO2, and MoO). After low-temperature treatment , the scale includes, over its whole depth, a metallic phase of variab le composition containing the main alloy components: Fe, Cr, Ni, V, an d Mo. After high-temperature treatment, pure Ni-0 is present in the sc ale at depths of more than 10 Angstrom from its surface. At room tempe rature, the surface monoatomic layer of the spinel (a approximate to 8 .5 Angstrom) is oxidized and partially hydrated to Ni(OH)(2). The quan tity of chemosorbed oxygen is calculated; it is a maximum (similar to 56 at. %) at the scale surface after low-temperature treatment and a m inimum (similar to 10 at. %) at a depth l similar to 10 Angstrom after high-temperature treatment.