Abstract: In order to determine the effect of different oxidation time and chromium content on microstructure and thickness of the iron oxide scale on steel surface at high temperature, Fe-5Cr steel and Fe-10Cr steel were oxidized at 1000 ℃ under air condition for 60-180 min. The oxidation kinetics curves of this two tested steels were drawn by using mass gain method, and the cross-section morphologies and phases of the steel scales were studied by means of metallographic microscope, energy spectrum and X-ray diffractometer. The results show that the oxidation of two steels is a “gas-solid” reaction in the early stage, and is a “gas phase-iron oxide scale-solid phase” reaction in the middle and late stages. The structure of the oxide scale is composed of outer oxide layer, inner oxide layer and inner oxide zone. When the oxidation time is 180 min, the internal oxide Cr₂O₃ is detected in the Fe-10Cr steel. The oxygen in the air diffuses inward and reacts with chromium in the substrate to produce internal oxide Cr₂O₃, which reacts with FeO in the iron oxide to produce spinel structure product FeCr₂O₄. With the increase of oxidation time, the internal oxide Cr₂O₃ is continuously wrapped by the internal oxide layer and converted to external oxidation. The transformation of internal and external oxidation causes the substrate to be continuously corroded, and the thickness of the iron oxide scale continues to increase.

Key words: Fe-Cr steel, high temperature oxidation, oxidation kinetics, iron oxide scale, oxidation time