High-temperature oxidation of Fe-Cr alloys in air

The scales formed on the Fe-Cr (19, 29.5, and 46.8 at % Cr) and Fe₇₀Cr₁₅B₁₅ alloys in the initial stages of oxidation in air at 1470 K have been analyzed using depth profiling. The results demonstrate that the Cr/(Fe + Cr) ratio in the scales on the Fe-Cr alloys is close to 30 at %, independent of the bulk chromium concentration. The top scale layers on the Fe-Cr alloys consist predominantly of M₂O₃ (M = Cr, Fe) solid solutions and CrO₂, the intermediate layers are composed of (Fe, Cr)₃O₄ and FeO, and the internal oxidation and nitridation layers consist of chromium oxides and nitrides. The top scale layers on the oxidized Fe₇₀Cr₁₅B₁₅ alloy contain, in addition, boron oxides, and the internal nitridation product is BN instead of chromium nitrides.