SUS430ステンレス鋼の高温酸化に対するランタン水酸化物コーティングの効果

概要

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Rare earth elements are effective for improving the oxidation resistance of stainless steels and chromia forming alloys at high temperatures. The reasons for the improvement, however, are not well understood and the effect of the content of rare earth elements is not clear. In this study, Type 430 stainless steel covered with a lanthanum hydroxide overlayer with various lanthanum contents was oxidized at 1373K in a 1.671×10^4Pa O_2-2.026×10^4Pa H_2O-balanced N_2 atmosphere. It was found that the critical amount of lanthanum atoms, at which reduction of the oxidation rate becomes obvious, was about 10^<16> atoms cm^<-2>. This value is similar to published data obtained from studies of Fe-Cr alloys coated with different rare earth elements by surface coating or ion implantation. Observation of the oxide surfaces at the initial stages of oxidation showed that lanthanum oxide does not provide a physical barrier for oxidant migration through oxide films nor preferential nucleation sites for Cr rich oxides. An explanation for the increased oxidation resistance with lanthanum is a change in the growth mode of the oxide from a predominance of cation to anion diffusion, at the critical amount of lanthanum of 10^<16> atoms cm^<-2> in the surface oxide film.
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