HIGH-TEMPERATURE CORROSION OF IRON CHROMIUM-ALLOYS BY TELLURIUM

The reaction rates between iron-chromium alloys (1.17, 5.65 and 11.96 a/0Cr) and tellurium were measured in the temperature range of 873 approximately 1,023 K at 350 Pa of tellurium vapor pressure, and in the tellurium vapor pressure range of 66.7 approximately 600 Pa at 923 K. The electron probe microanalysis, marker experiment and X-ray diffractometry were employed to clarify the mechanism of the telluride scale growth. The reaction rates between iron-chromium alloys and tellurium obeyed the parabolic rate law. It was observed that the telluride scale consists of the inner, the intermediate and the outer layers; chromium is concentrated in the inner layer which may grow by the inward diffusion of tellurium; the intermediate layer consists of mainly beta-iron telluride, and the outer layer consists of both delta- and delta'-iron tellurides below 980 and delta-iron telluride alone above 980 K. The intermediate and outer layers grow by the outward diffusion of iron in iron tellurides. The protection effect by chromium may be explained by the fact that chromium telluride covered over the surface of iron-chromium alloy interrupts the outward diffusion of iron. The reaction between iron-chromium alloy and tellurium can be classified into three regions from the view point of the activation energy. From the comparison of the activation energy of the reaction between iron-chromium alloy and tellurium with that for the diffusion of iron in iron tellurides, the rate-determining step for the scale formation for each region was discussed.