Kinetics and Mechanism of Telluride Scale Growth on Iron.

The reaction rates between iron and tellurium were measured in the temperature range of 873~1,023 K at the tellurium vapor pressure of 350 Pa and in the tellurium vapor pressure range of 66.7~1,000 Pa at 923 K. The electron probe micro-analyzer, marker experiment and X-ray diffractometer were used to clarify the mechanism of the telluride scale growth. The reaction rate between iron and tellurium obeyed the parabolic rate law and the platinum marker was observed at the phase boundary between iron and telluride scale. The telluride scale consisted of an inner and an outer layers; the former was β-iron telluride grown in parallel with a- and b-axes, and the latter consisted of δ-iron telluride above 980 K and δ- and δ′ -iron tellurides below 980 K. By comparing the activation energy for the reaction between iron and tellurium at constant tellurium vapor pressure with those for the diffusion of iron in β δ- and δ′-iron tellurides at constant composition, it was found that the rate-determining step of the reaction between iron and tellurium below 980 K was the diffusion of iron in δ′-iron telluride, but that above 980 K could not be determined. The reaction rate constant at 923 K and the tellurium vapor pressure of 350 Pa obtained in this study was in fairly good agreement with that calculated by Wagner's and Yurek's theories using diffusion coefficient of iron in iron telluride measured by the present authors.