Diffusion Behavior of D₂O in the Film on Fe Oxidized at High Temperature in Air

Abstract

Diffusion coefficients of D₂O in the films on Fe oxidized at high temperature in air have been determined in order to estimate corrosion rate of carbon steel for the overpack in ground water with quite less dissolved oxygen. Fe plate specimens were heated at 573, 723 and 873 K in air to form oxide films on them. The films on the specimens were characterized with X-ray diffraction method and scanning electron microscopy. After that, the specimen was contacted with D₂O for several periods to 5184 ks, followed by being subjected to thermal desorption gas spectroscopy to obtain an amount of D₂O absorbing into the film. As a result, single-layered film of Fe₃O₄ was formed at 573 and 723 K, and double-layered film of Fe₂O₃ and Fe₃O₄ was formed at 873 K. It was found from the D₂O absorption test that an amount of D₂O in the film correlated linearly with a square root of the absorption period, and that the amount was steady for a long period. From the experimental evidence and Fick’s second law, apparent diffusion coefficients of D₂O were estimated as 9.5×10^－13 cm^2.s^－1 for the Fe₃O₄ film, and 5.4×10^－13 cm^2.s^－1 to 2.1×10^－12 cm^2.s^－1 for Fe₂O₃ film.