High temperature oxidation of Hastelloy X, to be used as in-pile tubing material in a high-temperature helium gas loop (OGL-1), has been tested in high-velocity imprified helium gas at 800°, 900° and 1, 000°C. It was observed that, under these conditions, the outer layer of the oxide film covering the metal surface tended to be blown away, and that both ends of the specimen oxidized to appreciable extent. The oxide film was found upon examination to be constituted of three layers -(a) a boundary zone between base metal and the oxide film proper, (b) an inner layer consisting of Cr₂O₃, and (c) an outer layer mainly comprising spinel oxides of Cr and Mn. It was also revealed that, under the same experimental conditions, the oxidation penetration rate obeyed the cubic rate law at 900°C and the parabolic rate law at 1, 000°C. This difference of behavior according to temperature could be ascribed to a change occurring in the type of oxygen diffusion in the alloy phase.

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