The influence of size distribution of intermetallic precipitates on hydrogen uptake behavior of Zircaloy during oxidation in steam has been examined in the pre-transition region to clarify the role of precipitates in the hydrogen transport through the coherent oxide layer. Two different types of specimens containing coarse or fine precipitates are prepared with heat treatments for both Zircaloy-2 and -4. These specimens are oxidized in 0.1-MPa steam at 623 to 723 K. The amount of hydrogen uptake is measured with a hot extraction method. The concentration profiles of hydrogen in the oxide film is measured with secondary ion mass spectroscopy (SIMS). Detailed spatial distribution of hydrogen is observed with tritium microautoradiography.

The results obtained for Zircaloy-2 and -4 are qualitatively similar to each other: The amount of hydrogen uptake of the specimen containing coarse precipitates is larger than that of the specimen containing fine ones, and the difference between the former and the latter increases with decreasing oxidation temperature. The concentration of hydrogen in the oxide film is gradient from the surface to the oxide-metal interface, and the gradient in the oxide film of the former is smaller than that of the latter. The tritium microautoradiographs of a cross section of the oxide film show that hydrogen is concentrated in the intermetallic precipitates.

Summarizing these experimental results, the following can be concluded: 1. The rate of hydrogen uptake is controlled by the transport process through the oxide film. 2. The intermetallic precipitates accommodated in the oxide film play an important role. 3. Hydrogen is transported through not only the zirconium oxide phase but also the intermetallic precipitate phase, in which the latter acts as the supplementary expressway of the hydrogen transport through the oxide film.