The microstructures of the oxide layers formed on near (0001) plane of Zircaloy-4 were investigated by autoclave tests at 360°C in lithiated aqueous solution. Oxygen-rich regions with hcp structure were observed at the undulating O/M interface, and the inner surface morphology of the oxide layers formed on (0001) was only concaveconvex. Monoclinic, tetragonal and cubic phases and a kind of zirconium sub-oxide with bcc structure were detected in the oxide layer near the metal matrix. This zirconium sub-oxide layer had a coherent relationship with a-Zr matrix, and the growth direction of the zirconium sub-oxide layer was nearly parallel to the [0001] direction of a-Zr regardless of the orientation of metal matrix. The orientations scattering of columnar grains formed on near (0001) plane differ from that formed on near (10 " 10) plane.
The effect of surface state on the nodular corrosion resistance of Zircaloy-4 alloy was investigated in superheated steam at 500°C/10.3 MPa by autoclave tests. The microstructures of oxide films on the corroded specimens were observed by TEM and SEM. The results indicate that surface strained layer delays the appearance of nodular spots on the specimen surfaces and improves the nodular corrosion resistance. The columnar grains orientation of the oxide films formed on the specimens with surface strained layer was more consistent than that on the specimens without surface strained layer when a comparison was made on the same orientation of the grain surfaces. Such a kind of oxide microstructure formed on the specimens with surface strained layer can hinder the diffusion of oxygen ions along the grain boundaries and delay the growth of oxide films, therefore retard the formation process of nodular spots. This indicates that the microstructure of the initial oxide films has an important influence on the subsequent growth of the oxide films.
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