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The structure of the metal-oxide interface and its effect on the stability of the corrosion layers, hydrogen uptake, and the global oxidation kinetics are studied using a systematic multi-scale characterization of the oxide layers formed on autoclaved Zircaloy-4 samples. The multi-scale approach made use of complementary tools: metallography on cross sections, SEM coupled to image-processing analysis, TEM with EDX analysis, and ionic imaging by SIMS. In addition to these local characterizations, impedance measurements were used to give global information about the oxide protective characteristics. SEM observations show a wavy interface and the existence of lateral cracks distributed throughout the oxide layer. Image-processing analysis of 15 randomly chosen images of each sample allows a correlation to be established between the roughness of the interface and the formation of cracks. On the metal-oxide interface, a band-like region composed of grains having a ω-Zr hexagonal structure and containing large amounts of oxygen is occasionally seen by TEM and EDX, also indicating oxygen flux variations along the interface. Ionic imaging shows a correlation between the localization sites of hydrogen in the oxide and the presence of intermetallic precipitates, which is also linked with the detection of zirconium hydrides in the metal underneath. This is understood as an indication of preferred routes for hydrogen ingress through the metal-oxide interface. This multi-scale approach highlights some differences: the oxide appears to be composed of two distinct sublayers having different properties in terms of conductivity and transport, and the oxygen and hydrogen fluxes do not seem homogeneous through the inner protective layer and across the metal-oxide interface. The link between these different results and the global oxidation kinetics is discussed.

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