Microstructural characterization of oxide films formed on zirconium-based alloys was performed by use of scanning and transmission electron microscopy. Examination of pre-transition films formed on Zircaloy-4 oxidized in 360°C (680°F) water revealed a small grain size of approximately 100 Å (10 nm) diameter. In addition, a gradation of grain size was observed throughout the film thickness, such that at the oxide-water surface (oldest oxide), a grain size of less than 50 Å (5nm) was observed, and at the metal-oxide surface (newest oxide), the grain size was approximately 200 Å (20 nm). In post-transition films the outermost oxide still possessed the very fine 50 Å (5 nm) diameter grain size. However, the newest oxide of post-transition films consisted of relatively large grains, with grain diameters of 1000 to 5000 Å (100 to 500 nm). At the midthicknesses of these oxides intermediate grain sizes were observed. The bulk of the post-transition films was highly porous. Pore sizes ranged from approximately 10 to 150 Å (1 to 15 nm), and many connected pores were concentrated at the grain boundaries. Under more severe oxidizing conditions, imposed by increasing the corrosion temperature to 427°C (800°F), the growth of large grains at the metal-oxide interface was unstable and film growth proceeded by the nucleation of finer grains.

The results obtained conclusively establish the belief that the bulk of the post-transition oxide film is of nonprotective character as is obvious from the connected pore networks. A film growth model consistent with the experimental observations is that initial oxide formation is by nucleation at many sites. Growth of grains of certain orientations is favored, however, and these select grains expand in size at the expense of the less favorably oriented grains as the film thickens. Grain growth proceeds until a limiting size of 2000 to 5000 Å (200 to 500 nm) is reached. Further film growth induces nucleation of new grains at the metal-oxide interface. In thick scales, sequences of grain nucleation and growth are believed to occur.