Depth profile of oxide volume fractions of Zircaloy-2 in high-temperature steam: An in-situ synchrotron radiation study

To study the steam oxidation behavior of Zircaloy-2, a high-energy synchrotron X-ray diffraction technique was applied to perform an in-situ oxidation measurement. The depth profiles of oxide volume fractions were obtained at both 600 and 800 °C. Multiple layers, including ZrO2 scale, (α + β) Zr matrix with ZrO2 incursions, and (α + β) Zr matrix, were mapped according to the volume fraction of each phase. The volume fractions of these phases were observed to change gradually with different distances to the surface, without a sharp edge distinguishing each of the layers. The ZrO2 consisted of tetragonal and monoclinic crystal structures, which were observed to coexist with different ratios of volume fractions in depth. The higher amount of tetragonal ZrO2 observed in the very inner region of the oxidizing Zircaloy sample indicates that the tetragonal crystal structure is the ab initio phase type, in which new oxide molecules form at the metal-oxide interface.