Microstructural basis of uniform corrosion in zirconium alloys

In this thesis research, oxide layers formed on Zr-based alloys in high temperature water and steam autoclave environments are characterized in an effort to understand the oxide growth mechanism. The study included oxides formed on Zircaloy-4, ZIRLO, Zr-2.5Nb and Zr-2.5Nb-0.5Cu alloys in 360°C pure water, 360°C lithiated water and 400--426°C steam autoclave environments, which is pressurized to 2200 psi. The oxides are characterized using microbeam x-ray diffraction, microbeam x-ray fluorescence, transmission electron microscopy, transmitted light optical microscopy and scanning electron microscopy.; The variation of the crystal structure and the chemical composition across the oxide layers are determined on the sub-micron (∼0.2 mum) scale by using the high flux micro-x-ray beam on the 2-IDD beamline at Advanced Photon Source at Argonne National Laboratory. This unique synchrotron radiation facility allowed us to examine the oxides in a more detailed manner than previously done as it combines high flux with spatial resolution to produce new and detailed information on oxide structure.; A marked periodicity was observed in various oxide features (cracks, diffraction intensity, white/black bands etc.) as a function of distance from the oxide/metal interface was observed by the several techniques used in this study. The observed microstructural periodicity was linked to the periodicity observed in the corrosion data.; The structure of the oxide/metal interface is different from that in the bulk of the oxide and does not change significantly as the oxide advances, indicating a restructuring of the oxide in the region behind the oxide front to maintain a self-similar structure at the oxide front as the oxide advances. A highly oriented tetragonal zirconia phase was observed at the oxide/metal interface region which is the precursor of the highly oriented monoclinic phase in the bulk of the oxide layer, which forms to minimize stresses. The measured tetragonal phase fraction higher near the oxide/metal interface region than in the bulk of the oxide. These tetragonal grains are oriented randomly and their size is smaller than the highly oriented tetragonal grains at the oxide/metal interface. The structure of the region in the metal near the oxide/metal interface (called the "suboxide" region) is different than the bulk of the oxide layer and the metal. A Zr3O suboxide phase with alpha-Zr and small equiaxed tetragonal ZrO2 grain was identified in this region and its region of existence corresponds well to a region where oxygen content is high as determined by transmission electron microscopy analysis. A comparison between the oxides formed in different alloys shows systematic differences: oxide that grows slowly exhibit well-oriented, and comparatively thicker columns of monoclinic oxide than the oxide that grows faster. Oxides formed in lithiated water are more porous than those formed in pure water.; These observations helped us to propose a mechanism for the oxide growth which can be made to understand the influence of alloy microstructure on corrosion rate.