Volume 8, Issue 9 (October 2011)
Understanding Crack Formation at the Metal/Oxide Interface During Corrosion of Zircaloy-4 Using a Simple Mechanical Model
It has been established in previous works that corrosion kinetics in primary water of various zirconium alloys are periodic. Each period is associated with a layer of cracks parallel to the metal-oxide interface. These observations have been made either in autoclave or in pile. This indicates that corrosion processes in autoclave and under irradiation are of similar nature though their absolute kinetics might be different. Taking advantage of this correlation between cracks and corrosion kinetics, the present work aims at identifying the main microstructural parameters controlling cracks appearance in the oxide layer under well-controlled conditions. In order to achieve this, Zircaloy-4 was heat-treated to obtain various metallurgical states (stress-relieved versus recrystallised with different grain sizes) followed by corrosion tests in primary water. The key metallurgical parameters for the various conditions have been analysed (texture, precipitate sizes and grain sizes and distributions) using electron microscopy and synchrotron X-ray diffraction techniques. Corrosion kinetics of the various Zircaloy-4 microstructures are distinct as expected from the literature. Crack morphology in the oxide layer has been analysed and quantified using a dual beam scanning electron microscope/focused ion beam. Crack layers are evident even at small scale of observation. Three dimensional (3D) images of the oxide structure are presented. Cracks observed in this way are typically penny-shaped with a radius of about 100 nm. Near the metal-oxide interface, they are mainly found at the top of metal protrusions in the oxide. The roughness of the metal-oxide interface was measured. It does not exhibit any periodicity. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. A model of the oxide breaking at the point of transition has been developed. It is based on mechanical considerations and the existence of compressive stress in the oxide layer.