Crystal structure, corrosion kinetics of new zirconium alloys and residual stress analysis of oxide films

Corrosion kinetics of NZ2 alloy were investigated after autoclave treatments in 360 °C/18.6 MPa lithiated water and 400 °C/10.3 MPa steam. The crystal structure and the residual stress of oxide films of NZ2 alloys after corroded in both conditions were investigated by XRD method. The kinetics analysis indicates that the resistance of NZ2 alloy treated in 360 °C lithiated water is higher than that treated in 400 °C steam. The crystal structure analysis shows that the content of tetragonal t-ZrO2 in the oxide films decreases smoothly and the content of monoclinic m-ZrO2 increases with the duration of corrosion time, independent of the kinetics transition. Stress measurements show that high compressive stresses were developed in the oxide layers. Furthermore, the transitions of kinetics can be associated with the sudden decrease of macroscopic compressive stresses in the oxide films. The higher t-ZrO2 content is, the higher compressive stress in the oxide film is, the lower is the corrosion rate. Therefore it is considered that t-ZrO2 is mainly stabilized by the macroscopic compressive stresses in the oxide films. In addition, local stresses in the oxide films, grain size and the oxygen vacancies play an important role in the t-ZrO2 stabilization.