Research on the existence and stability of interfacial tetragonal zirconia formed on zirconium alloys

Abstract

Corrosion of zirconium claddings is one of the key properties that must be considered when designing new zirconium alloys. Tetragonal phase is formed on the zirconium matrix during high temperature corrosion and is mainly stabilized near the metal/oxide interface. This phase has a tremendous influence on the corrosion behavior of zirconium alloys. T-ZrO2 phase formed on Zr-0.5Sn-0.15Nb-0.5Fe-0.2 V (N2) and Zr-1.0Sn-1.0Nb-0.3Fe (N36) claddings corroded in 773 K/10.3 MPa steam were studied. A Micro Raman spectrometer was used to analyze the existence and stability of oxide phase before and after dissolution of the zirconium matrix. The result indicates that interfacial t-ZrO2 is not transformed after the removal of the zirconium matrix. Even though most of the stress in m-ZrO2 is released, the lattice distortion in t-ZrO2 still exists, causing this phase to deviate from normal state and maintain its stability. We suggest that the possible reason for stabilizing t-ZrO2 is not the released macroscopic compressive stress, but the existence of small tetragonal grains or large amount of vacancies which can also introduce lattice distortions. The results highlight the important role of oxygen vacancies. Therefore, the stability of t-ZrO2 after matrix dissolution provides us with a chance to observe the microscopic evolution of tetragonal phase at the inner face. T-ZrO2 grows with increasing corrosion time and its intensity drops down after transition. Its microscopic distribution seems to be related to oxide particles and interfacial waviness.