Quantitative Measurement of Displacement and Strain Fields in the ZrO2 Layer during the Transition in Oxidation Kinetics

A number of excellent properties, such as high resistance to corrosion by water, small cross-section for capture of thermal neutrons, adequate strength, and formability are required for Zr based alloys to be used as fuel cladding in light water reactors (LWRs). Among those properties, corrosion behavior of Zr based fuel cladding is a key concern since it could greatly affect both the operating performance during normal operation and the safety margins under transient or accident conditions. Therefore, corrosion tests under various conditions should be performed to predict the long term corrosion behavior and improve understanding of corrosion mechanism of Zr based alloys. In this respect, corrosion behavior of Zr based alloy has been extensively studied. Interestingly, despite various test conditions in different research groups, transition in oxidation kinetics was a common characteristic of all Zr base alloys, regardless of the oxidizing condition, temperature, and their composition. The transition can be explained as an abrupt change in the oxidation kinetics of Zr based alloys from an approximately parabolic to a linear rate law. The representative phenomena observed after transition are nodular and breakaway oxidation. Nodular oxides having a lenticular shape are formed at local region on the surface of uniform ZrO2 layer while whitish oxides are eventually developed in the entire surface in case of breakaway oxidation. These two phenomena have different appearance and reasons for their occurrence but they exhibit very similar post-transition characteristics in several points. Once the nodular or breakaway oxidation occurred after the transition point, several changes are observed in the oxide layer on the surface of the Zr based alloy metals. The superficial layer of Zr oxide changes color from the black to the white. In both cases, Zr oxide layer formed after transition lost its protectiveness. Oxygen deficient and non-stoichiometric ZrO2-x is the main species in initial black uniform oxide, while stoichiometric ZrO2 is the main oxide species in the whitish oxide. Another noticeable change during transition to breakaway is crystalline structure of oxide layer. The proportion of oxide with metastable tetragonal phase decreases, and most of tetragonal phase is replaced by the monoclinic form in the end. Recrystallized microstructure in oxide layer, micropores, and cracks can be also observed in the oxide layer formed after transition point. Even though several mechanisms with different explanation for the occurrence of nodular corrosion or breakaway oxidation have been proposed in a number of literatures, controversies remain, and it is still unclear how these abnormal corrosion behaviors occur. Mechanical failure of a protective oxide layer under the compressive stresses generated during oxidation is one of the reliable causes for initiation of transition in oxidation kinetics among a number of plausible theories. In this report, microstructural characterization and mechanism of nodular oxide were investigated by using high resolution transmission electron microscopy (HRTEM) and strain in the oxide layer was analyzed by geometric phase analysis (GPA) method