Microstructural characterization of ZrO2 layers formed during the transition to breakaway oxidation

In order to characterize the microstructure of oxide layers formed on Zircaloy-4 tubes during the breakaway transition, oxidation tests in a flowing steam environment were performed at 1000 °C with a different oxidation time. It was found that breakaway oxidation occurred after the oxidation time of 3000 s, and zirconium dioxide layers existed in two mixed crystallographic forms of the tetragonal and monoclinic phase in all samples. The zirconium oxide layers showed enhanced crystallinity, increase in grain size, and fine pores at the grain boundary after breakaway oxidation. We found that the initiation of breakaway oxidation instability originated from these microstructural changes.