Raman investigation of pre- and post-breakaway oxide scales formed on Zircaloy-4 and M5® in air at high temperature

For nuclear facility safety analysis purposes, the Zircaloy-4 and M5® air oxidation scales produced in the 800–1000°C temperature range have been investigated with micro-Raman spectroscopy. The oxides were characterized to determine the microstructures associated with protective and non-protective behaviors. Systematic scans, from the oxide/metal interface to the scale surface, were performed to study the variations in crystal structure and composition as a function of position in the oxide. The results presented here tend to indicate that the crystallographic phases that are present in the scales are strongly dependent on alloy composition. In particular, looking at scales grown at 800°C, the crystallographic phases observed for the M5® alloy, t-ZrO2 and m-ZrO2, are similar in many respects to those already identified in scales grown in pure oxygen or steam. This is not the case for Zircaloy-4, since we clearly observed additional Raman signatures which most probably track the presence of nitrogen in the layers well before the occurrence of the kinetic transition. Indeed, the Raman measurements first show the presence of cubic zirconia in the layers, and strongly suggest the presence of zirconium nitride and oxynitride. Increasing the oxidation temperature, zirconium nitride is clearly detected in the scales well before the occurrence of the kinetic transition. Thus, any mechanism proposed to explain this oxidation behavior must consider the fact that both nitrogen and oxygen participate in the scaling reaction.