The corrosion of Zr(Fe, Cr)₂ and Zr₂Fe secondary phase particles in Zircaloy-4 under 350 °C pressurised water conditions

Highlights: •The oxidation of Secondary Phase Particles (SPPs) in Zircaloy-4 is elucidated. •Upon oxidation Zr2Fe SPPs transform to a nanocrystalline homogeneous mixed oxide. •Zr(Fe, Cr)2 Laves-phase SPPs oxidise into a Zr-Cr oxide and metallic Fe. •Differences in expansion of SPPs on oxidation compared to the Zr cracks the scale. •The cracking may affect oxygen supply through the scale and its growth rate. -- Abstract: Using Scanning Transmission Electron Microscopy (STEM) coupled with Dual Electron Energy Loss Spectroscopy (DualEELS) and scanned diffraction, the corrosion and incorporation of Secondary Phase Particles (SPPs) in the oxide layer of Zircaloy-4 material has been investigated. This study focuses on mapping the corrosion of Zr₂Fe and Zr(Fe, Cr)₂ precipitates during the oxidation process and depicting their morphology as the oxidation front advances through the material. It has been found that Zr₂Fe SPPs retain the same general shape as in their pre oxidation stage, and transform to a nanocrystalline homogeneous mixed oxide, with a strong crystallographic texture, but hitherto unknown structure. The Zr(Fe, Cr)₂ Laves-phase SPPs however, oxidise in a notably more complicated manner. As the α-Zr around an SPP begins to oxidise, the SPP is completely encapsulated by the ZrO₂ whilst much of the SPP remains initially unoxidised. But, on oxidation, significant elemental segregation takes place, usually leaving a Cr₂O₃-rich cap, a nanocrystalline Zr,Cr mixed oxide body and veins of well-crystallised metallic iron. Both forms of SPP have a different expansion on oxidation compared to the Zr, resulting in cracking of the ZrO₂.