Breakaway oxidation of Zr-1Nb-1Sn-0.3Fe alloy in PWR primary water: Interfacial geometric thresholds and defect percolation

In this study, the oxidation behavior and degradation mechanisms of a Zr-1Nb-1Sn-0.3Fe alloy in simulated pressurized water reactor (PWR) primary water at 360℃ were systematically investigated. The macroscopic kinetics exhibited a three-stage progression: a protective parabolic regime featuring an extended plateau, a kinetic transition at ~2500 h with a critical oxide thickness of ~5.1 µm, and a subsequent linear breakaway regime. To elucidate the physical origin of this transition, a quantitative geometric probability model was established, directly correlating metal-oxide interfacial undulations with scale failure. Statistical evaluations reveal that once the local interfacial aspect ratio (amplitude-to-wavelength, A/L ) exceeds a critical threshold of ~0.3, the probability of convex crack initiation at the undulation crests abruptly surpasses 50%. Statistical results demonstrate that the macroscopic transition is triggered when the global fraction of cracked undulations breaches a 2/5 threshold; the intensification of these undulations at 1900h—prior to macroscopic kinetic deviation—confirms this cracking as a key precursor to breakaway oxidation. Furthermore, nanoscale characterizations demonstrate that these undulation-induced cracks percolate with various pre-existing isolated defects within the bulk oxide, encompassing both intrinsic lateral cracks and secondary phase particles induced crescent microcracks. The coalescence of these localized flaws into an interconnected short-circuit network dissipates the protective interfacial compressive stress. Ultimately, this stress relaxation triggers a widespread t-ZrO 2 to m-ZrO 2 martensitic transformation, finalizing the structural and crystallographic degradation of the macroscopic transport barrier.