Iron Content Effect on Oxidation Kinetics and Oxide Phase Stability of Zr-Nb and Zr-Sn Alloys

This study investigates the influence of iron (Fe) content on the oxidation kinetics and oxide layer microstructural evolution of zirconium alloys under autoclave conditions. Four alloys were evaluated: three Zr-Nb alloys (300, 400, and 600 ppm Fe) and one Zr-Sn alloy (1100 ppm Fe). The experimental approach integrated ICP-OES, oxidation kinetics modeling, CALPHAD thermodynamic simulations up to 360 °C, x-ray diffraction, Raman spectroscopy, and SEM-EDS. Oxidation parameters were selected to produce stable, adherent oxide scales, enabling high-resolution characterization of the pre-transition regime. ICP-OES confirmed nominal compositions, allowing Fe to be treated as the primary variable within the Zr-Nb system. XRD with Rietveld refinement and Raman analysis revealed microstructural heterogeneity, with Fe- and Nb-enriched oxides concentrated in localized dark regions, associated with the oxidation of iron-bearing second-phase particles. Raman spectroscopy identified monoclinic ZrO2 as the dominant phase, with localized contributions of tetragonal ZrO2, niobium oxides (NbO2, Nb2O5), and iron oxides (Fe3O4, Fe2O3). Increasing Fe content from 300 to 600 ppm in Zr-Nb alloys led to an approximately 12% reduction in final weight gain, resulting in thinner and more homogeneous oxide layers. The Zr-Nb alloy with 600 ppm Fe exhibited the best performance within its system, while the Zr-Sn alloy formed the most compact and stable oxide barrier. These results demonstrate that increasing Fe content promotes oxide densification and phase stabilization during early-stage oxidation, thereby improving corrosion resistance.