Mechanistic understanding of intergranular oxidation and passivation in Si/Al-doped Fe–Cr ODS steels exposed to liquid Pb via advanced characterization

This paper aims to investigate the oxidation mechanisms of Al/Si-doped Fe-Cr ODS steels exposed to liquid lead containing 10⁻⁶ wt% oxygen at 600 °C. To achieve this, the oxide scales of 12Cr1.5Si2Al0.5Zr-ODS and 9Cr0.5Si4.5Al0.6Zr-ODS steels after 5000 h of exposure were systematically characterized using a state-of-the-art characterization approach that combines synchrotron transmission micro-X-ray diffraction (TμXRD) and site-specific transmission electron microscopy (TEM). In 12Cr1.5Si2Al0.5Zr-ODS steel, the oxide scale consisted of a thin, multilayered structure, accompanied by pronounced intergranular oxidation (∼30 μm). This severe oxidation was attributed to the formation of discontinuous amorphous Al₂O₃ and SiO₂ layers, which were insufficient to block inward oxygen diffusion. Additionally, nano-cavities within the intergranular amorphous SiO₂ layer acted as rapid diffusion pathways, further accelerating oxygen ingress and exacerbating intergranular attack. In contrast, 9Cr0.5Si4.5Al0.6Zr-ODS steel developed a similar thin, multilayered structure, but it included a continuous, low-crystallinity Al₂O₃ layer that acted as an effective diffusion barrier, resulting in stable passivation. Moreover, the formation mechanisms of these complex oxide scales were further interpreted through oxidation thermodynamics and the internal-to-external oxidation transition theory.