Protective Al-rich oxide scale formation in low-Cr alumina-forming martensitic steels under liquid lead corrosion

Two alumina-forming martensitic (AFM) steels containing less than 8 wt.% Cr were exposed for over 1000 h at 550°C and 650°C in static liquid Pb with controlled oxygen concentration to evaluate the formation of an alumina oxide scale and its protective capacity against corrosion. The AFM-1 steel (3.6 wt.% Al, 7.8 wt.% Cr) formed a continuous, protective oxide scale that effectively resisted Pb penetration under all conditions, particularly at 650°C, where performance improved due to the dissolution of B2-NiAl precipitates during prolonged exposure, releasing Al that migrated to the surface and enabled the formation of an Al-rich oxide layer, ensuring sustained protection and mitigating the localized nodular oxidation observed at 550 °C. In contrast, the AFM-2 steel (2.9 wt.% Al, 7.5 wt.% Cr) failed to develop a complete protective oxide layer, allowing molten Pb to penetrate and react with the substrate even at 650 °C, causing severe oxidation. These results demonstrate that, beyond a high Ni content (12 wt.%), achieving excellent corrosion resistance in liquid Pb requires a synergistic combination of an optimal Al/Cr balance, dissolution of B2-NiAl precipitates as an Al source, and enhanced atomic diffusion facilitated by the high density of subgrain boundaries in the martensitic microstructure.