The Corrosion Behavior of Al/Al2O3 Composite Films with Ultra-Dense Structure Exposed to Lead-Bismuth Eutectic at 450 to 650 °C

Al2O3 coatings are the most promising candidate material for mitigating (lead-bismuth eutectic) LBE corrosion at elevated temperatures, but preventing inward diffusion of Pb, Bi, and O for the ceramic coating remains a critical challenge. Here, we have fabricated an amorphous Al2O3 coating with an ultra-dense structure by continuous high-power magnetron sputtering (C-HPMS). After LBE corrosion at 550 °C for 2000 h, nanocavities induced by the phase transformation from amorphous to γ-Al2O3 provide the diffusion path for Fe, O, Pb, and Bi in which the corrosion products, such as Fe3O4, PbO2, or their mixed oxides, form. Furthermore, the diffusion of Pb to the substrate and Cr segregation at the interface between the coating and substrate are observed for the sample exposed to LBE at 550 °C for 4000 h. Additionally, the hardness and interface bonding strength are enhanced after LBE corrosion. Moreover, pit corrosion was found to be the main failure mode of coating, and pits that merged with each other induced large area failure at a temperature of 650 °C. The corrosion mechanism of Al2O3 includes element diffusion, phase transformation, and chemical reaction. This work not only provides a deep understanding of the corrosion mechanism of amorphous Al2O3 coatings, but also shows the optimization method on the corrosion resistance of Al2O3 coating.