ODS FeCrAl alloys endure liquid metal flow at 600 °C resembling a fusion blanket environment

Fusion reactors, a promising source of sustainable energy, require advanced materials that can withstand extreme temperatures and corrosive environments created by liquid metal coolants such as lithium and lithium-lead (LiPb) alloy. These coolants are essential in fusion reactors to extract heat and breed tritium, but their corrosive nature threatens the integrity of the structural materials used. LiPb is particularly aggressive, as it has a high concentration of lithium, which reacts with structural materials, causing corrosion and material degradation over time.

ODS FeCrAl alloys, known for their excellent high-temperature strength and corrosion resistance, have been proposed as promising candidates for fusion reactors and other high-temperature applications like concentrated solar power systems. These alloys rely on the formation of protective oxide layers, such as α-Al₂O₃, which offers stability and durability under high temperatures. However, in a liquid LiPb environment, the chemical interactions between the alloy and the coolant raise concerns about the stability and longevity of these protective layers.

In this view, a team of researchers from the Institute of Science Tokyo (Science Tokyo), led by Associate Professor Masatoshi Kondo in collaboration with Yokohama National University, Nippon Nuclear Fuel Development and Department of Research, National Institute for Fusion Science, conducted corrosion tests on oxide layers formed on ODS FeCrAl alloys under prolonged exposure to flowing liquid LiPb at elevated temperatures. Their study was published in the journal Corrosion Science on September 17, 2024.

The researchers carried out corrosion tests using two types of ODS FeCrAl alloys: SP10 and NF12. The tests were performed under both static and stirred-flow conditions at 873 K to simulate realistic scenarios in fusion reactor coolant systems. They employed advanced metallurgical analysis techniques, including scanning transmission electron microscopy coupled with electron energy loss spectroscopy, to investigate the composition and microstructure of the protective oxide layers formed on the alloy surfaces.

They found that the pre-formed α-Al₂O₃ layer effectively suppressed initial corrosion but partially transformed into α-/γ-LiAlO₂ due to the adsorption of lithium. Interestingly, even without pre-oxidation, the ODS alloys in situ developed a durable γ-LiAlO₂ layer, which served as a self-forming protective barrier. Microstructural analysis using advanced electron microscopy revealed the penetration of lithium into the α-Al₂O₃ layer, leading to the chemical transformation. Despite this, both α-Al₂O₃ and γ-LiAlO₂ layers demonstrated strong resistance to exfoliation. Micro-scratch tests confirmed that these layers adhered strongly to the alloy surface, with minimal degradation, even under high thermal stresses caused by LiPb solidification.

"The lithium-aluminum oxide layer's durability shows that these alloys could last longer in high-temperature, high-stress settings. This layer serves as a sustainable shield that continues protecting reactor components even after initial wear," explains Kondo.

As nuclear technology evolves, these findings bring us one step closer to developing reactors that can run safely for extended duration, making sustainable energy sources more feasible. "Our findings show that ODS FeCrAl alloys, with their ability to form durable protective layers, could play a vital role in the future of fusion reactors and other high-temperature power systems," says Kondo, highlighting the impact of the research study.