A team at Stockholm's KTH Royal Institute of Technology reports that stainless steel corrosion in liquid lead begins with the formation of an ultrathin liquid lead film, as thin as about one micron, on the steel surface. This film drives rapid dissolution at the steel interface and helps explain the steep increase in degradation rates observed when steel is exposed to liquid lead.
The work, published in the journal Corrosion Science, addresses a central materials challenge for lead-cooled reactors. Liquid lead is strongly corrosive to conventional structural steels, so the development of more resistant steel grades and composite structures has become a key requirement for nuclear systems that rely on lead as a coolant.
The study examined an austenitic stainless steel alloy known as AISI 316L, which is widely used in water-cooled nuclear facilities because of its mechanical strength at high temperatures and its suitability for fabrication and welding of complex reactor components. This alloy contains significant amounts of nickel along with chromium and other elements, which give it the austenitic structure exploited in many current reactor designs.
The researchers found that, rather than forming stable surface layers, the austenitic structure destabilizes at the elemental scale when exposed to liquid lead. Earlier interpretations suggested that ferrite grows directly on the austenite and that liquid lead infiltrates later, but the new results indicate that the transformation begins within the austenitic matrix itself.
According to KTH researchers Kin Wing Wong and Peter Szakalos, nickel atoms, which have high solubility in liquid lead, diffuse out of the steel and dissolve into the coolant. The remaining iron and chromium then reorganize into a ferritic phase, but this newly formed ferrite is weak and porous, creating pathways that can fill with lead.
"Under flowing lead, these porous, lead-filled paths are easily torn away, dramatically accelerating material loss," Wong says. "This explains why liquid flowing lead strips away metal at such a high rate - sometimes several millimeters per year - faster than anyone thought."
These findings indicate that developing a single austenitic stainless steel that completely resists corrosion in liquid lead is unlikely to succeed. Because the lead does not simply remain on the surface but penetrates into the steel, removing chromium and nickel from within, the austenitic structure gradually breaks down regardless of detailed compositional adjustments.
Instead, the team proposes that layered or composite structures combining different types of steel offer a more realistic route for lead-cooled reactors. In such designs, each layer would provide a specific function, with one material optimized for corrosion resistance and others for mechanical strength and manufacturability.
Wong points to a class of alumina-forming ferritic steels (FeCrAl), developed at KTH by Szakalos, as a candidate for the corrosion-resistant layer. These steels form a thin, self-healing alumina (Al2O3) film that has shown robust corrosion resistance in liquid lead at temperatures up to 800 degrees Celsius, well above typical operating temperatures for lead-cooled reactors.
"When used together with conventional austenitic steels as layered materials, these materials could provide the long-lasting protection needed for tomorrow's lead-cooled reactors," Wong says.
Research Report:Mechanistic insight into the ferritization of austenite in Pb via a discontinuous reaction governed by a migrating liquid film
Related Links
KTH Royal Institute of Technology
Nuclear Power News - Nuclear Science, Nuclear Technology
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