Influence of Preoxidation on the Corrosion of Steels in Liquid Lead-Bismuth Eutectic Available to Purchase

Oxidation studies of martensitic/ferritic and austenitic steels have been conducted in static lead bismuth eutectic (LBE). Samples were preoxidized in an air/H₂O gas mixture prior to immersion in LBE. Preoxidation films grown on HT-9 at 800°C for 48 h had a bilayer structure, an outer Fe-rich layer, and an inner Cr-rich layer. Glancing angle x-ray diffraction data found that two distinct structures were present in this oxide: Fe_1+xCr_2-xO₄ spinel (cubic, face-centered cubic [fcc]) and (Cr,Fe)₂O₃ (rhombohedral, corundum). Magnetite formation (Fe₃O₄) was ruled out. Immersion in LBE resulted in the growth of an Fe-rich film on top of the preoxidation layer. It was concluded that the growth mechanism was Fe⁺⁺⁺ interstitial transport from the metal/oxide interface. Preoxidized films grown on Type 316 (UNS S31600) stainless steel (SS) at 800°C for 64 h resulted in a Cr-rich inner layer and Fe-rich outer layer. X-ray diffraction found both Fe_1+xCr_2-xO₄ spinel (cubic) and (Cr,Fe)₂O₃ corundum (rhombohedral). In contrast to HT-9, angle-resolved x-ray diffraction found that the concentration of the Fe_1+xCr_2-xO₄ spinel decreased with distance from the metal oxide interface. Immersion of preoxidized Type 316 SS in LBE resulted in oxide growth; however, the new film formed in LBE was indistinguishable from the preoxidation layer. It was proposed that the enhanced oxidation of Type 316 SS as compared to HT-9 formed a Fe_1+xCr_2-xO₄ spinel layer near the oxide/metal interface results in a boundary-to-cation interstitial transport.