Corrosion behavior of a high-carbon austenitic stainless steel with ultrafine Cr23C6 precipitates in molten FLiNaK salt

The influence of having a high fraction of ultrafine, intrinsically corrosion resistant Cr ₂₃C ₆ type carbide precipitates in an AISI 316 L austenitic stainless steel matrix on its corrosion behavior in molten FLiNaK (LiF-NaF-KF, 46.5–11.5–42 mol%) salt at 750°C has been investigated. The alloy was fabricated by mechanically alloying gas atomized 316 L stainless steel powders with 0.8 wt% graphite followed by consolidation using spark plasma sintering. The sintered alloy had an ultrafine grain size (200–500 nm), with a uniform distribution of Cr ₂₃C ₆ precipitates (< 500 nm). Despite the high intrinsic stability of Cr ₂₃C ₆ in the molten fluoride salt environment, it was found that the alloy experienced more corrosion in comparison to a reference 316 H stainless steel. The large interfacial and grain boundary area fraction in the alloy was found to offer a pathway for rapid outward diffusion of Cr, and formation of voids in the material. It was found that corrosion-induced shifts in thermodynamic equilibrium and associated compositional redistribution in the sub-surface regions of the alloy force the Cr ₂₃C ₆ precipitates to lose Cr to the matrix despite their intrinsic thermodynamic stability in the salt environment, while Mo partitions to the carbide phase. In other words, the otherwise stable Cr ₂₃C ₆ phases themselves serve as source of Cr, which then dissolves into the matrix via the alloy matrix.