Effect of salt purity on the corrosion of 316 L SS: Long-term studies in molten FLiNaK and ThF4 - LiF

• Salt purity strongly governs 316L stainless steel corrosion in molten fluoride salts. • Purified FLiNaK and LiThF enable long-term stability of 316L SS at 600-700°C. • Impurities accelerate Cr and Fe depletion through intergranular corrosion mechanisms. • Chromium carbide formation in purified FLiNaK may act as a protective diffusion barrier. • Results demonstrate purification as key to material compatibility in molten salt reactors. This study investigated the influence of impurities on the corrosion behaviour of 316L stainless steel in molten FLiNaK at 600°C and LiThF at 700°C for up to 3000 hours. Coupons exposed to untreated FLiNaK containing moisture and oxides exhibited significantly higher mass loss compared to those exposed to purified salt. This coincides with the depletion of chromium (Cr) and iron (Fe) from the steel, as confirmed by ICP-OES analysis of post-experiment salts. SEM analysis identified intergranular corrosion as the primary attack mode in untreated FLiNaK, with corrosion depths up to 112 μm. Conversely, coupons exposed to purified FLiNaK displayed excellent corrosion resistance, despite trace levels of metallic impurities detected in the salt. These impurities are believed to originate from the purification process itself and may contribute to a small degree of observed corrosion. The typical mode of grain boundary Cr dissolution was observed in coupons tested in untreated FLiNaK salts, while this was absent in coupons tested in purified FLiNaK salts. Interestingly, XRD analysis identified chromium carbide phases on coupons from purified FLiNaK tests. Similar trends were observed in LiThF salt, where untreated salt resulted in severe corrosion compared to purified salt. Overall, this study highlights the critical role of salt purity in minimizing corrosion of 316L stainless steel in molten fluoride salts. The data from the present study indicates that a purified molten FLiNaK at 600°C can be a long-term non-corrosive environment for 316L stainless steel.