Non-uniform corrosion of UNS N10003 alloy induced by trace SO4 2- in molten FLiNaK salt

Highlights: • A unique non-uniform corrosion induced by trace SO₄^2- was observed for UNS N10003 alloy exposed to molten FLiNaK salt. • Multi-methods were used to characterize the changes in microstructure and composition of the alloy induced by corrosion. • SO₄^2- caused both surface depletion of Cr and Fe, and internal sulfidation of Cr and Mo in the alloy. An un-expected non-uniform corrosion was observed for the UNS N10003 alloy immersed in static molten FLiNaK salt at 650 °C for 400 h. Results show that corrosion nodules with pores, embedded particles and boundaries inside form at some surface areas of the alloy, whereas other surface areas remain flat. Meanwhile, fine Cr₃S₄ precipitates form at grain boundaries and carbide-matrix phase boundaries beneath the surface. The non-uniform corrosion is mainly induced by the trace amount of SO₄^2- in the salt, which causes the surface depletion of Cr and Fe, and internal sulfidation of Cr and Mo in the alloy.