Effect of high temperature molten salt corrosion on the microstructure of a Co-Mo-Cr-Si wear resistant alloy

Highlights: • A novel low Cr content wear-resistant alloy was fabricated and its corrosion behavior in molten fluoride salt was tested. • Complex changes in microstructure induced by molten salt corrosion were characterized by XRD, SEM, TEM, EDS and EPMA methods. • The non-uniform corrosion of the alloy was mainly induced by the trace amount of water presented in the salt. The corrosion behavior of a novel low Cr content Laves phase strengthened wear-resistant alloy (Co-27.3Mo-2.9Cr-3.1Si) in molten FLiNaK salt (LiF–NaF–KF: 46.5–11.5–42 mol.%) was investigated by static immersion test at 700 °C. The microstructure and phase composition of the alloy after corrosion were characterized by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and transmission electron microscopy with energy dispersive spectrometry (TEM/EDS). Results show that the alloy surface exhibit either pitted or roughened morphology after corrosion. The preferential corrosion of the Laves phase contributes to the pitting, whereas the formation of Co₃Mo particles and Co layers contributes to the roughening. Carburization occurs in both the Laves phase and alloy matrix in the corrosion affected zone, forming Co₃Mo₃C carbides of different sizes. In addition, the thermal exposure effect during corrosion causes phase transformation in the entire alloy matrix and form Co₃Mo stripes. The observed non-uniform corrosion is mainly induced by the trace amount of H₂O in the salt, and it suggests that the corrosion resistance of the alloy is not satisfying in the test condition.