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

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.