The effect of microstructure on the corrosion resistance of VDM® alloy C-4 in molten salts

High-temperature corrosion of VDM® Alloy C-4 was investigated in 3LiCl–2KCl and acidic KCl–AlCl3 melts in a wide temperature range (450–750 °C). The corrosion resistance of the alloy in molten salts depends on the oxidizing properties of the electrolyte and temperature. Holding VDM® Alloy C-4 above 650 °C may lead to the formation of secondary phases at the grain boundaries inducing undesirable intergranular corrosion. The secondary phase is represented by intermetallic sigma-phase; its chemical composition corresponds to the stoichiometric Ni(Mo,Cr)2. Time-temperature-precipitation and time-temperature-sensitization diagrams describing formation of sigma-phase in VDM ® Alloy C-4 were built to predict maximal working temperature and time of the material's application. The phenomenon of simultaneous decreasing ductility and increasing strength of the alloy caused by the long range order transformation was noticed after prolonged exposure of the material in the range from 550 to 650 °C. It was shown that unlike precipitation at the grain boundaries, formation of the long range ordered Ni2(Cr,Mo) phase did not increase the corrosion rate of VDM® Alloy C-4 in halide melts due to absence of micro galvanic pairs and growing of interatomic interaction force.