Corrosion behavior of a wear resistant Co-Mo-Cr-Si alloy in molten fluoride salts

Abstract

Evaluating the corrosion performances of wear resistant alloys is important for designing the molten salt-wetted bearings and valves in molten salt reactors. In this study, the corrosion behavior of wear-resistant Tribaloy T-400 alloy (Co-29Mo-8.5Cr-2.8Si) in molten FLiNaK salt (LiF–NaF–KF: 46.5–11.5–42 mol.%), with and without the presence of UNS N10003 alloy, were investigated through static immersion tests at 650 °C. After 400 h corrosion, T-400 alloy showed greater weight loss than UNS N10003 alloy, and the presence of UNS N10003 alloy further increased the weight loss of T-400 alloy. Electron microscopic and salt chemistry analyses showed that Cr and Co were selectively dissolved from T-400 alloy matrix after corrosion, resulting in a porous structure. On the other hand, the Laves phase precipitates were hardly corroded but showed signs of carbide formation on the surface. In addition, when tested with UNS N10003 alloy, some of the Co dissolved from T-400 alloy transferred to the surface of UNS N10003 alloy, hence promoted the corrosion of T-400 alloy. The results have indicated that even T-400 alloy has a relatively low Cr content among high temperature wear-resistant hardfacing alloys, its corrosion resistance is still inadequate in a typical molten salt reactor environment where UNS N10003 alloy and graphite are also present.