Compatibility of Ni-Mo-Cr alloys with fluoride salt at elevated temperature

Ni-Mo-Cr based alloys have emerged as a potential candidate for applications in the molten salt breeder reactors, a Generation IV reactor concept aimed at providing safer, long lasting, proliferation-resistant and economically viable nuclear energy. In present study, the Ni-Mo-Cr alloys, with Mo/Cr ratios 2.28 and 0.4 respectively were exposed to 46.5 mol. % LiF–11.5 mol. % NaF–42 mol. % KF (FLiNaK salt) at 973 K for 72 h in a controlled environment and the resultant microstructural modifications were investigated. It is observed that the alloy with higher Cr content underwent higher mass loss and dissolution in the salt attributed to higher Cr- concentration gradient in the alloy (from alloy interior to surface). In addition, direct evidence of Li ingress into the alloy has also been observed for the first time using secondary ion mass spectroscopic (SIMS) depth profile measurements. Moreover, the Ni-Mo-Cr alloy with Mo/Cr ratio 2.28 was exposed to FLiNaK salt at 973 K for 3 h at ambient atmosphere to investigate the effect of oxygen on the corrosion mechanism. In presence of oxygen, the alloy develops oxide layers that remain partially protective as they form galvanic micro-couples with the exposed surface making the exposed alloy surface more prone to attack. This results in a higher depth of Cr depletion in the alloy