A comparative study of surface layer formation in Ni-based alloys with varying Cr contents exposed to high temperature fluoride environment

Abstract

Microstructural and surface characterization of Ni-based alloys viz. Alloy 690, Alloy 693 and Ni-17 wt.% Mo- 7wt.% Cr exposed to molten FLiNaK salt under air atmosphere at 973 K was carried out. Patches of mixed oxide layers of type NiO + Cr2O3, NiO + Al2O3 +Cr2O3 and MoO3 + NiO + Cr2O3 were observed for Alloy 690, alloy 693 and Ni-Mo-Cr alloy, respectively. Corrosion susceptibility index and calculated Cr diffusivity in the alloys followed the order: Alloy 690 > Alloy 693>Ni-Cr-Mo alloy. The oxide layers developed are only partially protective as they form a galvanic micro-couple with exposed surface which makes the later more prone to attack. The lower Cr depletion in case of Ni-Mo-Cr alloy as compared to Alloy 690 and 693 can be attributed to the fact that MoO3 layer is formed over the areas devoid of oxide patches which may prevent/reduce galvanic coupling between Ni-,Cr-rich oxides and exposed surface.