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

Highlights: • Ni-based alloys exposed to molten FLiNaK salt under air atmosphere develop patches of mixed oxides at surface. • Alloy 690 and Alloy 693 developed mixed oxide layers of the types NiO +Cr₂O₃ and NiO + Al₂O₃+Cr₂O₃, respectively. • These oxides formed galvanic micro-couples with exposed alloy surface leading to increase in Cr depletion from the alloy. • Ni-17 wt.% Mo-7wt.% Cr alloy formed MoO₃+NiO + Cr₂O₃ type mixed oxide layer. • A thin MoO₃ layer present on the exposed alloy surface reduces the effect of galvanic coupling thereby lowering Cr depletion. - 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.