High-temperature corrosion of nanocrystalline Ni with varying grain sizes in FLiNaK salt and corrosion-induced surface faceting

Nanocrystalline nickel (NCNi) coatings are currently under investigation to enhance the corrosion resistance of structural components in molten salt reactors (MSRs). The corrosion behavior of NCNi with varying grain sizes of 20 nm, 100 nm and 200 nm in a FLiNaK environment is studied. Interestingly, as the grain size increases, there is a systematic increase in weight loss. Studies conducted using electron backscattered diffraction (EBSD) on corrosion-tested and annealed samples revealed the significant role played by various microstructural attributes in enhancing corrosion resistance. These attributes include 〈100〉 crystallographic orientation, a high fraction of twin boundaries, low random boundary connectivity, high J₂/(1-J₃) value and low grain boundary length per area. The NCNi with the 20 nm grain size showed exceptional corrosion resistance due to a higher prevalence of these attributes. Furthermore, corrosion-induced facets, such as pyramids, terraces, steps, etc., are observed for the first time in FLiNaK solution, with grain interiors and boundaries exhibiting various morphologies. The impurities from the salt result in surface anisotropy leading to surface reconstruction and formation of facets. The study presents new findings that are significant for the MSR community: in systems where the protective oxide layer dissolves readily and lacks sustainability in the corroding medium, all the above-mentioned microstructural attributes are vital. Their predominance leads to enhanced corrosion resistance.