Effect of Thermomechanical Treatment of Cast Nickel Alloy on Corrosion Resistance in Molten FLiBe

Abstract The molten fluoride salts used in Molten Salt Reactors (MSRs) are known for being a challenging environment for construction materials. Nickel alloys are usually used in this application for their corrosion and temperature resistance. The goal of this work was to test MoNiCr alloy in a molten mixture of lithium fluoride and beryllium fluoride (FLiBe) and to evaluate the effect of laser shock peening (LSP) and hot isostatic pressing (HIP) methods of thermomechanical treatment on its corrosion resistance. The material was first processed by LSP, and then a HIP heat treatment followed. The aim is to create a modified surface layer through a combination of these processes. The corrosion test itself was performed by immersion of specimens in molten FLiBe at 700 °C for 1000 h. The results showed that both LSP and HIP have observable influence on corrosion of MoNiCr alloy. Although the depth of corrosion damage did not differ greatly, the mechanism varied considerably. Regardless of the treatment method, all samples showed chromium depletion to a depth of approximately 30 μm. A zone of finer grains was formed in the same area. Material processed with LSP alone was more susceptible to intergranular corrosion, while the combination of LSP and HIP suppressed it. The number of passes and type of ablation layer in LSP process have a big role. The best results were achieved with LSP processing with a higher number of repeated passes (10) and without ablation polymer tape. In combination with HIP, intergranular corrosion was completely suppressed.