Corrosion Performance And Its Mechanism In Weld Joint Of Ni-base Alloy In Molten Fluorides Salt

The Molten Salt Reactor（MSR） is one of the Gen-IV（Generation Four Initiative） nuclear reactors. Because of its high thermodynamic efficiency, intrinsic safety, effective utilization of nuclear power, MSR attracts so much attention. A MSR uses molten fluoride salts as primary reactor coolants and fuel. However, molten fluoride salts are highly corrosive at high temperature, materials corrosion has been recognized as a severe issue in MSR. Plant experiences and laboratory experiments have shown, in water reactors, that the weld joints of Ni-based alloys and stainless steels are more susceptible to corrosion failure. In MSR, welding technique has been applied extensively in nuclear reactors as pipes, exchangers, pressure vessels, tanks, cores, etc. Thus, it’s significant to investigate the corrosion behaviors of the joint in molten fluorides.Nickel-base alloys possess excellent corrosion resistance, and Hastelloy N is used as a candidate material for MSR metallic structures. The GH3535 alloy possesses similar composition, and comparable performance with Hastelloy N alloy, which is used for replacing Hastelloy N. According to the studies of ONRL, the corrosion mechanism of Ni-based alloy in molten salt is controlled by chromium diffusion. The main driving force come from the impurity ions in molten salt, impurities react with alloying elements, resulting in a rapid corrosion in the early stage.Though so many studies have been done to learn the corrosion mechanism of the alloys in molten salt, limited reports were available about corrosion performance and its mechanism in weld joint of GH3535 alloy in molten fluorides salt. Based on the above background, a series of static immersion corrosion tests were performed to investigate the corrosion performance of the joint of GH3535 alloy under differement temperatures. What’s more, we also studied the behavior of the joint of GH3535 alloy when introducing different impurities. Several analytical methods were applied in a combine way to investigate the mechanism of weld joint of GH3535 alloy in molten salt environments. The main results and conclusions are summarized as follows:The corrosion of GH3535 alloy was heavier when the temperature went up from 550 °C to 850 °C. The corrosion process was Cr diffusion, and pitting in the alloy surface. Fusion zone and base metal of corroded samples showed little difference for their corrosion behavior, which suggested that welding process has little impact on the diffusion-controlled corrosion. When the temperature was high, Fe from the salt replaced the Cr in the alloy through a substitution reaction and was deposited onto the surface. However, the thickness of Cr-depleted layer increased, which incidated that the corrosion was heavier when the temperature was higher.It was found that the corrosion attack became heavier of weld joint of GH3535 alloy when introducing impurities into molten salt. The weight loss increased sharply when introducing SO₄^2- by Na2SO4, the pitting in the joint surface and corrosion defects increased when the addition was higher. Introducing Ni²⁺ by Ni F2, the results showed that, when the addition was low, it enhanced the corrosion. When the addition was 500 ppm and 1000 ppm, there was Ni deposition on the surface, then formed a protection coat for the joint, prevented the Cr dissolving, which resulted in a drop of the weight loss. By introducing impurities(SO₄^2- and Ni²⁺) experiment, it indicated that the type and the amount of the ions could change the performance of the alloy in molten salt. Thus, it’s important to study the corrosion behavior and the corrosion mechanism when introducing impurities, and it can use as a reference to controll the quality of molten salt.