Dendritic Corrosion of 316L Stainless Steel Weld Joint in NaCl–MgCl₂ Molten Salt Loop
The corrosion of structural materials in molten salt environments remains a significant challenge in the development and deployment of molten salt reactors. These reactors, a promising solution for next-generation nuclear power, rely on molten salts as coolant or fuel-bearing salts. As molten salts offer improved safety, efficiency, and sustainability in nuclear systems, understanding the material degradation within these environments is critical for ensuring the longevity and reliability of reactor components.
One of the most commonly investigated materials for MSR applications is 316L stainless steel, particularly for its excellent mechanical properties, corrosion resistance, and relatively low cost. However, when exposed to molten salts, including the NaCl–MgCl₂ eutectic salt, the corrosion mechanisms can differ significantly from those seen in conventional environments, particularly when structural weld joints are involved. This study, led by Yafei Wang, Cody Falconer, and Adrien Couet, examines the specific corrosion characteristics of 316L stainless steel and its weld joints under flow conditions in a NaCl–MgCl₂ molten salt loop.
Molten Salt Reactors and NaCl–MgCl₂ Eutectic Salt
Molten salt reactors are a type of Generation-IV nuclear reactor being developed for their potential to provide low-cost, carbon-free energy. Unlike traditional reactors, MSRs use molten salts as coolants, which have several benefits, such as higher heat transfer capabilities and the potential for operating at higher temperatures. This is ideal for enhancing the efficiency of nuclear power generation.
Among the various molten salts being considered for use in MSRs, chloride salts, including the NaCl–MgCl₂ eutectic mixture, have emerged as key candidates due to their lower melting points, higher solubilities for actinide elements, and ability to accommodate significant amounts of transuranic materials. These attributes make NaCl–MgCl₂ an attractive option for advanced nuclear reactors. However, the behavior of materials, especially structural alloys like 316L stainless steel, when exposed to these molten salts under operational conditions, remains a subject of intensive study.
Corrosion Behavior of 316L Stainless Steel and its Welds
In this research, the corrosion of 316L stainless steel and its weld joint in a NaCl–MgCl₂ molten salt thermal convection loop was examined over a period of about 260 hours. The study highlights the differences in corrosion behavior between the base metal and the welded joint, which were subjected to similar thermal and flow conditions.
1. Formation of Voids and Dendritic Structures
The results showed that corrosion in the 316L stainless steel in the molten salt loop caused the formation of voids within the material. Interestingly, while the base metal exhibited a high concentration of these voids, the weld joint developed dendritic corrosion structures, which are a type of corrosion that creates branched, tree-like patterns on the surface. These structures were more discretely spread throughout the weld joint, as compared to the denser void formation in the base metal. This distinction between the two regions is crucial because it indicates different corrosion resistance levels and suggests that the welded areas of the reactor components may degrade more rapidly than the base metal.
2. Material Characterization of the Weld and Base Metal
Before conducting the corrosion experiments, a detailed characterization of the welded 316L stainless steel was performed. This included optical microscopy, electron backscatter diffraction, X-ray diffraction, and scanning electron microscopy with energy dispersive X-ray spectroscopy. These techniques were used to examine the microstructure and elemental distribution in the weld joint and base metal.
• Optical Microscopy: The images obtained from optical microscopy showed a clear difference between the microstructures of the base metal and the weld joint. In the fusion zone, the welded area, the grains were larger and more defined, while the base metal exhibited a fine, equiaxed grain structure.
• Electron Backscatter Diffraction: EBSD mapping revealed a marked contrast between the base metal and the fusion zone. The base metal displayed small, equiaxed grains, while the fusion zone primarily consisted of larger, columnar grains, a feature typically seen in welded materials.
• X-ray Diffraction: The XRD analysis indicated that both the base metal and weld joint consisted of a single FCC phase, although the (222) diffraction peak, characteristic of the base metal, was absent in the weld joint.
• SEM and EDS Analysis: The elemental distribution of the major constituents of 316L stainless steel, iron, chromium, nickel, and molybdenum, was found to be uniform in both the base metal and the weld joint. This suggests that the elemental composition remained consistent during the welding process but did not prevent the significant microstructural changes observed in the fusion zone.
Corrosion Performance in the Molten Salt Loop
In the natural circulation loop, the molten NaCl–MgCl₂ salt was allowed to flow continuously for approximately 260 hours, simulating the flow conditions in an operational MSR. During this time, corrosion developed in both the base metal and weld joints. While the corrosion of the base metal led to void formation, the weld joint showed more pronounced and discrete dendritic corrosion. This could be due to the differing microstructures and mechanical properties of the base metal and weld zone, which might influence how each area reacts to the aggressive molten salt environment.
The formation of dendritic corrosion in the weld joint could indicate a vulnerability in the weld area of MSR components. This is significant because welded joints are integral to the structural integrity of the reactor components. Understanding the corrosion behaviors in these areas is essential for designing more durable materials for MSRs.
Implications for MSR Material Selection and Alloy Development
The findings from this study provide important insights into the corrosion resistance of 316L stainless steel in molten salt environments, especially regarding the performance of weld joints. The higher corrosion rates observed in the weld joints compared to the base metal underline the need for better understanding and improvement of the corrosion resistance in welded areas.
For the successful deployment of MSRs, it is crucial to not only select corrosion-resistant alloys but also to consider how welding techniques and joint areas may behave differently in these high-temperature molten salt environments. Future studies and alloy development efforts will likely focus on enhancing the corrosion resistance of welded joints, possibly through new welding techniques, coatings, or alloy compositions, to mitigate the challenges posed by molten salt corrosion.
This study on the corrosion behavior of 316L stainless steel in NaCl–MgCl₂ molten salt loops underscores the complexity of material degradation in advanced nuclear reactors. The differences in corrosion between the base metal and the weld joint are crucial for improving the design and operation of MSRs, ensuring both safety and longevity.
