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Intergranular corrosion is a type of local corrosion. This is mainly due to the differences in chemical composition between the grain surface and the interior, as well as the presence of impurities at grain boundaries or internal stresses. Intergranular corrosion can occur in the heat-affected zone (HAZ) of a welded joint, in the weld itself, or on the fusion line; intergranular corrosion that occurs on the fusion line is also known as knife-line corrosion (KLA). Intergranular corrosion is a type of corrosion that occurs in stainless steel under the action of corrosive agents, between the crystal grains. Stainless steel that is prone to intergranular corrosion will fracture along the grain boundaries when subjected to stress, resulting in almost complete loss of strength; this is one of the most dangerous forms of failure for stainless steel. The necessary condition for stainless steel to have corrosion resistance is that the chromium content must be greater than 10–12%. As the temperature rises, the diffusion rate of carbon within the stainless steel grains is greater than that of chromium. Since the solubility of carbon in austenite at room temperature is very low, at around 0.02%–0.03%, the carbon content in typical austenitic stainless steels exceeds this value. As a result, the excess carbon continuously diffuses toward the boundaries of the austenite grains, where it combines with chromium to form chromium carbide compounds such as (CrFe)23C6. Data show that the activation energy for chromium diffusion along grain boundaries is 162–252 KJ/mol, whereas the activation energy for chromium diffusion within grains is approximately 540 KJ/mol. In other words, the diffusion rate of chromium within grains is slower than that along grain boundaries; as a result, chromium within the grains does not have enough time to diffuse to the grain boundaries. Therefore, the chromium required for the formation of chromium carbide at the grain boundaries comes mainly from areas near the grain boundaries rather than from within the austenite grains. This leads to a significant reduction in the chromium content in those areas near the grain boundaries. When the chromium mass fraction in these areas drops below 12%, so-called “chromium-deficient regions” are formed. Under the action of corrosive agents, these chromium-deficient regions lose their corrosion resistance, leading to intergranular corrosion. 01 Intergranular corrosion of welds: In multi-layer welding of ordinary austenitic stainless steels, the heat-affected zone of the subsequent weld corresponds to the weld metal of the previous weld. When this \"heat-affected zone\" reaches the sensitization temperature, chromium carbides tend to precipitate at the grain boundaries, resulting in chromium-deficient grain boundaries; when this area comes into contact with corrosive agents, intergranular corrosion occurs. The most direct and effective way to prevent intergranular corrosion in welds is to select welding materials resistant to intergranular corrosion. Currently, the most commonly used welding materials are ultra-low carbon ones containing Nb (or Ti) as stabilizing elements, along with a small amount of ferrite (with a ferrite content generally required to be 4%–12%). Depending on the base material and corrosion environment, materials that meet all three of the above requirements can also be selected (such as e347l welding rods). In terms of welding processes, it is necessary to control the interlayer temperature, which should generally be below 150°C. Using welding methods with low heat input along with appropriate process measures can reduce the temperature range in which sensitization occurs, preventing grain growth and the precipitation of carbides, thereby reducing the tendency for intergranular corrosion in the weld. 02 Intergranular corrosion in the temperature-sensitive zone of the heat-affected zone: Whether it is a single weld seam or multiple weld seams, a sensitized zone exists within the heat-affected zone, which poses a risk of intergranular corrosion. In terms of welding processes. Choose a welding method with lower wire energy, or reduce the welding parameters as much as possible to control the interpass temperature. Increase the cooling rate. Minimize the residence time within the sensitization temperature range to reduce the width of the heat-affected sensitized zone, thereby mitigating intergranular corrosion. The best way to prevent intergranular corrosion in the heat-affected zone is to choose an appropriate base material, such as stabilized austenitic stainless steels containing Ti and Nb, austenitic stainless steels with a certain amount of ferrite, and ultra-low carbon austenitic stainless steels. 03 Knife edge corrosion: It can sometimes be difficult to eliminate or reduce the risk of \"knife edge corrosion\" in welded joints of austenitic steels stabilized with Nb and Ti, but in the design of such welded joints, cross welds should be avoided as much as possible. In terms of welding sequence, care should be taken to ensure that the side prone to \"knife erosion\" does not come into contact with corrosive agents. If the workpiece is small and the material and structure of the workpiece permit it, post-weld solution treatment (i.e., at 1065–1120°C followed by water quenching or rapid cooling) can also be an effective method to prevent intergranular corrosion. However, in actual production, solid solution treatment cannot be carried out due to the large size of the workpieces, their complex structure, and other materials.