What is intergranular corrosion?

A type of local corrosion. Corrosion that spreads inward along the boundaries between metal grains. 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 destroys the bond between grains, **reducing the mechanical strength of the metal. Moreover, after corrosion occurs, the surface of metals and alloys still retains a certain metallic luster, with no signs of damage visible; however, the bonding strength between the grains is significantly reduced, and their mechanical properties deteriorate to the point that they cannot withstand impact. Therefore, it is a very dangerous type of corrosion. It usually occurs in brass, hard aluminum alloys, and some stainless steels and nickel-based alloys. Intergranular corrosion of stainless steel welds is a major problem in chemical plants. Intergranular corrosion of stainless steel: A corrosion phenomenon that occurs between the grain boundaries of stainless steel under the action of corrosive agents is known as intergranular corrosion. 　　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. Intergranular corrosion can occur in the heat-affected zone (HAZ) of welded joints, as well as in the weld seam or fusion line. Intergranular corrosion that occurs on the fusion line is also known as knife-line corrosion (KLA). 　　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 ordinary 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 from within the grains does not have 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 zones” are formed. Under the action of corrosive agents, these chromium-deficient zones lose their corrosion resistance, leading to intergranular corrosion. 　　Sensitization of stainless steel and preventive measures Unstable austenitic stainless steels with a carbon content of over 0.03% (i.e., 0Cr18Ni9 stainless steel without titanium or niobium) are prone to intergranular corrosion in certain environments if not heat-treated properly. These steels suffer from intergranular corrosion when heated between 425–815°C, or when cooled slowly through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization), and it leads to chromium depletion in the adjacent areas, making those areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent local corrosion in the weld heat-affected zone. 　　The most common method for testing the sensitivity of stainless steel is the 65% nitric acid corrosion test. During the test, the steel specimens were placed in boiling 65% nitric acid solution for a period of 48 hours per cycle, for a total of 5 cycles, with the weight loss measured at the end of each cycle. As a general rule, the average corrosion rate over 5 test cycles should not exceed 0.05 mm/month. 　　Intergranular corrosion in welded austenitic stainless steel structures can be prevented by the following methods: ① Use low-carbon grades such as 00Cr19Ni10 (304L) or 00Cr17Ni14Mo2 (316L), or stable grades such as 0Cr18Ni11Ti (321, commonly used in Europe) or 0Cr18Ni11Nb (347, commonly used in the United States). The use of these stainless steel grades helps to prevent the precipitation of carbides in amounts that could have harmful effects during welding. 　　②If the structural component is small and can be heat-treated in a furnace, it can be heat-treated at 1040–1150°C to dissolve chromium carbide, and then rapidly cooled in the range of 425–815°C to prevent carbon precipitation. 　　Welded ferritic stainless steels can also suffer from intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and strain in the metal lattice when steel is cooled rapidly from above 925°C; stress-relief heat treatment after welding can eliminate this stress and restore corrosion resistance. Adding titanium in an amount more than 8 times the carbon content to 1Cr17 stainless steel can generally reduce intergranular corrosion of welded steel structures in certain media. However, adding titanium in concentrated nitric acid is not effective.

The dielectrics that can cause intergranular corrosion in stainless steel are mainly acidic media: such as industrial acetic acid, formic acid, chromic acid, lactic acid, nitric acid, phosphoric acid, hydrochloric acid, sulfuric acid, sulfurous acid, ammonium carbamate, etc

Note: A type of local corrosion. Corrosion that spreads inward along the boundaries between metal grains. 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 destroys the bond between grains, **reducing the mechanical strength of the metal. Moreover, after corrosion occurs, the surface of metals and alloys still retains a certain metallic luster, with no signs of damage visible; however, the bonding strength between the grains is significantly reduced, and their mechanical properties deteriorate to the point that they cannot withstand impact. Therefore, it is a very dangerous type of corrosion. It usually occurs in brass, hard aluminum alloys, and some stainless steels and nickel-based alloys. Intergranular corrosion of stainless steel welds is a major problem in chemical plants. 　　Intergranular corrosion of stainless steel: A corrosion phenomenon that occurs between the grain boundaries of stainless steel under the action of corrosive agents is known as intergranular corrosion. 　　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. Intergranular corrosion can occur in the heat-affected zone (HAZ) of welded joints, as well as in the weld seam or fusion line. Intergranular corrosion that occurs on the fusion line is also known as knife-line corrosion (KLA). 　　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 ordinary 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 from within the grains does not have 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 zones” are formed. Under the action of corrosive agents, these chromium-deficient zones lose their corrosion resistance, leading to intergranular corrosion. 　　Sensitization of stainless steel and preventive measures Unstable austenitic stainless steels with a carbon content of over 0.03% (i.e., 0Cr18Ni9 stainless steel without titanium or niobium) are prone to intergranular corrosion in certain environments if not heat-treated properly. These steels suffer from intergranular corrosion when heated between 425–815°C, or when cooled slowly through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization), and it leads to chromium depletion in the adjacent areas, making those areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent local corrosion in the weld heat-affected zone. 　　The most common method for testing the sensitivity of stainless steel is the 65% nitric acid corrosion test. During the test, the steel specimens were placed in boiling 65% nitric acid solution for a period of 48 hours per cycle, for a total of 5 cycles, with the weight loss measured at the end of each cycle. As a general rule, the average corrosion rate over 5 test cycles should not exceed 0.05 mm/month. 　　Intergranular corrosion in welded austenitic stainless steel structures can be prevented by the following methods: ① Use low-carbon grades such as 00Cr19Ni10 (304L) or 00Cr17Ni14Mo2 (316L), or stable grades such as 0Cr18Ni11Ti (321, commonly used in Europe) or 0Cr18Ni11Nb (347, commonly used in the United States). The use of these stainless steel grades helps to prevent the precipitation of carbides in amounts that could have harmful effects during welding. 　　②If the structural component is small and can be heat-treated in a furnace, it can be heat-treated at 1040–1150°C to dissolve chromium carbide, and then rapidly cooled in the range of 425–815°C to prevent carbon precipitation. 　　Welded ferritic stainless steels can also suffer from intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and strain in the metal lattice when steel is cooled rapidly from above 925°C; stress-relief heat treatment after welding can eliminate this stress and restore corrosion resistance. Adding titanium in an amount more than 8 times the carbon content to 1Cr17 stainless steel can generally reduce intergranular corrosion of welded steel structures in certain media. However, adding titanium in concentrated nitric acid is not effective. 　　Relevant standards: China already has the GB/T 4334.(1–5)—2000 standards for testing the intergranular corrosion sensitivity of stainless steels (the appropriate standard is selected based on the sensitivity of the respective material). There is also GB/T 15260—1994, which specifies the methods for testing the intergranular corrosion sensitivity of nickel alloys. Additionally, there is GB/T 21433-2008, which deals with the inspection of intergranular corrosion sensitivity in stainless steel pressure vessels