Question 22 in August: Intergranular corrosion

English name: intergranular corrosion; intercrystalline corrosion Description: 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 impacts. 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 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 a welded joint, as well as in the weld seam or the 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 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 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 these areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent localized 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 specimen was placed in boiling 65% nitric acid solution for 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 quantities 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 experience 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.

GBT21433-2008 – Inspection for intergranular corrosion susceptibility in stainless steel pressure vessels. It is explained very clearly and comprehensively there

It is a common type of local corrosion. Corrosion occurs along the grain boundaries of a metal or alloy or in the areas adjacent to them; since the grains themselves are only slightly corroded, this type of corrosion is known as intergranular corrosion. Solution: Use targeted steel materials.

Intergranular corrosion is a common type of local corrosion. Metal corrosion occurs along the grain boundaries of a metal or alloy, or in the areas adjacent to them; when the grains of the material themselves are only slightly corroded, this type of corrosion is known as intergranular corrosion. 　　This corrosion weakens the bonding force between grains, and in severe cases it can cause the material to lose all of its mechanical strength. Metals such as stainless steel that suffer from this type of corrosion still appear shiny on the surface, but they break into fine particles upon even slight impact. Metals are difficult to inspect due to intergranular corrosion; therefore, the sudden failure of equipment made from such materials poses a great risk. Metals such as stainless steel, nickel-based alloys, aluminum alloys, and magnesium alloys are all materials with high susceptibility to intergranular corrosion. Intergranular corrosion can occur in metals when they are used under heat or during welding processes. Metallic corrosion begins with intergranular corrosion; under the combined action of stress and environmental factors, materials such as stainless steel and metal aluminum alloys can develop intergranular stress corrosion. Therefore, intergranular corrosion is sometimes a precursor to stress corrosion. 　　Under normal corrosion conditions, the grain boundaries in the passivated alloy structure of the material are not very active. However, when the material is susceptible to intergranular corrosion, its grain boundaries become highly active; that is, there is a certain potential difference between the lattice grains and the grain boundaries. This is mainly caused by changes in the material’s structure when it is exposed to excessive heat. Intergranular corrosion in metals is a type of local corrosion caused by electrochemical heterogeneity in the microstructure. In addition, when there are impurities at the grain boundaries in metals, the material can also suffer from intergranular corrosion in certain media. Intergranular corrosion of metals is the main form of corrosion in austenitic stainless steels; this occurs due to differences in composition or stress between the grain boundaries and the interior of the grains, which result in a significant decrease in the electrode potential at those grain boundary areas

Adding molybdenum seems to provide resistance to intergranular corrosion

During heat treatment of stainless steel when it passes through the sensitization temperature range, carbon precipitates at the grain boundaries and forms compounds with Cr, resulting in chromium depletion at those boundaries; this leads to intergranular corrosion in the presence of corrosive environments. Corrosion cracks propagate along grain boundaries, causing damage; intergranular corrosion can lead to severe damage in stainless steel, and it often occurs in welds and heat-affected zones. There are 3 solutions: First, use ultra-low carbon stainless steel in situations where intergranular corrosion is likely to occur ； II. Solid solution heat treatment of stainless steel ； III. Add stabilizing elements, such as titanium and niobium.

A type of local corrosion in austenitic stainless steels that develops along the grain boundaries and can ultimately lead to the destruction of the crystal structure.

Intergranular corrosion is a common type of localized corrosion； Corrosion develops along the grain boundaries or in the areas adjacent to them; when the grains of the material themselves are only slightly corroded, this type of corrosion is referred to as intergranular corrosion ； Intergranular corrosion weakens the bond between grains, and in severe cases it can cause the material to lose its mechanical strength entirely. 　Stainless steel affected by this type of corrosion still appears shiny on the surface, but it breaks into fine particles upon even slight impact. Metals are difficult to inspect due to intergranular corrosion. Stainless steel, nickel-based metal alloys, and similar materials are all highly susceptible to intergranular corrosion ； In particular, use under heating conditions can cause intergranular corrosion. Metallic corrosion originates from intergranular corrosion; under the combined action of stress and environmental factors, this can induce intergranular stress corrosion in stainless steel.

Grain boundaries are regions of disordered misalignment between grains with different crystal orientations; therefore, they are favorable areas for the segregation of various solute elements in metals or for the precipitation of metal compounds such as carbides and σ phases. In certain corrosive media, the grain boundaries may be corroded first. This corrosion that occurs along the grain boundaries of a material, leading to a local loss of cohesion between the grains, is known as intergranular corrosion. Currently, there are three main measures to prevent intergranular corrosion: 1. Solution treatment ； 2. Reduce the carbon content in steel ； 3. Elements that add stable carbides.

Intergranular corrosion is a common phenomenon in the modern chemical industry, and it is difficult to deal with. Currently, the best approaches are to select the right materials or modify the process conditions.