Moving Forward Every Day — April 11, 2010

1# baiyunfbb Cracking of metal and alloy materials in alkaline solutions, resulting from the combined effect of tensile stress and the corrosive medium. It is a type of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. 　　　　Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement. 　　For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is tensile stress, which can be caused by external loads, residual stresses, or a combination of both. Although the magnitude of tensile stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; localized tensile stress is what most easily leads to alkaline embrittlement. Alkali embrittlement usually occurs in high-temperature pressure-bearing components such as the boiler drum, as these components may meet all three conditions necessary for alkali embrittlement to occur: under normal operation, they are subjected to high temperatures and tensile stresses, while local areas such as the nozzles also experience uneven tensile stresses. As for the alkali concentration in the boiler water, although it does not reach a level that would cause alkaline embrittlement, local areas often experience an increase in the water’s alkali concentration due to the accumulation of sodium hydroxide. For example, in areas such as riveting, tube expansion, and other locations where gaps exist, once water enters, it tends to become gradually more concentrated, thus very likely reaching the concentration required for stress corrosion. Therefore, the majority of alkali embrittlement in the boiler drum occurs at the riveted or expansion-jointed seams. China has experienced boiler alkali embrittlement explosions on more than one occasion, and such severe accidents have also occurred frequently abroad. In China, there have also been cases of ultra-high-pressure vessels experiencing alkali embrittlement due to localized concentration of dilute alkaline solutions, which led to explosions.

Alkali embrittlement, also known as caustic embrittlement, is a special form of corrosion in metals. Since the main factor causing this corrosion is caustic soda in water, which renders the corroded metal brittle, it is known as caustic embrittlement. Also, since this type of corrosion involves cracks that form along the grain boundaries, it is also known as intergranular corrosion. For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is the stress greater than the material’s yield limit, a condition that is met at both expansion joints and riveted connections. It can be stress caused by external loads, residual stress, or a combination of the two. Although the magnitude of stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; local tensile stress is what most easily leads to alkaline embrittlement. 　　The danger of caustic embrittlement lies in the fact that this type of corrosion is not easy to detect in its early stages; it does not form ulcerated spots nor cause the metal to thin out. Once such corrosion occurs, the metal is quickly damaged. By the time cracks are detected, the metal damage has already reached a severe level, whereas the mechanical properties of the metal (such as plasticity and tensile strength) generally do not change at all. 　　Both low-carbon steel and low-alloy steel are susceptible to alkali embrittlement cracks under conditions of alkaline concentration and high tensile stress. The higher the temperature, the lower the concentration required for stress corrosion cracking to occur. Severe alkali embrittlement can cause explosions and fractures in boilers, pressure vessels, pressure pipelines, or other load-bearing components. Alkali embrittlement should be detected and addressed through various non-destructive testing methods.

Cracking in metal and alloy materials in alkaline solutions, resulting from the combined effect of tensile stress and the corrosive medium. It is a type of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. 　　 　　Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement. 　　For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is tensile stress, which can be caused by external loads, residual stresses, or a combination of both. Although the magnitude of tensile stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; localized tensile stress is what most easily leads to alkaline embrittlement. Alkali embrittlement usually occurs in high-temperature pressure-bearing components such as the boiler drum, as these components may meet all three conditions necessary for alkali embrittlement to occur: under normal operation, they are subjected to high temperatures and tensile stresses, while local areas such as the nozzles also experience uneven tensile stresses. As for the alkali concentration in the boiler water, although it does not reach a level that would cause alkaline embrittlement, local areas often experience an increase in the water’s alkali concentration due to the accumulation of sodium hydroxide. For example, in areas such as riveting, tube expansion, and other locations where gaps exist, once water enters, it tends to become gradually more concentrated, thus very likely reaching the concentration required for stress corrosion. Therefore, the majority of alkali embrittlement in the boiler drum occurs at the riveted or expansion-jointed seams. China has experienced boiler alkali embrittlement explosions on more than one occasion, and such severe accidents have also occurred frequently abroad. In China, there have also been cases of ultra-high-pressure vessels experiencing alkali embrittlement due to localized concentration of dilute alkaline solutions, which led to explosions.

Mechanism: Stress corrosion cracking of low-carbon steel and low-alloy steel in hot concentrated sodium hydroxide solutions is of the anodic dissolution type. The shape of the cracks is dendritic, with a width of 1–2 mm; their fracture potential is generally around -700 mV. The intergranular activation zone is caused by the segregation of carbon, nitrogen, and other harmful impurities such as sulfur, phosphorus, and arsenic at the grain boundaries, rather than by Fe3C ； On the one hand, relative to the matrix, Fe3C is the cathodic phase; on the other hand, cracks propagate along the interface between the matrix and Fe3C. Under external stress, grain boundary cracks are formed in the film, causing the newly exposed Fe to undergo selective dissolution from Fe to FeO22-. The hydrogen generated by this chemical reaction can penetrate into the metal without a film. Control measures: ① Select an appropriate carbon steel by taking into account strength, plasticity, and susceptibility to alkali embrittlement; normalized steel with a carbon content of about 0.2% is the most suitable carbon steel. ②Minimize residual stresses during assembly and welding. Reduce misalignment, edge angles, and gap leakage, and perform stress-relieving heat treatment after welding. ③For boilers, it is necessary to minimize the content of free sodium hydroxide. ④Corrosion inhibitors are added, with common ones including Na3PO4, NaNO3, NaNO2, Na2SO4, etc ； Alkali embrittlement can be prevented when the NaNO2/NaOH ratio is greater than 0.4 and the Na2SO4/NaOH ratio is greater than 5.

Alkali embrittlement, also known as caustic embrittlement, is the cracking of metal and alloy materials in alkaline solutions due to the combined effect of tensile stress and the corrosive medium. This type of corrosion involves cracks forming along the grain boundaries; it is a special form of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is tensile stress, which can be caused by external loads, residual stresses, or a combination of both. Although the magnitude of tensile stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; localized tensile stress is what most easily leads to alkaline embrittlement. Alkali embrittlement usually occurs in high-temperature pressure-bearing components such as the boiler drum, as these components may meet all three conditions necessary for alkali embrittlement to occur: under normal operation, they are subjected to high temperatures and tensile stresses, while local areas such as the nozzles also experience uneven tensile stresses. As for the alkali concentration in the boiler water, although it does not reach a level that would cause alkaline embrittlement, local areas often experience an increase in the water’s alkali concentration due to the accumulation of sodium hydroxide. For example, in areas such as riveting, tube expansion, and other locations where gaps exist, once water enters, it tends to become gradually more concentrated, thus very likely reaching the concentration required for stress corrosion. Therefore, the majority of alkali embrittlement in the boiler drum occurs at the riveted or expansion-jointed seams. China has experienced boiler alkali embrittlement explosions on more than one occasion, and such severe accidents have also occurred frequently abroad. In China, there have also been cases of ultra-high-pressure vessels experiencing alkali embrittlement due to localized concentration of dilute alkaline solutions, which led to explosions. Alkali embrittlement of stainless steel generally occurs at temperatures above its boiling point, but it can also crack at temperatures below the boiling point in 50% NaOH. Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement.

Alkali embrittlement, also known as caustic embrittlement, is a special form of corrosion in metals. Since the main factor causing this corrosion is caustic soda in water, which renders the corroded metal brittle, it is known as caustic embrittlement. Also, since this type of corrosion involves cracks that form along the grain boundaries, it is also known as intergranular corrosion. For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is the stress greater than the material’s yield limit, a condition that is met at both expansion joints and riveted connections. It can be stress caused by external loads, residual stress, or a combination of the two. Although the magnitude of stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; local tensile stress is what most easily leads to alkaline embrittlement. 　　The danger of caustic embrittlement lies in the fact that this type of corrosion is not easy to detect in its early stages; it does not form ulcerated spots nor cause the metal to thin out. Once such corrosion occurs, the metal is quickly damaged. By the time cracks are detected, the metal damage has already reached a severe level, whereas the mechanical properties of the metal (such as plasticity and tensile strength) generally do not change at all. 　　Both low-carbon steel and low-alloy steel are susceptible to alkali embrittlement cracks under conditions of alkaline concentration and high tensile stress. The higher the temperature, the lower the concentration required for stress corrosion cracking to occur. Severe alkali embrittlement can cause explosions and fractures in boilers, pressure vessels, pressure pipelines, or other load-bearing components. Alkali embrittlement should be detected and addressed through various non-destructive testing methods.

Cracking in metal and alloy materials in alkaline solutions, resulting from the combined effect of tensile stress and the corrosive medium. It is a type of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. 　　 　　Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement. 　　For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is tensile stress, which can be caused by external loads, residual stresses, or a combination of both. Although the magnitude of tensile stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; localized tensile stress is what most easily leads to alkaline embrittlement. Alkali embrittlement usually occurs in high-temperature pressure-bearing components such as the boiler drum, as these components may meet all three conditions necessary for alkali embrittlement to occur: under normal operation, they are subjected to high temperatures and tensile stresses, while local areas such as the nozzles also experience uneven tensile stresses. As for the alkali concentration in the boiler water, although it does not reach a level that would cause alkaline embrittlement, local areas often experience an increase in the water’s alkali concentration due to the accumulation of sodium hydroxide. For example, in areas such as riveting, tube expansion, and other locations where gaps exist, once water enters, it tends to become gradually more concentrated, thus very likely reaching the concentration required for stress corrosion. Therefore, the majority of alkali embrittlement in the boiler drum occurs at the riveted or expansion-jointed seams. China has experienced boiler alkali embrittlement explosions on more than one occasion, and such severe accidents have also occurred frequently abroad. In China, there have also been cases of ultra-high-pressure vessels experiencing alkali embrittlement due to localized concentration of dilute alkaline solutions, which led to explosions. 1# baiyunfbb

Alkali embrittlement, also known as caustic embrittlement, is the cracking of metal and alloy materials in alkaline solutions due to the combined effect of tensile stress and the corrosive medium. This type of corrosion involves cracks forming along the grain boundaries; it is a special form of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. For the alkaline embrittlement of steel, three conditions generally need to be met simultaneously. One is a solution of sodium hydroxide at a high concentration. Tests have shown that an alkali solution with a concentration of over 10% is sufficient to cause alkaline embrittlement in steel. Second is the higher temperature; the temperature range for alkali embrittlement is wide, but the temperature at which alkali embrittlement occurs most easily is around the boiling point of the solution. Third is tensile stress, which can be caused by external loads, residual stresses, or a combination of both. Although the magnitude of tensile stress is a factor affecting alkaline embrittlement, a more important factor is whether the stress is uniform; localized tensile stress is what most easily leads to alkaline embrittlement. Alkali embrittlement usually occurs in high-temperature pressure-bearing components such as the boiler drum, as these components may meet all three conditions necessary for alkali embrittlement to occur: under normal operation, they are subjected to high temperatures and tensile stresses, while local areas such as the nozzles also experience uneven tensile stresses. As for the alkali concentration in the boiler water, although it does not reach a level that would cause alkaline embrittlement, local areas often experience an increase in the water’s alkali concentration due to the accumulation of sodium hydroxide. For example, in areas such as riveting, tube expansion, and other locations where gaps exist, once water enters, it tends to become gradually more concentrated, thus very likely reaching the concentration required for stress corrosion. Therefore, the majority of alkali embrittlement in the boiler drum occurs at the riveted or expansion-jointed seams. China has experienced boiler alkali embrittlement explosions on more than one occasion, and such severe accidents have also occurred frequently abroad. In China, there have also been cases of ultra-high-pressure vessels experiencing alkali embrittlement due to localized concentration of dilute alkaline solutions, which led to explosions. Alkali embrittlement of stainless steel generally occurs at temperatures above its boiling point, but it can also crack at temperatures below the boiling point in 50% NaOH. Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement.

Cracking in metal and alloy materials in alkaline solutions, resulting from the combined effect of tensile stress and the corrosive medium. It is a type of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. 　　 Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement

Cracking in metal and alloy materials in alkaline solutions, resulting from the combined effect of tensile stress and the corrosive medium. It is a type of stress corrosion cracking. Alkali embrittlement mainly occurs due to the alkalinity resulting from water softening in boilers, which then concentrates in the gaps of the boilers leading to boiler failure; it also occurs in carbon steel, low-alloy steel, and austenitic stainless steel equipment that comes into contact with caustic alkalis. 　　 Measures to prevent alkali embrittlement include: (1) adding inhibitors such as trisodium phosphate, sodium nitrate, etc ； (2) Reduce the operating temperature as much as possible ； (3) Minimize the load stress as much as possible ； (4) Perform stress-relief heat treatment to remove residual stresses generated during welding, assembly, and machining ； (5) Use materials such as high-nickel cast iron and nickel alloys that are less prone to alkali embrittlement. 1# baiyunfbb