This is what you should do to deal with corrosion and perforation of stainless steel heat exchanger tubes!
By: mxgy,
The process of emergency stop loss, root cause investigation, targeted repair/replacement, and system optimization prevention should be followed to deal with corrosion perforation of stainless steel heat exchange tubes. The key is to first terminate the risk of leakage, and then analyze the cause of corrosion to avoid problem recurrence
Firstly, emergency stop loss and system isolation:
Corrosion perforation can cause leakage of the heat exchange medium (such as mutual connection between the tube side and shell side media), and it is necessary to stop the machine immediately, close the inlet and outlet valves on both sides of the heat exchange tube, and cut off the medium flow; If it is not possible to stop the machine immediately, the flow rate can be switched through the bypass pipeline to reduce the impact of the leaking medium on the system. Afterwards, the residual medium in the pressure relief discharge pipe should be removed (taking care to avoid direct discharge of corrosive media and pollution), and the pipe should be rinsed with neutral cleaning agents (such as clean water, weak alkaline solution, to avoid aggravating stainless steel corrosion) to remove residual corrosive media (such as chloride ions, acidic substances), clearing obstacles for subsequent inspection and repair, and preventing the perforation area from expanding due to continuous corrosion of residual media.
Next is root cause investigation:
Clarify the type and causes of corrosion perforation – Corrosion perforation of stainless steel heat exchange tubes is not a single cause, and it needs to be determined through visual inspection, medium analysis, and working condition review in order to solve it targetedly. Common types and triggers include:
Pitting corrosion/crevice corrosion: It is often caused by the presence of chloride ions in the medium (such as seawater, chlorine containing cooling water), or the presence of gaps at the expansion/welding joints between the heat exchange tube and the tube plate (residual accumulation of medium, forming a local high concentration corrosion environment), which leads to the destruction of the passive film on the surface of stainless steel, forming small corrosion holes that gradually expand into perforations. Observe whether there are “needle like” holes at the perforation site, or whether there are corrosion marks at the connection of the tube plate.
Stress Corrosion Cracking (SCC): If there are residual stresses in the heat exchange tube during installation (such as excessive expansion, failure to perform stress relief heat treatment after welding), and the medium contains chloride ions, hydroxide ions, etc., when the temperature rises (usually exceeding 60 ℃), the stress and corrosive medium work together, causing the tube to crack along grain boundaries or transgranular boundaries, ultimately forming perforations. The cracked area often appears in a “branched” or “straight” shape, with a relatively neat edge of the perforation.
Intergranular corrosion: If the material selection of stainless steel heat exchange tubes is improper (such as 304 stainless steel staying in the sensitization temperature range (450-850 ℃) for too long, or not cooled in time after welding), carbon and chromium elements combine to form chromium carbide, resulting in a decrease in chromium content at grain boundaries and loss of corrosion resistance. The medium will corrode along the grain boundaries, causing a decrease in the strength of the tube, and ultimately leading to perforation due to pressure or vibration. The perforated part of the tube is prone to “powdery” detachment.
Erosion corrosion: If the flow velocity of the medium inside the tube is too high (exceeding the design value, such as exceeding 3m/s), or if the medium contains solid particles, it will continue to erode the inner wall of the heat exchange tube, damage the passivation film, form a “groove like” corrosion, gradually deepen into perforation, and the perforation site is mostly located at the inlet end or turning point of the medium.
The third step is targeted repair or replacement:
Select a solution based on the degree of perforation and corrosion type – taking into account the number and location of perforations, the degree of pipe damage, and the cause of corrosion, and choose an economical and reliable treatment method:
Single/small number of perforations (≤ 5% of total number of perforations): Tube blockage treatment – If the number of perforated tubes is small and does not affect the overall heat transfer efficiency, “mechanical plugs” or “welded plugs” can be used to block both ends of the perforated tubes (ensuring that the plugs are tightly sealed with the tube sheet to avoid medium leakage from the plug gaps). Attention: Before blocking the pipe, it is necessary to confirm that there is no obvious stress corrosion or intergranular corrosion in the pipe (otherwise adjacent pipes may still corrode after blocking), and the total number of blocked pipes should not be too high (usually not exceeding 10%) to prevent a significant decrease in heat transfer efficiency.
Multiple perforations or perforations in critical areas: Replace heat exchange tubes – If there are a large number of perforated tubes, or if the perforations are located in the middle of the heat exchange tubes and are difficult to seal, the damaged heat exchange tubes need to be replaced as a whole. When replacing, attention should be paid to: ① Material matching: Choose a more corrosion-resistant stainless steel material according to the cause of corrosion (such as 316L (containing molybdenum, resistant to chloride ions) for severe pitting corrosion, and 2205 duplex stainless steel for stress corrosion); ② Installation process control: Control the expansion pressure during expansion to avoid residual stress caused by excessive expansion; After welding, stress relief heat treatment (such as solution treatment and stabilization treatment) is required to eliminate welding stress and prevent intergranular corrosion; ③ Tube sheet gap treatment: After expansion or welding, seal the gap between the tube sheet and the heat exchange tube (such as injecting sealant) to avoid gap corrosion.
Localized mild corrosion (unperforated but with corrosion pits): Repair treatment – If only localized corrosion pits or small cracks exist, “welding repair” (using stainless steel welding wire of the same material to repair the corrosion pits, polishing and stress relief treatment after welding) or “passivation treatment” (passivating with nitric acid hydrofluoric acid solution after welding to restore the passivation film on the stainless steel surface) can be used, but it is necessary to ensure that there is no residual stress in the repaired area and thoroughly rinse after passivation to avoid residual corrosion of passivation solution.
The fourth step is system recovery and verification:
Ensure safe operation after repair – After repair or replacement, the heat exchange system needs to be strictly inspected to avoid hidden dangers and residues: ① Pressure test: Conduct water pressure tests on the tube side and shell side respectively (test pressure is 1.25-1.5 times the design pressure), hold the pressure for 30 minutes, and check whether the plugs, welds, and new pipes leak; ② System flushing: Rinse the pipe and shell sides again with neutral water to remove residual welding slag, oil stains, etc. during construction, to avoid impurities blocking the pipeline or inducing corrosion; ③ Trial operation monitoring: After starting the system, continuously monitor the inlet and outlet temperature, pressure, and medium composition (such as chloride ion concentration), observe for 24-48 hours, confirm that there are no leaks, heat transfer efficiency is normal, and there are no new signs of corrosion.
Finally, long-term prevention:
Avoiding corrosion perforation from the source – After solving the immediate problem, it is necessary to optimize the system based on the causes of corrosion to prevent recurrence: ① Medium control: reduce the content of corrosive ions in the medium (such as dechlorination treatment of cooling water, controlling the concentration of chloride ions ≤ 20ppm), adjust the pH value of the medium to neutral or weakly alkaline (avoid acidic media); ② Optimization of working conditions: Control the flow rate of the medium inside the pipe within the design range (usually 1-3m/s) to avoid erosion and corrosion caused by high flow rate, and to avoid the temperature being in the stainless steel sensitization temperature range for a long time; ③ Regular testing: Inspect the inner wall of the heat exchange tube every 6-12 months using methods such as endoscopy and ultrasonic thickness measurement to promptly detect early signs of corrosion; ④ Material upgrade: If corrosion occurs repeatedly, consider upgrading the material of the heat exchange tube to a more corrosion-resistant type (such as Hastelloy or titanium alloy, depending on the corrosiveness of the medium).
In short, the key to solving the corrosion perforation of stainless steel heat exchange tubes is to “first stop the leakage, then investigate the cause, then repair, and finally prevent” – not only through emergency treatment and repair to terminate the current fault, but also through root cause analysis and system optimization to eliminate the corrosion causes from the aspects of material, process, and working conditions, in order to ensure the long-term stable operation of the heat exchange system.
This is what you should do to deal with corrosion and perforation of stainless steel heat exchanger tubes!
By: mxgy,
The process of emergency stop loss, root cause investigation, targeted repair/replacement, and system optimization prevention should be followed to deal with corrosion perforation of stainless steel heat exchange tubes. The key is to first terminate the risk of leakage, and then analyze the cause of corrosion to avoid problem recurrence
Firstly, emergency stop loss and system isolation:
Corrosion perforation can cause leakage of the heat exchange medium (such as mutual connection between the tube side and shell side media), and it is necessary to stop the machine immediately, close the inlet and outlet valves on both sides of the heat exchange tube, and cut off the medium flow; If it is not possible to stop the machine immediately, the flow rate can be switched through the bypass pipeline to reduce the impact of the leaking medium on the system. Afterwards, the residual medium in the pressure relief discharge pipe should be removed (taking care to avoid direct discharge of corrosive media and pollution), and the pipe should be rinsed with neutral cleaning agents (such as clean water, weak alkaline solution, to avoid aggravating stainless steel corrosion) to remove residual corrosive media (such as chloride ions, acidic substances), clearing obstacles for subsequent inspection and repair, and preventing the perforation area from expanding due to continuous corrosion of residual media.
Next is root cause investigation:
Clarify the type and causes of corrosion perforation – Corrosion perforation of stainless steel heat exchange tubes is not a single cause, and it needs to be determined through visual inspection, medium analysis, and working condition review in order to solve it targetedly. Common types and triggers include:
Pitting corrosion/crevice corrosion: It is often caused by the presence of chloride ions in the medium (such as seawater, chlorine containing cooling water), or the presence of gaps at the expansion/welding joints between the heat exchange tube and the tube plate (residual accumulation of medium, forming a local high concentration corrosion environment), which leads to the destruction of the passive film on the surface of stainless steel, forming small corrosion holes that gradually expand into perforations. Observe whether there are “needle like” holes at the perforation site, or whether there are corrosion marks at the connection of the tube plate.
Stress Corrosion Cracking (SCC): If there are residual stresses in the heat exchange tube during installation (such as excessive expansion, failure to perform stress relief heat treatment after welding), and the medium contains chloride ions, hydroxide ions, etc., when the temperature rises (usually exceeding 60 ℃), the stress and corrosive medium work together, causing the tube to crack along grain boundaries or transgranular boundaries, ultimately forming perforations. The cracked area often appears in a “branched” or “straight” shape, with a relatively neat edge of the perforation.
Intergranular corrosion: If the material selection of stainless steel heat exchange tubes is improper (such as 304 stainless steel staying in the sensitization temperature range (450-850 ℃) for too long, or not cooled in time after welding), carbon and chromium elements combine to form chromium carbide, resulting in a decrease in chromium content at grain boundaries and loss of corrosion resistance. The medium will corrode along the grain boundaries, causing a decrease in the strength of the tube, and ultimately leading to perforation due to pressure or vibration. The perforated part of the tube is prone to “powdery” detachment.
Erosion corrosion: If the flow velocity of the medium inside the tube is too high (exceeding the design value, such as exceeding 3m/s), or if the medium contains solid particles, it will continue to erode the inner wall of the heat exchange tube, damage the passivation film, form a “groove like” corrosion, gradually deepen into perforation, and the perforation site is mostly located at the inlet end or turning point of the medium.
The third step is targeted repair or replacement:
Select a solution based on the degree of perforation and corrosion type – taking into account the number and location of perforations, the degree of pipe damage, and the cause of corrosion, and choose an economical and reliable treatment method:
Single/small number of perforations (≤ 5% of total number of perforations): Tube blockage treatment – If the number of perforated tubes is small and does not affect the overall heat transfer efficiency, “mechanical plugs” or “welded plugs” can be used to block both ends of the perforated tubes (ensuring that the plugs are tightly sealed with the tube sheet to avoid medium leakage from the plug gaps). Attention: Before blocking the pipe, it is necessary to confirm that there is no obvious stress corrosion or intergranular corrosion in the pipe (otherwise adjacent pipes may still corrode after blocking), and the total number of blocked pipes should not be too high (usually not exceeding 10%) to prevent a significant decrease in heat transfer efficiency.
Multiple perforations or perforations in critical areas: Replace heat exchange tubes – If there are a large number of perforated tubes, or if the perforations are located in the middle of the heat exchange tubes and are difficult to seal, the damaged heat exchange tubes need to be replaced as a whole. When replacing, attention should be paid to: ① Material matching: Choose a more corrosion-resistant stainless steel material according to the cause of corrosion (such as 316L (containing molybdenum, resistant to chloride ions) for severe pitting corrosion, and 2205 duplex stainless steel for stress corrosion); ② Installation process control: Control the expansion pressure during expansion to avoid residual stress caused by excessive expansion; After welding, stress relief heat treatment (such as solution treatment and stabilization treatment) is required to eliminate welding stress and prevent intergranular corrosion; ③ Tube sheet gap treatment: After expansion or welding, seal the gap between the tube sheet and the heat exchange tube (such as injecting sealant) to avoid gap corrosion.
Localized mild corrosion (unperforated but with corrosion pits): Repair treatment – If only localized corrosion pits or small cracks exist, “welding repair” (using stainless steel welding wire of the same material to repair the corrosion pits, polishing and stress relief treatment after welding) or “passivation treatment” (passivating with nitric acid hydrofluoric acid solution after welding to restore the passivation film on the stainless steel surface) can be used, but it is necessary to ensure that there is no residual stress in the repaired area and thoroughly rinse after passivation to avoid residual corrosion of passivation solution.
The fourth step is system recovery and verification:
Ensure safe operation after repair – After repair or replacement, the heat exchange system needs to be strictly inspected to avoid hidden dangers and residues: ① Pressure test: Conduct water pressure tests on the tube side and shell side respectively (test pressure is 1.25-1.5 times the design pressure), hold the pressure for 30 minutes, and check whether the plugs, welds, and new pipes leak; ② System flushing: Rinse the pipe and shell sides again with neutral water to remove residual welding slag, oil stains, etc. during construction, to avoid impurities blocking the pipeline or inducing corrosion; ③ Trial operation monitoring: After starting the system, continuously monitor the inlet and outlet temperature, pressure, and medium composition (such as chloride ion concentration), observe for 24-48 hours, confirm that there are no leaks, heat transfer efficiency is normal, and there are no new signs of corrosion.
Finally, long-term prevention:
Avoiding corrosion perforation from the source – After solving the immediate problem, it is necessary to optimize the system based on the causes of corrosion to prevent recurrence: ① Medium control: reduce the content of corrosive ions in the medium (such as dechlorination treatment of cooling water, controlling the concentration of chloride ions ≤ 20ppm), adjust the pH value of the medium to neutral or weakly alkaline (avoid acidic media); ② Optimization of working conditions: Control the flow rate of the medium inside the pipe within the design range (usually 1-3m/s) to avoid erosion and corrosion caused by high flow rate, and to avoid the temperature being in the stainless steel sensitization temperature range for a long time; ③ Regular testing: Inspect the inner wall of the heat exchange tube every 6-12 months using methods such as endoscopy and ultrasonic thickness measurement to promptly detect early signs of corrosion; ④ Material upgrade: If corrosion occurs repeatedly, consider upgrading the material of the heat exchange tube to a more corrosion-resistant type (such as Hastelloy or titanium alloy, depending on the corrosiveness of the medium).
In short, the key to solving the corrosion perforation of stainless steel heat exchange tubes is to “first stop the leakage, then investigate the cause, then repair, and finally prevent” – not only through emergency treatment and repair to terminate the current fault, but also through root cause analysis and system optimization to eliminate the corrosion causes from the aspects of material, process, and working conditions, in order to ensure the long-term stable operation of the heat exchange system.
