How to protect stainless steel heat exchange tubes from corrosive environments?
By: mxgy,
When stainless steel heat exchange tubes encounter corrosive environments such as contact with media containing chloride ions or sulfides, or are subjected to high temperature, high humidity, and alternating stress conditions, protective measures should be taken from four aspects: strengthening the corrosion resistance of the material, optimizing the surface state, avoiding corrosion causes, and actively suppressing corrosion.
1. Accurately match corrosion-resistant materials
Select the appropriate stainless steel grade based on the characteristics of the corrosive medium. If the medium contains chloride ions (such as seawater or saltwater), preference should be given to 316L and 317L stainless steel containing molybdenum, or duplex stainless steel with stronger resistance to chloride ions (such as 2205 and 2507). These materials enhance their resistance to pitting corrosion caused by chloride ions through molybdenum elements; If the medium contains sulfides or is in a high-temperature and high-pressure environment, nickel based alloys (such as Inconel 600, Hastelloy C-276) can be selected, which have much higher resistance to intergranular corrosion and stress corrosion than ordinary stainless steel; For mildly corrosive environments (such as neutral water and weak acids), 304L stainless steel can be used to ensure corrosion resistance while controlling costs and avoiding waste caused by excessive material selection.
2. Strengthen surface passivation and modification treatment
Repair and enhance the passivation film (Cr ₂ O3 layer) on the surface of stainless steel. Before installation or during regular maintenance, use a 5% -20% concentration nitric acid solution (or nitric acid hydrofluoric acid mixed passivation solution, strictly control the ratio to prevent corrosion) to passivate the inner and outer surfaces of the heat exchanger tube. The soaking time should be adjusted according to the thickness of the tube wall (usually 10-30 minutes). After passivation, thoroughly rinse the residual liquid with deionized water, then dry or blow dry to form a denser and more stable passivation film; For scenarios with extremely high corrosion risks (such as strongly oxidizing media), surface coating technology can be used to coat a layer of titanium, ceramic, or polytetrafluoroethylene (PTFE) on the inner wall of the heat exchange tube. A physical isolation layer is used to block direct contact between the medium and stainless steel, avoiding chemical corrosion. The coating must ensure no pinholes, no detachment, and meet the standard of adhesion.
3. Optimize structural design and installation process
Reduce local corrosion triggers. During design, avoid the formation of narrow gaps between heat exchange tubes, tube sheets, and baffles (such as using expansion welding bonding technology, ensuring that the tube wall and tube sheet holes are tightly adhered during expansion, cleaning welding slag after welding, and preventing gap corrosion caused by liquid accumulation in the gaps); The opening size of the baffle plate should be accurately matched with the outer diameter of the heat exchange tube (with a gap controlled at 0.1-0.3mm), to avoid the formation of vortices and erosion of the tube wall by the medium at the gap. At the same time, the flow channel design should be optimized (such as adding guide plates and reducing fluid flow velocity to 1.5-3m/s) to reduce erosion and corrosion; During installation, avoid excessive bending or residual stress in the heat exchange tubes (such as using flexible supports and reserving space for thermal expansion and contraction) to prevent stress corrosion cracking. Especially for thin-walled heat exchange tubes (wall thickness<2mm), handle them gently to avoid collision and surface scratches that may damage the passivation film.
4. Control the medium environment and operating parameters
Reduce corrosion intensity. If the concentration of chloride ions and sulfides in the medium is too high, pre-treatment devices (such as ion exchange resin dechlorination and desulfurization tower desulfurization) can be installed in the system to control the concentration of harmful ions within a safe range (such as chloride ion concentration<200mg/L, specific reference should be made to the corrosion resistance threshold of stainless steel grades); Adjust the pH value of the medium to neutral or weakly alkaline (pH 6-9) to avoid accelerating electrochemical corrosion in acidic media; During operation, avoid overheating and overpressure (strictly control the temperature within the material’s tolerance range, such as long-term use of 316L stainless steel with a temperature not exceeding 800 ℃ and a pressure not exceeding 1.2 times the design pressure), prevent high temperature and high pressure from exacerbating corrosion reactions, and regularly discharge sedimentary impurities (such as sludge and scale) in the system to avoid corrosion under scale.
5. Adopt active anti-corrosion technology and regular operation and maintenance
Continuously monitor the corrosion status. For external corrosion (such as contact with corrosive cooling water on the outer surface of heat exchange tubes), cathodic protection can be applied in the system (such as sacrificial anode method, using zinc alloy anode to form an electric couple with the heat exchange tube, preferentially corroding the anode protection tube; or external current method, using DC power to make the heat exchange tube become the cathode). Cathodic protection needs to be regularly tested for polarization potential to ensure protection effectiveness; In daily operation and maintenance, ultrasonic thickness gauges are used regularly (every 3-6 months) to detect the wall thickness of heat exchange tubes and check for local thinning (if the wall thickness thinning rate exceeds 10%, it needs to be replaced in a timely manner). At the same time, the inner wall of the tube is observed through an endoscope for pitting corrosion and cracks, and any problems are promptly dealt with; When shutting down, it is necessary to empty the medium inside the pipe, blow it with dry nitrogen, and seal the pipe end to avoid residual moisture causing corrosion.
The above comprehensive protective measures can effectively reduce the corrosion rate of stainless steel heat exchange tubes in corrosive environments, extend their service life, and ensure the stable operation of the heat exchange system. Specific measures need to be flexibly adjusted according to the actual corrosion type (such as pitting corrosion, crevice corrosion, stress corrosion) and working conditions. If necessary, the protective effect can be verified through hanging plate tests before comprehensive application.
How to protect stainless steel heat exchange tubes from corrosive environments?
By: mxgy,
When stainless steel heat exchange tubes encounter corrosive environments such as contact with media containing chloride ions or sulfides, or are subjected to high temperature, high humidity, and alternating stress conditions, protective measures should be taken from four aspects: strengthening the corrosion resistance of the material, optimizing the surface state, avoiding corrosion causes, and actively suppressing corrosion.
1. Accurately match corrosion-resistant materials
Select the appropriate stainless steel grade based on the characteristics of the corrosive medium. If the medium contains chloride ions (such as seawater or saltwater), preference should be given to 316L and 317L stainless steel containing molybdenum, or duplex stainless steel with stronger resistance to chloride ions (such as 2205 and 2507). These materials enhance their resistance to pitting corrosion caused by chloride ions through molybdenum elements; If the medium contains sulfides or is in a high-temperature and high-pressure environment, nickel based alloys (such as Inconel 600, Hastelloy C-276) can be selected, which have much higher resistance to intergranular corrosion and stress corrosion than ordinary stainless steel; For mildly corrosive environments (such as neutral water and weak acids), 304L stainless steel can be used to ensure corrosion resistance while controlling costs and avoiding waste caused by excessive material selection.
2. Strengthen surface passivation and modification treatment
Repair and enhance the passivation film (Cr ₂ O3 layer) on the surface of stainless steel. Before installation or during regular maintenance, use a 5% -20% concentration nitric acid solution (or nitric acid hydrofluoric acid mixed passivation solution, strictly control the ratio to prevent corrosion) to passivate the inner and outer surfaces of the heat exchanger tube. The soaking time should be adjusted according to the thickness of the tube wall (usually 10-30 minutes). After passivation, thoroughly rinse the residual liquid with deionized water, then dry or blow dry to form a denser and more stable passivation film; For scenarios with extremely high corrosion risks (such as strongly oxidizing media), surface coating technology can be used to coat a layer of titanium, ceramic, or polytetrafluoroethylene (PTFE) on the inner wall of the heat exchange tube. A physical isolation layer is used to block direct contact between the medium and stainless steel, avoiding chemical corrosion. The coating must ensure no pinholes, no detachment, and meet the standard of adhesion.
3. Optimize structural design and installation process
Reduce local corrosion triggers. During design, avoid the formation of narrow gaps between heat exchange tubes, tube sheets, and baffles (such as using expansion welding bonding technology, ensuring that the tube wall and tube sheet holes are tightly adhered during expansion, cleaning welding slag after welding, and preventing gap corrosion caused by liquid accumulation in the gaps); The opening size of the baffle plate should be accurately matched with the outer diameter of the heat exchange tube (with a gap controlled at 0.1-0.3mm), to avoid the formation of vortices and erosion of the tube wall by the medium at the gap. At the same time, the flow channel design should be optimized (such as adding guide plates and reducing fluid flow velocity to 1.5-3m/s) to reduce erosion and corrosion; During installation, avoid excessive bending or residual stress in the heat exchange tubes (such as using flexible supports and reserving space for thermal expansion and contraction) to prevent stress corrosion cracking. Especially for thin-walled heat exchange tubes (wall thickness<2mm), handle them gently to avoid collision and surface scratches that may damage the passivation film.
4. Control the medium environment and operating parameters
Reduce corrosion intensity. If the concentration of chloride ions and sulfides in the medium is too high, pre-treatment devices (such as ion exchange resin dechlorination and desulfurization tower desulfurization) can be installed in the system to control the concentration of harmful ions within a safe range (such as chloride ion concentration<200mg/L, specific reference should be made to the corrosion resistance threshold of stainless steel grades); Adjust the pH value of the medium to neutral or weakly alkaline (pH 6-9) to avoid accelerating electrochemical corrosion in acidic media; During operation, avoid overheating and overpressure (strictly control the temperature within the material’s tolerance range, such as long-term use of 316L stainless steel with a temperature not exceeding 800 ℃ and a pressure not exceeding 1.2 times the design pressure), prevent high temperature and high pressure from exacerbating corrosion reactions, and regularly discharge sedimentary impurities (such as sludge and scale) in the system to avoid corrosion under scale.
5. Adopt active anti-corrosion technology and regular operation and maintenance
Continuously monitor the corrosion status. For external corrosion (such as contact with corrosive cooling water on the outer surface of heat exchange tubes), cathodic protection can be applied in the system (such as sacrificial anode method, using zinc alloy anode to form an electric couple with the heat exchange tube, preferentially corroding the anode protection tube; or external current method, using DC power to make the heat exchange tube become the cathode). Cathodic protection needs to be regularly tested for polarization potential to ensure protection effectiveness; In daily operation and maintenance, ultrasonic thickness gauges are used regularly (every 3-6 months) to detect the wall thickness of heat exchange tubes and check for local thinning (if the wall thickness thinning rate exceeds 10%, it needs to be replaced in a timely manner). At the same time, the inner wall of the tube is observed through an endoscope for pitting corrosion and cracks, and any problems are promptly dealt with; When shutting down, it is necessary to empty the medium inside the pipe, blow it with dry nitrogen, and seal the pipe end to avoid residual moisture causing corrosion.
The above comprehensive protective measures can effectively reduce the corrosion rate of stainless steel heat exchange tubes in corrosive environments, extend their service life, and ensure the stable operation of the heat exchange system. Specific measures need to be flexibly adjusted according to the actual corrosion type (such as pitting corrosion, crevice corrosion, stress corrosion) and working conditions. If necessary, the protective effect can be verified through hanging plate tests before comprehensive application.
