In-situ online chemical plating method for copper alloy heat transfer tubes in heat exchangers

Abstract

The present invention relates to an in-situ online chemical plating method for a copper alloy heat transfer tube of a heat exchanger. The present invention relates to the technical field of heat transfer tube inner surface protection and repair, and in particular to an in-situ online chemical plating process for heat transfer tubes in a heat exchanger. The in-situ online Ni-Cu-P-La-Y five-element chemical plating is performed on the copper alloy heat transfer tube of the heat exchanger to repair the corroded copper alloy heat transfer tube of the heat exchanger, improve its corrosion resistance, and extend its service life.

Description

Technical Field

[0001] The present invention relates to the technical field of heat transfer tube inner surface protection and repair, and in particular to an in-situ, online chemical plating process for heat transfer tubes within heat exchangers. Specifically, the present invention relates to a method for in-situ, online chemical plating of copper alloy heat transfer tubes within heat exchangers without removing the end caps. This method is used to repair corroded copper alloy heat transfer tubes online, improving their corrosion resistance and extending their service life. Background Art

[0002] Heat exchangers play a vital role in industry and are key equipment in heat exchange systems. They are widely used in fields such as electricity, petrochemicals, refrigeration, air conditioning, and seawater desalination. The performance of heat exchangers directly affects the efficiency and stability of the entire system. Copper alloy, as a common material for heat transfer tubes in heat exchangers, has good thermal conductivity and corrosion resistance. However, in flowing seawater media, copper alloys are extremely susceptible to erosion corrosion damage and cause local corrosion perforation, which seriously affects the performance and safe operation of the heat exchanger. Traditional repair methods usually require disassembly of the head and complex replacement or mechanical repair at a specialized maintenance site, which is not only time-consuming and labor-intensive, but also increases maintenance costs and equipment downtime. In addition, the disassembly and reinstallation process may cause secondary damage to the equipment itself, further affecting its long-term stability and reliability.

[0003] Heat exchangers contain hundreds or even thousands of heat transfer tubes. Disassembling and replacing the headers to repair a few severely corroded tubes inevitably results in significant waste of manpower and material resources. Therefore, developing a technology that can effectively repair corroded copper alloy heat transfer tubes on-site, without removing the headers, thereby extending their corrosion service life is extremely urgent and important, with significant industrial and strategic significance. This online, in-situ repair technology not only reduces downtime and maintenance costs, but also protects heat exchangers and their critical components from corrosion and provides more reliable and long-lasting equipment support for industrial production. Summary of the Invention

[0004] This invention patent is mainly aimed at the on-site in-situ online Ni-Cu-P-La-Y five-element chemical plating of the heat exchanger copper alloy heat transfer tube, in order to repair the corroded heat exchanger copper alloy heat transfer tube, improve its corrosion resistance and extend its service life.

[0005] The structure of the heat exchanger is mainly shell and tube heat exchanger (for your convenience, a brief schematic diagram is shown as follows Figure 1 As shown), the heat transfer tube is a copper alloy heat transfer tube. The present invention provides the following technical solutions:

[0006] Step 1: Use a truncated cone-shaped rubber plug with an axial through-hole passing through the axis to tightly plug the inlet and outlet of the single heat transfer tube to be repaired, thereby sealing the edge of the tube opening.

[0007] Insert one end of the silicone tube into (or through) the hole of the inlet rubber plug, and tightly plug the rubber plug to the inlet of the heat transfer tube. Connect the other end to the water outlet of a diaphragm pump that is resistant to acid, alkali, corrosion, and high temperature. This pipe serves as the water inlet pipe. Insert the other silicone tube into (or through) the hole of the outlet rubber plug, and tightly plug the rubber plug to the outlet of the heat transfer tube. Connect the other end to the inside of the container containing the solution. This pipe serves as the return pipe. The water inlet of the diaphragm pump is connected to the solution inside the container through a pipe (such as a silicone tube), and the diaphragm pump drives the solution to circulate between the heat transfer tube and the container.

[0008] Step 2: Fill the container with etching solution (5wt%~15wt% hydrochloric acid or 1wt%~5wt% sulfuric acid) to etch the inner wall of the heat transfer tube;

[0009] The inner diameter of a single heat transfer tube is 10~20mm and the length is 1~3m;

[0010] The container is placed in a constant temperature water bath for temperature control; the etching solution circulation time is 2-5 minutes, the temperature is 25-45°C, the flow rate is 1 L / min-3 L / min, and the etching solution volume is 1 L-1.5 L / m of heat transfer tube length;

[0011] After etching, fill the container with water and rinse with clean water for 3-5 minutes. The clean water volume is 1.5-2.5L / m of heat transfer tube length, the temperature is room temperature, and the flow rate is 2-3 L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to dry the inside of the heat transfer tube.

[0012] Step 3: Use a stainless steel brush or pipe brush (preferably a wire brush with bristles equal to or longer than the tube's inner diameter or radius) to polish the inner wall of the heat transfer tube until the inner wall is free of dirt and the base metal is exposed. After polishing, use an internal diameter gauge to measure the inner diameter of the heat transfer tube. You will find that the inner diameter of the heat transfer tube has increased significantly due to severe corrosion. Then, install inlet and outlet rubber plugs at the inlet and outlet of the heat transfer tube and fill the container with water. Rinse with clean water for 3-5 minutes at a volume of 1.5-2.5 L / m of heat transfer tube length, room temperature, and a flow rate of 2-3 L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to blow dry the heat transfer tube.

[0013] Step 4: Install inlet and outlet rubber stoppers at the inlet and outlet of the heat transfer tube. Place a roughening solution (20%-30% by volume nitric acid solution (65wt%-68wt%) + 5%-10% by volume hydrochloric acid solution (36wt%-38wt%) + balance water, or 10%-20% by volume nitric acid solution (65wt%-68wt%) + 10%-20% by volume sulfuric acid solution (95wt%-98wt%) + balance water) in a container to roughen the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control.

[0014] The roughening time is 2-5 min, the temperature is 25-45 °C, the flow rate is 1 L / min-3 L / min, and the roughening liquid volume is 1 L-1.5 L / m of heat transfer tube length;

[0015] After roughening, fill the container with water and rinse with clean water for 3-5 minutes. The clean water volume should be 1.5-2.5L / m of heat transfer tube length, the temperature should be room temperature, and the flow rate should be 2-3 L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to blow dry the heat transfer tube.

[0016] Step 5: Place iron-zinc alloy blocks inside the heat transfer tube. The iron-zinc alloy consists of 70 wt% to 90 wt% iron and the balance 10 wt% to 30 wt% zinc. Use polytetrafluoroethylene wire to string the iron-zinc alloy blocks together (preferably at equal intervals). The number of iron-zinc alloy blocks should be 5 to 10 per meter of heat transfer tube length. Ensure that the iron-zinc alloy blocks are large enough to fit inside the heat transfer tube and make contact with the tube inner wall. Do not remove the blocks during the entire electroless plating process.

[0017] Step 6: Install inlet and outlet rubber stoppers at the inlet and outlet of the heat transfer tube. Fill the container with chemical plating solution (in water, nickel salt 15-30 g / L, sodium hypophosphite 20-40 g / L, buffer 20-40 g / L, chelating agent 20-30 g / L, copper sulfate 0.5-2 g / L, lanthanum salt 0.1-0.2 g / L, yttrium salt 0.05-0.1 g / L, pH 7-8) to perform Ni-Cu-P-La-Y five-element chemical plating on the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. The chemical plating time is 0.5 h to 3 h, the temperature is 70-90 °C, the flow rate is 0.5 L / min to 2.5 L / min, and the chemical plating solution volume is 2 L to 5 L per meter of heat transfer tube length.

[0018] After the electroless plating is completed, remove the iron-zinc alloy block; fill the container with water and rinse with clean water for 3-5 minutes. The clean water volume is 1.5-2.5L / m of the heat transfer tube length, the temperature is room temperature, and the flow rate is 2-3 L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to blow dry the heat transfer tube.

[0019] The nickel salt is one or both of nickel sulfate and nickel chloride;

[0020] The buffer is one or more of sodium acetate, ammonium acetate, and boric acid;

[0021] The complexing agent is one or both of sodium citrate and lactic acid;

[0022] The lanthanum salt is one or both of lanthanum nitrate and lanthanum chloride;

[0023] The yttrium salt is one or both of yttrium nitrate and yttrium chloride;

[0024] The present invention has the following beneficial effects and advantages:

[0025] 1. Carry out in-situ online protection of the copper alloy heat transfer tubes of the heat exchanger on site.

[0026] Reduce downtime and production losses and save maintenance costs: Removing and reinstalling heat transfer tubes is not only time-consuming but also requires additional manpower and equipment resources.

[0027] Maintaining equipment integrity and stability: Inline protection technology does not require the removal or disassembly of major equipment components, such as heads, thus maintaining the integrity and stability of the equipment. This is particularly important for industrial equipment with high uptime requirements and cannot be easily shut down.

[0028] 2. Use five-element chemical plating instead of simple one-element or two-element plating.

[0029] The addition of Cu, La, and Y significantly improves the corrosion resistance of the coating. Copper has good corrosion resistance, while La and Y improve the crystal structure and grain refinement of the coating, thereby reducing defects and increasing the hardness of the coating, improving its wear resistance and abrasion resistance.

[0030] 3. Combine mechanical grinding and roughening in the pre-treatment process instead of using only one process.

[0031] Comprehensive surface roughness control:

[0032] Mechanical grinding typically smoothes the surface, removing larger bumps and defects, thereby reducing localized extreme unevenness. Roughening, on the other hand, creates more tiny grooves or bumps at the microscopic level, increasing the overall surface roughness. Combining mechanical grinding and roughening effectively adds microstructure to a smoother surface, achieving both a certain degree of flatness and an appropriate degree of roughness, which is crucial for improving the adhesion of electroless plating.

[0033] Surface Cleaning and Activity Enhancement:

[0034] Mechanical polishing can completely remove surface oxides, dirt, and deposits, providing a clean substrate for subsequent chemical treatments or coatings. Roughening processes can improve the adhesion and effectiveness of coatings or chemical treatments by increasing surface active sites and roughness. Combining mechanical polishing and roughening processes, the clean surface combined with a rough microstructure can enhance surface activity while strengthening the physical and mechanical anchoring effect of the chemical plating layer, significantly improving the bonding strength between the coating and the substrate.

[0035] 4. Use a special iron-zinc alloy to induce the copper alloy heat transfer tube. During the chemical plating process, the iron-zinc alloy is always kept induced and not removed.

[0036] By precisely controlling the composition of the iron-zinc alloy, its electrochemical performance during the electroless plating process is improved. This includes better electron release and higher reduction capacity for ions in the plating solution, thereby promoting uniform deposition of the coating and improving the quality of the coating.

[0037] Not removing the iron-zinc alloy can lead to continuous electron release, improve the uniformity and thickness of the coating, and save operating costs and time.

[0038] 5. Carry out chemical plating under full flow conditions.

[0039] Uniform coating formation: Continuous liquid flow effectively promotes uniform mixing and distribution of the plating solution within the pipeline. This helps reduce liquid stagnation and local concentration differences within the pipeline, thereby improving the uniformity of the coating.

[0040] Reduce bubble phenomenon: Flow can effectively reduce the aggregation and formation of bubbles in the heat transfer tube and reduce the number of pinholes in the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the heat exchanger device and solution pipeline. In the figure, 1 is the sealing joint, 2 is the manhole / handhole, 3 is the water inlet pipeline, 4 is the acid, alkali, corrosion and high temperature resistant diaphragm pump, 5 is the device solution (etching solution, roughening solution, clean water, chemical plating solution) container, 6 is the return water pipeline, and 7 is the heat transfer pipe. DETAILED DESCRIPTION

[0042] Example 1

[0043] This method is used for in-situ electroless plating of copper alloy heat transfer tubes in heat exchangers. The tubes are made of B30 alloy, have a length of 2 meters, and an inner diameter of 12 mm and are covered with copper rust and dirt. This method eliminates the need to disassemble or move any components of the heat exchanger, saving time during the overall repair process and reducing downtime while also helping to maintain the integrity and stability of the equipment. The specific process flow is as follows:

[0044] Step 1: Use a truncated cone-shaped rubber plug with an axial through-hole in the middle to tightly plug the inlet and outlet of the single heat transfer tube to be repaired, thereby sealing the edge of the tube opening.

[0045] Insert one end of the silicone tube into (or through) the hole of the inlet rubber plug, and tightly plug the rubber plug to the inlet of the heat transfer tube. Connect the other end to the water outlet of a diaphragm pump that is resistant to acid, alkali, corrosion, and high temperature. This pipe serves as the water inlet pipe. Insert the other silicone tube into (or through) the hole of the outlet rubber plug, and tightly plug the rubber plug to the outlet of the heat transfer tube. Connect the other end to the inside of the container containing the solution. This pipe serves as the return pipe. The water inlet of the diaphragm pump is connected to the solution inside the container through a pipe (such as a silicone tube), and the diaphragm pump drives the solution to circulate between the heat transfer tube and the container.

[0046] Step 2: Fill the container with etching solution (10wt% hydrochloric acid) and etch the inner wall of the heat transfer tube;

[0047] The container was placed in a constant temperature water bath for temperature control; the etching solution was circulated for 2 min at a temperature of 25°C, a flow rate of 1 L / min, and a volume of 2 L / m;

[0048] After etching, fill the container with water and rinse with clean water for 3 minutes. The clean water volume is 4L, the temperature is room temperature, and the flow rate is 3L / min. After cleaning, remove the inlet and outlet rubber plugs, and use an air compressor to blow air dry inside the heat transfer tube.

[0049] Step 3: Use a stainless steel pipe brush (the length of a single steel wire brush is equal to the inner diameter of the tube) to polish the inner wall of the heat transfer tube until the inner wall of the tube is free of dirt and the base metal is exposed. After polishing, use an internal diameter gauge to measure the inner diameter of the heat transfer tube. It can be found that the inner diameter of the heat transfer tube has increased significantly due to severe corrosion. Then, install inlet and outlet rubber plugs at the inlet and outlet of the heat transfer tube, and fill the container with water. Rinse with clean water for 3 minutes. The volume of clean water is 4L, the temperature is room temperature, and the flow rate is 3L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to blow dry the heat transfer tube.

[0050] Step 4: Install inlet and outlet rubber stoppers at the inlet and outlet of the heat transfer tube. Place a container in a container and use a roughening solution (10% by volume nitric acid solution (67 wt%) + 20% by volume sulfuric acid solution (97 wt%) + the balance water) to roughen the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. Roughening time: 3 minutes, temperature: 35°C, flow rate: 2 L / min, and the roughening solution volume: 3 L.

[0051] After roughening, fill the container with water and rinse with clean water for 4 minutes. The clean water volume is 4L, the temperature is room temperature, and the flow rate is 3L / min. After cleaning, remove the inlet and outlet rubber plugs, and use an air compressor to blow dry the heat transfer tube with air.

[0052] Step 5: Place iron-zinc alloy blocks inside the heat transfer tube. The iron-zinc alloy is composed of 90wt% iron and 10wt% zinc. Use polytetrafluoroethylene wire to string together 20 iron-zinc alloy blocks (preferably at equal intervals). Ensure that the iron-zinc alloy blocks are large enough to fit inside the heat transfer tube and come into contact with the inner wall of the tube. Do not remove the blocks during the entire electroless plating process.

[0053] Step 6: Install inlet and outlet rubber stoppers at the inlet and outlet of the heat transfer tube. Fill the container with chemical plating solution (in water, 20 g / L nickel sulfate, 40 g / L sodium hypophosphite, 25 g / L sodium acetate, 30 g / L sodium citrate, 0.6 g / L copper sulfate, 0.12 g / L lanthanum chloride, 0.07 g / L yttrium chloride, pH 7) to perform Ni-Cu-P-La-Y five-element chemical plating on the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. The chemical plating time is 2 hours, the temperature is 80°C, the flow rate is 2.5 L / min, and the chemical plating solution volume is 4 L.

[0054] After the chemical plating is completed, the iron-zinc alloy block is taken out; water is filled into the container and cleaned with clean water for 5 minutes. The clean water volume is 4L, the temperature is room temperature, and the flow rate is 3L / min; after cleaning, the inlet and outlet rubber plugs are removed, and the heat transfer tube is blown dry with air using an air compressor.

[0055] Example 2

[0056] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating solution composition in step 6 is changed to 15 g / L nickel sulfate and 20 g / L sodium hypophosphite, and the remaining operations are consistent with step 6.

[0057] Example 3

[0058] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating solution composition in step 6 is changed to 30 g / L nickel sulfate and 40 g / L sodium hypophosphite, and the remaining operations are consistent with step 6.

[0059] Example 4

[0060] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating temperature in step 6 is changed to 90° C., and the remaining operations are the same as those in step 6.

[0061] Example 5

[0062] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating temperature in step 6 is changed to 3 h, and the remaining operations are the same as those in step 6.

[0063] Example 6

[0064] The process and conditions were the same as in Example 1, except that the heat transfer tube was 1 m long and had an inner diameter of 10 mm;

[0065] Step 2: Fill the container with an etching solution (5wt% hydrochloric acid) and etch the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. Circulate the etching solution for 3 minutes at a temperature of 35°C, a flow rate of 2L / min, and a volume of 1.2L / m. After etching, fill the container with water and rinse with clean water for 4 minutes. The volume of clean water is 1.5L, the temperature is room temperature, and the flow rate is 2L / min.

[0066] Step 3: Rinse with clean water for 4 minutes. The volume of clean water is 1.5 L, the temperature is room temperature, and the flow rate is 2 L / min.

[0067] Step 4: Roughen the inner surface of the heat transfer tube using a roughening solution (15% volume fraction nitric acid solution (65wt%) + 10% volume fraction sulfuric acid solution (95wt%) + balance water). Place the container in a constant temperature water bath for temperature control. Roughening time: 2 min, temperature: 25°C, flow rate: 1 L / min, and volume of roughening solution: 1 L.

[0068] After roughening, clean water was used for washing for 3 min. The volume of clean water was 1.5 L, the temperature was room temperature, and the flow rate was 2 L / min.

[0069] Step 5: Place five iron-zinc alloy blocks in the heat transfer tube. The iron-zinc alloy consists of 70 wt% iron and 30 wt% zinc.

[0070] Step 6: Use a chemical plating solution (in water, nickel sulfate 15g / L, sodium hypophosphite 20g / L, sodium acetate 20g / L, sodium citrate 20g / L, copper sulfate 0.5g / L, lanthanum chloride 0.1g / L, yttrium chloride 0.05g / L, pH 7.5) to perform Ni-Cu-P-La-Y five-element chemical plating on the inner wall of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. The chemical plating time is 0.5h, the temperature is 70°C, the flow rate is 1.5L / min, and the chemical plating solution volume is 3L.

[0071] After the chemical plating is completed, clean water is used for cleaning for 3 minutes. The volume of clean water is 1.5L, the temperature is room temperature, and the flow rate is 2L / min.

[0072] Example 7

[0073] The process and conditions are the same as in Example 1, except that the heat transfer tube is 3 m long and has an inner diameter of 20 mm;

[0074] Step 2: Fill the container with an etching solution (15wt% hydrochloric acid) and etch the inner surface of the heat transfer tube. Place the container in a constant temperature water bath for temperature control. Circulate the etching solution for 5 minutes at a temperature of 45°C, a flow rate of 3L / min, and a volume of 4.5L / m. After etching, fill the container with water and rinse with clean water for 5 minutes. The volume of clean water is 7.5L, the temperature is room temperature, and the flow rate is 2.5L / min.

[0075] Step 3: Rinse with clean water for 5 minutes. The volume of clean water is 7.5 L, the temperature is room temperature, and the flow rate is 2.5 L / min.

[0076] Step 4: Roughen the inner surface of the heat transfer tube using a roughening solution (20% volume fraction nitric acid solution (68wt%) + 15% volume fraction sulfuric acid solution (98wt%) + balance water). Place the container in a constant temperature water bath for temperature control. Roughening time: 5 min, temperature: 45°C, flow rate: 3 L / min, and volume of roughening solution: 3.6 L.

[0077] After roughening, clean water was used for washing for 5 min. The volume of clean water was 7.5 L, the temperature was room temperature, and the flow rate was 2.5 L / min.

[0078] Step 5: Place 21 iron-zinc alloy blocks in the heat transfer tube. The iron-zinc alloy consists of 80 wt% iron and 20 wt% zinc.

[0079] Step 6: Use a chemical plating solution (in water, 30 g / L nickel sulfate, 30 g / L sodium hypophosphite, 40 g / L sodium acetate, 25 g / L sodium citrate, 2 g / L copper sulfate, 0.2 g / L lanthanum chloride, 0.1 g / L yttrium chloride, pH 8) to electrolessly plate the inner wall of the heat transfer tube with a nickel-copper-phosphite-ladenine-yellow ...

[0080] After the chemical plating is completed, clean water is used for cleaning for 4 minutes. The volume of clean water is 7.5L, the temperature is room temperature, and the flow rate is 2.5L / min.

[0081] Performance evaluation:

[0082] Corrosion resistance: Cut a 1cm long section radially from the plated heat transfer tube and seal it with Wacker SN waterproof glass glue (Germany WACKER (WACKER) Glass glue ) Cover all areas except the inner wall of the tube and let it stand for 24 hours. After the waterproof sealing glass glue is completely solidified, the sample is obtained. The sample is immersed in a solution with a mass concentration of 3.5% NaCl for 7 days. The mass of the coated sample before and after immersion is measured using a balance. The inner diameter of the sample before immersion is measured using a vernier caliper. The corrosion rate is calculated using the following formula, where R is the corrosion rate (mm / a), M is the mass of the sample before immersion (g), M1 is the mass of the sample after immersion (g), and S is the surface area of ​​the inner wall of the sample (cm 2 ), T is the immersion time (h), D is the density of the coating material, which is 8.2g / cm 3 ;

[0083] .

[0084] Bonding Strength: Cut a 0.5cm long section radially from a plated heat transfer tube and split it longitudinally in the middle axially. Use the two sections as specimens. Use pliers to bend the specimen as quickly as possible, first to one side, then to the other, until it breaks. Examine the fracture surface to determine if the deposited layer has delaminated or can be removed with a knife or chisel. Results are as follows: strong bonding (no delamination), moderate bonding (point-like delamination), or poor bonding (strip-like delamination). Among them, point peeling refers to the appearance of small peeling areas similar to small holes or spots. The diameter of small point peeling is about 0.1-0.5mm, the diameter of medium point peeling is about greater than 0.5 to 2mm, and the diameter of large point peeling is greater than 2mm to 5mm; strip peeling refers to the appearance of linearly distributed strip peeling areas. The width of narrow strip peeling is about 0.1-1mm and the length is 5-20mm. The width of medium strip peeling is about greater than 1 to 5mm and the length is 10-50mm. The width of wide strip peeling exceeds 5mm and the length is 50-100mm.

[0085] Hardness: (1) Cut a 0.3 cm long tube segment from the plated heat transfer tube along the radial direction, place the tube segment flat in a metallographic cold mounting mold, and pour Osborne 150 epoxy resin liquid into the mold ( Osborne 150 epoxy resin Potting glue), let it stand for 24 hours to allow the epoxy resin to completely solidify, and obtain a cross-sectional sample. Then, use 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 sandpaper to grind the cross section in sequence. Each time the sandpaper is changed, the grinding direction should be changed by 90°. The grinding process is continued until the scratches in the direction of the previous model are completely eliminated.

[0086] (2) Apply diamond polishing paste with a diamond particle size of 1 to 3.5 μm on the polishing wheel and polish the cross section to eliminate all scratches for the final observation.

[0087] (3) Use a micro Vickers hardness tester to measure the cross-sectional hardness of the Ni-P coating. Set the indenter load to 200 mN, the load loading and unloading time to 20 s, select 10 locations for testing, and take the average value.

[0088] Porosity: Cut a 1cm long section radially from the plated heat transfer tube and split it longitudinally in the middle along the axial direction. The two sections are used as samples. First, prepare an aqueous solution containing 10g / L potassium ferrocyanide and 10g / L sodium chloride. Soak the filter paper with this solution. After cleaning the sample, apply the soaked filter paper to the inner wall of the sample. After 5 minutes, remove the filter paper from the sample surface, rinse it with distilled water, and place it on glass. After drying, calculate the porosity. Porosity = n / s (voids / cm 2), where n is the total number of pores (number), s is the area of ​​the coating under inspection (cm 2 When the spot diameter is ≤1 mm, one spot is counted as one pore; when the diameter is between 1 and 3 mm, one spot is counted as three pores; when the diameter is greater than 3 mm, one spot is counted as 10 pores.

[0089] Thickness: (1) Cut a 0.3 cm long section of the plated heat transfer tube radially, place the section flat in a metallographic cold mounting mold, pour Osborne 150 epoxy resin liquid into the mold, let it stand for 24 hours until the epoxy resin is completely solidified, and obtain a cross-sectional sample. Then use 240, 320, 500, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 sandpaper to grind the cross section in sequence. Each time the sandpaper is changed, the grinding direction should be changed by 90°. Grind until the scratches in the previous model direction are completely eliminated.

[0090] (2) Apply diamond polishing paste with a diamond particle size of 1 to 3.5 μm on the polishing wheel and polish the cross section to eliminate all scratches for the final observation.

[0091] (3) Use a high-magnification metallographic microscope to observe the overall profile of the sample to determine the coating thickness, and select 5 locations to measure the coating thickness and take the average value.

[0092] Uniformity: Prepare 5 cross-section samples to evaluate the coating thickness and calculate the standard deviation SD of the 5 coating thicknesses, where T 1~ T 5 are the thickness values ​​of 5 coating layers respectively. A The average thickness of the five coating layers is as follows: when SD ≤ 0.5 μm, the uniformity is excellent; when 0.5 μm < SD ≤ 1 μm, the uniformity is medium; and when 1 μm < SD, the uniformity is poor.

[0093]

[0094] Comparative Example 1:

[0095] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating solution composition in step 6 is changed to 20 g / L nickel sulfate, 40 g / L sodium hypophosphite, 25 g / L sodium acetate, 30 g / L sodium citrate, and the pH value is 7, and the remaining operations are consistent with step 6.

[0096] Comparative Example 2:

[0097] The process and conditions are the same as those in Example 1, except that: Steps 1 to 5 are the same as those in Example 1, and in Step 6, after the heat transfer tube is filled with the chemical plating solution using a diaphragm pump, the pump is stopped and the chemical plating is allowed to stand. The remaining operations are the same as those in Step 6.

[0098] Comparative Example 3:

[0099] The process and conditions are the same as those in Example 1, except that steps 1 to 4 and 6 are the same as those in Example 1, and the special alloy composition in step 5 is changed to pure iron (100% iron).

[0100] Comparative Example 4:

[0101] The process and conditions are the same as those in Example 1, except that steps 1, 2, 4, 5, and 6 are the same as those in Example 1, and step 3 is deleted. That is, steps 1, 2, 4, 5, and 6 are performed in sequence.

[0102] Comparative Example 5:

[0103] The process and conditions are the same as those in Example 1, except that: Step 1, Step 2, Step 3, Step 5 and Step 6 are the same as those in Example 1, and Step 4 is deleted, that is, Step 1, Step 2, Step 3, Step 5 and Step 6 are performed in sequence.

[0104] Comparative Example 6:

[0105] The process and conditions are the same as those in Example 1, except that: steps 1 to 3 and step 6 are the same as those in Example 1, and the iron-zinc alloy in step 5 is removed after the chemical plating time in step 6 lasts for 1 hour, and then the chemical plating is continued for 2 hours. The remaining operations are the same as those in step 6.

[0106] Comparative Example 7:

[0107] The process and conditions are the same as those in Example 1, except that: steps 1 to 5 are the same as those in Example 1, the chemical plating solution composition in step 6 is changed to 10 g / L nickel sulfate, 50 g / L sodium hypophosphite, 10 g / L sodium acetate, 40 g / L sodium citrate, 1.1 g / L copper sulfate, 0.4 g / L lanthanum chloride, 0.3 g / L yttrium chloride, and the pH value is 5.5, and the remaining operations are the same as those in step 6.

[0108] Comparative Example 8:

[0109] The process and conditions are the same as those in Example 1, except that: steps 1 to 5 are the same as those in Example 1, the chemical plating solution composition in step 6 is changed to 40 g / L nickel sulfate, 50 g / L sodium hypophosphite, 45 g / L sodium acetate, 35 g / L sodium citrate, 0.9 g / L copper sulfate, 0.3 g / L lanthanum chloride, 0.2 g / L yttrium chloride, and the pH value is 8.5, and the remaining operations are the same as those in step 6.

[0110] Comparative Example 9:

[0111] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the flow rate in step 6 is changed to 3 L / min, and the remaining operations are the same as those in step 6.

[0112] Comparative Example 10:

[0113] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the chemical plating temperature in step 6 is changed to 50° C., and the remaining operations are the same as those in step 6.

[0114] Comparative Example 11:

[0115] The process and conditions are the same as those in Example 1, except that steps 1 to 5 are the same as those in Example 1, the electroless plating time in step 6 is changed to 5 hours, and the remaining operations are the same as those in step 6.

[0116] Table 1 Performance comparison

[0117] Case Corrosion rate (mm / a) Binding force Hardness (HV) <![CDATA[Porosity (n / cm 2 )]]> Thickness (μm) Uniformity Example 1 0.005 powerful 698 0.01 25 excellent Example 2 0.006 powerful 659 0.01 20 excellent Example 3 0.003 powerful 712 0.01 30 excellent Example 4 0.002 powerful 683 0 35 excellent Example 5 0.003 powerful 670 0 35 excellent Example 6 0.004 powerful 704 0.01 10 excellent Example 7 0.005 powerful 723 0 37 excellent Comparative Example 1 0.12 middle 390 0.6 18 middle Comparative Example 2 0.15 Difference 500 1.5 20 Difference Comparative Example 3 0.13 middle 420 0.8 10 middle Comparative Example 4 0.23 Difference 300 1.8 12 middle Comparative Example 5 0.20 Difference 325 1.4 13 middle Comparative Example 6 0.14 middle 410 1.0 15 Difference Comparative Example 7 0.19 middle 396 0.9 20 middle Comparative Example 8 0.25 Difference 399 0.3 19 Difference Comparative Example 9 0.33 Difference 300 1.2 21 Difference Comparative Example 10 0.27 Difference 320 1.1 10 Difference Comparative Example 11 0.24 Difference 344 1.6 40 Difference

[0118] Examples 1 to 7 respectively used different process parameters within the scope of the claims, and the performance results showed that the examples within the scope of protection of the claims had excellent coating effects and rationality.

[0119] For Comparative Example 1, based on Example 1, the chemical plating layer was changed from Ni-Cu-P-La-Y five-element plating layer to Ni-P two-element plating layer. The performance results showed that the corrosion resistance and hardness of the chemical plating layer were significantly deteriorated, which indicates that the Cu, La and Y elements can significantly improve the corrosion resistance of the coating, reduce defects and enhance the hardness of the coating.

[0120] Comparative Example 2, based on Example 1, switched from flow plating to static plating. Performance results showed that the porosity and uniformity of the electroless plating layer significantly deteriorated. This demonstrates that flow electroless plating effectively reduces liquid stagnation and local concentration variations within the pipe, thereby improving the uniformity of the coating. It also effectively reduces the accumulation and formation of bubbles within the heat transfer tube, reducing porosity.

[0121] For Comparative Example 3, based on Example 1, the iron-zinc alloy was replaced with pure iron. The performance results showed that the uniformity of the chemical plating layer was significantly deteriorated, which indicates that the iron-zinc alloy can promote the uniform deposition of the coating and improve the quality of the coating.

[0122] For Comparative Examples 4 and 5, based on Example 1, the roughening process and mechanical polishing process were deleted respectively. The performance results showed that the bonding strength of the chemical plating layer was significantly reduced, which indicates that the combination of mechanical polishing and roughening processes can effectively improve the bonding strength between the coating and the substrate.

[0123] For Comparative Example 6, based on Example 1, the iron-zinc alloy was removed during the chemical plating process. The performance results showed that the thickness and uniformity of the chemical plating layer deteriorated significantly, which indicates that not removing the iron-zinc alloy can lead to continuous electron release to improve the uniformity and thickness of the plating layer.

[0124] For Comparative Examples 7 to 11, based on Example 1, unreasonable chemical plating solution composition, chemical plating flow rate, chemical plating temperature and chemical plating time were selected respectively. The performance results showed that the various properties of the coating were significantly reduced, which indicates that the process parameters within the scope of protection of the claims have excellent performance effects.

Claims

1. An in-situ online chemical plating method for copper alloy heat transfer tubes of heat exchangers, characterized by: Step 1): Use a truncated cone-shaped rubber plug with an axial through-hole passing through the center axis to tightly plug the inlet and outlet of a single heat transfer tube to be repaired, thereby sealing the edge of the tube opening; the heat transfer tube has an inner diameter of 10-20 mm, a length of 1-3 m, and is made of B30 alloy; Insert one end of the silicone tube into or through the hole of the inlet rubber plug, and tightly plug the rubber plug to the inlet of the heat transfer tube. Connect the other end to the water outlet of a diaphragm pump that is resistant to acid, alkali, corrosion, and high temperature. This pipe serves as the water inlet pipe. Insert one end of the other silicone tube into or through the hole of the outlet rubber plug, and tightly plug the rubber plug to the outlet of the heat transfer tube. Connect the other end to the inside of the container containing the solution. This pipe serves as the return pipe. The water inlet of the diaphragm pump is connected to the solution inside the container through the pipe, and the diaphragm pump drives the solution to circulate between the heat transfer tube and the container. Step 2): Fill the container with etching liquid and etch the inner wall of the heat transfer tube; After etching is completed, fill the container with water and use the water to clean the inside of the heat transfer tube. After cleaning, blow dry the inside of the heat transfer tube. Step 3): Use a stainless steel brush or pipe brush to polish the inner wall of the heat transfer tube until there is no dirt on the inner wall of the tube and the base metal is exposed; Then, install the inlet and outlet rubber plugs at the inlet and outlet of the heat transfer tube, fill the container with water, use the water to clean the inside of the heat transfer tube, and blow dry the inside of the heat transfer tube after cleaning; Step 4): Installing inlet and outlet rubber plugs at the inlet and outlet of the heat transfer tube, respectively. A roughening liquid is placed in a container and used to roughen the inner wall of the heat transfer tube. The roughening liquid comprises a 10% to 20% by volume nitric acid solution, a 10% to 20% by volume sulfuric acid solution, and water. The concentration of the nitric acid solution is 65% to 68% by weight, and the concentration of the sulfuric acid solution is 95% to 98% by weight. After the roughening is completed, fill the container with water and use the water to clean the inside of the heat transfer tube. After cleaning, blow dry the inside of the heat transfer tube. Step 5): Place iron-zinc alloy blocks in the heat transfer tube. The iron-zinc alloy consists of 70-90 wt% iron and 10-30 wt% zinc. Use polytetrafluoroethylene thread to string the iron-zinc alloy blocks together. The number of iron-zinc alloy blocks is 5-10 per meter of heat transfer tube length. Ensure that the iron-zinc alloy blocks are large enough to fit inside the heat transfer tube and contact the inner wall of the tube. Do not remove them during the entire electroless plating process. Step 6): Install inlet and outlet rubber plugs at the inlet and outlet of the heat transfer tube, place a chemical plating solution in a container, and perform Ni-Cu-P-La-Y five-element chemical plating on the inner wall of the heat transfer tube using the chemical plating solution; Chemical plating solution composition: in water, nickel sulfate 15-30g / L, sodium hypophosphite 20-40g / L, buffer sodium acetate 20-40g / L, complexing agent sodium citrate 20-30g / L, copper sulfate 0.5-2g / L, lanthanum chloride 0.1-0.2g / L, yttrium chloride 0.05-0.1g / L, pH value 7-8; The container is placed in a constant temperature water bath for temperature control; the electroless plating time is 0.5h to 3h, the temperature is 70 to 90°C, the flow rate is 0.5L / min to 2.5L / min, and the volume of the electroless plating solution is 2L to 5L / m of heat transfer tube length; After the chemical plating is completed, water is filled into the container and the inside of the heat transfer tube is cleaned with water. After cleaning, the inside of the heat transfer tube is blown dry.

2. The in-situ online chemical plating method for heat exchanger copper alloy heat transfer tubes according to claim 1, characterized in that: In step 2), the etching solution is 5 wt% to 15 wt% hydrochloric acid; The container is placed in a constant temperature water bath for temperature control; the etching solution circulation time is 2 to 5 minutes, the temperature is 25 to 45°C, the flow rate is 1L / min to 3L / min, and the etching solution volume is 1L to 1.5L / m of heat transfer tube length.

3. The in-situ online chemical plating method for heat exchanger copper alloy heat transfer tubes according to claim 1, characterized in that: In step 4), the container is placed in a constant temperature water bath for temperature control; the roughening time is 2 to 5 minutes, the temperature is 25 to 45° C., the flow rate is 1 L / min to 3 L / min, and the roughening liquid volume is 1 L to 1.5 L / m of heat transfer tube length.

4. The in-situ online chemical plating method for heat exchanger copper alloy heat transfer tubes according to claim 1, characterized in that: In step 2), 3), 4) or 6), the process of filling the container with water, cleaning the interior of the heat transfer tube with the water, and then drying the interior of the heat transfer tube after cleaning is as follows: Fill the container with water and use water to clean for 3 to 5 minutes. The water volume is 1.5-2.5L / m of heat transfer tube length, the temperature is room temperature, and the flow rate is 2-3L / min. After cleaning, remove the inlet and outlet rubber plugs and use an air compressor to blow dry the heat transfer tube with air.

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