Anti-leakage treatment process for welded inner wall of tank body

Abstract

The invention discloses an anti-leakage treatment process for a welded inner wall of a tank body. The process sequentially comprises the following steps: degreasing and removing oil by adopting an alkali wash solution; argon tungsten-arc welding root welding and surface welding are carried out under the protection of internal argon filling; 100% radiographic testing and penetration testing are carried out after welding; a welding seam is ground and polished to the specified roughness; pickling paste containing nitric acid and hydrofluoric acid is used for local pickling; carrying out full-tank passivation by adopting a citric acid / hydrogen peroxide or nitric acid solution; leak detection and repair are carried out under the vacuum box condition; and finally, carrying out a water containing test and blue point method detection. According to the process, through combination of chemical treatment and physical leakage detection, the pitting corrosion resistance and the sealing performance of the weld joint are improved, the problems that in a traditional process, the blue point reject ratio is high, and the water test rework rate is large are solved, and the process has the advantages of being clear in process parameter, high in repeatability and high in industrial applicability.

Description

Technical Field

[0001] This invention belongs to the field of surface chemical treatment technology, specifically relating to a process for preventing leakage of the welded inner wall of a tank. Background Technology

[0002] Storage tanks are widely used in the petrochemical, grain and oil, and chemical industries. If leakage occurs on their welded inner walls, it will not only cause loss of media, but also pose safety and environmental risks. Existing post-weld leakage prevention processes mostly rely on conventional cleaning, pickling, and detection methods. However, under complex operating conditions, it is often difficult to balance weld strength, inner wall passivation effect, and leakage detection accuracy.

[0003] Most existing food-grade stainless steel tanks control quality through GTAW (Glass Surface Abrasive) pretreatment, radiographic testing (RT) / PT (Penetrating Permeability) testing, and water-filling tests. However, the heat-affected zone of the weld is prone to chromium depletion and micropores, leading to failure in the blue dot test or leakage risks after long-term service. Therefore, there is a need to propose a parameterized, highly operable process that can significantly improve the corrosion resistance and sealing performance of the tank to address the shortcomings of existing processes in the application of food-grade stainless steel tanks.

[0004] A search revealed a Chinese invention patent with publication number CN111717556A, which discloses a construction process for preventing oil tank leakage. This process, involving rust removal, coating, and leakage detection of the tank wall and weld areas, primarily addresses leakage problems caused by weld defects and corrosion during the service life of oil tanks. The method emphasizes ease of construction and the overall protection of large-area oil tank structures.

[0005] The technical comparison between the above application and this application is as follows: That application focuses on leak-proof measures during the construction of the overall structure of oil tanks, and its detection methods mainly rely on conventional leak inspection and anti-corrosion coating protection. In contrast, this application targets the leak-proof treatment of the inner wall of stainless steel storage tanks after welding, emphasizing the refinement of post-weld surface treatment, the integrity of the passivation film, and micro-defect leak detection methods. The technical objects and key control parameters are fundamentally different.

[0006] A search revealed that Chinese invention patent CN109306493A discloses an acid pickling and passivation surface treatment process for UF6 storage and transportation containers. This technology focuses on the high requirements of radioactive material containers, proposing a multi-step process involving alkaline washing, acid washing, overall acid washing, and passivation to ensure that the container's interior does not corrode or become contaminated during the storage of UF6.

[0007] The technical comparison between the above application and this application is as follows: That application pertains to the field of transport containers for radioactive materials, addressing the issues of material purity and corrosion resistance in highly corrosive environments containing radioactive media. Its focus is on chemical formulations for decontamination and surface passivation. In contrast, this application targets leak-proof treatment of the weld zone after welding, emphasizing a system combining weld defect elimination, passivation film detection, and hydrostatic testing with vacuum leak detection. The two applications differ significantly in their target audience, technical objectives, and testing methods.

[0008] A search revealed that Chinese invention patent CN110261042A discloses a process for detecting leaks in the welds of oil storage tanks. This process focuses on using improved detection methods to quickly identify potential leaks in the welds of oil storage tanks, ensuring the long-term safe operation of the tanks. The detection methods include vacuum leak detection and oil penetration testing.

[0009] The technical comparison between the above application and this application is as follows: That application emphasizes improvements in detection methods to address the difficulty in timely detection of weld leaks in oil storage tanks, focusing on the sensitivity and efficiency of leak detection. In contrast, this application's process route not only includes vacuum leak detection but also systematically integrates post-weld grinding, pickling and passivation, neutralization cleaning, water-filling tests, and passivation film testing, forming a complete closed-loop leak prevention treatment system. Compared to the two, this application places greater emphasis on optimizing the entire process from welding to surface treatment to detection.

[0010] Therefore, this application proposes a process for preventing leakage of the welded inner wall of a tank, which aims to solve the problems of insufficient post-weld treatment and inadequate combination of leak detection and passivation in the prior art, and to ensure the long-term sealing and safety of the tank under complex working conditions. Summary of the Invention

[0011] To address the aforementioned problems, this invention aims to provide a process for preventing leakage on the welded inner wall of a tank, thereby resolving the issues present in the prior art.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows: A process for preventing leakage of the welded inner wall of a tank includes the following steps: S1. Use an alkaline cleaning solution containing 1~3wt% sodium hydroxide to degrease and remove oil from the inner wall of the S31603 stainless steel storage tank at 60℃ for 15~30min, followed by rinsing with deionized water with a chloride ion concentration ≤25 mg / L. S2. Under internal argon protection conditions (residual oxygen <0.1%), root welding is performed using tungsten inert gas welding with a line energy of 0.6~1.2 kJ / mm and filler wire of ER316L. Then, manual arc welding or gas shielded welding is used for the cover welding. S3. Within 24 hours after welding, critical welds shall be inspected by 100% RT and 100% PT. Any non-conforming welds shall be repaired until they are qualified. S4. Grind the weld to be flush with or slightly convex with the base material, with a weld toe radius R ≥ 3 mm and an inner surface roughness Ra ≤ 1.6 μm; S5. Apply a pickling paste containing 10-15 wt% nitric acid and 6-8 wt% hydrofluoric acid evenly to the weld and heat-affected zone at 25-35°C. The coating thickness is 1-2 mm. Keep it for 10-30 minutes, and then rinse with deionized water until pH≈6-7. S6. After draining the rinsing solution from the inner wall of the tank by circulating spraying or soaking, neutralize it with 1-3 wt% sodium carbonate solution for 10-15 minutes, and then rinse with deionized water until the conductivity is <30 μs / cm. S7. Perform vacuum chamber leak detection on all welds under negative pressure of 20~30kPa. If a leak is found, perform secondary sealing welding and repeat steps S5 and S6. S8. Inject water with a chloride ion concentration ≤25 mg / L into the tank at a temperature ≥5 ℃ and a pressure holding time ≥48 h. Confirm that there is no leakage or abnormal deformation. Then, perform PT testing on the key welds for re-inspection and test the passivation film using the blue dot method. The absence of blue dots indicates that the test is qualified.

[0013] As an improvement of the present invention, in step S2, the argon protection flow rate is 10~15L / min, the protection length is not less than 150% of the weld length, and the argon purity is ≥99.99%.

[0014] As an improvement of the present invention, in step S4, the inner surface roughness Ra of the weld after grinding is ≤0.8μm.

[0015] As an improvement of the present invention, in step S5, the corrosion inhibitor in the pickling paste has a mass fraction of 0.1~0.5wt%, and the thickener is silica gel with a mass fraction of 1~3wt%.

[0016] As an improvement of the present invention, the washing solution in step S6 is a passivation solution, which is prepared by using 6~10 wt% citric acid and 0.3~0.8 wt% hydrogen peroxide, and the spraying or soaking time is 30~60 min, and the temperature is 45~60℃.

[0017] As an improvement of the present invention, the washing solution in step S6 is Use a 15-25 wt% nitric acid solution for spraying or soaking for 30-45 minutes at a temperature of 25-30°C.

[0018] As an improvement of the present invention, the ambient temperature during the water-filling test in step S8 is 15~35℃, and the water is drained within 2 hours after the test and the inner wall of the tank is dried with hot air at 50~60℃.

[0019] As an improvement of the present invention, in step S8, the blue dot method inspection is performed on 10-20% of the total weld length inside the tank, and the inspection locations include longitudinal seams, circumferential seams, and T-joints.

[0020] The beneficial effects of this invention are as follows: This invention, through a combination of pickling, passivation, and blue dot retesting, effectively eliminates the chromium-depleted layer in the heat-affected zone of the weld and improves the integrity of the passivation film. Experimental data shows that the pass rate of this process using the blue dot method can reach over 99.5%.

[0021] This invention, through vacuum chamber leak detection and secondary repair, can detect minute leaks in advance before the water-filling test, reducing the probability of repairs during the water test stage and avoiding project delays and increased costs caused by repairs due to water discharge.

[0022] This invention provides specific standards for surface roughness control and parametric operation, ensuring a smooth transition between the weld and the base material, with a roughness Ra controlled within the range of 0.8~1.6 μm. This meets the cleanliness requirements of food processing environments while facilitating operator implementation. This process is not only suitable for food cans but also for clean storage tanks in the chemical and pharmaceutical industries, demonstrating broad application value. Detailed Implementation

[0023] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0024] Example 1 S1. Pretreatment: The inner wall of the S31603 stainless steel storage tank is cleaned by circulating an alkaline cleaning solution containing 2wt% NaOH at 60℃ for 20 minutes; then rinsed with deionized water with a chloride ion concentration ≤25mg / L until pH≈7.

[0025] S2. Welding: Under internal argon protection conditions (residual oxygen <0.1%, argon protection flow rate of 12L / min), root welding is performed using tungsten inert gas welding with a line energy of 0.8kJ / mm and filler wire of ER316L, followed by manual arc welding; S3. First non-destructive testing: Within 24 hours after welding, perform 100% RT testing and 100% PT testing on critical welds, and rework any non-conforming areas until they are qualified. S4. Weld grinding and polishing: Grind the weld to be flush with the base material, with a weld toe radius R≥3mm and an inner surface roughness Ra=0.8μm; S5. Local pickling: Apply a pickling paste (with the remainder being deionized water) containing 12wt% nitric acid, 7wt% hydrofluoric acid, 2wt% silica and 0.2wt% corrosion inhibitor evenly to the weld and heat-affected zone at 28°C. The coating thickness is 1.5mm. Keep it for 15min, and then rinse with deionized water until pH≈6.5. S6. Full tank passivation: The inner wall of the tank is sprayed with a passivation solution consisting of 8 wt% citric acid and 0.5 wt% hydrogen peroxide. The spraying time is 45 min and the temperature is 50 ℃. After draining, the solution is neutralized with 2 wt% sodium carbonate solution for 10 min and then rinsed with deionized water until the conductivity is <30 μs / cm. S7. Vacuum chamber leak detection: Perform vacuum chamber leak detection on all welds under a negative pressure of 30 kPa. The leak detection fluid is a 2 wt% surfactant aqueous solution with a film thickness of 1 mm. If a leak is found, perform secondary sealing welding and repeat steps S5 and S6. S8. Water filling test and retest: Water with a chloride ion concentration ≤25mg / L and a temperature of 20℃ is injected into the tank and pressure is maintained for 48 hours to confirm that there is no leakage or abnormal deformation. Then, the key welds are retested by PT test, with a pass rate of 100%. 15% of the welds are randomly inspected by the blue dot method, and no blue dots are found.

[0026] Example 2 It is basically the same as Example 1, except that: Step S5: Pickling temperature 25℃, time 25min; Step S6: Passivation temperature 45℃, time 60min; Step S7: Vacuum chamber leak test at a negative pressure of 25 kPa.

[0027] Example 3 It is basically the same as Example 1, except that: Step S6: Full tank passivation is performed using a 20wt% HNO3 solution, with circulating spraying at 28°C for 30 minutes; neutralization and rinsing are the same as in Example 1.

[0028] Example 4 It is basically the same as Example 1, except that: Step S4: Polish the inner surface to Ra=1.6μm.

[0029] Comparative Example A conventional process for preventing leakage of the inner wall of a tank, specifically including: S1. Pretreatment: Rinse with water, do not use alkaline washing.

[0030] S2. Welding: GTAW is used for the root pass, manual arc welding is used for the top pass, the back is not argon-filled for protection, and the welding line energy is not controlled.

[0031] S3. Non-destructive testing: The RT sampling rate for butt welds is 10%, and PT sampling is implemented for some key parts.

[0032] S4. Weld grinding and polishing: roughness Ra=1.6–3.2μm.

[0033] S5. Pickling and passivation: Local pickling is not performed. The formula and conditions for full-tank passivation are not fixed, and this step is omitted for some projects.

[0034] S6. Leak detection and testing: A water-filling test was conducted directly, without vacuum chamber leak detection; the chloride ion content of the test water was not tested.

[0035] S7. Retest: Blue dot method testing will not be performed after the water test.

[0036] The experimental data of Examples 1-4 and Comparative Example 1 are compared in the table below:

[0037] As shown in the table, micro-leaks were detected in Examples 1-4 before the water test using vacuum chamber leak detection, and the water test passed on the first attempt after repair. The comparative example showed leakage during the water test and required repair. The blue dot method pass rate of Example 3 was slightly lower than that of Example 1, indicating that the lower the roughness control, the higher the blue dot pass rate. The blue dot method pass rate of the comparative example was only 90.4%, indicating that the traditional process lacked an acid pickling and passivation step, resulting in insufficient corrosion resistance.

[0038] It should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. For those skilled in the art, several improvements and modifications can be made on the basis of the above embodiments without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A process for preventing leakage on the welded inner wall of a tank, characterized in that, Includes the following steps: S1. Use an alkaline cleaning solution containing 1~3wt% sodium hydroxide to degrease and remove oil from the inner wall of the S31603 stainless steel storage tank at 60℃ for 15~30min, followed by rinsing with deionized water with a chloride ion concentration ≤25 mg / L. S2. Under internal argon protection conditions, root welding is performed using tungsten inert gas welding with a line energy of 0.6~1.2 kJ / mm and filler wire of ER316L. Then, manual arc welding or gas shielded welding is used for the cover welding. S3. Within 24 hours after welding, 100% radiographic testing and 100% penetrant testing shall be carried out on critical welds, and any unqualified areas shall be repaired until qualified. S4. Grind the weld to be flush with or slightly convex with the base material, with a weld toe radius R ≥ 3 mm and an inner surface roughness Ra ≤ 1.6 μm; S5. Apply a pickling paste containing 10-15 wt% nitric acid and 6-8 wt% hydrofluoric acid evenly to the weld and heat-affected zone at 25-35°C. The coating thickness is 1-2 mm. Keep it for 10-30 minutes, and then rinse with deionized water until pH≈6-7. S6. After draining the rinsing solution from the inner wall of the tank by circulating spraying or soaking, neutralize it with 1-3 wt% sodium carbonate solution for 10-15 minutes, and then rinse with deionized water until the conductivity is <30 μs / cm. S7. Perform vacuum chamber leak detection on all welds under negative pressure of 20~30kPa. If a leak is found, perform secondary sealing welding and repeat steps S5 and S6. S8. Inject water with a chloride ion concentration ≤25 mg / L into the tank at a temperature ≥5 ℃ and a pressure holding time ≥48 h. Confirm that there is no leakage or abnormal deformation. Then, conduct a penetrant test on the key welds and test the passivation film using the blue dot method. The absence of blue dots indicates that the test is qualified.

2. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: In step S2, the argon purging flow rate is 10~15L / min, the protection length is not less than 150% of the weld length, and the argon purity is ≥99.99%.

3. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: In step S4, the surface roughness Ra of the weld after grinding is ≤0.8μm.

4. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: In step S5, the corrosion inhibitor in the pickling paste has a mass fraction of 0.1~0.5 wt%, and the thickener is silica gel with a mass fraction of 1~3 wt%.

5. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: In step S6, the washing solution is a passivation solution, which is prepared by using 6~10 wt% citric acid and 0.3~0.8 wt% hydrogen peroxide. The spraying or soaking time is 30~60 min and the temperature is 45~60℃.

6. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: The washing solution in step S6 is... Use a 15-25 wt% nitric acid solution for spraying or soaking for 30-45 minutes at a temperature of 25-30°C.

7. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: During the water-filling test in step S8, the ambient temperature is 15~35℃. After the test, the water is drained and the inner wall of the tank is dried with hot air at 50~60℃.

8. The anti-leakage treatment process for the welded inner wall of the tank according to claim 1, characterized in that: In step S8, the blue dot method inspection is performed on 10-20% of the total weld length inside the tank. The inspection locations include longitudinal seams, circumferential seams, and T-joints.

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