A method for anticorrosion touch-up painting of welded areas of metal pipes

Abstract

The application relates to a metal pipe welding anticorrosion repair painting method, and belongs to the technical field of metal pipe anticorrosion construction, which comprises the following steps: dividing the metal pipe surface around the welding point into an inner ring area, a middle ring area and an outer ring area along the radial direction from inside to outside; detecting the temperatures of the inner ring area, the middle ring area and the outer ring area after welding; when the temperature of the inner ring area is at a first temperature, coating a first anticorrosion coating layer in the inner ring area; when the coating of the first anticorrosion coating layer is completed and the temperature of the middle ring area is at a second temperature, coating a second anticorrosion coating layer in the middle ring area; when the coating of the second anticorrosion coating layer is completed and the temperature of the outer ring area is at a third temperature, coating a third anticorrosion coating layer in the outer ring area; the first temperature is higher than the second temperature, and the second temperature is higher than the third temperature. In this way, coating is completed in the corresponding solidification temperature window of each area, the gradient accurate matching of the anticorrosion performance and the microstructure characteristics of each heat affected zone is realized, and the anticorrosion quality and long-term reliability are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of metal pipe anti-corrosion construction technology, and specifically relates to a method for anti-corrosion repair paint application at the welded joints of metal pipes. Background Technology

[0002] Aluminothermic welding is widely used in the construction of cathodic protection cable connections for buried metal pipelines due to its advantages such as no need for external power supply, convenient operation, and high welding efficiency. The reaction temperature of aluminothermic welding is extremely high (up to 2500℃ or more). During welding, a significant temperature gradient is formed from the inside to the outside around the weld point, resulting in three heat-affected zones on the surface of the metal sleeve along the radial direction: the fusion zone, the heat-affected zone, and the base material zone. The microstructure, residual stress state, and corrosion susceptibility of each zone are significantly different.

[0003] In cathodic protection systems, corrosion protection and repair of cable weld joints is a crucial step in ensuring the long-term effective operation of the system. Before welding, a 75mm x 75mm area of ​​the anti-corrosion coating on the surface of the protected metal sleeve must be removed to expose the metal substrate. After welding, this area must be recoated and repaired; otherwise, the weld joint will become a weak point where corrosion is most likely to occur.

[0004] Existing post-weld touch-up painting methods typically apply uniform coating process parameters (such as uniform application timing and curing temperature) to the entire weld area. However, aluminothermic welding creates a temperature gradient that decreases from the inside out around the weld point. This results in the inner ring (fusion zone) having higher residual welding stress and more severe grain coarsening, requiring a much denser coating than the outer ring. A single curing condition cannot adequately address the differentiated needs of each area. If high-temperature curing is used based on the inner ring, the outer ring will be cured at excessively high temperatures, leading to over-curing, brittleness, and reduced adhesion. Conversely, if room-temperature curing is used based on the outer ring, the inner ring will be cured at too low temperatures, resulting in insufficient cross-linking density and reduced protective performance. Therefore, existing post-weld touch-up painting methods for metal pipes do not consider the differences in microstructure and corrosion susceptibility caused by the temperature gradient in each area, making it difficult to guarantee the corrosion protection performance of the welded area. Summary of the Invention

[0005] This invention provides a method for anti-corrosion paint repair at the welded joint of a metal pipe, which solves the problem that existing post-weld paint repair methods for metal pipes do not take into account the differences in microstructure and corrosion sensitivity caused by temperature gradients in the aforementioned areas, making it difficult to guarantee the anti-corrosion performance of the welded area.

[0006] This invention is achieved through the following technical solution: a method for anti-corrosion and repair painting of welded joints of metal pipes, comprising:

[0007] The surface of the metal tube around the welding point is divided radially from the inside out into an inner ring region, a middle ring region, and an outer ring region.

[0008] After welding is completed, the temperature of the inner ring area, middle ring area and outer ring area is monitored;

[0009] When the temperature of the inner ring region is at a first temperature, a first anti-corrosion coating is applied to the inner ring region.

[0010] When the first anti-corrosion coating is applied and the temperature of the middle ring area is at the second temperature, a second anti-corrosion coating is applied to the middle ring area.

[0011] When the second anti-corrosion coating is applied and the temperature of the outer ring area is at the third temperature, the third anti-corrosion coating is applied to the outer ring area.

[0012] Wherein, the first temperature is higher than the second temperature, and the second temperature is higher than the third temperature.

[0013] Furthermore, in order to better realize the present invention, the radial distance of the inner ring region from the center of the solder joint is 0-5mm;

[0014] The radial distance from the center of the solder joint to the central ring area is 5-20mm.

[0015] The radial distance from the outer ring area to the center of the solder joint is more than 20mm.

[0016] Furthermore, in order to better realize the present invention, both the inner ring region and the middle ring region are solidified using the residual heat of welding after aluminothermic welding;

[0017] The first temperature is 80℃-120℃, the second temperature is 40℃-60℃, and the third temperature is the ambient temperature.

[0018] Furthermore, in order to better realize the present invention, the first anti-corrosion coating is a high-temperature curing coating, and its curing temperature window includes the first temperature;

[0019] The second anti-corrosion coating is a medium-temperature curing coating, and its curing temperature window includes the second temperature;

[0020] The third anti-corrosion coating is a room temperature curing coating, and its curing temperature window includes the third temperature.

[0021] Furthermore, to better realize the present invention, the method for temperature detection of the inner ring region, middle ring region, and outer ring region is specifically as follows:

[0022] The temperature of the inner ring area should be checked within 1-3 minutes after welding is completed;

[0023] The temperature of the middle ring area should be checked within 3-15 minutes after welding is completed;

[0024] The temperature of the outer ring area was measured 15 minutes after welding was completed.

[0025] Furthermore, in order to better realize the present invention, the average value of the measurement results of four detection points is taken when the temperature of the inner ring area is detected;

[0026] When measuring the temperature in the central area, the average value of the measurement results from six measurement points is taken;

[0027] When measuring the temperature in the outer ring area, the average value of the measurement results from eight measurement points is taken.

[0028] Furthermore, to better realize the present invention, after the paint touch-up is completed, it also includes:

[0029] Electrical spark leakage detection was performed on the inner ring area, middle ring area, and outer ring area respectively.

[0030] Furthermore, to better realize the present invention, after the leakage current detection in each area is completed and all are qualified, it further includes:

[0031] A surface paint layer is simultaneously applied to the inner ring area, the middle ring area, and the outer ring area, the surface paint layer covering the inner ring area, the middle ring area, and the outer ring area.

[0032] Furthermore, in order to better realize the present invention, the surface coating is a slow-curing epoxy coating.

[0033] Furthermore, in order to better realize the present invention, the metal pipe is a buried metal sleeve, and the welding is aluminothermic welding.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] The anti-corrosion and repair coating method for welded metal pipes provided by this invention divides the surface of the metal pipe around the weld point radially from the inside out into an inner ring region, a middle ring region, and an outer ring region. Temperature monitoring is used to apply the coating sequentially when each region is at a first, second, and third temperature, with the three temperatures decreasing sequentially. The inner ring region utilizes the residual heat after welding to cure at a high temperature, obtaining a dense coating to cope with the harsh corrosive environment of grain coarsening and residual stress concentration in the fusion zone. The outer ring region is applied after cooling to a lower temperature to avoid brittleness and reduced adhesion of the paint film caused by high-temperature curing. The middle ring region is applied at an intermediate temperature, serving as a transition between the inner and outer rings.

[0036] In this way, each area is coated within its corresponding curing temperature window, achieving a gradient and precise match between the anti-corrosion performance and the microstructure characteristics of each heat-affected zone. This effectively improves the overall anti-corrosion quality and long-term reliability of the welded area. By applying the corresponding anti-corrosion coating sequentially according to a gradient temperature from high to low, the differences in metal microstructure and corrosion sensitivity of each area are adapted to. This solves the drawback of existing uniform coating processes that cannot meet the curing requirements of each area. It utilizes the residual heat of welding to fully cross-link and densify the coating in the inner ring area, which has the highest corrosion risk, while avoiding the paint film in the outer ring area from becoming brittle and peeling off due to overheating. At the same time, no additional heating is required, and the layered coating forms a composite protective barrier, effectively improving the anti-corrosion capability of the welded points and ensuring the long-term stable operation of the cathodic protection system for buried pipelines. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0038] Example:

[0039] The anti-corrosion paint repair method for metal pipe weld joints provided in this embodiment is used to perform paint repair operations around the weld joint of the metal pipe after the welding of the cathodic protection cable of the buried metal sleeve is completed. The welding method is aluminothermic welding. The method includes:

[0040] Step 1: Divide the surface of the metal sleeve surrounding the weld point radially from the inside out into an inner ring region, a middle ring region, and an outer ring region. The radial distance from the center of the weld point to the inner ring region is 0-5mm (e.g., 0.1mm, 2.5mm, 5mm, etc.); the radial distance from the center of the weld point to the middle ring region is 5-20mm (e.g., 5.1mm, 10mm, 15mm, 20mm, etc.); and the radial distance from the center of the weld point to the outer ring region is 20mm or more. In other words, the inner ring region corresponds to the fusion zone around the weld point, the middle ring region corresponds to the heat-affected zone around the weld point, and the outer ring region corresponds to the base material zone around the weld point.

[0041] Step 2: After welding, temperature monitoring is performed on the inner, middle, and outer ring areas. Anti-corrosion coating is then applied based on the monitoring results. Specifically:

[0042] When the temperature of the inner ring region reaches the first temperature, a first anti-corrosion coating is applied to the inner ring region. Specifically, the temperature of the inner ring region is measured within 1-3 minutes after welding. During the measurement, four measurement points are taken in the inner ring region, evenly distributed around the welding point. The average value of the four measurement points is taken as the temperature of the inner ring region. In practice, the aforementioned first temperature is 80℃-120℃ (e.g., 80℃, 90℃, 100℃, 110℃, 120℃, etc.). When the temperature of the inner ring region reaches the first temperature, a first anti-corrosion coating is applied to the inner ring region. The aforementioned first anti-corrosion coating is a high-temperature curing coating (e.g., epoxy resin coating), and its curing temperature window includes the first temperature.

[0043] When the first anti-corrosion coating is applied and the temperature of the middle ring area reaches the second temperature, the second anti-corrosion coating is applied to the middle ring area. Specifically, the temperature of the middle ring area is monitored 3-15 minutes after welding. Six monitoring points are evenly distributed around the welding point, and the average value of these six points is taken as the temperature of the middle ring area. The second temperature is 40℃-60℃ (e.g., 40℃, 50℃, 60℃, etc.). When the temperature of the middle ring area reaches the second temperature, the second anti-corrosion coating is applied to the middle ring area. The second anti-corrosion coating is a medium-temperature curing coating (e.g., a medium-temperature curing epoxy coating), and its curing temperature window includes the second temperature.

[0044] When the second anti-corrosion coating is applied and the temperature of the outer ring area reaches the third temperature, the third anti-corrosion coating is applied to the outer ring area. Specifically, the temperature of the outer ring area is measured 15 minutes after welding. Eight measurement points are taken evenly around the welding point, and the average value of these eight points is taken as the temperature of the outer ring area. In practice, the third temperature is the ambient temperature. The third anti-corrosion coating is applied when the temperature of the outer ring area reaches the third temperature. The third anti-corrosion coating is a room-temperature curing coating (e.g., a room-temperature curing epoxy ester coating), and its curing temperature window includes the third temperature.

[0045] The first temperature is higher than the second temperature, and the second temperature is higher than the third temperature. In practice, after welding, the inner ring area cools to the first temperature first, and the middle ring area cools to the second temperature later. The residual heat from the aluminothermic welding then cures the anti-corrosion coatings in the inner and middle ring areas respectively. It is worth noting that the inner ring area is coated before the middle ring area, and the middle ring area is coated before the outer ring area.

[0046] Step 3: After the paint touch-up is completed, perform electrical discharge testing on the inner, middle, and outer ring areas to ensure that there is no leakage in any of them.

[0047] Step 4: After the leakage current testing in each area is completed and all tests are passed, the following steps are also included:

[0048] A surface coating is applied simultaneously to the inner, middle, and outer ring areas, covering all three areas. Specifically, this surface coating is a slow-curing epoxy coating.

[0049] Of course, if the leakage current test fails, the corresponding area needs to be repainted and retested.

[0050] The method provided in this embodiment precisely divides the surface of the metal pipe surrounding the welding point radially into inner, middle, and outer ring regions: 0-5mm, 5-20mm, and over 20mm, corresponding to the fusion zone, heat-affected zone, and base material zone, respectively. Coating is applied sequentially within three progressively decreasing temperature windows: 80℃-120℃, 40℃-60℃, and ambient temperature. The inner ring region utilizes the residual heat from the aluminothermic welding to cure at high temperatures, obtaining a dense coating to cope with the harsh corrosive environment of grain coarsening and residual stress concentration in the fusion zone. The middle ring region cures at an intermediate temperature, serving as a transition between the inner and outer rings. The outer ring region is applied after cooling to ambient temperature, avoiding brittleness and reduced adhesion caused by high-temperature curing. Thus, each region is coated within its corresponding curing temperature window, achieving a precise gradient match between anti-corrosion performance and the microstructure characteristics of each region.

[0051] Existing uniform coating processes, if using high-temperature curing with the inner ring as the reference, result in excessively high curing temperatures for the outer ring, leading to over-curing, brittle paint films and reduced adhesion. Conversely, if using room-temperature curing with the outer ring as the reference, the inner ring suffers from insufficient cross-linking density due to lower curing temperatures, resulting in decreased protective performance. This embodiment employs a zoned, time-based, and temperature-based construction strategy. The inner ring achieves a fully cross-linked, dense coating at high temperatures, while the outer ring achieves a flexible coating with good adhesion at room temperature. This simultaneously satisfies the high density requirements of the inner ring and the adhesion requirements of the outer ring, overcoming the limitation of single curing conditions that cannot meet the diverse needs of different areas.

[0052] This embodiment employs four, six, and eight evenly distributed testing points around the welding points in the inner, middle, and outer rings, respectively. The average value of the measurements from each point is taken as the judgment temperature for that area. This multi-point layout effectively eliminates temperature measurement deviations caused by uneven heat dissipation around the welding points, ensuring consistency between the actual construction temperature and the target temperature window in each area, and providing a reliable foundation for subsequent coating quality control. In this embodiment, the number of testing points increases with the area (4 in the inner ring, 6 in the middle ring, and 8 in the outer ring), keeping the testing density consistent across areas. This avoids blind spots or redundancy caused by different area sizes, controlling the workload while ensuring temperature detection accuracy.

[0053] The method provided in this embodiment only requires monitoring the temperature of each area after welding and coating sequentially when the temperature reaches the corresponding window, without the need for additional heating or cooling equipment. The inner and middle rings utilize the residual heat from the aluminothermic welding as the curing heat source, requiring no external energy; the outer ring is applied after natural cooling to ambient temperature, also without forced cooling. The entire construction process is completed sequentially during the natural cooling process after welding, without extending the construction cycle, making it simple to operate and suitable for on-site construction environments.

[0054] The method provided in this embodiment performs electrical discharge testing on the inner, middle, and outer rings after the paint touch-up is completed. This allows for verification of the paint touch-up quality area by area, ensuring that there are no leakage defects in any area. After passing the test, a slow-curing epoxy surface coating is simultaneously applied to the surfaces of the three areas. This slow-curing coating has a long application period, facilitates large-area uniform application, and simultaneously provides coverage and protection for the three primer layers, compensating for minor interface defects between the primer layers and forming a dense composite protective barrier, further enhancing the overall corrosion resistance of the weld points.

[0055] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope described in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for anti-corrosion and repair painting of welded joints of metal pipes, characterized in that, include: The surface of the metal tube around the welding point is divided radially from the inside out into an inner ring region, a middle ring region, and an outer ring region. After welding is completed, the temperature of the inner ring area, middle ring area and outer ring area is monitored; When the temperature of the inner ring region is at a first temperature, a first anti-corrosion coating is applied to the inner ring region. When the first anti-corrosion coating is applied and the temperature of the middle ring area is at the second temperature, a second anti-corrosion coating is applied to the middle ring area. When the second anti-corrosion coating is applied and the temperature of the outer ring area is at the third temperature, the third anti-corrosion coating is applied to the outer ring area. Wherein, the first temperature is higher than the second temperature, and the second temperature is higher than the third temperature.

2. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 1, characterized in that: The radial distance from the center of the inner ring area to the solder joint is 0-5mm; The radial distance from the center of the solder joint to the central ring area is 5-20mm. The radial distance from the outer ring area to the center of the solder joint is more than 20mm.

3. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 2, characterized in that: Both the inner and middle ring regions are cured using the residual heat from aluminothermic welding. The first temperature is 80℃-120℃, the second temperature is 40℃-60℃, and the third temperature is the ambient temperature.

4. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 3, characterized in that: The first anti-corrosion coating is a high-temperature curing coating, and its curing temperature window includes the first temperature; The second anti-corrosion coating is a medium-temperature curing coating, and its curing temperature window includes the second temperature; The third anti-corrosion coating is a room temperature curing coating, and its curing temperature window includes the third temperature.

5. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 3, characterized in that, The specific method for temperature detection of the inner, middle, and outer ring regions is as follows: The temperature of the inner ring area should be checked within 1-3 minutes after welding is completed; The temperature of the middle ring area should be checked within 3-15 minutes after welding is completed; The temperature of the outer ring area was measured 15 minutes after welding was completed.

6. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 5, characterized in that: When measuring the temperature in the inner ring area, the average value of the measurement results from four measuring points is taken. When measuring the temperature in the central area, the average value of the measurement results from six measurement points is taken; When measuring the temperature in the outer ring area, the average value of the measurement results from eight measurement points is taken.

7. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 1, characterized in that, After the paint touch-up is completed, it also includes: Electrical spark leakage detection was performed on the inner ring area, middle ring area, and outer ring area respectively.

8. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 7, characterized in that, After completing the leakage current detection in each area, the following is also included: A surface paint layer is simultaneously applied to the inner ring area, the middle ring area, and the outer ring area, the surface paint layer covering the inner ring area, the middle ring area, and the outer ring area.

9. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 8, characterized in that: The surface coating is a slow-curing epoxy coating.

10. The method for anti-corrosion and repair painting of welded joints of metal pipes according to claim 1, characterized in that: The metal pipe is a buried metal sleeve, and the welding is aluminothermic welding.