Non-destructive testing method for corrosion of painted structure and maintenance evaluation method

Abstract

The invention relates to a painted structure corrosion nondestructive testing method and a painted structure maintenance evaluation method.The painted structure corrosion nondestructive testing method comprises the steps that the crawling track of a crawler is preset, after the crawler crawls one step every time, an ultrasonic probe scans in the direction perpendicular to the stepping direction, and after full-stroke scanning is completed, the crawler advances; after the crawl device crawl for one step, a local visual detection system on the crawl device photographs a to-be-scanned area of the ultrasonic probe to obtain a surface image of a local painted structure; in the moving process of the ultrasonic probe, an ultrasonic reflection signal is received; performing corrosion identification on the surface layer image of the local painted structure to obtain a surface corrosion image identification result; meanwhile, potential corrosion identification is conducted on the ultrasonic reflection signals, and a potential corrosion identification result is obtained; and fusing the surface corrosion image recognition result and the hidden corrosion recognition result to obtain a paint structure corrosion detection result. The method has the advantages that the corrosion condition of the painted structure is accurately evaluated, and the method is high in accuracy, easy to implement and the like.

Description

Technical Field

[0001] The invention relates to the technical field of nondestructive testing, and in particular to a nondestructive testing method for corrosion of a painted structure and a maintenance assessment method. Background Art

[0002] There are many kinds of large painted structures such as storage tanks and cruise ships in the prior art. However, these large painted structures are exposed to the natural environment for a long time, which is prone to surface corrosion and greatly reduces their service life. Therefore, these large painted structures need to be maintained on a daily basis, and the daily maintenance includes regular inspection, cleaning, and rust removal and anti-corrosion treatment.

[0003] For example: large storage tanks generally adopt the method of regular tank cleaning and setting up peripheral scaffolding to achieve rust removal and anti-corrosion coating of the entire storage tank. This method can achieve better maintenance effects, but the maintenance interval is long, the construction cycle is long, and the cost is high. Regular maintenance cannot perform targeted maintenance work according to the surface corrosion of each storage tank, which will cause a certain waste of resources. In addition, sometimes fixed-point maintenance is performed on the surface anti-corrosion coating shedding and rusted areas based on the results of manual visual inspection, but it is impossible to find that the coating layer is not damaged and rust has occurred under the coating, resulting in the failure to effectively anti-corrosion treatment of the rusted area in time. Therefore, how to achieve non-destructive detection of the corrosion of the outer wall of the storage tank is of great practical significance to the production safety of the use area.

[0004] At present, there are many diagnostic technologies for tank wall quality, such as infrared thermal imaging detection, eddy current detection, X-ray detection, ultrasonic detection and visual detection. Each method is based on different detection principles and has its own advantages and disadvantages in actual detection, including:

[0005] (1) Infrared thermal imaging detection technology

[0006] Infrared thermal imaging detection technology is based on infrared physics. It determines the damage of the tank wall surface according to the different thickness of the side wall surface, which affects the temperature field distribution. The advantages are that it can realize large-area rapid detection of various types of damage, with intuitive image display and no need to contact the workpiece. The disadvantages are that the detection is greatly affected by the temperature factors in the environment, and it is relatively difficult to characterize the defects and the quantitative accuracy of the defects is low.

[0007] (2) Eddy current testing technology

[0008] Eddy current testing is a detection method based on the principle of electromagnetic induction and applied to the surface and near-surface of conductive media. The eddy current probe is used to approach the side wall of the tank to generate induced eddy currents in the wall. When there are rust defects on the tank wall, the phase and amplitude of the eddy currents change, thus realizing the detection of rust defects. The advantage is that there is no need to clean the tank, and direct contact and coupling agent are available. It has a high sensitivity to surface and near-surface defects, but it is difficult to make quantitative determination of defects due to interference from the surface morphology.

[0009] (3) X-ray detection technology

[0010] X-ray detection technology uses the different absorption rates of the intact area and the corroded area of ​​the tank wall to show different degrees of ray attenuation at the receiving end. The advantage is that the situation of the corroded area is intuitive, the sensitivity is high, and it is not affected by the structure. However, the tank needs to be cleaned before detection, and the X-ray radiation range should be paid attention to during detection;

[0011] (4) Ultrasonic testing

[0012] The principle of ultrasonic testing is that according to the different acoustic impedances between different materials or media, ultrasonic waves will produce reflection and scattering phenomena when acting on different media. When defects occur in the material, the amplitude and phase of the emitted ultrasonic waves will change, and the defects can be evaluated by the echo amplitude, etc. Traditional ultrasonic testing methods are mostly used for macroscopic and large-size defects, with moderate testing costs. In addition, they can also be combined with the positioning information of mechanical devices to achieve C-scan imaging of internal defects, making the test results more intuitive.

[0013] (5) Visual inspection

[0014] Visual inspection is one of the simplest and most widely used methods. It usually requires professional inspectors and is greatly influenced by subjective factors. It can detect surface corrosion that has already appeared, but it is powerless to detect corrosion hidden under the surface paint layer.

[0015] At present, the corrosion detection of the inner wall of the tank still relies on ultrasonic detection. The ultrasonic sound velocity and propagation time are used to calculate the tank wall thickness, which can timely detect the corrosion defects of the inner wall of the tank. It has high detection accuracy for large-area macro-scale defect detection of the inner wall of the tank; the rust detection of the outer wall of the tank relies on manual vision, which is powerless for the rust buried under the paint layer and not exposed. There is no more suitable non-destructive testing method in the industry. It can only be manually identified by removing the surface paint layer during staged maintenance. However, due to the differences in rust removal technology, personnel and equipment during daily maintenance and cleaning of the outer wall rust, it is difficult to ensure the consistency of the rust removal quality of the tank. After applying anti-rust paint, the generation and expansion of internal rust are difficult to predict. Therefore, a non-destructive testing method is needed to conduct non-destructive quantitative evaluation of the rust condition of the outer wall of the painted structure (tank), especially the rust condition below the paint layer. Summary of the invention

[0016] The technical problem to be solved by the present invention is to provide a nondestructive detection method for corrosion of painted structures which can automatically realize rust detection with high accuracy in view of the above-mentioned prior art.

[0017] The technical problem to be solved by the present invention is to provide a method for evaluating the maintenance of a painted structure using the above-mentioned nondestructive testing method for corrosion of a painted structure.

[0018] The first technical solution adopted by the present invention to solve the above technical problems is: a nondestructive detection method for corrosion of a painted structure, characterized by comprising a global visual detection system and an ultrasonic detection system, wherein the ultrasonic detection system comprises a crawler capable of crawling on the painted structure, an ultrasonic probe mounted on the crawler, and a local visual detection system;

[0019] The nondestructive testing method for corrosion of painted structures specifically includes the following steps:

[0020] Step 1: Place the crawler on the surface of the painted structure to be inspected, and determine the placement position of the global vision inspection system so that the shooting field of view of the global vision inspection system can cover the crawling area of ​​the crawler;

[0021] Step 2: Start the global visual inspection system. At this time, the global visual inspection system starts to collect images within the field of view, and identifies and locates the crawler in the image, identifies the center position of the crawler and the scanning area of ​​the ultrasonic probe;

[0022] Step 3: Preset the crawling trajectory of the crawler. After each step of the crawler, the ultrasonic probe scans in a direction perpendicular to the stepping direction. After the full-stroke scanning is completed, the crawler moves forward one more step. After each step of the crawler, the local visual detection system on the crawler will take a picture of the area to be scanned by the ultrasonic probe to obtain a surface image of the local painted structure. During the movement of the ultrasonic probe, the ultrasonic reflection signal will be received.

[0023] Step 4: perform rust recognition on the surface image of the local painted structure to obtain a surface rust image recognition result; at the same time, perform latent rust recognition on the ultrasonic reflection signal to obtain a latent rust recognition result; and fuse the surface rust image recognition result and the latent rust recognition result to obtain a painted structure corrosion detection result.

[0024] Preferably, the crawler is further provided with a scanning bracket, which is arranged perpendicular to the moving direction of the crawler, and the ultrasonic probe is constrained on the scanning bracket in a manner that it can move along the extension direction of the scanning bracket.

[0025] Preferably, the specific steps of identifying the ultrasonic probe scanning area in step 2 are:

[0026] The crawler is fixed at the detection starting position of the painted structure to be detected, and the ultrasonic probe is located at the starting point of the scanning bracket. The global visual detection system is started, and the ultrasonic probe is moved from the starting point of the scanning bracket to the end point of the scanning bracket. The first image when the ultrasonic probe is at the starting point of the scanning bracket is collected by the global visual detection system, and the second image when the ultrasonic probe is at the end point of the scanning bracket is collected by the global visual detection system;

[0027] The first image and the second image are binarized respectively, and then an XOR operation is performed on the binarized first image and the binarized second image. The area with a pixel value of 1 in the image after the XOR operation is the area where the ultrasound probe is located. The position where the pixel value of the area is 1 in the binarized first image corresponds to the starting position of the ultrasound probe, and the position where the pixel value of the area is 1 in the binarized second image corresponds to the end position of the ultrasound probe. The connecting area between the starting position and the end position of the ultrasound probe is the ultrasound probe scanning area.

[0028] Preferably, the specific steps of identifying the center position of the crawler are:

[0029] An AND operation is performed on the binarized first image and the binarized second image, and the horizontal coordinates and vertical coordinates of all pixel points in the images after the AND operation are averaged, so as to obtain the horizontal coordinate and vertical coordinate of the center position of the crawler.

[0030] Preferably, the specific process of performing rust identification on the surface image of the local painted structure in step 4 is:

[0031] Firstly, a global visual inspection system is used to collect the overall appearance image of the painted structure to be inspected, and the initial paint color of the painted structure to be inspected is determined by the color statistics method, and the brightness difference of the paint color at each position under the current light is realized;

[0032] In the local painted structure surface image collected by the local visual inspection system, the paint color of the current painted structure surface to be inspected is RGB (r, g, b). By collecting the RGB image of the entire painted structure surface to be inspected and performing histogram statistics on RGB respectively, the mean value of the paint surface color RGB is calculated. and standard deviation δ, judge Whether it is within ±k*δ, k is a constant greater than 0. If so, the current paint color is considered to be a normal color; if not, the current paint color is considered to be an abnormal color. The rust area in the paint surface is divided by the normal color and the abnormal color.

[0033] Preferably, the specific process of identifying latent corrosion on the ultrasonic reflection signal in step 4 is as follows:

[0034] The ultrasonic emission signal includes interface waves, bottom waves and noise. It is determined whether the maximum amplitude of the interface wave is greater than a times the average amplitude of the noise and the maximum amplitude of the bottom wave is lower than b times the average amplitude of the noise, where a and b are both constants, and a>b. If so, it is determined that the area detected by the current ultrasonic probe is rusted under the paint layer; if not, it is determined that the area detected by the current ultrasonic probe is not rusted under the paint layer.

[0035] Preferably, the specific process of fusing the surface corrosion image recognition result and the latent corrosion recognition result in step 4 is as follows:

[0036] The rusted area in the latent rust recognition result is written into the rusted area in the surface rust image recognition result, so as to realize the fusion of the surface rust image recognition result and the latent rust recognition result.

[0037] The second technical solution adopted by the present invention to solve the above-mentioned technical problem is: a method for evaluating the maintenance of a painted structure using the above-mentioned nondestructive testing method for corrosion of a painted structure, characterized in that it includes: calculating the rust ratio of the painted structure, and judging whether the rust ratio of the painted structure exceeds a preset maintenance setting value, if so, the painted structure needs to be maintained; if not, the painted structure does not need to be maintained.

[0038] Compared with the prior art, the advantages of the present invention are: the local visual detection system on the crawler will take pictures of the area to be scanned by the ultrasonic probe to obtain the surface image of the local painted structure; and the ultrasonic reflection signal is received by the ultrasonic probe, so as to realize the accurate assessment of the corrosion condition of the painted structure by combining visual and ultrasonic detection, which has the advantages of high accuracy and easy implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic diagram of an ultrasonic detection system in an embodiment of the present invention;

[0040] Figure 2 for Figure 1 Schematic diagram from another perspective;

[0041] Figure 3 It is a schematic diagram of an ultrasonic detection system detecting a vertical steel storage tank in an embodiment of the present invention;

[0042] Figure 4 It is a schematic diagram of an ultrasonic detection system detecting a horizontal steel storage tank in an embodiment of the present invention;

[0043] Figure 5 A diagram showing the relationship between the moving trajectory of the crawler and the moving trajectory of the ultrasonic probe in an embodiment of the present invention;

[0044] Figure 6Schematic diagram of surface corrosion image recognition results and latent corrosion recognition results in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The present invention is further described in detail below with reference to the accompanying drawings.

[0046] The nondestructive detection method for corrosion of painted structures in this embodiment includes a global visual detection system and an ultrasonic detection system, such as Figure 1 and Figure 2 As shown, the ultrasonic detection system includes a crawler 3 that can crawl on a painted structure, an ultrasonic probe 1 and a local visual detection system 2 mounted on the crawler 3; the crawler 3 is also provided with a scanning bracket 4, and the scanning bracket 4 is arranged perpendicular to the travel direction of the crawler 3, and the ultrasonic probe 1 is constrained on the scanning bracket 4 in a manner that it can move along the extension direction of the scanning bracket 4. The crawler 3 in this embodiment is a magnetic adsorption crawler, which includes a magnetic adsorption roller 31; the magnetic adsorption crawler is a prior art, and its structure is a prior art, which will not be elaborated here. The global visual detection system and the local visual detection system 2 in this embodiment are both CCD cameras.

[0047] The nondestructive testing method for corrosion of painted structures specifically includes the following steps:

[0048] Step 1: Place the crawler on the surface of the painted structure to be inspected, and determine the placement position of the global vision inspection system so that the shooting field of view of the global vision inspection system can cover the crawling area of ​​the crawler;

[0049] Step 2: Start the global visual inspection system. At this time, the global visual inspection system starts to collect images within the field of view, and identifies and locates the crawler in the image, identifies the center position of the crawler and the scanning area of ​​the ultrasonic probe;

[0050] The specific steps to identify the ultrasound probe scanning area are:

[0051] The crawler is fixed at the detection starting position of the painted structure to be detected, and the ultrasonic probe is located at the starting point of the scanning bracket. The global visual detection system is started, and the ultrasonic probe is moved from the starting point of the scanning bracket to the end point of the scanning bracket. The first image when the ultrasonic probe is at the starting point of the scanning bracket is collected by the global visual detection system, and the second image when the ultrasonic probe is at the end point of the scanning bracket is collected by the global visual detection system;

[0052] The first image and the second image are binarized respectively, and then an XOR operation is performed on the binarized first image and the binarized second image. The area with a pixel value of 1 in the image after the XOR operation is the area where the ultrasound probe is located.

[0053] The position where the pixel value of the region is 1 in the first binarized image corresponds to the starting position of the ultrasound probe, and the position where the pixel value of the region is 1 in the second binarized image corresponds to the end position of the ultrasound probe. The connection area between the starting position and the end position of the ultrasound probe is the ultrasound probe scanning area.

[0054] The specific steps to identify the center position of the crawler are:

[0055] Performing an AND operation on the binarized first image and the binarized second image, and calculating the average of the horizontal coordinates and vertical coordinates of all pixel points in the images after the AND operation, that is, obtaining the horizontal coordinate and vertical coordinate of the center position of the crawler;

[0056] Step 3: Preset the crawling trajectory of the crawler. After each step of the crawler, the ultrasonic probe scans in a direction perpendicular to the stepping direction. After the full-stroke scanning is completed, the crawler moves forward one more step. After each step of the crawler, the local visual detection system on the crawler will take a picture of the area to be scanned by the ultrasonic probe to obtain a surface image of the local painted structure. During the movement of the ultrasonic probe, the ultrasonic reflection signal will be received.

[0057] Step 4: perform rust recognition on the surface image of the local painted structure to obtain a surface rust image recognition result; at the same time, perform latent rust recognition on the ultrasonic reflection signal to obtain a latent rust recognition result; and fuse the surface rust image recognition result and the latent rust recognition result to obtain a corrosion detection result of the painted structure;

[0058] The specific process of rust identification on the surface image of a local painted structure is as follows:

[0059] First, a global visual inspection system is used to collect an overall appearance image of the painted structure to be inspected, and the initial paint color of the painted structure to be inspected is determined by a color statistics method, and the brightness difference of the paint color at each position under the current light is realized; in this embodiment, the global visual inspection system determines the initial paint color of the painted structure to be inspected in order to prevent the local visual inspection system from making mistakes in recognition;

[0060] In the local painted structure surface image collected by the local visual inspection system, the paint color of the current painted structure surface to be inspected is RGB (r, g, b). By collecting the RGB image of the entire painted structure surface to be inspected and performing histogram statistics on RGB respectively, the mean value of the paint surface color RGB is calculated. and standard deviation δ, judge Is it within ±k*δ, k is a constant greater than 0, if yes, the current paint color is considered normal; if no, the current paint color is considered abnormal; the rusted area in the paint surface is divided by normal color and abnormal color; in this embodiment, k=3;

[0061] The specific process of identifying latent corrosion using ultrasonic reflection signals is as follows:

[0062] The ultrasonic emission signal includes an interface wave, a bottom wave and noise. It is determined whether the maximum amplitude of the interface wave is greater than a times the average amplitude of the noise and the maximum amplitude of the bottom wave is less than b times the average amplitude of the noise. Both a and b are constants, and a>b. If so, it is determined that the area currently detected by the ultrasonic probe has rust under the paint layer; if not, it is determined that the area currently detected by the ultrasonic probe has no rust under the paint layer. In this embodiment, a=2, b=1.5, and the specific values ​​of a and b can be determined based on experiments or experience.

[0063] The specific process of fusing the surface corrosion image recognition results and the latent corrosion recognition results is as follows:

[0064] The rusted area in the latent rust recognition result is written into the rusted area in the surface rust image recognition result, so as to realize the fusion of the surface rust image recognition result and the latent rust recognition result.

[0065] This embodiment also relates to a method for evaluating the maintenance of a painted structure using the above-mentioned nondestructive testing method for corrosion of a painted structure, including: calculating the corrosion ratio of the painted structure, determining whether the corrosion ratio of the painted structure exceeds a preset maintenance setting value, if so, the painted structure needs to be maintained; if not, the painted structure does not need to be maintained. The preset maintenance setting value in this embodiment is determined based on experience, and the corrosion ratio of the painted structure is the ratio between the area of ​​the rusted region on the painted structure and the total area of ​​the painted structure.

[0066] In this embodiment, the painted structure is used as a storage tank, the global visual detection system is recorded as CCD0, and the local visual detection system is recorded as CCD1. Figure 3 As shown, when the storage tank is a vertical steel storage tank, the crawler adopts a bow-shaped scanning trajectory. The crawler first moves in steps along the circumferential direction. After each step, the scanning bracket carries the ultrasonic probe to scan perpendicular to the stepping direction. After the full-stroke scanning is completed, the crawler moves forward one step, and the scanning bracket moves to scan, and repeats. When the crawler moves from the starting end to the end of the current circumferential trajectory, the crawler completes a 360-degree U-turn, moves down one step, and then scans back along the circumferential direction, repeating the process until the scanning of the area is completed;

[0067] like Figure 4As shown, when the storage tank is a horizontal steel storage tank, the crawler adopts a bow-shaped scanning trajectory. The crawler first moves in steps along the circumferential direction. After each step, the scanning bracket carries the ultrasonic probe to scan perpendicular to the stepping direction. After the full-stroke scanning is completed, the crawler moves forward one step, and the scanning bracket moves to scan, and repeats. When the crawler moves from the starting end to the end of the current circumferential trajectory, the crawler completes a 360-degree U-turn, moves down one step, and then scans back along the circumferential direction, repeating the process until the scanning of the area is completed;

[0068] like Figure 5 As shown in FIG. 1 , it is a diagram showing the relationship between the moving trajectory of the crawler and the moving trajectory of the ultrasonic probe, where the black dot represents the center position O1 of the crawler, and the white dot represents the center position of the ultrasonic probe; at each black dot position, CCD1 will first take a picture of the area to be scanned by the ultrasonic probe;

[0069] like Figure 6 As shown, the surface rust image recognition results and the latent rust recognition results are fused to obtain the corrosion detection results of the painted structure.

[0070] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A nondestructive testing method for corrosion of painted structures, characterized in that It includes a global visual inspection system and an ultrasonic inspection system. The ultrasonic inspection system includes a crawler capable of crawling on a painted structure, an ultrasonic probe mounted on the crawler, and a local visual inspection system; The nondestructive testing method for corrosion of painted structures specifically includes the following steps: Step 1: Place the crawler on the surface of the painted structure to be inspected, and determine the placement position of the global vision inspection system so that the shooting field of view of the global vision inspection system can cover the crawling area of ​​the crawler; Step 2: Start the global visual inspection system. At this time, the global visual inspection system starts to collect images within the field of view, and identifies and locates the crawler in the image, identifies the center position of the crawler and the scanning area of ​​the ultrasonic probe; Step 3: Preset the crawling trajectory of the crawler. After each step of the crawler, the ultrasonic probe scans in a direction perpendicular to the stepping direction. After the full-stroke scanning is completed, the crawler moves forward one more step. After each step of the crawler, the local visual detection system on the crawler will take a picture of the area to be scanned by the ultrasonic probe to obtain a surface image of the local painted structure. During the movement of the ultrasonic probe, the ultrasonic reflection signal will be received. Step 4: performing rust recognition on the surface image of the local painted structure to obtain a surface rust image recognition result; and performing latent rust recognition on the ultrasonic reflection signal to obtain a latent rust recognition result; The surface rust image recognition results and the latent rust recognition results are fused to obtain the corrosion detection results of the painted structure.

2. The method for nondestructive testing of corrosion of painted structures according to claim 1, characterized in that: The crawler is also provided with a scanning bracket, which is arranged perpendicular to the moving direction of the crawler. The ultrasonic probe is constrained on the scanning bracket in a manner that it can move along the extending direction of the scanning bracket.

3. The method for nondestructive testing of corrosion of painted structures according to claim 2, characterized in that: The specific steps of identifying the ultrasonic probe scanning area in step 2 are: The crawler is fixed at the detection starting position of the painted structure to be detected, and the ultrasonic probe is located at the starting point of the scanning bracket. The global visual detection system is started, and the ultrasonic probe is moved from the starting point of the scanning bracket to the end point of the scanning bracket. The first image when the ultrasonic probe is at the starting point of the scanning bracket is collected by the global visual detection system, and the second image when the ultrasonic probe is at the end point of the scanning bracket is collected by the global visual detection system; The first image and the second image are binarized respectively, and then an XOR operation is performed on the binarized first image and the binarized second image. The area with a pixel value of 1 in the image after the XOR operation is the area where the ultrasound probe is located. The position where the pixel value of the area is 1 in the binarized first image corresponds to the starting position of the ultrasound probe, and the position where the pixel value of the area is 1 in the binarized second image corresponds to the end position of the ultrasound probe. The connecting area between the starting position and the end position of the ultrasound probe is the ultrasound probe scanning area.

4. The method for nondestructive testing of corrosion of painted structures according to claim 3, characterized in that: The specific steps to identify the center position of the crawler are: An AND operation is performed on the binarized first image and the binarized second image, and the horizontal coordinates and vertical coordinates of all pixel points in the images after the AND operation are averaged, so as to obtain the horizontal coordinate and vertical coordinate of the center position of the crawler.

5. The method for nondestructive testing of corrosion of painted structures according to any one of claims 1 to 4, characterized in that: The specific process of performing rust identification on the surface image of the local painted structure in step 4 is as follows: Firstly, a global visual inspection system is used to collect the overall appearance image of the painted structure to be inspected, and the initial paint color of the painted structure to be inspected is determined by the color statistics method, and the brightness difference of the paint color at each position under the current light is realized; In the local painted structure surface image collected by the local visual inspection system, the paint color of the current painted structure surface to be inspected is RGB (r, g, b). By collecting the RGB image of the entire painted structure surface to be inspected and performing histogram statistics on RGB respectively, the mean value of the paint surface color RGB is calculated. and standard deviation δ, judge Whether it is within ±k*δ, k is a constant greater than 0. If so, the current paint color is considered to be a normal color; if not, the current paint color is considered to be an abnormal color. The rust area in the paint surface is divided by the normal color and the abnormal color.

6. The method for nondestructive testing of corrosion of painted structures according to claim 5, characterized in that: The specific process of identifying latent corrosion using ultrasonic reflection signals in step 4 is as follows: The ultrasonic emission signal includes interface waves, bottom waves and noise. It is determined whether the maximum amplitude of the interface wave is greater than a times the average amplitude of the noise and the maximum amplitude of the bottom wave is lower than b times the average amplitude of the noise, where a and b are both constants, and a>b. If so, it is determined that the area detected by the current ultrasonic probe is rusted under the paint layer; if not, it is determined that the area detected by the current ultrasonic probe is not rusted under the paint layer.

7. The method for nondestructive testing of corrosion of painted structures according to claim 6, characterized in that: The specific process of fusing the surface corrosion image recognition result and the latent corrosion recognition result in step 4 is as follows: The rusted area in the latent rust recognition result is written into the rusted area in the surface rust image recognition result, so as to realize the fusion of the surface rust image recognition result and the latent rust recognition result.

8. A method for evaluating the repair of a painted structure using the nondestructive testing method for corrosion of a painted structure as described in any one of claims 1 to 7, characterized in that Including: calculating the rust ratio of the painted structure, judging whether the rust ratio of the painted structure exceeds a preset maintenance setting value, if so, the painted structure needs to be repaired and maintained; if not, the painted structure does not need to be repaired and maintained.

Images