Multi-mode detection method for integrity of coating on inner wall of heat exchanger tube bundle

Abstract

The invention discloses a heat exchanger tube bundle inner wall coating integrity multi-mode detection method, and belongs to the technical field of metal surface protective coating performance evaluation and corrosion monitoring. According to the method, electrochemical impedance spectroscopy (EIS) serves as a core, potentiodynamic polarization (PDP) and open circuit potential (OCP) monitoring are combined, endoscope visual inspection is assisted, and lossless, localization and dynamic evaluation and service life prediction of the inner wall coating of the heat exchanger tube bundle are achieved. By adopting the design of the multi-end bunchy electrode, the overall performance evaluation and local defect positioning of the coating can be realized in the tube bundle, and the problems of single detection means, incapability of real-time early warning and difficulty in positioning in the traditional method are solved. The system is high in integration level, and is suitable for online intelligent monitoring and maintenance management of an industrial field.

Description

Technical Field

[0001] This invention relates to the field of performance evaluation and corrosion monitoring technology for protective coatings on metal surfaces, specifically to a multimodal intelligent detection method for the integrity of coatings inside heat exchanger tube bundles. Background Technology

[0002] Heat exchangers are critical equipment in chemical, power, and shipbuilding industries, and their tube bundles are often exposed to high-temperature, high-pressure, high-flow-rate, and corrosive media environments. To extend their service life, anti-corrosion coatings, such as epoxy and phenolic coatings, are often sprayed onto the inner walls of the tube bundles. However, the small space within the tube bundles makes conventional spraying methods difficult, easily creating spray dead zones and resulting in defects such as missed areas and pinholes. Furthermore, after a certain operating cycle, the coating surface is prone to cracking and damage. These defects severely impact the coating's service life, leading to coating failure. Currently, methods for detecting coating defects on the inner walls of heat exchanger tube bundles have many limitations. Performance evaluation often relies on physical testing methods, such as endoscopic inspection. Traditional pressure drop methods, eddy current methods, or endoscopy can only detect macroscopic damage and cannot predict early failures; a single EIS (Electrochemical Impedance Spectroscopy) only provides "average" information, making it difficult to pinpoint the exact location. While existing technologies employ electrochemical impedance spectroscopy or polarization curves for coating evaluation, these are mostly limited to laboratory environments and fail to integrate with on-site online monitoring, intelligent early warning, and lifespan prediction, thus lacking a systematic multimodal intelligent detection system. Therefore, there is an urgent need for an intelligent evaluation method that uses impedance detection as its core and integrates multiple modes. Summary of the Invention

[0003] The purpose of this invention is to provide a multimodal intelligent detection and evaluation method for the integrity of the coating inside a heat exchanger tube bundle, so as to solve the problems of single detection methods, inability to monitor in real time, and difficulty in defect location in the existing technology.

[0004] The multimodal detection method for the integrity of the inner wall coating of heat exchanger tube bundles described in this invention is an intelligent multimodal detection method for the integrity of the inner coating of heat exchanger tube bundles. It is a non-destructive testing and life evaluation method for the integrity of the inner wall coating of tube bundles, which is based on electrochemical impedance spectroscopy (EIS) and integrates potentiodynamic polarization (PDP), open circuit potential (OCP) and local impedance imaging. Combined with tube bundle endoscopy, it can accurately and intuitively locate defects in the inner wall coating.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A multimodal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle includes the following steps: 1) Measurement Electrode Installation and Inspection: A specially designed multi-terminal bundled electrode is used, with a design that considers both overall evaluation and defect localization. During use, the inner wall of the heat exchanger tube bundle is filled with an electrolyte solution, typically a 1M NaCl solution. Both ends are sealed with rubber stoppers. The multi-terminal bundled electrode passes through the middle of the rubber stopper, traversing the entire interior of the heat exchanger tube bundle.

[0006] A multi-terminal bundled electrode is used to achieve overall coating evaluation and defect location. The multi-terminal bundled electrode design is shown in the attached figure. Figure 1 As shown, multiple locally conductive electrodes S1, S2, S3, etc., are integrated. The length of a single electrode can vary depending on the tube bundle design, but is generally around 5m. Each electrode uses copper wire, with measurement ports at both ends for connection to the measuring equipment. Single electrode S1 has a 0.2m section exposed at 0.8m from the starting position, while the rest remains sealed and insulated. The exposed area is platinum-plated, completely covering it. Single electrode S2 has a 0.2m platinum-plated area at 1.8m from the starting position, while the rest remains sealed and insulated. This process continues, with different single electrodes selected and integrated into a multi-terminal bundle electrode structure based on different pipeline lengths.

[0007] During measurement, a multi-channel electrochemical measurement device is used, with the tube bundle serving as the working electrode and connected to the working electrode end of the multi-channel electrochemical measurement device; the multi-terminal bundled counter electrode serves as both the counter electrode and the reference electrode and is connected to the working counter electrode and reference electrode end of the multi-channel electrochemical measurement device.

[0008] 2) Multimodal electrochemical detection: a) Early warning of coating defects through continuous open-circuit potential monitoring: Connect the multi-terminal bundled electrode and the pipeline to the electrochemical measurement equipment. Connect the pipeline to the working electrode of the electrochemical measurement equipment; connect the multi-terminal bundled electrode to the counter electrode and reference electrode of the electrochemical measurement equipment. Collect the potential sequentially every 5 minutes.

[0009] Result determination: The coating is considered intact if each OCP is consistently above -0.4 V and fluctuates by less than ±20 mV. If the OCP negative shift is ≥50 mV and continues to decrease within 24 hours, it indicates the appearance of scratches or pitting corrosion, and maintenance can be arranged in advance.

[0010] b) Non-destructive assessment of the coating using electrochemical impedance spectroscopy (EIS): Defects are located using localized electrochemical impedance spectroscopy. Small sinusoidal perturbations (typically 10–20 mV) are applied to the coating / metal system in the range of 10 mHz–100 kHz. Parameters such as film resistance Rp, pore resistance Rpo, and charge transfer resistance Rct are obtained by fitting.

[0011] Result determination: Low frequency (0.1 Hz) |Z| ≥ 10^8 Ω·cm 2 The coating is considered "excellent shielding." If a significant decrease in impedance is observed at a single electrode in the corresponding area, the immersion time is increased, and the measurement is repeated to characterize the change in impedance over time. If |Z| decreases by ≤0.2 orders of magnitude after immersion for 2 hours, the coating is deemed to have no through-hole defects.

[0012] Defects in tube bundles can be rapidly evaluated and measured using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). Based on this, potentiodynamic polarization (PDP) is used as the final criterion for identifying abnormal data or significant data fluctuations.

[0013] c) Potentiometric polarization (PDP) final assessment of corrosion status and coating protection condition: When anomalies are found in the impedance measurement of the coating inside the tube bundle, the anodic / cathode branches are recorded at a rate of 1 mV / s from -0.3 V (relative open circuit). This serves as the final electrochemical criterion for assessing the damage to the coating inside the tube bundle and the effectiveness of its protection.

[0014] Result determination: A comprehensive analysis of the potentiodynamic polarization measurement results and the open-circuit potential (OCP) measurement results showed that when the self-corrosion potential Ecorr shifted positively by ≥100 mV, the surface coating exhibited good protective effects, and local defects in the surface coating did not lead to large-area coating failure. Corrosion current Icorr was measured using the linear polarization method in the weak polarization region, and the coating protection status was quantitatively determined based on the changes and fluctuations of Icorr at different locations.

[0015] Note: The scanning range should not be too wide to avoid artificially damaging the coating; the results should complement OCP and EIS, and should not be used as the sole basis for acceptance.

[0016] d) Final positioning and coating morphology acquisition using the endoscopic probe: Based on the above test results, monitor areas with abnormal data, such as those corresponding to multi-ended bundled electrodes (S1, S2, S3, etc.). Electrochemical measurement data may exceed the specified range, and the data from surrounding bundled electrodes may fluctuate significantly. The corresponding location can be identified by the serial number. For example, if the S3 measurement result is abnormal, the probability of coating abnormalities is high in the approximately 3m area at the beginning of the corresponding tube bundle inner wall. In this case, use the endoscopic probe to perform detailed image acquisition in the corresponding area, record the image data file, and complete the coating integrity test.

[0017] The beneficial effects of this invention are: The multimodal detection method for the integrity of the inner wall coating of heat exchanger tube bundles described in this invention can realize multi-dimensional, dynamic, and non-destructive evaluation of coating performance; it has the ability to locate micro-defects and evaluate performance; it has a high degree of system integration and is suitable for intelligent monitoring and management systems in industrial sites. Attached Figure Description

[0018] Figure 1 Single multi-terminal bundled electrode; Figure 2 Schematic diagram of heat exchanger tube bundle measurement; Figure 2 In the diagram, 1 is a heat exchanger with a coating on its inner wall, 2 is a multi-terminal bundled electrode, and 3 is an electrochemical measurement device. Figure 3 Examples of test results: a) Impedance measurement results of an intact coating; b) Measurement results of a coating with local defects. Detailed Implementation

[0019] The present invention will be further described below with reference to embodiments, but is not limited thereto.

[0020] Example Example 1 Integrity testing was conducted on the inner wall epoxy coating of a heat exchanger tube bundle (6m in length, 20mm in inner diameter) in a chemical plant before its commissioning. Multi-terminal bundled electrodes (6 single electrodes in total, with exposed sections spaced 1m apart, as shown in the attached diagram) were used. Figure 1 (As shown). Inject 1M NaCl solution and connect to the electrochemical workstation, as shown in the attached diagram. Figure 2 As shown in the figure. Overall test results show that OCP monitoring remained stable at -0.25 V ±10mV for 24 hours; EIS testing showed that the low-frequency impedance was greater than 10^8 Ω·cm. 2 As attached Figure 3 As shown in Figure a; PDP scanning showed an Ecorr of -0.22 V. Endoscopic examination revealed no obvious defects, and the coating was rated as "excellent".

[0021] Example 2 The coating of a heat exchanger tube bundle (8m in length) at a power plant, which had been in operation for two years, was suspected of having localized damage. Testing using a multi-terminal bundled electrode (8 single electrodes) revealed that the overall OCP fluctuated wildly over time, with a fluctuation range exceeding 100mV within 4 hours. The overall trend was downward. Single electrode testing showed that the OCP at electrode S4 shifted negatively by 80mV within 12 hours, and EIS showed that the low-frequency impedance at this location decreased to 10^5 Ω·cm. 2 Left and right, as shown in the attached document Figure 3 As shown in Figure b, the corrosion current decreased by an order of magnitude after immersion for 2 hours. PDP analysis showed that the Ecorr in this area was -0.45 V, indicating a significant increase in corrosion current. Endoscopy revealed a 2cm long strip of coating peeling in the corresponding 4m area, confirming localized damage.

[0022] Example 3 Acceptance testing was conducted on the newly coated phenolic resin tube bundles (5m in length) of the ship's heat exchanger. Testing of the multi-terminal bundled electrodes (5 single electrodes) showed that the OCP at all points was higher than -0.3 V, the EIS impedance uniformity was good, and the low-frequency impedance was >5×10^7 Ω·cm. 2 PDP scanning showed that the Ecorr difference at each point was less than 50 mV. Endoscopic sampling revealed no pinholes or missed coating. The system determined that the coating uniformity was good and met the application standards.

[0023] Example 4 An early warning was issued for a 12m long tube bundle in a high-temperature heat exchanger during operation. OCP monitoring revealed a slow, continuous negative shift in the potential of electrode regions S7 and S8. EIS showed a slow, but not abrupt, decrease in impedance in this region. The system indicated "coating aging, observation recommended." After 3 months, the impedance in this region dropped to 10^6 Ω·cm. 2 The endoscope shows a microcrack. Immediately stop the machine for maintenance to prevent leakage.

[0024] Matters not covered in this invention are common knowledge.

[0025] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A multimodal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle, characterized in that, Includes the following steps: (1) Fill the heat exchanger tube bundle with electrolyte solution and insert multi-ended bundled electrodes. The electrodes are composed of multiple single electrodes with exposed platinum-plated sections spaced apart. The exposed sections of each electrode are distributed along the length of the tube bundle. (2) The tube bundle is used as the working electrode, and the multi-ended bundle-shaped counter electrode is used as the counter electrode and reference electrode, and connected to a multi-channel electrochemical measurement device. (3) Perform multimodal electrochemical detection, including: a) Continuous monitoring of open-circuit potential enables early warning of coating defects; b) Electrochemical impedance spectroscopy (EIS) is used to assess the coating condition and locate defects. c) Potentiodynamic polarization test to ultimately evaluate the coating's protective effect and corrosion status; (4) Determine the abnormal section based on the electrochemical detection results, and use an endoscope for visual localization and morphological recording.

2. The multi-modal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, The length of a single electrode of the multi-terminal bundled electrode is 3-10m, the length of the exposed platinum-plated section is 0.1-0.5m, and the interval between adjacent exposed sections is 0.5-2m.

3. The multi-modal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, The electrolyte solution is a 1M NaCl solution.

4. The multi-modal detection method for the integrity of the inner wall coating of a heat exchanger tube bundle according to claim 1, characterized in that, The open circuit potential is monitored every 1-10 minutes, and early damage to the coating is judged by the magnitude and trend of negative potential shift.

5. The multi-modal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, The electrochemical impedance spectroscopy test was conducted in the frequency range of 10 mHz-100 kHz. The shielding performance of the coating was evaluated by the low-frequency impedance modulus, and defects were located by the local impedance changes.

6. The multi-modal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, The potentiodynamic polarization scan rate is 0.5-2 mV / s, and the scan range is based on the open circuit potential and is within ±0.3 V.

7. The multi-modal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, It also includes obtaining one or more parameters from film resistance, pore resistance, and charge transfer resistance by fitting electrochemical impedance spectroscopy.

8. The multimodal detection method for the integrity of the coating on the inner wall of a heat exchanger tube bundle according to claim 1, characterized in that, The endoscope probe can be located to a specific section within the tube bundle for image acquisition based on the electrode number corresponding to the electrochemical abnormal signal.

Images