Method for evaluating integrity of silane electrophoresis film of automobile sheet based on conjoint analysis of surface topography characteristics and conductivity

Abstract

The invention provides a method for evaluating the integrity of a silane electrophoresis film of an automobile sheet based on conjoint analysis of surface topography characteristics and conductivity. The method comprises the following steps: carrying out silanization treatment on a sample; a sample is placed on the low-vacuum SEM sample table, and metal coating is not needed; performing SEM (scanning electron microscope) morphology analysis; counting the proportion of a cavity area in the SEM image; carrying out conductivity test on the sample by adopting a four-probe conductivity test method; calculating the conductivity range of each point on the sample in the 3 * 3 neighborhood; marking a conductivity gradient area on the sample; counting the number of connected domains of the low-conductivity region with sigma < 4S / m in the unit area of the sample; when the SEM image shows a cavity and the corresponding area sigma is less than 4S / m, judging that the defect is a film penetration defect; when the conductivity gradient hot spot is matched with the SEM microcrack morphology, the defect is judged as a hidden crack defect; if the defect cluster density is greater than 5 / or the cavity area proportion is greater than 0.5%, the film layer of the sample is unqualified. The method solves the problems that a method for detecting the corrosion resistance of the scratches of the automobile sheet after silane electrophoresis is large in destructiveness and cannot quantify the core defects in real time.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and more particularly to a method for evaluating the integrity of silane electrophoretic films on automotive panels based on joint analysis of surface morphology and conductivity. Background Technology

[0002] In the automotive manufacturing industry, silane electrophoresis treatment of galvanized steel sheets is a core anti-corrosion process, and the quality of its film directly affects the lifespan of the vehicle body. Currently, the industry relies on scratch corrosion tests (such as the ISO 9227 salt spray test) as a quality assessment standard. This method has a fundamental flaw: it has an extremely long cycle, requiring continuous testing for 720-1000 hours (30-42 days) before corrosion results can be observed, which seriously hinders new product development and production line release processes. Destructive testing: Samples are completely scrapped after being scratched and corroded by salt spray, resulting in high costs and only allowing for sampling inspection (missed detection rate ≥15%); results are severely delayed: by the time corrosion failure is detected, the same batch of materials has already entered the final assembly stage. To shorten the testing time, the industry has tried various alternative solutions, but all have failed to be industrialized due to technical bottlenecks: Electrochemical impedance spectroscopy (EIS): requires immersing the sample in an electrolyte, destroying the integrity of the film layer; operation is complex and data interpretation depends on expert experience; FTIR: can only analyze silane film chemical bonds (such as Si-O-Si), has no ability to detect physical defects (microcracks, peeling), and has a high false positive rate. Therefore, existing technologies cannot simultaneously meet the requirements of rapid, non-destructive, and quantitative testing, resulting in the automotive sheet metal anti-corrosion process being in a state of "blind testing" for a long time. Summary of the Invention

[0003] To address the core technical problems of existing methods for detecting scratch corrosion resistance after silane electrophoresis on automotive panels, such as long processing time, high destructiveness, and inability to quantify in real time, this invention provides a method for assessing the integrity of silane electrophoretic films on automotive panels based on joint analysis of surface morphology features and conductivity. This method rapidly evaluates the integrity of the silane film, replacing traditional scratch corrosion resistance testing methods. This invention achieves, for the first time, the following by integrating low-vacuum SEM morphology quantification and four-probe conductivity spatial mapping technology: gold-spray-free online SEM detection (samples can be reused); AI-automatic extraction of morphology features (percentage of abnormal areas, texture direction, etc.); conductivity-SEM data coordinate alignment (cross-validation of defect authenticity); and a corrosion risk index model (directly outputting equivalent salt spray time).

[0004] The technical means employed in this invention are as follows: A method for evaluating the integrity of silane electrophoretic films on automotive panels based on joint analysis of surface morphology and conductivity, specifically including the following steps: (1) Sample preparation: Cold-rolled steel or galvanized steel with a conductive substrate is subjected to silanization treatment; (2) Sample pretreatment and positioning: The silanized sample is placed on a low-vacuum SEM sample stage without the need for metal coating; (3) Mark the reference points of the QR code on the edge of the sample to realize the coordinate system of SEM and four-probe scanning; (4) SEM morphology analysis: SEM images of the samples were acquired using secondary electron mode; (5) Calculate the proportion of hollow areas in the SEM images; (6) The conductivity of the sample was tested using the four-probe conductivity test method. During the process of scanning the sample with the four probes, the coordinates (x, y) of each point on the sample and the corresponding conductivity σ (S / m) were recorded simultaneously. (7) Local range: Calculate the range of conductivity of each point on the sample in a 3×3 neighborhood Δσ=max(σ)-min(σ); (8) Gradient hotspots: Mark regions on the sample where the conductivity gradient |Δσ / Δx|>1S / mm, where Δx represents the range of the x-coordinates of points within that region; (9) Defect cluster density: The number of connected domains in the low conductivity region with σ < 4S / m per unit area of ​​the sample (number / ) ); (10) Joint analysis: Establish the spatial mapping relationship between SEM morphology and conductivity: when the SEM image shows voids and the corresponding region σ < 4S / m, it is determined to be a film penetration defect; when the conductivity gradient hotspot matches the SEM microcrack morphology, it is determined to be a hidden crack defect. Overall quality score: Defect cluster density > 5 / If the percentage of void areas is greater than 0.5%, the sample's film layer is considered unqualified.

[0005] Furthermore, in step (1), the temperature of the silane treatment is 35°C and the treatment time is 2 min.

[0006] Furthermore, in step (2), the silanized sample is cut into 50×50mm pieces, and the vacuum degree of the low vacuum SEM sample stage is ≤50Pa.

[0007] Further, in step (4), in the secondary electron mode of SEM, the accelerating voltage is 10~15kV, the beam current is 1~5nA, and the working distance is 8~15mm; the acquisition parameters of SEM image are: resolution 2048×1536, single frame imaging time ≤8 seconds; when acquiring images, contrast enhancement: adaptive histogram equalization is used to highlight the surface microstructure.

[0008] Furthermore, in step (7), a 1-2 mm step matrix scan is used, and the probe pressure is 0.5-1.0 N.

[0009] Compared with the prior art, the present invention has the following advantages: 1. The method for evaluating the integrity of silane electrophoretic films for automotive panels based on joint analysis of surface morphology and conductivity provided by this invention reduces the single-sample detection time from more than 720 hours in salt spray tests to 1 hour, supports full inspection of the production line, reduces the cost of a single test, and achieves zero sample scrap due to its non-destructive characteristics.

[0010] 2. The integrity assessment method for silane electrophoretic films on automotive panels based on joint analysis of surface morphology and conductivity provided by this invention can detect micropores with a resolution of up to 200 nm (the limit of traditional microscopes is 1 μm); and the original crack connectivity index (density of skeleton branch points) predicts corrosion channels with an accuracy of 98.7% (compared to only 62% in salt spray tests); the density of conductivity drop points has a positioning accuracy of 5 μm, enabling precise diagnosis of interface defects.

[0011] 3. The integrity assessment method for silane electrophoretic films of automotive panels based on joint analysis of surface morphology features and conductivity provided by this invention is applicable to fields such as automotive galvanized steel sheets and aluminum alloys. It can output the integrity index (0-100 points) in real time and automatically trace the root cause: when the index is <75, it triggers process optimization (such as increasing the silane concentration) to reduce the defect rate. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 The image shown is a SEM image of the experimental group in Embodiment 1 of the present invention.

[0014] Figure 2 for Figure 1 Enlarged view of a local defect.

[0015] Figure 3 This is the SEM image of the control group in Example 1 of the present invention.

[0016] Figure 4 This is an enlarged view of the SEM image of the control group in Example 1 of the present invention.

[0017] Figure 5 The image shown is a SEM image of the experimental group in Embodiment 2 of the present invention.

[0018] Figure 6 for Figure 5 Enlarged view of a local defect.

[0019] Figure 7 This is the SEM image of the control group in Example 2 of the present invention.

[0020] Figure 8 This is an enlarged view of the SEM image of the control group in Example 2 of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a method for evaluating the integrity of silane electrophoretic films on automotive panels based on joint analysis of surface morphology and conductivity, specifically including the following steps: (1) Sample preparation: Cold-rolled steel or galvanized steel with a conductive substrate is subjected to silanization treatment; (2) Sample pretreatment and positioning to obtain SEM sample preparation without gold spraying: The silanized sample is placed on a low vacuum SEM sample stage without the need for metal coating. (3) Mark the reference points of the QR code on the edge of the sample to realize the coordinate system of SEM and four-probe scanning, and the positioning error is ≤ ±0.1mm; (4) SEM morphology analysis: SEM images of the samples were acquired using secondary electron mode; (5) Statistically determine the proportion of abnormal areas such as holes in SEM images; (6) The conductivity of the sample was tested using the four-probe conductivity test method. During the process of scanning the sample with the four probes, the coordinates (x, y) of each point on the sample and the corresponding conductivity σ (S / m) were recorded simultaneously. (7) Local range: Calculate the range of conductivity of each point on the sample in a 3×3 neighborhood Δσ=max(σ)-min(σ); (8) Gradient hotspots: Mark regions on the sample where the conductivity gradient |Δσ / Δx|>1S / mm, where Δx represents the range of the x-coordinates of points within that region; (9) Defect cluster density: The number of connected domains in the low conductivity region with σ < 4S / m per unit area of ​​the sample (number / ) ); (10) Joint analysis: Establish the spatial mapping relationship between SEM morphology and conductivity: when the SEM image shows voids and the corresponding region σ < 4S / m, it is determined to be a film penetration defect; when the conductivity gradient hotspot matches the SEM microcrack morphology, it is determined to be a hidden crack defect. Overall quality score: Defect cluster density > 5 / If the percentage of void areas is greater than 0.5%, the sample's film layer is considered unqualified.

[0023] Furthermore, in step (1), the temperature of the silane treatment is 35°C and the treatment time is 2 min.

[0024] Furthermore, in step (2), the silanized sample is cut into 50×50mm pieces, and the vacuum degree of the low vacuum SEM sample stage is ≤50Pa.

[0025] Further, in step (4), in the secondary electron mode of SEM, the accelerating voltage is 10~15kV, the beam current is 1~5nA, and the working distance is 8~15mm. Preferably, the accelerating voltage is 10kV, the beam current is 1nA, and the working distance is 10mm. The acquisition parameters of SEM image are: resolution 2048×1536 (adapted to 100×150mm size), single frame imaging time ≤8 seconds. When acquiring images, contrast enhancement: adaptive histogram equalization (CLAHE, ClipLimit=3.0) is used to highlight the surface microstructure.

[0026] Furthermore, in step (7), a 1-2 mm step matrix scan is used, and the probe pressure is 0.5-1.0 N; for a 50×50 mm sample, a 1 mm step matrix scan (50×50=2,500 points) is used, and the probe pressure is 0.8 N (to ensure stable contact resistance).

[0027] Example 1 This embodiment uses the method provided by the present invention to perform silanization quality testing on defective galvanized steel sheets.

[0028] 1. Test subjects: Experimental group: defective steel plates (size = 50×50mm); Control group: normal steel plates from the same batch as the experimental group (size = 50×50mm); 2. The experimental group samples and the control group samples were processed according to steps (1) to (3) of the method described in this invention, and then SEM / conductivity combined scanning was performed according to steps (4) to (10): SEM image acquisition: single sample morphology scanning for 20 min (2500 points full coverage); four-probe conductivity scanning: single sample for 15 min (automatic platform moving speed 2 points / second); 3. Comparison of SEM / conductivity combined scanning results between the experimental group and the control group:

[0029] 4. Defect location verification: Figure 1 The image shows the SEM image of the experimental group. After local SEM magnification (5000×) of the gradient hotspot region (coordinates x=12.3mm, y=7.5mm) in the experimental group, as shown below. Figure 2 As shown: Microcracks with a width of about 200 nm can be observed (matching the conductivity abrupt change region), and the low conductivity region (σ=2.1S / m) corresponds to the 10μm-level void clusters in the SEM image; 5. Conclusion: The SEM images and magnified local images of the control group are as follows: Figure 3 and Figure 4 As shown, the control group has an intact surface film. Compared with the control group, the experimental group has insufficient curing temperature, resulting in: insufficient cross-linking of silane film → increased microcracks (the number of gradient hot spots increased by 800%); decreased film density → increased void rate by over 590% and increased defect cluster density by 314%. In summary, it can be determined that the batch of products in the experimental group is unqualified and the curing process needs to be adjusted.

[0030] Example 2 This embodiment provides a detection case of silane film failure caused by zinc dross defects on the steel plate surface, illustrating that the method described in this invention can accurately identify film quality problems caused by substrate impurities.

[0031] An automotive steel sheet manufacturer discovered abnormal pitting corrosion in silanized galvanized steel sheets during salt spray testing. Traditional visual inspection could not pinpoint the root cause. The following method, described in this invention, was used for integrity assessment: 1. Test subjects: Experimental group: Salt spray test failure plates (concentrated corrosion areas); Control group: Normal plates from the same batch as the experimental group; 2. Process the experimental group samples and control group samples according to steps (1) to (3) of the method described in this invention, and then perform SEM image acquisition and analysis according to steps (4) to (6). The SEM images of the experimental group and control group are as follows: Figure 5 and 7 As shown in the enlarged view, Figure 6 and Figure 8 As shown, the experimental group has defects such as incomplete surface film caused by the falling zinc slag (Zn-Fe alloy particles) and voids formed by the falling zinc slag, while the control group has an intact surface film. The SEM morphology analysis results of the experimental group and the control group are shown in the table below:

[0032] 3. The experimental group samples and the control group samples were subjected to four-probe conductivity scanning and analysis according to steps (7) to (10) of the method described in this invention. The results are shown in the table below:

[0033] 4. The conductivity distribution diagram of the experimental group shows that the conductivity at the zinc slag protrusions increases sharply (zinc slag has better conductivity than silane film) → corresponding to the annular gradient hot spots (|Δσ / Δx| reaches 5.2 S / mm) appearing at the protrusion-protrusion edges in the SEM morphology → SEM confirms that the film is cracked. Figure 2 (At the arrow) - the conductivity at the center of the void approaches 0 (the film layer is completely peeled off, and the probe contacts the insulating substrate). 5. Mechanism of zinc dross defects: Zinc dross inclusions in the substrate → discontinuous silane film coverage → local bulging of the film layer → stress concentration during curing → circumferential cracking at the edge → intrusion of corrosive media; 6. Defect verification: Energy dispersive spectroscopy (EDS) analysis of the experimental group revealed Zn / Fe / O peaks (characteristics of zinc slag oxidation) in the raised areas. Polishing of the experimental group section: confirmed that the zinc dross depth was 15μm (embedded in the zinc plating layer), and the top silane film was broken; Accelerated corrosion experiments were conducted on the experimental group: the corrosion expansion diameter in the zinc slag area after 24 hours of salt spray was approximately 200 μm (50 times that of the normal area).

[0034] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for assessing the integrity of silane electrophoretic films in automotive panels based on joint analysis of surface morphology and conductivity, characterized in that, Specifically, the following steps are included: (1) Sample preparation: Cold-rolled steel or galvanized steel with a conductive substrate is subjected to silanization treatment; (2) Sample pretreatment and positioning: The silanized sample is placed on a low-vacuum SEM sample stage without the need for metal coating; (3) Mark the reference points of the QR code on the edge of the sample to realize the coordinate system of SEM and four-probe scanning; (4) SEM morphology analysis: SEM images of the samples were acquired using secondary electron mode; (5) Calculate the proportion of hollow areas in the SEM images; (6) The conductivity of the sample was tested using the four-probe conductivity test method. During the process of scanning the sample with the four probes, the coordinates (x, y) of each point on the sample and the corresponding conductivity σ (S / m) were recorded simultaneously. (7) Local range: Calculate the range of conductivity of each point on the sample in a 3×3 neighborhood Δσ=max(σ)-min(σ); (8) Gradient hotspots: Mark regions on the sample where the conductivity gradient |Δσ / Δx|>1S / mm, where Δx represents the range of the x-coordinates of points within that region; (9) Defect cluster density: The number of connected domains in the low conductivity region with σ < 4S / m per unit area of ​​the sample (number / ) ); (10) Joint analysis: Establish the spatial mapping relationship between SEM morphology and conductivity: when the SEM image shows voids and the corresponding region σ < 4S / m, it is determined to be a film penetration defect; when the conductivity gradient hotspot matches the SEM microcrack morphology, it is determined to be a hidden crack defect. Overall quality score: Defect cluster density > 5 / If the percentage of void areas is greater than 0.5%, the sample's film layer is considered unqualified.

2. The method for evaluating the integrity of silane electrophoretic films in automotive panels based on joint analysis of surface morphology and conductivity as described in claim 1, characterized in that, In step (1), the temperature of the silane treatment is 35°C and the treatment time is 2 min.

3. The method for evaluating the integrity of silane electrophoretic films in automotive panels based on joint analysis of surface morphology and conductivity as described in claim 1, characterized in that, In step (2), the silanized sample is cut into 50×50mm pieces, and the vacuum degree of the low vacuum SEM sample stage is ≤50Pa.

4. The method for evaluating the integrity of silane electrophoretic films in automotive panels based on joint analysis of surface morphology and conductivity as described in claim 1, characterized in that, In step (4), in the secondary electron mode of SEM, the accelerating voltage is 10~15kV, the beam current is 1~5nA, and the working distance is 8~15mm; the acquisition parameters of SEM image are: resolution 2048×1536, single frame imaging time ≤8 seconds; when acquiring images, contrast enhancement: adaptive histogram equalization is used to highlight the surface microstructure.

5. The method for evaluating the integrity of silane electrophoretic films in automotive panels based on joint analysis of surface morphology and conductivity as described in claim 1, characterized in that, In step (7), a 1-2 mm step matrix scan is used, and the probe pressure is 0.5-1.0 N.

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