Metal material surface pretreatment method for improving binding force of epoxy primer coating

Abstract

The invention discloses a metal material surface pretreatment method for improving the binding force of an epoxy primer coating, which comprises the following steps: carrying out surface polishing and grinding treatment on a metal material sample to remove a surface original oxide layer, processing burrs and oil stain impurities and obtain a flat and clean reference surface; the polished sample is subjected to annealing treatment so as to eliminate the internal stress of the material and homogenize the surface structure; carrying out surface impact treatment on the sample subjected to polishing, grinding and annealing pretreatment by adopting submerged cavitation jet; performing hot air drying on the sample subjected to cavitation jet treatment; performing low-temperature plasma graft polymerization treatment on the dried sample to construct an interface transition layer; and spraying epoxy primer on the sample treated by the transition layer. And the problem that oxidation rust is easily generated on the metal surface in a traditional cavitation jet flow process is effectively solved.

Description

Technical Field

[0001] This invention relates to a surface pretreatment method for metal materials to improve the adhesion of epoxy primer coatings. Background Technology

[0002] In aerospace, marine engineering, and rail transportation, metallic materials are constantly exposed to a combination of corrosive environments, including high salt spray, high humidity, and strong ultraviolet radiation. Once the protective coating on the surface peels off, it will directly lead to a decline in the mechanical properties of the components or even failure. Epoxy primers are widely used in these scenarios due to their excellent shielding and adhesion properties. However, their initial bond strength on bare metal materials such as aluminum alloys and steel is generally low (cross-cut test ≤2, pull-out strength ≤6 MPa), making them prone to blistering and peeling during service life, thus becoming a weak link in the protective system.

[0003] To improve coating adhesion, industrial applications commonly use sandblasting, laser impact, or chemical conversion coatings as pretreatments. 1. Sandblasting: Using hard abrasives to impact the surface can quickly form a rough morphology, but abrasive residue, dust pollution and subsequent waste sand disposal bring secondary pollution, and it can easily cause deformation of thin plates or precision components. 2. Laser shock: High-energy pulses induce micro-pits and residual compressive stress on the surface, significantly improving the bonding strength. However, the equipment is expensive, the process window is narrow, and the local temperature rise can easily cause micro-melting or phase transformation of aluminum alloys, requiring additional cooling and secondary cleaning. 3. Chemical conversion coatings (phosphating, chromating): These form a microcrystalline conversion layer on the metal surface, providing chemical bonding sites. However, they contain harmful substances such as chromium and phosphorus, have high wastewater treatment costs, and have a long process chain (degreasing → washing → surface conditioning → conversion → washing → drying), making it difficult to meet the requirements of green manufacturing.

[0004] In recent years, cavitation jet technology has been explored for surface cleaning. It utilizes microjets and shock waves generated by the collapse of cavitation bubbles in high-speed water flow to simultaneously remove oxide layers, old coatings, and contaminants, leaving micro-pits on the material surface at the 50-200 μm level. However, existing cavitation jet processes only focus on "cleanliness" or "mass loss," without systematically optimizing coating adhesion. They also fail to address issues such as flash rust, stress concentration, and poor controllability of microstructure after cavitation. This results in limited improvement in the bonding strength of subsequent epoxy primers, making it difficult to meet the requirements of aerospace and marine engineering for Class 1 adhesion (ISO 2409) and ≥8 MPa pull-out strength.

[0005] Therefore, developing a highly efficient, environmentally friendly metal surface pretreatment method that can significantly improve adhesion and has a short process chain has become a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] The present invention provides a method for pretreatment of metal material surfaces to improve the adhesion of epoxy primer coatings in order to solve the problems existing in the prior art.

[0007] The technical solutions adopted in this invention are as follows: A method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings includes the following steps: (1) Polishing and grinding the surface of the metal material sample to remove the original oxide layer, burrs and oil stains on the surface, and obtain a flat and clean reference surface. (2) Anneal the polished sample to eliminate internal stress and homogenize the surface structure. (3) For the samples that have been polished, ground and annealed, the surface impact treatment is carried out by submerged cavitation jet; (4) The sample after cavitation jet treatment is dried with hot air; (5) The dried sample was subjected to low-temperature plasma grafting polymerization treatment to construct an interface transition layer; (6) Spray epoxy primer onto the sample after the transition layer treatment.

[0008] Furthermore, in step (1), the surface roughness Ra after polishing is 0.5-3.2 µm.

[0009] Furthermore, in step (2), the aluminum alloy is annealed at 250-350 ℃ and held for 60-90 min; the medium and low carbon steel is annealed at 550-600 ℃ and held for 90-120 min, and both are cooled with the furnace.

[0010] Furthermore, in step (3), the submerged cavitation jet uses an angle nozzle with a throat diameter of 0.7 mm, and the process parameters are set as follows: the jet pressure is 10 MPa to 40 MPa, the target distance is 20 mm to 50 mm, and the jet pressure and the target distance are positively correlated; the scanning speed of the submerged cavitation jet treatment is fixed at 55-65 mm / min, and the processing time is fixed at 1-3 hours.

[0011] Furthermore, in step (4), the hot air temperature is ≤80℃.

[0012] Further, in step (5), the low-temperature plasma graft polymerization treatment specifically involves: using low-temperature argon plasma with a power of 50-100W and a treatment time of 5-8min to activate the sample surface; then spraying an epoxy group acrylic prepolymer with a solid content of 10%-15% to form an interface transition layer of 2-5μm.

[0013] Further, in step (3), a DC electric field is applied in a pure water immersion environment with a voltage of 5-30 V and a current of 0.2-1 A. The workpiece is used as the cathode. At the same time, CO2 microbubbles are injected into the nozzle outlet area in a pulse manner with a pulse frequency of 50-200 Hz, a duty cycle of 30%, and a CO2 flow rate of 0.1-0.5 L / min.

[0014] The present invention has the following beneficial effects: (1) Effectively improves the oxidation and rust problem that is easily generated on the metal surface in the traditional cavitation jet process. Through the synergistic effect of electric field assistance and CO2 microbubble enhancement, while improving the cavitation effect, a temporary protective environment is built for the metal surface, and the uniformity and cleanliness of the surface micromorphology are optimized.

[0015] (2) By adding hot air drying and low temperature plasma grafting polymerization processes, a composite bonding mechanism of "physical anchoring + chemical bridging" is formed, which enhances the reliability and long-term effectiveness of the bonding between the metal substrate and the epoxy primer coating and reduces the risk of coating peeling.

[0016] (3) The annealing process parameters were optimized for different metal substrates, and were adapted to medium and low carbon steel materials such as 70 series aluminum alloy and 45 steel, which broadened the application scenarios of the technical solution and improved the versatility of the process.

[0017] (4) The process steps are logically coherent and the parameters are controllable. The entire process from surface pretreatment to coating formation has clear operating specifications, requiring no complex or special equipment. It balances the treatment effect with the feasibility of industrial application, making it easy to promote and implement in actual production. The process design takes into account both the surface modification effect and operational safety, avoids the generation of additional pollutants, meets the basic requirements of green processing, and improves the stability of the entire treatment process, reducing the risk of quality fluctuations during production. Attached Figure Description

[0018] Figure 1 This is a flowchart of the present invention.

[0019] Figure 2 shows the microstructure of the metal material surface observed by a high-resolution laser confocal microscope after step 3 (submerged cavitation jet treatment) of the present invention. Detailed Implementation

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] The overall process flow of this invention is shown in Figure 1. It requires the sequential completion of processes such as surface polishing and grinding, annealing, electric field and CO2 microbubble-assisted submerged cavitation jet impact, hot air drying, low-temperature plasma grafting polymerization, and epoxy primer spraying. It can be adapted to medium and low carbon steel metal components such as 70 series aluminum alloy and 45 steel. The following uses 70 series aluminum alloy as the core verification substrate to describe this invention in detail.

[0022] First, the 70 series aluminum alloy sample (100mm×100mm×5mm, belonging to the Al-Zn-Mg-Cu series high-strength wrought aluminum alloy) underwent surface polishing. Initially, 180-grit coarse sandpaper was used to grind the sample surface, removing large areas of the original oxide layer, machining burrs, and attached blocky oil stains. Then, 400-grit, 800-grit, and 1200-grit fine sandpaper were used for progressive fine grinding. During fine grinding, the sandpaper was kept parallel to the sample surface and the force applied evenly. Anhydrous ethanol was used for continuous wiping to reduce grinding debris residue. After fine grinding, a laser roughness meter was used to perform multi-point testing on the sample surface to ensure that the final surface roughness Ra was stable within the range of 0.5-3.2 µm, obtaining a smooth, clean, and uniformly rough reference surface, laying a consistent substrate foundation for subsequent processes.

[0023] After polishing and grinding, a targeted annealing treatment is carried out on the 70 series aluminum alloy substrate, specifically as follows: The polished sample is placed in a box annealing furnace and heated to a set temperature in the range of 250-350°C at a heating rate of 5°C / min. In this embodiment, 300°C is selected. The sample is then held for 60-90 minutes (75 minutes in this embodiment). After the holding time is completed, the furnace heating system is turned off, and the sample is allowed to cool naturally to room temperature. This eliminates the internal stress accumulated in the aluminum alloy during the early processing such as rolling and cutting, optimizes the surface microstructure, reduces the segregation of second phase particles such as Al-Zn-Mg, and improves the uniformity of surface mechanical properties.

[0024] If the sample is a medium- or low-carbon steel such as 45 steel, it is placed in an annealing furnace and heated to 550-600°C (580°C in this example) at a heating rate of 8°C / min, held for 90-120 min (105 min in this example), and then cooled to room temperature in the furnace to avoid secondary stress caused by rapid cooling.

[0025] After annealing and cooling to room temperature, the 70 series aluminum alloy samples were subjected to submerged cavitation jet surface impact treatment. This was carried out in a pure water submerged environment using an angle nozzle with a throat diameter of 0.7 mm. To enhance the cavitation effect and suppress flash rust on the substrate surface, a DC electric field was applied in the water environment. The workpiece was connected to the cathode of the power supply, and the electric field voltage was controlled at 5-30V (20V in this example) and the current at 0.2-1A (0.5A in this example). This process utilizes the hydrogen evolution reaction at the cathode to form additional cavitation nuclei while simultaneously providing temporary cathodic protection to the substrate.

[0026] A pulsed microbubble generator was simultaneously activated, continuously injecting CO2 microbubbles into the nozzle outlet area. The microbubble pulse frequency was controlled at 50-200Hz (120Hz in this embodiment), the duty cycle at 30%, and the CO2 gas flow rate at 0.1-0.5L / min (0.3L / min in this embodiment). The CO2 microbubbles acted as cavitation nuclei, forming a weakly acidic passivation environment in the water. To verify the effect of different jet parameters on the treatment effect, four sets of gradient experiments were set up. Each set followed the principle that the jet pressure was positively correlated with the target distance, and the scanning speed was controlled at 55-65mm / min (60mm / min in this embodiment), and the treatment time was controlled at 1-3h (2h in this embodiment).

[0027] The specific matching of jet pressure and target distance for the four sets of experiments is as follows: Experiment 1 uses a jet pressure of 20 MPa corresponding to a target distance of 40 mm (optimal parameters); Experiment 2 uses a jet pressure of 15 MPa corresponding to a target distance of 30 mm; Experiment 3 uses a jet pressure of 25 MPa corresponding to a target distance of 45 mm; and Experiment 4 uses a jet pressure of 10 MPa corresponding to a target distance of 20 mm.

[0028] The instantaneous high-pressure microjets and shock waves generated by cavitation bubble collapse simultaneously complete contaminant stripping and the construction of 50-200 μm-scale micropit structures on the sample surface. High-resolution laser confocal microscopy revealed the following surface micromorphology of the four groups of samples after cavitation jet treatment: Figure 2 As shown, the sample surface of the optimal parameter group (Experiment 1) formed uniformly distributed micron-level pits and nano-level textures of appropriate depth. With the gradient adjustment of jet pressure and target distance, the density and depth of the micro-pit structure in the other experimental groups changed regularly. This kind of multi-scale microstructure provides a key morphological basis for the physical anchoring of the subsequent coating.

[0029] After the cavitation jet treatment, the four groups of aluminum alloy samples were subjected to hot air drying treatment. Hot air with a temperature of ≤80℃ (65℃ in this embodiment) was used to blow evenly on each area of ​​the sample. At the same time, the surface humidity was monitored in real time by an infrared moisture detector until the sample surface was completely free of residual moisture and reached a dry state. This completely avoids the risk of oxidation and rust on the metal surface after cavitation and ensures the cleanliness and dryness of the substrate in subsequent processes.

[0030] After drying, four groups of samples underwent low-temperature plasma graft polymerization to construct an interface transition layer. The samples were first placed in a plasma treatment chamber, argon gas was introduced into the chamber, and the plasma generator was activated. The plasma power was adjusted to 50-100W (75W in this embodiment), and the sample surface was activated for 5-8 minutes (6 minutes in this embodiment). High-energy argon plasma particles bombarded the substrate surface, generating a large number of active groups such as hydroxyl and carboxyl groups, significantly increasing the substrate surface energy. After activation, a high-pressure airless spraying device was used to spray an epoxy-based acrylic prepolymer with a solid content of 10%-15% (12% in this embodiment) onto the sample surface. During spraying, the distance between the spray gun and the sample surface was controlled at 20-30cm, and the spraying pressure at 0.3-0.5MPa. Finally, a uniform interface transition layer with a thickness of 2-5μm (3μm in this embodiment) was formed on the sample surface. This transition layer can form stable chemical bonds with the metal substrate through active groups, while also providing crosslinking sites for subsequent epoxy primer.

[0031] After the interface transition layer has fully cured, epoxy primer is sprayed onto the four groups of aluminum alloy samples using the air spraying method. The epoxy primer is thoroughly stirred according to the mixing ratio and then uniformly coated onto the sample surface at a spraying pressure of 0.4-0.6MPa and a spray gun distance of 30-40cm. After spraying, the samples are placed in a constant temperature and humidity environment of 25℃ and 50% relative humidity for more than 24 hours to cure, thus completing the entire pretreatment and primer coating process for 70 series aluminum alloys.

[0032] To verify the applicability of the scheme to different substrates, a 45 steel sample was selected and processed in the whole process according to the above-mentioned optimal parameters (20MPa jet pressure, 40mm target distance).

[0033] To quantitatively verify the treatment effect of this invention, the treatment of 70 series aluminum alloys was performed according to ISO 2409-2013 "Paints and varnishes—Cross-cut test" and the pull-out test specification. Four groups of experimental samples and two groups of blank control samples without electric field / CO2 microbubble / plasma grafting processes were tested for surface roughness (Ra, Rz, Rt, Rq, Rmax) and coating adhesion. For the cross-cut test, a 10×10 grid was formed using a cross-cutting tool with a 1mm tooth spacing, and the grid was rated after being peeled off with tape. For the pull-out test, a test column was bonded with high-strength epoxy resin, cured for 24 hours, and then stretched at a rate of 10mm / min until separation, and the maximum tensile force was recorded. The specific test results are shown in Table 1 below.

[0034] Table 1 Performance test results for different substrates and experimental groups Experiment number Ra (µm) Rz (µm) Rt (µm) Rq (µm) Rmax (µm) Bonding force value (MPa) Bonding strength level 1 22.694 102.342 158.092 29.814 127.086 10.527 1 2 18.047 87.906 109.22 22.93 100.671 9.793 1 3 13.292 73.828 98.703 17.792 98.703 9.235 1 4 10.701 58.009 65.048 13.31 61.069 8.412 1 average value 16.1835 80.52125 107.76575 20.9615 96.88225 9.49175 1 Blank control 1 3.133 23.666 25.543 4.219 24.326 5.759 2 Blank control 2 2.894 20.216 25.826 3.725 22.42 6.132 2 Depend on Figure 2 As shown in Table 1, the four experimental samples of 70 series aluminum alloy treated with the original cavitation jet process have formed multi-scale microstructures on their surfaces. The coating adhesion strength grade of all samples reached level 1, and the average pull-out force reached 9.49175 MPa. Furthermore, the addition of electric field assistance, CO2 microbubble reinforcement, and low-temperature plasma grafting processes in this invention can further improve the uniformity of the microstructure and the interfacial chemical bonding strength, resulting in a stable adhesion strength exceeding 10 MPa and better consistency in surface roughness parameters.

[0035] To verify the applicability of the scheme to 45 steel, another 45 steel sample was selected and processed in the whole process according to the optimal parameters (20MPa jet pressure, 40mm target distance). The test results were: Ra14.26μm, Rz72.38μm, bonding force value of 9.21MPa, bonding force grade 1, which is far better than the blank control group of 45 steel (bonding force 5.98MPa, grade 2).

[0036] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings, characterized in that: Includes the following steps: (1) Polishing and grinding the surface of the metal material sample to remove the original oxide layer, burrs and oil stains on the surface, and obtain a flat and clean reference surface. (2) Anneal the polished sample to eliminate internal stress and homogenize the surface structure. (3) For the samples that have been polished, ground and annealed, the surface impact treatment is carried out by submerged cavitation jet; (4) The sample after cavitation jet treatment is dried with hot air; (5) The dried sample was subjected to low-temperature plasma grafting polymerization treatment to construct an interface transition layer; (6) Spray epoxy primer onto the sample after the transition layer treatment.

2. The method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings as described in claim 1, characterized in that: In step (1), the surface roughness Ra after polishing is 0.5-3.2 µm.

3. The method for pretreatment of metal material surfaces to improve the adhesion of epoxy primer coating as described in claim 1, characterized in that: In step (2), aluminum alloys are annealed at 250-350 ℃ and held for 60-90 min; low and medium carbon steels are annealed at 550-600 ℃ and held for 90-120 min, and both are cooled with the furnace.

4. The method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings as described in claim 1, characterized in that: In step (3), the submerged cavitation jet uses an angle nozzle with a throat diameter of 0.7 mm. The process parameters are set as follows: the jet pressure is 10 MPa to 40 MPa, the target distance is 20 mm to 50 mm, and the jet pressure and the target distance are positively correlated; the scanning speed of the submerged cavitation jet treatment is fixed at 55-65 mm / min, and the processing time is fixed at 1-3 hours.

5. The method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings as described in claim 1, characterized in that: In step (4), the hot air temperature is ≤80℃.

6. The method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings as described in claim 1, characterized in that: In step (5), the low-temperature plasma graft polymerization treatment specifically involves: using low-temperature argon plasma with a power of 50-100W and a treatment time of 5-8min to activate the sample surface; then spraying an epoxy group acrylic prepolymer with a solid content of 10%-15% to form an interface transition layer of 2-5μm.

7. The method for pretreatment of metallic material surfaces to improve the adhesion of epoxy primer coatings as described in claim 1, characterized in that: In step (3), a DC electric field is applied in a pure water submerged environment with a voltage of 5-30 V and a current of 0.2-1 A. The workpiece is used as the cathode. At the same time, CO2 microbubbles are injected into the nozzle outlet area in a pulse manner with a pulse frequency of 50-200 Hz, a duty cycle of 30%, and a CO2 flow rate of 0.1-0.5 L / min.

Images