A low pressure cathodic electrophoretic coating process for 5xxx and 6xxx aluminum alloy automotive panels

Abstract

The application provides a low-pressure cathode electrophoretic coating process for 5XXX and 6XXX aluminum alloy automobile coverings, which comprises the following steps: pretreatment, electrophoresis, pure water washing, drying, curing and inspection. In the pretreatment, an acid pickling process is used to replace phosphorization, the ash produced in the degreasing process on the surface of the aluminum alloy workpiece is removed, the surface cleanliness of the workpiece is improved, a good substrate condition for electrophoresis is created, the electrophoresis construction voltage is adjusted and reduced, and the electrophoretic coating film thickness is ensured; the electrophoretic solution is newly prepared, and a promoter is added, so that the interface bonding strength between the electrophoretic coating and the aluminum alloy part is effectively improved, the film forming voltage is reduced, the coating film after treatment is uniform and delicate, and the use requirement is met.

Description

Technical Field

[0001] This invention relates to the field of surface treatment technology, and in particular to a low-pressure cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels. Background Technology

[0002] With the advancement of lightweight technology, 5xxx and 6xxx aluminum alloys are increasingly used in automotive body panels. Replacing steel with aluminum can reduce vehicle weight by approximately 35%, becoming a future development direction for the automotive industry. To improve the corrosion resistance and decorative properties of aluminum alloy body panels, electrophoretic coating is often applied. Generally, the coating process is as follows:

[0003] Degreasing → Water washing → Surface conditioning (pure water) → Phosphating (film pretreatment) → Water washing → Electrophoresis → Water washing → Drying → Curing → Inspection

[0004] Phosphating is used as a primer before painting to improve the adhesion of the paint film and is widely used in automotive cab panel coating production lines that primarily use cold-rolled steel sheets. When switching to aluminum alloy substrates, automakers often employ co-line pretreatment processes for steel and aluminum to save costs and improve production efficiency. These methods include adding fluoride ions and two-step phosphating-passivation processes, with the specific process chosen based on the aluminum alloy content of the body components. However, due to the potential difference and varying chemical reactivity between steel and aluminum, the phosphating effect after co-line application is unstable, increasing the difficulty of process management. Furthermore, increasingly stringent national environmental policies on phosphorus emissions will lead to the gradual phasing out of traditional phosphating processes.

[0005] If electrophoretic coating is performed directly without phosphating, the electrophoretic process needs to be improved to ensure the bonding strength between the electrophoretic paint and the substrate. Generally, increasing the electrophoretic voltage can be used. However, high-voltage electrophoresis not only consumes a lot of energy, but also produces uneven electrophoretic paint films on complex-shaped aluminum alloy stamping parts, resulting in differences in corrosion resistance at different locations on the same workpiece. Summary of the Invention

[0006] This invention proposes a low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels. In the pretreatment, acid pickling replaces phosphating to remove slag generated during degreasing of the aluminum alloy workpiece surface and further cleans surface oil, improving surface cleanliness and creating favorable substrate conditions for electrophoresis. Simultaneously, the electrophoresis application voltage is adjusted and reduced to ensure the thickness of the electrophoretic paint film. The electrophoretic solution is re-prepared, and by adding an accelerator, the interfacial bonding strength between the electrophoretic paint and the aluminum alloy parts is effectively improved, and the film-forming voltage is reduced. The resulting paint film is uniform and fine, meeting the application requirements.

[0007] To address the problems mentioned above in the background section, the present invention is achieved through the following technical solution:

[0008] A low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels includes the following steps: pretreatment → electrophoresis → pure water washing → drying → curing → inspection; First, the aluminum alloy body panels are pretreated, and then the pretreated aluminum alloy body panels are subjected to electrophoresis. The electrophoresis method is as follows: the electrophoresis solution is poured into a PVC tank, stainless steel or lead plates are used as anodes, and aluminum alloy body panels are used as cathodes, with a plate spacing of 10-15 cm. Then, the power supply is turned on, and the voltage is linearly increased to 60-100V in 30 seconds for electrophoresis. The electrophoresis time is 3-3.5 minutes, and the temperature is 29-35℃.

[0009] Preferably, the drying and curing methods are as follows: after electrophoresis, the aluminum alloy cover is removed, cleaned with deionized water until there is no floating paint on the surface, placed in an oven, and cured at 180°C for 20-30 minutes.

[0010] Preferably, the inspection method is as follows: the adhesion and standard film thickness between the cured electrophoretic coating and the aluminum alloy cover are inspected; the adhesion is inspected using the cross-cut test; and the standard film thickness is inspected using the salt spray test.

[0011] Preferably, the electrophoretic solution comprises an epoxy resin-based cathodic electrophoretic paint with a mass percentage concentration of 17±2%; a co-solvent of n-butanol with a mass percentage concentration of 3-4%; a regulator of acetic acid; an accelerator of polyethyleneimine (PEI) at a dosage of 10-20 mL / L; and the remainder being deionized water.

[0012] Preferably, the method for preparing the electrophoresis solution is as follows:

[0013] 1) Take epoxy resin-type cathodic electrophoresis paint and n-butanol, add deionized water and mix well;

[0014] 2) Take polyethyleneimine, add an appropriate amount of deionized water, and heat to dissolve it;

[0015] 3) Pour the dissolved polyethyleneimine solution into step 1), and add the remaining deionized water to replenish it;

[0016] 4) Adjust the pH of the electrophoresis solution to 5.4-6.1 using acetic acid to obtain the electrophoresis solution.

[0017] Preferably, the pretreatment steps are: degreasing → water washing → acid washing → water washing → pure water washing.

[0018] Preferably, the pickling is an immersion pickling method, and the pickling steps are: aluminate removal → water washing.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects:

[0020] 1. In the pretreatment, pickling is used instead of phosphating to remove the ash and slag generated on the surface of aluminum alloy workpieces during the degreasing process, and to clean the surface oil again, thereby improving the surface cleanliness of the workpieces and creating good substrate conditions for electrophoresis. At the same time, the electrophoresis construction voltage is adjusted and reduced to ensure the thickness of the electrophoretic paint film.

[0021] 2. The electrophoretic solution was remade, and an accelerator was added to effectively improve the interfacial bonding strength between the electrophoretic paint and the aluminum alloy cover, and to reduce the film-forming voltage. The treated paint film was uniform and fine, meeting the usage requirements. Attached Figure Description

[0022] Figure 1 This is a process step diagram of the present invention. Detailed Implementation

[0023] Example 1

[0024] A low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels includes the following steps: pretreatment → electrophoresis → pure water washing → drying → curing → inspection; First, the aluminum alloy body panels are pretreated, and then the pretreated aluminum alloy body panels are subjected to electrophoresis. The electrophoresis method is as follows: the electrophoresis solution is poured into a PVC tank, stainless steel or lead plates are used as anodes, and aluminum alloy body panels are used as cathodes, with a plate spacing of 10-15 cm. Then, the power supply is turned on, and the voltage is linearly increased to 60-100V in 30 seconds for electrophoresis. The electrophoresis time is 3-3.5 minutes, and the temperature is 29-35℃.

[0025] The drying and curing method is as follows: After electrophoresis, remove the aluminum alloy cover, clean it with deionized water until there is no floating paint on the surface, place it in an oven, and cure it at 180℃ for 20-30 minutes.

[0026] The testing methods are as follows: the adhesion between the cured electrophoretic coating and the aluminum alloy cover and the standard film thickness are tested; the adhesion is tested using the cross-cut test; and the standard film thickness is tested using the salt spray test.

[0027] The electrophoresis solution includes an epoxy resin-based cathodic electrophoretic paint with a mass percentage concentration of 17±2%; a co-solvent of n-butanol with a mass percentage concentration of 3-4%; a modifier of acetic acid; an accelerator of polyethyleneimine (PEI) at a concentration of 10-20 mL / L; and the remainder of deionized water.

[0028] The preparation method of the electrophoresis solution is as follows:

[0029] 1) Take epoxy resin-type cathodic electrophoresis paint and n-butanol, add deionized water and mix well;

[0030] 2) Take polyethyleneimine, add an appropriate amount of deionized water, and heat to dissolve it;

[0031] 3) Pour the dissolved polyethyleneimine solution into step 1), and add the remaining deionized water to replenish it;

[0032] 4) Adjust the pH of the electrophoresis solution to 5.4-6.1 using acetic acid to obtain the electrophoresis solution.

[0033] The pretreatment steps are: degreasing, water washing, acid washing, water washing, and pure water washing.

[0034] Pickling is an immersion pickling method. The pickling steps are: aluminum acid removal → water washing.

[0035] Acid pickling is used instead of phosphating to remove slag and residue generated during degreasing of aluminum alloy workpieces and to further clean surface oil, improving surface cleanliness and creating favorable substrate conditions for electrophoresis. Epoxy resin-based cathodic electrophoresis paint is used as the electrophoresis solution. The electrophoresis voltage is adjusted and reduced, and the electrophoresis time is appropriately extended by adjusting the travel speed of the production line fixtures or lifting equipment to ensure the thickness of the electrophoretic paint film. No major adjustments to the process equipment are required; ensuring the water washing and spraying effect is sufficient. The cleanliness of the substrate and the degreasing effect can be ensured by adjusting the acid pickling time and concentration, and cross-contamination between degreasing and acid pickling tanks can be prevented by optimizing the production line equipment. The adhesion between the cured electrophoretic coating and the substrate, as well as the corrosion resistance of the standard film thickness, are tested. Adhesion is tested using a cross-cut test, with a standard of 0-1. Level 0 indicates smooth lines with no coating peeling; Level 1 indicates slight peeling at the intersections of the grid, affecting less than 5% of the cross-cut area. The corrosion resistance of the standard film thickness is tested using a salt spray test, with a standard of 500-800 hours.

[0036] By adding accelerators, the interfacial bonding strength between the electrophoretic paint and the aluminum alloy parts is effectively improved, and the film-forming voltage is reduced. The treated paint film is uniform and fine, meeting the usage requirements.

[0037] Aluminate degreasing agent, also known as aluminum degreaser or aluminum oil remover, is a highly efficient aluminum treatment agent. This product is scientifically formulated with organic acids, inorganic acids, oxidants, corrosion inhibitors, and other degreasing aids and surfactants. The product does not contain environmentally harmful substances, heavy metals, nitrites, or other controlled components.

[0038] The specific steps of pickling are as follows: First, prepare a 2%-6% cleaning solution by mixing the aluminum acid degreasing solution with clean water (tap water or purified water) according to the amount of oil stains on the aluminum alloy parts to be cleaned; immerse the aluminum alloy parts to be cleaned in the prepared cleaning solution for 3-5 minutes; remove the immersed aluminum alloy parts from the cleaning solution and rinse the aluminum workpieces thoroughly with clean water.

[0039] Example 2

[0040] Specifically, electrophoresis can be further divided into:

[0041] Example 1: The electrophoresis solution uses epoxy resin-based cathodic electrophoresis paint with a mass percentage concentration of 17±2%, n-butanol with a mass percentage concentration of 3-4%, and PEI accelerator at a concentration of 10 mL / L. The pH is adjusted to 5.4-6.1 using acetic acid. The electrophoresis solution is poured into a PVC tank. Stainless steel or lead plates are used as anodes, and aluminum alloy covers are used as cathodes. The distance between the plates is 10-15 cm. Then, the power is turned on, and the voltage is linearly increased to 100V in 30 seconds for electrophoresis. The electrophoresis time is 3-3.5 min, and the temperature is 29-35℃.

[0042] Example 2: The electrophoresis solution uses epoxy resin-based cathodic electrophoresis paint with a mass percentage concentration of 17±2%, n-butanol with a mass percentage concentration of 3-4%, and PEI accelerator at a concentration of 15 mL / L. The pH is adjusted to 5.4-6.1 using acetic acid. The electrophoresis solution is poured into a PVC tank. Stainless steel or lead plates are used as anodes, and aluminum alloy covers are used as cathodes. The distance between the plates is 10-15 cm. Then, the power is turned on, and the voltage is linearly increased to 80V in 30 seconds for electrophoresis. The electrophoresis time is 3-3.5 min, and the temperature is 29-35℃.

[0043] Example 3: The electrophoresis solution uses epoxy resin-type cathodic electrophoresis paint with a mass percentage concentration of 17±2%, n-butanol with a mass percentage concentration of 3-4%, and PEI accelerator at a volume of 20 mL / L. The pH is adjusted to 5.4-6.1 with acetic acid. The electrophoresis solution is poured into a PVC tank. Stainless steel or lead plates are used as anodes, and aluminum alloy covers are used as cathodes. The distance between the plates is 10-15 cm. Then, the power is turned on, and the voltage is linearly increased to 60V in 30 seconds for electrophoresis. The electrophoresis time is 3-3.5 min, and the temperature is 29-35℃.

Claims

1. A low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels, characterized in that: Includes the following steps: Pretreatment → Electrophoresis → Pure water washing → Drying → Curing → Inspection; First, the aluminum alloy cover is pretreated, and then the pretreated aluminum alloy cover is subjected to electrophoresis. The electrophoresis method is as follows: the electrophoresis solution is poured into a PVC tank, stainless steel or lead plate is used as the anode, and the aluminum alloy cover is used as the cathode. The distance between the plates is 10~15cm. Then the power is turned on, and the voltage is linearly increased to 60~100V in 30s for electrophoresis. The electrophoresis time is 3~3.5min, and the temperature is 29~35℃. The electrophoresis solution includes an epoxy resin-based cathodic electrophoresis paint with a mass percentage concentration of 17% ± 2%; a co-solvent of n-butanol with a mass percentage concentration of 3-4%; a regulator of acetic acid; an accelerator of polyethyleneimine (PEI) at a dosage of 10-20 mL / L; and the remainder of deionized water.

2. The low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels according to claim 1, characterized in that, The drying and curing methods are as follows: After electrophoresis, the aluminum alloy cover is removed, cleaned with deionized water until there is no floating paint on the surface, and placed in an oven to cure at 180°C for 20-30 minutes.

3. The low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels according to claim 1, characterized in that, The testing method is as follows: the adhesion between the cured electrophoretic coating and the aluminum alloy cover and the standard film thickness corrosion resistance are tested; the adhesion is tested using the cross-cut test; and the standard film thickness corrosion resistance is tested using the salt spray test.

4. The low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels according to claim 1, characterized in that, The method for preparing the electrophoresis solution is as follows: 1) Take epoxy resin-type cathodic electrophoresis paint and n-butanol, add deionized water and mix well; 2) Take polyethyleneimine, add an appropriate amount of deionized water, and heat to dissolve it; 3) Pour the dissolved polyethyleneimine solution into step 1), and add the remaining deionized water to replenish it; 4) Use acetic acid to adjust the pH of the electrophoresis solution to 5.4~6.1 to obtain the electrophoresis solution.

5. The low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels according to claim 1, characterized in that, The pretreatment steps are: degreasing → water washing → acid washing → water washing → pure water washing.

6. The low-voltage cathodic electrophoretic coating process for 5XXX and 6XXX aluminum alloy automotive body panels according to claim 5, characterized in that, The pickling method is an immersion pickling method, and the pickling steps are: aluminate removal → water washing.

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