Aluminum alloy and multi-gradient sulfuric acid anodic oxidation and sealing method thereof

Abstract

The invention discloses an aluminum alloy and a multi-gradient sulfuric acid anodic oxidation and sealing method thereof. The method comprises the steps that an aluminum alloy matrix is cleaned; the cleaned aluminum alloy matrix is placed in a sulfuric acid solution, and anodic oxidation treatment is conducted in a multi-gradient power supply mode; the aluminum alloy matrix subjected to anodic oxidation treatment is cleaned and then placed in a sealing agent to be subjected to sealing treatment; and finally, the aluminum alloy base body obtained after sealing treatment is subjected to drying treatment. Under the condition that the thickness index of a traditional oxidation film is reserved, it is guaranteed that the oxidation film does not crack in the high-temperature environment, and the problems of cracking and dust sticking of the oxidation film in the high-temperature environment are thoroughly solved.

Description

Technical Field

[0001] This application relates to a surface treatment process for aluminum alloys. Background Technology

[0002] Aluminum alloys have been widely used as structural and functional materials. Anodizing aluminum alloys is almost an essential processing technology for achieving various excellent properties. It can enhance the interfacial bonding between the topcoat and the aluminum substrate, improve corrosion resistance, construct multilayer solid supercapacitors, assemble self-constrained nanomaterials, and grow functional films using anodized pre-coatings, etc.

[0003] CN114059127A discloses a method for preparing a high-temperature resistant anodized film. The film prepared by this method will not peel off after being subjected to prolonged impact at 200°C, thereby improving the product's protective capability and extending its service life. CN117626379A discloses a method for preparing an aluminum alloy sulfuric acid anodized film that does not crack at 230°C, solving the problems of oil leakage along cracks and poor sealing performance.

[0004] In existing aluminum alloy hard anodizing and sealing processes, anodizing can improve corrosion and wear resistance through thick films, and sealing treatment can optimize the protective effect, but there are still obvious defects: due to the large difference in thermal expansion coefficients between the anodized film and the substrate, internal stress is easily generated at high temperatures, leading to cracking, and conventional processes are difficult to adapt to complex components and precision requirements; traditional sealing processes rely on heavy metal salts to pollute the environment, and environmentally friendly sealing processes have problems such as insufficient protective stability and low adaptability, and poor synergy between sealing and hard anodized film, which can easily aggravate internal stress or lead to incomplete sealing of deep pores. These shortcomings greatly limit its application in high-temperature, precision and environmentally demanding fields. Summary of the Invention

[0005] The purpose of this invention is to provide a surface treatment process for aluminum alloys.

[0006] According to a first aspect of the present invention, a surface treatment method for aluminum alloy is provided, comprising: Clean aluminum alloy substrate; The cleaned aluminum alloy substrate was placed in a sulfuric acid solution and anodized using a multi-gradient power supply method. After cleaning, the anodized aluminum alloy substrate is placed in a sealant for sealing treatment. Finally, the sealed aluminum alloy substrate is dried.

[0007] According to a preferred embodiment of the present invention, the concentration of the sulfuric acid solution can be 190~200 g / L, and the temperature can be 18~20℃.

[0008] The multi-gradient power supply method can be a three-step voltage boosting method: the first step is to boost to 15 V in 3 minutes; the second step is to boost to 16 V in 1 minute; and the third step is to boost to 17 V in 1 minute, with each step of anodizing taking 8 minutes.

[0009] The multi-gradient power supply method can also be a seven-step voltage boosting method: the first step is to boost to 15V in 3 minutes; the second step is to boost to 15.5V in 10 seconds; the third step is to boost to 16V in 10 seconds; the fourth step is to boost to 16.5V in 10 seconds; the fifth step is to boost to 17V in 10 seconds; the sixth step is to boost to 17.5V in 10 seconds; and the seventh step is to boost to 18V in 10 seconds. The anodizing time for each step is 3 minutes.

[0010] According to a preferred embodiment of the present invention, the sealing agent comprises 16-17 g / L Ce(NO3)3·6 H2O and 0.9-1 g / L HNO3, the sealing temperature is 85-95°C, and the sealing time is 25-35 min.

[0011] The type of aluminum alloy matrix in this invention is not particularly limited; for example, any suitable type of aluminum alloy, such as 6061 aluminum alloy, can be used.

[0012] This invention employs a multi-gradient power supply method for aluminum alloy anodizing. By progressively controlling the current density and electric field strength, the oxide film formation process is optimized, overcoming the shortcomings of traditional constant voltage / constant current processes and achieving a comprehensive improvement in oxide film performance, production efficiency, and process stability.

[0013] Furthermore, compared to traditional sealing processes (such as hot water sealing and steam sealing), this invention uses a cerium salt-containing sealing agent to generate insoluble precipitates within the pores of the oxide film, achieving pore blockage and enhanced film performance. In other words, by replacing traditional physical hydration with chemical deposition, it solves the problem of easy cracking at high temperatures inherent in traditional sealing processes, meeting the requirements for higher corrosion resistance, wear resistance, or special applications, and significantly improving the ability of the oxide film to remain crack-free under high-temperature conditions.

[0014] According to the present invention, cleaning an aluminum alloy substrate may include: Chemical degreasing: Immerse the substrate in ethanol and ultrasonically clean for 10 minutes. This removes soluble oil and some loose contaminants from the surface, which is beneficial for subsequent cleaning and pickling of the aluminum alloy, and promotes uniform oxide film growth.

[0015] Hot water cleaning: Wash in deionized water at 50~80 ℃ for 10 minutes. This can remove residual ethanol and some water-soluble contaminants, accelerate cleaning with the heat effect, rinse off residual ethanol on the surface, and prevent ethanol from reacting with sodium hydroxide degreasing solution.

[0016] Cold water rinsing: Rinse in deionized water at room temperature for 3 minutes. This quickly cools the water, stopping the dissolution process of hot water rinsing, preventing substrate oxidation and affecting the pickling effect; the temperature difference of cold water helps to remove residual oil and impurities from the surface, ensuring surface cleanliness.

[0017] Pickling: Immerse in 450 g / L HNO3 at room temperature for 3 minutes. This removes the natural oxide film and residual oxide debris from polishing, and helps dissolve metallic impurities on the aluminum alloy surface, facilitating removal and cleaning. Furthermore, immersion at room temperature avoids accelerated corrosion due to high temperatures, and short pickling time prevents "over-corrosion pits" on the substrate surface. High concentration ensures rapid dissolution of the oxide film without severely corroding the substrate, balancing "film removal efficiency" and "substrate protection."

[0018] Rinse with cold water: Rinse with deionized water at room temperature for 3 minutes. This thoroughly removes residual nitric acid and reaction products. Cold water rinsing also quickly removes residual acid, terminates the acid pickling reaction, and promotes uniform oxide film growth.

[0019] Thorough degreasing: Immerse in 50 g / L NaOH solution at 60℃ for 20 seconds. This removes stubborn oil stains, as well as the passivation film and surface impurities remaining from acid washing. Saponification occurs, thoroughly removing stubborn oil stains and accelerating the dissolution of the passivation film.

[0020] Hot water rinse: Rinse in deionized water at 50~80℃ for 5 minutes. This can quickly dissolve the sodium fatty acids after degreasing, which is beneficial for rinsing off residual NaOH and saponification products, neutralizing some of the alkalinity, and preventing localized corrosion of the substrate caused by residual alkaline substances during subsequent cold water rinsing.

[0021] Cold water rinse: Rinse in deionized water at room temperature for 3 minutes. This quickly cools the surface and thoroughly rinses away any residual alkaline substances. Hot water rinsing results in a higher substrate temperature, while cold water rinsing rapidly cools the surface to room temperature, stopping any potential residual alkaline corrosion reaction and preventing the formation of bubbles when alkaline substances react with nitric acid during subsequent acid pickling, thus avoiding "bubble defects" on the surface.

[0022] Second pickling: Immerse in 450 g / L HNO3 at room temperature for 3 min. This removes the new oxide film formed after alkaline washing, obtains a uniformly activated substrate surface, which is beneficial for the "uniform nucleation sites" of subsequent anodic oxidation of the oxide film, ensuring that the oxide film grows uniformly from the substrate surface and avoiding stress concentration cracking of the film layer caused by uneven nucleation.

[0023] Final rinse with cold water: Rinse in deionized water at room temperature for 3 minutes. This thoroughly removes residual nitric acid from the substrate surface, resulting in a clean, uniform aluminum alloy surface free of oxide film, oil, and impurities.

[0024] According to another aspect of the present invention, an aluminum alloy workpiece having a surface film layer obtained according to the above-described processing method is also provided.

[0025] This invention ensures that the oxide film does not crack under high temperature conditions while retaining the thickness (8-10μm) of traditional oxide films, thus completely solving the problems of cracking and dust accumulation of oxide films under high temperature conditions. Attached Figure Description

[0026] Figure 1 This is a graph showing the relationship between current, voltage, and time during the anodizing process according to Embodiment 1 of the present invention.

[0027] Figure 2 This is a graph showing the relationship between current, voltage, and time during the anodizing process according to Embodiment 2 of the present invention. Detailed Implementation

[0028] The invention will be further explained below through specific embodiments. An aluminum alloy (model 6061) sheet (25.4*7.6cm) was used as the experimental substrate.

[0029] Example 1 Aluminum alloy substrate cleaning treatment The substrate was ultrasonicated in ethanol for 10 min; then cleaned in hot water (65℃) for 10 min and in cold water (room temperature) for 3 min. The cleaned substrate was then placed in a 450 g / L nitric acid solution (HNO3) for 3 min and rinsed in cold water for 3 min. Then soak in a 50 g / L sodium hydroxide solution (NaOH) at 60℃ for 20 seconds; Next, rinse in hot water for 5 minutes, then rinse in cold water for 3 minutes. Then wash for another 3 minutes in a 450 g / L nitric acid solution (HNO3); Finally, rinse in cold water for 3 minutes.

[0030] Anodizing treatment The cleaned aluminum alloy substrate was immersed in an anodizing bath for anodizing. The anodizing bath solution was a 195 g / L sulfuric acid solution (H2SO4), and the treatment temperature was 19℃. A seven-step voltage increase method was used: 3 min to 15V, 3 min; 10 s to 15.5V, 3 min; 10 s to 16V, 3 min; 10 s to 16.5V, 3 min; 10 s to 17V, 3 min; 10 s to 17.5V, 3 min; 10 s to 18V, 3 min. Each step of anodizing lasted 3 min, for a total of 21 min. Figure 1The graph shows the relationship between current, voltage, and time during the anodizing process. It can be seen that in the initial constant voltage range (15 V, 200–400 s), the current decreases significantly, which is due to the rapid film formation and increased resistance in the early stage of anodizing. In the middle and late stages from 400 to 1600 s, the system implements multi-gradient voltage increases (0.5 V per step), and each voltage increase causes a momentary increase in current, which enhances film growth. Subsequently, at each constant voltage plateau, the current decreases as the growth and dissolution of the oxide film tend to reach a dynamic equilibrium, indicating that the rate of increase in film thickness gradually slows down.

[0031] Anodized film sealing treatment After anodizing, the aluminum alloy substrate is cleaned with deionized water for 3 minutes and then immediately immersed in a sealant at 90°C for 30 minutes. After that, it is treated in a vacuum drying oven at 60°C for 30 minutes to complete the sealing process.

[0032] The sealing agent used was prepared according to the following procedure: (1) Add 3000ml of deionized water to a 5L beaker, and then add 50g of cerium nitrate hexahydrate (Ce(NO3)3·6H2O) and stir until completely dissolved.

[0033] (2) Add 2 ml of nitric acid (HNO3) with a concentration of 1.4 g / ml and stir until completely mixed.

[0034] Inspection The film was heated to 230℃ and held for 2 hours. Examination using an electron microscope revealed no cracks on the film surface. Corrosion resistance was assessed according to the requirements of "Test Methods for Anodized Films and Organic Polymer Films of Aluminum and Aluminum Alloys - Part 3: Salt Spray Test".

[0035] Example 2 The process is the same as in Example 1, except that the seven-step voltage boosting method in the anodizing process is replaced with a three-step voltage boosting method: 3 min to 15 V, 1 min to 16 V, and 1 min to 17 V. Each anodizing step lasts 8 min, for a total of 24 min. Figure 2The diagram illustrates the current, voltage, and time relationship during the anodizing process described above. It shows a multi-step voltage increase: initially stabilizing at 15 V for surface activation (200-600 s), followed by phased increases (600 s to 16 V, 1200 s to 17 V). This design counteracts the continuous increase in oxide film resistance, providing the necessary electric field driving force for the film formation reaction. The corresponding current response exhibits an overall decreasing trend, a direct result of the increased film resistance. At each voltage increase, a brief pulse in current is generated, characterizing the instantaneous enhancement effect of the electric field on ion transport; after the pulse, the current recovers and decreases as the film continues to grow, indicating that the system returns to a dynamic equilibrium state dominated by film resistance.

[0036] The aluminum alloy surface treatment method of the present invention can make the obtained oxide film (layer) not crack or accumulate dust at high temperature, and no rust is generated after 336 hours of neutral salt spray test, showing good corrosion resistance.

Claims

1. A surface treatment method of an aluminum alloy, comprising: cleaning an aluminum alloy substrate; subjecting the cleaned aluminum alloy substrate to an anodization treatment in a sulfuric acid solution by a multi-gradient power feeding mode; cleaning the aluminum alloy substrate after the anodization treatment and subjecting the cleaned aluminum alloy substrate to a sealing treatment in a sealing agent; and finally drying the aluminum alloy substrate after the sealing treatment in a vacuum drying oven.

2. The method according to claim 1, wherein the concentration of the sulfuric acid solution is 190-200 g / L and the temperature is 18-20℃.

3. The method according to claim 2, wherein the multi-gradient power feeding mode is a three-step power feeding mode: the first step is to increase to 15 V in 3 min; the second step is to increase to 16 V in 1 min; and the third step is to increase to 17 V in 1 min, and the anodization time of each step is 8 min.

4. The method according to claim 2, wherein the multi-gradient power feeding mode is a seven-step power feeding mode: the first step is to increase to 15 V in 3 min; the second step is to increase to 15.5 V in 10 s; the third step is to increase to 16 V in 10 s; the fourth step is to increase to 16.5 V in 10 s; the fifth step is to increase to 17 V in 10 s; the sixth step is to increase to 17.5 V in 10 s; and the seventh step is to increase to 18 V in 10 s, and the anodization time of each step is 3 min.

5. The method according to claim 1, wherein the sealing agent comprises 16-17 g / L of Ce(NO3)3·6H2O and 0.9-1 g / L of HNO3, the sealing temperature is 85-95℃, and the time is 25-35 min.

6. An aluminum alloy workpiece having a surface oxide film layer prepared by the treatment method according to any one of claims 1-5.

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