Aluminum or aluminum alloy anodic oxidation method and metal workpiece

Abstract

The invention provides an aluminum or aluminum alloy anodic oxidation method and a metal workpiece. The anodic oxidation method comprises the steps that a metal matrix of aluminum or aluminum alloy is subjected to first-section anodic oxidation, a first layer of anodic oxidation film is formed on the surface of the metal matrix, the voltage of the first-section anodic oxidation ranges from 19 V to 27 V, the stable current density of the first-section anodic oxidation is I1, the peak current density of the first-section anodic oxidation is I2, and I2 is larger than 1.5 I1 and smaller than 2I1; and the metal matrix is subjected to second-section anodic oxidation, a second layer of anodic oxidation film is formed on the surface of the metal matrix, the second layer of anodic oxidation film is located between the metal matrix and the first layer of anodic oxidation film, and the voltage of the second-section anodic oxidation is 15-18 V. According to the method, sectional anodic oxidation is utilized, the internal stress and thermal stress of the anodic oxide film are reduced, the anodic oxide film low in cracking risk and high in density is formed, and the method can be suitable for base materials of complex structures.

Description

Technical Field

[0001] The present application relates to the technical field of metal surface treatment, and in particular to a method for anodizing aluminum or aluminum alloys and a metal workpiece. Background Art

[0002] Existing anodizing technology primarily targets flat substrates. However, for substrates with irregular shapes such as curvature or sharp corners, the oxide film is susceptible to severe stress cracking. The current approach to addressing this issue is to round off sharp corners to increase the curvature and reduce the risk of cracking. However, this approach can lead to dimensional deviations in the product and does not fundamentally address the stress cracking issue.

[0003] In order to solve the above problems, it is urgent to develop new technical solutions to effectively reduce the cracking risk of the anodic oxide film formed on complex structure substrates during the anodic oxidation process, while ensuring the product dimensional accuracy and surface quality. Summary of the Invention

[0004] In view of this, in order to solve at least one of the above technical problems, an embodiment of the present application provides a novel method for anodizing aluminum or aluminum alloy.

[0005] In addition, embodiments of the present application also provide a metal workpiece to which the aforementioned method of anodizing aluminum or aluminum alloy is applied.

[0006] In a first aspect, an embodiment of the present application provides a method for anodizing aluminum or aluminum alloys, the method comprising: performing a first-stage anodizing on a metal substrate of aluminum or aluminum alloy to form a first layer of anodized film on the surface of the metal substrate, wherein the voltage of the first-stage anodizing is 19V~27V, the stable current density of the first-stage anodizing is I1, the peak current density of the first-stage anodizing is I2, and 1.5 I1<I2<2 I1; and performing a second-stage anodizing on the metal substrate to form a second layer of anodized film on the surface of the metal substrate, the second layer of anodized film being located between the metal substrate and the first layer of anodized film, wherein the voltage of the second-stage anodizing is 15V~18V.

[0007] In some possible embodiments, the stable current density I1 of the first stage of anodization is in the range of 1 A / dm 2 ~2A / dm 2 and / or the peak current density I2 of the first anodic oxidation is in the range of 1.5A / dm 2 ~4A / dm 2 .

[0008] In some possible embodiments, in the first anodizing step, a voltage rise rate of the first anodizing step is 1.5 V / min to 8 V / min.

[0009] In some possible embodiments, the surface of the metal substrate includes a non-planar area, and the first anodized film is formed in the non-planar area.

[0010] In some possible embodiments, the oxidation time of the first anodization stage is greater than the oxidation time of the second anodization stage.

[0011] In some possible embodiments, the first anodizing period lasts for 45 minutes to 53 minutes; and / or the second anodizing period lasts for 18 minutes to 25 minutes.

[0012] In some possible embodiments, the thickness of the first anodized film layer accounts for 60% to 90% of the total thickness of the first anodized film layer and the second anodized film layer.

[0013] In some possible embodiments, the thickness of the first anodized film is 10 μm to 14 μm; and / or the thickness of the second anodized film is 1 μm to 3 μm.

[0014] In some possible embodiments, the electrolytes of the first anodizing stage and the second anodizing stage both include oxalic acid, sulfuric acid and Al 3+ , where the concentration of oxalic acid is 30g / L~180g / L, the concentration of sulfuric acid is 20g / L~120g / L, and Al 3 + The concentration of oxalic acid is 0~10g / L, the mass concentration ratio of oxalic acid and sulfuric acid is sulfuric acid: oxalic acid = 1: (1.5~3), and the temperature of the first anodizing stage and the second anodizing stage are both 21℃~27℃.

[0015] In some possible embodiments, after the step of subjecting the metal substrate to a second stage of anodization, the method further comprises: subjecting the metal substrate after the second stage of anodization to a sealing treatment and a heat treatment.

[0016] In some possible embodiments, the sealing treatment includes: sealing the metal substrate after the second anodizing treatment with a sealing agent for 30 minutes to 90 minutes at a temperature of 80°C to 90°C and a pH of 5 to 6, wherein the sealing agent includes nickel acetate.

[0017] In some possible embodiments, the heat treatment includes: washing and drying the metal substrate after the sealing treatment, the washing temperature is 60℃~85℃, the drying temperature is 60℃~80℃, and the drying time is 20min~30min; and boiling and blow-drying the metal substrate after drying, the boiling temperature is 90℃~95℃, the boiling time is 30min~120min, the blow-drying temperature is 20℃~50℃, and the blow-drying time is 15min~40min.

[0018] In the second aspect, the present application also provides a metal workpiece, including a metal substrate and an anodized film formed on the metal substrate, the anodized film is prepared by the aforementioned method of anodizing aluminum or aluminum alloy, and the anodized film includes a second layer of anodized film and a first layer of anodized film stacked sequentially on the surface of the metal substrate.

[0019] Compared with the prior art, the method for anodizing aluminum or aluminum alloy provided in the embodiment of the present application, through segmented anodizing, first performs a first segment of anodizing with a higher voltage, and controls the peak current density I2 of the first segment of anodizing to be in the range of 1.5 I1<I2<2 I1, where I1 is the stable current density of the first segment of anodizing, so as to slow down the initial growth rate of the anodized film and reduce the generation of growth stress of the first layer of anodized film; then performs a second segment of anodizing with a lower voltage. Under low voltage, the generation and dissolution rate of the second layer of anodized film slows down, the density of the second layer of anodized film is improved, and the Joule heat generated by the second segment of anodizing is lower, and the thermal stress of the anodized film is reduced. Therefore, the embodiment of the present application utilizes segmented anodizing to reduce the growth stress and thermal stress of the anodized film, forming an anodized film with low cracking risk and high density. This method can be applied to substrates with complex structures, and can reduce the cracking risk of the anodized film and improve the quality of the anodized film while ensuring the dimensional accuracy of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a test chart of current change during constant voltage anodizing of aluminum or aluminum alloy.

[0021] Figure 2 This is a process flow chart of a method for anodizing aluminum or aluminum alloy provided in one embodiment of the present application.

[0022] Figure 3 A schematic diagram of an anodic oxide film provided in one embodiment of the present application.

[0023] Figure 4 This is the SEM photograph of the anodized film provided in Example 1 of the present application.

[0024] Figure 5This is the SEM photo of the anodized film provided for Comparative Example 1 of this application. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present application in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0026] The inventors have found that when the voltage is fixed for anodization, the current changes as follows: Figure 1 As shown in Figure 1, the growth of an oxide film can be divided into four stages: (I) Rapid formation of a barrier layer causes a sharp drop in current. When the barrier layer thickness is maximum, the resistance is maximum, and the current is minimum; (II) The barrier layer current is uneven, dissolving, forming a porous layer, causing the resistance to decrease and the current to rebound; (III) The porous structure expands, causing the resistance to increase and the current to decrease; and (IV) The pores continue to grow inward, and the current remains stable. In stages (I) and (II), the rapid growth of the aluminum oxide in the barrier layer generates growth stress, which squeezes the molecules. Furthermore, during anodization, the Joule heat generated by the current causes the metal substrate to expand due to the different expansion coefficients of the aluminum or aluminum alloy substrate and the aluminum oxide (anodized film). For example, the expansion coefficient of aluminum alloy is approximately 5 to 10 times that of aluminum oxide. This causes the Joule heat generated by the current to expand the metal substrate, subjecting the anodic oxide film grown in situ on the metal substrate surface to strong tensile stress (thermal stress). When the anodic oxide film grows in a planar manner, the film grows continuously, the stress at each point is balanced, and the film orientation is continuous. The anodic oxide film can release stress by adjusting the pore size of the pore structure in the anodic oxide film. However, when the substrate surface has curvature, the anodic oxide film is discontinuous, and stress is concentrated at the sharp corners. Due to the curvature and spatial limitations, high stress is generated. The inability to release stress may lead to radial cracks. In addition, if the surface curvature is too small, the outward-expanding anodic oxide film cannot fully cover the metal substrate surface, which is also prone to cracks and anodic oxide film cracking. Cracks or even ruptures in the anodic oxide film will lead to a decrease in the corrosion resistance of the product, affecting product quality and appearance.

[0027] To do this, see Figure 2 The present invention provides a novel method for anodizing aluminum or aluminum alloy. The method specifically comprises the following steps: Step S1: pre-treating a metal substrate of aluminum or aluminum alloy.

[0028] Before anodizing, the metal substrate can be pretreated. The pretreatment may include at least one of degreasing, alkali biting, chemical polishing, and black film stripping to effectively remove dirt, natural oxide film and minor defects on the surface of the metal substrate, which is conducive to the smooth progress of the anodizing process.

[0029] Degreasing can be done with a solution of 60% sodium pyrophosphate, 20% borax, and 20% sodium phosphate at 55±5°C for 3-5 minutes. Degreasing the metal substrate is accomplished through a series of processes, including saponification, emulsification, dispersion, wetting, solubilization, foaming, and washing, separating dirt from the metal substrate. Degreasing can improve the decorative properties of the surface treatment and facilitate the subsequent anodizing process.

[0030] Alkali etching can be treated with a strong alkaline solution (such as a sodium hydroxide solution with a concentration of 30g / L±3g / L) at a temperature of 50±5°C for 20s~60s. The aluminum on the metal substrate reacts with the strong alkaline solution, which is mainly composed of NaOH, to remove the natural oxide film on the metal substrate surface, eliminate minor surface defects of the workpiece, and make the metal substrate surface smooth. At the same time, pay attention to the temperature and time of the treatment to avoid excessive alkaline cleaning and maintain the dimensional accuracy of the metal substrate.

[0031] Black film stripping can be performed using a sulfuric acid solution with a concentration of 200g / L ± 20g / L at a temperature of 20°C to 35°C for 3 to 5 minutes. After treatment with alkali, a layer of brown or gray ash forms on the metal surface. This ash is primarily composed of metallic compounds or reaction products such as copper, iron, and silicon within the metal matrix. Black film stripping removes this layer of insoluble impurities, ensuring a clean, uniform oxide film after anodizing.

[0032] The polishing liquid used in chemical polishing includes 60% phosphoric acid and 20% Al 3+ The metal substrate is treated with a mixture of 20% water and 20% water at a temperature of 60°C to 80°C for 1 to 4 minutes. During chemical polishing, the metal substrate surface undergoes selective dissolution, with microscopic protrusions being dissolved preferentially and concave areas being dissolved at a slower rate, thereby achieving a smooth and bright surface. It is understood that after chemical polishing, the black film can be stripped again to remove dust from the polished metal substrate surface and maintain a clean surface.

[0033] In the pretreatment, after any of degreasing, alkali biting, chemical polishing or black film stripping, it can be cleaned with pure water at least once.

[0034] In step S2, the metal substrate is subjected to a first stage of anodization to form a first layer of anodized film on the surface of the metal substrate, wherein the voltage of the first stage of anodization is 19V~27V, the stable current density of the first stage of anodization is I1, the peak current density of the first stage of anodization is I2, and 1.5 I1<I2<2 I1.

[0035] Specifically, a metal substrate of aluminum or aluminum alloy is placed in an electrolyte, with the metal substrate serving as an anode, and the voltage is increased to a first-stage anodic oxidation voltage of 19V to 27V. The stable current density of the first-stage anodic oxidation is I1, the peak current density of the first-stage anodic oxidation is I2, and the peak current density of the first-stage anodic oxidation is controlled to be 1.5 I1 < I2 < 2 I1. The first-stage anodic oxidation treatment is then performed at a constant voltage to form a first anodic oxide film on the surface of the metal substrate. The surface of the metal substrate may include planar regions and non-planar regions, especially non-planar regions, such as, for example, the surface of the metal substrate includes irregular structures such as curved surfaces or sharp corners. The first-stage anodic oxidation treatment, which is performed by slowly increasing the voltage, forms the first anodic oxide film on the non-planar regions, which is beneficial for reducing the growth rate and current heat generation of the first anodic oxide film, and reducing the stress in the initial stage of the first anodic oxide film formation, thereby alleviating the discontinuous formation of the first anodic oxide film caused by the presence of the non-planar regions and the stress concentration at the irregular structures (such as sharp corners), thereby reducing the risk of rupture of the first anodic oxide film and improving the corrosion resistance of the product.

[0036] The first anodizing voltage is 19V to 27V. A higher voltage is conducive to generating a larger pore structure, forming a first layer of anodized film with an ordered porous structure, and improving the decorative properties of the anodized film. The first anodizing voltage can illustratively be 19V, 20V, 21V, 22V, 23V, 24V, 25V, 26V, 27V, or any value within a range consisting of any two of the above values. The first anodizing voltage can further be 23V to 27V.

[0037] However, high voltage can easily lead to excessive growth of the anodic oxide film or local overheating of the electrolyte, causing the anodic oxide film to be ablated (powdered) or cracked during the anodic oxidation process.

[0038] Therefore, the present application reduces the growth rate of the first anodized film and the Joule heat generated by the current by controlling the peak current density I2 of the first stage of anodization to be in the range of 1.5 I1<I2<2I1, where I1 is the stable current density of the first stage of anodization, thereby reducing the growth stress and thermal stress of the first anodized film and avoiding the first anodized film from cracking or pulverizing under high voltage. During the anodizing process, when the first stage anodizing voltage, electrolyte, type of metal substrate and metal substrate area are constant, the stable current density I1 is not affected by factors such as the boost rate or boost time, and is a fixed value, while the peak current density I2 is affected by factors such as the boost rate and temperature. The peak current density I2 of the first stage of anodization is regulated to meet the following conditions: 1.5 I1<I2<2 I1, indicating that the peak current is controlled within an appropriate range, which is beneficial to further improve the quality of the anodized film and the stability of the anodizing process. The peak current density I2 does not exceed 2I1, which is beneficial to reducing the increase in growth stress and thermal stress of the first layer of anodic oxide film caused by excessive current, such as excessive growth or local overheating of the first layer of anodic oxide film, resulting in an increase in the risk of film cracking or peeling; the peak current density I2 is higher than 1.5I1, which is beneficial to the smooth formation of the anodic oxide film, reducing the uneven growth or slow growth of the first layer of anodic oxide film due to insufficient current, and can also reduce or avoid problems such as loose structure and decreased hardness of the first layer of anodic oxide film due to insufficient current.

[0039] Furthermore, the stable current density I1 of the first stage of anodization can be in the range of 1A / dm 3 ~2A / dm 3 The peak current density I2 of the first stage of anodization can be in the range of 1.5A / dm 2 ~4A / dm 2 When I2 is within this range, the stress in the first anodized film can be reduced without reducing the hardness and density of the first anodized film. The peak current density I2 of the first anodized film can be 1.5A / dm 2 , 2A / dm 2 , 2.5A / dm 2 、3A / dm 2 , 3.5A / dm 2 , 4A / dm 2 Or any value within the numerical range formed by any two of the above values.

[0040] In some embodiments, in order to control the peak current density I2 of the first stage of anodization, the voltage ramp rate of the first stage of anodization can be controlled to reduce the Joule heat generated by the current and avoid cracking or pulverization of the first layer of anodized film under high voltage. Before the voltage is constant to the voltage value of the first stage of anodization, the lower the voltage ramp rate, the longer the time for slowly increasing the voltage, the smaller the peak current density I2, and the slower the growth rate of the first layer of anodized film, thereby effectively reducing the growth stress of the first layer of anodized film. In addition, by controlling the voltage ramp rate, the voltage is slowly increased to avoid an excessively rapid increase in current, thereby reducing the peak current density I2, and thus the Joule heat generated by the current is also reduced, and the thermal stress generated between the anodized film and the metal substrate is also reduced. Therefore, the voltage ramp rate of the first stage of anodization can be controlled to be 1.5V / min~8V / min, which can effectively reduce the generation of stress within the anodized film and reduce cracking or pulverization of the first layer of anodized film.

[0041] The voltage rise rate of the first anodization stage can be adjusted according to the voltage of the first anodization stage. For example, when the voltage of the first anodization stage is 19V~21V, the voltage rise rate of the first anodization stage can be 1.5V / min~6V / min; when the voltage of the first anodization stage is 22V~24V, the voltage rise rate of the first anodization stage can be 1.8V / min~7V / min; when the voltage of the first anodization stage is 25V~27V, the voltage rise rate of the first anodization stage can be 2V / min~8V / min.

[0042] It is understood that the voltage ramp-up time of the first anodization stage = the voltage of the first anodization stage / the voltage ramp-up rate of the first anodization stage. Therefore, the voltage ramp-up time of the first anodization stage can be 2.4 minutes to 18 minutes, and further can be 3 minutes to 15 minutes. The voltage ramp-up time of the first anodization stage within the above range is conducive to slowly increasing the voltage and reducing cracking of the first anodized film.

[0043] In some embodiments, the first anodization period (i.e., the time during which the voltage is ramped up to the first anodization voltage and then anodized at a constant voltage) can be 45 to 53 minutes, forming a first anodized film approximately 10 μm to 14 μm thick, which is beneficial for improving the decorative properties, hardness, corrosion resistance, and wear resistance of the anodized film. It is understood that the anodization period after the voltage is ramped up to the first anodization voltage can vary depending on the desired thickness of the first anodized film of the product.

[0044] In some embodiments, the electrolyte for the first stage of anodization may include oxalic acid and sulfuric acid, wherein the concentration of oxalic acid is 30 g / L to 180 g / L, and the concentration of sulfuric acid is 20 g / L to 120 g / L, and the mass concentration ratio of oxalic acid to sulfuric acid is sulfuric acid: oxalic acid = 1: (1.5 to 3). In addition, the electrolyte also includes Al2O3 at a concentration of 0 to 10 g / L. 3+ , provides the acidic environment and Al required for the first stage of anodization 3+ ion.

[0045] In some embodiments, the temperature of the first anodization stage is 21°C to 27°C, which is beneficial for regulating the growth rate of the first anodized film and reducing thermal stress generated during the first anodization process. The temperature of the first anodization stage can illustratively be 21°C, 22°C, 23°C, 24°C, 25°C, 26°C, 27°C, or any value within a range consisting of any two of these values.

[0046] In step S3, the metal substrate is subjected to a second anodization step to form a second anodized film on the surface of the metal substrate. The second anodized film is located between the metal substrate and the first anodized film. The voltage of the second anodization step is 15V to 18V.

[0047] Specifically, the metal substrate is continued to be used as the anode, the voltage is adjusted to a voltage value of 15V~18V for the second stage of anodization, and then the second stage of anodization is performed at a constant voltage to form a second layer of anodized film on the surface of the metal substrate. The second layer of anodized film is located between the metal substrate and the first layer of anodized film. The first layer of anodized film and the second layer of anodized film constitute a complete anodized film. The positional relationship between the first layer of anodized film and the second layer of anodized film in the anodized film can be referred to Figure 3 It is understandable that in this step, the first layer of anodized film formed will be partially corroded by the electrolyte, and its thickness will be slightly reduced.

[0048] The second stage of anodization voltage is 15V~18V, which is a relatively low voltage. At low voltage, the Joule heat of the current is reduced, the growth rate of the second layer of anodized film is slowed down, and the lateral expansion of the pore structure is suppressed, thereby achieving a reduction in pore size and an increase in pore density, thereby improving the density performance of the second layer of anodized film and the bonding strength between the second layer of anodized film and the metal substrate; at the same time, low voltage can slow down the dissolution of the first layer of anodized film and reduce the corrosion of the first layer of anodized film by the electrolyte; in addition, low voltage can also reduce the stress accumulation in the process of forming the anodized film, further reducing the risk of cracking of the anodized film. The second stage of anodization voltage can illustratively be 15V, 16V, 17V, 18V or any value within the numerical range composed of any two of the above values. The second stage of anodization voltage can further be 15V~16V.

[0049] In some embodiments, the oxidation time of the first anodization stage can be greater than the oxidation time of the second anodization stage, which can reduce the corrosion of the first layer of anodized film during the second anodization stage, and can improve the quality of the anodized film while achieving the required film thickness of the anodized film, and can provide favorable guarantees for downstream processes (such as dyeing), thereby improving product quality.

[0050] Specifically, the second anodizing time (i.e., the time of anodizing after the voltage is ramped up to the second anodizing voltage) can be 18 to 25 minutes, forming a dense second anodized film approximately 1 μm to 3 μm thick. It is understood that the time of anodizing after the voltage is ramped up to the second anodizing voltage can vary depending on the thickness requirements of the second anodized film of the product.

[0051] In the anodized film obtained by controlling the segmented anodizing under the aforementioned conditions, the thickness of the first anodized film can account for 60% to 90% of the total thickness of the first anodized film and the second anodized film. The thicker first anodized film can serve as an outer barrier, effectively blocking the corrosion of the metal substrate by the external environment such as water, oxygen, and corrosive media, thereby improving corrosion resistance. Correspondingly, the second anodized film can account for 10% to 40% of the total thickness of the first anodized film and the second anodized film. The denser second anodized film can serve as an underlying support structure, improving the bonding strength with the metal substrate, dispersing external stress, and improving the overall mechanical strength and impact resistance of the anodized film. By controlling the proportion of the first anodized film and the second anodized film in the anodized film within the aforementioned range, the anodized film can have both good mechanical properties and corrosion resistance, achieving the best balance of various properties of the anodized film.

[0052] In some embodiments, the electrolyte for the second stage of anodization may include oxalic acid, sulfuric acid, and Al 3+ , which includes oxalic acid with a concentration of 30g / L~180g / L and sulfuric acid with a concentration of 20g / L~120g / L. The mass concentration ratio of oxalic acid to sulfuric acid is sulfuric acid: oxalic acid = 1: (1.5~3). In addition, the electrolyte also includes Al2O3 with a concentration of 0~10g / L. 3+ , providing the acidic environment and Al required for the second stage of anodization 3+ ion.

[0053] In some embodiments, the temperature of the second anodization stage is 21°C to 27°C, which is beneficial for regulating the growth rate of the second anodized film and reducing thermal stress generated during the second anodization process. The temperature of the second anodization stage can illustratively be 21°C, 22°C, 23°C, 24°C, 25°C, 26°C, 27°C, or any value within a range consisting of any two of these values.

[0054] In some embodiments, after the second stage of anodization, the metal substrate may be cleaned with pure water at least once to remove impurities on the surface of the second anodized film, such as electrolyte or by-products.

[0055] Step S4, performing sealing treatment and heat treatment on the metal substrate after the second stage of anodizing treatment.

[0056] Specifically, after the second stage of anodization, the anodic oxide film (including the first anodic oxide film and the second anodic oxide film) on the surface of the metal substrate is chemically or physically treated to improve the corrosion resistance and pollution resistance of the anodic oxide film.

[0057] In some embodiments, the pore sealing treatment may include: sealing the metal substrate after the second stage of anodization treatment with a pore sealing agent for 30 minutes to 90 minutes at a temperature of 80°C to 90°C and a pH of 5 to 6, wherein the pore sealing agent may include nickel acetate. Nickel acetate has strong permeability and can evenly seal complex pore structures. In water above 80°C, water and aluminum oxide react to form AlOOH with a boehmite structure, and nickel ions react with hydroxyl groups (OH - ) reacts to form Ni(OH)2 which is deposited in the pore structure, thereby filling the pores and sealing the surface to improve the corrosion resistance, wear resistance, insulation and appearance quality of the anodized film. The chemical reaction equation is as follows: Al2O3 (anodic oxide film) + H2O = 2AlOOH (boehmite) Ni 2+ + 2OH - = Ni(OH)2(↓) Furthermore, after the sealing treatment is complete, the sealing reaction is not yet fully completed. The sealing agent needs time to penetrate the porous layer of the anodic oxide film and chemically react with the pore walls. Therefore, a period of aging exceeding 24 hours is often required after sealing. The sealing process specifically includes: first sealing the pore opening. After the sealing treatment is completed, the sealing reaction continues along the pore walls and then expands toward the center of the pore diameter. After a period of aging, the sealing reaction is completely complete.

[0058] Higher temperature and humidity can accelerate the reaction and penetration of the sealing agent and accelerate aging. Moreover, the volume expansion of the anodized film after heating can, to a certain extent, seal the pore structure or microcracks generated by anodization, thereby improving the sealing effect. Therefore, the metal substrate with the anodized film can also be heat-treated after the sealing treatment. The heat treatment after the anodized film sealing step is completed can provide a high temperature to make the molecules, crystals, etc. in the anodized film active, rearrange, and release the previously accumulated stress. At the same time, the movement of molecules and crystals tends to a natural orientation to achieve equilibrium, thereby further eliminating the internal stress of the anodized film and further reducing the risk of cracking of the anodized film.

[0059] Specifically, the heat treatment may include: washing and drying the metal substrate after the sealing treatment, the washing temperature is 60℃~85℃, the drying temperature is 60℃~70℃, and the drying time is 20min~30min; and boiling and blow-drying the metal substrate after the drying treatment, the boiling temperature is 90℃~95℃, the boiling time is 30min~120min, the blow-drying temperature is 20℃~50℃, and the blow-drying time is 15min~40min.

[0060] Water washing and drying help remove the sealing agent on the metal substrate surface and remove free water molecules within the pores during the sealing process. Some AlOOH will shrink due to water loss during drying, generating stress, which can be released during boiling. In addition, drying causes the free water at the top of the pores to be lost, creating pores, which facilitates the reintroduction of water and heat during boiling, accelerating aging. It should be noted that the surface temperature of the metal substrate after the sealing treatment is still relatively high. If the water washing temperature is too low at this time, it is easy to cause the anodic oxide film to shrink and crack due to cold. Therefore, a water washing temperature of 60°C to 85°C is used.

[0061] The metal substrate after the sealing treatment is then boiled and blown dry. Since oven drying will remove the moisture inside the anodized film, which is not conducive to the aging of the sealing, the product is dried by blowing after boiling.

[0062] Compared with the prior art, the method for anodizing aluminum or aluminum alloy provided in the embodiments of the present application has the following beneficial effects: 1. The stable current density of the first anodization stage is I1, and the peak current density of the first anodization stage is I2. The first anodization stage is performed under the conditions of 19V to 27V and 1.5 I1 < I2 < 2 I1 to form a first anodized film with a porous structure, effectively reducing the generation of internal stress in the first anodized film, reducing the risk of cracking of the first anodized film, and improving the decorative properties, corrosion resistance, and hardness of the anodized film. Furthermore, a slow voltage increase rate can be used to achieve a higher first anodization voltage to control the peak current density I2 of the first anodization stage.

[0063] 2. By adopting a lower second-stage anodizing voltage and carrying out the second-stage anodizing at a lower voltage, the Joule heat of the current is reduced, the growth rate of the second layer of anodized film is slowed down, the pore size of the pore structure is reduced, and the pore density is increased, thereby forming a dense second layer of anodized film, improving the bonding strength between the anodized film and the metal substrate, and further reducing the stress accumulation of the anodized film.

[0064] 3. The aforementioned segmented anodizing treatment of the metal substrate reduces the growth and thermal stresses of the anodic oxide film, resulting in a dense anodic oxide film with a low cracking risk. This method is particularly suitable for substrates with special shapes, including those with curved surfaces or sharp corners. While ensuring product dimensional accuracy, it effectively reduces the risk of cracking and improves the quality of the anodic oxide film. This solution is applicable to the anodizing of all aluminum-based metal substrates with curved surfaces (including 7-series aluminum alloys), and the specific process parameters can be adjusted for different aluminum or aluminum alloys.

[0065] 4. By regulating the duration of the first or second anodizing stage, the thickness of the first and second anodized films can be controlled. Further controlling the proportion of the first and second anodized films in the anodized film can achieve an optimal balance between the mechanical properties and corrosion resistance of the anodized film.

[0066] 5. By sealing and heat treating the metal substrate after anodizing, the corrosion resistance and pollution resistance of the anodized film are further improved, and the internal stress of the anodized film is further reduced, thereby reducing the risk of cracking of the anodized film.

[0067] See also Figure 3 The present application also provides an anodic oxide film, which is prepared using the aforementioned method for anodizing aluminum or an aluminum alloy. The anodic oxide film includes a first anodic oxide film layer and a second anodic oxide film layer. The first anodic oxide film layer serves as an outer barrier, effectively preventing corrosion of the metal substrate from external environments such as water, oxygen, and corrosive media, thereby improving corrosion resistance. The denser second anodic oxide film layer serves as an underlying support structure, dispersing external stress and improving the overall mechanical strength and impact resistance of the anodic oxide film.

[0068] In some embodiments, the thickness of the first anodized film layer can account for 60% to 90% of the total thickness of the first anodized film layer and the second anodized film layer, and correspondingly, the second anodized film layer can account for 10% to 40% of the total thickness of the first anodized film layer and the second anodized film layer. By controlling the proportion of the first anodized film layer and the second anodized film layer in the anodized film layer within the aforementioned range, the anodized film can have both good mechanical properties and corrosion resistance, achieving an optimal balance of various properties of the anodized film.

[0069] In addition, embodiments of the present application further provide a metal workpiece comprising a metal substrate and an anodic oxide film formed on the metal substrate. The anodic oxide film is produced using the aforementioned method for anodizing aluminum or an aluminum alloy, and comprises a second anodic oxide film layer and a first anodic oxide film layer sequentially stacked on the surface of the metal substrate. The metal workpiece can be a special metal workpiece having a special-shaped structure, including a metal workpiece with curved surfaces or sharp corners. While ensuring the dimensional accuracy of the metal workpiece, the risk of cracking of the anodic oxide film can be effectively reduced, and the quality of the anodic oxide film can be improved, thereby obtaining a metal workpiece having excellent hardness and corrosion resistance.

[0070] The above-mentioned method of anodizing aluminum or aluminum alloy and the metal workpiece are further described below through specific embodiments.

[0071] Example 1 Step S1: performing pretreatments of degreasing, alkali biting, chemical polishing, and black film stripping on the curved aluminum alloy metal substrate.

[0072] Step S2: using oxalic acid 40g / L, sulfuric acid 20g / L, Al 3+ The concentration of 5g / L is used as the electrolyte, the pretreated metal substrate is used as the anode, and the graphite is used as the cathode. At 23°C, the metal substrate is subjected to the first stage of anodization. The voltage rise rate of the first stage of anodization is 8V / min. After reaching the voltage of the first stage of anodization of 27V, constant voltage anodization is performed for 50min. The stable current density I1 of the first stage of anodization is 1.9A / dm 2 The peak current density of the first stage of anodization is I2, which is 3.7A / dm 2 , a first layer of anodized film with a thickness of 11.6 μm is formed on the surface of the metal substrate.

[0073] Step S3: The electrolyte and temperature remain unchanged, and the metal substrate is subjected to a second stage of anodization. The voltage of the second stage of anodization is 18 V, and constant voltage oxidation is performed for 20 minutes. A second layer of anodized film with a thickness of 2.5 μm is formed on the surface of the metal substrate. The second layer of anodized film is located between the metal substrate and the first layer of anodized film.

[0074] Step S4: Using nickel acetate as a sealing agent, the metal substrate is sealed at a pH of 5.5 and a temperature of 85°C for 60 minutes. The metal substrate after the sealing treatment is washed and dried at a water temperature of 70°C and a drying temperature of 75°C for 20 to 30 minutes. The metal substrate is then boiled and air-dried at a water temperature of 90°C for 90 minutes and air-dried at a temperature of 20 to 50°C for 15 to 40 minutes to obtain a metal workpiece with an anodic oxide film.

[0075] Example 2 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 6.5 V / min, the peak current density of the first stage of anodization is I2 is 3.3 A / dm 2 , other steps refer to Example 1.

[0076] Example 3 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 5 V / min, the peak current density of the first stage of anodization is I2 is 3.0 A / dm 2 , other steps refer to Example 1.

[0077] Example 4 The steps are basically the same as those in Example 1, with the main difference being that in step S2, the voltage rise rate of the first stage of anodization is 4 V / min, and the peak current density of the first stage of anodization is I2, which is 2.9 A / dm 2 , other steps refer to Example 1.

[0078] Example 5 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 5 V / min, the voltage of the first stage of anodization is 25 V, and the stable current density I1 of the first stage of anodization is 1.5 A / dm 2 The peak current density of the first stage of anodization is I2, which is 2.7A / dm 2 , other steps refer to Example 1.

[0079] Example 6 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 1.5 V / min, the voltage of the first stage of anodization is 19 V, and the stable current density I1 of the first stage of anodization is 1 A / dm 2 The peak current density of the first stage of anodization is I2, which is 1.5A / dm 2 , other steps refer to Example 1.

[0080] Example 7 The steps are basically the same as those in Example 1, with the main difference being that in step S3, the voltage of the second stage of anodization is 15 V. The other steps refer to Example 1.

[0081] Example 8 The steps are basically the same as those in Example 1, with the main differences being that in step S2, the first anodization time is 55 minutes, and the thickness of the first anodized film is 12 μm; and in step S3, the second anodization time is 25 minutes, and the thickness of the second anodized film is 3 μm. The other steps are the same as those in Example 1.

[0082] Comparative Example 1 Step S1: performing pretreatments of degreasing, alkali biting, chemical polishing and black film stripping on the aluminum alloy metal substrate.

[0083] Step S2: using oxalic acid 40g / L, sulfuric acid 20g / L, Al 3+ The concentration is 5g / L as the electrolyte, the pretreated metal substrate is used as the anode, and the graphite is used as the cathode. The metal substrate is anodized at 27°C, wherein the anodization voltage rise rate is 8V / min. After reaching the anodization voltage of 27V, constant voltage anodization is performed for 60 minutes to form an anodized film with a thickness of 13.2μm on the surface of the metal substrate.

[0084] Step S3: Using nickel acetate as a sealing agent, the metal substrate is sealed at a pH of 5.5 and a temperature of 85°C for 60 minutes. The metal substrate after the sealing treatment is washed and dried at a water temperature of 70°C and a drying temperature of 75°C for 20 to 30 minutes. The metal substrate is then boiled and air-dried at a water temperature of 90°C for 90 minutes and air-dried at a temperature of 20 to 50°C for 15 to 40 minutes to obtain a metal workpiece with an anodic oxide film.

[0085] Comparative Example 2 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 9 V / min, the peak current density of the first stage of anodization is I2 is 3.9 A / dm 2 , other steps refer to Example 1.

[0086] Comparative Example 3 The steps are basically the same as those in Example 1, except that in step S2, the voltage rise rate of the first stage of anodization is 1.3 V / min, and the peak current density of the first stage of anodization is I2, which is 2.2 A / dm 2 , other steps refer to Example 1.

[0087] Corresponding tests were performed on the anodic oxide films obtained in Examples 1-8 and Comparative Examples 1-3.

[0088] Test method: 1. Scanning electron microscope (SEM) test: A JSM-IT210 scanning electron microscope was used. This instrument has high-resolution imaging capabilities and can clearly observe the microscopic morphology and structural characteristics of the anodized film.

[0089] 2. Observation of the performance of the anodic oxide film: (1) RT sectioning to observe the cracking or powdering of the anodic oxide film; Surface hardness of the anodic film: (2) Use a regular quadrangular pyramid diamond indenter (the angle between the two opposite sides of the top is 136°) to press into the sample surface under the action of a specified test force F. After maintaining the test force for a specified time, remove the test force and calculate the Vickers hardness by measuring the length d of the two diagonals of the indentation on the sample surface, that is, HV≈0.1891×(F / d 2 ).

[0090] 3. Anodic oxide film corrosion resistance test: The product is cyclically immersed in artificial sweat of a certain ratio, and then a constant temperature and humidity cabinet is used to provide certain temperature and humidity conditions. The various chemical components in the artificial sweat will chemically corrode the anodic oxide film structure.

[0091] The relevant test results of Examples 1-8 and Comparative Examples 1-3 are shown in Table 1.

[0092] The above results show: like Figure 4 The SEM picture of the anodic oxide film in Example 1 is shown in FIG. Figure 4 It can be seen that the anodic oxide film of the metal workpiece in Example 1 includes a first anodic oxide film and a second anodic oxide film, and the anodic oxide film is continuous and dense without any cracks. Figure 5 This is the SEM image of the anodized film of the metal workpiece in Comparative Example 1. There are obvious cracks on the curved surface, which affects the quality of the anodized film. Figure 4 and Figure 5 , indicating that the anodizing method provided in Example 1 of the present application can effectively reduce the internal stress of the anodized film and reduce the risk of cracking of the anodized film.

[0093] The results in Table 1 show that Examples 1-8 adopted a segmented anodizing method, controlling the boost rate and pressure of the first anodizing stage, as well as the pressure of the second anodizing stage. Compared with the one-step anodizing at a constant voltage in Comparative Example 1, the cracking of the anodized film was significantly improved, and the corrosion resistance of the anodized film was also improved. In Comparative Example 2, the boost rate of the first anodizing stage was too fast, resulting in an increase in the peak current, too fast growth of the first layer of anodized film, increased growth stress and thermal stress, and an increased risk of cracking of the anodized film. In Comparative Example 3, the boost rate of the first anodizing stage was too low, resulting in a loose structure of the first layer of anodized film, decreased hardness and corrosion resistance, and pulverization of the anodized film.

[0094] In Examples 1-6, by controlling the ramp rate and voltage of the first anodization stage and adjusting the peak current density I2 of the first anodization stage, the growth stress and thermal stress of the anodized film are reduced, thereby reducing the risk of cracking of the anodized film. Compared with Example 1, Example 7 changes the voltage of the second anodization stage, thereby increasing the hardness of the second anodized film and thus the overall hardness of the anodized film. Compared with Example 1, Example 8 increases the thickness of the first and second anodized films by regulating the duration of the two anodization stages.

[0095] It can be seen from this that Examples 1-8 of the present application use a segmented anodizing method, and control the boost rate and pressure of the first anodizing stage, as well as control the pressure of the second anodizing stage. Compared with the existing anodizing methods, they can reduce the risk of cracking of the anodized film and improve the overall performance of the anodized film, such as hardness and corrosion resistance. They are applicable to metal substrates whose surfaces include non-planar areas, such as metal substrates with special-shaped structures such as curved surfaces or sharp corners.

[0096] The description of the above embodiments is only used to help understand the method of this application and its core ideas; in addition, for ordinary technicians in this field, various other corresponding changes and modifications can be made according to the technical concept of this application, and all these changes and modifications should fall within the scope of protection of the claims of this application.

Claims

1. A method for anodizing aluminum or an aluminum alloy, characterized in that: include: Performing a first-stage anodization on a metal substrate of aluminum or aluminum alloy to form a first layer of anodized film on the surface of the metal substrate, wherein the voltage of the first-stage anodization is 19V to 27V, the stable current density of the first-stage anodization is I1, the peak current density of the first-stage anodization is I2, and 1.5 I1 < I2 < 2 I1; and The metal substrate is subjected to a second stage of anodization to form a second layer of anodized film on the surface of the metal substrate, wherein the second layer of anodized film is located between the metal substrate and the first layer of anodized film, wherein the voltage of the second stage of anodization is 15V~18V.

2. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The range of the stable current density I1 of the first stage of anodization is 1A / dm 2 ~2A / dm 2 and / or The peak current density I2 of the first stage of anodization is in the range of 1.5A / dm 2 ~4A / dm 2 .

3. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: In the first anodizing step, the voltage rise rate of the first anodizing step is 1.5 V / min to 8 V / min.

4. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The surface of the metal substrate includes a non-planar area, and the first anodized film is formed in the non-planar area.

5. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The oxidation time of the first anodization stage is greater than the oxidation time of the second anodization stage.

6. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The first anodizing period is 45 to 53 minutes; and / or The second stage of anodizing lasts for 18 to 25 minutes.

7. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The thickness of the first anodized film accounts for 60% to 90% of the total thickness of the first anodized film and the second anodized film.

8. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The thickness of the first anodic oxide film is 10 μm to 14 μm; and / or The thickness of the second anodic oxide film is 1 μm to 3 μm.

9. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: The electrolytes of the first anodizing stage and the second anodizing stage both include oxalic acid, sulfuric acid and Al 3+ , where the concentration of oxalic acid is 30g / L~180g / L, the concentration of sulfuric acid is 20g / L~120g / L, and Al 3+ The concentration of oxalic acid is 0~10g / L, the mass concentration ratio of oxalic acid and sulfuric acid is sulfuric acid: oxalic acid = 1: (1.5~3), and the temperature of the first anodizing stage and the second anodizing stage are both 21℃~27℃.

10. The method for anodizing aluminum or aluminum alloy according to claim 1, characterized in that: After the step of performing a second anodizing step on the metal substrate, the method further comprises: The metal substrate after the second stage anodizing treatment is subjected to sealing treatment and heat treatment.

11. The method for anodizing aluminum or aluminum alloy according to claim 10, characterized in that: The sealing treatment includes: sealing the metal substrate after the second anodizing treatment with a sealing agent for 30 minutes to 90 minutes at a temperature of 80° C. to 90° C. and a pH of 5 to 6, wherein the sealing agent includes nickel acetate.

12. The method for anodizing aluminum or aluminum alloy according to claim 10, characterized in that: The heat treatment comprises: Washing and drying the metal substrate after the sealing treatment, wherein the washing temperature is 60° C. to 85° C., the drying temperature is 60° C. to 80° C., and the drying time is 20 min to 30 min; and The dried metal substrate is boiled in water and blown dry, wherein the boiling temperature is 90° C. to 95° C., the boiling time is 30 min to 120 min, the drying temperature is 20° C. to 50° C., and the drying time is 15 min to 40 min.

13. A metal workpiece, characterized in that: It includes a metal substrate and an anodized film formed on the metal substrate, the anodized film is prepared by the anodizing method according to any one of claims 1 to 12, and the anodized film includes a second anodized film and a first anodized film sequentially stacked on the surface of the metal substrate.

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