A micro-arc oxidation composite coating for thermally broken aluminum profiles and its preparation process

Abstract

This invention relates to the field of aluminum alloy surface treatment technology, and discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles. The process includes: Step 1: Pre-treatment with zinc phosphate chemical conversion on the surface of the aluminum profile to form a zinc phosphate crystallization conversion film; Step 2: Micro-arc oxidation treatment with a ternary alkaline electrolyte containing trisodium phosphate, calcium nitrate, and sodium silicate, depositing a Ca-P-Si composite sealing phase with hydroxyapatite as the main component in situ within the discharge channels; Step 3: Complexation and sealing treatment with sodium citrate solution. The resulting composite coating consists of a zinc phosphate conversion film bottom layer, a micro-arc oxidation ceramic composite layer containing the Ca-P-Si composite sealing phase, and a citrate metal complex sealing top layer, from the aluminum substrate to the outer surface. This effectively alleviates the thermal expansion coefficient mismatch problem between the aluminum substrate and the micro-arc oxidation ceramic layer, and provides long-term stable protective performance under temperature-changing cycling conditions.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy surface treatment technology, and more specifically, to a preparation process of a micro-arc oxidation composite coating for thermally broken aluminum profiles. Background Technology

[0002] Thermally broken aluminum profiles are widely used in building doors, windows, and curtain walls. While aluminum alloy substrates naturally form an alumina passivation layer in the atmosphere, this passivation layer is thin and porous, resulting in insufficient corrosion resistance under coastal salt spray, acid rain, and industrial pollution atmospheres. This leads to pitting and filiform corrosion on the aluminum profile surface, affecting the long-term service life of the profiles and the building's appearance. Micro-arc oxidation (MAO) is an electrochemical treatment process that grows an alumina ceramic coating in situ on the aluminum alloy surface. The resulting coating has high hardness and strong adhesion to the substrate, making it an effective means to improve the corrosion resistance and wear resistance of aluminum profiles. However, the inherent discharge channels in the MAO coating create open pores, allowing corrosive media to penetrate into the substrate along these pores. Existing post-sealing processes (such as hot water sealing and chromate sealing) are applied independently of the MAO process, increasing the number of steps. Furthermore, the sealing material differs significantly in chemical composition from the MAO phosphate ceramic layer, resulting in weak interfacial bonding and making the pores prone to cracking and failure during service.

[0003] The coefficient of thermal expansion (CTE) of the aluminum alloy matrix is ​​approximately The CTE of the alumina ceramic layer formed by micro-arc oxidation is approximately The difference between the two is approximately When thermally broken aluminum profiles are used in northern regions, they face a temperature-changing environment with an annual temperature difference of 60-70℃ and a diurnal temperature difference of 20-30℃. Thermal stress accumulates at the interface between the aluminum substrate and the ceramic coating, inducing microcracks and creating channels for corrosive media penetration, accelerating coating failure. If the coefficient of thermal expansion of the pore-sealing filling phase differs significantly from that of the MAO alumina matrix, micro-gaps will also form between the pore wall and the filler during temperature cycling, causing a long-term decrease in the density of the discharge channel seal.

[0004] Zinc phosphate chemical conversion treatment, as a base treatment process before coating aluminum alloy profiles, can generate zinc phosphate in situ on the surface of the aluminum alloy substrate. Zinc phosphate conversion films improve the interfacial adhesion between the coating and the substrate, and have engineering application value in the pretreatment process of architectural aluminum profiles. However, zinc phosphate conversion films undergo localized surface dissolution in alkaline environments (pH 10–12), releasing... It rapidly converts to zinc hydroxide under alkaline conditions. Zinc deposits accumulate in the pores or inner walls of the contacted channels. When aluminum profiles are pretreated with zinc phosphate and then subjected to micro-arc oxidation in an alkaline electrolyte, the above process leaves zinc-containing deposits within the micro-arc oxidation discharge channels. When corrosive media penetrate, these zinc-containing deposits can dissolve to form soluble zinc salts, which then migrate outwards, posing a threat to the long-term stability of the sealing system. Existing sealing processes (such as hot water sealing and chromate sealing) do not include specific treatment methods for zinc-containing deposits within the channels.

[0005] Existing technologies have not yet proposed a process that can simultaneously solve the following problems: thermal cracking caused by the mismatch in thermal expansion coefficients between the aluminum substrate and the MAO ceramic layer, weak interfacial bonding caused by poor chemical compatibility between the sealing phase and the MAO ceramic layer, and insufficient long-term stability of the sealing layer caused by zinc deposits in the pores. Summary of the Invention

[0006] This invention provides a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, in order to solve the problems of thermal cracking caused by the mismatch of thermal expansion coefficients between the aluminum substrate and the micro-arc oxidation ceramic layer, the weak interfacial bonding due to poor chemical compatibility between the sealing phase and the ceramic layer, and the insufficient long-term stability of the sealing layer caused by zinc deposits in the pores.

[0007] This invention provides a process for preparing a micro-arc oxidation composite coating for thermally broken aluminum profiles, comprising the following steps:

[0008] Step 1: After surface pretreatment of the thermally broken aluminum profile, it is placed in a zinc phosphate chemical conversion treatment solution containing phosphoric acid, zinc nitrate, and sodium nitrite to generate zinc phosphate on the surface of the aluminum alloy substrate. Crystallization conversion membrane;

[0009] Step Two: The aluminum profile treated in Step One is placed in a ternary alkaline electrolyte containing trisodium phosphate, calcium nitrate, and sodium silicate. Using the aluminum profile as the anode and the titanium plate as the cathode, micro-arc oxidation is performed in a constant current mode. This causes phosphate, calcium, and silicate ions in the electrolyte to deposit in situ within the discharge channels, generating hydroxyapatite (… ) is the main component, calcium silicate ( A Ca-P-Si composite sealing phase, with Ca as the minor component, is filled into the discharge channels of the micro-arc oxidation ceramic layer to obtain a micro-arc oxidation ceramic composite layer containing the Ca-P-Si composite sealing phase.

[0010] Step 3: After cleaning the aluminum profiles processed in Step 2, immerse them in a sodium citrate aqueous solution. This allows the citrate ions to complex with the zinc ions dissolved from the residual zinc deposits on the inner wall of the discharge channel and the residual calcium ions in the channel, respectively, forming zinc citrate complexes and calcium citrate complexes. These complexes are then deposited in situ within the discharge channel to fill the residual gaps between the Ca-P-Si composite sealing phase particles. After cleaning and drying, a micro-arc oxidation composite coating for the thermally broken aluminum profiles is obtained.

[0011] Preferably, the surface pretreatment in step one includes, in sequence, using trisodium phosphate (… Degreased by immersing in a 20–40 g / L solution at 50–70°C for 35 min, followed by rinsing with water; then alkali-etched by immersing in a 35% NaOH solution at 15–35°C for 1–3 min, followed by thorough rinsing with water; finally, brightened by immersing in a 15–25% nitric acid solution at 15–35°C for 30–90 s, followed by thorough rinsing with water.

[0012] Preferably, the zinc phosphate chemical conversion treatment solution in step one contains 5-20 g / L phosphoric acid, 10-30 g / L zinc nitrate and 0.1-0.5 g / L sodium nitrite, with a pH of 2-3, a treatment temperature of 40-60℃, a treatment time of 5-15 min, and the resulting zinc phosphate crystallization conversion film has a thickness of 1-5 μm.

[0013] Preferably, the ternary alkaline electrolyte in step two contains 10-30 g / L trisodium phosphate, 3-10 g / L calcium nitrate and 3-8 g / L sodium silicate, and has a pH of 10-12.

[0014] Preferably, in step two, the ternary alkaline electrolyte is continuously mechanically stirred at a speed of 50-200 rpm during the micro-arc oxidation process to ensure that the suspended particles are uniformly dispersed in the electrolyte.

[0015] Preferably, the current density of the micro-arc oxidation treatment in step two is 5-15 A / dm², the pulse frequency is 50-1000 Hz, and the duty cycle is 10-30%.

[0016] Preferably, the electrolyte temperature for the micro-arc oxidation treatment in step two is 20–40°C, the treatment time is 20–60 min, and the thickness of the resulting micro-arc oxidation ceramic composite layer is 20–60 μm.

[0017] Preferably, in step three, the concentration of sodium citrate in the sodium citrate aqueous solution is 5-15 g / L, the pH is adjusted to 6-8 with citric acid, the immersion treatment temperature is 40-60°C, and the treatment time is 15-30 min.

[0018] Preferably, after cleaning in step three, the product is dried at 60–80°C for 20–40 minutes.

[0019] This invention also provides a micro-arc oxidation composite coating for thermally broken aluminum profiles obtained by the above-described preparation process. The composite coating, from the aluminum substrate to the outer surface, consists of: a zinc phosphate crystallization conversion film bottom layer with a thickness of 1–5 μm; a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of 20–60 μm; and a sealing top layer composed of zinc citrate complex and calcium citrate complex. The micro-arc oxidation ceramic composite layer is composed of an alumina ceramic matrix and a Ca-P-Si composite sealing phase filled within the discharge channels. The Ca-P-Si composite sealing phase is composed of hydroxyapatite (… ) is the main component, calcium silicate ( () is a minor component.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. Ca-P-Si composite sealing phase (mainly hydroxyapatite, CTE approximately...) After being filled into the micro-arc oxidation discharge channels, the effective thermal expansion coefficient of the micro-arc oxidation ceramic composite layer containing the Ca-P-Si filling phase is higher than that of the pure alumina micro-arc oxidation ceramic layer without this filling phase (CTE approximately). ), aluminum substrate (CTE approximately The difference in the coefficient of thermal expansion between the aluminum substrate and the micro-arc oxidation ceramic composite layer is reduced, which reduces the thermal stress concentration at the aluminum substrate-ceramic layer interface during temperature change cycles, reduces the generation of microcracks at the interface, and improves the long-term service stability of the coating under temperature change environment.

[0022] 2. Hydroxyapatite (CTE approx.) ) and alumina pore walls (CTE approximately The difference in the coefficients of thermal expansion between them is approximately The tendency for micro-gaps to form between the pore wall and the filler during temperature cycling is lower, and the density of the discharge channel sealing decreases less over service time.

[0023] 3. The deposition of phosphate and Ca-P-Si phases occurs simultaneously with the growth of the alumina ceramic layer. The sealing function is built into the micro-arc oxidation process, reducing the need for separate sealing steps and simplifying the process flow.

[0024] 4. The zinc phosphate substrate and the phosphate-based micro-arc oxidation ceramic layer both belong to the inorganic phosphate chemical system. The chemical composition at the interface is continuous, which improves the abrupt change in chemical composition between the substrate and the ceramic layer compared to micro-arc oxidation directly on the aluminum substrate, thus reducing the risk of interlayer delamination.

[0025] 5. Citrate complexation sealing step: This step utilizes a weakly acidic sodium citrate solution (pH 6-8) to seal any residual pores on the inner wall of the pores. After partial dissolution, it is immediately complexed and fixed by citrate ions into a low-solubility zinc citrate complex, eliminating the risk of dissolution and expanded corrosion under corrosive media. At the same time, the zinc citrate and calcium citrate complexes fill the residual gaps between Ca-P-Si particles, further reducing the effective permeability of the pores, resulting in a composite protective coating with a more complete pore sealing effect than the existing single-step sealing process. Attached Figure Description

[0026] Figure 1 This is a cross-sectional schematic diagram of the micro-arc oxidation composite coating structure of the thermally broken aluminum profile of the present invention;

[0027] Figure 2 This is a BSE-SEM cross-sectional topography image (backscattered electron mode, magnification approximately 3000×) of the sample obtained in Example 1 of this invention.

[0028] Figure 3 This is a comparison of the surface morphology of the sample obtained in Example 1 of the present invention before and after sodium citrate sealing treatment (secondary electron mode, magnification of about 5000×).

[0029] Figure 4 These are XRD phase analysis comparison spectra of the sample obtained in Example 1 of this invention and the control sample;

[0030] Figure 5 This is a comparison chart of the neutral salt spray corrosion resistance test results of various samples;

[0031] Figure 6 This is a graph showing the change of low-frequency impedance modulus of electrochemical impedance spectroscopy with the number of cycles after temperature cycling for each comparative sample.

[0032] In the figure: 1-Aluminum alloy substrate; 2-Zinc phosphate crystallization conversion film bottom layer; 3-Micro-arc oxidation ceramic composite layer containing Ca-P-Si composite sealing phase; 4-Citrate metal complex sealing top layer; 5-Discharge channel; 6-Ca-P-Si composite sealing phase (hydroxyapatite main phase + calcium silicate secondary phase). Detailed Implementation

[0033] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the spirit and essence of the invention. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0034] Example 1

[0035] This embodiment discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, which includes the following steps:

[0036] Step 1: Pretreatment of Zinc Phosphate Chemical Conversion

[0037] Take aluminum alloy profiles with thermal break (alloy grade 6063) and sequentially perform degreasing, alkaline etching, brightening, and zinc phosphate chemical conversion treatment. Degreasing: with trisodium phosphate (… Immerse in 30 g / L NaOH solution at 60°C for 4 min, then rinse with water; Alkaline etching: Immerse in 4% NaOH solution at 25°C for 2 min, then rinse thoroughly with water; Brightening: Immerse in 20% nitric acid solution (… The aluminum profile was immersed in a solution at 25°C for 60 seconds, then thoroughly rinsed with water to obtain a clean surface. The aluminum profile was then placed in a zinc phosphate chemical conversion treatment solution containing phosphoric acid (…). 12g / L, zinc nitrate ( 20g / L, sodium nitrite ( 0.3 g / L, pH 2.5, was immersed at 50 °C for 10 min to form a zinc phosphate layer with a thickness of approximately 2.5 μm on the surface of an aluminum alloy substrate. The crystallization conversion membrane should be thoroughly rinsed with water before use.

[0038] Step 2: Micro-arc oxidation treatment with a ternary electrolyte containing phosphate-calcium salt-silicate

[0039] Preparation of ternary alkaline electrolyte: trisodium phosphate 20g / L, calcium nitrate ( ) 6g / L, sodium silicate ( Add 5 g / L of NaOH to adjust the pH to 11, and continuously stir mechanically at 100 rpm. Place the aluminum profile treated in step one into the above electrolyte, using the aluminum profile as the anode and the titanium plate as the cathode, and perform constant current micro-arc oxidation treatment under the conditions of current density 10 A / dm², pulse frequency 500 Hz, duty cycle 20%, and electrolyte temperature 30°C for 40 min. This forms a micro-arc oxidation ceramic composite layer with a thickness of about 35 μm containing a Ca-P-Si composite sealing phase on the surface of the aluminum profile.

[0040] Step 3: Citrate complexation followed by pore sealing

[0041] After the aluminum profiles processed in step two are removed from the electrolyte, thoroughly rinsed with water, and then placed in sodium citrate ( The aluminum profile was immersed in an aqueous solution (concentration 10 g / L, pH adjusted to 7.0 with citric acid) at 50°C for 20 min, then rinsed with water and dried at 70°C for 30 min to obtain a micro-arc oxidation composite coating.

[0042] The resulting composite coating, from the aluminum substrate to the outer surface, consists of: a zinc phosphate crystallization conversion film base layer with a thickness of approximately 2.5 μm; a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of approximately 35 μm (wherein the Ca-P-Si composite sealing phase is mainly composed of hydroxyapatite and secondarily of calcium silicate, filling the discharge channels); and a zinc citrate complex (…). ) and calcium citrate complex ( The top layer of the sealing layer, as shown in the schematic diagram of the coating cross-section, is composed of... Figure 1 As shown, the BSE-SEM cross-sectional morphology of the obtained sample is shown in [reference needed]. Figure 2 .

[0043] Example 2

[0044] This embodiment discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, which includes the following steps:

[0045] Step 1: Pretreatment of Zinc Phosphate Chemical Conversion

[0046] Aluminum alloy profiles with thermal break (alloy grade 6063) were subjected to degreasing, alkaline etching, brightening, and zinc phosphate chemical conversion treatment in sequence. Degreasing: Immersion in trisodium phosphate solution (20 g / L) at 50°C for 3 min, followed by rinsing with water. Alkaline etching: Immersion in 3% NaOH solution at 15°C for 1 min, followed by thorough rinsing with water. Brightening: Immersion in 15% nitric acid solution at 15°C for 30 s, followed by thorough rinsing with water, resulting in a clean aluminum profile. The aluminum profile was then placed in a zinc phosphate chemical conversion treatment solution containing 5 g / L phosphoric acid, 10 g / L zinc nitrate, and 0.1 g / L sodium nitrite, with a pH of 2.2, and immersed at 40°C for 5 min. This generated a zinc phosphate crystallization conversion film approximately 1 μm thick on the aluminum alloy substrate surface, followed by thorough rinsing with water for later use.

[0047] Step 2: Micro-arc oxidation treatment with a ternary electrolyte containing phosphate-calcium salt-silicate

[0048] Prepare a ternary alkaline electrolyte: trisodium phosphate 10 g / L, calcium nitrate 3 g / L, sodium silicate 3 g / L, adjust the pH to 10 with NaOH, and continuously stir mechanically at 50 rpm. Place the aluminum profile treated in step one into the above electrolyte, using the aluminum profile as the anode and the titanium plate as the cathode, and perform constant current micro-arc oxidation treatment under the conditions of current density 5 A / dm², pulse frequency 100 Hz, duty cycle 10%, and electrolyte temperature 20℃ for 20 min. This forms a micro-arc oxidation ceramic composite layer with a thickness of approximately 20 μm containing a Ca-P-Si composite sealing phase on the surface of the aluminum profile.

[0049] Step 3: Citrate complexation followed by pore sealing

[0050] After the aluminum profiles that have completed step two are removed from the electrolyte, thoroughly rinsed with water, and then placed in a sodium citrate aqueous solution (concentration 5g / L, pH adjusted to 6.5 with citric acid) for immersion treatment at 40℃ for 15min. After removal, rinsed with water and dried at 60℃ for 20min, a micro-arc oxidation composite coating for thermally broken aluminum profiles is obtained.

[0051] The resulting composite coating consists of, from the aluminum substrate to the outer surface, the following layers: a zinc phosphate crystallization conversion film with a thickness of about 1 μm, a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of about 20 μm, and a sealing top layer composed of zinc citrate complex and calcium citrate complex.

[0052] Example 3

[0053] This embodiment discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, which includes the following steps:

[0054] Step 1: Pretreatment of Zinc Phosphate Chemical Conversion

[0055] Aluminum alloy profiles with thermal break (alloy grade 6063) were subjected to degreasing, alkaline etching, brightening, and zinc phosphate chemical conversion treatment in sequence. Degreasing: Immersion in trisodium phosphate solution (40 g / L) at 70℃ for 5 min, followed by rinsing with water. Alkaline etching: Immersion in 5% NaOH solution at 35℃ for 3 min, followed by thorough rinsing with water. Brightening: Immersion in 25% nitric acid solution at 35℃ for 90 s, followed by thorough rinsing with water, resulting in a clean aluminum profile. The aluminum profile was then placed in a zinc phosphate chemical conversion treatment solution containing 18 g / L phosphoric acid, 28 g / L zinc nitrate, and 0.5 g / L sodium nitrite, with a pH of 2.8, and immersed at 58℃ for 15 min. This generated a zinc phosphate crystallization conversion film approximately 5 μm thick on the aluminum alloy substrate surface, followed by thorough rinsing with water for later use.

[0056] Step 2: Micro-arc oxidation treatment with a ternary electrolyte containing phosphate-calcium salt-silicate

[0057] Prepare a ternary alkaline electrolyte: trisodium phosphate 30 g / L, calcium nitrate 10 g / L, sodium silicate 8 g / L, adjust the pH to 12 with NaOH, and continuously stir mechanically at 200 rpm. Place the aluminum profile treated in step one into the above electrolyte, using the aluminum profile as the anode and the titanium plate as the cathode, and perform constant current micro-arc oxidation treatment under the conditions of current density 15 A / dm², pulse frequency 1000 Hz, duty cycle 30%, and electrolyte temperature 40℃ for 60 min. This forms a micro-arc oxidation ceramic composite layer with a thickness of approximately 55 μm containing a Ca-P-Si composite sealing phase on the surface of the aluminum profile.

[0058] Step 3: Citrate complexation followed by pore sealing

[0059] After the aluminum profiles that have completed step two are removed from the electrolyte, thoroughly rinsed with water, and then placed in a sodium citrate aqueous solution (concentration 15g / L, pH adjusted to 7.5 with citric acid), immersed at 60℃ for 30min, removed, rinsed with water, and dried at 80℃ for 40min to obtain the micro-arc oxidation composite coating of the thermally broken aluminum profiles.

[0060] The resulting composite coating consists of, from the aluminum substrate to the outer surface, the following layers: a zinc phosphate crystallization conversion film with a thickness of approximately 5 μm, a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of approximately 55 μm, and a sealing top layer composed of zinc citrate complex and calcium citrate complex.

[0061] Example 4

[0062] This embodiment discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, which includes the following steps:

[0063] Step 1: Pretreatment of Zinc Phosphate Chemical Conversion

[0064] Aluminum alloy profiles with thermal break (alloy grade 6063) were subjected to degreasing, alkaline etching, brightening, and zinc phosphate chemical conversion treatment in sequence. Degreasing: Immersion in trisodium phosphate solution (concentration 35g / L) at 55℃ for 5min, followed by rinsing with water; Alkaline etching: Immersion in 3.5% NaOH solution at 30℃ for 2min, followed by thorough rinsing with water; Brightening: Immersion in 22% nitric acid solution at 20℃ for 60s, followed by thorough rinsing with water, resulting in a clean aluminum profile. The aluminum profile was then placed in a zinc phosphate chemical conversion treatment solution containing 15g / L phosphoric acid, 25g / L zinc nitrate, and 0.2g / L sodium nitrite, with a pH of 2.5, and immersed at 45℃ for 12min. This generated a zinc phosphate crystalline conversion film with a thickness of approximately 3μm on the aluminum alloy substrate surface, followed by thorough rinsing with water for later use.

[0065] Step 2: Micro-arc oxidation treatment with a ternary electrolyte containing phosphate-calcium salt-silicate

[0066] Prepare a ternary alkaline electrolyte: trisodium phosphate 15 g / L, calcium nitrate 8 g / L, sodium silicate 6 g / L, adjust the pH to 11.5 with NaOH, and continuously stir mechanically at 150 rpm. Place the aluminum profile treated in step one into the above electrolyte, using the aluminum profile as the anode and the titanium plate as the cathode, and perform constant current micro-arc oxidation treatment under the conditions of current density 8 A / dm², pulse frequency 300 Hz, duty cycle 25%, and electrolyte temperature 35℃ for 50 min. This forms a micro-arc oxidation ceramic composite layer with a thickness of approximately 42 μm containing a Ca-P-Si composite sealing phase on the surface of the aluminum profile.

[0067] Step 3: Citrate complexation followed by pore sealing

[0068] After the aluminum profiles that have completed step two are removed from the electrolyte, thoroughly rinsed with water, and then placed in a sodium citrate aqueous solution (concentration 12g / L, pH adjusted to 6.5 with citric acid) for immersion treatment at 55℃ for 25min. After removal, rinsed with water and dried at 65℃ for 35min, a micro-arc oxidation composite coating for thermally broken aluminum profiles is obtained.

[0069] The resulting composite coating consists of, from the aluminum substrate to the outer surface, the following layers: a zinc phosphate crystallization conversion film with a thickness of approximately 3 μm, a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of approximately 42 μm, and a sealing top layer composed of zinc citrate complex and calcium citrate complex.

[0070] Example 5

[0071] This embodiment discloses a preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, which includes the following steps:

[0072] Step 1: Pretreatment of Zinc Phosphate Chemical Conversion

[0073] The thermally broken aluminum profiles underwent surface pretreatment in sequence, including degreasing, alkaline etching, brightening, and water washing: using trisodium phosphate (... The substrate was immersed in a 30 g / L degreasing solution at 60°C for 4 minutes to remove oil stains, and then rinsed with water. Subsequently, it was immersed in a 4% NaOH solution at 25°C for 2 minutes (alkaline etching) to remove the natural oxide film on the aluminum alloy substrate surface, and then thoroughly rinsed with water. Finally, it was immersed in a 20% nitric acid solution (…). The aluminum profile is immersed in the solution at 25°C for 60 seconds (to remove the aluminum dust remaining on the surface of the aluminum alloy after alkaline etching) and then thoroughly rinsed with water to obtain a clean aluminum profile.

[0074] The aluminum profiles were then placed in a zinc phosphate chemical conversion treatment solution for chemical conversion treatment. The treatment solution contained phosphoric acid (…). Concentration 12g / L), zinc nitrate ( (concentration 20g / L) and sodium nitrite ( Zinc phosphate (ZP) was generated on the surface of an aluminum alloy substrate at a concentration of 0.3 g / L (used as an oxidation promoter to accelerate the phosphating reaction), pH 2.5, treatment temperature 50℃, and treatment time 10 min. A crystallization conversion film, approximately 2–3 μm thick, is applied. After the chemical conversion treatment is completed, the aluminum profile is thoroughly rinsed with clean water to remove any residual phosphating solution from the surface, and then set aside for use.

[0075] The zinc phosphate chemical conversion pretreatment is a known technique in the field. The purpose of this step is to form a crystalline conversion film belonging to the inorganic phosphate system on the surface of the aluminum substrate. This conversion film, along with the phosphate electrolyte used in the subsequent step two and the generated micro-arc oxidation ceramic layer, belongs to the same inorganic phosphate chemical system. The chemical composition at the interface is continuous, providing a transitional inorganic phosphate underlayer in terms of chemical composition between the aluminum substrate and the micro-arc oxidation ceramic layer, so that the chemical composition of the interface between the aluminum substrate and the micro-arc oxidation ceramic layer changes from abrupt to continuous transition.

[0076] The difference from existing technologies: Existing technologies typically involve directly subjecting aluminum profiles to micro-arc oxidation after degreasing and alkaline etching. This results in a sudden shift in chemical composition at the interface between the aluminum substrate and the alumina ceramic layer, lacking a continuous inorganic phosphate underlayer with a smooth chemical transition. This step introduces a zinc phosphate chemical conversion pretreatment, forming an inorganic phosphate underlayer with a chemically continuous structure with the subsequent diphosphate-based micro-arc oxidation ceramic layer. This is a significant difference from existing technologies.

[0077] Step 2: Micro-arc oxidation treatment with a ternary electrolyte containing phosphate-calcium salt-silicate

[0078] This step is the core and critical step of this process.

[0079] 2.1 Preparation of ternary electrolyte

[0080] Prepare an alkaline electrolyte solution (pH 11) containing the following three components: - trisodium phosphate ( ), concentration 20 g / L; - Calcium nitrate ( ), concentration 6 g / L; - Sodium silicate ( ), concentration 5g / L.

[0081] All three components are inorganic salts and can be directly dissolved in water for preparation. Because in the above alkaline electrolyte (pH 11), and When coexisting, the solubility product of calcium phosphate salts may exceed that of calcium phosphate salts, resulting in the formation of a small number of suspended particles. During use, the electrolyte should be continuously mechanically stirred at 100 rpm to ensure that the suspended particles are evenly dispersed in the electrolyte, guaranteeing the micro-arc oxidation process. , , The three types of ions are continuously and stably supplied to the discharge channel.

[0082] 2.2 Micro-arc oxidation treatment

[0083] The aluminum profile processed in step one was placed in the aforementioned ternary electrolyte. Using the aluminum profile as the anode and the titanium plate as the cathode, micro-arc oxidation was performed in a constant current mode with a current density of 10 A / dm², a pulse frequency of 500 Hz, a duty cycle of 20%, and the electrolyte temperature controlled at 30°C for 40 minutes. Under these conditions, an alumina ceramic layer containing discharge channels gradually formed on the surface of the aluminum substrate, and the thickness of the resulting micro-arc oxidation ceramic layer was approximately 35 μm.

[0084] 2.3 In-situ formation of Ca-P-Si composite sealing phase

[0085] The principle behind this step is that by simultaneously introducing [the substance] into the electrolyte... , , Three types of ions are deposited in situ within the discharge channels using the unique local high-temperature-rapid cooling process during micro-arc oxidation discharge channel formation, forming a Ca-P-Si composite sealing phase that fills the discharge channels. Specifically, at the instant of micro-arc discharge, the local temperature within the channels rises sharply to approximately 1000–4000℃, and the electrolyte... , , and in alkaline electrolyte The material is introduced into the high-temperature region of the discharge channel, existing in a molten or high-temperature dissolved state within the channel. After the discharge stops, the channel cools at an extremely rapid rate. When the temperature drops to the stable temperature range of hydroxyapatite (approximately 800–1250 °C), the material within the channel... , , Co-crystallization produces hydroxyapatite (HAP, chemical formula...) The sedimentary phase with ) as the main component; and During the cooling process, calcium silicate, a minor component, precipitates out. Together, they constitute the Ca-P-Si composite sealing phase filling the discharge channels.

[0086] The above-mentioned in-situ deposition reaction occurs simultaneously with the growth of the alumina ceramic layer, eliminating the need for an additional independent sealing process. This achieves synchronous self-sealing of the discharge channels by the phosphate electrolyte system during the process.

[0087] 2.4 Improvement of the effective thermal expansion coefficient of micro-arc oxidation ceramic layer by Ca-P-Si composite sealing phase filling

[0088] The Ca-P-Si composite sealing phase is dominated by HAP, and the coefficient of thermal expansion of HAP is approximately [missing value]. (This value represents a known property of HAP materials and is supported by extensive literature data in the field of bioceramics.) This value is higher than the coefficient of thermal expansion of the alumina ceramic matrix formed by micro-arc oxidation. - The CTE is approximately Furthermore, its coefficient of thermal expansion is lower than that of the aluminum alloy matrix (CTE is approximately...). ).

[0089] The Ca-P-Si composite sealing phase is distributed in the form of a filler within the discharge channels of the micro-arc oxide layer, giving the micro-arc oxide layer an alumina matrix (CTE approximately). ) and Ca-P-Si pore-filling phase (CTE approximately A composite structure with coexisting Ca-P-Si phases. Compared with pure alumina micro-arc oxidation ceramic layers without Ca-P-Si filler phases, the effective thermal expansion coefficient of micro-arc oxidation ceramic composite layers containing Ca-P-Si filler phases is improved, thereby reducing the CTE step difference between the aluminum substrate and the micro-arc oxidation ceramic layer, reducing the thermal stress concentration at the interface between the aluminum substrate and the micro-arc oxidation ceramic layer during temperature change cycling, and reducing the generation of interface microcracks.

[0090] Meanwhile, the Ca-P-Si composite sealing phase fills the discharge channels, and the filling material (mainly HAP, CTE approximately) The coefficient of thermal expansion of the alumina matrix on the pore wall is higher than that of the alumina matrix (CTE approximately). ), and the difference in CTE between the well wall and the pore wall (approximately The CTE difference between the filler and the pore wall is much smaller than that of a single sealing phase with a CTE similar to that of an aluminum matrix, thereby reducing the generation of micro-gap between the pore wall and the filler during temperature cycling and improving the long-term sealing density of the discharge channel during temperature cycling.

[0091] Differences from existing technologies: 1. Existing technologies typically use a single phosphate electrolyte for micro-arc oxidation, resulting in limited filling rate of the single phosphate phase deposited within the pores. Furthermore, the CTE is close to that of the alumina matrix, insufficient to improve the effective CTE of the micro-arc oxidation ceramic composite layer. 2. Existing sealing processes (hot water sealing, chromate sealing, etc.) are applied separately after micro-arc oxidation. The sealing material and the micro-arc oxidation ceramic layer have significantly different chemical compositions, leading to weak interfacial bonding. 3. This process simultaneously introduces... , , By utilizing the in-situ deposition conditions within the micro-arc oxidation discharge channels, a Ca-P-Si composite sealing phase with CTE between the aluminum substrate and the alumina ceramic is formed within the pores. This combines the two functions of sealing and improving the effective CTE of the micro-arc oxidation ceramic composite layer into the same process, which is a technical approach that has not been proposed in the prior art.

[0092] Step 3: Citrate complexation followed by pore sealing

[0093] This step is the second key step in this process.

[0094] After the micro-arc oxidation treatment is completed, the aluminum profile is removed from the electrolyte and thoroughly rinsed with water to remove any residual alkaline electrolyte from the surface. Then, the aluminum profile is placed in sodium citrate ( The solution (concentration 10 g / L; pH adjusted to 7 with citric acid) was immersed in an aqueous solution at 50°C for 20 min, then removed, rinsed with water, and dried at 70°C for 30 min to obtain the top layer of the citrate metal complex for sealing the pores, thus completing the preparation of the entire coating.

[0095] 3.1 Citrate ions and discharge channels Sediments and Residues Complexation to form sealing complexes

[0096] The phosphate electrolyte is alkaline (pH 11). During contact with the alkaline electrolyte, the zinc phosphate conversion film formed in step one undergoes partial dissolution of its surface layer, releasing [resources / products]. Since the pH of the MAO electrolyte is 11, Under these alkaline conditions, it does not exist stably in a free state, but rapidly converts into zinc hydroxide (Zn(OH)2). The electrolyte is deposited on the inner wall of the discharge channel in the form of [a specific substance]. Rinsing with water after micro-arc oxidation can remove residual alkaline electrolyte adhering to the aluminum profile surface, but the electrolyte deposited on the inner wall of the channel remains. It is difficult to completely remove with flushing water and remains inside the channels. If the above... Without treatment, when corrosive media (such as aqueous solutions containing chlorides) invade the channels, It can dissolve to form soluble products such as zinc chloride and migrate outwards, further eroding the inner wall of the pores and expanding the corrosion range.

[0097] This step involves immersing the aluminum profile in a sodium citrate (pH 7) solution for post-sealing, utilizing the weakly acidic environment to seal the inner walls of the pores. Partial dissolution occurs, releasing... The latter is then reacted with citrate ions in the solution ( It contains three carboxyl groups. ) complexes to form zinc citrate complex ( The citrate ions are deposited in situ within the pores; simultaneously, citrate ions react with free Ca-P-Si phases that did not fully participate in the deposition within the discharge pores. A complexation reaction occurs, forming a calcium citrate complex ( The aforementioned zinc citrate and calcium citrate complexes have low solubility under neutral to weakly acidic conditions. After in-situ formation within the pores, they act as solid fillers, removing residual material from the pore walls. It is converted in situ into a stable sealing complex, eliminating the potential for dissolution and expanded corrosion when corrosive media invade.

[0098] 3.2 Organic complexes fill the residual gaps between inorganic sealing phase particles

[0099] The Ca-P-Si composite sealing phase generated in step two is deposited in the pores as solid particles, inevitably resulting in tiny gaps between adjacent particles. Citrate, a small-molecule organic acid anion (molecular weight approximately 189), has a molecular size much smaller than the particle size of the inorganic particles within the pores, allowing it to penetrate the tiny gaps between the Ca-P-Si particles and interact with the residual... , After complexation to form a complex, it is deposited in situ, forming a physical blockage of the aforementioned micro-gaps, further reducing the effective permeability of the discharge channels and improving the overall blockage density of the discharge channels.

[0100] After the above three steps, the resulting composite coating consists of the following layers from the aluminum substrate to the outer surface: zinc phosphate chemical conversion film bottom layer (thickness approximately 2-3 μm) → micro-arc oxidation ceramic composite layer containing Ca-P-Si composite sealing phase (thickness approximately 35 μm) → citrate metal complex sealing top layer.

[0101] Differences from existing technologies: 1. Existing sealing processes do not include a citrate complexation step; the zinc phosphate underlayer dissolves in the alkaline MAO electrolyte. Deposits remaining on the inner wall of the pores can dissolve and form soluble zinc salts when corrosive media penetrate, posing a corrosion hazard. 2. This step, through the introduction of citrate, partially dissolves the zinc phosphate substrate from step one, and then reacts it in an alkaline MAO electrolyte. The zinc source deposited on the inner wall of the pores in the form of zinc dissolves in situ in a weakly acidic sodium citrate solution and is in-situ complexed and fixed by citrate ions to form a low-solubility zinc citrate complex. This realizes the internal linkage of the process to "convert the in-process pore deposits into sealing fillers", which is a technical means not recorded in the prior art; 3. The citrate complex sealing and the Ca-P-Si inorganic sealing in step two form an organic-inorganic synergistic supplementary sealing. The two cooperate to cover pore spaces of different scales, and the sealing integrity is better than that of a single inorganic sealing system.

[0102] Experimental verification

[0103] 1. Comparison of sample grouping and preparation

[0104] Four comparative sample groups were set up. Except for the differences listed in the table below, the operating conditions of each group were the same, and the substrate used was uniformly 6063-T5 aluminum alloy profile (50mm×50mm×3mm, from the same batch). The number of parallel samples in each group was no less than 3.

[0105]

[0106] Note: The MAO electrical parameters of S1 and S2 are the same as those of S3 (10A / dm², 500Hz, 20%, 30℃, 40min), only the electrolyte composition is different (single-component vs. ternary). The complete parameters of S3 are completely consistent with those of Examples 1 and 5.

[0107] 2. Coating morphology and phase characterization

[0108] 2.1 SEM cross-sectional morphology (backscattered electron mode)

[0109] Metallographic sections were prepared using S3 (cold-mounted resin fixation → grinding with 400# → 800# → 1500# → 2000# sandpaper → polishing with 0.05μm alumina suspension), sputtered with gold, and observed under a scanning electron microscope (accelerating voltage 15kV, backscattered electron BSE mode). Images were taken at approximately 3000× magnification (scale bar 10μm) to focus on recording the interface characteristics of the aluminum matrix / zinc phosphate underlayer / MAO layer, as well as the filling state of the Ca-P-Si composite sealing phase within the discharge channels. The results are as follows: Figure 2 As shown.

[0110] 2.2 SEM surface morphology (secondary electron mode, comparison before and after sealing)

[0111] After completing steps two (MAO, unsealed) and three (after sodium citrate sealing and drying), the surface of sample S3 was observed using SE-SEM (accelerating voltage 5kV, secondary electron mode), at a magnification of approximately 5000× (scale bar 5μm). The surface morphology before and after sealing is compared as follows: Figure 3 As shown.

[0112] 2.3 XRD Phase Analysis

[0113] After grinding the MAO ceramic layer of sample S3 (scraping off the cross-section to remove the aluminum matrix), the sample was subjected to X-ray diffraction (Cu-Kα radiation). The tube voltage is 40kV and the tube current is 40mA. Analysis was performed on a scale of 0.02° step size and 2° / min scan rate. The results were compared with those of S1 (single component). Comparison of electrolyte MAO layer powder.

[0114] The following characteristic peaks can be identified in the XRD diffraction pattern of S3 (Miller index is in parentheses):

[0115] ( ), ( ), ( ), ( ), ( ), ( ): Belongs to - (Corundum phase, PDF#10-0173), is the main phase of MAO alumina ceramic matrix;

[0116] ( ), ( ), ( ), ( ), ( ), ( ): Classified as hydroxyapatite (HAP, (PDF#09-0432), confirming the presence of the HAP phase in the Ca-P-Si composite sealing phase;

[0117] , (Weak peak): Attributable to calcium silicate ( Wollastonite (PDF#43-1460) is a minor component in the Ca-P-Si composite sealing phase.

[0118] (Broadened diffuse scattering peak): Attributable to the amorphous state that is unavoidably present in the MAO layer. (γ or amorphous phase, usually distributed in the low-temperature region outside the MAO layer).

[0119] Control sample S1 (single component) Only the XRD pattern of the MAO layer of the electrolyte appears - Diffraction peaks and amorphous diffuse scattering peaks, without HAP and The characteristic diffraction peaks further confirm HAP and The generation depends on the ternary electrolyte ( + + (Simultaneous existence) conditions. The XRD patterns of S3 and S1 are as follows: Figure 4 As shown.

[0120] 3. Neutral Salt Spray Corrosion Resistance Test

[0121] Test standard: GB / T10125—2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", Neutral Salt Spray (NSS) Test.

[0122] Test conditions: Sodium chloride solution concentration 50 g / L (mass fraction approximately 5%), chamber temperature 35 ± 2℃, pH 6.5-7.2, salt spray deposition rate 12 mL / (80 cm²·h), continuous exposure.

[0123] Evaluation method: According to GB / T6461—2002 "Rating of specimens and test pieces of metal and other inorganic coatings on metal substrates after corrosion test"; record the time when visible corrosion (white rust spots or blistering) first appears on the surface of each sample; take a group of samples at 168h, 500h, 720h and 1000h respectively, take pictures and record the surface corrosion status (evaluated by the percentage of corrosion area to total area).

[0124] Results (mean of 3 parallel samples in each group):

[0125]

[0126] The comparison results of the salt spray resistance of each sample are shown in the figure. Figure 5 The time for the first corrosion to appear in S3 is twice that of S2, and the corrosion area after 1000 hours is much smaller than that in S2. This indicates that the long-term salt spray resistance of the ternary electrolyte MAO + citrate complex sealing system of the present invention is significantly better than that of the traditional single-component MAO process for hot water sealing.

[0127] 4. Test on the retention of coating protective performance after temperature change cycling

[0128] Test Objective: To evaluate the effect of Ca-P-Si composite sealing on the effective thermal expansion coefficient of the micro-arc oxidation ceramic layer, and its contribution to the maintenance of the coating's protective performance under temperature cycling. S1 and S2 (single-component) without Ca-P-Si phase. Electrolyte MAO and S3 (ternary MAO) should exhibit different degrees of coating performance degradation under the same temperature cycling conditions.

[0129] Temperature cycling parameters: Programmable cooling chamber, -30℃ (hold for 30 min) to +70℃ (hold for 30 min) → cooling, 100 cycles in total (single cycle temperature difference 100℃, covering the annual temperature difference range of thermally broken aluminum profiles in northern building exterior scenarios). Excludes S0 (no coating, thermal cycling test is meaningless).

[0130] Evaluation criteria:

[0131] Visual inspection (every 10 cycles, 10× magnifying glass): Record whether visible cracks, blistering, or peeling appear on the coating surface;

[0132] Cross-cut adhesion (tested once before and after 100 cycles): According to GB / T9286—2021 "Paints and Varnishes Cross-cut test", the cross-cut spacing is 1mm, 6×6 grid, transparent tape peeling method, rating 0 (best) to 5 (worst);

[0133] Electrochemical impedance spectroscopy (EIS): Measured in 3.5% (mass fraction) NaCl solution (three-electrode system: sample as working electrode, platinum sheet as counter electrode, saturated calomel electrode (SCE) as reference electrode, test frequency...). The amplitude of the sinusoidal disturbance (10mV vs. OCP) was measured after 0, 50, and 100 cycles, respectively, at the low-frequency end. (Unit: Ω·cm²) characterizes the shielding performance of the coating.

[0134] Cross-cut adhesion results:

[0135]

[0136] EIS low-frequency impedance modulus results:

[0137]

[0138] Results Analysis: EIS Low-Frequency Impedance Modulus of Each Sample ( The trend of change with the number of temperature cycles is shown in the figure. Figure 6 After 100 temperature cycles (-30℃ to +70℃), the low-frequency impedance of coatings S1 and S2 decreased to 6.0% and 20.2% of their initial values, respectively, and their adhesion grades decreased to level 3 and level 2, respectively. This indicates that single-component MAO coatings without the Ca-P-Si composite sealing phase (regardless of whether they are hot-water sealed) undergo significant interfacial microcrack propagation and pore reopening during temperature cycling, leading to a substantial decline in protective performance. In contrast, the low-frequency impedance of S3 (in this invention, containing the Ca-P-Si composite sealing phase) decreased by only about 0.14 orders of magnitude, with an impedance retention rate of 72.2%. After 100 cycles, the adhesion grade remained at 0, and no visible cracks were observed on the surface. These results indicate that the Ca-P-Si composite sealing phase (mainly HAP, with a CTE of approximately...) is effective in preventing and controlling corrosion. The effect of improving the effective thermal expansion coefficient of the micro-arc oxidation ceramic composite layer significantly improves the long-term protective stability of the coating during temperature cycling.

[0139] 5. Summary of Experimental Verification

[0140]

[0141] The process of this invention is significantly superior to the traditional process of single-component MAO (with or without hot water sealing) in key indicators such as salt spray resistance, adhesion after temperature cycling, and electrochemical impedance retention rate. It verifies the effectiveness of the two key steps of in-situ sealing with ternary electrolyte Ca-P-Si and sealing after citrate complexation.

[0142] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A preparation process for a micro-arc oxidation composite coating on thermally broken aluminum profiles, characterized in that, Includes the following steps: Step 1: After surface pretreatment of the thermally broken aluminum profile, it is placed in a zinc phosphate chemical conversion treatment solution containing phosphoric acid, zinc nitrate, and sodium nitrite to generate zinc phosphate on the surface of the aluminum alloy substrate. Crystallization conversion membrane; Step Two: The aluminum profile treated in Step One is placed in a ternary alkaline electrolyte containing trisodium phosphate, calcium nitrate, and sodium silicate. Using the aluminum profile as the anode and the titanium plate as the cathode, micro-arc oxidation is performed in a constant current mode. This causes phosphate, calcium, and silicate ions in the electrolyte to deposit in situ within the discharge channels, generating hydroxyapatite (… ) is the main component, calcium silicate ( A Ca-P-Si composite sealing phase, with Ca as the minor component, is filled into the discharge channels of the micro-arc oxidation ceramic layer to obtain a micro-arc oxidation ceramic composite layer containing the Ca-P-Si composite sealing phase. Step 3: After cleaning the aluminum profiles processed in Step 2, immerse them in a sodium citrate aqueous solution. This allows the citrate ions to complex with the zinc ions dissolved from the residual zinc deposits on the inner wall of the discharge channel and the residual calcium ions in the channel, respectively, forming zinc citrate complexes and calcium citrate complexes. These complexes are then deposited in situ within the discharge channel to fill the residual gaps between the Ca-P-Si composite sealing phase particles. After cleaning and drying, a micro-arc oxidation composite coating for the thermally broken aluminum profiles is obtained.

2. The preparation process according to claim 1, characterized in that, The surface pretreatment described in step one includes, in sequence, the following steps: using trisodium phosphate ( Degreased by immersing in a 20–40 g / L solution at 50–70°C for 35 min, followed by rinsing with water; then alkali-etched by immersing in a 35% NaOH solution at 15–35°C for 1–3 min, followed by thorough rinsing with water; finally, brightened by immersing in a 15–25% nitric acid solution at 15–35°C for 30–90 s, followed by thorough rinsing with water.

3. The preparation process according to claim 1, characterized in that, The zinc phosphate chemical conversion treatment solution in step one contains 5-20 g / L phosphoric acid, 10-30 g / L zinc nitrate and 0.1-0.5 g / L sodium nitrite, with a pH of 2-3, a treatment temperature of 40-60℃, a treatment time of 5-15 min, and the resulting zinc phosphate crystallization conversion film has a thickness of 1-5 μm.

4. The preparation process according to claim 1, characterized in that, The ternary alkaline electrolyte in step two contains 10-30 g / L trisodium phosphate, 3-10 g / L calcium nitrate, and 3-8 g / L sodium silicate, with a pH of 10-12.

5. The preparation process according to claim 4, characterized in that, In step two, the ternary alkaline electrolyte is continuously mechanically stirred at a speed of 50-200 rpm during the micro-arc oxidation process to ensure that the suspended particles are uniformly dispersed in the electrolyte.

6. The preparation process according to claim 1, characterized in that, The current density of the micro-arc oxidation treatment in step two is 5–15 A / dm², the pulse frequency is 50–1000 Hz, and the duty cycle is 10–30%.

7. The preparation process according to claim 6, characterized in that, In step two, the electrolyte temperature for the micro-arc oxidation treatment is 20–40°C, the treatment time is 20–60 min, and the thickness of the resulting micro-arc oxidation ceramic composite layer is 20–60 μm.

8. The preparation process according to claim 1, characterized in that, In step three, the sodium citrate aqueous solution has a sodium citrate concentration of 5–15 g / L, the pH is adjusted to 6–8 with citric acid, the immersion temperature is 40–60 °C, and the treatment time is 15–30 min.

9. The preparation process according to claim 1, characterized in that, After cleaning in step three, dry at 60-80℃ for 20-40 minutes.

10. A micro-arc oxidation composite coating for thermally broken aluminum profiles obtained by the preparation process described in any one of claims 1 to 9, characterized in that, The composite coating consists of, from the aluminum substrate to the outer surface, the following layers: a zinc phosphate crystallization conversion film with a thickness of 1–5 μm as the bottom layer; a micro-arc oxidation ceramic composite layer containing a Ca-P-Si composite sealing phase with a thickness of 20–60 μm; and a sealing top layer composed of zinc citrate complex and calcium citrate complex. The micro-arc oxidation ceramic composite layer is composed of an alumina ceramic matrix and a Ca-P-Si composite sealing phase filled within the discharge channels. The Ca-P-Si composite sealing phase is composed of hydroxyapatite (… ) is the main component, calcium silicate ( () is a minor component.

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