A method for preparing a high corrosion-resistant micro-arc oxidation coating based on synergism of cerium salt valence and voltage

Abstract

This invention discloses a method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage. The method involves adding trivalent cerium salt as a modifier to the micro-arc oxidation electrolyte and performing oxidation treatment within a suitable voltage range. Through the synergistic effect of valence state and voltage, a highly corrosion-resistant coating is obtained. The coating prepared by this invention has a uniform surface, a dispersed cerium-rich phase distribution, a dense cross-section, a cross-sectional porosity as low as below 3.11%, and a surface roughness (Sa) of approximately 14.2 nm. The main component of the coating is γ-Al₂O₃, with Ce as the predominant component. 3+ and Ce 4+ The coexistence of two compound states in the coating effectively refines the coating grains, increases the coating density, reduces micro-defects on the coating surface, and ensures that Ce is distributed relatively uniformly in the coating without aggregation.

Description

Technical Field

[0001] This invention relates to the field of metal surface coating technology, and in particular to a method for preparing highly corrosion-resistant micro-arc oxidation coatings based on the synergistic effect of cerium salt valence state and voltage. Background Technology

[0002] Aluminum alloys, due to their low density, high specific strength, and recyclability, are widely used in critical fields such as aerospace, marine engineering, transportation, machinery manufacturing, and chemical equipment. During service, aluminum alloys face severe corrosion challenges. While a natural oxide passivation film forms on their surface, this film is often too thin to provide long-term effective protection. More importantly, second phases or intermetallic compounds present in the alloy can form micro-galvanic couples, inducing localized damage such as pitting corrosion, intergranular corrosion, and stress corrosion cracking. - This problem is particularly prominent in corrosive environments such as those containing aluminum alloys. Therefore, effective surface strengthening treatment of aluminum alloys is crucial to ensuring their long-term service safety and reliability.

[0003] Micro-arc oxidation (MAO) technology, an electrochemical method for in-situ growth of ceramic coatings on valve metal surfaces, is an effective way to improve the corrosion resistance of aluminum alloys. This technology induces plasma discharge through a high-voltage electric field, generating a high-hardness ceramic coating on the aluminum alloy surface, primarily composed of γ-Al₂O₃ and metallurgically bonded to the substrate. However, traditional MAO coatings typically exhibit a porous structure, with inherent micropores and microcracks becoming channels for corrosive media penetration, limiting their long-term protective capabilities.

[0004] To improve coating density, the introduction of rare earth element cerium (Ce) for modification has become an effective strategy. Cerium has two common valence states, +3 and +4. Its compounds (such as Ce(NO3)3 and Ce(SO4)2) may exhibit fundamentally different chemical behaviors in electrolytes, migration in plasma discharge environments, and incorporation mechanisms. This directly affects the uniformity of its distribution, its morphology, and its final modification effect on the coating structure. Currently, most studies focus only on the effects of adding cerium salts with a single valence state, or vaguely discuss the benefits of "cerium doping," lacking in-depth research that systematically compares the effects of different valence states of cerium salts on coating growth kinetics, microstructure, and corrosion resistance under different oxidation voltages. Therefore, how to scientifically select the valence state of cerium salts based on process conditions such as oxidation voltage to maximize the modification efficiency of cerium is a crucial issue that urgently needs to be addressed in the development of high-performance micro-arc oxidation coatings. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing highly corrosion-resistant micro-arc oxidation coatings based on the synergistic effect of cerium salt valence state and voltage. This invention utilizes the behavioral differences of cerium salts with different valence states within a specific voltage range to determine the optimal combination of cerium valence state and voltage process, thereby producing micro-arc oxidation coatings with a dense structure, uniform cerium distribution, and significantly improved corrosion resistance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage is disclosed. The method includes adding trivalent cerium salt as a modifier to the micro-arc oxidation electrolyte and performing oxidation treatment within a suitable voltage range. Through the synergistic effect of valence state and voltage, a highly corrosion-resistant coating is obtained.

[0008] Preferably, the trivalent cerium salt is any one or a combination of two or more of cerium nitrate hexahydrate, cerium oxalate, or cerium octanoate, and its concentration in the electrolyte is 1-2 g / L.

[0009] Preferably, the oxidation treatment is performed at a voltage of 350-450V within the specified voltage range.

[0010] Preferably, the basic composition of the micro-arc oxidation electrolyte includes 10 g / L sodium silicate (Na2SiO3·9H2O), 8 g / L potassium hydroxide (KOH), and 10 mL / L ethanol (C2H5OH).

[0011] Preferably, the micro-arc oxidation process uses a constant voltage mode with a dual-pulse power supply, setting the voltage to 400V, the duty cycle to 6%, the frequency to 500 Hz, and the oxidation time to 5 minutes.

[0012] Preferably, the present invention provides a method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage, comprising the following steps:

[0013] (1) After grinding the aluminum alloy substrate to 2000 grit with silicon carbide sandpaper, ultrasonically clean it with distilled water and ethanol, and then dry it with cold air for later use.

[0014] (2) Prepare the electrolyte, and then use the treated aluminum alloy as the anode and the stainless steel electrolytic cell as the cathode, and use a dual-pulse power supply for micro-arc oxidation treatment.

[0015] (3) After the reaction is complete, take out the sample, sonicate it with deionized water, and dry it in an oven to obtain the finished product.

[0016] Preferably, the micro-arc oxidation process uses a constant voltage mode with a dual-pulse power supply, setting the voltage to 400V, the duty cycle to 6%, the frequency to 500 Hz, and the oxidation time to 5 minutes.

[0017] Preferably, the aluminum alloy substrate is polished sequentially with 400# to 2000# silicon carbide sandpaper, and then ultrasonically cleaned with distilled water and ethanol for 10-20 minutes.

[0018] Preferably, the sample is ultrasonically treated with deionized water for 5-10 minutes and then dried in an oven at 60-65℃ for 5-6 hours.

[0019] The advantages of this invention compared to the prior art are as follows:

[0020] 1. This invention establishes a strong interaction between the valence state of cerium salts and the oxidation voltage in a micro-arc oxidation system. Within a medium voltage range of 350-450V, the trivalent cerium salt (Ce...)... 3+ Because it exists primarily in a free state in the electrolyte, it has strong migration capabilities, enabling more uniform transport and incorporation into the coating. Ultimately, it mainly forms catalytically active CeO2, which then forms a dense structure with the substrate oxide; this overcomes the limitations of tetravalent cerium salts (CeO2). 4+ This leads to the formation of complexes, hindering migration and causing localized aggregation in the coating. This invention solves the coating corrosion problem by selecting trivalent cerium salts and matching them with a suitable voltage range, achieving "ion migration-uniform doping-structural densification".

[0021] 2. The coating prepared by this invention exhibits a significant leap in corrosion resistance. At 400V, the coating prepared using 2 g / L Ce(NO3)3·6H2O shows a corrosion current density (Icorr) as low as 9.98 × 10⁻⁶ in a 3.5 wt.% NaCl solution. -6 A·cm -2 The low-frequency impedance modulus (|Z| 0.01Hz) is as high as 5.99 × 10⁻⁶. 6 The coating strength is significantly higher than that of coatings prepared under the same conditions using tetravalent cerium salts and a voltage of 500V, as well as coatings prepared using existing technologies.

[0022] 3. The coating prepared by this invention has a uniform surface, a dispersed cerium-rich phase distribution, a dense cross-section, and a porosity as low as 3.11%. The surface roughness (Sa) is approximately 14.2 nm. The main component of the coating is γ-Al₂O₃, with Ce as the primary component. 3+ and Ce 4+ The coexistence of two compound states in the coating effectively refines the coating grains and improves the coating density. The coating surface has few micro-defects, and Ce is relatively uniformly distributed without aggregation. The coating growth is relatively uniform, with an average thickness of 4.97 μm and minimal error. The potentiodynamic polarization curve fitting results in 3.5% NaCl solution show that the corrosion potential of the coating is -1.17 V, and the corrosion current density is 1.83 × 10⁻⁶. -5 A / cm 2The AC impedance curve shows that its low-frequency modulus impedance is as high as 5.99 × 10⁻⁶. 6 Ω·cm 2 .

[0023] 4. The method of preparation of the present invention has a wide process window and good repeatability, and provides a highly corrosion-resistant and efficient surface treatment technology solution to solve the problem of long-term protection of aluminum alloy parts in harsh corrosive environments (such as aerospace, marine engineering and transportation). Attached Figure Description

[0024] Figure 1 Figure 1 shows the XRD patterns of the coatings, where Figure 2a shows the XRD patterns of the coatings prepared in Examples 1 and 2 of this invention; and Figure 3b shows the XRD patterns of the coatings prepared in Examples 3 and 4 of this invention.

[0025] Figure 2 XPS curves of the elements in the coating prepared in Example 1 of this invention.

[0026] Figure 3 XPS curves of the elements in the coating prepared in Example 2 of this invention.

[0027] Figure 4 Figures a and b' show the surface microstructure of the coatings prepared in Example 1 of the present invention; figures b and b' show the surface microstructure of the coatings prepared in Example 2 of the present invention; figures c and c' show the surface microstructure of the coatings prepared in Example 3 of the present invention; and figures d and d' show the surface microstructure of the coatings prepared in Example 4 of the present invention.

[0028] Figure 5 Figure 1 shows the AFM images of the coatings, where Figure a is the AFM image of the coating prepared in Example 1 of the present invention; Figure b is the AFM image of the coating prepared in Example 2 of the present invention; Figure c is the AFM image of the coating prepared in Example 3 of the present invention; and Figure d is the AFM image of the coating prepared in Example 4 of the present invention.

[0029] Figure 6 Figure 1 shows the cross-sectional microstructure of the coating, where Figure a is the cross-sectional microstructure of the coating prepared in Example 1 of the present invention; Figure b is the cross-sectional microstructure of the coating prepared in Example 2 of the present invention; Figure c is the cross-sectional microstructure of the coating prepared in Example 3 of the present invention; and Figure d is the cross-sectional microstructure of the coating prepared in Example 4 of the present invention.

[0030] Figure 7 Figure a shows the coating thickness and cross-sectional porosity of the coatings prepared in Examples 1 and 2 of the present invention; Figure b shows the cross-sectional porosity of the coatings prepared in Examples 3 and 4 of the present invention.

[0031] Figure 8 Figure 1 shows the potentiodynamic polarization curves of the coatings, where Figure 2a is the potentiodynamic polarization curve of the coatings prepared in Examples 1 and 2 of the present invention; and Figure 3b is the potentiodynamic polarization curve of the coatings prepared in Examples 3 and 4 of the present invention.

[0032] Figure 9 Figure 1 shows the Nyquist and Bode curves of the coatings, where Figure a is the Nyquist curve of the coatings prepared in Examples 1 and 2 of the present invention; Figure b is the Bode curve of the coatings prepared in Example 1 compared to Example 2 of the present invention; Figure c is the Nyquist curve of the coatings prepared in Example 3 compared to Example 4 of the present invention; and Figure d is the Bode curve of the coatings prepared in Example 3 compared to Example 4 of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and preferred embodiments. However, it should be noted that many details listed in the specification are merely to provide the reader with a thorough understanding of one or more aspects of the present invention, and these aspects of the invention can be implemented even without these specific details.

[0034] Example 1

[0035] A method for preparing a micro-arc oxidation coating by adding trivalent cerium at 400V includes the following steps:

[0036] (1) Substrate treatment: Select 6063 aluminum alloy plate, and polish it with 400# to 2000# silicon carbide sandpaper until the surface is smooth and free of scratches. Then, ultrasonically clean it in distilled water and anhydrous ethanol for 10 minutes each, and dry it with cold air for later use.

[0037] (2) Micro-arc oxidation treatment: Prepare an electrolyte with the following composition: 10 g / L Na2SiO3·9H2O, 8 g / L KOH, 10 mL / L C2H5OH, and 2 g / L Ce(NO3)3·6H2O (trivalent cerium salt). Use the treated aluminum alloy as the anode and the stainless steel electrolytic cell as the cathode. Use a dual-pulse power supply, set the oxidation voltage to 400 V (constant voltage mode), the frequency to 500 Hz, the duty cycle to 6%, and the oxidation time to 5 minutes.

[0038] (3) Post-treatment: After the reaction is completed, the sample is taken out, ultrasonically cleaned with deionized water for 5 minutes, and dried in an oven at 60°C for 6 hours to obtain a highly corrosion-resistant micro-arc oxidation coating.

[0039] The prepared coating was characterized, and the results are as follows:

[0040] The data obtained by X-ray diffraction (XRD) analysis are as follows: Figure 1 As shown in (a), the main phase structure of the coating is γ-Al₂O₃. Figure 2 XPS results show that Ce has been successfully doped into the coating, and in Ce... 3+ and Ce 4+ Compounds with mixed valence states exist, but the Ce doping level is not high, so the Ce peak shape is not obvious.

[0041] like Figure 4 (a, a') show the microscopic surface morphology of the micro-arc oxidation coating with added trivalent cerium at 400V. The coating surface can be divided into two typical regions: a porous region with larger pores and a relatively loose structure, and a dense region with closed central pores and a relatively smooth surface. The increased proportion of dense regions helps to enhance the overall barrier performance of the coating. In addition, fine deposits with bright contrast were observed on both coating surfaces, and they were evenly distributed on the coating surface.

[0042] Figure 5 The AFM results in (a) show that the grains formed on the surface are relatively fine, and the surface roughness Sa is 14.2 nm.

[0043] Figure 6 The microstructure of the cross-section in (a) shows that although the coating thickness is small, Figure 7 The measurement result in (a) is 4.97 μm, but the coating distribution is relatively uniform, with fewer defects and better density. Figure 7 (b) The test results showed that its surface porosity reached 3.11%.

[0044] Corrosion resistance testing: Potentiodynamic polarization curve scanning experiments were conducted on the coating surface at a scan rate of 0.01 mV / s, within the open circuit potential (OCP) range of +1.5 V to -1.5 V. When the OCP was approximately 200 mV, the polarization curves were processed using the Tafel fitting method. Electrochemical impedance spectroscopy (EIS) was performed in 3.5% NaCl solution using a three-electrode setup equipped with a frequency response analyzer (FRA), with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the auxiliary electrode, and the micro-arc oxidation coating as the working electrode. The results are as follows: Figure 8 As shown in curve (a), its corrosion potential is -1.17 V and its corrosion current density is 9.98 × 10⁻⁶. -6 A / cm 2 The polarization resistance is 1.16 × 10⁻⁶. 7 Ω·cm 2 . Figure 9The EIS results in (b) show that its low-frequency modulus resistance (|Z| 0.01 Hz) reaches 5.99 × 10⁻⁶. 6 Ω·cm 2 .

[0045] Example 2

[0046] A method for preparing a micro-arc oxidation coating by adding tetravalent cerium at 400V includes the following steps:

[0047] (1) Substrate treatment: Select 6063 aluminum alloy plate, and polish it with 400# to 2000# silicon carbide sandpaper until the surface is smooth and free of scratches. Then, ultrasonically clean it in distilled water and anhydrous ethanol for 10 minutes each, and dry it with cold air for later use.

[0048] (2) Micro-arc oxidation treatment: The electrolyte was prepared with the following composition: 10 g / L Na2SiO3·9H2O, 8 g / L KOH, 10 mL / L C2H5OH, and 1.86 g / L Ce(SO4)2·4H2O (tetravalent cerium salt) with an equimolar cerium content. The treated aluminum alloy was used as the anode, and the stainless steel electrolytic cell was used as the cathode. A dual-pulse power supply was used, and the oxidation voltage was set to 400 V (constant voltage mode), the frequency to 500 Hz, the duty cycle to 6%, and the oxidation time to 5 minutes.

[0049] (3) Post-treatment: After the reaction is completed, the sample is taken out, ultrasonically cleaned with deionized water for 5 minutes, and dried in an oven at 60°C for 6 hours to obtain a highly corrosion-resistant micro-arc oxidation coating.

[0050] The prepared coating was characterized, and the results are as follows:

[0051] The data obtained by X-ray diffraction (XRD) analysis are as follows: Figure 1 As shown in (a) below, the main phase structure of the coating is γ-Al2O3. Figure 3 XPS results show that Ce has been successfully doped into the coating, and in Ce... 3+ and Ce 4+ Compounds with mixed valence states exist, but the Ce doping level is not high, so the Ce peak shape is not obvious.

[0052] like Figure 4 (b, b') show the microscopic surface morphology of the micro-arc oxidation coating with tetravalent cerium added at 400V. The coating surface can be divided into two typical regions: one is a porous region with larger pores and a relatively loose structure; the other is a dense region with closed central pores and a relatively smooth surface. Increasing the proportion of dense regions helps to enhance the overall barrier performance of the coating. In addition, fine deposits with bright contrast were observed on both coating surfaces, but their distribution was uneven, showing local aggregation.

[0053] Figure 5 The AFM results in (b) show that the surface roughness Sa = 16.6 nm of the added tetravalent cerium coating is due to the aggregation of the cerium-rich phase.

[0054] Figure 6 An increase in coating thickness was observed in the cross-sectional microstructure of (b). Figure 7 The measurement result in (a) is 5.12 μm, but the error range of its thickness measurement is significantly larger, indicating more defects in the coating and a decrease in density compared to the trivalent cerium coating. Figure 7 (b) The test results showed that the surface porosity increased to 3.22%.

[0055] Corrosion resistance testing: Potentiodynamic polarization curve scanning experiments were conducted on the coating surface at a scan rate of 0.01 mV / s, within the open circuit potential (OCP) range of +1.5 V to -1.5 V. When the OCP was approximately 200 mV, the polarization curves were processed using the Tafel fitting method. Electrochemical impedance spectroscopy (EIS) was performed in 3.5% NaCl solution using a three-electrode setup equipped with a frequency response analyzer (FRA), with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the auxiliary electrode, and the micro-arc oxidation coating as the working electrode. The results are as follows: Figure 8 As shown in curve (a), its corrosion potential is -1.33 V and its corrosion current density is 1.51 × 10⁻⁶. -5 A / cm 2 The polarization resistance is 4.44 × 10⁻⁶. 6 Ω·cm 2 . Figure 9 The EIS results for (b) show that its low-frequency modulus resistance (|Z| 0.01 Hz) is reduced to 2.39 × 10⁻⁶ compared to the trivalent cerium coating. 6 Ω·cm 2 .

[0056] Example 3

[0057] A method for preparing a micro-arc oxidation coating by adding trivalent cerium at 500V includes the following steps:

[0058] (1) Substrate treatment: Select 6063 aluminum alloy plate, and polish it with 400# to 2000# silicon carbide sandpaper until the surface is smooth and free of scratches. Then, ultrasonically clean it in distilled water and anhydrous ethanol for 10 minutes each, and dry it with cold air for later use.

[0059] (2) Micro-arc oxidation treatment: Prepare an electrolyte with the following composition: 10 g / L Na2SiO3·9H2O, 8 g / L KOH, 10 mL / L C2H5OH, and 2 g / L Ce(NO3)3·6H2O (trivalent cerium salt). Use the treated aluminum alloy as the anode and the stainless steel electrolytic cell as the cathode. Use a dual-pulse power supply, set the oxidation voltage to 500 V (constant voltage mode), the frequency to 500 Hz, the duty cycle to 6%, and the oxidation time to 5 minutes.

[0060] (3) Post-treatment: After the reaction is completed, the sample is taken out, ultrasonically cleaned with deionized water for 5 minutes, and dried in an oven at 60°C for 6 hours to obtain a highly corrosion-resistant micro-arc oxidation coating.

[0061] The prepared coating was characterized, and the results are as follows:

[0062] The data obtained by X-ray diffraction (XRD) analysis are as follows: Figure 1 As shown in (b), the main phase structure of the coating is γ-Al2O3.

[0063] like Figure 4 (c, c') show the microscopic surface morphology of the micro-arc oxidation coating with added trivalent cerium at 500V. The molten pool size is significantly increased, and the proportion of closed central pores is further improved. Porous and dense regions also exist on the surface. However, due to the dramatic increase in discharge energy at high voltage, the number and size of electric sparks are greater, and the reaction is more intense, resulting in an increase in the area ratio of loose and porous regions in the coating, an increase in the number of surface microcracks, and a decrease in overall density.

[0064] Figure 5 The AFM results in (c) show that the surface roughness of the added trivalent cerium coating at 500V increases significantly to Sa=44.85 nm.

[0065] Figure 6 An increase in coating thickness was observed in the cross-sectional microstructure of (c). Figure 7 The measurement result in (a) is 5.93 μm. When the voltage is increased to 500 V, the number of visible microcracks and defects in the coating cross-section increases, the continuity of the coating decreases, and the density decreases compared to the coating at 400 V. Figure 7 (b) The test results showed that the surface porosity increased to 3.83%.

[0066] Corrosion resistance testing: Potentiodynamic polarization curve scanning experiments were conducted on the coating surface at a scan rate of 0.01 mV / s, within the open circuit potential (OCP) range of +1.5 V to -1.5 V. When the OCP was approximately 200 mV, the polarization curves were processed using the Tafel fitting method. Electrochemical impedance spectroscopy (EIS) was performed in 3.5% NaCl solution using a three-electrode setup equipped with a frequency response analyzer (FRA), with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the auxiliary electrode, and the micro-arc oxidation coating as the working electrode. The results are as follows: Figure 8 As shown in curve (b), its corrosion potential is -1.30 V and its corrosion current density is 1.09 × 10⁻⁶. -5 A / cm 2 The polarization resistance is 8.52 × 10⁻⁶. 6 Ω·cm 2 . Figure 9 The EIS results of (d) show that the modulus resistance value (|Z|0.01Hz) of the low-frequency group is lower than that of the coating prepared at 400V, and is 2.81×10. 6 Ω·cm 2 .

[0067] Example 4

[0068] The micro-arc oxidation method for preparing tetravalent cerium at 500V includes the following steps:

[0069] (1) Substrate treatment: Select 6063 aluminum alloy plate, and polish it with 400# to 2000# silicon carbide sandpaper until the surface is smooth and free of scratches. Then, ultrasonically clean it in distilled water and anhydrous ethanol for 10 minutes each, and dry it with cold air for later use.

[0070] (2) Micro-arc oxidation treatment: The electrolyte was prepared with the following composition: 10 g / L Na2SiO3·9H2O, 8 g / L KOH, 10 mL / L C2H5OH, and 1.86 g / L Ce(SO4)2·4H2O (tetravalent cerium salt) with an equimolar cerium content. The treated aluminum alloy was used as the anode, and the stainless steel electrolytic cell was used as the cathode. A dual-pulse power supply was used, and the oxidation voltage was set to 500 V (constant voltage mode), the frequency to 500 Hz, the duty cycle to 6%, and the oxidation time to 5 minutes.

[0071] (3) Post-treatment: After the reaction is completed, the sample is taken out, ultrasonically cleaned with deionized water for 5 minutes, and dried in an oven at 60°C for 6 hours to obtain a highly corrosion-resistant micro-arc oxidation coating.

[0072] The prepared coating was characterized, and the results are as follows:

[0073] The data obtained by X-ray diffraction (XRD) analysis are as follows: Figure 1 (b) As shown below, the main phase structure of the coating is γ-Al2O3.

[0074] like Figure 4 (d, d') shows the microscopic surface morphology of the micro-arc oxidation coating with tetravalent cerium added at 500V. The excessive discharge caused by the high voltage also leads to more significant melting and splashing and rapid solidification, resulting in a significant increase in the size of the molten pool, which in turn introduces more microscopic defects and weakens the uniformity of the coating structure.

[0075] Figure 5 The AFM results in (d) show that the surface roughness of the tetravalent cerium coating increased significantly at 500V, but was lower than that of the trivalent cerium coating, Sa=42.37 nm.

[0076] Figure 6 In the cross-sectional microstructure of (c), it was observed that although the coating thickness was relatively large, the number of defects was the highest. Figure 7 The coating thickness measured in (a) is 7.11 μm. When the voltage increases to 500 V, the number of visible microcracks and defects in the coating cross-section increases, the continuity of the coating decreases, and the density decreases compared to the trivalent cerium coating. Figure 7 (b) The test results showed that its surface porosity was slightly lower than that of trivalent cerium, down to 3.75%.

[0077] Corrosion resistance testing: Potentiodynamic polarization curve scanning experiments were conducted on the coating surface at a scan rate of 0.01 mV / s, within the open circuit potential (OCP) range of +1.5 V to -1.5 V. When the OCP was approximately 200 mV, the polarization curves were processed using the Tafel fitting method. Electrochemical impedance spectroscopy (EIS) was performed in 3.5% NaCl solution using a three-electrode setup equipped with a frequency response analyzer (FRA), with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the auxiliary electrode, and the micro-arc oxidation coating as the working electrode. The results are as follows: Figure 8 As shown in curve (b), its corrosion potential is -1.34 V and its corrosion current density is 1.81 × 10⁻⁶. -5 A / cm 2 The polarization resistance is 5.65 × 10⁻⁶. 5 Ω·cm 2 . Figure 9 The EIS results for (d) show that its low-frequency modulus impedance (|Z| 0.01 Hz) is only 19858 Ω·cm. 2 Furthermore, it exhibits inductive reactive characteristics in the low-frequency region, indicating poor corrosion resistance.

[0078] In summary, the preparation methods and characterization results of Examples 1-4 show that Example 1, using trivalent cerium salt and 400 V oxidation, exhibits superior ion mobility compared to tetravalent cerium salts (such as cerium sulfate) at this voltage. This allows for uniform doping and distribution of Ce in the coating, which, combined with the mild discharge energy, results in a dense coating with few defects. Example 1 addresses the problems of uneven cerium distribution, high porosity, and insufficient corrosion resistance in coatings caused by improper selection of cerium salt valence state or poor voltage matching. The micro-arc oxidation coating prepared by the method in Example 1 demonstrates significantly improved corrosion resistance. At 400 V, the coating prepared using trivalent cerium salt exhibits a corrosion current density (Icorr) as low as 9.98 × 10⁻⁶ in 3.5 wt.% NaCl solution. -6 A·cm -2 The low-frequency impedance modulus (|Z| 0.01Hz) is as high as 5.99 × 10⁻⁶. 6 Ω·cm 2 The performance of the coatings prepared under the same voltage using tetravalent cerium salts (Example 2) or higher voltages (500V) (Examples 3 and 4) is superior to that prepared under the same voltage (Example 1). The coating prepared in Example 1 also exhibits low porosity (≤3.5%) and good surface uniformity. The results of Examples 1-4 clarify the synergistic influence mechanism between cerium valence state and oxidation voltage, providing a stable and effective method for preparing high-performance micro-arc oxidation coatings, which is particularly suitable for surface protection of aluminum alloy components in aerospace, marine engineering, and other fields with stringent corrosion resistance requirements.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing highly corrosion-resistant micro-arc oxidation coatings based on the synergistic effect of cerium salt valence state and voltage, characterized in that, This method involves adding trivalent cerium salt as a modifier to a micro-arc oxidation electrolyte and performing oxidation treatment within a suitable voltage range. Through the synergistic effect of valence state and voltage, a highly corrosion-resistant coating is obtained.

2. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 1, characterized in that, The trivalent cerium salt is any one or a combination of two or more of cerium nitrate hexahydrate, cerium oxalate, or cerium octanoate, and its concentration in the electrolyte is 1-2 g / L.

3. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 1, characterized in that, The oxidation process is performed at a voltage of 350-450V within the specified voltage range.

4. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 1, characterized in that, The basic composition of the micro-arc oxidation electrolyte includes sodium silicate (Na2SiO3·9H2O) 10 g / L, potassium hydroxide (KOH) 8 g / L, and ethanol (C2H5OH) 10 mL / L.

5. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 1, characterized in that, Micro-arc oxidation treatment: The constant voltage mode of the dual-pulse power supply is adopted, with the voltage set at 400V, the duty cycle at 6%, the frequency at 500 Hz, and the oxidation time at 5 minutes.

6. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 1, characterized in that, Includes the following steps: (1) After grinding the aluminum alloy substrate to 2000 grit with silicon carbide sandpaper, ultrasonically clean it with distilled water and ethanol, and then dry it with cold air for later use. (2) Prepare the electrolyte, and then use the treated aluminum alloy as the anode and the stainless steel electrolytic cell as the cathode, and use a dual-pulse power supply for micro-arc oxidation treatment. (3) After the reaction is complete, take out the sample, sonicate it with deionized water, and dry it in an oven to obtain the finished product.

7. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 6, characterized in that, Micro-arc oxidation treatment: The constant voltage mode of the dual-pulse power supply is adopted, with the voltage set at 400V, the duty cycle at 6%, the frequency at 500 Hz, and the oxidation time at 5 minutes.

8. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 6, characterized in that, The aluminum alloy substrate is polished sequentially with 400# to 2000# silicon carbide sandpaper, and then ultrasonically cleaned with distilled water and ethanol for 10-20 minutes.

9. The method for preparing a highly corrosion-resistant micro-arc oxidation coating based on the synergistic effect of cerium salt valence state and voltage according to claim 6, characterized in that, The sample was ultrasonically treated with deionized water for 5-10 minutes, and then dried in an oven at 60-65℃ for 5-6 hours.

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