A method for constructing a self-healing and corrosion-resistant composite film on the surface of an aluminum-lithium alloy based on microfluidic technology and its product

Abstract

The present invention provides a method for constructing a self-healing and corrosion-resistant composite film on the surface of an aluminum-lithium alloy based on microfluidic technology and its product, which relates to the technical field of organic-inorganic hybrid coating composite materials. First, the aluminum-lithium alloy is subjected to electrochemical oxidation treatment, and composite micro-droplets of cerium dioxide and epoxy resin prepared by microfluidic technology are added to the epoxy resin solution. The obtained solution system is coated on the surface of the aluminum-lithium alloy after electrochemical oxidation treatment to prepare the composite film. Among them, the electrochemical oxidation treatment adopts two-step electrochemical oxidation: the electrolyte used in the first-step electrochemical oxidation is sulfuric acid electrolyte, and the electrolyte used in the second-step electrochemical oxidation is a mixed electrolyte of phosphoric acid and oxalic acid. The present invention can realize the preparation of a self-healing and corrosion-resistant composite film on the surface of an aluminum-lithium alloy, achieving efficient protection of the aluminum-lithium alloy.

Description

Technical Field

[0001] The present invention relates to the technical field of organic-inorganic hybrid coating composites, and particularly to a method for constructing a self-healing and corrosion-resistant composite film on the surface of an aluminum-lithium alloy based on microfluidic technology and its product. Background Art

[0002] The characteristics of aluminum-lithium alloys lie in their light weight, so they are increasingly widely used in aerospace. However, the incorporation of lithium also leads to poor plastic toughness and serious anisotropy problems. Its fracture toughness is low in the direction with high strength, and the crack propagation rate is high in the direction with low strength. It is relatively easier to fracture compared to other aluminum alloys. Therefore, it is particularly crucial to solve the problem of initial crack repair. Also, due to the harshness and complexity of the aerospace environment, the threats to be faced include corrosion, mechanical damage, etc. Protection against aluminum-lithium alloys is extremely critical. Therefore, it is of great significance to develop a multifunctional coating with self-healing function.

[0003] As one of the most important anti-corrosion means, organic coatings can effectively provide corrosion protection performance for the metal surface through physical shielding. However, in the face of external mechanical damage, the coating will be damaged and small cracks will occur. If not repaired in time and effectively, these defects will significantly reduce the protective effect of the coating on the metal substrate. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for constructing a self-healing and corrosion-resistant composite film on the surface of an aluminum-lithium alloy based on microfluidic technology and its product to solve the problems existing in the above-mentioned prior art and achieve efficient protection of the aluminum-lithium alloy.

[0005] To achieve the above purpose, the present invention provides the following solution:

[0006] The present invention provides a method for constructing a composite film on the surface of an aluminum-lithium alloy, including the following steps:

[0007] Perform electrochemical oxidation treatment on the aluminum-lithium alloy, add composite micro-droplets of cerium dioxide and epoxy resin prepared by microfluidic technology into the epoxy resin solution, and coat the obtained solution system on the surface of the aluminum-lithium alloy after electrochemical oxidation treatment to prepare the composite film;

[0008] The thickness of the composite film in the present invention is 100 - 150 μm.

[0009] The electrochemical oxidation treatment adopts two-step electrochemical oxidation: the electrolyte used in the first-step electrochemical oxidation is sulfuric acid electrolyte, and the electrolyte used in the second-step electrochemical oxidation is a mixed electrolyte of phosphoric acid and oxalic acid.

[0010] As a further preference of the present invention, in the preparation process of the composite microdroplets, a mixed solution of epoxy resin and nano-cerium dioxide is used as the inner-phase solution, a curing agent solution is used as the middle-phase solution, and a surfactant solution is used as the outer-phase solution.

[0011] As a further preference of the present invention, the content of epoxy resin in the inner-phase solution is 70 wt%, and the content of nano-cerium dioxide is 1.5 wt%; the content of curing agent in the middle-phase solution is 70 wt%, and the concentration of surfactant in the outer-phase solution is 1-2 wt%.

[0012] In the present invention, the solvents used for the inner-phase solution, the middle-phase solution and the outer-phase solution are xylene, and the surfactant used is polyvinyl alcohol.

[0013] As a further preference of the present invention, the flow rate of the inner-phase solution is 0.2-0.8 ml / h, the flow rate of the middle-phase solution is 0.4-1.6 ml / h, and the flow rate of the outer-phase solution is 4.0-10.0 ml / h.

[0014] As a further preference of the present invention, the mass of the nano-cerium dioxide is 1-2% of the total mass of the epoxy resin and the curing agent; the mass ratio of the curing agent to the epoxy resin in the middle-phase solution is 1:1.

[0015] In the present invention, the composite microdroplets prepared by the microfluidic technology are core-shell microdroplets.

[0016] As a further preference of the present invention, the two-electrode system is adopted for the electrochemical oxidation treatment, the working electrode is the aluminum-lithium alloy, and the counter electrode is the graphite electrode.

[0017] As a further preference of the present invention, the concentration of the sulfuric acid electrolyte is 160-200 ml / dm 3 ; in the mixed electrolyte, the concentration of phosphoric acid is 0.8-1.6 M, and the concentration of oxalic acid is 0.1-0.185 M.

[0018] As a further preference of the present invention, the oxidation and condensation temperature of the first-step electrochemical oxidation is (-2 to 2 ± 0.5) °C, the current density is 3 A / dm 2 ~3.5 A / dm 2 and the oxidation time is 60-90 min; the oxidation and condensation temperature of the second-step electrochemical oxidation is (-2 to 2 ± 0.5) °C, the oxidation voltage is 120-140 V, and the oxidation time is 4-8 h.

[0019] As a further preference of the present invention, in the solution system obtained by adding the composite microdroplets to the epoxy resin solution, the content of the epoxy resin is 35 wt%, the content of the curing agent is 35 wt%, the content of the composite microdroplets is 20 wt%, and the balance is 10 wt% of a solvent, i.e., a diluent, preferably xylene.

[0020] In the epoxy resin solution, the content of the epoxy resin is 35 wt%, the content of the curing agent is 35 wt%, the addition amount of the composite microdroplets in the epoxy resin solution is 20 wt%, and the balance is 10 wt% of xylene as a diluent.

[0021] The present invention also provides an aluminum-lithium alloy, on the surface of which a composite film prepared by the above preparation method is attached.

[0022] More preferably, the epoxy resin used in the present invention is epoxy resin E-44.

[0023] The electrochemical oxidation treatment of the present invention can construct a surface with a periodic micro-nano structure and a high activation energy on the surface of the aluminum-lithium alloy.

[0024] More preferably, the aluminum-lithium alloy of the present invention is pretreated, and the pretreatment includes steps of grinding, polishing, alkali washing, acid washing and water washing. Specifically: the aluminum-lithium alloy is ground and polished successively with metallographic sandpapers of 800 mesh, 1000 mesh and 1200 mesh; the polished electrode is pretreated, including alkali washing, acid washing and water washing. For alkali washing, a 0.5-1 mol / L NaOH solution is used, the electrode is placed in the solution for 2-3 min, and after taking out the electrode, it is placed in deionized water and left standing for 2-3 min to remove the residual NaOH solution on the surface; then the treated electrode is placed in a 0.5-1 mol / L sulfuric acid solution for 2-3 min to remove the residual sodium hydroxide and some hanging ash in the previous process; after the treatment is completed, finally, the surface of the specimen is flushed with deionized water to remove the residual acid solution on the surface, and then it is dried with cold air for standby.

[0025] The present invention can realize the preparation of a self-repairing corrosion-resistant composite film on the surface of the aluminum-lithium alloy. The more specific preparation steps of the composite film are as follows:

[0026] (1) Electrode pretreatment

[0027] The aluminum-lithium alloy was polished successively with 800-mesh, 1000-mesh, and 1200-mesh metallographic sandpapers; the polished electrode was pretreated, including alkali washing, acid washing, and water washing. For alkali washing, a 0.5 - 1 mol / L NaOH solution was used. The electrode was placed in the solution for 2 - 3 min, and after taking out the electrode, it was placed in deionized water and left standing for 2 - 3 min to remove the residual NaOH solution on the surface. Then the treated electrode was placed in a 0.5 - 1 mol / L sulfuric acid solution for 2 - 3 min to remove the residual sodium hydroxide and some ash from the previous process. After the treatment was completed, finally, the surface of the specimen was rinsed with deionized water to remove the residual acid solution on the surface, and then it was dried with cold air for standby.

[0028] (2) Electrochemical oxidation

[0029] The compositions of the electrolytes for the two micro-nano oxide films were respectively an electrolyte of 160 - 200 ml / dm 3 of sulfuric acid and a mixed electrolyte of 0.8 - 1.6 M phosphoric acid and 0.1 - 0.185 M oxalic acid; the oxidation method adopted a two-electrode system. The working electrode was the aluminum-lithium alloy, and the counter electrode was a graphite electrode. The working electrode and the counter electrode were fixed in the electrolytic cell, and then the electrolyte was added to the electrolytic cell for oxidation. The preparation of the micro-nano oxide film was divided into two steps: first, in the sulfuric acid electrolyte, with the oxidation condensation temperature of (-2 - 2 ± 0.5) °C, the current density was adjusted to 3 A / dm 2 ~3.5 A / dm 2 , and the oxidation time was 60 - 90 min to prepare an oxide film with small pores; then it was oxidized with the mixed electrolyte, the condensation temperature was (-2 - 2 ± 0.5) °C, by controlling the change of the anodic oxidation voltage from 120 - 140 V, and the oxidation time was 4 - 8 hours to prepare a micro-nano structure oxide film; finally, the prepared oxide film was rinsed with distilled water, dried, and then placed in an oven for drying for subsequent preparation; the drying temperature was 40 - 60 °C, and the time was 20 - 30 min.

[0030] (3) Preparation of composite coating solution

[0031] 5 - 10 g of epoxy resin and 0% - 2% (not 0%) of nano-CeO2 based on the total mass of the epoxy resin and the epoxy curing agent were added, and an appropriate amount of xylene was added for dilution to form a transparent yellow viscous solution to prepare the internal phase solution; an epoxy curing agent with a mass ratio of 1:1 to the epoxy resin was taken and an appropriate amount of xylene was added to prepare the middle phase solution; finally, 1 - 2 wt% of polyvinyl alcohol was added to pure water to prepare the external phase solution.

[0032] (4) Preparation of micro-droplets and filling them into the coating

[0033] Put the three-phase solution into three syringes respectively, connect them to a three-phase microfluidic chip with a hose, and connect the syringes to three independent propulsion pumps. Adjust the flow rates of the three phases: the inner-phase flow rate is 0.2 - 0.8 ml / h, the middle-phase flow rate is 0.4 - 1.6 ml / h, and the outer-phase flow rate is 4.0 - 10.0 ml / h. After curing the prepared CeO2 / epoxy micro-droplets, put them into a beaker, mix them into the epoxy resin and curing agent mixed solution, and then spray them onto the alloy substrate obtained after drying in step (1). After the spraying is completed, take out the sample, air-dry it naturally for 24 - 48 h and then store it.

[0034] The present invention also provides an aluminum-lithium alloy, on the surface of which there is attached a composite film prepared by the above preparation method.

[0035] The present invention conducts electrochemical oxidation on the aluminum-lithium alloy to grow a dense oxide film on its surface, thereby constructing different morphologies and roughnesses on the substrate. By studying the roughness, hydrophobicity, and surface energy of oxide films with different morphologies and pore sizes, the most suitable oxidation method for epoxy resin spraying is selected, and the improvement of the bonding strength and corrosion resistance after spraying is explored based on this.

[0036] In a large number of current studies, the method of doping oxide particles in the coating is to use air gun spraying or directly use the wire bar coating method. These methods will cause easy agglomeration of micro / nano particles, and the preparation process is cumbersome, the distribution of particles is uneven, and a micro / nano layered structure will be formed inside the coating, resulting in a reduction in the corrosion resistance and physical properties of the coating. The present invention selects to introduce nano-CeO2 particles as fillers into the epoxy resin, and coats a CeO2 / epoxy resin composite coating on the surface of the lithium-aluminum alloy through microfluidic technology. While enhancing the corrosion resistance by adding nano-CeO2 particles to fill the voids of the epoxy resin, when the nano-CeO2 particles encounter water and air after the coating is cracked, insoluble substances such as cerium hydroxide will be generated to fill the cracks to achieve self-repair.

[0037] The present invention uses microfluidic technology to prepare micro-droplets, which can make the addition amount and dispersibility of cerium dioxide controllable, avoid polymerization between droplets, and can effectively embed. There is currently no report on a self-repairing corrosion-resistant composite coating prepared by microfluidic technology to prepare a micro-droplet coating containing nano-CeO2 particles after electrochemical oxidation of an alloy substrate.

[0038] The present invention discloses the following technical effects:

[0039] The present invention first proposes to prepare a self-healing corrosion-resistant composite coating with micro-droplet coating containing nano-CeO2 particles by using microfluidic technology after electrochemically oxidizing an aluminum-lithium alloy substrate. A surface with periodic micro-nano structure and high activation energy is constructed on the aluminum-lithium alloy surface by an electrochemical method, and the corrosion resistance of the sprayed oxide film is improved by improving the roughness, hydrophobicity and surface energy of the micro-nano oxide film on the aluminum-lithium alloy surface.

[0040] The present invention uses microfluidic technology to prepare micro-droplets, which can control the addition amount and dispersion of cerium dioxide, avoid the polymerization between droplets, and can effectively embed; it can reduce the number of heterogeneous interfaces / coating porosity between particles and the organic coating, better play the role of self-healing characteristics, and increase the corrosion resistance of the coating.

[0041] The self-healing corrosion-resistant composite coating prepared by the present invention has the characteristics of multiple functions integrated and strong practicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0043] Figure 1 SEM image of the micro-nano oxide film prepared in Example 1 (under the condition of 130V) of the present invention;

[0044] Figure 2 SEM image of the oxide film prepared in Comparative Example 1 of the present invention;

[0045] Figure 3 Contact angle of the micro-nano oxide film prepared in Example 1 of the present invention under different voltage conditions;

[0046] Figure 4 Surface energy of the micro-nano oxide film prepared in Example 1 of the present invention under different voltage conditions;

[0047] Figure 5 Schematic structural diagram of the microfluidic chip used in the present invention;

[0048] Figure 6 Principle diagram of the formation of micro-droplets in the present invention;

[0049] Figure 7 Mechanism diagram of the self-healing of micro-droplets in the present invention;

[0050] Figure 8 SEM image of the coating prepared in Example 2 of the present invention;

[0051] Figure 9 EDS diagram of the coating prepared in Example 2 of the present invention;

[0052] Figure 10 EDS diagram of the coating prepared in Comparative Example 3 of the present invention;

[0053] Figure 11 Polarization curve diagrams of the coatings prepared in Example 1, Example 5 and Comparative Examples 1 and 2 of the present invention;

[0054] Figure 12 Self-healing SEM diagrams of the coating prepared in Example 1 of the present invention; (a) before immersion and (b) after immersion. Detailed implementation manners

[0055] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and implementation schemes of the present invention.

[0056] It should be understood that the terms used in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0057] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0058] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the present invention specification, which are obvious to those skilled in the art. Other implementation manners obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are only exemplary.

[0059] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, that is, they are meant to include but not be limited to.

[0060] The drugs used in the following examples and comparative examples of the present invention were purchased from Hangzhou Gaojing Fine Chemical Co., Ltd.

[0061] The microfluidic chip used in the embodiments of the present invention is as Figure 5 shown, which is composed of three capillary glass tubes (one square tube and two round tubes) adhered together. The size of the square tube is 1.10mm * 1.10mm, and the diameters of the two round tubes are both 1.05mm. The two round tubes are inserted into the square tube from both sides as the input tube and the collection tube respectively, and the inner end diameters of the infusion round tube and the collection round tube are controlled to be 40 - 60μm and 150 - 200μm respectively by the methods of tube drawing and tube cutting, and the distance between the orifices of the two round tubes in the square tube is controlled to be 150 - 200μm. The purpose is to narrow the channel to increase the liquid flow rate and thus generate shear force.

[0062] The schematic diagram of the micro-droplet formation principle of the present invention is as Figure 6 , which uses the shear force generated by high flow rate to shear the low flow rate fluid into uniform small droplets; the repair mechanism of the micro-droplets in the composite coating of the present invention is as Figure 7 shown. After the coating is broken under the action of external force, the microcapsules therein crack, exposing the nano-ceria. When encountering water and air, insoluble substances are formed to fill the cracks.

[0063] Example 1

[0064] (1) Electrode pretreatment

[0065] The aluminum-lithium alloy is polished successively with 800-mesh, 1000-mesh and 1200-mesh metallographic sandpapers; the polished electrode is pretreated, including alkali washing, acid washing and water washing.

[0066] For alkali washing, a 1mol / L NaOH solution is used. The electrode is placed in the solution for 2min, taken out and placed in deionized water for 2min to remove the residual NaOH solution on the surface; then the treated electrode is placed in a 0.5mol / L sulfuric acid solution for 2min to remove the residual sodium hydroxide and some hanging ash in the previous process; after the treatment is completed, finally, the surface of the specimen is rinsed with deionized water to remove the residual acid solution on the surface, and then dried with cold air for use.

[0067] (2) Electrochemical oxidation

[0068] Two-step electrochemical oxidation is adopted: the electrolyte in the first step uses deionized water as the solvent and is mixed with 20% dilute sulfuric acid to form a sulfuric acid electrolyte of (180ml / dm 3 ); the electrolyte in the second step also uses deionized water as the solvent and is mixed with phosphoric acid and oxalic acid to form a mixed electrolyte of 1.1M phosphoric acid and 0.185M oxalic acid.

[0069] The oxidation method adopts a two - electrode system. The working electrode is an aluminum - lithium alloy, and the counter electrode is a graphite electrode. The working electrode and the counter electrode are fixed in the electrolytic cell, and then an electrolyte is added to the electrolytic cell for oxidation.

[0070] The first - step electrochemical oxidation is carried out in a sulfuric acid electrolyte. During the oxidation process, the oxidation condensation temperature is (0 ± 0.5) °C, and the current density is adjusted to 3.5 A / dm 2 , and the oxidation times are 90 min respectively to prepare a porous oxide film; then a phosphoric acid electrolyte is used for secondary oxidation, where the oxidation condensation temperature is (0 ± 0.5) °C. By controlling the anodic oxidation voltage at 130 V and the oxidation time at 4 h, a micro - nano oxide film with excellent periodicity is prepared; and the obtained aluminum - lithium alloy oxide film is heat - treated, that is, baked in an oven at 40 °C for 20 min.

[0071] (3) Preparation of composite coating solution

[0072] 5 g of epoxy resin E - 44 and 1% of nano - CeO2 based on the total mass of epoxy resin E - 44 and epoxy curing agent 650 are added and diluted with an appropriate amount of xylene to form a transparent yellow viscous solution, preparing an inner - phase solution (the content of epoxy resin in the inner - phase solution is 70 wt%, and the content of nano - CeO2 is 1.5 wt%); take epoxy curing agent 650 with a mass ratio of 1:1 to the epoxy resin and add an appropriate amount of xylene to prepare a middle - phase solution (the content of the curing agent in the middle - phase solution is 70 wt%), and finally add 1 wt% of polyvinyl alcohol to pure water to prepare an outer - phase solution.

[0073] (4) Preparation of micro - droplets and filling into the coating

[0074] The three - phase solutions are respectively put into three syringes, connected to a three - phase microfluidic chip with a hose, and the syringes are connected to three independent propulsion pumps. Adjust the three - phase flow rates (inner - phase flow rate 0.8 ml / h, middle - phase flow rate 0.4 ml / h, outer - phase flow rate 8 ml / h) to prepare CeO2 / epoxy resin micro - droplets. The prepared CeO2 / epoxy resin micro - droplets are heated and cured at 60 °C for 10 min and then put into a beaker. Then the CeO2 / epoxy resin micro - droplets are added to the epoxy resin solution. In the obtained solution system, the content of epoxy resin is 35 wt%, the content of the curing agent is 35 wt%, the content of CeO2 / epoxy resin micro - droplets is 20 wt%, and the remaining is 10 wt% of xylene diluent. The above solution system is sprayed onto the alloy substrate obtained after drying in step (2). After spraying, the sample is taken out, placed in a drying dish and air - dried naturally for 24 h and then stored to obtain an aluminum - lithium alloy with a composite film (coating) thickness of 150 μm.

[0075] In the present invention, under the condition that the other conditions in Example 1 remain unchanged and only the voltage is changed, the properties of the oxide film and the coating under different voltages are explored:

[0076] The morphology diagram of the oxide film prepared by electrochemical oxidation (under the condition of 130V) in Example 1 is as follows Figure 1 shown. It can be seen from the figure that the overall oxide film has a periodic structure and its morphology is in the shape of hilly and gully (the micro-nano oxide film with the best periodicity in the embodiments of the present invention), and the size of its structure is distributed in 2-10μm.

[0077] The contact angle of the oxide film in Example 1 is as follows Figure 3 shown. As the voltage increases, the contact angle decreases significantly. The voltage condition of 130V is the condition for the most hydrophilic oxide film.

[0078] The surface energy of the oxide films obtained by treating in Example 1 at different voltages (120V, 130V, 140V) is as follows Figure 4 shown.

[0079] The polarization curve of the coating prepared under the voltage condition of 130V in Example 1 is as follows Figure 11 shown. The corrosion current density is two orders of magnitude lower than that in Example 5, and the corrosion potential also shifts to the right. It can be seen that its corrosion resistance has been significantly improved, which is the condition with the best corrosion resistance.

[0080] The self-healing performance of the coating prepared in Example 1 is as follows Figure 12 shown. After the treated aluminum-lithium alloy is immersed in a 3.5% sodium chloride solution at room temperature for 24h and then taken out and dried at room temperature, it can be found from the front and back SEM comparison diagrams that the cracks are filled with impurities. The principle is that cerium dioxide nanoparticles react in a 3.5% sodium chloride solution to generate insoluble substances such as cerium hydroxide, so that the cracks are repaired, indicating its self-healing performance.

[0081] Example 2

[0082] (1) Electrode pretreatment

[0083] The aluminum-lithium alloy is polished successively with 800-mesh, 1000-mesh and 1200-mesh metallographic sandpapers; the polished electrode is pre-treated, including alkali cleaning, acid cleaning and water washing.

[0084] Alkali cleaning is carried out with 1mol / L NaOH solution. The electrode is placed in the solution for 2min, and after taking out the electrode, it is placed in deionized water and left standing for 2min to remove the residual NaOH solution on the surface; then the treated electrode is put into 0.5mol / L sulfuric acid solution for 2min to remove the residual sodium hydroxide and some hanging ash in the previous process; after the treatment is completed, finally, the surface of the specimen is rinsed with deionized water to remove the residual acid solution on the surface, and then dried with cold air for standby.

[0085] (2) Electrochemical oxidation

[0086] Two-step electrochemical oxidation is adopted: the electrolyte in the first step uses deionized water as the solvent and is prepared into a sulfuric acid electrolyte with 20% dilute sulfuric acid (180 ml / dm 3 ). The electrolyte in the second step also uses deionized water as the solvent and is prepared into a mixed electrolyte of 1.1 M phosphoric acid and 0.185 M oxalic acid by adding phosphoric acid and oxalic acid.

[0087] The oxidation method adopts a two-electrode system. The working electrode is an aluminum-lithium alloy, and the counter electrode is a graphite electrode. The working electrode and the counter electrode are fixed in the electrolytic cell, and then the electrolyte is added to the electrolytic cell for oxidation.

[0088] The first-step electrochemical oxidation is carried out in the sulfuric acid electrolyte. During the oxidation process, the oxidation condensation temperature is (0 ± 0.5) °C, and the current density is adjusted to 3.5 A / dm 2 . The oxidation times are 90 min respectively to prepare a porous oxide film. Then, the phosphoric acid electrolyte is used for secondary oxidation. Among them, the oxidation condensation temperature is (0 ± 0.5) °C, and by controlling the anodic oxidation voltage at 130 V and the oxidation time at 4 h, a micro-nano oxide film with excellent periodicity is prepared. And the obtained aluminum-lithium alloy oxide film is heat-treated, that is, baked in an oven at 40 °C for 20 min.

[0089] (3) Preparation of composite coating solution

[0090] 5 g of epoxy resin E-44 and 1% of nano-CeO2 by the total mass of epoxy resin E-44 and epoxy curing agent 650 are added to an appropriate amount of xylene for dilution to prepare a transparent yellow viscous solution, which is prepared into an internal phase solution (the content of epoxy resin in the internal phase solution is 70 wt%, and the content of nano-CeO2 is 1.5 wt%). Take epoxy curing agent 650 with a mass ratio of 1:1 to the epoxy resin and add an appropriate amount of xylene to prepare a middle phase solution (the content of the curing agent in the middle phase solution is 70 wt%). Finally, 1 wt% of polyvinyl alcohol is added to pure water to prepare an external phase solution.

[0091] (4) Preparation of micro-droplets and filling into the coating

[0092] Put the three-phase solution into three syringes respectively, connect them to the three-phase microfluidic chip with hoses, and connect the syringes to three independent propulsion pumps. Adjust the three-phase flow rates (inner-phase flow rate 0.4 ml / h, middle-phase flow rate 0.4 ml / h, outer-phase flow rate 10 ml / h) to prepare CeO2 / epoxy microdroplets; heat and cure the prepared CeO2 / epoxy microdroplets at 60 °C for 10 min and then put them into a beaker. Then add the CeO2 / epoxy microdroplets into the epoxy resin solution. In the resulting solution system, the content of epoxy resin is 35 wt%, the content of curing agent is 35 wt%, the content of CeO2 / epoxy microdroplets is 20 wt%, and the remaining is 10 wt% xylene diluent. Spray the above solution system onto the alloy substrate obtained after drying in step (2). After spraying, take out the sample, place it in a drying dish and air dry it naturally for 24 h and then store it to obtain an aluminum-lithium alloy with a composite film (coating) thickness of 150 μm.

[0093] The morphology diagram of the coating prepared in Example 2 is as Figure 8 shown, which is a uniform and smooth surface; and the distribution of cerium in the coating is as Figure 9 shown, and the distribution is very uniform.

[0094] Example 3

[0095] (1) Electrode pretreatment

[0096] Grind and polish the aluminum-lithium alloy successively with 800-mesh, 1000-mesh and 1200-mesh metallographic sandpapers; perform pretreatment on the polished electrode, including alkali washing, acid washing and water washing.

[0097] For alkali washing, use 1 mol / L NaOH solution, place the electrode in the solution for 2 min, take out the electrode and put it in deionized water and let it stand for 2 min to remove the residual NaOH solution on the surface; then put the treated electrode into 0.5 mol / L sulfuric acid solution for 2 min to remove the residual sodium hydroxide and some hanging ash in the previous process; after the treatment is completed, finally rinse the surface of the specimen with deionized water to remove the residual acid solution on the surface, and then dry it with cold air for standby.

[0098] (2) Electrochemical oxidation

[0099] Two-step electrochemical oxidation is adopted: the electrolyte in the first step uses deionized water as the solvent and is prepared into a sulfuric acid electrolyte with 20% dilute sulfuric acid (180 ml / dm 3 ); the electrolyte in the second step also uses deionized water as the solvent and is prepared into a mixed electrolyte of 1.1 M phosphoric acid and 0.185 M oxalic acid.

[0100] The oxidation method adopts a two - electrode system. The working electrode is an aluminum - lithium alloy, and the counter electrode is a graphite electrode. The working electrode and the counter electrode are fixed in the electrolytic cell, and then an electrolyte is added to the electrolytic cell for oxidation.

[0101] The first - step electrochemical oxidation is carried out in a sulfuric acid electrolyte. During the oxidation process, the oxidation condensation temperature is (0 ± 0.5)°C, and the current density is adjusted to 3.5 A / dm 2 , and the oxidation time is 90 min respectively to prepare a porous oxidation film; then a phosphoric acid electrolyte is used for secondary oxidation. Among them, the oxidation condensation temperature is (0 ± 0.5)°C, and by controlling the anodic oxidation voltage at 130 V and the oxidation time at 4 h, a micro - nano oxidation film with excellent periodicity is prepared; and the obtained aluminum - lithium alloy oxidation film is heat - treated, that is, baked in an oven at 40°C for 20 min.

[0102] (3) Preparation of the composite coating solution

[0103] 10 g of epoxy resin E - 44 and 2% of nano - CeO2 by the total mass of epoxy resin E - 44 and epoxy curing agent 650 are added and diluted with an appropriate amount of xylene to form a transparent yellow viscous solution to prepare an inner - phase solution (the content of epoxy resin in the inner - phase solution is 70 wt%, and the content of nano - CeO2 is 1.5 wt%); it is diluted with an appropriate amount of xylene to form a transparent yellow viscous solution to prepare an inner - phase solution; an epoxy curing agent 650 with a mass ratio of 1:1 to the epoxy resin is taken and diluted with an appropriate amount of xylene to prepare a middle - phase solution (the content of the curing agent in the middle - phase solution is 70 wt%), and finally 1 wt% of polyvinyl alcohol is added to pure water to prepare an outer - phase solution.

[0104] (4) Preparation of micro - droplets and filling them into the coating

[0105] The three - phase solutions are respectively put into three syringes, connected to a three - phase microfluidic chip with a hose, and the syringes are connected to three independent propulsion pumps. The three - phase flow rates are adjusted (inner - phase flow rate 0.4 ml / h, middle - phase flow rate 0.4 ml / h, outer - phase flow rate 10 ml / h) to prepare CeO2 / epoxy resin micro - droplets. The prepared CeO2 / epoxy resin micro - droplets are heated and cured at 60°C for 10 min and then put into a beaker. Then the CeO2 / epoxy resin micro - droplets are added to the epoxy resin solution. In the obtained solution system, the content of epoxy resin is 35 wt%, the content of the curing agent is 35 wt%, the content of CeO2 / epoxy resin micro - droplets is 20 wt%, and the remaining is 10 wt% of xylene diluent. The above - mentioned solution system is sprayed onto the alloy substrate obtained after drying in step (2). After spraying, the sample is taken out, placed in a drying dish and air - dried naturally for 24 h and then stored to obtain an aluminum - lithium alloy with a composite film (coating) thickness of 150 μm.

[0106] Example 4

[0107] (1) Electrode pretreatment

[0108] The aluminum-lithium alloy was polished successively with 800-mesh, 1000-mesh, and 1200-mesh metallographic sandpapers; the polished electrode was pretreated, including alkali washing, acid washing, and water washing.

[0109] For alkali washing, a 1 mol / L NaOH solution was used. The electrode was placed in the solution for 2 min, taken out and placed in deionized water for 2 min to remove the residual NaOH solution on the surface; then the treated electrode was placed in a 0.5 mol / L sulfuric acid solution for 2 min to remove the residual sodium hydroxide and some ash in the previous process; after the treatment was completed, finally, the surface of the specimen was rinsed with deionized water to remove the residual acid solution on the surface, and then dried with cold air for standby.

[0110] (2) Electrochemical oxidation

[0111] Two-step electrochemical oxidation was adopted: the electrolyte for the first step used deionized water as the solvent and was prepared into a sulfuric acid electrolyte with 20% dilute sulfuric acid (180 ml / dm 3 ); the electrolyte for the second step also used deionized water as the solvent and was prepared into a mixed electrolyte of 0.8 M phosphoric acid and 0.185 M oxalic acid.

[0112] The oxidation method adopted a two-electrode system. The working electrode was the aluminum-lithium alloy, and the counter electrode was the graphite electrode. The working electrode and the counter electrode were fixed in the electrolytic cell, and then the electrolyte was added to the electrolytic cell for oxidation.

[0113] The first-step electrochemical oxidation was carried out in the sulfuric acid electrolyte. During the oxidation process, the oxidation condensation temperature was (0 ± 0.5) °C, and the current density was adjusted to 3.5 A / dm 2 , and the oxidation time was 90 min respectively to prepare a small-hole oxide film; then the phosphoric acid electrolyte was used for secondary oxidation, in which the oxidation condensation temperature was (0 ± 0.5) °C, and by controlling the anodic oxidation voltage at 130 V and the oxidation time at 4 h, a micro-nano oxide film with the best periodicity was prepared; and the obtained aluminum-lithium alloy oxide film was heat-treated, that is, baked in an oven at 40 °C for 20 min.

[0114] (3) Preparation of composite coating solution

[0115] Add 5 g of epoxy resin E-44 and 1% of nano-CeO₂ by the total mass of epoxy resin E-44 and epoxy curing agent 650 into an appropriate amount of xylene for dilution to form a transparent yellow viscous solution, and prepare an internal phase solution (the content of epoxy resin in the internal phase solution is 70 wt%, and the content of nano-CeO₂ is 1.5 wt%); take epoxy curing agent 650 with a mass ratio of 1:1 to the epoxy resin and add an appropriate amount of xylene to prepare a middle phase solution (the content of the curing agent in the middle phase solution is 70 wt%). Finally, add 1 wt% of polyvinyl alcohol into pure water to prepare an external phase solution.

[0116] (4) Prepare micro-droplets and fill them into the coating

[0117] Put the three-phase solutions into three syringes respectively, connect them to a three-phase microfluidic chip with a hose, and connect the syringes to three independent propulsion pumps. Adjust the flow rates of the three phases (the internal phase flow rate is 0.8 ml / h, the middle phase flow rate is 0.4 ml / h, and the external phase flow rate is 8.0 ml / h) to prepare CeO₂ / epoxy resin micro-droplets. Heat and cure the prepared CeO₂ / epoxy resin micro-droplets at 60 °C for 10 min and then put them into a beaker. Then add the CeO₂ / epoxy resin micro-droplets into the epoxy resin solution. In the obtained solution system, the content of epoxy resin is 35 wt%, the content of the curing agent is 35 wt%, the content of CeO₂ / epoxy resin micro-droplets is 20 wt%, and the remaining is 10 wt% of xylene diluent. Spray the above solution system onto the alloy substrate obtained after drying in step (2). After spraying, take out the sample, place it in a drying dish and air-dry it naturally for 24 h and then store it to obtain an aluminum-lithium alloy with a composite film (coating) thickness of 150 μm.

[0118] Example 5

[0119] (1) Electrode pretreatment

[0120] Grind and polish the aluminum-lithium alloy successively with 800-mesh, 1000-mesh and 1200-mesh metallographic sandpapers; perform pretreatment on the polished electrode, including alkali washing, acid washing and water washing.

[0121] For alkali washing, use 1 mol / L NaOH solution, place the electrode in the solution for 2 min, take out the electrode and put it in deionized water and let it stand for 2 min to remove the residual NaOH solution on the surface; then put the treated electrode into 0.5 mol / L sulfuric acid solution for 2 min to remove the residual sodium hydroxide and some ash in the previous process; after the treatment is completed, finally rinse the surface of the specimen with deionized water to remove the residual acid solution on the surface, and then dry it with cold air for standby.

[0122] (2) Preparation of composite coating solution

[0123] 5 g of epoxy resin E-44 and 1% of nano-CeO2 by the total mass of epoxy resin E-44 and epoxy curing agent 650 were added, and an appropriate amount of xylene was added for dilution to form a transparent yellow viscous solution, which was prepared into an internal phase solution (the content of epoxy resin in the internal phase solution was 70 wt%, and the content of nano-CeO2 was 1.5 wt%); an epoxy curing agent 650 with a mass ratio of 1:1 to the epoxy resin was taken and an appropriate amount of xylene was added to prepare a middle phase solution (the content of the curing agent in the middle phase solution was 70 wt%). Finally, 1 wt% of polyvinyl alcohol was added to pure water to prepare an external phase solution.

[0124] (3) Prepare microdroplets and fill them into the coating

[0125] The three-phase solutions were respectively put into three syringes, connected to a three-phase microfluidic chip with a hose, and the syringes were connected to three independent propulsion pumps. The three-phase flow rates were adjusted (the internal phase flow rate was 0.8 ml / h, the middle phase flow rate was 0.4 ml / h, and the external phase flow rate was 8 ml / h) to prepare CeO2 / epoxy resin microdroplets. The prepared CeO2 / epoxy resin microdroplets were heated and cured at 60 °C for 10 min and then put into a beaker. Then, the CeO2 / epoxy resin microdroplets were added to the epoxy resin solution. In the obtained solution system, the content of epoxy resin was 35 wt%, the content of the curing agent was 35 wt%, the content of CeO2 / epoxy resin microdroplets was 20 wt%, and the remaining was 10 wt% of xylene diluent. The above solution system was sprayed onto the alloy substrate obtained after drying in step (1). After spraying, the sample was taken out, naturally air-dried in a drying dish for 24 h and then stored, and an aluminum-lithium alloy with a composite film (coating) thickness of 150 μm was obtained.

[0126] Example 5 was the same as Example 1 in other conditions, except that the step of electrochemical oxidation of the aluminum-lithium alloy substrate was missing, and its corrosion resistance was as Figure 11 shown.

[0127] Comparative Example 1

[0128] Same as Example 1, the difference was only that the voltage of the second oxidation was 70 V.

[0129] The anodic oxidation film prepared in this comparative example was as Figure 2 shown. Although there were also many large flaky structures on the surface, it did not have good periodicity, and there were only a few small holes formed on the surface, with different sizes and less than micron-level, which should be due to the low voltage resulting in incomplete oxidation. Compared with Example 1, the corrosion resistance of this comparative example was as Figure 11 shown, which was relatively poor.

[0130] Comparative Example 2

[0131] Same as Example 1, the difference was only that the composite coating did not contain nano-CeO2 particles.

[0132] The polarization curve of the coating prepared in this comparative example is as Figure 11 shown. Compared with the corrosion current density of the composite coating added with nano-CeO2 particles, the substrate is 1-2 orders of magnitude higher, and the corrosion potential also decreases; moreover, this coating does not have self-healing performance, and the cracks do not change at all under the same conditions.

[0133] Comparative Example 3

[0134] Same as Example 2, the difference is only that the nano-CeO2 particles in the composite coating are directly added.

[0135] The coating prepared in this comparative example directly added nano-CeO2 particles to epoxy resin. As Figure 10 shown, the distribution of cerium elements in this composite coating is not uniform enough, and micro / nano particles are prone to agglomeration, forming a micro / nano layered structure inside the coating; moreover, nano-cerium dioxide particles are prone to deposition during the coating preparation process, resulting in a significant decrease in the actual content compared to the added amount in the end.

[0136] The present invention improves the hydrophobicity and adhesion of the coating by constructing an oxide film with a periodic micro-nano surface structure. When the nano-cerium dioxide particles in the cerium dioxide composite coating encounter an environment of water and oxygen, insoluble substances such as cerium hydroxide are generated to fill the cracks to achieve self-healing, thereby demonstrating the corrosion resistance and self-healing performance of the composite film of the present invention.

[0137] The above-described embodiments are only descriptions of the preferred modes of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for constructing a composite film on the surface of an aluminum-lithium alloy, characterized in that, It includes the following steps: Perform electrochemical oxidation treatment on the aluminum-lithium alloy, add the composite microdroplets of cerium dioxide and epoxy resin prepared by the microfluidic technology into the epoxy resin solution, and coat the obtained solution system on the surface of the aluminum-lithium alloy after electrochemical oxidation treatment to prepare a composite film; The electrochemical oxidation treatment adopts two-step electrochemical oxidation: the electrolyte used in the first-step electrochemical oxidation is a sulfuric acid electrolyte, and the electrolyte used in the second-step electrochemical oxidation is a mixed electrolyte of phosphoric acid and oxalic acid; In the preparation process of the composite microdroplets, a mixed solution of epoxy resin and nano-ceria is used as the inner-phase solution, a curing agent solution is used as the middle-phase solution, and a surfactant solution is used as the outer-phase solution.

2. The method according to claim 1, characterized in that, The content of epoxy resin in the inner-phase solution is 70wt%, and the content of nano-ceria is 1.5wt%; the content of curing agent in the middle-phase solution is 70wt%, and the concentration of surfactant in the outer-phase solution is 1-2wt%.

3. The method according to claim 2, wherein The flow rate of the inner-phase solution is 0.2-0.8 ml / h, the flow rate of the middle-phase solution is 0.4-1.6 ml / h, and the flow rate of the outer-phase solution is 4.0-10.0 ml / h.

4. The method according to claim 1, wherein The mass of the nano-ceria is 1-2% of the total mass of the epoxy resin and the curing agent.

5. The method according to claim 1, characterized in that, The electrochemical oxidation treatment adopts a two-electrode system, the working electrode is the aluminum-lithium alloy, and the counter electrode is a graphite electrode.

6. The method according to claim 1, wherein The concentration of the sulfuric acid electrolyte is 160 - 200 ml / dm 3 ; in the mixed electrolyte, the concentration of phosphoric acid is 0.8 - 1.6 M, and the concentration of oxalic acid is 0.1 - 0.185 M.

7. The method according to claim 6, wherein The oxidation and condensation temperature of the first-step electrochemical oxidation is (-2 to 2 ± 0.5) °C, and the current density is 3 A / dm 2 ~3.5 A / dm 2 , and the oxidation time is 60 to 90 min; the oxidation and condensation temperature of the second-step electrochemical oxidation is (-2 to 2 ± 0.5) °C, the oxidation voltage is 120 to 140 V, and the oxidation time is 4 to 8 h.

8. The method according to claim 1, wherein In the solution system, the content of epoxy resin is 35wt%, the content of curing agent is 35wt%, the content of the composite microdroplets is 20wt%, and the balance is solvent.

9. An aluminum-lithium alloy, characterized in that, The surface of the aluminum-lithium alloy is attached with a composite film prepared by the preparation method according to any one of claims 1-8.

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