CN108315800A - A kind of preparation method of the differential arc oxidation of magnesium/magnesium alloy-alumina composite coating - Google Patents

Abstract

The present invention provides a kind of preparation methods of the differential arc oxidation alumina composite coating of magnesium/magnesium alloy, the post-processing of the configuration of pretreatment, micro-arc oxidation electrolyte, the preparation of arc differential oxide ceramic layer and atomic layer deposition including magnesium/magnesium alloy, the primary raw material for being directed to micro-arc oxidation electrolyte has phytic acid, sodium hydroxide and glucose, atomic layer deposition primary raw material to have trimethyl aluminium, ultra-pure water, hydrogen peroxide, oxygen and ozone.The arc differential oxide ceramic layer that with porous structure, chemical composition is mainly magnesia is first deposited on magnesium/magnesium alloy matrix surface using differential arc oxidation method, recycles the composite coating of technique for atomic layer deposition deposition of aluminium oxide.The present invention is prepared simply, and film layer is easily-controllable, and obtained composite coating has the characteristics that compact structure, strong adhesive force, corrosion resisting property are good, can be used for the protection to magnesium/magnesium alloy.

Description

A preparation method of micro-arc oxidation-alumina composite coating on magnesium/magnesium alloy

technical field

The invention relates to the field of coating preparation, and relates to a preparation method of a magnesium/magnesium alloy micro-arc oxidation-alumina composite coating.

Background technique

Magnesium and its alloys have the characteristics of light weight, high specific strength, and good shock resistance. They are a new generation of light metal structural materials and have broad application prospects in the fields of 3C products, automobiles, high-speed rail, and aerospace. However, the excessive corrosion rate of magnesium alloy is still the main factor limiting its application. The high activity of magnesium makes magnesium alloy structural parts prone to corrosion during use. If magnesium alloy components are connected with dissimilar metals, they are prone to galvanic corrosion. Therefore, developing an effective anti-corrosion coating and technology is an urgent problem to be solved in promoting the large-scale application of magnesium alloys.

At present, people mainly start from two aspects to solve this problem. The first idea is to improve the corrosion resistance of magnesium alloy itself. The common ways are: (1) preparation of high-purity magnesium, (2) addition of alloying elements, (3) rapid solidification and other technologies. The second idea is to modify the surface of magnesium alloys, including anodic oxidation and chemical conversion treatment. Anodizing of magnesium alloys is one of the most effective surface techniques. Magnesium alloy anodic oxidation is to use electrochemical method to form a stable oxide film on the surface of magnesium alloy substrate to improve the corrosion resistance of magnesium alloy. The oxide film is grown in situ on the surface of the magnesium alloy substrate, and the obtained film layer has excellent bonding force with the substrate.

Microarc oxidation (MAO) surface treatment technology, also known as plasma electrolyte oxidation (PEO), refers to the use of arc discharge to enhance and activate the reaction on the anode on the basis of anodic oxidation. A method for forming a ceramic film on the surface of valve metals (Al, Ti and Mg) and their alloy workpieces. In other words, by adjusting the parameters of the electrolyte and the power supply, under the action of the instantaneous high temperature generated by the micro-arc discharge, a ceramic film layer consisting mainly of matrix elements and supplemented by electrolyte components is grown on the surface of the valve metal. The ceramic layer has the advantages of high hardness and simple preparation method.

At present, magnesium alloy micro-arc oxidation technology is developing rapidly. From the perspective of electrolyte and process, it is constantly developing in the direction of energy saving, green and environmental protection. Its development process has mainly gone through three stages:

1. In the 1950s, micro-arc oxidation began to be applied to the field of magnesium alloys. Generally, alkaline electrolyte is the main solution. The main problem is that the electrolyte contains hexavalent chromium ions, which is seriously harmful to the environment;

Second, since the 1980s, fluoride ions have been used as the main component of the electrolyte. Fluoride ions are still harmful to the environment and pose potential safety hazards to operators;

3. Since the beginning of the 21st century, the green and environment-friendly magnesium alloy micro-arc oxidation electrolyte has become the focus of research. Generally, various additives are added to the basic system of alkaline electrolyte to improve the performance of the film layer. Additives mainly include three types: aluminate, silicic acid and phosphoric acid. Among them, the aluminum acid system can promote the growth of the oxide film layer, form a dense film layer, reduce the self-corrosion current, and thus improve the corrosion resistance. However, the alumina-based electrolyte is not easy to control, and the solution is easily turbid, which seriously affects the film-forming effect. The silicic acid-based electrolyte is stable, can promote the oxidation of the matrix alloy, and form insoluble compounds, but requires relatively high equipment voltage, high power consumption, and high energy consumption. Phosphate-based electrolytes can be prepared at lower voltages, and the formed oxide film is dense and stable.

However, magnesium alloys are prone to destructive spark discharges when they are anodized in an environmentally friendly electrolyte composed of inorganic salts, resulting in large surface roughness of the oxide film, uneven distribution of micropores and large pore diameters, and the appearance of through holes. , microcracks and localized ablation. The micro-arc oxidation layer prepared by using these electrolytes has relatively high porosity, obvious micro-cracks and a certain number of through holes, resulting in poor long-term corrosion resistance, thereby reducing the service life of anodized film structural parts.

Studies have found that organic additives such as triethanolamine, ethylenediaminetetraacetic acid (EDTA), and amino acids can inhibit spark discharge, increase the breakdown voltage of the oxide film, and increase the thickness and surface finish of the oxide film. For example, L-ornithine acetate can form a coordination bond with the Mg surface through the N atom (Mg-N) of δ-NH2 and the O atom (Mg-O) of the carboxyl group in the alkaline anodizing electrolyte. And produce strong chemical adsorption (Gou Yining, Rare Metal Materials and Engineering, 2017, 4: 1103).

It is worth noting that there are micropores and microcracks in the micro-arc oxidation film on the surface of magnesium alloy, and post-treatment such as hole sealing is required to further improve the corrosion resistance and protective effect of the film layer. At present, there are many ways to seal the pores of the micro-arc oxidation film layer. According to the principle, the sealing methods of magnesium alloy oxide film mainly include hydration reaction, alkali treatment, inorganic filling and organic filling.

Hydration reaction and alkali treatment are to treat the anodized film in boiling water or alkaline solution (such as sodium hydroxide, NaHCO3) to form hydroxide (such as magnesium hydroxide) and produce volume expansion, so as to achieve the purpose of sealing. Such treatments have very limited effects. Because in a dry environment, magnesium hydroxide can also lose water molecules to form an oxide film, and the pore size returns to its original size.

Inorganic fillings generally use sealing agents, including chromates, silicates, phosphates, and sol-gels. Although chromate process such as HAE has good corrosion resistance, hexavalent chromium is toxic and easy to cause cancer. Silicate sealing is also called water glass sealing. Silicates and phosphates are more environmentally friendly. However, the silicate sealing can not maintain the high hardness of the ceramic membrane because the water glass is soft. Sol-gel is a physical sealer. The types of organic coatings can be divided into paraffin series, thermoplastic resin series such as vinyl resin, thermosetting resin series such as epoxy resin, fluororesin series such as polytetrafluoroethylene resin, organic polymer series such as silicone resin, etc. Organic coating is a physical sealing method, which is affected by the wettability of oxide film and coating material. It is necessary to choose a sealing agent with a large degree of wetting, so that they can penetrate deeply into the pores of the oxide film. On the other hand, considering the effect of surface phenomena, the surface tension of the coating should be relatively small, which is conducive to the coating on the surface of the oxide film. The spreading and coating enter the pores of the oxide film through capillary action.

Chinese patent "Sealing Method of Magnesium Alloy Micro-arc Oxidation Film" (201510076185.9) first configure PVDF solution, then add sodium orthosilicate, potassium hydroxide or sodium hydroxide, and then selectively add dibenzoyl peroxide (BPO) And divinylbenzene DVB, after soaking and drying the system. The Chinese patent "A Method for Constructing a Superhydrophobic Film on the Surface of a Magnesium Alloy Micro-arc Oxidation Ceramic Layer" (201510527099.5) uses fluorosilane for modification and curing. The Chinese patent "Phytic acid micro-arc anodic oxidation film and polylactic acid coating and process on the surface of magnesium alloy for medical use" (201210184704.X) introduces the method of sealing the polylactic acid film on the surface of the anodic oxidation film of magnesium alloy. A porous polylactic acid film was formed on it. However, Zeng Rongchang et al. (ACS Applied Materials & Interfaces, 2016, 8:10014) showed that the effective life of phytic acid micro-arc anodic oxidation film in simulated body fluid (hank's) is only more than 50 hours. And after soaking for 140 hours, the polylactic acid film appeared bubbling and shedding phenomenon. The main reason is that the organic coating polylactic acid has a high water permeability, and the combination with the anodized film is mechanical or physical, so the binding force is weak. Although these methods have a certain protective effect, the film layer is thicker, the binding force is not strong, and it is easy to fall off. For the organic coating, the hydrolysis is too fast, so it cannot play the role of long-term protection.

Atomic Layer Deposition (ALD) technology is characterized by a self-limiting surface growth method. Atomic layer deposition can achieve controllable film thickness at the level of single atomic layer, and can achieve 100% uniform and conformal film coverage on three-dimensional structures with large aspect ratios.

Atomic layer deposition technology was mainly used in the field of semiconductors to prepare high-K materials (that is, high dielectric constant materials) and IC (integrated circuit integrated circuit) interconnection technology. At present, there is no search for using atomic layer deposition technology to seal holes in magnesium alloys. Oxidation patents, but there are reports (Nanoscale, 2017 No. 9) that use atomic layer deposition technology to deposit Zn-Al-O coatings on porous surfaces. This coating has the characteristics of compact structure, but it is harmful to corrosion resistance Sexual improvements are negligible. Therefore, in order to further improve the performance of micro-arc oxidation, it is necessary to select an atomic layer deposition process with better effect.

Contents of the invention

In order to solve the problem that the service time of magnesium/magnesium alloy is short, the corrosion resistance of the micro-arc oxidation layer is poor, and it cannot meet the needs of production, the invention provides a method for preparing a micro-arc anodic oxidation-alumina composite coating of magnesium/magnesium alloy . The present invention mainly provides (1) a kind of optimum phytic acid-glucose composite electrolyte, and provides optimum aging time; (2) a kind of better, simpler atomic layer deposition process method, with Provide better protection.

Phytic acid (commonly known as phytic acid, molecular formula C ₆ H ₁₈ O ₂₄ P ₆ ) used in the present invention also belongs to the main component of phosphoric acid system, which is environmentally friendly and widely used. The weakly acidic phytic acid provides phosphate radicals, and at the same time, the damage to the magnesium alloy substrate and the corrosion degree of the micro-arc oxidation film are small, and the quality of the formed oxide film is relatively dense and has good corrosion resistance. Glucose (chemical formula C ₆ H ₁₂ O ₆ ) widely exists in plant starch and cellulose, and is the most widely distributed and most important monosaccharide in nature. It is a polyhydroxy aldehyde containing five hydroxyl groups and one aldehyde group , this structure makes it easy to adsorb with other molecules. Based on the above discussion, it can be assumed that in the electrolyte solution containing phytic acid and glucose, the corrosion resistance of magnesium alloy MAO coating will be better than that in phytic acid or glucose solution. The main technical parameters that need to be considered here include the ratio of phytic acid to glucose, pH value, and electrolyte aging time.

Atomic layer deposition splits a complete reaction into two steps. Two different precursor sources are fed into the chamber in turn, and layer-by-layer growth at the atomic level is achieved through supersaturated adsorption and chemical reaction. Because each half-reaction is a chemical combination, the atomic layer deposition film has the characteristics of nanoscale, strong binding force, high density, and high three-dimensional shape retention. This film layer can block microcracks, improve corrosion resistance, and retain the functionality of the micro-arc oxidation porous layer. The present invention selects aluminum oxide as the modification layer, mainly because compared with oxides of the same series (such as: tantalum oxide, magnesium oxide, zinc oxide, etc.), aluminum oxide is the easiest to deposit by atomic layer deposition technology, and the raw material is easy to get, and the preparation The conditions are simple, the growth rate is fast, and the prepared alumina layer is dense and pollution-free.

The present invention takes the following technical solutions:

A method for preparing a magnesium/magnesium alloy micro-arc oxidation-alumina composite coating, comprising the following steps:

(1) The magnesium or magnesium alloy substrate is sequentially mechanically polished, washed with sodium hydroxide solution and deionized water, and quickly dried in warm air to remove oxides and impurities on the surface of the substrate;

(2) Configure the micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide, and glucose, and age it for subsequent use;

(3) Clamp the pretreated magnesium/magnesium alloy on the electrode, immerse in the electrolyte solution, slowly increase the voltage under the condition of constant current, and grow a micro-arc oxidation ceramic layer on the surface of the magnesium/magnesium alloy;

(4) When the voltage rises to the maximum and the current drops below 0.02A, the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample obtained in step (4) is placed in the reaction chamber of the atomic layer deposition equipment, the chamber is sealed, nitrogen or argon inert gas is introduced and vacuumized, the temperature is gradually raised to the reaction temperature, and the chamber pressure is controlled at 0.1-1.0 Torr, the temperature is controlled at 80-150°C, and atomic layer deposition is grown on the micro-arc oxidation ceramic layer grown on the surface of magnesium/magnesium alloy;

A cycle of the atomic layer deposition growth includes the following four links: 1. The first reaction precursor, trimethylaluminum, is introduced into the reaction chamber for a time of t1, and supersaturated adsorption occurs with the sample in the chamber. A chemical reaction occurs to release by-products; ② Stop feeding the first reaction precursor, the inert carrier gas will clean up the excess precursor and by-products, and the carrier gas purging time is t2; The first reaction precursor is introduced at t3, which reacts with the first precursor to release by-products; ④ stop feeding the second reaction precursor, the inert carrier gas will clean up the excess precursor and by-products, and the carrier gas will blow The sweep time is t4.

Preferably, the mechanical polishing in step (1) is to use water-grinding sandpaper to polish to more than 1500 mesh.

Preferably, the concentrations of phytic acid, sodium hydroxide, and glucose in the micro-arc oxidation electrolyte solution in step (2) are 5-10 g/L, 6.25-12.5 g/L, and 8-12 g/L in sequence.

Preferably, the weight ratio of phytic acid to sodium hydroxide is 4:5, and the pH of the final electrolyte is 9.5-11.

Preferably, the aging time in step (2) is 0-48 hours.

Preferably, the aging time in step (2) is 12-24 hours.

Preferably, the constant current in step (3) is 0.1-0.25A.

Preferably, the voltage in step (4) is 200-350V.

Preferably, the second reaction precursor in step (5) is ultrapure water, hydrogen peroxide, oxygen or ozone.

Preferably, the time of the four links in the atomic layer deposition growth process in step (5) is t1: 0.015-0.1s, t2: 10-120s, t3: 0.015-0.2s, t4: 10-120s.

Preferably, the number of cycles of atomic layer deposition growth in step (5) is 1-5000.

Compared with the beneficial effects of the prior art the present invention is:

(1) Ingeniously combine the existing micro-arc oxidation technology with atomic layer deposition technology, and use the high bonding force and three-dimensional shape retention of atomic layer deposition technology to prepare a composite coating with strong bonding force and density, At the same time, it can retain the microstructure and play a good corrosion resistance effect; (2) The micro-arc oxidation electrolyte solution contains phytic acid and glucose, which can effectively reduce the arcing voltage of the electrolyte during the micro-arc oxidation process, and the phytic acid and glucose The compound has higher corrosion resistance. Glucose can further inhibit spark discharge in the phytic acid electrolyte, making the oxide film dense, with uniform micropore distribution and smaller pore size. Compared with the oxide film formed without adding glucose, It has very good performance, which is mainly due to the fact that there are more hydroxyl groups containing lone pair electrons in the glucose molecule, which can combine with the empty orbitals of magnesium ions, effectively adsorb on the surface of magnesium alloys, and form an adsorption layer, which can more effectively suppress spark discharge , to reduce the appearance of macropores formed by spark discharge, so that the number of micropores on the surface of the microarc oxidation coating is reduced, and the size of the micropores is reduced; (3) the atomic layer deposition technology is used to deposit on the microarc oxidation porous layer The aluminum oxide film layer has excellent surface coverage and three-dimensional shape retention; (4) The thickness of the nanoscale film layer is accurately and controllable, which can completely cover the crack defects of micro-arc oxidation, thereby playing a protective role.

Description of drawings

Fig. 1 is a graph comparing the polarization curves of the micro-arc oxidation coatings prepared in Example 1 and Example 2.

FIG. 2 is a comparison chart of AC impedance Bode of the micro-arc oxidation coatings prepared in Example 1 and Example 2.

Fig. 3 is a SEM comparison diagram of the microscopic morphology of MAO prepared in Example 2 and MAO/Al ₂ O ₃ .

Fig. 4 is a comparison diagram of the polarization curves of MAO, MAO/Al ₂ O ₃ composite coating samples prepared in Example 2 and AZ31 substrate in sodium chloride solution.

Fig. 5 is a comparison chart of AC impedance Bode of MAO, MAO/Al ₂ O ₃ composite coating samples prepared in Example 2 and AZ31 substrate in sodium chloride solution.

Fig. 6 is a comparison chart of the hydrogen evolution rate between the MAO/Al ₂ O ₃ composite coating sample prepared in Example 2 and the AZ31 substrate in sodium chloride solution.

Fig. 7 is a comparison diagram of the macroscopic morphology of the MAO, MAO/Al ₂ O ₃ composite coating samples prepared in Example 2 and the AZ31 substrate soaked in sodium chloride solution for 14 days.

Among them, in Fig. 3, a is the microscopic appearance diagram of MAO (coating treated only by micro-arc oxidation), b is MAO/Al ₂ O ₃ (after micro-arc oxidation treatment, atomic layer deposition aluminum oxide layer Composite coating) in Figure 7; a is the macroscopic appearance of the AZ31 substrate soaked in sodium chloride solution for two weeks, and b is the MAO (coating only treated by micro-arc oxidation) The macroscopic image after soaking in sodium chloride solution for two weeks, c is the composite coating of MAO/Al ₂ O ₃ (a composite coating of atomic layer deposition aluminum oxide layer after micro-arc oxidation treatment) soaked in sodium chloride solution for two weeks Macroscopic image after one week.

Detailed ways

Atomic Layer Deposition (ALD) technology is a nanostructure manufacturing and surface modification technology, which is based on the idea of bottom-up assembly, through periodic control of gaseous reaction precursors and matrix material supersaturated chemistry Adsorption, reaction, and growth of controllable thin films with atomic-level precision are realized. The unique high coverage of ALD technology allows this technology to be used in porous materials to play a role in high-depth coverage. The present invention uses ALD technology to modify the surface of the porous microarc oxidation (MAO) coating, and within a certain deposition period, a complete and uniform coating film is obtained, and the microstructure of the porous layer remains unchanged.

After a large number of experimental verifications, the performance of MAO will first increase and then decrease with the aging time of the micro-arc oxidation electrolyte solution. The best time is 12-24 hours (25°C), and the atomic layer is wrapped on the MAO porous layer. The performance of the deposited film is gradually improved with the prolongation of the atomic layer deposition cycle. For the cladding material involved in the present invention, the thickness of the film layer formed by each atomic layer deposition cycle is between 0.08-0.13nm. By repeating a certain period of atomic layer deposition, the MAO porous layer can be well protected.

The instrument used for the above atomic layer deposition is Jiaxing Kemin Electronic Equipment and Instrument Company, model: KmicroT-ALD-150A.

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Example 1:

(1) Prepare MAO coating on the surface of AZ31 by micro-arc oxidation method. The specific steps are: use 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper to mechanically polish the AZ31 substrate, and then pass through 1mol/L Wash with sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Configure the micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide and glucose, the concentration is 8g/L, 10g/L, 0g/L (i.e. without adding glucose), the mass ratio of phytic acid and sodium hydroxide is 4:5, the pH of the electrolyte is 10, aged at 25°C for 24 hours for later use;

(3) Clamp the AZ31 substrate on the electrode, after immersing in the electrolyte, slowly increase the voltage to 220V under the condition of a constant current of 0.2A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 100°C; chamber pressure: 0.2Torr; reaction precursor source: trimethylaluminum and ultrapure water, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity nitrogen ; Process source and purging time, cycle period: trimethylaluminum source time is 0.02s, purging time is 30s; ultrapure water source time is 0.015s, purging time is 30s, cycle period is 500 cycles , to obtain a uniform and dense aluminum oxide film.

Example 2

(1) Prepare MAO coating on the surface of AZ31 by micro-arc oxidation method. The specific steps are: use 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper to mechanically polish the AZ31 substrate, and then pass through 1mol/L Wash with sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Prepare the micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide, and glucose. The concentrations are 8g/L, 10g/L, and 10g/L in sequence. The pH is 10, aged at 25°C for 24 hours for later use;

(3) Clamp the AZ31 substrate on the electrode, after immersing in the electrolyte, slowly increase the voltage to 220V under the condition of a constant current of 0.2A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 100°C; chamber pressure: 0.2Torr; reaction precursor source: trimethylaluminum and ultrapure water, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity nitrogen ; Process source and purge time, cycle period: trimethylaluminum source time is 0.02s, purge time is 30s; water source time is 0.015s, purge time is 30s, cycle is 500 cycles, get Uniform and dense aluminum oxide film.

Example 3:

(1) Prepare MAO coating on the surface of AZ31 by micro-arc oxidation method. The specific steps are: use 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper to mechanically polish the AZ31 substrate, and then pass through 1mol/L Wash with sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Prepare the micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide, and glucose. The concentrations are 8g/L, 10g/L, and 10g/L in sequence. The pH is 10, aged at 25°C for 24 hours for later use;

(3) Clamp the AZ31 substrate on the electrode, after immersing in the electrolyte, slowly increase the voltage to 220V under the condition of a constant current of 0.2A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 100°C; chamber pressure: 0.2Torr; reaction precursor source: trimethylaluminum and ultrapure water, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity nitrogen ; Process source and purge time, cycle period: the source time of trimethyl aluminum is 0.02s, and the purge time is 30s; the water source time is 0.015s, the purge time is 30s, and the cycle period is 1000 cycles, Uniform and dense aluminum oxide film.

Example 4:

(1) Prepare MAO coating on the surface of magnesium alloy by micro-arc oxidation method. The specific steps are: the AZ31 substrate is mechanically polished with 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper, and then passed through 1mol/ Wash with L sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Prepare micro-arc oxidation electrolyte solutions of phytic acid, sodium hydroxide, and glucose. The liquid pH is 9, aged at 25°C for 0h for later use;

(3) Clamp the AZ31 substrate on the electrode, after immersing in the electrolyte, slowly increase the voltage to 200V under the condition of a constant current of 0.1A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 80°C; chamber pressure: 0.1Torr; reaction precursor source: trimethylaluminum and hydrogen peroxide, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity nitrogen; process Source and purge time, cycle period: trimethylaluminum source time is 0.015s, purge time is 10s; hydrogen peroxide source time is 0.015s, purge time is 10s, cycle period is 5000 cycles, and uniform and dense aluminum oxide film.

Example 5:

(1) Prepare MAO coating on the surface of magnesium substrate by micro-arc oxidation method. The specific steps are: the AZ31 substrate is mechanically polished with 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper, and then passed through 1mol/ Wash with L sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Prepare micro-arc oxidation electrolyte solutions of phytic acid, sodium hydroxide, and glucose, the concentrations of which are 10g/L, 12.5g/L, and 12g/L in turn, and the mass ratio of phytic acid to sodium hydroxide is 10:12.5. The liquid pH is 11, aged at 25°C for 12 hours for later use;

(3) Clamp the AZ31 substrate on the electrode, and after immersing in the electrolyte, slowly increase the voltage to 350V under the condition of a constant current of 0.3A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 150°C; chamber pressure: 1Torr; reaction precursor source: trimethylaluminum and oxygen, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity nitrogen; Source and purge time, cycle period: trimethylaluminum source time is 0.1s, purge time is 120s; oxygen source time is 0.2s, purge time is 120s, cycle period is 1 cycle, and uniform and dense aluminum oxide film.

Embodiment 6:

(1) Prepare MAO coating on the surface of magnesium by micro-arc oxidation method. The specific steps are: the AZ31 substrate is mechanically polished with 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper, and then passed through 1mol/L Wash with sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) Prepare micro-arc oxidation electrolyte solutions of phytic acid, sodium hydroxide, and glucose. The pH of the solution is 10, and it is aged for 36 hours at 25°C for later use;

(3) Clamp the AZ31 substrate on the electrode, and after immersing in the electrolyte, slowly increase the voltage to 260V at a constant current of 0.15A to grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 120 ° C; chamber pressure: 0.8 Torr; reaction precursor source: trimethylaluminum and ozone precursor kept at room temperature; carrier gas: 8 sccm high-purity argon; process Source and purging time, cycle period: trimethylaluminum source time is 0.08s, purging time is 100s; ozone source time is 0.15s, purging time is 100s, cycle period is 100 cycles, and uniform and dense aluminum oxide film.

Embodiment 7:

(1) Prepare MAO coating on the surface of magnesium by micro-arc oxidation method. The specific steps are: the AZ31 substrate is mechanically polished with 150#, 400#, 800#, 1200#, 1500# water-grinding sandpaper, and then passed through 1mol/L Wash with sodium hydroxide solution and deionized water, and quickly dry in warm air to remove oxides and impurities on the surface;

(2) The micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide, and glucose is configured, the concentrations are 9g/L, 11.25g/L, and 11g/L in sequence, and the mass ratio of phytic acid to sodium hydroxide is 9:11.25, The pH of the electrolyte solution is 9.5, aged at 25°C for 48 hours for later use;

(3) Clamp the AZ31 substrate on the electrode, after immersing in the electrolyte, slowly increase the voltage to 320V at a constant current of 0.25A, and grow the micro-arc oxidation ceramic layer;

(4) After the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use;

(5) The sample is placed in the reaction chamber of the atomic layer deposition equipment, and the preparation of the aluminum oxide nano film layer is carried out;

The set parameters of atomic layer deposition are: deposition temperature: 140°C; chamber pressure: 0.4Torr; reaction precursor source: trimethylaluminum and hydrogen peroxide, the precursor is kept at room temperature; carrier gas: 8 sccm high-purity argon; Process source and purging time, cycle period: trimethylaluminum source time is 0.06s, purging time is 60s; hydrogen peroxide source time is 0.02s, purging time is 60s, cycle period is 3000 cycles, and uniform dense aluminum oxide film.

Sample testing and results:

For micro-arc oxidation, it is generally believed that the spark discharge phenomenon is the result of an applied voltage higher than the breakdown voltage of the existing film on the electrode surface. The occasional electronic discharge before the spark discharge causes the thin and dense amorphous oxide film formed on the electrode surface to be locally heated, causing local crystallization. When the oxide film layer reaches a certain critical value, the partial electronic discharge develops into a large-area continuous electronic avalanche, the oxide film is locally severely damaged, and a spark discharge phenomenon occurs. As the oxidation progresses, the applied voltage increases continuously, and the spark transitions from a micro-arc to an arc. However, the micro-arc and arc light are mainly concentrated on the edge of the sample at first, and then evenly distributed on the entire sample surface. Compared with the micro-arc oxidation stage, the number of sparks in the arc oxidation stage is significantly reduced, the size is increased, and the duration is prolonged. Corresponding to the spark change process, the number of pores on the surface of the oxide film decreases and the diameter gradually increases. After the oxidation time exceeds a certain period of time, the surface of the oxide film presents an inconspicuous porous structure. Therefore, the suppression of arc formation is the key to the preparation of high-quality MAO films.

Representative patents include: "An Environment-friendly Magnesium Alloy Micro-arc Oxidation and Micro-arc Oxidation Method" (CN101386982) and "Magnesium and Magnesium Alloy Environment-friendly Anodizing Electrolyte and Its Application" (CN 01114075). Zhang Rongfa et al. (Functional Materials, 2007, A10: 3762) found that adding phytic acid to alkaline sodium silicate solution can reduce the number of pores per unit area, increase the thickness of the oxide film, and improve the corrosion resistance of the oxide film. However, the thickness of the phytic acid oxide film is generally thin (<20 μm), and the pore size of the oxide film increases, thereby increasing the probability of through holes. Zeng Rongchang et al. (Journal of Alloys and Compounds, 2017, 695: 2464) found that through holes are the main reason for the galvanic corrosion between the oxide film and the magnesium alloy substrate, and the accelerated chemical corrosion and electrochemical corrosion of the magnesium alloy oxide film. .

Our previous research (Scientific Reports, 2015, 5, 13026) found that the aldehyde group of glucose can be acidified in aqueous solution to form gluconic acid, which can combine with magnesium on the surface of magnesium alloy to form magnesium gluconate. Therefore, glucose can form chemical adsorption on the surface of magnesium alloy. Tu Xiaohua et al. (Chinese Journal of Nonferrous Metals, 2013, 3: 727) showed that in an electrolyte system composed of NaOH, Na ₂ SiO ₃ , and Na ₂ B ₄ O ₇ , glucose can effectively inhibit the sparking of the anodized film of magnesium alloy AZ31. Discharge makes the oxide film dense, the micropore distribution is uniform and the pore size becomes smaller.

Electrochemical performance testing was carried out on the above examples. The micro-arc oxidation electrolyte was aged at 25°C after adding glucose. The performance of the sample prepared when the aging time was 12-24h was the best. Fully mixed reaction; samples processed by atomic layer deposition technology become denser and denser with the prolongation of the cycle, and at the same time show good corrosion resistance.

Embodiment 1 and 2 are used as representative examples, and the morphology, the polarization curve, AC impedance and hydrogen evolution pH changes in sodium chloride solution are introduced, and the results are as follows:

Fig. 1 is two kinds of different electrolytes of embodiment 1 and example 2, the polarization curve of film corrosion resistance test after micro-arc oxidation, it can be seen that the phytic acid-glucose composite electrolyte after adding glucose, the prepared sample The self-corrosion potential increased, and the self-corrosion current decreased significantly from 1.11×10 ^-5 A to 3.00×10 ^-6 A (this is the most obvious sign of the improvement of corrosion resistance).

Table 1 corresponds to the fitting results of the polarization curve in Figure 1

Figure 2 is the impedance-frequency diagram corresponding to the electrochemical test in Figure 1. It can also be found that the impedance of the sample prepared after adding glucose increases, and the corrosion resistance effect is better. The electrochemical test shows that this phytic acid-glucose compound electrolyte has a better effect.

Figure 3 is the SEM comparison of the microstructure of MAO and MAO/Al ₂ O ₃ prepared in Example 2, that is, only the micro-arc oxidation treatment and the atomic layer deposition Al ₂ O ₃ layer after the micro-arc oxidation treatment Comparison of the microscopic morphology of the treated surface.

As shown in Figure 3, the microcracks and microporous structure of the coating after ALD treatment are significantly reduced. EDS analysis was performed on the composite coating in Figure 3, and the results are shown in Table 2:

Table 2 corresponds to the EDS analysis of the positions of points A and B of the block structure in Figure 3

The element content at A and B was tested. The results showed that the Al element increased significantly, and the Mg and P elements decreased significantly, indicating that the alumina coating covered the micro-arc oxidation substrate, and the thickness gradually increased with the increase of the deposition cycle.

Fig. 4 is a comparison diagram of the polarization curves of the samples of the micro-arc oxidation coating and composite coating prepared in Example 2 and the AZ31 substrate in sodium chloride solution.

After fitting by powersuit software, the data in the table below are obtained. After micro-arc oxidation treatment, the self-corrosion current of the sample dropped from 3.16×10 ^-4 to 3.55×10 ^-6 A/cm ² , and then processed by atomic layer deposition technology The corrosion resistance of the sample was significantly improved when the temperature dropped to 6.66×10 ^-9 A/cm ² .

Table 3 corresponds to the fitting results of the polarization curve in Figure 4

Fig. 5 is a comparison chart of AC impedance of samples of the micro-arc oxidation coating and composite coating prepared in Example 2 and the AZ31 substrate in sodium chloride solution. From the bode diagram, we can see that the corrosion resistance of the samples treated by micro-arc oxidation and the samples treated by atomic layer deposition technology are respectively increased by 2 and 5 orders of magnitude relative to the substrate.

Fig. 6 is a comparison chart of the hydrogen evolution rate of the samples of the micro-arc oxidation coating and composite coating prepared in Example 2 and the AZ31 substrate in sodium chloride solution. After long-term immersion, it was found that the corrosion resistance of the samples treated by MAO and ALD was much better than that of the substrate. Then, comparing the micro-arc oxidation coating and the atomic layer deposition composite layer, it was found that the atomic layer deposition composite coating had a lower corrosion rate. , with a better effect.

In Figure 7, a, b, and c are the substrate, MAO (only the coating treated by micro-arc oxidation treatment), MAO/Al ₂ O ₃ (the atomic layer deposition aluminum oxide layer after the micro-arc oxidation treatment) in Example 2, respectively. Composite coating) macroscopic morphology after immersion in sodium chloride solution for two weeks. From the macroscopic pictures after corrosion, it can be seen intuitively that the MAO/Al ₂ O ₃ composite coating has not been damaged and has a good protective effect.

Of course, the above descriptions are not intended to limit the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or replacements made by those skilled in the art within the scope of the present invention shall also belong to the present invention. protection scope of the invention.

Claims (10)

1. a preparation method of the micro-arc oxidation-alumina composite coating of magnesium/magnesium alloy, is characterized in that, comprises the following steps: (1) The magnesium or magnesium alloy substrate is sequentially mechanically polished, washed with sodium hydroxide solution and deionized water, and quickly dried in warm air to remove oxides and impurities on the surface of the substrate; (2) Configure the micro-arc oxidation electrolyte solution of phytic acid, sodium hydroxide, and glucose, and age it for subsequent use; (3) Clamp the pretreated magnesium/magnesium alloy on the electrode, immerse in the electrolyte solution, slowly increase the voltage under the condition of constant current, and grow a micro-arc oxidation ceramic layer on the surface of the magnesium/magnesium alloy; (4) After the voltage rises to the maximum value U and the current drops below 0.02A, the preparation of the micro-arc oxidation ceramic layer is completed, the power is cut off, the sample is taken out, cleaned in deionized water, and dried for later use; (5) The sample obtained in step (4) is placed in the reaction chamber of the atomic layer deposition equipment, the chamber is sealed, nitrogen or argon inert gas is introduced and vacuumized, the temperature is gradually raised to the reaction temperature, and the chamber pressure is controlled at 0.1-1.0 Torr, the temperature is controlled at 80-150°C, and atomic layer deposition is grown on the micro-arc oxidation ceramic layer grown on the surface of magnesium/magnesium alloy; A cycle of the atomic layer deposition growth includes the following four links: 1. The first reaction precursor, trimethylaluminum, is introduced into the reaction chamber for a time of t1, and supersaturated adsorption occurs with the sample in the chamber. A chemical reaction occurs to release by-products; ② Stop feeding the first reaction precursor, the inert carrier gas will clean up the excess precursor and by-products, and the carrier gas purging time is t2; The first reaction precursor is introduced at t3, which reacts with the first precursor to release by-products; ④ stop feeding the second reaction precursor, the inert carrier gas will clean up the excess precursor and by-products, and the carrier gas will blow The sweep time is t4. 2. the preparation method of the micro-arc oxidation-alumina composite coating of a kind of magnesium/magnesium alloy according to claim 1, is characterized in that, in the micro-arc oxidation electrolyte solution described in step (2), phytic acid, The concentrations of sodium hydroxide and glucose are 5-10g/L, 6.25-12.5g/L, and 8-12g/L in sequence. 3. the preparation method of the micro-arc oxidation-alumina composite coating of a kind of magnesium/magnesium alloy according to claim 2, is characterized in that, the weight ratio of phytic acid and sodium hydroxide is 4:5, and final electrolytic solution The pH is 9.5-11. 4. The preparation method of a micro-arc oxidation-alumina composite coating of a magnesium/magnesium alloy according to claim 1, characterized in that, the aging time in step (2) is 0-48 hours. 5 . The method for preparing a magnesium/magnesium alloy micro-arc oxidation-alumina composite coating according to claim 4 , wherein the aging time in step (2) is 12-24 hours. 6 . The method for preparing a magnesium/magnesium alloy micro-arc oxidation-alumina composite coating according to claim 1 , wherein the constant current in step (3) is 0.1-0.3A. 7 . The method for preparing a magnesium/magnesium alloy micro-arc oxidation-alumina composite coating according to claim 1 , wherein the maximum voltage U in step (4) is 200-350V. 8. the preparation method of the micro-arc oxidation-alumina composite coating of a kind of magnesium/magnesium alloy according to claim 1 is characterized in that, the second reaction precursor described in step (5) is ultrapure water , hydrogen peroxide, oxygen or ozone. 9. the preparation method of the micro-arc oxidation-alumina composite coating of a kind of magnesium/magnesium alloy according to claim 1, is characterized in that, four links in the atomic layer deposition growth process described in step (5) The times are t1: 0.015-0.1s, t2: 10-120s, t3: 0.015-0.2s, t4: 10-120s. 10. the preparation method of the micro-arc oxidation-alumina composite coating of a kind of magnesium/magnesium alloy according to claim 1, is characterized in that, the cycle number of atomic layer deposition growth described in step (5) is 1- 5000.