Preparation method of magnesium alloy super-hydrophobic LDHs / MAO composite coating

Abstract

The application discloses a preparation method of a magnesium alloy surface super-hydrophobic LDHs / MAO composite coating and belongs to the technical field of magnesium alloy surface treatment. The method comprises the following steps: pretreating an AZ31 magnesium alloy base material; placing the pretreated base material in a micro-arc oxidation electrolyte to obtain a micro-arc oxidation (MAO) layer; placing the micro-arc oxidation sample in an LDHs growth solution to perform a hydrothermal reaction, thereby preparing an LDHs / MAO composite coating; placing the LDHs / MAO composite coating in a hydrothermal reaction kettle to realize intercalation of aspartic acid (ASP); and finally modifying the LDHs-ASP / MAO to obtain a composite coating with a super-hydrophobic function on the surface of the magnesium alloy. The super-hydrophobic coating can significantly improve the corrosion resistance of the magnesium alloy, has the characteristics of green environmental protection, easy operation and low cost, and can achieve good economic and environmental benefits.

Description

[0001] SPECIFICATION TECHNICAL FIELD

[0002] The present application relates to magnesium alloy surface treatment technology, in particular to a kind of preparation method of magnesium alloy superhydrophobic LDHs / MAO composite coating. BACKGROUND

[0003] Magnesium and its alloys have excellent radiation resistance, good electromagnetic shielding, machining performance and other characteristics, have been applied in aerospace, electronic communication, transportation and other fields, but the poor corrosion resistance of magnesium alloy is one of the reasons restricting its wide application.

[0004] At present, the means for improving the corrosion resistance of magnesium alloy mainly include alloying and surface treatment.Taking the micro-arc oxidation (MAO) technology in the existing surface treatment technology, the ceramic film prepared has the advantages of corrosion resistance, wear resistance, good insulation and strong matrix bonding force, but in the preparation process, the micro-arc oxidation layer surface usually forms micro-pores and micro-cracks, which will affect the corrosion resistance of the coating.

[0005] The method for growing LDHs layer on the surface of MAO layer to seal its micro-pores has attracted much attention.LDHs is a kind of inorganic functional material with layered structure, which has unique ion exchange capacity, not only can capture the corrosion anions (Cl - ) invading from the outside environment, but also can be used as a nano container for loading corrosion inhibitor, which can effectively improve the corrosion resistance of the substrate.However, due to the hydrophilic hydroxyl group on the surface of LDHs, the corrosion resistance and functionality of the coating are still insufficient to cope with harsh environments, so the LDHs / MAO composite coating needs to be further improved. SUMMARY

[0006] The present application proposes a preparation method of magnesium alloy superhydrophobic LDHs / MAO composite coating to solve the problem of hydrophilicity of LDHs / MAO composite coating, which can provide more excellent corrosion protection effect for magnesium alloy.

[0007] To achieve the above purpose, the preparation method of magnesium alloy superhydrophobic LDHs / MAO composite coating comprises the following steps:

[0008] (1) Pretreatment: polishing the surface of the substrate, cleaning, blowing dry, and obtaining the pretreated magnesium alloy substrate;

[0009] (2) Preparation of micro-arc oxidation (MAO) layer: placing the substrate obtained in step (1) in an electrolyte composed of 20 g / L Na2SiO3, 10 g / L Na3PO4 and 2 g / L NaOH, and carrying out micro-arc oxidation to obtain the MAO layer;

[0010] (3) Growth of LDHs: The micro-arc oxidation sample is placed in a LDHs growth solution configured by 0.375 g of Al(NO3)3·9H2O, 0.36 g of NH4NO3 and NaOH to perform a hydrothermal reaction, and is reacted at 125 DEG C for 24 hours to obtain a LDHs / MAO composite coating;

[0011] (4) Interlayer corrosion inhibitor: the LDHs / MAO composite coating is placed in a solution configured by 0.088 g of vanillin L-aspartic acid and deionized water to intercalate ASP by ion exchange, the ion exchange reaction temperature is 125 DEG C, and the time is 1 hour. A LDHs-ASP / MAO layer is obtained;

[0012] (5) Hydrophobic modification: one of lauric acid (La), myristic acid (Ma) and stearic acid (Sa) is used to modify the LDHs-ASP / MAO layer to obtain a super-hydrophobic coating LDHs-ASP / MAO-X (X=La, Ma, Sa). Among them, the LDHs-ASP / MAO-Sa layer can provide more excellent protection for the magnesium alloy substrate.

[0013] Further, the LDHs / MAO composite coating with excellent corrosion protection effect is prepared by the above method.

[0014] The present application has the advantages that:

[0015] (1) The present application prepares a MAO layer on the surface of a magnesium alloy, and uses LDHs to repair the micro-holes and micro-cracks of the MAO layer caused by the plasma discharge process, which helps to prolong the service time of the MAO layer.

[0016] (2) In the present application, LDHs as an inorganic functional material can not only seal the MAO layer, but also capture Cl - , and is also a nano-container for anionic corrosion inhibitor, when the LDHs layer is damaged, the interlayer corrosion inhibitor can be actively released to protect the substrate.

[0017] (3) The present application proposes to modify the LDHs by using a low surface energy substance to prepare a super-hydrophobic coating in view of the hydrophilicity of LDHs / MAO, the super-hydrophobic coating can trap air between the solid-liquid interface, reduce the interaction between the solid-liquid phases, and further delay the occurrence of corrosion reaction.

[0018] (4) The present application utilizes a series of processes such as preparing MAO layer on the surface of magnesium alloy, growing LDHs, intercalating corrosion inhibitor, modifying LDHs by using low surface energy substances, etc. to prepare composite coating, and each process plays a synergistic role, which greatly enhances the corrosion resistance of the composite coating. Compared with the prior art, the preparation process is easy to control, green, pollution-free, low in production cost, can significantly reduce the corrosion current density of the substrate, and widens the application range of magnesium alloy. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The process flow chart of the preparation method of the magnesium alloy super-hydrophobic LDHs / MAO composite coating of the present application is shown in the figure.

[0020] Figure 2 The XRD spectrum of the LDHs-ASP / MAO-X coating after hydrophobic modification is shown in the figure.

[0021] Figure 3 The infrared spectrum of the LDHs-ASP / MAO-X coating after hydrophobic modification is shown in the figure.

[0022] Figure 4 The water contact angles of the LDHs-ASP / MAO layer before and after hydrophobic modification are shown in the figure, wherein (a) is LDHs-ASP / MAO, (b) is LDHs-ASP / MAO-La, (c) is LDHs-ASP / MAO-Ma, and (d) is LDHs-ASP / MAO-Sa.

[0023] Figure 5 The surface morphology of the LDHs-ASP / MAO composite coating after hydrophobic modification is shown in the figure, wherein (a) is LDHs-ASP / MAO-La, (b) is LDHs-ASP / MAO-Ma, and (c) is LDHs-ASP / MAO-Sa.

[0024] Figure 6 The dynamic polarization curve of the LDHs-ASP / MAO-X coating after hydrophobic modification is shown in the figure. DETAILED DESCRIPTION

[0025] A magnesium alloy super-hydrophobic LDHs / MAO composite coating, the preparation process of which comprises the following steps:

[0026] (1) The pretreated AZ31 magnesium alloy substrate is placed in an electrolyte composed of 20 g / L Na2SiO3, 10 g / L Na3PO4 and 2 g / L NaOH for micro-arc oxidation, and the preparation of the micro-arc oxidation coating adopts a constant current mode, the current density is 2 A / dm 2, frequency is 800Hz, duty cycle is 40%, reaction time is 10min, the electrolyte temperature is kept below 25℃ by using the water cooling system of the low-temperature constant-temperature reaction bath during the reaction process;

[0027] (2) the micro-arc oxidation sample is placed in the LDHs growth solution configured by 0.375g Al(NO3)3·9H2O, 0.36g NH4NO3 and NaOH to carry out hydrothermal reaction, and is reacted for 24 hours at 125℃ to obtain the LDHs / MAO composite coating;

[0028] (4) one kind of low surface energy substance of lauric acid (La), myristic acid (Ma) and stearic acid (Sa) is used to carry out hydrophobic modification on the LDHs-ASP / MAO layer to obtain the super-hydrophobic coating LDHs-ASP / MAO-X (X=La, Ma, Sa).

[0029] Figure 1 The process flow chart of the preparation method of the magnesium alloy super-hydrophobic LDHs / MAO composite coating prepared by the application is shown in the figure.

[0030] In order to make the content of the application more convenient to understand, the technical solutions of the application will be further described in combination with specific embodiments, but the application is not limited to this.

[0031] Example 1

[0032] The preparation steps of the magnesium alloy super-hydrophobic LDHs-ASP / MAO-Sa composite coating are as follows:

[0033] (1) pretreatment: polishing the surface of the AZ31 magnesium alloy substrate, using 400#-1500# metallographic sandpaper to polish one by one, rinsing with deionized water, scrubbing with anhydrous ethanol, and then blowing dry with cold air for standby;

[0034] (2) preparing the micro-arc oxidation (MAO) layer: the pretreated substrate in step (1) is placed in the electrolyte composed of 20g / L Na2SiO3, 10g / L Na3PO4 and 2g / L NaOH, and the magnesium alloy is used as the anode and the stainless steel plate is used as the cathode to carry out micro-arc oxidation, the constant current mode is used in the micro-arc oxidation process, the current density is 2A / dm 2 , frequency is 800Hz, duty cycle is 40%, reaction time is 10min;

[0035] (3) Growth of LDHs: 0.375 g of Al(NO3)3.9H2O and 0.36 g of NH4NO3 were weighed and dissolved in 100 ml of deionized water. After stirring until uniform, 1 M NaOH was added to adjust the pH of the solution to 12. The LDHs growth solution and the sample obtained in step (2) were moved to a hydrothermal reactor, and reacted at 125°C for 24 hours to obtain an LDHs / MAO composite coating;

[0036] (4) Interlayered corrosion inhibitor: the LDHs / MAO composite coating was placed in a solution prepared from 0.088 g of vanillin L-aspartic acid and 100 ml of deionized water, and the sample and the mixed solution were moved into a hydrothermal reactor. Through a hydrothermal reaction, ion exchange interlayering ASP was achieved. The hydrothermal reaction temperature was 125°C, and the hydrothermal reaction time was 1 hour to obtain an LDHs-ASP / MAO composite coating.

[0037] (5) Hydrophobic modification: 2.84 g of stearic acid (Sa) was dissolved in a mixed solution of 100 ml of ethanol and 100 ml of deionized water. 1 M NaOH was added to adjust the pH of the solution to neutral. The LDHs-ASP / MAO composite coating and the above solution were moved to a hydrothermal reactor, and hydrothermal reaction was carried out at 70°C for 4 hours to obtain a super-hydrophobic coating LDHs-ASP / MAO-Sa.

[0038] Example 2

[0039] A preparation procedure of a magnesium alloy super-hydrophobic LDHs-ASP / MAO-Ma composite coating is as follows:

[0040] (1) Pretreatment: the surface of the AZ31 magnesium alloy substrate was polished using metallographic sandpaper from 400# to 1500#. The substrate was rinsed with deionized water and wiped with anhydrous ethanol. After being blown dry by cold air, it was ready for use.

[0041] (2) Preparation of micro-arc oxidation (MAO) layer: the pretreated substrate in step (1) was placed in an electrolyte composed of 20 g / L Na2SiO3, 10 g / L Na3PO4 and 2 g / L NaOH. The magnesium alloy was used as the anode, and the stainless steel plate was used as the cathode. Micro-arc oxidation was carried out. The constant current mode was used in the micro-arc oxidation process. The current density was 2 A / dm2, the frequency was 800 Hz, the duty cycle was 40%, and the reaction time was 10 min. 2

[0042] (3) Growth of LDHs: 0.375 g of Al(NO3)3.9H2O and 0.36 g of NH4NO3 were weighed and dissolved in 100 ml of deionized water. After stirring until uniform, 1 M NaOH was added to adjust the pH of the solution to 12. The LDHs growth solution and the sample obtained in step (2) were moved to a hydrothermal reactor, and reacted at 125°C for 24 hours to obtain an LDHs / MAO composite coating;​

[0043] (4) Interlayered corrosion inhibitor: LDHs / MAO composite coating is placed in a solution prepared from 0.088 g vanillin L-aspartic acid and 100 ml deionized water, and the sample and mixed solution are moved into a hydrothermal reaction kettle, ion exchange interlayering ASP is realized by means of hydrothermal reaction, the hydrothermal reaction temperature is 125 ℃, the hydrothermal reaction time is 1 hour, and LDHs-ASP / MAO composite coating is obtained;

[0044] (5) Hydrophobic modification: 2.28 g of myristic acid (Ma) is dissolved in a mixed solution of 100 ml ethanol and 100 ml deionized water, 1M NaOH is added to adjust the pH value of the solution to neutral, LDHs-ASP / MAO composite coating and the above-mentioned solution are moved into a hydrothermal reaction kettle, and the hydrothermal reaction is carried out at 70 ℃ for 4 hours, and super-hydrophobic coating LDHs-ASP / MAO-Ma is obtained.

[0045] Example 3

[0046] The preparation steps of a magnesium alloy super-hydrophobic LDHs-ASP / MAO-La composite coating are as follows:

[0047] (1) Pretreatment: The surface of the AZ31 magnesium alloy substrate is polished with 400#-1500# metallographic sandpaper one by one, rinsed with deionized water and wiped with anhydrous ethanol, and then dried with cold air and used after cooling;

[0048] (2) Preparation of micro-arc oxidation (MAO) layer: the pretreated substrate in step (1) is placed in an electrolyte composed of 20 g / L Na2SiO3, 10 g / L Na3PO4 and 2 g / L NaOH, with magnesium alloy as anode and stainless steel plate as cathode, and micro-arc oxidation is carried out. The constant current mode is adopted in the micro-arc oxidation process, the current density is 2 A / dm 2 , the frequency is 800 Hz, the duty cycle is 40%, and the reaction time is 10 min;

[0049] (3) Growth of LDHs: 0.375 g of Al(NO3)3·9H2O and 0.36 g of NH4NO3 are dissolved in 100 ml of deionized water, and after stirring uniformly, 1M NaOH is added to adjust the pH of the solution to 12. The LDHs growth solution and the sample obtained in step (2) are moved into a hydrothermal reaction kettle, and the reaction is carried out at 125 ℃ for 24 hours, and LDHs / MAO composite coating is obtained;

[0050] (4) Interlayered corrosion inhibitor: LDHs / MAO composite coating was placed in a solution prepared by 0.088 g vanillin L-aspartic acid and 100 ml deionized water, and the sample and mixed solution were moved into a hydrothermal reactor, ion exchange interlayered ASP was realized by means of hydrothermal reaction, the hydrothermal reaction temperature was 125 ℃, the hydrothermal reaction time was 1 hour, and LDHs-ASP / MAO composite coating was obtained;

[0051] (5) Hydrophobic modification: 1.86 g of lauric acid (La) was dissolved in a mixed solution of 100 ml ethanol and 100 ml deionized water, 1M NaOH was added to adjust the pH value of the solution to neutral, LDHs-ASP / MAO composite coating and the above-mentioned solution were moved to a hydrothermal reactor, and the hydrothermal reaction was carried out at 70 ℃ for 4 hours, and super-hydrophobic coating LDHs-ASP / MAO-La was obtained.

[0052] The composite coating prepared above was subjected to X-ray diffraction, and the diffraction pattern showed that the diffraction peaks of (003) and (006) crystal planes of LDHs were detected in the three samples, indicating that the LDHs layer was formed on the surface of the MAO layer after the hydrothermal reaction. Figure 2 In addition, the characteristic diffraction peaks of Sa, Ma and La three surfactants were also detected in the XRD pattern, indicating that the three low surface energy substances were successfully loaded on the LDHs-ASP / MAO composite coating. Figure 2 By comparing the three samples, it can be observed that the characteristic peaks belonging to the LDHs-ASP / MAO layer did not shift obviously, and the interplanar spacing did not change, that is, the LDHs-ASP / MAO layer did not appear ion exchange phenomenon after hydrophobic modification, and ASP was still interlayered in the LDHs layer.

[0053] Figure 3 The infrared spectra of the three super-hydrophobic coatings after modification are shown. The wide band centered at 3440 cm -1 corresponds to the stretching vibration of water molecules and -OH groups between the layers. The peaks at 2955 cm -1 correspond to the stretching vibration of methyl groups -CH3, while the peaks at 2919 cm -1 and 2852 cm -1 correspond to the symmetric and asymmetric stretching of -CH2, respectively. The peaks at 1537 cm -1 and 1456 cm -1 correspond to the vibration absorption of methyl and methylene in the three surfactants, respectively. The characteristic groups of the three surfactants are successfully detected in the infrared spectrum, which indicates that the three surfactants are successfully combined with the LDHs-ASP / MAO layer.

[0054] Figure 4The static water contact angles of LDHs-ASP / MAO composite coatings before and after hydrophobic modification. Under the action of surfactant, the contact angle of LDHs-ASP / MAO composite coating appears different degrees of promotion. The contact angle of LDHs-ASP / MAO layer is 30.07°, and the coating shows certain hydrophilicity, which can be attributed to the rich hydroxyl groups on the surface of LDHs. After loading Sa, Ma and La, the water contact angles of the coating are 153.44°, 152.67° and 150.03°, respectively, realizing the transformation from hydrophilicity to superhydrophobicity. Surfactant can reduce the surface energy of the coating and endow the coating with superhydrophobicity. Superhydrophobic coating can trap air between solid-liquid interface (Cassie-Baxter model theory), thereby reducing the interaction between solid-liquid phase and improving the corrosion resistance of the coating.

[0055] Figure 5 The surface morphologies of the three coatings, respectively. From the morphology, the modified LDHs sheets can still be observed obvious hexagonal structure, without aggregation phenomenon. The undulation of the coating surface is caused by the different growth rates of LDHs sheets. There are larger pores between the LDHs sheets in LDHs-ASP / MAO-La and LDHs-ASP / MAO-Ma, and part of the sheet is twisted, and part of the sheet structure forms a small piece on the top to form a flower-like structure, which can effectively seal the inherent defects on the MAO layer and hinder the corrosion medium from entering the magnesium alloy substrate through the defects to occur corrosion. Compared with the former two, the LDHs-ASP / MAO-Sa coating surface appears more dense LDHs sheets, and the porosity between the sheets is reduced, which can reduce the accumulation of electrolyte and delay the occurrence of magnesium alloy corrosion reaction.

[0056] Figure 6 The dynamic polarization curves of the three superhydrophobic coatings in 3.5wt% NaCl solution at room temperature are given, and the fitting parameters are shown in Table 1. Compared with the fitting parameters before hydrophobic modification, the dynamic potential polarization curves of the above three coatings move to the direction of more positive corrosion potential (E corr ) and lower corrosion current density (I corr ). The E corr of LDHs-ASP / MAO composite coating is -1.399V SCE , and the E corr of LDHs-ASP / MAO-Sa, LDHs-ASP / MAO-Ma and LDHs-ASP / MAO-La occurs different degrees of positive shift, which are -1.377V SCE , -1.389V SCE and -1.330V SCEThis indicates that the corrosion resistance of all coatings was improved after modification, with the LDHs-ASP / MAO-Sa layer showing the best corrosion resistance. Comparing the I values ​​before and after modification... corr , from 4.368×10 -5 A·cm -2 Reduced to 1.073×10 -7 A·cm -2 2.932×10 -5 A·cm -2 and 6.170×10 - 7 A·cm -2 The LDHs-ASP / MAO-Sa layer exhibited excellent corrosion protection, consistent with the above analytical results. Potentiodynamic polarization curve analysis showed that the LDHs-ASP / MAO-Sa composite coating had the best corrosion resistance, which is related to the properties of Sa itself. Comparing the carbon chain and relative molecular mass of the three surfactants, it is easy to see that Sa has the longest carbon chain and the largest relative molecular mass, exhibiting the strongest ability to reduce the surface energy of the coating. Therefore, among the three superhydrophobic coatings, the LDHs-ASP / MAO-Sa layer provides superior protection for magnesium alloy substrates.

[0057] Table 1. Dynamic polarization curves and fitting parameters of the surface-modified LDHs-ASP / MAO composite coating.

[0058]

[0059]

[0060] Finally, it should be noted that although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art should understand that changes, modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such changes should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a magnesium alloy superhydrophobic LDHs / MAO composite coating, characterized in that, The method comprises the following steps: (1) pretreatment: polishing the surface of the substrate, cleaning, blowing dry, and obtaining a pretreated magnesium alloy substrate; (2) preparing a micro-arc oxidation (MAO) layer: placing the substrate obtained in step (1) in an electrolyte composed of 20 g / L Na2SiO3, 10 g / L Na3PO4 and 2 g / L NaOH, and performing micro-arc oxidation to obtain the MAO layer; (3) growing LDHs: placing the micro-arc oxidation sample in an LDHs growth solution composed of 3.75 g / L Al(NO3)3·9H2O, 3.6 g / L NH4NO3 and 1M NaOH to perform a hydrothermal reaction, and reacting at 125℃ for 24 hours to obtain an LDHs / MAO composite coating; (4) intercalating corrosion inhibitors: placing the LDHs / MAO composite coating in a solution composed of 0.088 g vanillin L-aspartic acid and 100 ml deionized water to intercalate ASP by ion exchange, the ion exchange reaction temperature is 125℃, and the reaction time is 1 hour, and an LDHs-ASP / MAO layer is obtained; (5) hydrophobic modification: using a low surface energy substance X to perform hydrophobic modification on the LDHs-ASP / MAO layer to obtain a super-hydrophobic coating LDHs-ASP / MAO-X, the low surface energy substance X includes any one of lauric acid, myristic acid, and stearic acid.

2. The production method according to claim 1, characterized by, The magnesium alloy substrate in step (1) is AZ31 magnesium alloy.

3. A magnesium alloy superhydrophobic LDHs / MAO composite coating characterized in that, The super-hydrophobic coating is prepared by the method of any one of claims 1-2.

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