Micro-arc oxidation electrolyte for black corrosion-resistant ceramic coating on surface of magnesium alloy and preparation method of coating

Abstract

The invention provides a magnesium alloy surface black corrosion-resistant ceramic coating micro-arc oxidation electrolyte and a coating preparation method, and belongs to the technical field of magnesium alloy surface treatment. Electrolyte is composed of meta-aluminate, fluoride, copper salt, vanadium salt, tungsten salt, sodium potassium tartrate tetrahydrate, inorganic base and deionized water, and in the micro-arc oxidation process, the magnesium alloy is firstly ground, polished, cleaned and deoiled, and then the magnesium alloy is completely immersed in a composite electrolyte system. Magnesium alloy serves as an anode, stainless steel serves as a cathode, and the black composite ceramic coating is prepared on the surface of the magnesium alloy in a constant-voltage mode by adjusting micro-arc oxidation electrical parameters (voltage, frequency, duty ratio and time). The copper salt is used as the main coloring salt to color the micro-arc oxidation ceramic coating, the vanadium salt and the tungsten salt cooperate with the copper salt to remarkably enhance the color of the ceramic coating, so that the chromatic value of the black ceramic coating is reduced, and meanwhile, the product generated in the micro-arc oxidation reaction process of the vanadium salt is beneficial to improving the corrosion resistance of the ceramic coating.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnesium alloy surface treatment, and particularly relates to a magnesium alloy surface black corrosion-resistant ceramic coating electrolyte and a coating preparation method. BACKGROUND

[0002] Magnesium alloy has a wide application prospect in the fields of aerospace, national defense and military industry, new energy vehicles and biomedical treatment due to its low density, high specific strength, excellent thermal conductivity and recyclable characteristics. However, the magnesium alloy has a low electrode potential, a high affinity with oxygen and poor chemical stability, and is prone to spontaneously form a loose porous film (mainly composed of Mg(OH)2, MgO and MgCO3) on the surface. In addition, the second phase or other impurity elements in the alloy will cause micro-electric corrosion, thereby reducing the corrosion resistance, wear resistance and thermal stability, and seriously restricting the engineering application of the magnesium alloy.

[0003] The magnesium alloy surface micro-arc oxidation (MAO) treatment technology refers to arc discharge on the surface of a component in an electrolyte through high voltage, and then a ceramic coating mainly composed of magnesium oxide and supplemented by the alloy and other components of the electrolyte is formed. After the micro-arc oxidation treatment, the ceramic coating formed on the surface of the magnesium alloy can greatly improve the wear resistance, corrosion resistance and other properties of the magnesium alloy. In order to avoid the influence of stray light reflection on the optical system, the magnesium alloy also needs to be subjected to surface light absorption treatment when used as an optical support component. The black coating has an absorption performance for all wavelengths of visible light, and is the preferred solution for surface light absorption treatment. The black coating has high light absorption and infrared radiation performance, and is widely used in the fields of aerospace, automobile industry, optical devices, building and decoration.

[0004] The traditional surface treatment technology of the black coating includes spraying, chemical conversion film and electroplating, etc. However, the obtained coating has problems such as low hardness, poor adhesion to the substrate, easy discoloration under the action of ultraviolet light, poor durability and the like. The oxidation ceramic coating generated in situ on the surface of the magnesium alloy through the micro-arc oxidation technology not only has high adhesion to the substrate, but also has the advantages of simple process, high production efficiency and environmental friendliness. However, the micro-arc oxidation ceramic coating film prepared in a single electrolyte system is prone to have large pores and cracks, causing local defects. The corrosion medium can easily penetrate and erode the magnesium alloy substrate through the defect sites, thereby making it difficult for the coating to effectively protect the magnesium alloy substrate for a long time.

[0005] Therefore, by optimizing the electrolyte components and configuring a multi-component electrolyte system, the coating defects can be reduced, the corrosion resistance of the coating can be improved, and the coating can be uniformly colored, so as to expand the application boundary of the magnesium alloy. SUMMARY

[0006] The present application aims at the existing magnesium alloy difficult to have engineering protection and decoration demand, and proposes a magnesium alloy surface black corrosion-resistant ceramic coating micro-arc oxidation electrolyte and coating preparation method. The present application introduces vanadium salt, tungsten salt and copper salt in metaborate electrolyte through ternary salt composite modification strategy, successfully prepares the micro-arc oxidation ceramic coating with low blackness value and excellent corrosion resistance on the surface of magnesium alloy (taking AZ31B as an example).

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is:

[0008] Firstly, the present application proposes a magnesium alloy surface black corrosion-resistant ceramic coating micro-arc oxidation electrolyte, which is composed of deionized water and the following components with mass concentration: 16-20 g / L main film former metaborate, 10-13 g / L auxiliary film former fluoride, 1-2 g / L main colorant soluble copper salt, 1-3 g / L auxiliary colorant soluble vanadium salt, 3-5 g / L auxiliary colorant soluble tungsten salt, 2-4 g / L complexing agent potassium sodium tartrate and 4-6 g / L pH regulator inorganic alkali.

[0009] As a preferred technical scheme of the present application, the metaborate in the micro-arc oxidation electrolyte is one or a mixture of sodium metaborate and potassium metaborate, the fluoride is one or a mixture of ammonium fluoride, sodium fluoride and potassium fluoride, the soluble copper salt is one or a mixture of copper sulfate, copper pyrophosphate and copper acetate, the soluble vanadium salt is one or a mixture of ammonium metavanadate and sodium metavanadate, the soluble tungsten salt is sodium tungstate, and the inorganic alkali is one or a mixture of sodium hydroxide and potassium hydroxide.

[0010] Secondly, the present application proposes a method for preparing a magnesium alloy surface black corrosion-resistant ceramic coating based on the above-mentioned electrolyte, which is specifically as follows: a single polarity pulse bias power source is used for micro-arc oxidation of the magnesium alloy to prepare a ceramic coating with smooth surface, high blackness, uniform color and excellent corrosion resistance on the surface of the magnesium alloy, and the specific steps are as follows: the pretreated magnesium alloy is completely immersed in the electrolyte; the magnesium alloy is used as an anode, stainless steel is used as a cathode, and the power source is set to constant voltage mode for micro-arc oxidation.

[0011] As a preferred technical scheme of the present application, the magnesium alloy pretreatment step is as follows: the surface of the magnesium alloy is polished with sandpaper, and then ultrasonic cleaning is sequentially carried out in acetone, anhydrous ethanol and deionized water respectively to remove surface oil stains, and then dried for use.

[0012] As a preferred technical scheme of the present application, the magnesium alloy is completely immersed into the electrolyte; a stainless steel sheet with a thickness of 2-5 mm is used as the cathode, and the magnesium alloy is used as the anode; the power supply is set to a constant voltage mode, the voltage is set to 400 V, the oxidation treatment time is 6-10 min, the frequency is 1000 Hz, the duty cycle is 10%, the limited current is 30 A, the temperature of the electrolyte during the treatment process is controlled to be 15-25 DEG C, the magnesium alloy is subjected to micro-arc oxidation, and after the treatment, the sample is taken out, cleaned and dried. During the micro-arc oxidation process, the power supply is set to a constant voltage mode, thereby reducing the local defects caused by spark discharge and improving the compactness and corrosion resistance of the coating.

[0013] The present application prepares a corrosion-resistant black ceramic coating on the surface of the magnesium alloy in situ by preparing an alkali composite electrolyte based on a metallate and using a constant voltage micro-arc oxidation technology. The metallate is used as a main film-forming salt to form a spinel structure MgAl2O4 to improve the hardness, wear resistance and corrosion resistance of the micro-arc oxidation coating. CuSO4 in the electrolyte participates in the micro-arc oxidation reaction to generate CuO and CuAl2O4 to reduce the color difference and blackness value of the coating. NH4VO3 and Na2WO4 can inhibit the red phenomenon of the coating. MgV2O4 generated from NH4VO3 can fill the pores of the coating, reduce the corrosion current density of the coating and further enhance the corrosion resistance. The CuO phase is formed in the coating by using copper salt to reduce the blackness of the micro-arc oxidation coating. The tungsten salt is used to promote the decomposition of the vanadium salt to form more V2O3 phase to further reduce the chroma value (L*a*b) of the coating: the lightness |L*|≤30, the red-green axis chroma coordinate |a*|≤2 and the yellow-blue axis chroma coordinate |b*|≤2.

[0014] The present application utilizes the synergistic effect of the multi-component electrolyte in the micro-arc oxidation process to grow a dense and smooth black corrosion-resistant ceramic coating on the surface of the magnesium alloy in situ. Compared with the prior art, the present application has the following beneficial effects:

[0015] 1. The three coloring salts are added to the electrolyte prepared in the present application. The CuO and CuAl2O4 generated from CuSO4 in the micro-arc oxidation process significantly improve the blackness of the coating (color difference ΔE<0.5). The coupling effect of NH4VO3 and Na2WO4 effectively inhibits the red phenomenon of the coating, and the generation of CuO, MgV2O4 and WO3 increases the compactness of the coating and improves the corrosion resistance of the coating.

[0016] 2. The constant voltage mode is used in the present application. The magnesium alloy surface will quickly form a film. The coating hinders the direct contact of the magnesium alloy with the electrolyte, further reduces the probability of the occurrence of the ablation phenomenon, and as the thickness of the coating increases, the resistance of the entire circuit also increases, the current decreases, the local injection energy and the large arc discharge tendency during the transient discharge can be effectively reduced, thereby dispersing the arc and reducing the ablation phenomenon.

[0017] 3、The black ceramic coating prepared by the corrosion-resistant magnesium alloy surface micro-arc oxidation process has simple preparation process, relatively low cost, wide adaptability to micro-arc oxidation equipment, and high practical application value. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 SEM morphology diagram of the surface of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1.

[0019] Figure 2 SEM morphology diagram of the cross section of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1.

[0020] Figure 3 XRD diagram of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1.

[0021] Figure 4 XPS diagram of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1.

[0022] Figure 5 Current-time relationship curve (a) and coating thickness and solution conductivity comparison (b) in the micro-arc oxidation process of Example 1 and Comparative Example 1.

[0023] Figure 6 TaFel diagram of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1 and Comparative Example 1.

[0024] Figure 7 Nyquist diagram of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of the magnesium alloy in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0025] The application will be further described in detail below in combination with the embodiments and the drawings.

[0026] Example 1

[0027] The embodiment provides a method for preparing a black corrosion-resistant ceramic coating by micro-arc oxidation on the surface of a magnesium alloy, and the method comprises the following steps:

[0028] (1) Pretreatment Firstly, 1500# sandpaper is used to polish the surface of the AZ31B magnesium alloy, and then the magnesium alloy is sequentially cleaned in acetone, anhydrous ethanol and deionized water for 15 min by ultrasonic cleaning, so as to remove surface oil stains, and the magnesium alloy is naturally air-dried for use.

[0029] (2) Preparation of electrolyte: 20 g / L sodium metaaluminate, 12 g / L potassium fluoride dihydrate, 3 g / L ammonium metavanadate, 2 g / L copper sulfate anhydrous, 5 g / L sodium tungstate dihydrate, 5 g / L potassium hydroxide, 4 g / L potassium sodium tartrate tetrahydrate were dissolved in deionized water to obtain the micro-arc oxidation electrolyte.

[0030] (3) Micro-arc oxidation: AZ31B magnesium alloy was completely immersed into the electrolyte prepared in step (2); a stainless steel sheet (thickness 2 mm) was used as cathode and the magnesium alloy as anode; the power supply was set to constant voltage mode, the voltage was set to 400 V, the frequency was 1000 Hz, the duty cycle was 10%, and the limited current was 30 A; the magnesium alloy was subjected to micro-arc oxidation, and the sample was taken out after 7 min of work, washed, and dried.

[0031] Please refer to Figure 1 and Figure 2 , Figure 1 The SEM image of the surface of the black corrosion-resistant ceramic coating prepared on the surface of the magnesium alloy in Example 1 shows that the surface of the sample is smooth, and there are some holes, as shown in Figure 1 a. The local enlarged view of Figure 1 b shows that the surface of the coating is uneven, which is caused by the repeated electrical breakdown of the micro-arc oxidation coating under the action of high current density and the sudden cooling of the molten product when it meets water. The molten product further seals the pores in the coating, improving the density and corrosion resistance of the coating. In addition, the sample is anode during the micro-arc oxidation process, and carries positive electricity. The anions in the electrolyte move towards the anode under the action of the electric field, promoting the anions to enter the discharge channel, generating oxides, sealing the discharge channel, reducing the pores of the coating, greatly reducing the probability of contact between the corrosion medium and the magnesium alloy substrate, and improving the corrosion resistance of the magnesium alloy. Figure 2 The SEM image of the cross section of the micro-arc oxidation coating prepared in Example 1 shows that the coating has basically no pores inside and has high density, and therefore, the coating has good corrosion resistance.

[0032] Please refer to Figure 3 , which is the XRD image of the black corrosion-resistant ceramic coating prepared on the surface of the magnesium alloy in Example 1. As shown in Figure 3 , the black ceramic coating is composed of MgAl2O4, MgO, CuAl2O4, and MgV2O4. The characteristic peaks of MgAl2O4 and MgO are relatively obvious. In addition, due to the thin thickness and porous structure of the coating, the X-ray penetrates through the membrane layer, resulting in the appearance of the characteristic peaks of the Mg substrate. CuAl2O4 is the main reason for the blackening of the coating, and MgV2O4 can improve the corrosion resistance of the coating.

[0033] Please refer to Figure 4The figure shows the XPS image of the black corrosion-resistant ceramic coating prepared by micro-arc oxidation on the surface of magnesium alloy in Example 1. Figure 4 As can be seen from a, the micro-arc oxidation coating contains elements such as W, Al, V, O, F, Cu, and Mg, which are present in the Al 2p high-resolution spectrum ( Figure 4 (b) The characteristic peak at 74.2 eV corresponds to aluminum oxide, and the characteristic peak at 77.44 eV corresponds to AlF3. In the Cu 2p high-resolution spectrum ( Figure 4 c), the characteristic peaks with binding energies of 933.2 eV and 935.2 eV correspond to CuO and CuAl2O4, respectively. In the Mg 1s high-resolution spectrum ( Figure 4 d) The characteristic peak at 1303.8 eV corresponds to Mg, and the characteristic peak at 1304.7 eV corresponds to Mg. 2+ In the V 2p high-resolution spectrum ( Figure 4 e), there are four characteristic peaks: the characteristic peak at 517.3 eV corresponds to V in V2O5. 5 + The binding energy, and the characteristic peak at 516.5 eV, correspond to the V in MgV₂O₄. 3+ The binding energy. In the high-resolution spectrum of W4f ( Figure 4 f), the characteristic peaks at 35.4 eV and 37.5 eV correspond to WO bonds.

[0034] Comparative Example 1

[0035] To illustrate the effect of coloring salt on the micro-arc oxidation coating in the preparation method of black corrosion-resistant ceramic coating for magnesium alloy surface provided by the present invention, the preparation of micro-arc oxidation coating in this comparative example is basically the same as that in Example 1. The difference is that the vanadium salt, copper salt, complexing agent and tungsten salt components in the electrolyte in step (2) are removed to prepare a white micro-arc oxidation coating.

[0036] Please see Figure 5 Figure a shows the relationship between current and time during the micro-arc oxidation process in Example 1 and Comparative Example 1. Figure 5 Figure b shows the relationship between the coating thickness of Example 1 and Comparative Example 1 and the conductivity of the micro-arc oxidation electrolyte. As can be seen from the figure, the micro-arc oxidation coating thickness of Example 1 is slightly thicker than that of Comparative Example 1. This is related to the increase in conductivity caused by the introduction of Cu, V and W ions. Its thickness is between 13 and 14 μm. The current first increases and then decreases with the reaction time.

[0037] Table 1 shows the chromaticity values ​​and color differences of the micro-arc oxidation coatings of Example 1 and Comparative Example 1. The surface color difference of the micro-arc oxidation coatings prepared in both Example 1 and Comparative Example 1 is less than 0.5, indicating that both coatings have a uniform color distribution. Notably, the blackness value of Example 1 is significantly lower than that of Comparative Example 1. This result confirms that the introduction of copper salts, tungsten salts, and vanadium salts in the electrolyte can significantly reduce the chromaticity value of the micro-arc oxidation coating, while reducing the surface color difference and promoting the formation of a uniform and deeper black appearance on the coating surface.

[0038] Table 1. Colorimetric values ​​(L*a*b), blackness values, and color differences of the micro-arc oxidation coatings prepared in Example 1 and Comparative Example 1.

[0039] Sample a* b* L* Blackness value Color difference Example 1 1.28 0.93 24.07 25.51 0.133 Comparative Example 1 2.24 1.34 84.29 87.82 0.218

[0040] Please see Figure 6 , Figure 6 These are the TaFel curves of the micro-arc oxidation coatings prepared in Example 1 and Comparative Example 1. Figure 6 The polarization resistance was determined using the Stern–Geary equation:

[0041]

[0042] Where: β a β is the anodic polarization slope; b i is the cathode polarization slope. corr This is the corrosion current.

[0043] The corrosion potential, corrosion current, and polarization resistance of the samples are shown in Table 2.

[0044] Table 2. Corrosion potential, corrosion current, and polarization resistance of the micro-arc oxidation coatings prepared in Example 1 and Comparative Example 1.

[0045] Sample E corr (V)]]> i corr (A / cm)]]> [Rp (kΩ / cm -2 )]]> Example 1 -1.34 5.62 x 10 -8 ]] 290.74 Comparative Example 1 -1.27 1.35 x 10 -7 ]]> 107.33

[0046] As can be seen from Table 2, compared with Comparative Example 1, Example 1 has a lower corrosion current and a higher polarization resistance, resulting in a slower corrosion rate and excellent corrosion resistance.

[0047] Please see Figure 7 , Figure 7 The Nyquist curves for the micro-arc oxidation coatings prepared in Example 1 and Comparative Example 1 show that Example 1 has the lowest corrosion current and the largest diameter of the Nyquist semicircle, demonstrating that the corrosion resistance of Example 1 is stronger than that of Comparative Example 1, consistent with the results of the TaFel test. This is because the introduction of copper salts, tungsten salts, and vanadium salts increases the density of the micro-arc oxidation coating, thereby significantly enhancing its corrosion protection capability.

[0048] Comparative Example 2

[0049] The preparation method of the present comparative example is the same as that of Example 1, except that in step (3), the micro-arc oxidation electrical parameters are changed, the power supply mode is set to constant current mode, and the current parameter is set to 3.4 A / dm 2 , and other conditions remain unchanged. Compared with the micro-arc oxidation coating prepared in Example 1, the chroma value of the coating is not different from that of the sample in constant voltage mode, but the coating surface is ablated, reducing the quality of the coating. The reason for this situation is that the power supply mode is changed from constant voltage mode to constant current mode, which promotes the discharge energy in the later stage of the micro-arc oxidation process, resulting in large arc discharge, which causes the magnesium alloy surface micro-arc oxidation coating to have ablation points and the coating appearance to have defects.

[0050] Comparative Example 3

[0051] The preparation method of the present comparative example is the same as that of Example 1, except that in step (3), the micro-arc oxidation electrical parameters are changed, the voltage is set to 300 V, and other conditions remain unchanged. Compared with the micro-arc oxidation coating prepared in Example 1, the voltage is reduced in the present comparative example, resulting in a reduction in the thickness of the prepared micro-arc oxidation coating, which is about 3 μm, and an increase in the L*a*b* values, with the coating color being lighter. The electrochemical corrosion current of the present comparative example in 3.5 wt% NaCl solution is 1.45×10 -6 A, which is higher than that of Example 1, indicating a reduction in the corrosion resistance of the coating.

[0052] Comparative Example 4

[0053] The preparation method of the present comparative example is the same as that of Example 1, except that in step (3), the micro-arc oxidation electrical parameters are changed, the duty cycle is set to 20%, and other conditions remain unchanged. Compared with the micro-arc oxidation coating prepared in Example 1, the duty cycle is increased in the present comparative example, resulting in a reduction in the thickness of the prepared micro-arc oxidation coating, which is about 10 μm, and no change in the L*a*b* values, with the coating color being unchanged. The electrochemical corrosion current of the present comparative example in 3.5 wt% NaCl solution is 2.95×10 -7 A, which is higher than that of Example 1, indicating a reduction in the corrosion resistance of the coating.

Claims

1. A magnesium alloy surface black corrosion-resistant ceramic coating micro-arc oxidation electrolyte, characterized in that, The electrolyte is prepared by deionized water and the following components with the mass concentration: 16-20 g / L main film-forming agent meta-aluminate, 10-13 g / L auxiliary film-forming agent fluoride, 1-2 g / L main colorant soluble copper salt, 1-3 g / L auxiliary colorant soluble vanadium salt, 3-5 g / L auxiliary colorant soluble tungsten salt, 2-4 g / L complexing agent potassium sodium tartrate and 4-6 g / L pH regulator inorganic alkali.

2. The magnesium alloy surface black corrosion-resistant ceramic coating micro-arc oxidation electrolyte of claim 1, characterized in that, The meta-aluminate is one or a mixture of both of sodium meta-aluminate and potassium meta-aluminate, the fluoride is one or a mixture of more than one of ammonium fluoride, sodium fluoride and potassium fluoride, the soluble copper salt is one or a mixture of more than one of copper sulfate, copper pyrophosphate and copper acetate, the soluble vanadium salt is one or a mixture of both of ammonium metavanadate and sodium metavanadate, the soluble tungsten salt is sodium tungstate and the inorganic alkali is one or a mixture of both of sodium hydroxide and potassium hydroxide.

3. A method for producing a black corrosion-resistant ceramic coating on a magnesium alloy surface, characterized by, The micro-arc oxidation process is carried out by using the electrolyte as claimed in claim 1 or 2.

4. The method according to claim 3, wherein the voltage is 400 V or more. The magnesium alloy is micro-arc oxidized by using a unipolar pulse bias power supply to prepare a ceramic coating with smooth surface, high blackness, uniform color and excellent corrosion resistance on the surface of the magnesium alloy, and the specific steps are as follows: the pretreated magnesium alloy is completely immersed in the electrolyte; the magnesium alloy is used as an anode, a stainless steel is used as a cathode, and the power supply is set to a constant voltage mode for micro-arc oxidation.

5. The method according to claim 4, wherein the micro-arc oxidation is performed in a solution of 0.5% Na2SiO3 and 0.5% NaOH. The pretreatment steps of the magnesium alloy are as follows: the surface of the magnesium alloy is polished by using sandpaper, and then ultrasonic cleaning is sequentially carried out in acetone, anhydrous ethanol and deionized water to remove surface oil stains, and the magnesium alloy is dried for use.

6. The micro-arc oxidation preparation method according to claim 4, characterized in that, The magnesium alloy is completely immersed in the electrolyte; a stainless steel sheet with a thickness of 2-5 mm is used as a cathode, the magnesium alloy is used as an anode, the power supply is set to a constant voltage mode, the voltage size is set to 400 V, the oxidation treatment time is 6-10 min, the frequency is 1000 Hz, the duty cycle is 10%, the limited current is 30 A, the temperature of the electrolyte during the treatment process is controlled to be 15-25 ℃, the magnesium alloy is micro-arc oxidized, and after the treatment, the sample is taken out, cleaned and dried.

7. The method according to any one of claims 3 to 6, wherein the micro-arc oxidation is performed in a solution of 0.5 to 5 wt% Na2SiO3 and 0.5 to 5 wt% NaOH. The prepared black corrosion-resistant ceramic coating on the surface of the magnesium alloy exhibits specific color characteristics in the CIE 1976 L*a*b standard colorimetric system, i.e., the lightness |L*|≤30, the red-green axis color coordinate |a*|≤2 and the yellow-blue axis color coordinate |b*|≤2.

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