Method for directionally removing Al-Mn inclusions on surface layer of AZ31 magnesium alloy and dipping bright zinc

Abstract

The invention belongs to the field of magnesium alloy surface treatment, and relates to a method for directionally removing Al-Mn inclusions on the surface layer of AZ31 magnesium alloy and dipping bright zinc, which comprises the following steps: step 1, substrate pretreatment: grinding and cleaning the AZ31 magnesium alloy containing MgO and Al-Mn inclusions; 2, acid pickling is conducted, specifically, the magnesium alloy is subjected to acid pickling, and an oxidation film on the surface of a magnesium alloy matrix is removed; 3, activation is conducted, specifically, the magnesium alloy subjected to acid pickling is activated; 4, primary zinc immersion is conducted, specifically, primary zinc immersion is conducted on the activated magnesium alloy; 5, zinc removal is conducted, specifically, zinc removal is conducted on the magnesium alloy subjected to zinc immersion; 6, secondary zinc immersion is conducted, specifically, secondary zinc immersion is conducted on the magnesium alloy subjected to zinc removal; and 7, zincate zinc plating is conducted, specifically, zinc plating is conducted on the magnesium alloy subjected to secondary zinc dipping. According to the method, the quality of the coating is effectively improved by directionally etching the Al-Mn inclusions.

Description

Technical Field

[0001] The invention belongs to the field of magnesium alloy surface treatment, and in particular relates to a method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion. Background Art

[0002] The microstructure of AZ31 magnesium alloy is composed of α-Mg solid solution and non-equilibrium eutectic α-Mg+ Mg 17 Al 12 and a small amount of secondary Mg precipitated from the α-Mg solid solution 17 Al 12 However, magnesium alloys are prone to forming inclusions such as oxides, nitrides, and intermetallic compounds during smelting and casting. The presence of these inclusions seriously affects the corrosion resistance of magnesium alloys and the difficulty of surface treatment.

[0003] Magnesium alloys are highly active and are prone to corrosion and displacement plating in electroplating solutions, which deteriorates the bonding strength and corrosion resistance of metal coatings. Therefore, magnesium alloys usually need to be pre-galvanized to form a protective film on the substrate before electroplating to adapt to traditional electroplating solutions and reduce or inhibit the occurrence of displacement plating and corrosion reactions. However, the presence of inclusions on the surface of magnesium alloys seriously affects the quality of the zinc-plated layer and the bonding strength and corrosion resistance of the subsequent coating.

[0004] The AZ31 magnesium alloy used in the present invention has relatively serious MgO and Al-Mn alloy inclusions, and these impurities are difficult to remove during pickling and activation. During the zinc immersion process, MgO inclusions cannot dissolve to provide reducing electrons, and a metal zinc immersion layer is formed. Al-Mn inclusions can dissolve in the zinc immersion solution to provide reducing electrons, but cannot form a zinc immersion layer, and mainly undergo a dissolution reaction. Due to the presence of these two main impurities, there is no zinc immersion layer on the surface of the MgO inclusions after one zinc immersion. Al-Mn particles dissolve to form pits, and a zinc immersion layer can be formed after the Al-Mn in the pits is completely dissolved, while there is no zinc immersion layer when it is not completely dissolved, and the zinc immersion layer in the pits is relatively looser.

[0005] In order to form a uniform zinc protective layer on the surface of the magnesium alloy and facilitate the subsequent deposition of other coatings, the zinc-impregnated magnesium alloy is zinc-plated in alkaline zincate. After zinc plating, the surface of the MgO inclusions can be covered with a dense zinc plating layer. However, Al-Mn inclusions in the alkaline zinc plating solution have the problem of dissolving and exposing the magnesium matrix, resulting in poor bonding and poor density of the zinc plating layer at this location. Therefore, in order to obtain a uniform, strong bonding, and high-density zinc plating protective layer, it is necessary to eliminate or etch the Al-Mn inclusions in a directional manner. In addition, the current magnesium alloy surface treatment has the following problems: 1. The one-time zinc-plating process of magnesium alloy is affected by the presence of α and β phases, metal inclusions, oxide film, etc. in the magnesium alloy matrix. The one-time zinc-plating layer has problems such as poor uniformity, loose zinc-plating layer, and leakage, which leads to high porosity, low bonding strength, low flatness, poor brightness, and poor uniformity in the zincate zinc-plating layer.

[0006] 2. The density, flatness, uniformity and stability of the zinc coating of magnesium alloy pyrophosphate galvanizing are not as good as those of zincate galvanizing after zinc immersion.

[0007] 3. The potential difference between the chemical nickel plating on magnesium alloy and the substrate is large, the galvanic corrosion is serious, and the porosity of the chemical nickel layer is high. As a corrosion-resistant coating, the coating needs to be deposited thicker, which seriously increases the weight of the magnesium alloy. Summary of the invention

[0008] The object of the present invention is to provide a method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion to solve the above-mentioned technical problems.

[0009] In order to solve the above technical problems, the specific technical scheme of the method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion of the present invention is as follows: A method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion, comprising the following steps: Step 1: Matrix pretreatment: Grinding and cleaning of AZ31 magnesium alloy containing MgO and Al-Mn inclusions; Step 2: Pickling: Pickling the magnesium alloy to remove the oxide film on the surface of the magnesium alloy substrate; Step 3: Activation: Activate the pickled magnesium alloy; Step 4: One-time zinc immersion: zinc immersion is performed on the activated magnesium alloy once; Step 5: Stripping zinc: stripping zinc from the magnesium alloy after zinc immersion; Step 6: Secondary zinc immersion: The magnesium alloy after zinc stripping is subjected to secondary zinc immersion; Step 7: zincate galvanizing: zincate the magnesium alloy after secondary zinc immersion.

[0010] Furthermore, in step 1, the AZ31 magnesium alloy containing MgO and Al-Mn inclusions is sanded to 3000 mesh, and then ultrasonically cleaned and degreased with an organic solvent.

[0011] Furthermore, in step 2, the magnesium alloy is pickled with 20±5 g / L citric acid for 10-60 s to remove the oxide film on the surface of the magnesium alloy substrate, the pickling temperature is room temperature, and then washed with water three times.

[0012] Furthermore, in step 3, 100 g / L NH4HF2 is used to activate the pickled magnesium alloy for 1 to 4 min to form a uniform MgF2 protective film on its surface, followed by water washing 3 times, and the activation temperature is room temperature.

[0013] Furthermore, the zinc dipping in step 4 is performed in a zinc dipping solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H20, 5±2 g / L Na2CO3, and 5±2 g / L KF, with a zinc dipping temperature of 30-80°C, a zinc dipping time of 2-15 min, and a pH value of the zinc dipping solution of 10-10.5.

[0014] Furthermore, the zinc stripping in step 5 is carried out in a solution with a concentration ratio of 0.1-20 mL / L of hydrochloric acid, nitric acid or sulfuric acid and 50-200 g / L of potassium fluoride, sodium fluoride or ammonium bifluoride, the zinc stripping temperature is 10-35°C at room temperature, the time is 5-240 s, and the zinc-impregnated magnesium alloy product is evenly shaken during the zinc stripping process, or the liquid is evenly stirred.

[0015] Furthermore, the secondary zinc dipping in step 6 is performed in a zinc dipping solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H2O, 5±2 g / L Na2CO3, and 5g±2 g / L KF, the zinc dipping temperature is 30-80°C, the zinc dipping time is 5-20 min, the pH value of the zinc dipping solution is 10-10.5, and a uniform secondary zinc dipping layer with metallic luster is obtained after the secondary zinc dipping.

[0016] Furthermore, in step 7, the zincate zinc plating solution is composed of 110-150 g / L NaOH and 8-15 g / L Zn 2+ It is composed of 0.5~2 mL / L brightener, 5~15 mL / L softener and 5~15 mL / L purifier. The electroplating current density is 0.5~6 A / dm 2 , plating solution temperature 20~40 ℃, electroplating time 5~60 min.

[0017] The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion of the present invention has the following advantages: in the process of single zinc immersion and stripping of single zinc immersion layer of AZ31 magnesium alloy, the present invention directionally dissolves or etches Al-Mn inclusion particles, improves the surface consistency of magnesium alloy before secondary zinc immersion, and improves the coverage rate after secondary zinc immersion. Compared with single zinc immersion magnesium alloy, the corrosion resistance of secondary zinc immersion magnesium alloy in zincate plating solution and 3.5% NaCl solution is improved. At the same time, the hydrochloric acid or nitric acid or sulfuric acid and potassium fluoride or sodium fluoride or ammonium bifluoride stripping solution used in the present invention are conducive to obtaining a uniform, bright, and consistent secondary zinc immersion layer, and are conducive to obtaining a dense, uniform, and bright zincate zinc-plated layer. The corrosion resistance of the zincate zinc-plated layer in 3.5% NaCl solution is close to that of pure zinc plate, and its corrosion resistance is effectively improved. These results indicate that the presence of Al-Mn inclusions has a serious impact on the quality of zinc immersion and subsequent coatings. The present invention can effectively improve the quality of coatings by directional etching of Al-Mn inclusions. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a magnesium alloy zinc immersion process flow chart; Figure 2a Schematic diagram of inclusions in AZ31 magnesium alloy matrix; Figure 2b This is the element table of inclusion spectrum in AZ31 magnesium alloy matrix; Figure 3 This is the SEM image of inclusions on the surface of magnesium alloy after pickling and activation; Figure 4 This is the surface SEM image of AZ31 magnesium alloy containing Al-Mn inclusions after one zinc immersion; Figure 5a This is the surface morphology of AZ31 magnesium alloy containing Al-Mn inclusions after zinc stripping for 60s after zinc immersion once; Figure 5b The surface morphology of AZ31 magnesium alloy containing Al-Mn inclusions after secondary zinc immersion; Figure 6 The graph shows the test results of Tafel polarization curves of the zinc-dipped AZ31 magnesium alloy in the alkaline zincate bath after the zinc stripping time. Figure 7 The graph shows the test results of Tafel polarization curves of the secondary zinc-magnesium alloy galvanized samples in 3.5 wt.% NaCl solution after different de-zincification times; DETAILED DESCRIPTION

[0019] In order to better understand the purpose, method and function of the present invention, the following is a further detailed description of a method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to the present invention in conjunction with the accompanying drawings.

[0020] like Figure 1 As shown, the present invention forms a directional dissolution of Al-Mn inclusions during a single zinc immersion in a zinc immersion solution, and forms a passivation film MgF2 on the surface of the magnesium matrix in a zinc stripping solution of hydrochloric acid, nitric acid, sulfuric acid and potassium fluoride, sodium fluoride or ammonium bifluoride, while a passivation film cannot be formed on the surface of the Al-Mn inclusions, thereby forming a secondary directional etching effect of the Al-Mn inclusions, and finally achieving the purpose of directional removal of the Al-Mn inclusions and improving the quality of the zinc immersion layer and the zincate zinc plating layer.

[0021] Specifically, the method of directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion of the present invention comprises the following steps: Step 1: Substrate pretreatment The AZ31 magnesium alloy containing MgO and Al-Mn inclusions was sanded to 3000 mesh and ultrasonically cleaned and degreased with organic solvents such as acetone and anhydrous ethanol.

[0022] Step 2: Pickling The magnesium alloy was pickled with 20±5 g / L citric acid for 10-60 s to remove the oxide film on the surface of the magnesium alloy substrate. The pickling temperature was room temperature and then washed with water three times.

[0023] Step 3: Activation The pickled magnesium alloy was activated with 100 g / L NH4HF2 for 1-4 min to form a uniform MgF2 protective film on its surface, and then washed with water three times at room temperature.

[0024] Step 4: Primary zinc immersion The primary zinc dipping is carried out in a zinc dipping solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H20, 5±2 g / L Na2CO3, and 5±2 g / L KF. The zinc dipping temperature is 30~80 ℃, the zinc dipping time is 2~15 min, and the pH value of the zinc dipping solution is 10~10.5.

[0025] Step 5: Zinc removal Zinc stripping is done in a solution with a concentration ratio of 0.1~20 ml / L of hydrochloric acid, nitric acid or sulfuric acid, and 50~200 g / L of potassium fluoride, sodium fluoride or ammonium bifluoride. The zinc stripping temperature is room temperature 10~35 ℃, and the time is 5~240 s. During the zinc stripping process, the zinc-magnesium alloy product can be shaken evenly, or the liquid can be stirred evenly.

[0026] Step 6: Secondary zinc immersion The secondary zinc immersion is completed in a zinc immersion solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H2O, 5±2 g / L Na2CO3, and 5g±2 g / L KF, with a zinc immersion temperature of 30~80 ℃, a zinc immersion time of 5~20 min, and a zinc immersion solution pH value of 10~10.5. After the secondary zinc immersion, a uniform secondary zinc immersion layer with metallic luster can be obtained.

[0027] Step 7: Zincate Galvanizing The zincate zinc plating solution is composed of 110~150 g / L NaOH and 8~15 g / L Zn 2+ It is composed of 0.5~2 mL / L brightener, 5~15 mL / L softener and 5~15 mL / L purifier. The electroplating current density is 0.5~6 A / dm 2 , plating solution temperature 20~40 ℃, electroplating time 5~60 min.

[0028] Example 1: Inclusions in AZ31 magnesium alloy sheet (1) Substrate: 100×100×1 mm AZ31B magnesium alloy plate was sanded to 3000 mesh.

[0029] (2) Oil removal: ultrasonic cleaning with anhydrous ethanol.

[0030] (3) Pickling: Pickling with 20 g / L citric acid and 20 mL / L nitric acid for 30 s to corrode the magnesium matrix and expose inclusions.

[0031] Embodiment 1 Technical effect: Using Scheme 1 to pickle AZ31 magnesium alloy can expose the inclusions contained in the magnesium alloy matrix. Figure 2a , Figure 2b As shown in the figure, the AZ31 magnesium alloy specimen contains a large number of Al-Mn inclusion particles and a small amount of SiO2 inclusions.

[0032] Example 2: Pickling and activation cannot remove inclusions Substrate: AZ31B magnesium alloy plate with a size of 100 × 100 × 1 mm was sanded to 3000 mesh.

[0033] Degreasing: ultrasonic cleaning with anhydrous ethanol.

[0034] Acid washing: 20 g / L citric acid for 30 s at room temperature, then washed with water three times.

[0035] Activation: 100 g / L NH4HF2 for 2 min at room temperature, then washed with water 3 times.

[0036] Embodiment 2 Technical effect: The pickling activation scheme adopted in Example 2 cannot effectively remove the Al-Mn inclusions in the magnesium alloy. Figure 3 shown.

[0037] Example 3: Dissolution of Al-Mn inclusions by single zinc immersion Substrate: AZ31B magnesium alloy plate with a size of 100 × 100 × 1 mm was sanded to 3000 mesh.

[0038] Degreasing: ultrasonic cleaning with anhydrous ethanol.

[0039] Acid washing: 20 g / L citric acid for 30 seconds at room temperature, then washed with water three times.

[0040] Activation: 100 g / L NH4HF2 for 2 min at room temperature, then washed with water 3 times.

[0041] Primary zinc dipping: completed in a zinc dipping solution of 140 g / L K4P2O7, 45 g / L ZnSO4·7H2O, 5 g / L Na2CO3, and 6 g / L KF, with a zinc dipping temperature of 60°C, a zinc dipping time of 5 min, and a pH value of the zinc dipping solution of 10.2.

[0042] Embodiment 3 technical effect: Scheme 3: After zinc immersion once, Al-Mn inclusions dissolve faster and there is a zinc immersion layer in the pores after zinc immersion, but the zinc immersion layer is relatively loose. When Al-Mn inclusions are not completely dissolved, there is no zinc immersion layer on the surface, such as Figure 4 shown.

[0043] Example 4: Effect of removing Al-Mn inclusions after stripping and secondary zinc immersion Substrate: 100×100×1 mm AZ31 magnesium alloy plate was sanded to 3000 mesh with sandpaper.

[0044] Degreasing: Ultrasonic cleaning with anhydrous ethanol.

[0045] Acid washing: 20 g / L citric acid for 30 s at room temperature, then washed with water three times.

[0046] Activation: 100 g / L NH4HF2 for 2 min at room temperature, then washed with water 3 times.

[0047] Primary zinc immersion: completed in a zinc immersion solution of 140g / L K4P2O7, 45 g / L ZnSO4·7H2O, 5 g / L Na2CO3, and 6 g / L KF, with a zinc immersion temperature of 60°C, a zinc immersion time of 3 min, a zinc immersion solution pH of 10.2, and washed with water 3 times.

[0048] Zinc removal: Zinc removal was performed in a zinc removal solution consisting of 2 mL / L HNO3 and 50 g / L KF for 10, 30, 60 and 120 s at room temperature, and washed with water three times.

[0049] (7) Secondary zinc immersion: in a zinc immersion solution of 140 g / L K4P2O7, 45 g / L ZnSO4·7H2O, 5 g / L Na2CO3, and 6 g / L KF, at a temperature of 60 °C, for 6 min, at a pH of 10.2, and washed with water three times.

[0050] (8) The zincate zinc plating solution is composed of 120 g / L NaOH and 8 g / L Zn2+, 1 mL / L brightener, 10 mL / L softener and 10 mL / L purifier. The electroplating current density is 2 A / dm2, the plating temperature is 25 °C, and the electroplating time is 20 min.

[0051] Embodiment 4 technical effect: After zinc immersion and zinc stripping treatment, Al-Mn on the surface of magnesium alloy was effectively removed. Figure 5a As shown in the figure, the coverage of zinc immersion after secondary zinc immersion is effectively improved, such as Figure 5b The electrochemical corrosion performance of AZ31 magnesium alloy in zincate before zinc immersion, zinc immersion once, and zinc immersion twice after different de-zincification times is shown in Figure 2. Figure 6 As shown in Figure 2, after the Al-Mn inclusions are removed, the corrosion current of the zinc-immersed layer decreases, that is, the corrosion is weakened. Figure 7 As shown, as the stripping time increases, the corrosion current of the electro-galvanized layer shows a trend of gradually decreasing, and the corrosion current of the galvanized layer is close to that of the pure zinc plate when the stripping time is 120s. The galvanized layer and the electro-galvanized layer obtained by Example 4 both have high brightness and uniformity, providing a good base layer for subsequent electroplating of other coatings.

[0052] It is to be understood that the present invention is described by some embodiments, and it is known to those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the scope of protection of the present invention.

Claims

1. A method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion, characterized in that: The steps include: Step 1: Matrix pretreatment: Grinding and cleaning of AZ31 magnesium alloy containing MgO and Al-Mn inclusions; Step 2: Pickling: Pickling the magnesium alloy to remove the oxide film on the surface of the magnesium alloy substrate; Step 3: Activation: Activate the pickled magnesium alloy; Step 4: One-time zinc immersion: zinc immersion is performed on the activated magnesium alloy once; Step 5: Stripping zinc: stripping zinc from the magnesium alloy after zinc immersion; Step 6: Secondary zinc immersion: The magnesium alloy after zinc stripping is subjected to secondary zinc immersion; Step 7: zincate galvanizing: zincate the magnesium alloy after secondary zinc immersion.

2. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: In the step 1, the AZ31 magnesium alloy containing MgO and Al-Mn inclusions is sanded to 3000 mesh, and then ultrasonically cleaned and degreased with an organic solvent.

3. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: In step 2, the magnesium alloy is pickled with 20±5 g / L citric acid for 10-60 s to remove the oxide film on the surface of the magnesium alloy substrate. The pickling temperature is room temperature, and then the magnesium alloy is washed with water for 3 times.

4. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: In step 3, 100 g / L NH4HF2 is used to activate the pickled magnesium alloy for 1 to 4 min to form a uniform MgF2 protective film on its surface, followed by water washing 3 times. The activation temperature is room temperature.

5. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: The zinc immersion in step 4 is carried out in a zinc immersion solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H20, 5±2 g / LNa2CO3, and 5±2 g / L KF, with a zinc immersion temperature of 30-80°C, a zinc immersion time of 2-15 min, and a pH value of the zinc immersion solution of 10-10.

5.

6. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: The zinc stripping in step 5 is carried out in a solution with a concentration ratio of 0.1-20 mL / L of hydrochloric acid, nitric acid or sulfuric acid and 50-200 g / L of potassium fluoride, sodium fluoride or ammonium bifluoride. The zinc stripping temperature is 10-35°C at room temperature and the time is 5-240 s. During the zinc stripping process, the zinc-magnesium alloy product is evenly shaken or the liquid is evenly stirred.

7. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: The secondary zinc dipping in step 6 is carried out in a zinc dipping solution prepared by 140±10 g / L K4P2O7, 45±5 g / L ZnSO4·7H2O, 5±2 g / LNa2CO3, and 5g±2 g / L KF, with a zinc dipping temperature of 30-80°C, a zinc dipping time of 5-20 min, and a pH value of the zinc dipping solution of 10-10.

5. After the secondary zinc dipping, a uniform secondary zinc dipping layer with metallic luster is obtained.

8. The method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that: The zincate zinc plating solution in step 7 is composed of 110-150 g / L NaOH and 8-15 g / L Zn 2+ It is composed of 0.5~2 mL / L brightener, 5~15 mL / L softener and 5~15 mL / L purifier. The electroplating current density is 0.5~6 A / dm 2 , plating solution temperature 20~40 ℃, electroplating time 5~60 min.

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