CN119980388A

CN119980388A - A method for directional removal of Al-Mn inclusions on the surface of AZ31 magnesium alloy and bright zinc immersion

Info

Publication number: CN119980388A
Application number: CN202510089260.9A
Authority: CN
Inventors: 王宏涛; 刘进成; 李山林; 徐夺花
Original assignee: Cross Ion Hangzhou New Materials Technology Co ltd
Current assignee: Cross Ion Hangzhou New Materials Technology Co ltd
Priority date: 2025-01-21
Filing date: 2025-01-21
Publication date: 2025-05-13

Abstract

The invention belongs to the field of magnesium alloy surface treatment, and discloses a method for directionally removing Al-Mn inclusion and bright zinc dipping on an AZ31 magnesium alloy surface layer, which comprises the following steps of 1, preprocessing a matrix, polishing and cleaning the AZ31 magnesium alloy containing MgO and Al-Mn inclusion; the method comprises the steps of (1) pickling magnesium alloy to remove oxide films on the surface of a magnesium alloy matrix, (3) activating the pickled magnesium alloy, (4) primary zincating the activated magnesium alloy, (5) dezincating the zincating magnesium alloy, (6) secondary zincating the dezincating magnesium alloy, and (7) zincate zincating the secondary zincating magnesium alloy. The invention effectively improves the quality of the plating layer by directionally etching the Al-Mn inclusion.

Description

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

Technical Field

The invention belongs to the field of magnesium alloy surface treatment, and particularly relates to a method for directionally removing Al-Mn inclusion on an AZ31 magnesium alloy surface layer and dipping bright zinc.

Background

The structure of AZ31 magnesium alloy consists of alpha-Mg solid solution and crystal boundary unbalanced eutectic alpha-Mg+Mg ₁₇Al₁₂ and a small amount of secondary Mg ₁₇Al₁₂ separated from the alpha-Mg solid solution. However, magnesium alloy is extremely prone to form inclusions such as oxides, nitrides, and intermetallic compounds during smelting, casting, and the like. The presence of these inclusions severely affects the corrosion resistance of the magnesium alloy and the difficulty of surface treatment.

The magnesium alloy has higher activity, and is easy to generate corrosion and displacement plating in the plating solution, so that the binding force, corrosion resistance and other performances of the metal plating layer are deteriorated, and therefore, the magnesium alloy is usually required to be pre-galvanized to form a protective film on a substrate before plating so as to adapt to the traditional plating solution, and the occurrence of displacement plating and corrosion reaction is reduced or inhibited. However, the quality of the zinc dipping layer, the binding force and corrosion resistance of the subsequent plating layer are seriously affected by the existence of the inclusions on the surface layer of the magnesium alloy.

The AZ31 magnesium alloy adopted by the invention has relatively serious MgO and Al-Mn alloy inclusions, and the inclusions are difficult to remove in the pickling and activation processes. MgO inclusions cannot be dissolved to provide reducing electrons in the zincating process, so that a metal zincating layer is formed. Al-Mn inclusion can be dissolved in the zincating solution to provide reducing electrons, but cannot form a zincating layer, and mainly undergoes dissolution reaction. The existence of the two main impurities results in no zinc dipping layer on the surface of MgO inclusion after primary zinc dipping. The Al-Mn particles are dissolved to form pits, the zinc dipping layer can be formed after the Al-Mn particles in the pits are completely dissolved, the zinc dipping layer is not formed when the Al-Mn particles are not completely dissolved, and the zinc dipping layer in the pits is relatively loose.

In order to form a uniform zinc protection layer on the surface of the magnesium alloy, the zinc-coated magnesium alloy is favorable for the subsequent deposition of other plating layers, the surface of MgO inclusion can be covered with a compact zinc coating after zinc coating, however, the problems of dissolution and exposure of a magnesium matrix exist in the alkaline zinc coating liquid due to the inclusion of Al-Mn, and the problems of poor binding force, poor compactness and the like of the zinc coating at the position are caused. Therefore, the Al-Mn inclusion is directionally eliminated or etched to obtain the uniform, strong-binding and high-density galvanized protective layer. In addition, the current magnesium alloy surface treatment has the following problems:

1. The primary zinc-dipping process of the magnesium alloy is influenced by alpha and beta phases, metallic inclusions, oxide films and the like of a magnesium alloy matrix, and the primary zinc-dipping layer has the problems of poor uniformity, loose zinc-dipping layer, leakage dipping and the like, so that the zincate zinc-plating layer has the problems of high porosity, low binding force, low flatness, poor brightness, poor uniformity and the like.

2. Magnesium alloy pyrophosphate is galvanized, and compactness, flatness and uniformity of a galvanized layer and stability of a plating solution are inferior to those of zincate galvanized after zinc immersion.

3. The chemical nickel plating of magnesium alloy has great potential difference with the matrix, serious galvanic corrosion, high porosity of the chemical nickel layer, thicker plating layer as corrosion-resistant plating layer and serious weight gain for magnesium alloy.

Disclosure of Invention

The invention aims to provide a method for directionally removing Al-Mn inclusion on the surface layer of an AZ31 magnesium alloy and immersing bright zinc, so as to solve the technical problems.

In order to solve the technical problems, the specific technical scheme of the method for directionally removing Al-Mn inclusion on the surface layer of the AZ31 magnesium alloy and immersing bright zinc is as follows:

a method for directionally removing Al-Mn inclusion and bright zinc dipping on the surface layer of AZ31 magnesium alloy comprises the following steps:

Step 1, preprocessing a matrix, namely polishing and cleaning AZ31 magnesium alloy containing MgO and Al-Mn inclusion;

step2, pickling, namely pickling the magnesium alloy to remove an oxide film on the surface of a magnesium alloy matrix;

step 3, activating, namely activating the magnesium alloy after pickling;

step 4, primary zinc dipping, namely primary zinc dipping is carried out on the activated magnesium alloy;

step 5, dezincification, namely dezincification is carried out on the magnesium alloy subjected to zinc leaching;

step 6, secondary zinc dipping, namely, performing secondary zinc dipping on the magnesium alloy after zinc removal;

and 7, zincate galvanization, namely galvanizing the magnesium alloy subjected to secondary galvanization.

Further, the AZ31 magnesium alloy containing MgO and Al-Mn inclusion is sanded to 3000 meshes in the step 1, and ultrasonic cleaning and oil removal are carried out by using an organic solvent.

Further, in the step 2, the magnesium alloy is pickled for 10-60 s by using 20+/-5 g/L of citric acid, an oxide film on the surface of a magnesium alloy matrix is removed, the pickling temperature is room temperature, and then the magnesium alloy is washed for 3 times by water.

Further, in the step 3, 100 g/L NH ₄HF₂ is used for activating the pickled magnesium alloy for 1-4 min, so that a uniform MgF ₂ protective film is formed on the surface of the magnesium alloy, and then the magnesium alloy is washed with water for 3 times, wherein the activation temperature is room temperature.

Further, the primary zinc leaching in the step 4 is completed in a zinc leaching solution prepared by 140±10 g/L K₄P₂O₇、45±5 g/L ZnSO₄·7H₂0、5±2 g/L Na₂CO₃、5±2 g/L KF, the zinc leaching temperature is 30-80 ℃, the zinc leaching time is 2-15 min, and the PH value of the zinc leaching solution is 10-10.5.

And further, the zinc removal in the step 5 is completed in a solution with the concentration ratio of 0.1-20 mL/L of hydrochloric acid, nitric acid or sulfuric acid, 50-200 g/L of potassium fluoride, sodium fluoride or ammonium bifluoride, the zinc removal temperature is 10-35 ℃ at room temperature, the time is 5-240 s, and the zinc-immersed magnesium alloy product is uniformly shaken or the liquid is uniformly stirred in the zinc removal process.

Further, the secondary zinc dipping in the step 6 is completed in a zinc dipping liquid prepared by 140±10 g/L K₄P₂O₇、45±5 g/L ZnSO₄·7H₂O、5±2 g/L Na₂CO₃、5g±2 g/L KF, the zinc dipping temperature is 30-80 ℃, the zinc dipping time is 5-20 min, the PH value of the zinc dipping liquid is 10-10.5, and a secondary zinc dipping layer with metallic luster and uniformity is obtained after the secondary zinc dipping.

Further, the zincate galvanizing solution in the step 7 consists of 110-150 g/L NaOH and 8-15 g/L Zn ²⁺ matched with 0.5-2 mL/L brightening agent, 5-15 mL/L softening agent and 5-15 mL/L purifying agent, wherein the electroplating current density is 0.5-6A/dm ², the temperature of the galvanizing solution is 20-40 ℃, and the electroplating time is 5-60 min.

The method for directionally removing Al-Mn inclusion and bright zinc dipping on the surface layer of the AZ31 magnesium alloy has the advantages that Al-Mn inclusion particles are directionally dissolved or etched in the primary zinc dipping and primary zinc dipping layer stripping processes of the AZ31 magnesium alloy, so that the surface consistency of the magnesium alloy before secondary zinc dipping is improved, the coverage rate after secondary zinc dipping is improved, and the corrosion resistance of the secondary zinc dipping magnesium alloy in zincate plating solution and 3.5% NaCl solution is improved compared with that of the primary zinc dipping magnesium alloy. Meanwhile, the hydrochloric acid or nitric acid or sulfuric acid and potassium fluoride or sodium fluoride or ammonium bifluoride zinc removing liquid adopted by the invention is favorable for obtaining a uniform, bright and uniform secondary zinc dipping layer, is favorable for obtaining a compact, uniform and bright zincate zinc plating layer, and has the corrosion resistance close to that of a pure zinc plate in 3.5% NaCl solution, and the corrosion resistance of the zincate zinc plating layer is effectively improved. These results indicate that the presence of Al-Mn inclusion has a serious effect on the quality of zinc impregnation and subsequent plating, and the invention can effectively improve the quality of plating by directionally etching the Al-Mn inclusion.

Drawings

FIG. 1 is a flow chart of a zinc dipping process of the magnesium alloy of the invention;

FIG. 2a is a schematic diagram of inclusions present in an AZ31 magnesium alloy matrix;

FIG. 2b is a table of inclusion spectrum elements present in the AZ31 magnesium alloy matrix;

FIG. 3 is an SEM image of inclusions in a magnesium alloy surface layer after pickling and activation;

FIG. 4 is a surface SEM image of an AZ31 magnesium alloy containing Al-Mn inclusions after primary zincating;

FIG. 5a is a surface topography of a primary zincating post-zincating 60s AZ31 magnesium alloy containing Al-Mn inclusions;

FIG. 5b is a surface topography of a secondary zincating AZ31 magnesium alloy containing Al-Mn inclusions;

FIG. 6 is a graph showing the results of Tafel polarization curve test of a secondary zincating sample in an alkaline zincate plating solution after a primary zincating AZ31 magnesium alloy is dezincified for different times;

FIG. 7 is a graph showing the results of Tafel polarization curve test of a secondary zincating magnesium alloy galvanized sample in 3.5 wt% NaCl solution after various times of dezincification;

Detailed Description

In order to better understand the purposes, methods and functions of the invention, the method for directionally removing Al-Mn inclusion and immersing bright zinc on the surface layer of AZ31 magnesium alloy is further described in detail below with reference to the accompanying drawings.

As shown in figure 1, the invention forms directional dissolution to Al-Mn inclusion when primary zinc dipping in zinc dipping solution, and forms passivation film MgF ₂ on the surface of magnesium base in hydrochloric acid or nitric acid or sulfuric acid and potassium fluoride or sodium fluoride or ammonium bifluoride dezincification solution, while the surface of Al-Mn inclusion can not form passivation film, forming the effect of secondary directional etching of Al-Mn inclusion, finally achieving the purposes of directionally removing Al-Mn inclusion and improving the quality of zinc dipping layer and zincate zinc plating layer.

Specifically, the method for directionally removing Al-Mn inclusion and immersing bright zinc on the surface layer of the AZ31 magnesium alloy comprises the following steps:

Step 1, substrate pretreatment

AZ31 magnesium alloy containing MgO and Al-Mn inclusion is sanded to 3000 meshes, and ultrasonic cleaning and degreasing are carried out by using organic solvents such as acetone, absolute ethyl alcohol and the like.

Step 2, acid washing

And pickling the magnesium alloy with 20+/-5 g/L citric acid for 10-60 s, removing an oxide film on the surface of the magnesium alloy matrix, and then washing with water for 3 times at room temperature.

Step 3, activating

And (3) using 100 g/L NH ₄HF₂ to activate the pickled magnesium alloy for 1-4 min to form a uniform MgF ₂ protective film on the surface of the magnesium alloy, and then washing the magnesium alloy with water for 3 times, wherein the activation temperature is room temperature.

Step 4, one-time zinc dipping

The primary zinc dipping is completed in a zinc dipping liquid prepared by 140±10 g/L K₄P₂O₇、45±5 g/L ZnSO₄·7H₂0、5±2 g/L Na₂CO₃、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 liquid is 10-10.5.

Step 5, dezincification

The dezincification is completed in a solution with the concentration ratio of 0.1-20 ml/L of hydrochloric acid or nitric acid or sulfuric acid and 50-200 g/L of potassium fluoride or sodium fluoride or ammonium bifluoride. The zinc stripping temperature is 10-35 ℃ at room temperature, and the time is 5-240 s. And uniformly shaking the zinc-immersed magnesium alloy product in the zinc removing process, or uniformly stirring the liquid.

Step 6, secondary zinc dipping

The secondary zinc dipping is completed in zinc dipping liquid prepared by 140±10 g/L K₄P₂O₇、45±5 g/L ZnSO₄·7H₂O、5±2 g/L Na₂CO₃、5g±2 g/L KF, the zinc dipping temperature is 30-80 ℃, the zinc dipping time is 5-20 min, and the PH value of the zinc dipping liquid is 10-10.5. After the secondary zincating, a uniform secondary zincating layer with metallic luster can be obtained.

Step 7, zincate galvanization

The zincate zinc plating solution consists of 110-150 g/L NaOH and 8-15 g/L Zn ²⁺ matched with 0.5-2 mL/L brightening agent, 5-15 mL/L softening agent and 5-15 mL/L purifying agent. The electroplating current density is 0.5-6A/dm ², the temperature of the plating solution is 20-40 ℃, and the electroplating time is 5-60 min.

Example 1 AZ31 magnesium alloy sheet containing inclusions

(1) The substrate, 100X 1mm AZ31B magnesium alloy plate, is sanded to 3000 mesh.

(2) Deoiling, namely ultrasonic cleaning treatment by absolute ethyl alcohol.

(3) Pickling, namely pickling 30 s by 20 g/L citric acid and 20 mL/L nitric acid, so as to achieve the purpose of corroding magnesium substrates and exposing inclusions.

Example 1 technical effects:

The AZ31 magnesium alloy is pickled by adopting the scheme 1, so that inclusions contained in a magnesium alloy matrix can be exposed. As shown in FIG. 2a and FIG. 2b, the AZ31 magnesium alloy test piece contains more Al-Mn inclusion particles and a small amount of SiO ₂ inclusion.

Example 2 acid washing and activation to remove inclusions

The substrate, 100X 1mm AZ31B magnesium alloy plate, was sanded to 3000 mesh.

Deoiling, namely carrying out ultrasonic cleaning treatment by absolute ethyl alcohol.

Pickling, namely pickling 20 g/L citric acid with 30 s, washing with water for 3 times at room temperature.

Activation, namely 100 g/L NH ₄HF₂ activation 2 min, room temperature and water washing 3 times.

Example 2 technical effects:

the pickling activation scheme employed in example 2 was not effective in removing al—mn inclusions of magnesium alloy, as shown in fig. 3.

EXAMPLE 3 dissolution of Al-Mn inclusion by Primary zincate

The substrate, 100X 1mm AZ31B magnesium alloy plate, was sanded to 3000 mesh.

Pickling, namely pickling 20 g/L citric acid for 30s, washing with water for 3 times at room temperature.

Activating for 2min at room temperature with 100 g/L NH ₄HF₂, and washing with water for 3 times.

The primary zinc dipping is completed in 140g/L K₄P₂O₇、45 g/L ZnSO₄·7H₂O、5 g/L Na₂CO₃、6 g/L KF zinc dipping liquid, the zinc dipping temperature is 60 ℃, the zinc dipping time is 5min, and the PH value of the zinc dipping liquid is 10.2.

Example 3 technical effects:

After primary zincating in scheme 3, zincating layers are arranged in holes after zincating at positions where Al-Mn inclusion is dissolved faster, but the zincating layers are relatively loose. And when the Al-Mn inclusion is not completely dissolved, the surface thereof is free of a zincating layer, as shown in FIG. 4.

Example 4 removal of Al-Mn inclusion after dezincification and secondary zincification

The substrate, 100X 1mm AZ31 magnesium alloy plate, is sanded to 3000 mesh.

Deoiling, namely ultrasonic cleaning treatment by absolute ethyl alcohol.

The primary zinc dipping is finished in 140g/L K₄P₂O₇、45 g/L ZnSO₄·7H₂O、5 g/L Na₂CO₃、6 g/L KF zinc dipping liquid, the zinc dipping temperature is 60 ℃, the zinc dipping time is 3 min, the PH value of the zinc dipping liquid is 10.2, and the zinc dipping liquid is washed 3 times.

Dezincification, namely dezincification of 10, 30, 60 and 120 s in dezincification solution composed of 2 mL/L HNO ₃ and 50 g/L KF, and washing 3 times at room temperature.

(7) And (3) secondary zinc leaching, namely, completing zinc leaching in a zinc leaching solution of 140g/L K₄P₂O₇、45 g/L ZnSO₄·7H₂O、5 g/L Na₂CO₃、6 g/L KF, wherein the zinc leaching temperature is 60 ℃, the zinc leaching time is 6 min, the PH value of the zinc leaching solution is 10.2, and washing is carried out for 3 times.

(8) The zincate galvanizing solution consists of 120 g/L NaOH, 8 g/L Zn2+ matched 1 mL/L brightening agent, 10 mL/L softening agent and 10 mL/L purifying agent. The electroplating current density is 2A/dm 2, the plating solution temperature is 25 ℃ and the electroplating time is 20 min.

Example 4 technical effects:

After primary zincating and dezincating treatment, al-Mn on the surface of the magnesium alloy is effectively removed as shown in figure 5a, and the coverage rate of zincating after secondary zincating is effectively improved as shown in figure 5 b. The electrochemical corrosion performance of samples of AZ31 magnesium alloy, which are subjected to primary zincating before zincating and secondary zincating after different zincating time, in zincate is shown in figure 6, and after Al-Mn inclusion is directionally removed, the corrosion current of the zincating layer is reduced, namely the corrosion is weakened. Further, as shown in fig. 7, as the dezincification time is prolonged, the corrosion current of the electrogalvanized layer shows a tendency to gradually decrease, and at the dezincification time of 120s, the corrosion current of the electrogalvanized layer is close to that of the pure zinc sheet. The zincating layer and the electrogalvanizing layer obtained by the embodiment 4 have higher brightness and uniformity, and provide better priming layers for the subsequent electroplating of other plating layers.

It will be understood that the application has been described in terms of several embodiments, and that various changes and equivalents may be made to these features and embodiments by those skilled in the art without departing from the spirit and scope of the application. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the application without departing from the essential scope thereof. Therefore, it is intended that the application not be limited to the particular embodiment disclosed, but that the application will include all embodiments falling within the scope of the appended claims.

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 it comprises 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.

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 is characterized in that in step 1, the AZ31 magnesium alloy containing MgO and Al-Mn inclusions is sanded to 3000 mesh with sandpaper, and 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 is characterized in that the step 2 uses 20±5 g/L citric acid to pickle the magnesium alloy for 10~60 s to remove the oxide film on the surface of the magnesium alloy matrix, the pickling temperature is room temperature, and then washed with water 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 is characterized in that in step 3, 100 g/L NH ₄ HF ₂ is used to activate the pickled magnesium alloy for 1 to 4 min to form a uniform MgF ₂ protective film on its surface, followed by water washing 3 times, and 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/LK ₄ P ₂ O ₇ , 45±5 g/L ZnSO ₄ ·7H ₂ 0, 5±2 g/LNa ₂ CO ₃ , and 5±2 g/L KF, the zinc immersion temperature is 30~80 ℃, the zinc immersion time is 2~15 min, and the pH value of the zinc immersion solution is 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 is characterized in that the zinc stripping in step 5 is completed 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-immersed magnesium alloy product is evenly shaken during the zinc stripping process, or the liquid is evenly stirred.

7. The method for directionally removing Al-Mn inclusions from the surface layer of AZ31 magnesium alloy and bright zinc immersion according to claim 1, characterized in that the secondary zinc immersion in step 6 is carried out in a zinc immersion solution prepared by 140±10 g/LK ₄ P ₂ O ₇ , 45±5 g/L ZnSO ₄ ·7H ₂ O, 5±2 g/LNa ₂ CO ₃ , and 5g±2 g/L KF, the zinc immersion temperature is 30~80 ℃, the zinc immersion time is 5~20 min, the pH value of the zinc immersion solution is 10~10.5, and a uniform secondary zinc immersion layer with metallic luster is obtained after the secondary zinc immersion.

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⁺ , 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 , the plating solution temperature is 20-40 °C, and the electroplating time is 5-60 min.