CN102409383A - Magnesium Alloy Anodizing Method - Google Patents

Abstract

A magnesium alloy anode oxidation method uses electrolyte with the following components: 10-100g/L of sodium hydroxide or potassium hydroxide, 20-200g/L of sodium silicate or potassium silicate, 20-150g/L of sodium tetraborate or potassium tetraborate, saccharide and derivative additive thereof and water. The method comprises the following steps: pretreating the magnesium alloy; (2) anodizing the magnesium alloy: and (3) carrying out constant-current anodic oxidation under the stirring condition, wherein the power supply is a pulse power supply, the temperature of the electrolyte is controlled to be 5-40 ℃, and the time is 10-50 min. Because the electrolyte adopts the components, the solution is free of chromium, phosphorus and fluorine, and has no pollution to the environment. The addition of the additive saccharide substance into the basic electrolyte can slow down the reaction rate of magnesium and oxygen and the growth rate of an oxide film, so that the surface pores of the oxide film of the magnesium alloy are reduced, the surface smoothness is improved, and the corrosion resistance of the oxide film of the magnesium alloy is improved.

Description

Magnesium Alloy Anodizing Method

technical field

The invention relates to a magnesium alloy anodic oxidation method, which belongs to metal surface treatment technology. the

Background technique

Magnesium alloy has the advantages of low density, high specific strength, high specific stiffness, good shock absorption, electromagnetic shielding and radiation resistance, and has become a hot spot in the research and application of new materials. However, the corrosion resistance of magnesium alloy is the lowest among commonly used metal structural materials, and it is prone to galvanic corrosion and accelerated dissolution corrosion, which restricts the wide application of magnesium alloy. Surface treatment of magnesium alloys can effectively improve the corrosion resistance of alloy materials. The anodic oxidation technology of magnesium alloy can form an oxide film with good electrical insulation, strong binding force and good corrosion resistance on the surface of the material, which is the most promising surface treatment technology for magnesium alloy in industrial application. the

At present, most of the electrolytic solutions used in industrial production for surface treatment of magnesium alloys contain harmful elements such as Cr, P, and F, which seriously pollute the environment, and the treatment cost of waste liquid is high, which does not meet the requirements of sustainable development. Therefore, the environmentally friendly electrolyte and its technology without harmful elements such as Cr, P, and F have become the focus of current magnesium alloy anodic oxidation research. Environmentally friendly electrolyte systems mainly include KOH-Na ₂ SiO ₃ -NaAlO ₂ , NaOH-Al(OH) ₃ -Na ₂ SiO ₃ , NaOH-Na ₂ SiO ₃ -Na ₂ B ₄ O ₇ and other electrolyte systems. The surface roughness of the oxide film obtained by the environment-friendly electrolyte composed of inorganic salts is large, the micropore distribution is uneven, and the pore size is large, resulting in poor corrosion resistance, weak adhesion, uneven film formation, and easy cracks. Dealing with problems such as difficult voltage control and easy local ablation.

Contents of the invention

The present invention aims to propose a magnesium alloy anodic oxidation method, which can form an oxide film with uniform micropore distribution, small pore size, small roughness and good corrosion resistance on the surface of magnesium alloy, and the electrolyte meets the requirements of environmental protection without causing environmental pollution. Pollution, the surface treatment process of magnesium alloy is simple, the film formation is easy to control, and the raw materials are easy to obtain and cheap, so it is suitable for industrial production. the

In this magnesium alloy anodic oxidation method, the magnesium alloy is pretreated first, and then put into the electrolyte for anodic oxidation. The electrolyte used is composed of: sodium hydroxide or potassium hydroxide 10-100g/L, sodium silicate Or potassium silicate 20-200g/L, sodium tetraborate or potassium tetraborate 20-150g/L, additives and water, the additives are sugars and their derivatives glucose, lactose, sucrose, maltose, starch, dextrin , one or both of sorbitol and xylitol. the

In the electrolyte in this magnesium alloy anodic oxidation method, hydroxide is mainly used to adjust the solution resistance and pH value; silicate is the main film-forming agent, which participates in the anodic oxidation reaction of magnesium alloy to form insoluble compounds and make oxidation The film layer is thin and dense, and the uniformity is improved, which can significantly improve the corrosion resistance of the oxide film; borate is mainly used to increase the film forming speed of the oxide film and shorten the reaction time; the additive sugar and its derivatives are biopolyols , its single molecular structure contains multiple hydroxyl groups, and the lone pair of electrons on the hydroxyl group can combine with the empty orbital of the magnesium atom, so that it can be effectively adsorbed on the surface of the magnesium alloy, and play a shielding role on the surface of the magnesium alloy, thereby slowing down the magnesium alloy The reaction rate with oxygen and the growth rate of the oxide film make the compactness, uniformity and corrosion resistance of the oxide film significantly improved. the

Magnesium alloy anodizing method of the present invention compares with existing technology, mainly has the following advantages:

(1) Environmental protection. The electrolytic solution of the invention has no harmful substances such as chromium, fluorine, phosphorus, etc., and at the same time, the waste liquid is easy to handle, has no pollution to the environment, and belongs to a green and environment-friendly formula. the

(2) The additives are sugars and their derivatives, all of which are biomass compounds, which are non-polluting to the human body and the natural environment, and are easily decomposed by microorganisms. the

(3) An oxide film excellent in corrosion resistance can be obtained. The method of the invention can obtain smooth, dense, uniform and good corrosion resistance magnesium alloy anodic oxide film, and at the same time, the micropore distribution of the oxide film is uniform and the pore size is small. the

Description of drawings

Fig. 1 is the Tafel polarization curve of the AZ31 magnesium alloy sample and the AZ31 magnesium alloy substrate that are surface-treated through the method of the present invention, among the figure, a is the magnesium alloy sample curve without anodic oxidation, and b represents through the method of the present invention Treated magnesium alloy sample curve;

Figure 2 is a SEM photo of the surface morphology of the AZ31 magnesium alloy anodized film when starch is used as an additive;

Figure 3 is a SEM photo of the cross-sectional morphology of the anodized film of AZ31 magnesium alloy when starch is used as an additive. the

Detailed ways

The composition of the electrolyte used in this magnesium alloy anodic oxidation method is: sodium hydroxide or potassium hydroxide 10-100g/L, sodium silicate or potassium silicate 20-200g/L, sodium tetraborate or potassium tetraborate 20- 150g/L, additive and water, described additive is one or both in carbohydrate and its derivative glucose, lactose, sucrose, maltose, starch, dextrin, sorbitol and xylitol. the

When the additive is glucose, the addition amount can be 5-40g/L. the

When the additive is lactose, the added amount can be 5-60g/L. the

When the additive is sucrose, the added amount can be 10-40g/L. the

When the additive is maltose, the addition amount can be 10-50g/L. the

When the additive is dextrin, the added amount can be 5-50 g/L. the

When the additive is starch, the addition amount can be 5-50g/L. the

When the additive is sorbitol, the addition amount can be 10-40g/L. the

When the additive is xylitol, the addition amount can be 5-30g/L. the

When using the above electrolyte to anodize magnesium alloy, pulse power supply and constant current mode can be used. The specific steps are as follows: 

(1) Magnesium alloy pretreatment: grinding, drilling, cleaning with distilled water, ultrasonic cleaning with acetone, cleaning with distilled water, and drying. the

(2) Magnesium alloy anodization: use stainless steel as the cathode and magnesium alloy as the anode, under stirring conditions, constant current anodization, the temperature of the electrolyte is controlled at 5-40°C, and the time is 10-50min. The power supply used is a pulse power supply, its pulse frequency and duty ratio are continuously adjustable, the current density is 5mA/cm ² -40mA/cm ² , the frequency range is 100Hz-500Hz, and the duty ratio is 10%-30%.

Example 1

Magnesium alloy material: AZ31 magnesium alloy plate

The magnesium alloy material is made into a sample and undergoes the following pretreatments: grinding, drilling, cleaning with distilled water, ultrasonication with acetone, cleaning with distilled water, and drying. the

The above-mentioned magnesium alloy samples (a total of 7 samples) were anodized in the electrolyte described in Table 1, the anodizing conditions were: pulse power supply, constant current mode, current density 10mA/cm ² , anodizing time 15min , The pulse frequency is 100Hz, the duty cycle is 10%, and the temperature is controlled at 10-30°C. The whole anodizing process is carried out under stirring conditions.

Table 1 Electrolyte composition

Sample No 1 2 3 4 5 6 7 NaOH(g/L) 45 45 45 45 45 45 45 Na ₂ SiO ₃ (g/L) 70 70 70 70 70 70 70 Na ₂ B ₄ O ₇ (g/L) 90 90 90 90 90 90 90 Glucose (g/L) 10 the the the the the the Sucrose (g/L) the 10 the the the the the Maltose the the the the 15 the the Dextrin(g/L) the the 20 the the the the Lactose (g/L) the the the 15 the the the Sorbitol (g/L) the the the the the 15 the Xylitol (g/L) the the the the the the 20

Performance evaluation of magnesium alloy anodized film: see Table 2. the

Table 2 Magnesium alloy anodic oxidation film properties

The salt spray test is carried out according to ASTMB117 and ASTMB398 standards. The test temperature is 35.5±0.5°C, and the corrosion medium used is 5% NaCl solution with pH=7. Corrosion spots appeared on the AZ31 magnesium alloy without anodic oxidation treatment in salt spray for 8 hours; this program treated the AZ31 magnesium alloy in salt spray for 130 hours, and no corrosion spots appeared on the surface of the sample. the

Example 2

Magnesium alloy material: AZ31 magnesium alloy plate

The magnesium alloy material is made into samples and undergoes the following pretreatments: grinding, drilling, cleaning with distilled water, ultrasonication with acetone, cleaning with distilled water, and drying. the

Electrolyte composition:

NaOH: 45g/L,

Na ₂ SiO ₃ : 50g/L,

Na ₂ B ₄ O ₇ : 70g/L,

Glucose: 15g/L. the

The magnesium alloy sample was anodized in the above electrolyte, pulsed power supply, constant current mode, and 5 different anodizing process conditions as shown in Table 3 were used for anodizing. the

Table 3 Magnesium alloy anodizing process conditions

Sample No 1 2 3 4 5 Current density (mA/cm ² ) 5 10 10 20 40 Oxidation time (min) 25 15 20 10 10 Pulse frequency (Hz) 100 100 200 500 100 Duty cycle (%) 20 10 10 30 10

The performance evaluation of the obtained magnesium alloy anodized film is shown in Table 4. the

Table 4 Magnesium alloy anodic oxidation film properties

The salt spray test is carried out according to ASTMB117 and ASTMB398 standards. The test temperature is 35.5±0.5°C, and the corrosion medium used is 5% NaCl solution with pH=7. Corrosion spots appeared on the AZ31 magnesium alloy without anodic oxidation treatment in salt spray for 8 hours; this program treated the AZ31 magnesium alloy with salt spray for 120 hours, and no corrosion spots appeared on the surface of the sample

Example 3

Magnesium alloy material: AZ31 magnesium alloy plate

The magnesium alloy material is made into a sample and undergoes the following pretreatments: grinding, drilling, cleaning with distilled water, ultrasonication with acetone, cleaning with distilled water, and drying. the

Electrolyte composition:

NaOH: 45g/L,

Na ₂ SiO ₃ : 50g/L,

Na ₂ B ₄ O ₇ : 70g/L,

Soluble starch: 15g/L. the

The magnesium alloy samples were anodized in the above electrolyte. The anodizing conditions are: pulse power supply, constant current mode, current density 10mA/cm ² , anodizing time 20min, pulse frequency 200Hz, duty cycle 10%, temperature controlled at 5-40°C and the whole anodizing process is under stirring conditions next.

Oxide film appearance and performance: Magnesium alloy anodic oxide film is off-white, smooth, uniform and dense; the film thickness is 15-17 μm, and the roughness is 0.326 μm. the

The non-anodized magnesium alloy and the anodized magnesium alloy were evaluated by Tafel polarization curve in 3.5% sodium chloride solution, and the results are shown in Figure 1. In the figure, a represents a magnesium alloy sample without anodic oxidation, and b represents a magnesium alloy sample treated by the method of the present invention. a The corrosion potential is -1.496V, the self-corrosion current density is 338.84μA/cm ² , the corrosion potential of the optimized AZ31 magnesium alloy oxide film is -1.215V, the corrosion potential is positively shifted by 0.281V, and the self-corrosion current density is 0.14μA/cm 2 cm ² . The surface and cross-sectional morphology of the magnesium alloy workpiece are shown in Figure 2 and Figure 3. It can be seen from the figure that the oxide film is uniform and dense.

The salt spray test is carried out according to ASTMB117 and ASTMB398 standards. The test temperature is 35.5±0.5°C, and the corrosion medium used is 5% NaCl solution with pH=7. Corrosion spots appeared on the AZ31 magnesium alloy without anodic oxidation treatment in salt spray for 8 hours; this program treated the AZ31 magnesium alloy in salt spray for 150 hours, and no corrosion spots appeared on the surface of the sample. the

Example 4

Magnesium alloy material: AZ31 magnesium alloy plate. the

The magnesium alloy material is made into a sample and undergoes the following pretreatments: grinding, drilling, cleaning with distilled water, ultrasonication with acetone, cleaning with distilled water, and drying. the

Electrolyte composition:

NaOH: 45g/L,

Na ₂ SiO ₃ : 50g/L,

Na ₂ B ₄ O ₇ : 70g/L,

Soluble starch: 15g/L. the

Anodizing process: pulse power supply, constant current mode, current density 25mA/cm ² , anodizing time 25min, pulse frequency 100Hz, duty cycle 10%.

Obtain a gray-black, uniform oxide film with a film thickness of 55-70 μm. the

The unanodized magnesium alloy and the anodized magnesium alloy were evaluated by Tafel polarization curve in 3.5% sodium chloride solution. The corrosion potential of the magnesium alloy substrate was -1.496V, and the self-corrosion current density was 338.84 μA/cm ² , the corrosion potential of the black oxide film of the magnesium alloy is -1.284V, the corrosion potential is positively shifted by 0.112V, and the self-corrosion current density is 0.32μA/cm ² .

The salt spray test is carried out according to ASTMB117 and ASTMB398 standards. The test temperature is 35.5±0.5°C, and the corrosion medium used is 5% NaCl solution with pH=7. Corrosion spots appeared on the AZ31 magnesium alloy without anodic oxidation treatment in salt spray for 8 hours; this program treated the AZ31 magnesium alloy with salt spray for 130 hours, and no corrosion spots appeared on the surface of the sample. the

Example 5

Prepare the electrolyte according to Table 5, and perform anodic oxidation treatment on the surface of the AZ31 magnesium alloy. the

Table 5 Electrolyte formula

Sample No 1 2 3 4 5 6 7 8 9 NaOH(g/L) 20 20 45 45 30 25 the the the KOH(g/L) the 50 the the 30 40 65 65 100 Na ₂ SiO ₃ (g/L) the 70 the 30 70 the 50 100 the K ₂ SiO ₃ (g/L) 100 the 80 60 the 90 40 the 200 Na ₂ B ₄ O ₇ (g/L) the 80 60 20 90 40 90 90 the K ₂ B ₄ O ₇ (g/L) 150 the 20 70 the 60 the the 70 Glucose (g/L) 5 the 40 the the the the the the Sucrose (g/L) the 40 the 5 the 15 the the the Maltose (g/L) the the the the 10 the the 50 the Lactose (g/L) the 5 the 40 the the the the the Starch (g/L) the the 5 the the the the the 60 Dextrin(g/L) 50 the the the the 10 the the 5 Sorbitol (g/L) the the the the 40 the 10 the the Xylitol (g/L) the the the the the the 30 5 the

The performance evaluation of the obtained magnesium alloy anodized film is shown in Table 6

Table 6 Magnesium alloy anodic oxidation film properties

The salt spray test is carried out according to ASTMB117 and ASTMB398 standards. The test temperature is 35.5±0.5°C, and the corrosion medium used is 5% NaCl solution with pH=7. Corrosion spots appeared on the AZ31 magnesium alloy without anodic oxidation treatment in salt spray for 8 hours; this program treated the AZ31 magnesium alloy in salt spray for 110 hours, and no corrosion spots appeared on the surface of the sample. the

Claims (10)

1. anodic oxidation of magnetism alloy method; With the magnesiumalloy pre-treatment; Put into electrolytic solution then and carry out anodic oxidation; It is characterized in that consisting of of used electrolytic solution: sodium hydroxide or Pottasium Hydroxide 10-100g/L, water glass or potassium silicate 20-200g/L, sodium tetraborate or potassium tetraborate 20-150g/L, additive and water, described additive are one or both in carbohydrate and verivate glucose, lactose, sucrose, SANMALT-S, starch, dextrin, sorbyl alcohol and the Xylitol.

2. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a glucose, and add-on is 5-40g/L.

3. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a lactose, and add-on is 5-60g/L.

4. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a sucrose, and add-on is 10-40g/L.

5. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a SANMALT-S, and add-on is 10-50g/L.

6. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a dextrin, and add-on is 5-50g/L.

7. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a starch, and add-on is 5-50g/L.

8. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is a sorbyl alcohol, and add-on is 10-40g/L.

9. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described additive is an Xylitol, and add-on is 5-30g/L.

10. anodic oxidation of magnetism alloy method as claimed in claim 1 is characterized in that described magnesiumalloy pre-treatment is: polishing, boring, zero(ppm) water clean, acetone is ultrasonic, zero(ppm) water cleans, dries up; Described anode oxidation process is: use stainless steel to be negative electrode, magnesiumalloy is an anode, under agitation condition, and the constant current anodic oxidation, used power supply is the pulse power, and its pulse-repetition and dutycycle have continuously adjustable, and current density is 5mA/cm ²～50mA/cm ², range of frequency is 100Hz～500Hz, and dutycycle is 10%～30%, and electrolyte temperature is controlled at 5～40 ℃, and the time is 10～50min.

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