KR20180088966A - Process for surface treatment of magnesium - Google Patents

Abstract

The present invention relates to a surface treatment method of forming an oxide film on magnesium or magnesium alloy. The surface treatment method of the present invention comprises following steps of: preparing a magnesium material; mechanically polishing the surface of the magnesium material to remove a native oxide or contaminant from the surface of the magnesium material; immersing the magnesium material in acetone and applying ultrasonic waves to the magnesium material; acid treating the magnesium material with citric acid, sulfuric acid or oxalic acid at room temperature; forming the oxide film on the surface of the magnesium material by anodizing; and hydrating and sealing the surface of the magnesium material having the oxide film formed thereon.

Description

{PROCESS FOR SURFACE TREATMENT OF MAGNESIUM}

More particularly, the present invention relates to a method for forming an oxide film on the surface of magnesium or magnesium alloy by anodic oxidation.

The magnesium or magnesium alloy has a density of 1.8 g / cm, which is about two-thirds of that of the aluminum alloy. The magnesium or magnesium alloy has the lowest density among the commercial structural alloys and has excellent nodal strength and inelasticity coefficient. great. Also, it is excellent in absorbability against vibration and impact, has excellent electric and thermal conductivity, workability, fatigue at high temperature, impact property, and has an electromagnetic wave shielding property that can efficiently shield electromagnetic waves generated in various electric device parts.

Hereinafter, throughout this specification, magnesium and a magnesium alloy are collectively referred to as magnesium, and magnesium is defined as including a magnesium alloy.

Such magnesium has recently been widely applied to lightweight products such as computers, mobile phones, automobile parts, precision electronic device parts, and home appliances.

However, since magnesium is an active metal having a very low standard electrode potential of 2.37 V, it is easily oxidized from an acidic to a weakly alkaline region. When magnesium is contacted with another metal having a high standard electrode potential, magnesium is weak in corrosion resistance and low in hardness, It is a drawback.

For these reasons, although magnesium has many advantages, practical use of magnesium as a single material is severely limited. Therefore, in order to commercialize magnesium as a commercialization material, it is required to treat the surface of the product chemically, electrochemically, or physically to improve the corrosion resistance by improving the corrosion resistance.

The surface treatment of magnesium can be roughly divided into chemical method, electrochemical method and physical method.

The chemical method is a chemical conversion method in which a magnesium product is immersed in a chemical and the surface of the surface is chemically reacted to form a corrosion-resistant surface film.

Electrochemical methods include anodizing, which forms an oxide film with corrosion on the surface by applying electricity in a solution, and a plating method of applying a corrosion-resistant metal film on the surface.

As a physical surface treatment method, there is a coating method for applying a protective polymer layer on the surface.

Among these methods, the chemical treatment method, the anodic oxidation method and the plating method corresponding to the chemical and electrochemical methods have been known as the most advanced surface treatment methods of magnesium, and as a field requiring a high level of skill and knowledge in the treatment method and the principle aspect, Many studies have been actively conducted.

The anodic oxidation method refers to a method in which a metallic material to be surface-treated is made into an anode in an electrolytic solution and a coating film of the metal oxide is produced by an electrochemical method. The anodic oxidation is the most typical material of the anodic oxidation, and anodic oxidation can be performed on metal materials such as Mg, Zn, Ti, Hf and Nb. In recent years, the anodic oxidation treatment of magnesium and titanium materials is also increasingly used have.

The film obtained through the anodic oxidation is extremely hard, has a high corrosion resistance, becomes extremely porosity and fibrous, and can be colored with a desired color. It is used for the practicality of abrasion resistance and cosmetic treatment.

The structure of the film is different in the anodic oxidation process using various electrolytes. The reactions at anode and cathode by anodic oxidation are as follows.

To anodize magnesium, an anodic current is applied to form a minute film of magnesium hydroxide at a very low voltage, and when a voltage of about 10 V is applied, a magnesium oxide layer is formed. When the oxide layer is formed, resistance is increased, internal stress is concentrated on the magnesium oxide layer, the oxide layer is destroyed at a voltage of about 70 V, and a second porous oxide layer is formed when the voltage is raised again. The anodic oxidation of magnesium requires higher voltage than the anodic oxidation of aluminum.

When an anodized film is formed on the magnesium surface by the anodic oxidation, the corrosion resistance is improved, the appearance of the magnesium product is improved by the gloss of the anodized film, and the wear resistance of the magnesium is weak. In addition, tightly anodized coatings provide a chemically active surface in all paint systems to improve paintability. In addition, since the organic dye can be adsorbed in the pores of the coating film in the anodic oxide coating, coloring can be performed in various colors, so that the paintability is improved.

However, before the surface treatment, the magnesium material has various contaminants, high-temperature oxides, and naturally formed surface oxide films formed in processes such as machining, casting, and plastic working. These contaminants impair the formation of a useful oxide film by anodic oxidation and therefore need to be removed prior to surface treatment.

Document 1: Patent Publication No. 10-1440555 Document 2: Published Patent Application No. 10-2011-0100848

The present invention provides a surface treatment method for enhancing the corrosion resistance of magnesium having high corrosion resistance.

Particularly, the present invention aims to provide a pretreatment method for eliminating factors that inhibit the formation of various contaminants or oxide films on the surface accompanied by anodizing treatment as one method of enhancing corrosion resistance of magnesium.

The inventors of the present invention have conducted various experiments and researches to solve the above-mentioned problems, so that it is possible to obtain a good quality oxide film by anodic oxidation of magnesium by performing various pretreatments before formation of an oxide film by anodic oxidation of magnesium The conclusion is reached. Such pretreatment involves mechanical polishing and acid treatment.

A surface treating method for forming an oxide film on magnesium according to the present invention comprises the steps of: preparing a magnesium material; Mechanically polishing the surface of the magnesium material to remove the native oxide or contaminant from the surface of the magnesium material; Immersing the magnesium material in acetone and washing by applying ultrasonic waves; Treating the magnesium material with citric acid, sulfuric acid or oxalic acid at room temperature; Forming an oxide film on the surface of the magnesium material by an anodic oxidation treatment; And hydrating and sealing the surface of the magnesium material having the oxide film formed thereon.

In the magnesium surface treatment method according to the present invention, an oxide film of excellent quality is formed by mechanically polishing the magnesium surface and acid-treating it with citric acid and then forming an oxide film by anodic oxidation.

1 is a view showing a surface state of a magnesium alloy processed according to an embodiment of the present invention.
2 is a graph of X-ray diffraction analysis of a magnesium alloy specimen processed according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. A method of forming an oxide film on a magnesium specimen according to a surface treatment method of the present invention and a surface treatment method of a comparative example The test results for the oxide film of magnesium will be described.

Preparation of Magnesium Specimens

The magnesium specimen was an AZ31B alloy with the following composition: a plate with a thickness of 1 mm was cut into a size of 70 mm x 40 mm, exposed to an area of 40 mm x 40 mm, and subjected to anodic oxidation with or without acid treatment. For the cathode, the STS304 specimen was also cut into a size of 70 mm x 40 mm.

Aluminum (Al): 2.5 - 3.5 wt%

Copper (Cu): not more than 0.05% by weight

Iron (Fe): 0.005 wt% or less

Magnesium (Mg): Balance

Manganese (Mn): not less than 0.2% by weight

Nickel (Ni): 0.005 wt% or less

Silicon (Si): 0.1% by weight or less

Zinc (Zn): 0.6 - 1.4 wt%

Among the magnesium alloys, AZ91 series alloys have been extensively studied for anodic oxidation, but alloys of AZ31 series are suitable as materials for various parts due to their high elongation, but are relatively less studied for anodic oxidation.

Anodic oxidation treatment

Magnesium specimens were polished with # 1000, 2000 SiC abrasive paper to physically remove oxide films and contaminants from the surface of the material. Then, the polished magnesium specimen was immersed in acetone so as to prevent natural oxidation on the surface, and ultrasonic washing was conducted for 1 minute.

For the cleaning, acid treatment, anodic oxidation and sealing were carried out sequentially. Between each step, the previous step was rinsed with reverse osmosis pure water for 10 seconds to remove residual solution on the magnesium surface.

After anodizing, the sealing was immersed in distilled water at 100 ° C for 10 minutes, and after sealing, it was dried through natural drying.

Concentrations and treatment conditions of the chemicals used in the acid treatment and the anodizing process are shown in Table 1.

fair Furtherance density.
(g / L) Temperature
(° C) Immersion time (min) Pickling oxalic acid dehydrate, 99.5% (Comparative Example 1) 10 25 60 citric acid Monohydrate, 99.5% (this example) Sulfuric acid, 98.0% (Comparative Example 2) Anodic oxidation Potassium hydroxide Flake, 93.0%, 50 25 900 shilling D.I water 0.2 μm filter 100 600

The anodic oxidation was performed a plurality of times with different conditions as shown in Table 2 below.

Experimental order fair Remarks Primary Anodic oxidation KOH electrolyte, 25 DEG C, 20 min, 60 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (40 mm x 40 mm to 1T thickness)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) Irregularity when visually confirmed
Anodizing process confirms with multimeter Secondary Anodic oxidation KOH electrolyte, 25 DEG C, 30 min, 60 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (40 mm x 40 mm to 1T thickness)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) Irregularity when visually confirmed Third Anodic oxidation KOH electrolyte, 25 DEG C, 30 min, 10 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (40 mm x 40 mm to 1T thickness)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) Low voltage results in very thin oxide layers.
Melting of Specimen at 120V when Anodizing in Materials Laboratory Fourth Anodic oxidized KOH electrolyte, 25 DEG C, 15 min, 120 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (40 mm x 40 mm to 1T thickness)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) By cooperating with power supply
120V in the school.
Convert to 100V with arc generation 5th Anodic oxidation KOH electrolyte, 25 DEG C, 15 min, 100 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (40 mm x 40 mm to 1T thickness)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) There is no unevenness in the naked eye, but the front and back sides are different. Six Anodic oxidation KOH electrolyte, 25 DEG C, 15 min, 100 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (10mm X 40mm in thickness of 1T)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) Change the size of the specimen - Check the same thing with the naked eye Seventh Anodic oxidized KOH electrolyte, 25 DEG C, 30 min, 100 V
Specimen - (pickling x oxalic acid sulfuric acid citrate), (10mm X 40mm in thickness of 1T)
(Citric acid: 2 g / 200 mL, oxalic acid: 2 g / 200 mL, sulfuric acid: 2 g / 200 mL) Anodizing film is an island growth mechanism
Experiment by increasing the time 8th car Carrying out coloring under the same condition as 7th Check coloring by boiling water

Anodizing was performed after dividing the magnesium alloy AZ31B according to the acid treatment. The surface was analyzed by Scanning Electron Microscopy (SEM; JSM-6500F, JEOL, Japan) and X-ray diffraction (XRD; Siemens D-5005) The morphology of the film was observed at X1000 magnification and the crystal structure was measured and analyzed at a rate of 2 ° per minute. The thickness of the film was measured by using an eddy current using a film thickness meter (CM-8826FN).

The electrochemical analysis of the oxide film of AZ31B which had been subjected to the anodic oxidation process was carried out by immersing it in an aqueous solution of 3.5 wt% NaCl at 25 ° C for 16 cm ² . The analysis was carried out using the weight change rate for 3 days in 24 hour unit. In order to measure the resistance of the passive film, the frequency range was measured from 1 KHz to 1000 KHz using Impedance / gain-phase analyzer (SI1260, SCHLUMBERGER Solartron) and the current was applied at 10 mV. Finally, the hardness test was conducted using a micro Vickers tester to verify the formation of anodizing film.

The resistance of AZ31B alloy and AZ31B alloy subjected to anodizing were measured to determine the success of anodizing.

In anodized specimens, the resistance value was very high, 0.00 Ω, and 0.029 Ω in the control group. We confirmed the success of anodizing using a multimeter. The resistivity of AZ31B control was measured, but it was impossible to measure with 4 point probe measured up to KΩ for anodizing treated specimen. Coloring was carried out in red color and immediately after the anodic oxidation, the dye was dipped in the aqueous solution at a rate of 25 g / L. Initially, the dye was dissolved in D.I Water at 25 ° C, but the color was lost and the shape was not clear. Successful experiments were conducted by dissolving the dye in boiling water at 100 ° C.

Subsequently, the oxidized surface was analyzed using a scanning electron microscope.

First, the surface shape according to the presence or absence of 90 V pickling is shown in Fig. (c) is a specimen treated with oxalic acid according to Comparative Example 1, and (d) is a specimen treated with oxalic acid according to Comparative Example 2, Sulfuric acid treated specimens are shown.

1, it can be seen that all the specimens have an outer layer and an inner layer, so that the surface is uneven and the inner layer is formed in the short period of anodic oxidation and is denser than the outer layer .

Compared with the acid treatment, the surface roughness of the acid treated and non-acid treated specimens varies to such an extent that they can be distinguished visually by SEM photographs. For this reason, it is possible to form a dense film by inhibiting the generation of hydrogen gas during anodizing through an acid treatment, and foreign substances which are not dissolved or not emulsified in the oxide, the processing oil or water present on the surface are removed and the irregularities are relatively reduced, Respectively.

Next, X-ray diffraction analysis was performed on each specimen, and the results are shown in the graph of FIG. Figure 2 shows the XRD peak results for each control group of magnesium, untreated, sulfuric, citric, oxalic acid, and treated specimens.

The black is the peak of the untreated magnesium specimen, and the other colors are the anodized samples using the different acids. The highest peaks are the peaks of magnesium, and peaks with a color as compared to black show a decrease in magnesium content.

The oxidation film is too thin and its amount is too small compared to magnesium, so it is not known what XRD peak is, but the amount of magnesium is reduced, so that magnesium combines with other substances to form a compound, It can be inferred that the amount of magnesium is reduced.

In the case of anodic oxidation, the solubility of oxalic acid in water is relatively lower than that of the other two acids. In this case, oxalic acid is ionized (chelated) on the magnesium surface and the anodic oxidation film is not well adhered to the magnesium specimen. Is not achieved. The intensity of XRD is proportional to the quantity of the specimen. As shown in the XRD graph below, the XRD intensities of oxalic acid are relatively higher than those of other acid-treated XRD intensities. Therefore, it is believed that magnesium oxalate treated with oxalic acid has a relatively high amount of Mg due to anodic oxidation.

Next, the thickness of the anodized film was measured using a film thickness meter. The difference between the base of the AZ31B alloy and the current characteristics of anodizing was used, and the detailed results are shown in Table 3 below.

Psalm / Order One 2 3 4 5 Average Mg control 5.1 탆 6.1 ㎛ 8.5 탆 11.3 탆 5.1 탆 7.22 탆 Sansome City 14.5 탆 24.1 탆 26.2 탆 14.6 탆 22.2 탆 20.32 탆 Sulfate column 21.2 탆 25.1 탆 18.9 탆 22.0 탆 24.6 탆 22.36 탆 Oxalic acid wash 28.3 탆 32.3 탆 29.4 탆 16.5 탆 26.9 탆 26.68 탆 Citric acid wash 26.2 탆 27.2 탆 23.0 탆 26.6 탆 30.4 탆 26.68 탆

Thickness measurements showed the lowest thickness in the Mg control group, and the thickness of the other anodized specimens increased. However, it was concluded that the standard deviation was large in the comparison with the pickling solution, and the accuracy of the coating thickness was deteriorated depending on the pickling solution.

As an electrochemical analysis of the specimens, immersion experiments were carried out by exposure to 16 cm ² of a 3.5 wt% NaCl aqueous solution at 25 ° C. The analysis was carried out using the weight change rate for 3 days in 24 hour unit. Weight reduction and averages are shown in Table 4.

In the immersion experiment of NaCl 3.5 wt% aqueous solution, the anodized magnesium specimen showed less weight loss than the experimental control. The anodized magnesium specimen appears to have less oxidation due to the formation of a coating layer with a ceramic layer. As a result, the corrosion resistance of magnesium was improved.

The specimens pickled with oxalic acid seem to show a larger weight change than the other specimens. In the case of anodic oxidation, oxalic acid is relatively low in solubility in water compared to the other two acids. In this case, oxalic acid is ionized on the magnesium surface, and the anodic oxidation film is not easily adhered to the magnesium sample. Therefore, the specimens treated with oxalic acid do not have anodic oxidation well, and the corrosion of the ceramics layer formed through anodic oxidation is thinner than other specimens.

Initial Weight Day 1
Mass (decrease) Day 2
Mass (decrease) Day 3
Mass (decrease) Weight reduction average g / year Control group 5.479 g 5.471 g
(0.008) 5.461 g
(0.010) 5.440 g
(0.021) 13 mg / day 4.745 Pickling 5.208g 5.202 g
(0.006) 5.196 g
(0.004) 5.186 g
(0.01) 7 mg / day 2.555 Sulfuric acid wash 5.228 g 5.226 g
(0.002) 5.222 g
(0.004) 5.220 g
(0.002) 3 mg / day 1.095 Citric acid wash 5.215 g 5.214 g
(0.001) 5.211 g
(0.003) 5.209 g
(0.002) 2 mg / day 0.730 Oxalic acid wash 5.238 g 5.234 g
(0.004) 5.230 g
(0.004) 5.222 g
(0.008) 5 mg / day 1.825

As shown in the above results, citric acid showed the most excellent effect in the pickling prior to the formation of the oxide film by anodic oxidation, followed by sulfuric acid and oxalic acid in that order. The acid cleaning treatment has a great influence on the anodization and contributes to the improvement of corrosion resistance.

Claims (1)

A surface treatment method for forming an oxide film on magnesium,
Preparing a magnesium material;
Mechanically polishing the surface of the magnesium material to remove the native oxide or contaminant from the surface of the magnesium material;
Immersing the magnesium material in acetone and washing by applying ultrasonic waves;
Treating the magnesium material with citric acid, sulfuric acid or oxalic acid at room temperature;
Forming an oxide film on the surface of the magnesium material by an anodic oxidation treatment; And
A step of hydrating and sealing the surface of the magnesium material having the oxide film formed thereon
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