KR100853996B1 - Method for Treating the Surface on Magnesium and Its Alloys - Google Patents

Abstract

The present invention relates to a method for treating a surface of a metal body containing magnesium as a main component, the main configuration of which comprises: pretreating the surface of the metal body containing magnesium as a main component; Forming a coating layer on the surface of the metal body pretreated in the step; And forming one of a coating layer and a plating layer on the coating layer formed in the step. In the step of pretreating the surface of the metal body, the surface roughness of the metal body is 0.1 on the average using a sandblasting method. Maintaining at least 20 μm or less, and in the step of forming the coating layer, the coating layer is immersed in the electrolyte solution spaced apart from the electrode facing the metal body, each of which has a different polarity applied to the electrode facing the metal body The current waveform of the applied current is formed on the surface of the metal body by alternating the current waveform in the forward direction and the current waveform in the reverse direction with time, and the surface resistance of the coating layer is 0.001 Ω / sq. 2 Ω / sq. Or less, and the thickness of the coating layer is 0.1 μm or more and 10 μm or less.

Surface Treatment, Anodizing, Magnesium, Adhesive, Conductive

Description

Method for Treating the Surface on Magnesium and Its Alloys}

1 is a surface photograph photographed by a scanning electron microscope of a surface treated by a surface treatment method of a metal body containing magnesium as a main component using a conventional anodizing method,

FIG. 2 is a surface photograph taken by a scanning electron microscope of a surface treated by a surface treatment method of a metal body containing magnesium as a main component using a conventional non-chromate method,

3 is a photograph taken with a scanning electron microscope of the surface after coating in the method for surface treatment of a metal body mainly composed of magnesium according to the present invention.

The present invention relates to a surface treatment method of a magnesium metal body containing magnesium as a main component, in particular, a surface treatment method of a magnesium metal body containing magnesium as a main component to ensure that the surface of the metal body is excellent in corrosion resistance and paint adhesion, while It is about.

Magnesium has the lowest specific gravity (1.74) among structural metal materials, has a high specific strength, and is excellent in castability, machinability, dimensional stability, scratch resistance, etc. Increasingly, the use is increasing and the demand is increasing, and the demand for precision electronic components such as mobile phones, laptops, digital cameras, and various audio devices is rapidly increasing.

However, magnesium is the most chemically active metal among commercial metals, and its standard electrode potential is 2.363V NHE. In general, magnesium exhibits corrosion resistance in air or in solution when it is not surface treated. It is indispensable and in order to enhance the value of the product, it is generally used by coating the surface of magnesium metal body.

Therefore, various methods are currently used to make the surface of the magnesium metal body corrosion resistant and have a good coating adhesion with the coating layer, but among them, as a widely used technique, (1) Anodizing (2) Neutralization (3) Washing (4) Drying (5) Surface adjustment (6) Washing (7) Drying (8) Primer coating (9) Drying process is a surface pretreatment method.

However, the surface pretreatment method of the above anodization treatment is made of several steps, resulting in a decrease in productivity, difficulty in automating the process consistently, and inferior in workability. Not only is it a factor that greatly increases the defective rate of the product and there is a problem that a large amount of environmental waste occurs.

In view of the above, the present inventor simplified the complicated process generated during the pretreatment of the coating by the anodization method in Patent Application No. 10-2006-0028749 to lower the manufacturing cost, and wastewater of a solution containing various acids, alkalis and other heavy metals. In addition to eliminating treatment problems, poor working environment, etc., a method of surface treatment of a metal body containing magnesium as a main component excellent in corrosion resistance and paint adhesion has been proposed.

However, when a metal body mainly composed of magnesium is used as a case of an electronic device such as a mobile phone, a notebook PC, a digital camera, a camcorder, and an audio, the case itself must serve as an electrical grounding and at the same time, data communication within the device. In order to remove the noise of electromagnetic waves that may occur during the operation, the electromagnetic shielding role should also be performed. In order to perform this role, the surface inside the case should be electrically conductive so that current can flow easily. However, although the method of patent application 10-2006-0028749 filed by the inventor of the present invention can improve the adhesion of the coating layer formed on the surface of the metal body, the current conduction is required because the current conduction is not secured. There is a problem that is not useful when applying the above method to some electronic devices.

On the other hand, the non-chromate method has been proposed as a surface treatment method for imparting electrical conductivity to the surface of the metal body, but this method is not only poor in paint adhesion between the surface of the metal body and the coating layer, but also toxic. Since a strong chemical solution must be used, the working environment is poor and the formed coating layer itself has poor wear resistance.

Therefore, there is a need for a method of surface treatment of a metal body containing new magnesium as a main component that can solve the above problems.

The object of the present invention is to reduce the manufacturing cost by simplifying the complicated process generated during the pretreatment of the coating by anodization, to eliminate the problem of wastewater treatment of the solution containing various acids, alkalis and other heavy metals, poor working environment, etc. At the same time, there is provided a method for surface treatment of a metal body having magnesium as a main component, which is excellent in corrosion resistance and paint adhesion, and which can secure current conduction in the formed coating layer.

In order to achieve the above object, a method for surface treatment of a metal body containing magnesium as a main component comprises: pretreating the surface of the metal body containing magnesium as a main component; Forming a coating layer on the surface of the metal body pretreated in the step; And forming one of a coating layer and a plating layer on the coating layer formed in the step. In the step of pretreating the surface of the metal body, the surface roughness of the metal body is 0.1 on the average using a sandblasting method. Maintaining at least 20 μm or less, and in the step of forming the coating layer, the coating layer is immersed in the electrolyte solution spaced apart from the electrode facing the metal body, each of which has a different polarity applied to the electrode facing the metal body The current waveform of the applied current is formed on the surface of the metal body by alternating the current waveform in the forward direction and the current waveform in the reverse direction with time, and the surface resistance of the coating layer is 0.001 Ω / sq. 2 Ω / sq. Or less, and the thickness of the coating layer is 0.1 μm or more and 10 μm or less.

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It is preferable that the said electrolyte solution contains potassium hydroxide and sodium silicate.

The electrolyte may also contain sodium hydroxide and sodium silicate.

Hereinafter, a method of surface treatment of a metal body based on magnesium according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

According to the present invention, there is provided a method for treating a surface of a metal body containing magnesium as a main component, forming a coating layer on the surface of the metal body pretreated in the step, forming a coating layer on the coating layer formed in the step. Forming any one of the plating layer.

In the step of pretreating the surface of the metal body, the surface roughness of the metal body is maintained at 0.1 μm or more and 20 μm or less on average.

When the average value of the surface roughness is less than 0.1 μm, the surface may be too smooth to damage the coating adhesion after the subsequent coating layer is formed. When the average value of the surface roughness exceeds 20 μm, the coating formed on the surface of the metal body The shape of the layer surface becomes rough.

Various methods may be used as a method of pretreating the surface of the metal body, and a sandblast method may be used as a specific example.

In the forming of the coating layer, the coating layer, immersed the electrode facing the metal body in the electrolyte spaced apart, and energized so that different polarities are applied to the electrode facing the metal body, respectively, of the applied current The waveform is formed on the surface of the metal body by causing the current waveform in the forward direction and the current waveform in the reverse direction to appear alternately with time. At this time, by appropriately adjusting the electrolyte solution, the applied voltage, the applied current density and the applied frequency, the thickness and the surface resistance of the desired coating layer can be obtained.

It is preferable that the said metal body consists of magnesium or its alloy. The shape of the metal body does not need to be particularly limited.

The material of the electrode is generally corrosion-resistant, electrochemically stable material is used, a representative material is stainless steel, titanium, nickel or graphite may be used as the electrode material. In the present invention, stainless steel is used as an electrode material, and a metal body having magnesium as its main component corresponds to the opposite electrode. When the current is used as an alternating current, the polarity of each electrode is not determined. In the case of a direct current, stainless steel is used as the cathode, and a metal body containing magnesium as the main component is used as the anode.

The electrolyte consists essentially of a small amount of strongly alkaline aqueous solution and a combination of one or more of all kinds of water-soluble salts that can serve as a component of the coating layer. The electrolyte according to the present invention preferably contains potassium hydroxide (KOH) and sodium silicate (Na ₂ O—nSiO ₂ , n = 1 to 4). In addition, a strong alkaline solute such as sodium hydroxide (NaOH) may be used instead of potassium hydroxide, and the content of the electrolyte may be about 1 to 100 g / L. Here, instead of sodium silicate, a weakly alkaline solute such as sodium aluminate (Na ₂ AlO ₃ ) may be used.

In order to form the coating layer on the surface of the metal body in the electrolyte solution, an alternating current or direct current may be applied to the metal body and the electrode having different polarities. In this case, if the voltage, current density, and frequency applied are appropriately adjusted, a desired coating layer can be obtained. The appropriate applied voltage is in the range of 100 to 600 V. If the applied voltage is 100V or less, the coating layer having electrical conductivity is not formed, and when the voltage is 600V or higher, the coating layer is formed, but the electrical conductivity is not given, and economic efficiency is reduced due to excessive power consumption. Appropriate applied current density is preferably in the range of 0.5 to 30 A / dm ² . When the applied current density is 0.5 A / dm ² or less, it takes a long time to form a coating layer, so that the productivity is reduced, and when it is 30 A / dm ^{2 or} more, the economy is inferior due to excessive power consumption. The frequency applied is usually 10 Hz or more and 2 GHz or less. If the frequency is less than 10 Hz, the coating layer is not properly formed, and if the frequency is 2 GHz or more, the coating layer may be destroyed. The waveform of the applied current is adjusted so that the forward current waveform and the reverse current waveform appear alternately with time, and the respective holding times and amplitudes of the forward current wave and the reverse current wave may be appropriately adjusted. There may be a pause in which no current flows between current waves. In this case, the shape of the applied current wave may be a triangular wave, a square wave, a sawtooth wave, or an irregular combination thereof in addition to the sinusoidal wave. The coating layer formed from the above process is a complex oxide containing MgO as a main component, or forms a hydroxide containing Mg (OH) ₂ as a main component.

By appropriately adjusting the above electrolyte, applied voltage, applied current density and applied frequency, the desired coating layer thickness and surface resistance of the coating layer can be obtained, and the preferred coating layer thickness is 0.1 μm or more and 10 μm or less, and the coating layer has a thickness of 0.1 If it is less than μm it is difficult to secure sufficient paint adhesion during subsequent coating, and if the thickness of the coating layer exceeds 10 μm it is impossible to secure the electrical conductivity.

In addition, the surface resistance of the coating layer is 0.001 Ω / sq. More than 10.0 Ω / sq. It is preferable to set it as below, and surface resistance is 0.001 (ohm) / sq. To achieve a value less than 10.0 Ω / sq. In the above case, the electromagnetic shielding function and the grounding function, which will be described later, may not be sufficiently performed. The most preferable coating layer has a surface resistance of 0.001 Ω / sq. More than 2 Ω / sq. It is good to set it as the following range, and it has sufficient effect regarding the electrical conductivity of the said coating layer in this range of surface resistance.

The surface resistance, as defined in ASTM D-257 standard, is called area resistance or sheet resistance, and means a resistance value between cross sections of a square portion to be measured. At this time, even if the size of the square to be measured is arbitrarily changed, the resistance value does not theoretically change, and is measured using a surface resistance meter. The unit of surface resistance is Ω / Sq. (Ohm per square) or the same display unit Ω / □ is used.

The coating layer as described above has a corrosion resistance on the surface of the metal body, but does not peel off from the surface of the metal body, and serves as a medium for the subsequent coating or plating layer to be in close contact with the surface of the metal body, and at the same time the electrical conductivity ( When a metal body is used as a case of an electronic device such as a mobile phone, a notebook PC, a digital camera, a camcorder, or an audio, the case itself must serve as an electrical ground, and at the same time, noise of electromagnetic waves that may occur during data communication inside the device In order to remove the noise, the electromagnetic shielding role should also be performed. In order to perform this role, the electrical conduction of the surface inside the case can be easily flowed.

In the step of forming any one of a coating layer or a plating layer on the coating layer formed in the above step, an electrodeposition coating method may be used to obtain the coating layer, and a general electrodeposition plating method may be used to obtain the plating layer.

The electrodeposition coating layer is formed by using a general electrodeposition coating method in which a coating object (a metal body mainly composed of magnesium) and a tank (or an electrode opposed to the coating material) are used as both electrodes, and the coating film is coated with an electric current in a water-soluble coating in the tank. Let's do it. The electrodeposition coating is to be coated in order to have corrosion resistance on the surface of the metal body containing magnesium as a main component, when the electrodeposition coating method is directly applied to the surface of the metal body without forming a coating layer according to the present invention The adhesion between the coating film and the surface of the metal body is poor, causing the electrodeposition coating layer to peel off.

The electrodeposition plating layer is formed of a metal ion component contained in the plating liquid on the surface of the workpiece through a current through a plating liquid in a tank (or a metal body mainly composed of magnesium) as a cathode, and through a plating liquid in a tank (or an electrode facing the workpiece). It forms using the general electrodeposition plating method which forms the plating layer which makes a main component. The electrodeposition plating layer is formed to form a metallic plating layer on the surface of the metal body containing magnesium as a main component to obtain a surface coating film layer having a beautiful metallic luster on the surface of the metal body. However, when the electrodeposition plating method is carried out directly on the surface of the metal body without forming the coating layer according to the present invention, not only the adhesion between the electrodeposited coating film and the surface of the metal body is poor, but also between the surface of the metal body and the electrodeposition plating layer. When corrosion occurs and a predetermined time elapses, the electrodeposition plating layer is peeled off.

If the surface treatment process of the metal body which has magnesium as a main component of this invention as mentioned above is arranged in order, it is a sequence of (1) sandblasting (2) cleaning (3) coating layer formation (4) washing (5) drying process. . Therefore, the surface treatment method of the metal body which has magnesium as a main component which concerns on this invention is compared with the conventional coating pretreatment process with respect to the metal body which has magnesium as a main component which carries out anodic oxidation followed by surface adjustment and performs a primer treatment for painting. It can reduce the number of processes, improve productivity, reduce product defect rate, and improve the environmental wastewater treatment problem and the poor working environment at the same time as the existing paint pretreatment process, and also the surface of the metal body which contains magnesium as the main component At the same time, it is possible to greatly improve the corrosion resistance and paint adhesion, which are the biggest problems during the treatment, and to obtain the electrical conductivity which cannot be obtained in the anodizing surface treatment method.

1 is a surface photograph taken by a scanning electron microscope after the conventional anodizing surface treatment process, it can be seen that there are many cracks and a large number of puddles partially broken anodizing layer on a relatively flat surface. In addition, irregular cracks and puddles present on such a flat surface prevent the primer from effectively penetrating and binding to the anodizing surface upon subsequent primer application, thereby causing a loss of adhesion between the primer and the anodizing layer.

FIG. 2 is a photograph taken of a scanning electron microscope after the conventional non-chromate surface treatment process, and shows a representative defect shape inevitably seen on the non-chromate process. These defects result in a poor paint adhesion with the paint layer during subsequent coating.

3 is a photograph taken after the coating by the surface treatment process of the metal body according to the present invention, the surface of the surface by scanning electron microscopy, unlike the conventional anodizing process or non-chromate process irregularities that may damage the coating adhesion There are no cracks, and very fine pores are evenly distributed on all surfaces regardless of surface curvature, so that the coating penetrates into these fine pores during coating application, thereby maximizing adhesion between the coating layer and the coating layer by the coating.

The following is an embodiment according to the present invention, the metal body selected as the test target in each example was made of a plate of 70mm width, 50mm length, 0.6mm thickness, made of AZ91D, a magnesium alloy for die casting, and the surface roughness of the plate To adjust, the surface roughness of each specimen was adjusted by applying the sandblasting method according to the present invention. Thereafter, in order to form a coating layer on each specimen, a stainless electrode and a stainless steel electrode were placed inside an electrolytic cell containing an electrolyte consisting of a small amount of potassium hydroxide (KOH) and sodium silicate (Na ₂ O-nSiO ₂ )) After immersing the opposing plate specimens at the same time, the applied current is adjusted and energized so as to have a constant current density. Once energized, the applied voltage rises over time, and when the applied voltage reaches a constant voltage, the power is cut off and the specimen is taken out and washed twice with de-ionized water. do. After washing with water, the test piece is put into a drying line and dried at a temperature of 150 ° C. for 30 minutes. A urethane resin-based paint is applied to the coating layer according to the present invention by a spray method at a thickness of 20 μm, followed by baking at 80 ° C. for 20 minutes. Finish coating was performed.

<Example 1>

Surface roughness of metal: 2.1 μm on average

Coating conditions and average thickness of the coating layer:

-Electrolyte Composition: KOH 5g / l + Na ₂ O-SiO ₂ 8 g / l + Distilled Water

Power condition: Current density 2 A / dm ² , final applied voltage 250 V AC

-Average thickness of coating layer: 21 μm

<Example 2>

Surface roughness of metal: 2.2 μm average

Coating Conditions and Average Thickness of Coating Layer:

-Electrolyte composition: KOH 5g / l + Na ₂ O-SiO ₂ 3 g / l + distilled water

-Power condition: Current density 3 A / dm ² , final applied voltage 200 V AC

-Average thickness of coating layer: 22 μm

<Example 3>

Surface roughness of metal body: Average 2.3 μm

Coating conditions and average thickness of the coating layer:

-Electrolyte composition: KOH 3 g / l + Na ₂ O-SiO ₂ 5 g / l + distilled water

-Power condition: Current density 5 A / dm ² , Final applied voltage 250 V AC

-Average thickness of coating layer: 21 μm

<Example 4>

Surface roughness of metal: 2.4 μm average

Coating conditions and average thickness of the coating layer:

-Electrolyte composition: KOH 3 g / l + Na ₂ O-SiO ₂ 3 g / l + distilled water

-Power condition: Current density 5 A / dm ² , Final applied voltage 200 V AC

-Average thickness of coating layer: 21 μm

<Example 5>

Surface roughness of metal: 2.5 μm average

Coating conditions and average thickness of the coating layer:

-Electrolyte composition: KOH 10 g / l + Na ₂ O-SiO ₂ 15 g / l + distilled water

-Power condition: Current density 5 A / dm ² , Final applied voltage 250 V AC

-Average thickness of coating layer: 23 μm

Table 1 summarizes the surface treatment conditions of the metal body based on magnesium according to Examples 1 to 5 as follows.

<Table 1> Surface treatment conditions and surface resistance of the coating layer of the metal body (AZ91D) according to the embodiment

As shown in Table 1 above, the method specified in ASTM D-257 was applied to measure the surface resistance of the surface of the coating layer, and the surface resistance value of the coating layer measured using the surface resistor for each specimen ensured the electrical conductivity. Sufficient values were obtained for each.

Table 2. Coating Film Adhesion and Corrosion Resistance Test Results

In addition, the cross-cutting test was carried out on the specimens completed until the final finish coating in accordance with the test procedure specified in ISO 2409 in order to find the coating adhesion between the coating layer and the finish coating layer, and the test results are shown in Table 2. Indicated. As a result of the coating adhesion test, all the examples showed the best grade of 0 (when only 1 film out of 100 pieces was peeled off) or 1 grade (less than 5 out of 100 pieces of film as shown in Table 2). Results in the case where only the coating film is peeled off).

As described above, when the surface treatment method of the metal body mainly composed of magnesium according to the present invention is used as the coating pretreatment process, not only the paint adhesion that is a problem in the anodizing process, which is a conventional coating pretreatment process, can be improved. In addition, it can reduce production time and cost, and can eliminate various problems such as environmental wastewater treatment problem and poor working environment caused by the existing paint pretreatment process, and at the same time, make the surface of the metal body conductive. It works. The surface treatment method of the metal body containing magnesium as a main component according to the present invention can be applied to various communication devices including digital phones, digital cameras, video cameras, camcorders, MP3 players, and other audio devices using magnesium alloys as interior materials or exterior materials. In addition, it can be applied to the field of automobile, aircraft parts, etc. in pursuit of light weight.

Although the present invention has been described with reference to the embodiments shown in the accompanying drawings, this may be regarded as exemplary, and those skilled in the art may conceive various modifications and equivalent embodiments therefrom. Such equivalent embodiments should also be considered to be within the scope of the claims of the present invention.

Claims (5)

delete delete Pretreating the surface of the metal body based on magnesium; Forming a coating layer on the surface of the metal body pretreated in the step; And Forming any one of a coating layer and a plating layer on the coating layer formed in the step; In the step of pretreating the surface of the metal body, the surface roughness of the metal body is maintained on the average of 0.1 μm or more and 20 μm or less by using the sand blasting method, In the forming of the coating layer, the coating layer, immersed the electrode facing the metal body in the electrolyte spaced apart, and energized so that different polarities are applied to the electrode facing the metal body, respectively, of the applied current The waveform is formed on the surface of the metal body by causing the current waveform in the forward direction and the current waveform in the reverse direction to appear alternately with time. The surface resistance of the formed coating layer is 0.001 Ω / sq. 2 Ω / sq. Or less, wherein the coating layer has a thickness of 0.1 μm or more and 10 μm or less. The method of claim 3, The electrolyte solution is a surface treatment method of a metal body, characterized in that it contains potassium hydroxide and sodium silicate. The method of claim 3, The electrolyte solution surface treatment method of a metal body characterized in that it contains sodium hydroxide and sodium silicate.

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