JP2000219975A - Surface-treated Mg alloy and surface treatment method - Google Patents

Abstract

(57) [Summary] [Problem] Surface treated M excellent in design and paint adhesion
g alloy and its surface treatment method. On the surface of the A Mg alloy, there following metallic chromium particles diameter 15μm is 100,000 / mm ² or less, and the entire surface of the Mg alloy containing chromium metal fine particles 5 to 100
An Mg alloy covered with a chromate coating layer consisting of hydrate of trivalent chromium with a thickness of nm. The amount of chromium contained in the fine chromium particles and the chromate coating layer is preferably ¹ to 300 mg / m ² . It is obtained by performing cathodic electrolysis in an acidic aqueous solution containing at least 2 to 100 g / L of hexavalent chromium ions on the surface of the Mg alloy and having a pH adjusted to 0.5 to 3.0. This acidic aqueous solution can contain 2,000 ppm or less of sulfuric acid and 400 ppm or less of a fluorine compound. The cathodic electrolysis treatment has a current density of 0.1 to 20 A
/ Dm ² , and an energized amount of electricity of 10 to 1000 coulombs / dm ² .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated Mg alloy having excellent design properties and coating adhesion, and a method of treating the surface.

[0002]

2. Description of the Related Art In recent years, demand for Mg alloys for housings of home information appliances such as notebook personal computers, mobile phones, digital cameras and digital videos has been rapidly increasing.
This is because compared to the conventionally used plastic, M
This is because the g alloy has better electromagnetic wave shielding properties and heat radiation properties without increasing its weight. Further, since the recyclability of materials becomes more and more important due to recent environmental problems, Mg alloys are expected to be applied not only to the information home appliances but also to various fields.

[0003] As described above, Mg alloy has excellent characteristics as a material, but since the oxidation-reduction potential of Mg itself is lower than that of other metals, its chemical activity is extremely high in a humid environment at normal temperature and normal pressure. Easy corrosion has been an obstacle to industrialization.

Therefore, in order to use the Mg alloy industrially, it is indispensable to perform some surface treatment for preventing corrosion. When used, it must have a certain design property. For that purpose, for example, painting is used if a colored appearance is required, and plating is used if a metal skin is required.

[0005] As in the case of other practical metal materials such as steel and aluminum, even in the case of coating or plating with Mg alloy, a chemical pretreatment is performed to impart corrosion resistance and adhesion. . Moreover, in the case of Mg alloys, due to its high chemical activity, the performance of this chemical pretreatment step greatly affects the overall performance, and is generally more complicated than in the case of other practical metal materials. I have.

For example, as a pretreatment method for coating, Do
The mainstream is a chemical conversion treatment method in which a chromate film is formed on the surface of an Mg alloy by immersion in an aqueous solution containing chromic acid as a main component, as typified by the w7 method, the Dow20 method, and the Dow22 method. Furthermore, recently, a chromium-free surface treatment has been demanded due to environmental problems.
Japanese Patent Application Laid-Open No. Hei 7-204, one immersed in an aqueous solution containing permanganate or manganate as the main component
As disclosed in Japanese Patent Application Laid-Open No. 577, there is disclosed a device immersed in a nickel-free aqueous solution of manganese-modified zinc phosphate.

[0007] In the case of plating, as a pre-treatment before performing a target plating, after performing pickling, an activation treatment is performed with a mixed solution of phosphoric acid and hydrofluoric acid, and a displacement zinc plating treatment is performed.
Further, strike plating such as copper and nickel is applied.

However, Mg alloys have different alloy compositions and molding methods depending on the purpose. Therefore, the surface condition varies widely, and the above-mentioned surface treatment method may not be applicable in some cases. For example, the plating method is currently the most in demand AZ91 material (A
1: 9%, Zn: 1% Mg alloy) is not considered to be a sufficient method.
No. 0 discloses a method in which chemical etching and its neutralization treatment, and further, electroless nickel plating are performed as a pretreatment.

Further, in the pretreatment for coating, in practice, if the surface condition of the Mg alloy to be treated is not chemically uniform, good chemical conversion treatment cannot be performed. Processing steps are also important. As such a pretreatment step, a method of performing uniform etching using a pickling solution to which a persulfate is added, as disclosed in JP-A-53-102231, and a method of performing acid etching as disclosed in JP-A-6-220363. A method is disclosed in which desmutting is performed using a chelating agent after washing to obtain a clean surface.

As described above, painting and plating have been described with respect to the prior art for imparting a design property to the Mg alloy surface, but plating is generally more expensive as easily imagined. Also,
In the case of plating, it is necessary to form nickel or copper between the layers as strike plating, and if such a material is used, it cannot be recycled (when nickel or copper is M
(g) Corrosion resistance is remarkably deteriorated when a solid solution is formed in an alloy.

As a means to solve this problem by coating, a method of applying a clear coating after giving a mirror gloss to the Mg alloy by mechanical processing such as polishing in advance is considered. In this case, the essential condition is clear coating. In the pretreatment process of
That is, the gloss appearance of the Mg alloy surface is not substantially changed. However, according to the conventional technique, for example, a chromate-based or permanganate-based chemical conversion treatment gives a yellow appearance, and a phosphate-based chemical treatment gives a matte gray appearance. No surface treatment method has been found yet.

[0012]

Therefore, in the present invention, a colorless and transparent surface treatment method having good adhesion and corrosion resistance without impairing the design property when applying a clear coating to a specular gloss material is disclosed. It is an object of the present invention to provide a Mg alloy that has been subjected to heat treatment. Further, such a surface treatment is obviously effective for ordinary colored coatings, that is, it is also an object of the present invention to provide a wide range of surface treatment methods at the same time regardless of the kind of the target Mg alloy.

[0013]

Means for Solving the Problems As a result of the inventor's intensive studies to solve such technical problems, yellow coloration is observed in chromate conversion treatment.
This is because hexavalent chromium is contained in the film, and even if the same chromic acid-based treatment solution is used, the cathode electrolysis method is successfully applied, and the chromium in the metal state is added to the surface of the Mg alloy.
It has been newly found that a colorless and transparent appearance can be obtained by forming a surface treatment film containing valent chromium as a main component and substantially not containing hexavalent chromium.

That is, the surface-treated Mg alloy of the present invention is characterized in that fine metal chromium particles having a diameter of 15.0 μm or less are present at a frequency of 100,000 particles / mm ² or less on the surface of the Mg alloy material, and The entire surface of the Mg alloy material including the fine particles is covered with a chromate coating layer made of a hydrated oxide of trivalent chromium having a thickness of 5 to 100 nm.

The amount of chromium contained in the fine metal chromium particles and the chromate coating layer is preferably ¹ to 300 mg / m ² .

It is preferable that the surface of the Mg alloy material is polished in advance before forming the coating layer.

Further, it is preferable that a clear coating layer is provided on the outermost surface of the Mg alloy material.

Next, the surface treatment method of the Mg alloy according to the present invention comprises the steps of:
Cathodic electrolysis is carried out in an acidic aqueous solution containing 100 g / L and having a pH adjusted to 0.5 to 3.0.

The acidic aqueous solution further contains 2000 ppm
It is preferable to contain the following sulfuric acid and / or a fluorine compound of 400 ppm or less in terms of fluorine.

It is preferable that the cathodic electrolysis is carried out at a current density of 0.1 to 20 A / dm ² and a current flow of 10 to 1000 coulombs / dm ² .

Further, it is preferable that a step of polishing the surface of the Mg alloy before performing the cathodic electrolytic treatment is provided. Further, it is preferable that a clear coating is performed after the cathode electrolytic treatment is performed.

Hereinafter, the contents of the present invention will be described in detail. The Mg alloy to which the present invention can be applied is not particularly limited.
Most Mg alloys currently used, namely Mg
-Al-Zn alloy, Mg-Mn alloy and Mg-Zn alloy are the targets.

The surface-treated Mg alloy of the present invention is subjected to a surface treatment as described below so that its surface has a diameter of 15.0.
chromate in which fine particles of metallic chromium having a diameter of not more than 100 μm / mm ² are present at a frequency of 100,000 or less, and the entire surface of the Mg alloy containing the fine particles of metallic chromium is made of a hydrated oxide of trivalent chromium having a thickness of 5 to 100 nm. Must be covered with a coating layer.

Here, it is considered that the fine metal chromium particles contribute to corrosion resistance after coating, and the chromate coating layer contributes to coating film adhesion. Fine metal chromium particles with a diameter of 15.0
When the thickness exceeds μm or when the frequency of occurrence exceeds 100,000 / mm ² , the corrosion resistance is good, but the appearance of the surface changes from gray to black-brown, which is disadvantageous to the design ( However, there is no problem when colored painting is performed.) When used only in relatively mild corrosive environments such as information home appliances, the required characteristics are paint adhesion rather than corrosion resistance. The lower limit is not particularly limited. In an extreme example, even if almost no chromium metal particles are recognized, no problem is obtained.

On the other hand, a chromate coating layer made of a hydrated oxide of trivalent chromium is indispensable for the present invention. If the thickness of the chromate coating layer is less than 5 nm, it is impossible to uniformly cover the surface of the Mg alloy, and sufficient coating adhesion cannot be obtained. Although exceeding 100 nm does not cause any particular problem, it is economically disadvantageous because the adhesion is already saturated at this point. However, excessive thickness should be avoided because the strength of the chromate coating layer itself deteriorates and cracks may occur. Its limit is about 300 nm.

The amount of chromium contained in the fine metal chromium particles and the chromate coating layer is 1 to 3 by weight per unit area.
It is preferably set to 00 mg / m ² . In the case where the present invention is used industrially, by controlling the weight per unit area in this way, it is possible to easily measure by applying a fluorescent X-ray analysis or the like. Further, by forming the surface treatment film on the surface of the Mg alloy material which has been mirror-polished in advance, and further forming a clear coating film layer thereon, a Mg alloy having excellent design properties, which is the main object of the present invention, is obtained. Obtainable. 1 to 3 show images of the surface-treated Mg alloy surface of the present invention.

Next, the surface treatment method of the present invention will be described. Most of the current mainstream structural Mg alloy components are formed by die-casting or recently by thixomolding, and have a rapidly solidified layer of about 5 μm (a layer on which the added alloy component segregates) on the surface. The remaining of the organic release agent used at the time of the molding is observed. Therefore, the polishing of the Mg alloy surface applied to the method of the present invention is preferably such that at least these layers are removed. In the case of clear coating, the polishing further obtains a specular gloss. It is more preferable that it is possible. Then, the cathode electrolytic treatment of the present invention is preferably applied immediately after this polishing step.

On the other hand, when a normal colored coating is performed, a polishing step is not particularly required. However, even in such a case, it is preferable to remove the rapidly solidified layer and the organic release agent. Conventional pretreatment methods such as washing and surface conditioning can be applied. The cathode electrolytic treatment of the present invention may be applied thereafter.

The cathodic electrolysis treatment of the present invention is at least 6 times.
It contains 2 to 100 g / L of valent chromium ions and has a pH of 0.1.
It is necessary to carry out in an acidic aqueous solution adjusted to 5 to 3.0. Here, the concentration of hexavalent chromium ions is 2 g.
/ L, it is difficult to form a film by electrolysis.
Conversely, if it exceeds 100 g / L, there is no particular problem, but it is not economically preferable because the film formation efficiency is saturated. On the other hand, the film-forming efficiency tends to gradually decrease as the pH decreases. Therefore, it is not preferable that the film-forming efficiency be less than 0.5. It is rather preferable to increase the pH up to 3.0, but it is not preferable if the pH is exceeded because the efficiency of film formation sharply decreases.

The source of hexavalent chromium ions in the aqueous chromic acid solution is not particularly limited, but it is advantageous in terms of cost to dilute chromic acid with water. The pH can be adjusted using an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide or ammonia water. Therefore, a part of the chromic acid may be supplied as an alkali metal salt or an ammonium salt of dichromic acid so as to have a predetermined pH.

The chromic acid aqueous solution further contains 2,000
It is preferable that sulfuric acid of less than ppm and / or fluorine compound of less than 400 ppm in terms of fluorine be contained. By using these additives in combination, the frequency of precipitation of the metal chromium fine particles can be increased, and the corrosion resistance of the surface-treated Mg alloy of the present invention can be further improved. However, the addition of sulfuric acid exceeding 2000 ppm would rather hinder the precipitation of fine metal chromium particles,
The addition of a fluorine compound exceeding m is not preferable because the effect is saturated and the surface appearance of the Mg alloy may be deteriorated by corroding the surface.

The fluorine compound can be added in the form of hydrofluoric acid, hydrofluoric acid, zircon hydrofluoric acid, titanium hydrofluoric acid or the like, and alkali metal salts or ammonium salts thereof may also be used. Can be.

The cathodic electrolysis in the chromic acid aqueous solution is carried out at a current density of 0.1 to 20 A / dm ² and a supplied electricity of 10 to 1000 coulombs / dm ² . If the current density is less than 0.1 A / dm ² , the current efficiency decreases, which is not preferable. On the other hand, if it exceeds 20 A / dm ² , there is no particular problem, but the electrolysis time for obtaining a suitable amount of electricity becomes extremely short, so that it is not industrially practical. Also, the amount of electricity supplied is 10 coulombs / dm ²
Less than a not preferable because it not obtained the required amount of metallic chromium particles and chromate coating layer to be undesirable because 1000 metallic chromium particles and chromate coating layer from exceeding a coulomb / dm ² becomes excessive.

The temperature of the above-mentioned cathodic electrolysis treatment is as follows:
Usually, it is preferable to carry out in the range of 20 to 50 ° C. If the temperature is too low, hexavalent chromium is contained in the chromate film, and it may be colored yellow, which is not preferable. on the other hand,
If the temperature is too high, a chromate film may be formed without performing the cathode electrolytic treatment, which is not preferable. This is because such an electroless chromate film similarly contains hexavalent chromium.

The chromic acid aqueous solution used in the surface treatment of the present invention accumulates trivalent chromium ions as the cathodic electrolytic treatment is continued, but this does not cause any particular problem. Further, since the surface-treated Mg alloy of the present invention does not contain hexavalent chromium, it does not substantially pollute the environment even if it is distributed on the market. In order to use it, washing with water after surface treatment must be performed sufficiently. If the washing is not sufficient, a small amount of hexavalent chromium may remain on the surface layer.

After the above surface treatment step, a clear coating (or a colored coating) is applied. For the clear coating, not only a normal colorless and transparent clear paint, but also a color clear paint to which a slight amount of pigment is added can be used, and various design properties can be provided.

[0037]

According to the present invention, the fine metal chromium particles in the coating are responsible for corrosion resistance, and the trivalent chromate coating layer is responsible for coating adhesion. In particular, since such a chromate coating layer has excellent adhesion to most organic resins and ceramics, it is clear that it can be applied not only as a base treatment for coating but also as a base treatment for laminates and adhesives. ing. Furthermore, by applying a polishing step, a complicated chemical pretreatment step for surface cleaning as in the prior art is not required, so that a cost-effective and versatile surface treatment method is provided. Can be provided.

[0038]

EXAMPLES The contents of the present invention will be described more specifically with reference to examples and comparative examples.

The material is AZ91D plate (100 mm × 7
(0 mm × 2 mm), the surface of which was previously polished to a mirror surface, and then gently washed with water to remove a polishing powder to prepare a sample.

Example 1 The above sample was adjusted to a pH of 0.8 by adding aqueous ammonia to a hexavalent chromium ion concentration of 2
Cathodic electrolysis was performed in a 0 g / L chromic acid aqueous solution at a temperature of 40 ° C. and a constant current density of 8 A / dm ² for 60 seconds using SUS304 material as an anode, washed well with deionized water and dried. Did not show any change in the appearance.

When the amount of chromium adhering to the sample surface in Example 1 was analyzed by X-ray fluorescence analysis, it was confirmed that about 15 mg / m ² of chromium was present. Further, when the surface chemical state was analyzed by XPS, trivalent chromium was observed at a thickness of about 10 nm from the outermost surface, and a trace amount of metallic chromium was observed in the lower layer. On the other hand, when the sample surface was analyzed by EPMA, fine particles having a diameter of 1 to 6 μm were observed at a deposition frequency of about 5000 particles / mm ^{2 in} the electron secondary image, and elemental analysis revealed that it was chromium. became.

The sample of Example 1 was spray-coated with a 20 μm-thick acrylic clear paint, Magicalon, manufactured by Kansai Paint Co., Ltd., and baked at 150 ° C. for 20 minutes. next,
When 100 pieces of grids were formed by making a scratch reaching the substrate 10 pieces at a time in the orthogonal direction with a width of 1 mm using a sharp cutter, and these pieces were peeled off with scotch tape (hereinafter referred to as a primary adhesion test). No peeling was seen.
Further, after the coating sample was immersed in boiling deionized water for 4 hours, the grid was evaluated in the primary adhesion test, but no peeling was observed. A test performed by deteriorating the film is hereinafter referred to as a secondary adhesion test).

Example 2 The mirror-polished sample used in Example 1 was prepared by adding 400 ppm of sulfuric acid and 100 p of zircon hydrofluoric acid to a chromic acid aqueous solution having a hexavalent chromium ion concentration of 20 g / L.
SUS304 material was used as an anode in an aqueous solution whose pH was adjusted to 0.8 with aqueous ammonia.
Cathodic electrolysis was performed for 20 seconds at 0 ° C. and a constant current density of 3 A / dm ² , washed well with deionized water, and dried. As a result, a slight white haze was observed on the material surface.

When the amount of chromium adhering to the sample surface in Example 2 was analyzed by X-ray fluorescence analysis, the presence of 95 mg / m ² of chromium was confirmed. The surface was analyzed for its chemical state by XPS.
The peak of chromium of valence and the clear metal chromium in the lower layer were observed. On the other hand, when the sample surface was analyzed by EPMA, fine particles having a diameter of 1 to 3 μm were observed at a deposition frequency of about 40,000 particles / mm ^{2 in} the electron secondary image, and elemental analysis revealed that it was chromium. became.

Next, when the same coating as in Example 1 was applied to the sample of Example 2, the white haze on the material surface observed after the cathode electrolysis became almost inconspicuous. further,
A primary adhesion test and a secondary adhesion test by boiling water immersion were performed according to Example 1, and no peeling was observed in any case.

Comparative Example 1 The mirror-polished sample used in Example 1 was subjected to the same coating as in Example 1 without performing cathode electrolytic treatment or the like. When a primary adhesion test was performed on this sample, three out of 100 pieces peeled off. Similarly, a secondary adhesion test was conducted by immersion in boiling water.
55 of the pieces peeled off.

Comparative Example 2 The mirror-polished sample was Dow 2
Method 0 was applied. That is, sodium acid fluoride 15
g / L, 180 g / L of sodium bichromate, 10 g / L of sodium acetate, and 84 mL / L of nitric acid were prepared, immersed at room temperature for 30 seconds, sufficiently washed with deionized water, and dried. This sample had a matte tan appearance.

Example 3 The mirror-polished sample used in Example 1 was subjected to SUS304 in a chromic acid aqueous solution having a hexavalent chromium ion concentration of 5 g / L and adjusted to pH 0.8 by adding aqueous ammonia. Using the material as the anode, temperature 40 ° C, 3
Cathodic electrolysis was performed for 60 seconds under the condition of a constant current density of A / dm ² , washed well with deionized water, and dried. As a result, no change was observed in the appearance of the material.

When the amount of chromium adhering to the sample surface in Example 3 was analyzed by X-ray fluorescence analysis, the presence of about 7 mg / m ² of chromium was confirmed. The surface was analyzed for its chemical state by XPS.
Val chromium was observed, and a trace amount of metallic chromium was observed in the lower layer. On the other hand, when the sample surface was observed by SEM,
Fine particles having a diameter of 1 to 12 μm were observed at a deposition frequency of about 2000 particles / mm ² .

To the sample of Example 3, 10 μm of Parker-processed ceramic coating CERA-STATS124CL-3 was added.
It was spray-coated with a film thickness of m and baked at 200 ° C. for 20 minutes. Next, when this sample was immersed in a 5% saline solution at 25 ° C. for 2 hours, no peeling of the coating film was observed, but three out of four samples were repeatedly evaluated as having slightly dotted rust. Occurrence was observed.

Example 4 The mirror-polished sample used in Example 1 was prepared by adding 100 ppm of sulfuric acid and 80 ppm of zircon hydrofluoric acid to a chromic acid aqueous solution having a hexavalent chromium ion concentration of 5 g / L, and adjusting the pH with ammonia water. In the aqueous solution adjusted to 0.8, the SUS304 material was used as the anode, and the temperature was set to 40.
The cathode electrolysis was performed for 20 seconds at a constant current density of 3 A / dm ² for 20 seconds, washed well with deionized water, and dried.
A slight white haze was observed on the material surface.

When the amount of chromium adhering to the sample surface in Example 4 was analyzed by X-ray fluorescence analysis, the presence of 63 mg / m ² of chromium was confirmed. The surface was analyzed for its chemical state by XPS.
The peak of chromium of valence and the clear metal chromium in the lower layer were observed. On the other hand, when the sample surface was observed by SEM, fine particles having a diameter of 1 to 3 μm were found to be about 20,000 particles / m 2.
This was observed at a precipitation frequency of m ² .

Next, when the same coating as in Example 3 was applied to the sample of Example 4, the white haze observed on the material surface after the cathode electrolysis almost disappeared. Further, Example 3
When a salt water immersion test was performed according to the test, no abnormalities such as peeling of the coating film and generation of rust were observed at all.

Comparative Example 3 The mirror-polished sample used in Example 1 was subjected to the same coating as in Example 3 without performing cathode electrolytic treatment or the like. Further, when a salt water immersion test was carried out in accordance with Example 3, one of the four samples was repeatedly evaluated, and one of the samples was peeled off, and this portion was severely corroded. In the remaining three sheets, although no peeling was observed, dot-like rust was observed on the entire surface of the sample.

[0055]

As described above, it can be seen that the use of the surface treatment method of the present invention does not substantially change the appearance of the surface of the Mg alloy, and thus provides a wide range of possibilities for its design as a clear coating base treatment. . Moreover, since the pre-treatment does not depend on the type of mechanical polishing or conventional chemical method, it can be applied as a normal coating pre-treatment and provides a general-purpose surface treatment that does not depend on the type of Mg alloy can do. Furthermore, in the method of the present invention, an aqueous solution of hexavalent chromium is used. However, since the coating layer on the treated Mg alloy does not contain hexavalent chromium, it can be said that the surface treatment has a small environmental load.

[Brief description of the drawings]

FIG. 1 is a view showing a mirror-polished flat Mg alloy material.

FIG. 2 is a view showing an image of a surface of a Mg alloy subjected to cathode electrolytic treatment.

FIG. 3 is a view showing an image of a surface of a painted Mg alloy after a cathode electrolytic treatment.

[Explanation of symbols]

 1: Mirror-polished flat Mg alloy material 2: Chromate coating layer with hydrated oxide of trivalent chromium 3: Metal chromium fine particles 4: Clear coating film

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) C23C 22/57 C23C 22/57 F term (reference) 4D075 AE03 BB02X BB74X BB81Y BB92X CA33 CB00 DA06 DB01 DC18 EA02 EA43 EB16 EC01 EC53 4J038 HA066 KA14 NA13 NA19 PC08 4K026 AA01 BA06 BA11 BB06 BB08 CA13 CA18 CA20 CA28 CA33 DA01 DA16 EA03 EB08

Claims (10)

[Claims] The present invention is characterized in that fine metal chromium particles having a diameter of 15.0 μm or less are present at a frequency of 100,000 / mm ² or less on the surface of the Mg alloy material, and the entire surface of the Mg alloy material including the fine metal chromium particles is thick. A surface-treated Mg alloy, which is covered with a chromate coating layer of a hydrated oxide of trivalent chromium having a thickness of 5 to 100 nm. 2. The surface-treated Mg alloy according to claim 1, wherein the amount of chromium contained in the fine metal chromium particles and the chromate coating layer is 1 to 300 mg / m ² . 3. The Mg alloy material surface is polished before forming the coating layer.
The surface treated Mg alloy according to the above. 4. The surface-treated Mg alloy according to claim 3, which has a clear coating layer on the outermost surface of said Mg alloy material. 5. The cathode electrolysis treatment is carried out in an acidic aqueous solution containing 2 to 100 g / L of hexavalent chromium ions on the surface of the Mg alloy and adjusted to a pH of 0.5 to 3.0. , Mg alloy surface treatment method. 6. The acidic aqueous solution further contains 2,000 ppm.
The method for surface treating an Mg alloy according to claim 5, wherein the following sulfuric acid is contained. 7. The method according to claim 5, wherein the acidic aqueous solution further contains 400 ppm or less of a fluorine compound in terms of fluorine. 8. The method according to claim 1, wherein said cathodic electrolysis is performed at a current density of 0.1.
8. The method according to claim 5, wherein the heating is performed at an electric current of 10 to 1000 coulombs / dm ² and an electric current of 10 to 1000 coul / dm ² .
The surface treatment method of the Mg alloy according to the above. 9. The method according to claim 5, further comprising a step of polishing the surface of the Mg alloy before subjecting the Mg alloy to the cathodic electrolytic treatment. 10. The method according to claim 9, wherein a clear coating is further performed after the cathode electrolytic treatment.