JPH058278B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for surface treatment of magnesium, aluminum and alloys thereof. More specifically, the present invention uses an alkali metal or alkaline earth metal carbonate bath to anodize magnesium, aluminum, or an alloy thereof, thereby giving the surface a hard and corrosion-resistant surface. The present invention relates to a method for forming a white film.

Conventional technology Magnesium, aluminum, and their alloys have traditionally been used as important industrial materials because they have the lowest specific gravity among practical metals, have higher specific strength than steel, and have excellent sound absorption properties. It is used as a practical metal in the fields of, for example, automobile parts, leisure goods, audio parts, and even space equipment parts, and its demand is likely to increase further in the future.

By the way, as is well known, magnesium and aluminum are chemically unstable and their surfaces tend to corrode in the atmosphere, and it is necessary to prevent this from occurring in practice, so various surface treatment methods have been used to date. Proposed. For example, chromates, manganese salts,
Various methods have been proposed, including chemical conversion treatment in a bath using fluoride or a combination of these, surface treatment using a galvanic method using steel or stainless steel in a bath, and anodization treatment. There is.

However, although the above-mentioned chemical conversion treatment using chromates, etc. is effective for short-term temporary corrosion protection, long-term corrosion prevention effects cannot be expected, and furthermore, the treatment bath is harmful, so it is necessary to prevent pollution. It is economically disadvantageous as it requires various equipment. Furthermore, regarding the galvanic method, since the surface film obtained is a chemically unstable lower-grade oxide, corrosion resistance cannot be expected by itself, and treatment such as painting must be used in conjunction with it. , blisters after long-term use.
There are disadvantages such as the occurrence of Furthermore, the anodic oxidation method, which produces a surface film made of relatively stable oxides, has disadvantages such as significant roughening of the treated surface and difficulty in obtaining a uniform color tone due only to the salt formed. Therefore, it is actually only used as a base preparation for painting.

Additionally, a surface treatment method has been proposed in which aluminum is anodized in an alkaline carbonate solution with a concentration of about 10 to 150 g/l to which phosphate is added (Japanese Patent Application Laid-Open No. 1983-54238), but this method The film formed by this method is porous and lacks corrosion resistance, so it has the disadvantage of requiring pore sealing treatment.

As described above, the effects of conventional surface treatments for magnesium, aluminum, and alloys thereof are not necessarily satisfactory, and the development of an excellent surface treatment method has been desired.

Problems to be Solved by the Invention The purpose of the present invention is to meet these demands and to provide a method for extremely easily forming a dense white film that is hard and has excellent corrosion resistance on the surface of magnesium, aluminum, or an alloy thereof. The purpose of this invention is to provide a new surface treatment method.

Means for Solving the Problems The present inventors first applied an anodic oxidation treatment to magnesium or its alloy using an alkaline bath containing a specific amphoteric metal compound and an aluminate, thereby making the surface dense. However, as a result of further research, we found that the carbonate of an alkali metal or alkaline earth metal was contained at a concentration of 2N or more, and that alkali metal or alkaline earth metal carbonate was used as a film formation promoter. By anodizing magnesium or its alloys, or aluminum or its alloys, in a bath containing at least one selected from carbonates of alkaline earth metals, the surface of the magnesium or its alloys becomes crystalline. It has been discovered that a dense film of magnesium oxide and crystalline aluminum oxide can be easily formed and the above object can be achieved, and based on this knowledge, the present invention has been completed.

That is, in forming a corrosion-resistant film on the surface of magnesium, aluminum, or an alloy thereof by anodic oxidation, the present invention provides carbonates of alkali metals and alkaline earth metals selected from carbonates of alkali metals and alkaline earth metals with a concentration of 2 normal or more. The present invention provides a surface treatment method characterized by performing anodization treatment in a bath containing at least one kind of salt and an alkali metal halide.

The bath used in the present invention contains at least one selected from carbonates of alkali metals and alkaline earth metals, and from the viewpoint of solubility in water, sodium carbonate, potassium carbonate,
Alkali metal carbonates such as lithium carbonate are suitable. It is necessary to use these carbonates at a concentration of 2N or more. If the concentration is lower than this, a film with good crystallinity and high hardness cannot be obtained.

In addition, in order to form a dense film without pores on the surface of magnesium, aluminum, or their alloy, this bath also contains a film-forming accelerator.
It is necessary to add alkali metal halides such as potassium fluoride, potassium iodide, sodium fluoride, sodium iodide and the like. By using such a film-forming agent, a dense film that does not require pore sealing treatment can be obtained. Furthermore, an acidic oxide such as potassium permanganate can be added to this bath as necessary to promote the oxidation reaction in the anodizing treatment.

In the anodizing method of the present invention, either direct current or alternating current can be used as a power source.
When using direct current as a power source, use magnesium, aluminum, or an alloy thereof as an anode, a stainless steel plate or a lead plate as a cathode, and use the above bath to reduce the bath temperature to -10
Anodic oxidation treatment is performed by electrolyzing at ~80°C and a current density in the range of 0.1~20A/ ^dm2 . When alternating current is used, magnesium, aluminum, or an alloy thereof is used as the counter electrode, and the same treatment as above is performed.

This type of anodizing process creates a thick layer on the surface of magnesium, aluminum or their alloys.
A dense white film of about 0.5 to 50 μm is formed.
This film is mainly composed of crystalline magnesium oxide for magnesium and its alloys, and is mainly composed of crystalline aluminum oxide for aluminum and its alloys.

Effects of the Invention According to the surface treatment method for magnesium, aluminum, and alloys thereof of the present invention, a dense film having a uniform white color, hardness, and excellent corrosion resistance is formed on the surface of the magnesium, aluminum, and alloys of the present invention. The treatment method is an excellent method with high practical value.

EXAMPLES Next, the present invention will be explained in more detail with reference to practical examples.

Comparative example 1 Pure magnesium MI material is used as an anode, stainless steel plate (SUS304) is used as a cathode, Na ₂ CO ₃ 200g/l
Anodizing was carried out for 5 minutes under electrolytic conditions of a bath temperature of 20° C. and a DC current density of 0.2 A/dm ² using a carbonate bath consisting of: As a result, the surface of the MI material has a thickness of approx.
A dense white film of 3.0 μm was formed, but upon magnification and observation using an electron microscope, many fine pores were observed on the surface.

In addition, two sheets of pure magnesium MI material are used for the counter electrode,
Except for using alternating current and setting the current density to 0.7A/ ^dm2 ,
When anodic oxidation treatment was performed for 5 minutes under the same conditions as above, a dense white film with a thickness of 4 μm was formed, but when it was enlarged and observed with an electron microscope, many fine pores were observed on the surface. Ta. As a result of X-ray diffraction, these films were found to be mainly composed of magnesium oxide.

Comparative Example 2 When anodizing was performed in the same manner as in Example 1 except that magnesium alloy AZ31 was used instead of the pure magnesium MI material in Example 1, a dense white film was formed in both direct current and alternating current. However, when examined under magnification using an electron microscope, many fine pores were observed on the surface. As a result of X-ray diffraction, the film was found to be mainly composed of magnesium oxide.

Example 1 Anodizing treatment was carried out in exactly the same manner as in Example 1, except that the carbonate bath in Example 1 contained Na ₂ CO ₃ and KI at concentrations of 200 g/l and 10 g/l, respectively. When this film was enlarged with an electron microscope and observed under both direct current and alternating current, no fine pores were observed on the surface.

The thickness of the film was 4 μm in the case of direct current and 3 μm in the case of alternating current.

Example 2 Anodizing was carried out in the same manner as in Example 1 except that the bath temperature was changed from 20 to 70°C, and a dense white film was observed under an electron microscope under both direct current and alternating current. Enlarge with photos,
Upon observation, no micropores were observed on the surface. Furthermore, no significant change in film thickness was observed within this temperature range.

Comparative Example 3 Using pure aluminum 1100 material as an anode and a lead plate as a cathode, a carbonate bath consisting of 200 g/l of Na ₂ CO ₃ was used, and the electrolytic conditions were 10°C at a bath temperature of 40°C and a DC current density of 0.2 A/dm ^2. Anodization treatment was performed for a minute. As a result, a dense white film with a thickness of approximately 2 μm was formed on the surface of the pure aluminum 1100 material, but when observed under magnification using an electron microscope, many fine pores were observed on the surface.

In addition, when two sheets of pure aluminum 1100 material were used as the counter electrode and anodized for 10 minutes under the same conditions as above, except that the current density was 0.4 A/dm ² using alternating current, the result was a 2 μm thick layer. A white film was formed. As a result of X-ray diffraction, these films were found to be mainly composed of aluminum oxide.

Example 6 Anodizing was performed in the same manner as in Example 5, except that aluminum alloy 5052 was used instead of the pure aluminum 1100 material in Comparative Example 3.
A dense white film was formed in both direct current and alternating current, but upon magnification and observation using an electron microscope, many fine pores were observed on the surface. As a result of X-ray diffraction, the film was found to be mainly composed of aluminum oxide.

Example 4 Anodizing treatment was carried out in exactly the same manner as in Comparative Example 3, except that the carbonate bath in Comparative Example 3 contained Na ₂ CO ₃ and KI at concentrations of 200 g/l and 10 g/l, respectively. When applied, a dense white film was formed under both direct current and alternating current.When this film was enlarged and observed using an electron microscope, no micropores were observed on the surface.

The thickness of the film was 2 μm in the case of direct current and 2 μm in the case of alternating current.

Claims (1)

[Claims] 1. At least one salt selected from carbonates of alkali metals and alkaline earth metals with a concentration of 2 normal or more when forming a corrosion-resistant film on the surface of magnesium, aluminum, or their alloys by anodic oxidation. and an alkali metal halide.