JPH06116740A - Surface treatment of magnesium alloy product - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of the surfaces of Mg-Al alloy parts without deteriorating the dimensional accuracy of the parts by cleaning the surfaces of the parts, then treating the surfaces with a pyrophosphate soln. and an alkali hydroxide soln. CONSTITUTION:The Al-contg. alloy parts, such as MC 1, 2, 3 and MDC1A, 1B, 1C, 2B and 3A and others specified in JIS(Japanese Industrial Standards) H 5303 are first degreased by an org. solvent, such as trichloroethylene and are thereafter subjected to a surface treatment for 0.5 to 5minutes with an aq. soln. of 10 to 50g/l concn. of the Na salt or K salt of pyrophosphoric acid kept at room temp. to 70 deg.C. After these parts are washed with water, the parts are subjected to a surface treatment for 0.5 to 5 minutes with an aq. soln. of 10 to 100g/l concn. of alkaline metal hydroxides, such as KOH and NaOH, kept at room temp. to 70 deg.C. The Mg alloy parts are etched by the treatment with these solns., by which the Mg-Al alloy parts having the excellent corrosion resistance and the excellent surface characteristics are obtd. without degrading the dimensional accuracy and without subjecting the parts to a chromate treatment and coating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method for an aluminum-containing magnesium alloy product, and more particularly, to a surface-treating method for an aluminum-containing magnesium alloy product capable of achieving practical corrosion resistance without chromate treatment or painting. Regarding surface treatment method.

[0002]

2. Description of the Related Art Conventionally, as an anticorrosion treatment for aluminum-containing magnesium alloy products, a surface conditioning treatment and a cleaning treatment using a chromate solution, ferric nitrate solution, concentrated sodium hydroxide solution, hydrofluoric acid solution, etc. After that, chemical conversion treatment such as chromate treatment, anodization treatment, and coating were performed.

[0003]

In these surface conditioning treatments and cleaning treatments, the etching amount on the surface of the aluminum-containing magnesium alloy product is large, so that there is a problem of dimensional accuracy, and the corrosion resistance of the cleaned surface decreases. As a result, it cannot be used indoors. Therefore, following these surface conditioning treatments and cleaning treatments, it is necessary to inevitably perform chemical conversion treatments, paintings, etc. to make them corrosion resistant. In addition, since these surface conditioning treatments and cleaning treatments involve high treatment temperatures and use of fluoride-added baths,
The work environment may be damaged by the generation of mist,
There was also a problem in terms of pollution control.

The present invention has been made in view of the above problems of the prior art. The object of the present invention is to ensure dimensional accuracy, to process under mild conditions, and to prevent pollution problems. It is an object of the present invention to provide a surface treatment method for an aluminum-containing magnesium alloy product capable of simultaneously achieving cleaning and imparting corrosion resistance.

[0005]

The surface treatment method for an aluminum-containing magnesium alloy product of the present invention is characterized in that the aluminum-containing magnesium alloy product is treated with a pyrophosphate solution, and then with an alkali hydroxide solution. And preferably the pyrophosphate concentration is 10 to 50 g /
l aqueous solution and alkaline hydroxide concentration 10-100 g / l aqueous solution, and carry out treatment with pyrophosphate aqueous solution and alkaline hydroxide aqueous solution at room temperature to 70 ° C for 0.5 to 5 minutes, respectively. Is characterized by.

The aluminum-containing magnesium alloy product which is the object of the surface treatment method of the present invention is JIS H5303.
MC 1, 2, 3 and 5, MDC 1A, 1B, 1C, 2
A, 2B and 3A, MB1, 2 and 3, MT1 and 2, and the like, which are die castings, sand castings, die castings, wrought products, and cut products obtained by using aluminum-containing magnesium alloys. Is equally applicable to

In the surface treatment method of the present invention, practically, the surface of the aluminum-containing magnesium alloy product is degreased with an organic solvent such as trichlorethylene, ethanol, methanol or acetone to obtain a pyrophosphate concentration of 10 to 10.
It is treated with an aqueous solution of 50 g / l, washed with water, treated with an aqueous solution having an alkali hydroxide concentration of 10 to 100 g / l, washed with water, and then sufficiently dried. The treatment temperature and treatment time for the treatment with the aqueous pyrophosphate solution and the treatment with the aqueous alkali hydroxide solution are preferably room temperature to 70 ° C. and 0.5, respectively.
It is ~ 5 minutes, but it can be carried out outside this range.

In the surface treatment method of the present invention, since no bath containing fluoride is used, there is no problem of pollution, and it adheres to the surface of the alloy product despite the treatment under mild conditions. The impurities (release agent, oxide, etc.) present can be effectively removed, and since the treatment is performed under mild conditions, the etching amount is small, and therefore the dimensional accuracy can be secured.

In the surface treatment method of the present invention, mainly magnesium on the surface of the aluminum-containing magnesium alloy product is eluted by the treatment with the aqueous solution of pyrophosphate, so that the alloy components aluminum and zinc are concentrated and remain on the surface. Subsequent treatment with an alkali hydroxide aqueous solution turns magnesium into Mg (OH) ₂ and forms an extremely thin film on the surface of the alloy product, which never elutes. Further, aluminum and zinc concentrated on the surface also become oxides or hydroxides by the treatment with the alkali hydroxide aqueous solution, and form a film on the surface of the alloy product. Such a coating improves the corrosion resistance of the aluminum-containing magnesium alloy product, and provides sufficient corrosion resistance for indoor use.

When it is desired to further improve the corrosion resistance of the aluminum-containing magnesium alloy product treated by the surface treatment method of the present invention, the treated surface can be further chromated. In this chromate treatment, the effect can be sufficiently achieved even if the dichromate concentration is much lower than the conventional one. Although the reason for this is not clear, treatment with the aqueous solution of pyrophosphate and treatment with the aqueous solution of alkali hydroxide produce aluminum and magnesium hydroxide on the surface of the aluminum-containing magnesium alloy product, and the subsequent chromate treatment. It is considered that chromium oxide / hydroxide with good adhesion is easily formed by the treatment. Further, by lowering the concentration of dichromate in this way, it is possible to prevent the surface of the product from becoming rough. Further, the treatment with the aqueous solution of pyrophosphate and the treatment with the aqueous solution of alkali hydroxide remove the impurities adhering to the surface of the aluminum-containing magnesium alloy product, so that the life of the chromate solution can be extended. Further, by containing a small amount of anionic surfactant in the chromate treatment liquid, it is possible to prevent the generation of mist during the chromate treatment, and the working environment is not impaired. For example, dichromate concentration 35-8
A chromate treatment solution having a concentration of 0 g / l, nitric acid (60%) of 25 to 130 ml / l, and an anionic surfactant concentration of 0.1 to 0.6 g / l can be used, and a treatment temperature for this chromate treatment, The treatment time is preferably from room temperature to 35 ° C., 0.
It takes 5 to 2 minutes, but it can be carried out outside this range.

Subsequent to the above chromate treatment, the aluminum-containing magnesium alloy product is left in the air to complete the chromate film, and then the excess chromate treatment liquid is removed and washed with water.

[0012]

【Example】

Examples 1 to 18 and Comparative Example 1 JIS H5303 MC1, MC2, M described in Table 1
A molten metal of C5 or MC6 (comparative example 1 not containing aluminum) was cast into a mold to prepare a test piece having a length of 10 cm, a width of 5 cm and a thickness of 2 mm. The test pieces were degreased with trichlorethylene and then placed in an aqueous solution of pyrophosphate of the type and concentration listed in Table 1 at the temperatures listed in Table 1
It was immersed for the time described in 1) and then washed with water. Then, it was immersed in an aqueous solution of an alkali metal hydroxide of the type and concentration shown in Table 1 at the temperature shown in Table 1 for the time shown in Table 1, then washed with water, and dried sufficiently. Dimensional change of products obtained by such treatment and JIS Z2371 "Salt spray test method"
Evaluation by JIS H8502 after 8 hours of salt spray test by R. No (n = 5) was as shown in Table 1.

Comparative Example 2 A test piece having a length of 10 cm, a width of 5 cm and a thickness of 2 mm was prepared by casting a molten metal of JIS H5303 MC2 alloy in a mold. Degrease this test piece with trichlorethylene,
It was washed with water and dried thoroughly. The dimensional change of the product obtained by the above treatment and the evaluation by JIS H8502 after 8 hours of the salt spray test according to JIS Z2371 "Salt spray test method". No (n = 5) was as shown in Table 1.

Comparative Example 3 A molten sample of JIS H5303 MC2 alloy was cast into a mold to prepare a test piece having a length of 10 cm, a width of 5 cm and a thickness of 2 mm. Degrease this test piece with trichlorethylene,
Then _{CrO 3 180g / l, Fe (} NO 3) 3 · 9H 2 O 33g / l and Na
Treated in a solution containing 33 g / l F for 2 minutes at 30 ° C.,
It was washed with water and dried thoroughly. The dimensional change of the product obtained by the above treatment and JIS H8502 after 8 hours of the salt spray test according to JIS Z2371 "Salt spray test method"
Evaluation by R. No (n = 5) was as shown in Table 1.

[0015]

[Table 1] As is clear from the data in Table 1, the aluminum-containing magnesium alloy product treated by the surface treatment method of the present invention has a small dimensional change and has sufficient corrosion resistance for indoor use.

[0016]

According to the surface treatment method of the present invention, it is possible to obtain an aluminum-containing magnesium alloy product having a small dimensional change and sufficient corrosion resistance for indoor use.

Claims (2)

[Claims] 1. A method for surface treatment of an aluminum-containing magnesium alloy product, which comprises treating the aluminum-containing magnesium alloy product with a pyrophosphate solution and then treating with an alkali hydroxide solution. 2. An aqueous solution having a pyrophosphate concentration of 10 to 50 g / l and an alkali hydroxide concentration of 10 to 100 g / l are used, and the treatment with the pyrophosphate aqueous solution and the treatment with an alkali hydroxide aqueous solution are carried out at room temperature, respectively. 0.5 ~ 5 at ~ 70 ℃
The surface treatment method according to claim 1, wherein the surface treatment is performed for a period of time.