JP2823118B2 - Conversion coating treatment method for magnesium alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical conversion coating method for imparting corrosion resistance and paint adhesion to a magnesium alloy, especially a magnesium alloy die-cast product.

[0002]

[Prior art] Magnesium alloy is the lightest among practical metals,
Due to its high specific strength, its applications are expanding to reduce the weight of aircraft, automobiles, computer-related parts, and the like. However, since magnesium is a very chemically active metal, magnesium alloys have very poor corrosion resistance. Therefore, as a surface treatment method for imparting corrosion resistance and paint adhesion, Dow in the United States has been conventionally used.
The method according to the invention of Chemical Company is commonly practiced. These methods include Dow 7, Dow 2
There are a chemical conversion treatment method called 0 and an anodizing treatment method called Dow 17, which are widely practiced industrially.

[0003] However, the anodizing treatment method such as Dow 17 requires a relatively high bath voltage of 60 to 95 V, and requires high cooling equipment such as a refrigerator to control the bath temperature. There is. Also Dow
7, it is difficult to say that a chemical conversion coating method such as Dow 20 provides sufficient corrosion resistance, and these treatment baths contain hexavalent chromium and a fluorine compound. However, it is difficult to treat hexavalent chromium ions and fluorine ions at the same time during the treatment of wastewater and wastewater in the treatment bath, and the above-mentioned chemical conversion film treatment method has disadvantages such as high costs for wastewater treatment and wastewater treatment.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a magnesium alloy with excellent corrosion resistance and paint adhesion by a chemical conversion treatment method which is inexpensive in equipment and work cost. It is an object of the present invention to provide a surface treatment method using a treatment liquid that can impart the property and that can easily perform wastewater treatment and waste liquid treatment.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, after performing a known surface cleaning treatment, such as pickling and degreasing, on a magnesium alloy. By immersing in a new chemical conversion coating solution described below, it was found that magnesium alloys can be provided with excellent corrosion resistance and coating adhesion, which are markedly different from conventional conversion coating treatment methods. Completed the invention.

In the surface treatment method of the magnesium alloy of the present invention, it is a constitutional requirement of the present invention that the magnesium alloy is immersed in a chemical conversion coating solution having the following composition under the following processing conditions. Sodium dichromate _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 60~150g / L magnesium sulfate _{(MgSO 4 · 7H 2 O)} 30~75 g / L manganese sulfate _{(MnSO 4 · 5H 2 O)} 30~75 g / L zirconium sulfate (Zr (SO ₄ ) ₂ .4H ₂ O) 4 to 12 g / L ethylene glycol (OHCH ₂ .CH ₂ HO) 8 to 25 mL / L N-methyl-2-pyrrolidone 1.5 to 6 mL / L treatment conditions : temperature 60 to 80 ° C. immersion time 5 to 10 minutes A treatment liquid having a composition outside the above range cannot provide sufficient corrosion resistance and coating film adhesion. Further, the stability of the bath is deteriorated, and the life is shortened. And that conversion coating treatment solution having the following composition is especially preferred: sodium dichromate _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 80~120g / L magnesium sulfate _{(MgSO 4 · 7H 2 O)} 40~60 g / L manganese sulfate _{(MnSO 4 · 5H 2 O)} 40~60 g / L of zirconium sulfate _{(Zr (SO 4) 2 ·} 4H 2 O) 5~10 g / L ethylene glycol _{(OHCH 2 · CH 2 HO)} 10 ２０20 mL / L N-methyl-2-pyrrolidone 2-5 mL / L magnesium alloy is subjected to the chemical conversion treatment of the present invention.
A known surface cleaning treatment is performed as a pretreatment. Known surface cleaning treatments include, for example, steam degreasing, for example, degreasing using trichloroethane vapor; solvent degreasing, for example, degreasing using a thinner; alkali degreasing, for example, degreasing using an alkaline aqueous solution. There are mechanical methods such as shot peening and pickling, chemical methods such as activation and mixtures thereof, and after sufficiently cleaning the surface of the magnesium alloy product by these known methods, the above-mentioned chemical conversion coating solution When the immersion treatment is performed under the above conditions, a brown (chocolate-colored) chemical conversion film is formed on the magnesium alloy surface depending on the immersion time.

As a result of an electron probe microanalyzer (EPMA) analysis and an X-ray diffraction test, it was confirmed that this film was a film mainly composed of manganese dioxide, a zirconium compound and chromate. The zirconium sulfate added to the chemical conversion coating solution of the present invention has an effect of increasing the corrosion resistance of the chemical conversion coating by depositing a zirconium compound such as zirconium-chromate in the coating.

Further, the magnesium alloy treated with the chemical conversion coating of the present invention has much higher corrosion resistance and coating properties than those subjected to the conventionally known chemical conversion coating treatment, for example, Dow 7 or Dow 20. Adhesion was confirmed. Ethylene glycol and N-methyl-2-pyrrolidone added to the chemical conversion coating solution of the present invention suppress the radical and excessive reaction between the components of the processing solution and the surface of the magnesium alloy, and have the effect of forming a dense chemical conversion coating. When these were excluded, it was found that problems such as the formation of a rough film due to excessive reaction and the shortening of the service life of the treatment solution in which manganese dioxide was precipitated in the solution were found to occur.

Next, in order to explain the present invention in detail, examples and comparative examples of the conventional method will be described.

[0010]

[Example] Magnesium die casting alloy ASTM N
o. AZ91D is a test piece (dimensions 100 × 50 × 3m
m) was used in the experiment. Its chemical composition is as follows. Treatment Method 1 (Invention) After a test piece was subjected to alkali degreasing, pickling and activation by a known method, it was immersed in a chemical conversion coating solution of the present invention having the following composition.

[0011] sodium dichromate _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 80g / L of magnesium sulfate _{(MgSO 4 · 7H 2 O)} 40g / L manganese sulfate _{(MnSO 4 · 5H 2 O)} 40g / L of zirconium sulfate (Zr (SO ₄ ) ₂ .4H ₂ O) 8 g / L Ethylene glycol (OHCH ₂ .CH ₂ HO) 10 mL / L N-methyl-2-pyrrolidone 2 mL / L Temperature 80 ° C., Immersion time 5 minutes Brown (chocolate color) ) Was obtained.

[0012] treatment method 2 (present invention) dichromate Sodium _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 120g / L magnesium sulfate _{(MgSO 4 · 7H 2 O)} 60g / L manganese sulfate (MnSO ₄ · 5H ₂ O) 60 g / L zirconium sulfate (Zr (SO ₄ ) ₂ .4H ₂ O) 6 g / L ethylene glycol (OHCH ₂ .CH ₂ HO) 15 mL / L N-methyl-2-pyrrolidone 3 mL / L Temperature 60 ° C., immersion For 10 minutes, a brown, chocolate-colored, smooth, dense and uniform chemical conversion film was obtained.

Treatment Method 3 (Comparative Example) Dow 7 treatment, which is most commonly used as a chemical conversion coating treatment of a magnesium alloy, was carried out as a comparative example. Sodium dichromate _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 140g / L magnesium fluoride (MgF ₂₎ 2.5g / L Temperature 98-100 ° C., uniform chemical conversion coating film by immersion time tan 30 minutes smooth was gotten.

Treatment Method 4 (Comparative Example) Dow 20 which is a chemical conversion treatment method for a magnesium alloy
Processing was performed. Sodium dichromate _{(Na 2 Cr 2 O 7 ·} 2H 2 O) 180g / L sodium hydrogen fluoride (NaHF ₂₎ 15g / L of aluminum sulfate _{(Al 2 (SO 4) 3} .18H 2 O) 10g / L nitric acid ( (68% concentration) (HNO ₃ ) 85 mL / L Room temperature, immersion time 2 minutes A smooth and uniform conversion film of yellow rainbow color was obtained.

Corrosion resistance test: Treatment methods 1 and 2 of the above examples
The test pieces obtained in Comparative Examples 3 and 4 were subjected to a salt spray test according to JIS standard Z2371. The results were as follows. Coating film adhesion test: Epoxy-based cationic electrodeposition coating was applied to each of the above test pieces to a thickness of 20 μm, and a cross-cut reaching the base material was made on the coating film, and then a similar salt water spray test was performed. . After 500 hours of salt spray exposure, a coating film adhesion test using an adhesive tape was performed to measure the peeling width of the coating film.

[0016]

According to the corrosion test by the salt spray test described above and the coating film adhesion test after the salt spray exposure, the chemical conversion film treatment method of the present invention is compared with a conventionally known chemical conversion film treatment method. It was confirmed that the film exhibited remarkably excellent corrosion resistance and coating film adhesion. Since the chemical conversion coating solution according to the present invention does not contain fluoride in its components, the treatment of only hexavalent chromium and heavy metal ions is sufficient in the drainage treatment and waste liquid treatment. Therefore, a known chemical conversion coating treatment solution such as Dow It is extremely advantageous because there is no difficulty in wastewater treatment and waste liquid treatment caused by the coexistence of hexavalent chromium and fluorine ions as in No. 7 and Dow 20. As described in detail above, the present invention is an industrially extremely inventive invention.

Claims (2)

(57) [Claims] 1. After performing a surface cleaning treatment on a magnesium alloy, sodium dichromate 60 to 150 g / L, magnesium sulfate 30 to 75 g / L, manganese sulfate 30
-75 g / L, zirconium sulfate 4-12 g / L, ethylene glycol 8-25 mL / L, N-methyl-2
-A method for converting a magnesium alloy into a chemical conversion film, wherein the method is immersed in a processing solution containing 1.5 to 6 mL / L of pyrrolidone at a temperature of 60 to 80C for 5 to 10 minutes. 2. Sodium dichromate 80-120 g /
L, magnesium sulfate 40-60 g / L, manganese sulfate 40-60 g / L, zirconium sulfate 5-10 g / L
The method according to claim 1, wherein a treatment solution containing L, ethylene glycol 10 to 20 mL / L, and N-methyl-2-pyrrolidone 2 to 5 mL / L is used.