JP2002294466A - Conversion coating solution for magnesium alloy, surface treatment method, and magnesium-alloy base material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To give adhesiveness to a paint film, a rust-prevention property, and corrosion resistance onto a magnesium alloy. SOLUTION: This conversion coating solution for the magnesium alloy is characterized by including phosphate ions and permanganate ions, and having pH of 1.5-7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical conversion treatment solution for a magnesium alloy capable of imparting excellent corrosion resistance, rust prevention and coating film adhesion to a magnesium alloy, and a surface using the chemical conversion treatment solution. The present invention relates to a treatment method and an obtained magnesium alloy substrate. This surface treatment method is required for the surface treatment of magnesium alloy used for electronic devices such as mobile phones, personal computer housings, and home appliances such as televisions, which require corrosion resistance, rust prevention, coating film adhesion, and electrical conductivity. It is suitable. In this surface treatment method, at the same time as promoting the formation of a magnesium oxide film by the high oxidizing power of permanganate ions, phosphorus-manganese compounds and phosphorus-magnesium compounds are formed by the reaction of permanganate ions and magnesium ions with phosphate ions. A chemical conversion film having excellent corrosion resistance, rust prevention, and high coating film adhesion can be formed on a magnesium alloy by forming a film containing the same.

[0002]

2. Description of the Related Art As a chemical conversion treatment method of a magnesium alloy, a method of immersing a magnesium alloy in a chemical conversion treatment solution containing hexavalent chromium is generally used. For example, JIS-H8
651, MIL-M-3171. However, hexavalent chromium has an adverse effect on the environment and the human body. Therefore, the working environment for surface treatment is poor, and sophisticated drainage equipment is required to prevent hexavalent chromium emission into the environment. The use of hexavalent chromium not only has an adverse effect on the environment and the human body, but also has the problem of increasing the cost of surface treatment equipment. In addition, in the near future, its use will or will likely be banned in many areas. Therefore, there is a need to develop a magnesium chemical conversion treatment technique, which is a so-called non-chromium chemical conversion treatment technique, in place of the chromate treatment.

[0003] As a non-chromium chemical conversion treatment, Japanese Patent Application Laid-Open
Some are described in -126858. A treatment method based on a magnesium phosphate treatment and a phosphate treatment in which a metal such as zirconium other than chromium, titanium, and zinc are added as the prior art are cited. However, as pointed out in Japanese Patent Application Laid-Open No. Hei 7-126858, these surface treatment methods are not practical due to long treatment steps, require a long time for treatment, have sufficient corrosion resistance, rust preventive properties and coating films. There is a problem that adhesion cannot be provided.

As a technique for solving these problems, Japanese Patent Laid-Open No.
No. 126858 discloses a manganese phosphate treatment. The feature of this manganese phosphate treatment method is that, in addition to divalent manganese ions (Mn ²⁺ ) and phosphate ions, aliphatic amines, aromatic amines, and heterocyclic amine compounds are added to the chemical conversion bath. is there. Further, one kind selected from nitrate ion, sulfate ion and fluorine compound may be added to this solution. It is described that by this treatment method, a film having excellent corrosion resistance, rust prevention and coating film adhesion can be formed on a magnesium alloy. However, it is not environmentally preferable to add an amine compound, and this treatment The composition of the solution is complicated and there is a problem of complicated bath management.

JP-A-8-35073 discloses a chemical conversion treatment using permanganate ions.
It is described that a chemical conversion coating having good corrosion resistance can be formed by adding a promoter such as a mineral acid or a fluoride to a permanganic acid aqueous solution. However, the formed film is not manganese phosphate but a film containing manganese oxide and manganese hydroxide, and does not contain a phosphoric acid bond.

[0006]

These prior arts include:
(1) It contains hexavalent chromium, which has an adverse effect on the human body and the environment; (2) The treatment process is long and complicated; (3)
There remain problems such as that high temperature is required for the treatment, (4) sufficient corrosion resistance, anticorrosion, and coating film adhesion cannot be imparted like a chromate film, and (5) bath composition is complicated.
The present invention has been made to solve these problems.

[0007]

The present invention is a chemical conversion treatment solution for magnesium alloys, which contains phosphate ions and permanganate ions and has a pH of 1.5 to 7. The concentration of the compound serving as the phosphate ion supply source is as follows:
It is preferably 20 to 50 g / L, and the concentration of the compound serving as a source of permanganate ions is preferably 1 to 10 g / L. The present invention is also a surface treatment method comprising a step of bringing the above-mentioned chemical conversion treatment solution into contact with a magnesium alloy substrate. It is preferable that the magnesium alloy base material has been subjected to a degreasing treatment, a pickling treatment and a desmutting treatment in advance, and the pickling treatment is
The pickling treatment is more preferably performed using a treatment liquid containing at least one selected from the group consisting of sulfuric acid, nitric acid, and phosphoric acid and a fluorine compound. The present invention is also a magnesium alloy substrate treated by the above surface treatment method. Hereinafter, the present invention will be described in more detail.

[0008] The chemical conversion treatment solution for magnesium alloy of the present invention contains phosphate ions and permanganate ions. The role of the phosphate ions is to impart corrosion resistance by forming a phosphate compound and to improve coating film adhesion. Examples of the supply source of the phosphate ion include phosphorus such as sodium phosphate monobasic, sodium phosphate dibasic, ammonium phosphate monobasic, ammonium phosphate dibasic, potassium phosphate monobasic, and potassium phosphate dibasic. Acid compounds, orthophosphoric acid, and the like. The phosphate compound concentration in the chemical conversion bath is
Although it can be used within the range of 5 g / L within the upper limit of the solubility of the compound used, it is more preferably 20 g / L to 50 g / L. When the amount is less than 5 g / L, sufficient coating film adhesion cannot be obtained, and when it exceeds 50 g / L, no remarkable improvement in performance is observed, so that it is useless.

The role of the permanganate ion is to promote the formation of an oxide film on the surface of the magnesium alloy and to form a phosphorus manganese compound and a manganese oxide film having excellent corrosion resistance. Permanganate ions are supplied from permanganate compounds. Specifically, potassium permanganate, sodium permanganate, ammonium permanganate and the like can be dissolved in water to generate permanganate ions. The concentration of the permanganate salt compound in the chemical conversion bath can be used within the range of 1 g / L to the solubility of the compound used, but is preferably 1 g / L to 10 g / L. If it is less than 1 g / L, the deposition of the manganese compound film becomes insufficient, and sufficient corrosion resistance cannot be obtained. 10g
When / L is exceeded, no performance improvement is observed.

[0010] The chemical conversion solution for magnesium alloy of the present invention has a pH of 1.5 to 7. In the present invention, the pH
Excellent corrosion resistance and coating film adhesion can be exhibited in a wide range of 1.5 to 7. The pH can be adjusted mainly by adding an alkaline solution such as sodium hydroxide or an acidic solution such as orthophosphoric acid, but can also be controlled by the concentration ratio between the first sodium phosphate and the second sodium phosphate. If the pH is lower than 1.5, the dissolution of the magnesium alloy becomes intense, and the surface is damaged, and sufficient corrosion resistance cannot be obtained. If the pH exceeds 7, it is not suitable because sufficient corrosion resistance and coating film adhesion cannot be obtained. This is considered to be due to the extremely small amount of the film deposited and the decrease in the oxidizing power of permanganate ions.

The chemical conversion treatment liquid of the present invention can be suitably used for magnesium alloys. A surface treatment method including a step of bringing the above-mentioned chemical conversion treatment solution into contact with a magnesium alloy substrate is also one of the present invention. The magnesium alloy is a magnesium alloy produced mainly by a die casting method or a thixomold method, and AM50D, AM60D, and AZ91D are preferably used. Other metals for producing the magnesium alloy include, for example, aluminum, manganese, zinc, silver, rare earth elements and the like.

In some cases, these alloys are heavily contaminated with an emulsion oil called a release agent used at the time of casting, or an uneven layer exists on the surface depending on the casting conditions of the alloy. In these cases, an appropriate pretreatment is required to perform normal chemical conversion treatment on the magnesium alloy surface. Usually, surface treatment is performed in the sequence of degreasing, washing with water, pickling, washing with water, desmutting, washing with water, chemical conversion treatment, washing with water and drying.

The purpose of the degreasing treatment is to remove oil on the surface. When the degree of contamination is small, this degreasing step alone is sufficient. The degreasing agent used in the above degreasing treatment is roughly classified into an alkaline degreasing agent and an acidic degreasing agent.
In the present invention, the degreasing method is not particularly limited, but since magnesium is vigorously dissolved in an acidic aqueous solution,
It is preferable to use an alkaline degreaser because magnesium in the magnesium alloy may cause selective melting and damage the surface.

The above-mentioned pickling treatment is performed for the purpose of removing a deviated layer made of fine crystals deviated on the surface and a release agent permeating the deviated layer. In the present treatment method, it is preferable to use an aqueous solution obtained by adding a fluorine compound such as hydrosilicic acid to orthophosphoric acid, or an aqueous solution of an inorganic acid such as sulfuric acid or nitric acid. The concentration of these acids is preferably in the range of 0.3 to 20 g / L, more preferably about 0.3 to 5 g / L. When the amount is less than 0.3 g / L, the period for maintaining the cleaning effect when used repeatedly becomes short, and the solution is replenished.
If the replacement becomes complicated, and if the amount exceeds 20 g / L, the dissolution of the magnesium alloy becomes severe, and the alloy surface is damaged and severe smut is generated. The temperature of the pickling bath can be set in the range from room temperature to the boiling point of the aqueous acid solution. It is preferable to use them. In addition, when an aqueous solution of an organic acid having a carboxylic acid is used, a compound film is formed on the surface of the magnesium alloy, the cleaning effect cannot be sufficiently obtained, and a problem may occur in coating film adhesion. Also, a single aqueous solution of orthophosphoric acid, phosphorous acid or the like having a concentration of about 1 g / L or more phosphorylates the magnesium surface, does not provide a sufficient cleaning effect, and may cause a problem in coating film adhesion. As an acid to be used, an acid which is inactive against a magnesium alloy such as sulfuric acid or nitric acid is desirable.

The desmutting treatment is performed to remove contaminants (smut) on the surface of the magnesium alloy, and is usually washed with an alkaline aqueous solution such as sodium hydroxide. After performing the degreasing treatment, the pickling treatment, the desmutting treatment, and the chemical conversion treatment, they are washed with water according to a conventionally known method. The drying step can also be performed by a conventional method.

According to the surface treatment method of the present invention, excellent corrosion resistance, rust prevention and coating film adhesion can be imparted to a magnesium alloy substrate in a wide range of pH 1.5 to 7, so that a mobile phone or a personal computer housing can be provided. Applications can be expected in a wide range of fields such as electronic devices such as electronic devices and home electric appliances such as televisions. The magnesium alloy substrate thus obtained is also one of the present invention.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Several embodiments of the present invention will be described below. However, the present invention is not limited to this embodiment. Example 1 A chemical conversion film was formed on a magnesium alloy by the steps of degreasing, washing, pickling, washing with water, desmutting, washing with water, manganese phosphate treatment, washing with water, and drying under the following conditions. Magnesium alloy: AZ91D test piece (size:
Degreasing: 1% by weight "Surf Fine 100" (alkaline degreasing agent: Nippon Paint Co., Ltd.) aqueous solution, bath temperature 50 ° C, treatment time 2 minutes Pickling: orthophosphoric acid 0.4g / L, fluorosilicate Hydroic acid 0.03 g / L, rest is tap water, bath temperature 50 ° C., treatment time 2 minutes Desmutting: sodium hydroxide 20 g / L, sodium gluconate 3.1 g / L, bath temperature 60 ° C., treatment time 5 minutes Chemical conversion treatment: KMnO ₄ concentration 5.5 g / L, primary phosphoric acid concentration 45 g / L, orthophosphoric acid concentration 1.8 g / L, the rest being tap water, pH 2.8, bath temperature 50 ° C, treatment time 2 minutes Drying conditions : 10 minutes in a heating furnace at 100 ° C.

The uncoated corrosion resistance, corrosion resistance after coating, coating film adhesion, and film appearance were evaluated, and the surface electric resistance was measured. Note that, except for the post-painting corrosion resistance evaluation, the above-described obtained magnesium alloy test piece was carried out, and the post-painting corrosion resistance evaluation was carried out on the above-obtained magnesium alloy test piece, which was further subjected to powder coating. . The results are shown in Table 1.

(1) Corrosion resistance evaluation method. Unpainted corrosion resistance (corrosion resistance of magnesium alloy after chemical conversion coating treatment) was evaluated by a salt spray test (SST). The SST is a test for evaluating the corrosion occurrence rate after spraying a 5% by weight aqueous solution of sodium chloride in a tester kept at 35 ° C. for a predetermined time. The corrosion resistance was evaluated by visually observing the uncorroded portion residual ratio for 48 hours in SST.

(2) Corrosion resistance after coating The corrosion resistance after coating was evaluated by SST. After installing the cross-cut coated film on the SST tester for a predetermined time (96 h), a cellophane tape is pressure-bonded to the surface of the cross-cut portion, the tape is peeled off according to a specified method, and the maximum coating film from the cut portion The peel width was measured. The smaller the peel width, the better the corrosion resistance after painting. Gray epoxy powder coating (Magdyne PD-E, manufactured by Nippon Paint) was used as the paint.
The conditions for curing the coating film are 160 ° C. and 20 minutes, and the dry film thickness of the coating film is 40 μm.

(3) Coating adhesion The coating adhesion was evaluated by a hot water immersion test. The hot water immersion test means that a sample is immersed in warm water of 50 ° C. for a predetermined time (24 hours, 96 hours).
h) This is a method for evaluating the adhesion of the coating film after immersion. At this time, the adhesion of the coating film was determined by inserting a cross hatch consisting of 100 1 mm squares into the coating film, pressing a cellophane tape on the cross hatch surface and peeling off the tape by a prescribed method. The evaluation was made by a grid test.

(4) Surface Electric Resistance Measurement Method The surface electric resistance was measured by a two-terminal method. Nine locations were measured for one sample, and the average of the seven values excluding the maximum value and the minimum value was measured as the surface resistance value. For the measurement, a surface resistance measuring device (EP-T36)
0, manufactured by Keyence Corporation). (5) Evaluation method of film appearance The film was visually evaluated.

[0023]

[Table 1]

Example 2 The chemical conversion treatment bath of Example 1 was tested in the same manner as in Example 1 except that 75% by weight of orthophosphoric acid was added so as to have a pH of 1.8 while measuring with a pH meter. Pieces were prepared, evaluated and measured.

Example 3 A test piece was prepared and evaluated in the same manner as in Example 1 except that a 20% by weight aqueous sodium hydroxide solution was added to the chemical conversion treatment bath of Example 1 to adjust the pH to 6.9. Measurements.

Comparative Example 1 A test piece was prepared, evaluated and measured in the same manner as in Example 1 except that a 20% by weight aqueous solution of sodium hydroxide was added to the chemical conversion treatment bath of Example 1 to adjust the pH to 9. Was done.

Comparative Example 2 The procedure of Example 1 was repeated except that a 20% by weight aqueous solution of sodium hydroxide was added to the chemical conversion bath of Example 1 to adjust the pH to 12.
A test piece was prepared, evaluated and measured in the same manner as described above.

Example 4 A test piece was prepared, evaluated and measured in the same manner as in Example 3 except that the pickling bath in Example 3 was changed to a 1 g / L sulfuric acid aqueous solution.

Example 5 A test piece was prepared, evaluated and measured in the same manner as in Example 3, except that the pickling bath in Example 3 was changed to a 1 g / L nitric acid aqueous solution.

Comparative Example 3 In the chemical conversion treatment step of Example 3, the pH of the chemical conversion treatment bath was adjusted to 6.8 while monitoring the pH value of a 45 g / L aqueous sodium phosphate salt solution with a pH meter. A test piece was prepared, evaluated and measured in the same manner as in Example 3, except that a 20% by weight aqueous solution of sodium hydroxide was used. This was performed using a chemical conversion treatment solution to which no permanganate ion was added, and confirms the effect of the permanganate ion.

Comparative Example 4 A test was conducted in the same manner as in Example 3 except that in the chemical conversion treatment step of Example 3, a substance consisting of 5.5 g / L permanganate ion was used without adding phosphate ions. Pieces were prepared, evaluated and measured. This confirms the effect of phosphoric acid.

Comparative Example 5 A test piece was prepared, evaluated and measured in the same manner as in Example 1 except that the chemical treatment and the subsequent washing step were not performed in the process shown in Example 1.

[0033]

The chemical conversion solution for magnesium alloys of the present invention can stably impart coating film adhesion and corrosion resistance to a magnesium alloy in a wide range of pH 1.5 to 7, so that bath management is simple and easy to handle. . This is a great practical advantage. Further, the surface treatment method of the present invention can provide excellent corrosion resistance and coating film adhesion to a magnesium alloy, and also to a magnesium alloy produced by a casting method contaminated with a release agent. Therefore, it can be widely used for molded articles such as personal computer and mobile phone housings, parts having complicated shapes, automobile parts and the like.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsuyoshi Yamazoe 4-1-1-15 Minamishinagawa, Shinagawa-ku, Tokyo Inside Japan Paint Co., Ltd. (72) Kiyotada Yasuhara 4-1-1-15 Minamishinagawa, Shinagawa-ku, Tokyo Japan Paint Co., Ltd. F term (reference) 4K026 AA01 BA05 BB06 BB08 CA03 CA13 CA23 DA03 EA07

Claims (6)

[Claims] 1. A chemical conversion treatment solution for magnesium alloys, which contains phosphate ions and permanganate ions and has a pH of 1.5 to 7. 2. The concentration of a compound serving as a source of phosphate ions is 20 to 50 g / L, and the concentration of a compound serving as a source of permanganate ions is 1 to 10 g / L. A chemical conversion treatment liquid for a magnesium alloy as described in the above. 3. A surface treatment method comprising a step of bringing the chemical conversion treatment solution according to claim 1 into contact with a magnesium alloy substrate. 4. The surface treatment method according to claim 3, wherein the magnesium alloy substrate has been subjected to a degreasing treatment, an acid washing treatment and a desmutting treatment in advance. 5. The surface treatment method according to claim 4, wherein the pickling treatment is performed using a treatment liquid containing sulfuric acid, nitric acid, and at least one selected from the group consisting of phosphoric acid and a fluorine compound. . 6. A magnesium alloy substrate treated by the surface treatment method according to claim 3.