JP3529415B2 - Method for forming highly corrosion-resistant coating film of magnesium alloy material and magnesium alloy material - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a highly corrosion-resistant coating film on a magnesium alloy material and its magnesium alloy material , and more particularly to a magnesium alloy material such as a wheel for automobiles and parts underneath. The present invention relates to a method suitable for forming a highly corrosion resistant coating film and its magnesium alloy material .

[0002]

2. Description of the Related Art Magnesium alloys have low specific gravity and toughness, and therefore their use as materials for weight reduction is increasing. However, since the corrosion resistance is insufficient, there are restrictions on the application to members such as automobile wheels that require high corrosion resistance. Therefore, the development of a magnesium alloy material having a high corrosion resistance has been studied conventionally.

In Japanese Patent Laid-Open No. 63-250498, a specific high-purity magnesium alloy is used as a base material, the magnesium alloy material is subjected to chemical conversion treatment or anodization treatment, and then a synthetic resin coating film is formed by electrodeposition coating. , A method of obtaining a magnesium alloy material having excellent corrosion resistance is disclosed.

[0004]

However, the magnesium alloy material disclosed in the above publication has a salt spray test (S
ST: JIS-Z-2371) shows excellent corrosion resistance, but a corrosion acceleration test assuming a use environment such as automobile wheels, that is, a salt spray test, hot air drying and a combined corrosion cycle test (CCT) in high temperature and high humidity environment. ), It did not have sufficient corrosion resistance required.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a method for forming a highly corrosion-resistant coating film on a magnesium alloy material that can withstand not only SST but also CCT, and the magnesium alloy material . To provide.

[0006]

Means for Solving the Problems The inventors of the present invention have made extensive studies as to a method for forming a highly corrosion-resistant coating film on a magnesium alloy material in order to solve the above-mentioned conventional problems, and as a result, paint on a specific high-purity magnesium alloy material. We have found that it is possible to make a magnesium alloy material with a coating film with high corrosion resistance by applying a powder coating with an epoxy-based powder coating that uses a specific curing agent after applying a base treatment. The present invention has been completed.

That is, according to the present invention, the iron content is 0.0
Epoxy system with a carboxyl group-containing polyester resin as a curing agent after applying a coating base treatment to a magnesium alloy material having a nickel content of less than 04%, a nickel content of less than 0.002%, and a copper content of less than 0.015%. It is characterized in that powder coating is carried out by powder coating or epoxy powder coating using dibasic acid dihydrazide as a curing agent.

In the magnesium alloy material used in the present invention, the iron content is less than 0.004%, the nickel content is less than 0.002%, and the copper content is less than 0.015%. If these metals are contained in the above specified amount or more,
Satisfactory properties in CCT cannot be obtained even if powder coating is performed after applying a coating base treatment.

The coating surface treatment applied to the magnesium alloy material of the present invention is not particularly limited as long as it is a coating surface treatment capable of forming a chemical conversion film.
Chromate treatment and manganese-modified zinc phosphate treatment are preferable.

In the present invention, the powder coating to be powder coated after the surface treatment for coating is an epoxy powder coating using a carboxyl group-containing polyester resin as a curing agent or an epoxy powder coating using dibasic acid dihydrazide as a curing agent. Is.

As the epoxy resin component in these epoxy powder coatings, a general bisphenol A type epoxy resin can be used. The molecular weight is 360-10
000 is preferable, and 800-3000 is more preferable.
Is. If the molecular weight is too low, the physical properties of the coating film may deteriorate, and if the molecular weight is too high, the flowability may be insufficient and the appearance of the coating film may deteriorate.

The carboxyl group-containing polyester resin used as a curing agent in the present invention preferably has a molecular weight of 1,000 to 10,000, more preferably 1,500 to 8,000. If the molecular weight is too low, the film strength may be insufficient, and if the molecular weight is too high, the flowability may be insufficient and the appearance of the coating film may be deteriorated.

The acid value of the polyester resin having a carboxyl group is preferably 5 to 100 (mgKOH / g), more preferably 20 to 80 (mgKOH / g).
Is. If the acid value is too low, the film strength may be insufficient, and if the acid value is too high, curing may be significant and the film physical properties such as flexibility and impact resistance may be deteriorated.

The glass transition temperature (Tg) of the polyester resin having a carboxyl group is preferably 20 to 100 ° C, more preferably 30 to 80 ° C. If the glass transition temperature is too low, the stability of the coating, that is, blocking resistance may be reduced, and if the glass transition temperature is too high, the flowability may be reduced and the coating film appearance may be deteriorated.

In the present invention, the mixing ratio of the epoxy resin and the polyester resin as the curing agent is preferably 0.5 to 2.0 in terms of the ratio of glycidyl group of epoxy resin / carboxyl group of polyester resin (functional group equivalent ratio). . If the mixing ratio of the epoxy resin and the polyester resin is out of this range, the degree of curing of the coating film becomes too low and the film strength may be insufficient.

Examples of the dibasic acid dihydrazide used as the curing agent in the present invention include adipic acid dihydrazide, isophthalic acid dihydrazide, sebacic acid dihydrazide and the like.

The mixing ratio of the dibasic acid dihydrazide and the epoxy resin is preferably in the range of 0.5 to 4 as the ratio of glycidyl group of epoxy resin / hydrazide group of dibasic acid dihydrazide (functional group equivalent ratio). If this ratio is too low, curing will proceed too much, and the coating film performance such as flexibility and impact resistance may deteriorate. On the other hand, if this ratio is too high, the curing of the coating film becomes too low, and the coating performance such as flexibility and impact resistance may deteriorate.

In the powder coating material of the present invention, in addition to the above-mentioned components, pigments, plasticizers, catalysts and the like can be added within a range that does not impair the effects of the present invention.

The epoxy powder coating material used in the present invention can be produced by a usual method for producing a powder coating material, and can be produced by a dry production method or a wet production method. For example, in the case of manufacturing by a dry manufacturing method, it can be manufactured by mixing (premixing) a resin, a curing agent, a pigment, an additive, etc., dispersing this with a kneader, and then pulverizing it into a powder.

The method of powder coating the epoxy powder coating material in the present invention can be carried out by ordinary electrostatic spray coating, baking, or the like. The film thickness of the coating film formed by powder coating is preferably in the range of 15 to 200 μm. If the film thickness is too thin, the smoothness may deteriorate and a continuous film may not be formed. On the other hand, if the film thickness is too thick, it will exceed the limit of electrostatic coating alone, causing a coating film defect due to the occurrence of cracks, which is also economically disadvantageous.

As the coating base treatment in the present invention, the chromate treatment and the manganese-modified zinc phosphate treatment are preferable as described above. As the chromate treatment, a treatment liquid generally known in the past can be used.
rO ₃ = 0 to 10 g / liter and / or Cr ₂ O ₇
^2- = 0 to 150 g / liter, CrO ₃ + Cr ₂ O ₇
^2- ≧ 0.1 g / liter aqueous solution can be used,
In addition, F ion, Zr ion, Mo ion, K ion, M
It may contain n ions, NO ₃ ⁻ , SO ₄ ²⁻ and the like. If the content of CrO ₃ + Cr ₂ O ₇ ²⁻ is too small, the corrosion resistance decreases, and if the content of CrO ₃ + Cr ₂ O ₇ ²⁻ is too large, the effect becomes smaller than the ratio of the contents and it is economical. May be disadvantageous to. Generally, the treatment temperature is room temperature (20 ° C.) to 50 ° C., and the treatment time is generally 10 seconds to 5 minutes. As the treatment method, for example, a method such as dipping treatment or spraying treatment is adopted. As a commercially available processing solution, a processing agent equivalent to the Dow22 method or manufactured by Nippon Paint Co., Ltd.
An example of the product name is "Alsurf 600N".

In the present invention, the manganese-modified zinc phosphate treatment liquid used for the coating base treatment is zinc ion 0.5 to
A treatment liquid having a composition containing 2.5 g / liter, manganese ion 0.1 to 3 g / liter, phosphate ion 5 to 40 g / liter, fluorine compound (HF conversion) 0.05 to 3 g / liter, and a film formation accelerator. It is preferably used. It is preferable that the manganese-modified zinc phosphate treatment liquid does not substantially contain nickel ions, cobalt ions, and copper ions, and it is preferable that all of them are less than 0.01 g / liter.

If the zinc ion content is less than 0.5 g / liter, the corrosion resistance of the coated magnesium material in SST may decrease. On the other hand, if it exceeds 2.5 g / liter, the corrosion resistance in CCT of the coated magnesium material may decrease. The more preferable content of zinc ion is 0.8 to 1.5 g / liter.

If the content of manganese ions is less than 0.1 g / liter, the corrosion resistance in CCT of the coated magnesium material may decrease. If it exceeds 3 g / liter, the SST of the coated magnesium material may decrease. The more preferable content of manganese ion is 0.8 to 2 g / liter.

The manganese ion / zinc ion content ratio is preferably 1/4 to 4. If this ratio is less than 1/4, the C of the coated magnesium material is
The effect of improving corrosion resistance in CT may decrease.
If this ratio exceeds 4, the effect of improving the corrosion resistance of the coated magnesium material in SST may be reduced. A more preferable content ratio of manganese ion / zinc ion is 0.5 to 2.5.

If the content of phosphate ions is less than 0.5 g / liter, the bath composition may fluctuate greatly and a stable and good film may not be formed. Also, SS
The effect of improving the corrosion resistance at T decreases. Further, even if the content of phosphate ions exceeds 40 g / liter, no further particular improvement in the effect is achieved, which is economically disadvantageous. The more preferable content of phosphate ion is 10 to 20 g.
/ Liter.

Fluorine compound content (converted to HF) is 0.0
If it is less than 5 g / liter, the fluctuation of the bath composition becomes large, and it becomes impossible to stably form a good film. Moreover, even if the content exceeds 3 g / liter, there is no further improvement in the particular effect, which is economically disadvantageous. As the fluorine compound, for example, hydrofluoric acid, hydrosilicofluoric acid, borohydrofluoric acid, zirconium hydrofluoric acid, titanium hydrofluoric acid, and their alkali metal salts or ammonium salts can be used. The more preferable content of the fluorine compound is 0.3 to 1.5 g / liter in terms of HF.

As the film formation accelerator contained in the manganese-modified zinc phosphate treatment solution, for example, at least one selected from nitrite, hydrogen peroxide, and m-nitrobenzene sulfonate can be used. . When nitrite is used alone, it is preferably contained in an amount of 0.01 to 0.5 g / liter. When hydrogen peroxide is used alone, it is preferably contained in an amount of 0.5 to 10 g / liter. When m-nitrobenzenesulfonate is used alone, it is preferably contained in an amount of 0.05 to 5 g / liter. If the amount of these film formation accelerators is less than the above range, the corrosion resistance in SST may be reduced. Further, even if the content exceeds the above range, further remarkable effects cannot be obtained, which is economically disadvantageous.

Further, in the manganese-modified zinc phosphate treatment liquid, as described above, it is preferable that each of the nickel ion, the cobalt ion and the copper ion is less than 0.01 g / liter. If these ions are contained in an amount of 0.01 g / liter or more, the corrosion resistance performance in SST and CCT may deteriorate.

Further, the manganese-modified zinc phosphate treatment liquid may contain 2 to 20 g / liter of nitrate ions. Further, 0.05 to 2 g / liter of chlorate ion may be contained.

The total acidity of the manganese-modified zinc phosphate treatment liquid is 1
It is preferably 0 to 40 points. The total acidity can be determined by collecting 10 ml of the treatment liquid and titrating with 0.1N caustic soda using phenolphthalein as an indicator. If it is less than 10 points, the corrosion resistance in SST may be reduced, and if it exceeds 40 points, no particular effect can be obtained, which is economically disadvantageous.

Further, the free acidity of the manganese-modified zinc phosphate treatment liquid is preferably 0.5 to 2.0 points. The free acidity of the treatment liquid can be determined by collecting 10 ml of the treatment liquid and titrating with 0.1N caustic soda using bromphenol blue as an indicator. 0.5
If it is less than the point, the stability of the treatment liquid is lowered and sludge may be generated. Further, if it exceeds 2.0 points, the corrosion resistance performance in SST may be deteriorated.

The treatment temperature for treatment with the manganese-modified zinc phosphate treatment liquid can be appropriately selected within the range of room temperature (20 ° C.) to 70 ° C. The treatment time is not particularly limited, and is, for example, 10 seconds to 10 minutes, preferably 1 to 2 minutes. As the method of treatment using the manganese-modified zinc phosphate treatment solution, treatment methods such as dipping treatment and spraying treatment can be adopted as in the case of chromate treatment.

[0034]

According to the present invention, the specific high-purity magnesium alloy material is subjected to the coating pretreatment, and then the epoxy powder coating material using the specific curing agent is powder coated. The magnesium alloy material used in the present invention is stipulated so that the contents of iron, nickel and copper are lower than a predetermined value, and regulates the amount of impurities that cause the corrosion resistance to be reduced, which allows sufficient CCT. It has satisfactory corrosion resistance.

Further, in the present invention, the coating film of the epoxy-based powder coating material to be powder coated has a low internal stress, and a coating film having high adhesion with the magnesium alloy material subjected to the coating base treatment is formed. It Therefore, it exhibits more excellent corrosion resistance in CCT.

Further, since the coating film is formed by powder coating, it is not necessary to use a solvent, which is preferable from the viewpoint of environmental hygiene.

Further, in the present invention, when chromate treatment is adopted as the coating base treatment, electric energy cost is not required unlike the conventional general anodic oxidation treatment, and it is preferable from the viewpoint of working environment. Will be things.

Further, in the present invention, when the manganese-modified zinc phosphate treatment which does not substantially contain nickel ions is adopted as the coating base treatment, since a chromium compound which is a harmful substance is not used, it is further preferable from the viewpoint of environmental protection. It will be preferable. Further, when the present base treatment is carried out, the above-mentioned action and effect resulting from the improvement in the adhesion of the coating system are maximized.

[0039]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

(1) Magnesium alloy material As the magnesium alloy material, the following ASTM standards AZ of A and B are used.
The product equivalent to 80 was subjected to T6 heat treatment (400 ° C. × 10 hours → air cooling → 170 ° C. × 10 hours → air cooling).

Magnesium alloy material A : Al content 8.
0 wt%, Zn content 0.67 wt%, Mn content 0.
21% by weight, Si content 0.042% by weight, Fe content 0.002% by weight, Cu content 0.005% by weight, Ni
Content 0.001% by weight Magnesium alloy material B : Al content 8.0% by weight, Z
n content 0.67% by weight, Mn content 0.21% by weight,
Si content 0.042% by weight, Fe content 0.005% by weight, Cu content 0.08% by weight, Ni content 0.01
weight%

(2) Coating undercoat treatment liquid The following three types of treatment liquids were used for the coating undercoat treatment.

[0043] - chromated solution: (Dow22 method of treatment liquid) K ⁺ 1.23 g / l, Cr ^₂ O ₇ ^2- 13
0.48 g / l, MnO ₄ ^- 3.77 g / l, SO ₄ ^2- 1.00g / liter manganese-modified zinc phosphate treatment solution: Zn ²⁺ 1.0 g / l, Mn ²⁺ 1.0 g / l, SiF ₆ ^2- 1.0 g
/ L, PO ₄ ^3- 15.0 g / l, NO ₃ ^-
5.0 g / l, NO ₂ ^- 0.07 g / l,
Total acidity 21 points, free acidity 1.0 points, normal zinc phosphate treatment liquid : Zn ²⁺ 1.0 g / liter,
Mn ²⁺ 1.0 g / liter, SiF ₆ ²⁻ 1.0 g / liter, Ni ²⁺ 1.0 g / liter, PO ₄ ³⁻ 15.0 g
/ Liter, NO ₃ ^- 5.0g / liter, NO
₂ ^- 0.07 g / l, total acidity 21 points, a free acidity 1.0 points

(3) Undercoat coating The following three types of paints were used as the undercoat.

Epoxy powder 1 : (epoxy powder coating using a carboxyl group-containing polyester resin as a curing agent); bisphenol A type epoxy resin (molecular weight about 20)
00, manufactured by Tohto Kasei Co., Ltd. and a carboxyl group-terminated polyester resin (molecular weight about 4000, acid value 53 mgKOH / g,
Tg = 69 ° C., manufactured by Nippon Yupica Co., Ltd.) in a weight ratio of 50/50
Prepared by mixing with, and adding a pigment and additives to prepare a powder coating by the steps of premix-kneader-crushing-classification. Epoxy powder 2 : (epoxy-based powder coating using dibasic acid dihydrazide as a curing agent); the same bisphenol A type epoxy resin as epoxy powder 1 and adipic acid dihydrazide (A.D.H.) are weighted. Prepared by mixing at a ratio of 95/5, adding a pigment and an additive, and preparing a powder coating by the steps of premix->kneader->grinding-> classification. Epoxy electrodeposition : Urethane-modified epoxy cationic electrodeposition coating, trade name "Power Top U-1000", manufactured by Nippon Paint Co., Ltd.

(4) Intermediate coating and top coating As the intermediate coating and top coating, the following acrylic melamine-based coatings a to c were used.

A: (intermediate coating); acrylic melamine-based paint, trade name "OTO-4825", manufactured by Nippon Paint Co., Ltd., R: (topcoat base); acrylic melamine-based paint, trade name "OTO-520-1" Manufactured by Nippon Paint Co., Ltd .: (clear topcoat); acrylic melamine-based paint, trade name "OTO-563", manufactured by Nippon Paint Co., Ltd.

(5) Treatment and Coating Step The magnesium alloy material was treated and coated in the order of the following steps.

Degreasing → Washing → (In case of zinc phosphate treatment) Surface adjustment → Coating base treatment → Washing → Pure water washing → Drying → Undercoating → (If necessary) Intermediate topcoating Each treatment and coating are as follows. went.

Degreasing Alkaline degreasing agent ("Surf Cleaner 53" manufactured by Nippon Paint Co., Ltd., 2% by weight aqueous solution) is used at 40 to 45 ° C.
Was soaked for 3 minutes.

Washing with tap water Using a tap water, spray washing was carried out at room temperature for 15 seconds. For the surface-adjusted manganese-modified zinc phosphate treatment and the normal zinc phosphate treatment, a 0.1 wt% aqueous solution of "Surffine 5MZ-1" manufactured by Nippon Paint Co., Ltd. was used as a surface-conditioning agent, and the surface treatment was performed for 20 seconds at room temperature.

Coating Undercoat Treatment The treatment liquids shown in Table 1 were used, and the manganese-modified zinc phosphate treatment liquid and the ordinary zinc phosphate treatment liquid were immersed at 40 ° C. for 2 minutes. For the chromate treatment liquid, 25 ° C
Immersion treatment was performed for 1 minute.

Washing with tap water Using a tap water, spray washing was carried out at room temperature for 15 seconds. Washing with pure water Using ion-exchanged water, spray washing was performed for 15 seconds at room temperature. Drying It was dried with hot air at 80 ° C. for 10 minutes.

Undercoat Coating Using the paints for undercoat coating shown in Table 1, the undercoat coating was carried out by the following method. Epoxy powder 1 : electrostatic spraying method, film thickness 70-80 μm,
Baking 180 ° C. × 15 minutes Epoxy powder 2 : Electrostatic spraying method, film thickness 70-80 μm,
Baking 180 ° C x 15 minutes- Epoxy electrodeposition : Cationic electrodeposition coating, film thickness 25-30μ
m, baking 170 ° C for 20 minutes

Medium-top coating The coatings for medium-top coating shown in Table 1 were used for coating in the following manner.・Intermediate coating a : Electrostatic spray, film thickness 30-40 μm,
Baking 140 ° C x 20 minutes- Overcoat coating B and C : electrostatic spray, film thickness 15-20
μm (base) + 30-35 μm (clear), baking 1
40 ° C x 20 minutes

(5) Evaluation of Corrosion Resistance Using the coated plate obtained as described above, the corrosion resistance was evaluated by the following two methods.

Salt spray test (SST resistance, JIS-Z
-2371) Put a cross cut on the coated plate and add 5% salt water to 360,72.
After continuing spraying for 0,1000 hours, the maximum corrosion width (one side) from the cut portion was measured.

Composite corrosion cycle test (CCT resistance) A cross cut was put in a coated plate and salt spray (JIS-Z-
2371) 6 hours → Dry (50 ° C) 3 hours → Wet (50
C., relative humidity 95% or more) 14 hours → Blowing (room temperature) 1 hour was set as one cycle, and 10 cycles and 70 cycles were repeated to measure the maximum corrosion width (one side) from the cut portion.

Table 1 shows the corrosion resistance test results measured as described above.

[0060]

[Table 1]

As is clear from Table 1, Examples 1 to 4 in which a high-purity magnesium alloy material was used in accordance with the present invention, which was subjected to a coating base treatment and then undercoated with an epoxy resin using a specific curing agent, It can be seen that both are excellent not only in SST resistance but also in CCT resistance.

On the other hand, Comparative Examples 1 to 3 undercoated by epoxy electrodeposition coating were inferior in SST resistance and CCT resistance. Further, Comparative Examples 4 and 5 using a magnesium alloy material containing a large amount of impurities such as Fe, Cu, and Ni as the magnesium alloy material also have SST resistance and C resistance.
It was inferior in CT property.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI // C23C 22/00 C23C 22/00 B (72) Inventor Naoji Miyazaki 3-3 Shinchi Fuchu-cho, Aki-gun, Hiroshima Mazda Stock In-company (72) Shoji Hirahara 3-1, Shinchi Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) Inventor Taku Makino 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan Paint Co., Ltd. (72) Inventor Isao Kawasaki 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan Paint Co., Ltd. (56) Reference JP-A-55-132667 (JP, A) JP-A-54-11145 (JP, A) JP-A-4 -180867 (JP, A) JP 59-29154 (JP, A) JP 2-203970 (JP, A) JP 1-234573 (JP, A) JP 5-331658 (JP, A) ) JP-A-62-202083 (JP, A) Kai 63-250498 (JP, A) JP-B 51-42132 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) B05D 1/00-7/26 C22C 23/00- 23/06 C23C 22/00-22/86

Claims (4)

(57) [Claims] 1. A magnesium alloy material having an iron content of less than 0.004%, a nickel content of less than 0.002%, and a copper content of less than 0.015% is subjected to a coating pretreatment and then a carboxyl group. A method for forming a highly corrosion-resistant coating film on a magnesium alloy material, comprising powder coating with an epoxy-based powder coating using a polyester resin having a curing agent or an epoxy-based powder coating using dibasic acid dihydrazide as a curing agent. 2. The coating base treatment is 0.01 g / liter
The method for forming a highly corrosion-resistant coating film of a magnesium alloy material according to claim 1, wherein the coating base treatment is a treatment with a manganese-modified zinc phosphate treatment liquid having a nickel ion content of less than torr . 3. The coating base treatment is a coating base treatment performed with a chromate treatment liquid.
A method for forming a highly corrosion-resistant coating film on a magnesium alloy material according to 1. 4. A magnesium alloy material which is powder coated by the coating film forming method according to claim 1.