JP3224639B2 - Method for forming colorless chromate film on aluminum wheels - 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 pre-painting an aluminum wheel.

[0002]

2. Description of the Related Art Anodizing (alumite method), coloring treatment, and chemical film treatment (reaction chromate method, MBV method, boehmite method, etc.) are generally known as surface treatment methods for aluminum or aluminum alloys. When painting aluminum wheels, reactive chromate treatment, non-chromate treatment (organic metal salt film etc.), and anodic oxide film treatment are applied as pre-coating treatments to improve corrosion resistance and paint adhesion.

[0003] Considering the pretreatment for clear coating utilizing the glitter of aluminum wheels, the non-chromate treatment (organic metal salt film) is insufficient in corrosion resistance, and the anodic oxide film has good corrosion resistance, but the amount of film is good. However, it is not suitable as a pretreatment for clear coating, which emphasizes the glitter of the material. The reactive chromate film has excellent corrosion resistance, but when a colorless film appearance is required, the chromium adhesion amount is limited to 10 mg / m ² or less, resulting in insufficient rust corrosion resistance. Yarn rust is a state in which water penetrates into the metal surface through a coating film on the metal surface, locally causes electrolytic corrosion, and earthworm crawling rust with a width of about 0.1 to 0.5 mm progresses over time. To tell.

[0004]

As described above, no pretreatment method has yet been found which can maintain the brilliancy of the material of the aluminum wheel and satisfy the corrosion resistance and adhesion after coating. SUMMARY OF THE INVENTION An object of the present invention is to provide a pretreatment method for forming a pretreatment film that retains the brilliancy of an aluminum wheel material, has excellent corrosion resistance after coating (particularly, rust corrosion resistance), and has excellent adhesion.

[0005]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and as a result, the aluminum wheel contains hexavalent chromium ion, sulfate ion and fluorine ion at a specified concentration, and In an acidic aqueous solution adjusted to a specified pH, it was found that a colorless chromate film adapted for the above-mentioned purpose can be obtained by performing cathodic electrolysis by a specific electrolytic method, and the film has a rust corrosion resistance after coating, The inventors have found that they have excellent paint adhesion, and have completed the present invention.

[0006] That is, the present invention provides an aluminum wheel 6
Chromium ion 2g / l or more, sulfate ion 20-200
PH containing 0 ppm and 10 to 400 ppm of fluorine
Immersed in an acidic aqueous solution of 0.6 to 1.7, and 0.5 to 15 A /
A method for forming a colorless chromate film on an aluminum wheel, comprising performing cathodic electrolysis at a current density of dm ² for 4 seconds or more, and removing from the acidic aqueous solution 1 to 300 seconds after the end of energization. Hereinafter, the configuration of the present invention will be specifically described.

First, each component constituting the acidic aqueous solution will be described. Hexavalent chromium ions include chromic anhydride, dichromic acid, and alkali metal salts of dichromic acid, and one or more of these compounds can be arbitrarily selected and used, and the concentration range is as follows. 2 to 50 g / l
It is. If the hexavalent chromium ion concentration is less than 2 g / l, a chromate film having sufficient corrosion resistance cannot be obtained, and
Even if it exceeds / l, there is no problem in performance, but the burden of wastewater treatment and the like is high and uneconomical. In addition, trivalent chromium ions increase as the electrolytic treatment is performed. However, in carrying out the present invention, the concentration of trivalent chromium ions has little effect on film performance. There is no need to set a range.

As sulfate ions, sulfuric acid and alkali metal salts of sulfuric acid can be used, and the concentration range is from 20 to 2,000.
ppm. If the concentration is less than 20 ppm, a colorless chromate film will not be formed. On the other hand, when the content exceeds 2,000 ppm, the amount of chromium attached is reduced and the corrosion resistance is reduced, which is not preferable.

Fluorine exists in the form of fluorine ions and / or complex fluoride ions. The source of the fluorine ions is, for example, hydrofluoric acid, and the source of the complex fluoride ions is hydrosilicofluoric acid, zircon hydrogen fluoride. An acid, borofluoric acid, or the like can be arbitrarily selected and used, but zircon hydrofluoric acid is more preferably used. The concentration range as fluorine is 10 to 400 ppm. If it is less than 10 ppm, a chromate film excellent in corrosion resistance after coating is not formed, and if it exceeds 400 ppm, the formation of a chromate film by electrolysis is suppressed, so that it is difficult to obtain a desired amount of chromium coating, and even if the amount of chromium is relatively small, the film can be formed. It is unsuitable for clear coating because of coloring.

The pH of the acidic aqueous solution is set to a value of 0.6 to 1.
7 range. In order to control the pH, it can be arbitrarily selected from ammonium hydroxide, alkali metal hydroxide, alkali metal carbonate, chromic acid, sulfuric acid and the like and added to the aqueous solution. Even if the pH is less than 0.6, there is no problem in the coating performance, but since the pH of the processing solution is increased by the cathodic electrolysis, it is necessary to constantly adjust the pH in industrial production, and it is difficult to maintain and control the pH. . When the pH exceeds 1.7, the amount of chromate film adhered decreases sharply, and it becomes difficult to obtain an appropriate amount of chromium film adhered.

Although the temperature of the processing liquid is not specified,
In order to perform the film deposition reaction by electrolysis under certain conditions, 30
It is necessary to keep the temperature constant at a temperature selected from the range of 60C to 60C. Here, when the temperature is 30 ° C. or less, heat is generated by electrolysis, and cooling is required to maintain the temperature, which is uneconomical for industrial production. Further, even if the temperature is higher than 60 ° C., there is no significant change in the characteristics of the formed film. However, after the electrolytic treatment, the processing liquid adheres to the object to be processed and dries while moving to the next washing step. It is not preferable because the washing property in the process is lowered.

Next, in the electrolytic treatment, an object to be treated is used as a cathode, and an electrolytic treatment is performed using a lead-based alloy electrode, stainless steel, titanium platinum or the like as an anode. The conditions for the electrolytic treatment are such that the electrolytic current density at the cathode is in the range of 0.5 to 15 A / dm ² . When the current density is lower than 0.5 A / dm ^2, it is difficult to form a chromate film. When the current density is higher than 15 A / dm ² , the film is colored, and the film is redissolved. Cannot be obtained.

The energization in the electrolytic treatment is always started after the aluminum wheel is completely immersed in the acidic aqueous solution.
This takes several tens of seconds from the start of immersion of the aluminum wheel to the complete immersion in the acidic aqueous solution when performing the actual treatment. This is because a change in the electrolysis time makes it difficult to form a uniform film. The electrolysis time from the start of the electrolysis is controlled to 4 seconds or more in order to keep the formed film amount (the amount of chromium adhered) in a desired range. There are various factors that change the amount of the formed film (the amount of chromium adhered), but in the method of the present invention,
The concentration, pH, temperature, current density, etc., of each component of the treatment liquid are fixed under preferable conditions of the present invention, and the desired amount of chromium can be controlled by changing the electrolysis time. On the contrary, fix the electrolysis time,
By changing the current density, the amount of deposited chromium can be controlled. After the electrolysis has been performed for a predetermined time, the aluminum wheel is held in the acidic aqueous solution for 1 to 300 seconds in a state where the current is stopped. If the aluminum wheel is pulled up while energized, or if it is pulled up in less than 1 second even after the energization is stopped, the film formed on the aluminum wheel surface will turn yellow, and a film that is not suitable for a clear paint base turn into. On the other hand, when the film is held for more than 300 seconds, the film formed on the aluminum wheel exhibits a brown color, which is not a film suitable for a clear coating base. In consideration of the variation in the position of the aluminum wheel in the electrolytic solution on the production line, the cathode electrolysis is performed at a current density of 1 to 3 A / dm ² for 15 to 60 seconds in order to obtain a uniform and stable chromium deposition amount. Preferably, it is a treatment.

The amount of chromium deposited on the film formed in the present invention is not particularly limited, but is preferably in the range of 50 to 250 mg / m ² . When the chromium adhesion amount is less than 50 mg / m ² , the corrosion resistance after coating, particularly the rust resistance, is poor.
If it exceeds mg / m ² , the chromate film will be colored and not suitable for clear coating. However, in the case of colored paint, 250
mg / m ² or more can be used.

Further, colloidal silica,
By containing a compound such as fumed silica or an alkali metal silicate, a colorless and relatively thick chromate film can be formed.

The aluminum wheel which has been subjected to the cathodic electrolysis according to the present invention is washed with water, dried and applied as a coating base. If necessary, a post-treatment with an aqueous chromate solution, an organic compound, and the like, which are generally used between the water washing and drying, can be performed.

[0017]

Next, the present invention will be specifically described with reference to examples and comparative examples. The test plate is an aluminum casting alloy (JIS symbol AC4C, commonly used for aluminum wheels).
After grinding the surface of the plate (size 70 × 150 × 7 m / m), the surface was cleaned by alkali degreasing and cleaning, and then subjected to electrolytic treatment.

(Examples 1 to 3) Hexavalent chromium ions were 20 g / l with chromic anhydride and sulfate ions were 400 pp with sulfuric acid.
m, the pH of an acidic aqueous solution to which 100 ppm of fluorine was added with zircon hydrofluoric acid using ammonium hydroxide under the conditions shown in Table 1 (Example 1: pH 0.6, Example 2: p
H1.0, Example 3: pH 1.6). After the test plate was immersed in the acidic aqueous solution heated to 40 ° C., energization was started and cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. After the completion of energization, the sample was kept in the acidic aqueous solution for 5 seconds, the test plate was taken out, washed with water and washed with deionized water, and then washed for 100 seconds.
It dried for 5 minutes in the drying oven of ° C. Next, a thermosetting acrylic resin clear was applied to the test plate (30 μm in thickness) at 140 ° C.
After baking and drying for 0 minutes, a corrosion resistance test (yarn rust test, salt spray test) was performed. Table 1 shows the results.

(Comparative Examples 1-2) Hexavalent chromium ions were 20 g / l with chromic anhydride, and sulfate ions were 400 pp with sulfuric acid.
m, the pH of an acidic aqueous solution to which 100 ppm of fluorine was added with zircon hydrofluoric acid using ammonium hydroxide under the conditions shown in Table 1 (Comparative Example 1: pH 2.0, Comparative Example 2: p
H3.0), the same electrolytic treatment and coating as in Example 1 were performed, and a corrosion resistance test was performed. Table 1 shows the results.

(Comparative Example 3) The pH of an acidic aqueous solution in which hexavalent chromium ions were added with chromic anhydride at 20 g / l, sulfate ions at 10 ppm with sulfuric acid, and zircon hydrofluoric acid at 5 ppm with fluorine was adjusted to 1 pH using ammonium hydroxide. Was adjusted to 0.0, electrolytic treatment and coating were performed in the same manner as in Example 1, and a corrosion resistance test was performed. Table 1 shows the results.

Example 4 The pH of an acidic aqueous solution obtained by adding hexavalent chromium ions to chromic anhydride at 5 g / l, sulfuric acid ions to sulfuric acid at 100 ppm, and fluorosilicic acid to fluorine at 30 ppm was adjusted using ammonium hydroxide. Adjust to 1.0,
The same electrolytic treatment and painting as in Example 1 were performed, and a corrosion resistance test was performed. Table 1 shows the results.

Example 5 The pH of an acidic aqueous solution obtained by adding hexavalent chromium ion to chromic anhydride at 50 g / l, sulfate ion to sulfuric acid at 1 g / l, and zircon hydrofluoric acid to 400 ppm of fluorine was adjusted using ammonium hydroxide. And the same electrolytic treatment and coating as in Example 1 were performed, and a corrosion resistance test was performed. Table 1 shows the results.

Example 6 Hexavalent chromium ions were 20 g / l with chromic anhydride and sulfate ions were 400 ppm with sulfuric acid.
An acidic aqueous solution to which fluorine was added to be 110 ppm with hydrofluoric acid was used to adjust the pH to 1.0 using ammonium hydroxide.
0, and the same electrolytic treatment and coating as in Example 1 were performed.
A corrosion resistance test was performed. However, when the current density during energization is 1
The electrolysis time was 10 seconds at 0 A / dm ² . Table 2 shows the results
Shown in

Comparative Example 4 Hexavalent chromium ions were 20 g / l with chromic anhydride, sulfate ions were 400 ppm with sulfuric acid,
An acidic aqueous solution to which fluorine was added to be 110 ppm with hydrofluoric acid was used to adjust the pH to 1.0 using ammonium hydroxide.
0, and the same electrolytic treatment and coating as in Example 1 were performed.
A corrosion resistance test was performed. However, when the current density during energization is 2
The electrolysis time was 10 seconds at 0 A / dm ² . Table 2 shows the results
Shown in

(Example 7) An aluminum cast alloy (JIS symbol AC4C, size 70 × 450 × 7 m / m) plate was prepared assuming the actual aluminum wheel diameter in the length direction, and acidity under the same conditions as in Example 2 was prepared. In aqueous solution (pH 1.0)
Then, the tank was introduced at a tank entry speed in which the lengthwise direction of 450 mm was completely submerged in 60 seconds. Current density 2A / dm after the test plate is completely immersed
Cathodic electrolysis at ² for 30 seconds, and after completion of energization, holding in the acidic aqueous solution for 5 seconds, removing the test plate, washing with water and washing with deionized water, and drying in a drying oven at 100 ° C.
Dried for minutes. The upper part of the test plate thus prepared
00 mm and the lower 100 mm were cut, and each was separately coated with a thermosetting acrylic resin clear (film thickness 30 mm).
μ) After baking and drying at 140 ° C. for 30 minutes, a corrosion resistance test (yarn rust test, salt spray test) was performed. Table 3 shows the results.

(Comparative Example 5) An aluminum cast alloy (JIS symbol AC4C, size 70 × 450 × 7 m / m) plate whose length direction is assumed to be the actual aluminum wheel diameter was prepared, and a constant current electrolytic condition of 6.3 A was prepared. (Current density 2
A / dm ² ) was supplied in an acidic aqueous solution (pH 1.0) under the same conditions as in Example 2 at a bathing speed at which 450 mm in the length direction completely submerged in 60 seconds. After immersion, the battery was energized for 30 seconds. After the energization was completed, the test plate was kept in the acidic aqueous solution for 5 seconds. A 100 mm upper portion and a lower 100 mm portion of the test plate thus prepared were cut, and each was separately coated with a thermosetting acrylic resin clear (thickness: 30 μm) at 140 ° C.
After partial baking and drying, a corrosion resistance test (yarn rust test, salt spray test) was performed. Table 3 shows the results.

(Example 8) After immersing the test plate in an acidic treatment solution (pH 1.0) under the same conditions as in Example 2, energization was started and cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. After the completion of energization, the sample was kept in the acidic aqueous solution for 1 second, the test plate was taken out, washed with water and deionized water, and then dried in a drying oven at 100 ° C. for 5 minutes. Next, the coating was performed in the same manner as in Example 2, and a corrosion resistance test (fiber rust test, salt spray test) was performed. Table 3 shows the results.

Example 9 After the test plate was immersed in an acidic treatment solution (pH 1.0) under the same conditions as in Example 2, energization was started and cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. After the energization was completed, the sample was held in the acidic aqueous solution for 180 seconds, the test plate was taken out, washed with water and deionized water, and then dried in a drying oven at 100 ° C. for 5 minutes. Next, the coating was performed in the same manner as in Example 2, and a corrosion resistance test (fiber rust test, salt spray test) was performed. Table 3 shows the results.

Example 10 A test plate was immersed in an acidic treatment solution (pH 1.0) under the same conditions as in Example 2, and then current was started and cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. After the energization was completed, the plate was kept in the acidic aqueous solution for 300 seconds, the test plate was taken out, washed with water and deionized water, and then dried in a drying oven at 100 ° C. for 5 minutes. Next, the coating was performed in the same manner as in Example 2, and a corrosion resistance test (fiber rust test, salt spray test) was performed.
Table 3 shows the results.

(Comparative Example 6) After immersing the test plate in an acidic treatment solution (pH 1.0) under the same conditions as in Example 2, the current was started and the cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. After the energization was completed, the plate was kept in the aqueous acidic solution for 360 seconds, the test plate was taken out, washed with water and deionized water, and then dried in a drying oven at 100 ° C. for 5 minutes. Next, the coating was performed in the same manner as in Example 2, and a corrosion resistance test (fiber rust test, salt spray test) was performed. Table 3 shows the results.

(Comparative Example 7) After immersing the test plate in an acidic treatment solution (pH 1.0) under the same conditions as in Example 2, energization was started and cathodic electrolysis was performed at a current density of 2 A / dm ² for 30 seconds. At the end of energization, the test plate was taken out with energization, washed with water and deionized water, and dried in a drying oven at 100 ° C. for 5 minutes. Next, the coating was performed in the same manner as in Example 2, and a corrosion resistance test (fiber rust test, salt spray test) was performed. Table 3 shows the results.

(Comparative Example 8) A test plate was cleaned by alkali degreasing to clean the surface, then subjected to a reactive chromate treatment (using Alchrome 3703 manufactured by Nippon Parkerizing), washed with water and pure water, and then dried at 100 ° C. Dry in oven for 5 minutes. Next, a thermosetting acrylic resin clear was applied to the test plate (thickness: 30 μm), baked and dried at 140 ° C. for 30 minutes, and then subjected to a corrosion resistance test (fiber rust test, salt spray test). Table 3 shows the results.

(Comparative Example 9) A test plate was subjected to alkaline degreasing and washing,
After the surface is cleaned by caustic soda etching and desmutting, alumite treatment (180 g / l sulfuric acid, 5 g / l dissolved aluminum, bath temperature 25) is used.
Anodizing treatment was performed for 15 minutes at a temperature of 1 ° C. and a current density of 1 A / dm ² ), followed by washing with water and pure water, followed by drying in a drying oven at 100 ° C. for 5 minutes. Next, a thermosetting acrylic resin clear was applied to the test plate (thickness: 30 μm), baked at 140 ° C. for 30 minutes, dried, and then subjected to a corrosion resistance test (yarn rust test, salt spray test).
Was carried out. Table 3 shows the results.

Comparative Example 9 A test plate was cleaned by alkali degreasing to clean the surface, and then dried in a drying oven at 100 ° C. for 5 minutes. Next, a thermosetting acrylic resin clear was applied to the test plate (thickness: 30 μm), baked and dried at 140 ° C. for 30 minutes, and then subjected to a corrosion resistance test (fiber rust test, salt spray test). Table 3 shows the results.

(Evaluation test method) 1) Appearance of coating film JIS-Z8 using a color difference meter based on an untreated sample.
The color difference specified in 730 was measured and evaluated according to the following ranks. ◎ Color difference 0 to 3.2 (discoloration is not visually observed) ○ Color difference 3.2 to 6.5 (slight discoloration is observed) △ Color difference 6.5 to 13 (discoloration is clearly observed) × color difference 13 or more (strong discoloration is observed)

2) Chromium adhering amount Measured by a fluorescent X-ray analyzer.

3) Yarn rust test A coated test plate was cut with an NT cutter to reach the substrate, immersed in a corrosive solution (a mixture of 1N hydrochloric acid and 5% hydrogen peroxide solution) for 1 minute, and dried at room temperature. And let it stand for 24 hours.
After performing a wet test (temperature of 50 ° C., humidity of 80%) for 1000 hours, the length of the thread rust generated from the cut scratch is measured. ◎ Maximum rust length 2mm or less ○ 〃 3mm or less △ 以内 5mm or less × ５ 5mm or more

4) Salt Spray Test (SST) After making a cut on the coated test plate to reach the substrate with an NT cutter, performing the test for 1000 hours in accordance with JIS-Z2371, and then examining rust and blistering from the cut. Measure significant. ◎ Maximum blister rust width 1mm or less ○ 以内 2mm or less △ 以内 3mm or less × ３ 3mm or more

The following can be said from the examples and comparative examples. As is clear from Table 1 (composition of acidic aqueous solution), Table 2 (cathode electrolysis current density), and Table 3 (treatment method), in Examples 1 to 10 of the present invention, appearance and corrosion resistance after painting (fiber rust resistance) And salt spray resistance). On the other hand, Comparative Examples 1 to 4 and 6 which are out of the scope of the present invention, Comparative Example 8 using a reactive type chromate which is a conventional coating undercoating method, Comparative Example 9 using alumite, and a comparison using only alkali degreasing In Example 10, none was good in both appearance and corrosion resistance after painting. In the case where the tank was energized as in Comparative Example 5, the amount of the film was different between the upper and lower portions of the test plate, resulting in an uneven appearance, which was not preferable. In addition, as shown in Comparative Example 7, the one which was discharged from the tank in an energized state had a poor appearance and had poor corrosion resistance after painting.

[0040]

According to the present invention, it is possible to form a colorless chromate film which retains the brilliancy of the aluminum wheel material and has excellent corrosion resistance and adhesion after coating, and has a problem as a conventional pretreatment for clear coating. The corrosion resistance, especially the thread rust resistance, which has been improved, can be improved.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Akihiko Hasebe 1-15-1 Nihonbashi, Chuo-ku, Tokyo Inside Japan Parkerizing Co., Ltd. (72) Inventor Norifumi Hatano 1-15-1 Nihonbashi, Chuo-ku, Tokyo Sun (56) References JP-A-5-179488 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25D 11/38

Claims (1)

(57) [Claims] 1. An aluminum wheel is made of hexavalent chromium ion 2 g /
l or more, sulfate ion 20 to 2000 ppm and fluorine 1
The aluminum wheel was immersed in an acidic aqueous solution having a pH of 0.6 to 1.7 containing 0 to 400 ppm, and then immersed in the acidic aqueous solution at a current density of 0.5 to 15 A / dm ^2.
Sec cathodic electrolysis treatment, after application end, like stopping the energization
A method for forming a colorless chromate film on an aluminum wheel, wherein the colorless chromate film is held in the acidic aqueous solution for 1 to 300 seconds and then taken out of the acidic aqueous solution. .