JPH0257155B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a film on aluminum or aluminum alloy, in which aluminum or aluminum alloy is electrolytically etched in an aqueous solution of hydrochloric acid or nitric acid, and then etched in an aqueous solution of an acid such as hydrochloric acid, sulfuric acid, nitric acid or hydrofluoric acid. By immersing the etching in water for smut removal and chemical etching, the chemical etching is carried out in such a way that the pores generated by electrolytic etching remain, resulting in a film with a wide range of colors from gray to pale gray-white. The purpose of the present invention is to provide a method for forming a film on aluminum or aluminum alloy, which allows the film to be freely formed, has high corrosion resistance and weather resistance, and has extremely excellent adhesion to the paint film. .

For example, a method has been proposed in which aluminum or aluminum alloy is subjected to alternating current electrolytic treatment in an aqueous hydrochloric acid solution, then corroded with caustic alkali, and then anodized to form a milky white film on the surface. The coating is limited to a milky white color, and has the disadvantage that the desired color tone, from gray to pale grayish white, cannot be obtained.

The present inventor noticed that the electrolytically etched surface becomes grayish black or gray after electrolytically etching and desmatting aluminum or aluminum alloy (hereinafter simply referred to as aluminum), and as a result of intensive research on chemical etching after desmatting, the inventor found that When smut removal and chemical etching are performed by immersion in an aqueous solution containing hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid, the color tone of the surface chemical conversion film changes from gray to pale grayish white depending on the amount of etching, and the surface of the film is extremely pale. They discovered that it is fine-grained, has excellent adhesion to, for example, paint, and has excellent corrosion resistance and weather resistance.

That is, aluminum is electrolytically etched in an aqueous solution containing hydrochloric acid or nitric acid using a direct current, alternating current, or pulsed current, and the electrolytically etched aluminum is immersed in an aqueous solution of hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid. By chemically etching the base aluminum after removal, the color tone of the aluminum surface can be made fine and in any tone from gray to pale gray-white, and in particular, the base aluminum after the smut remaining on the aluminum surface is removed by the chemical etching solution. The amount of aluminum chemically etched is approximately 5 to 350 mg/ ^dm2 , preferably approximately 5 to 100 mg/dm2.
dm ² , or even after chemical etching, the unoxidized approximately 500 ~
After chemical etching is controlled to have micropores with a pore size of 1000 Å, if necessary, for example, the unoxidized micropores of 500 to 1000 Å are etched, even after undergoing oxidative dissolution during anodization. After forming an anodic oxide film with a thickness of 2 to 30 μm, a coating film was formed.

The electrolytic etching conditions mentioned in the above step are as follows: When using an aqueous hydrochloric acid solution, the hydrochloric acid concentration is 0.5 to 10%, preferably about 1 to 5%.
%, when using nitric acid, the nitric acid concentration is 1
~5%, preferably about 1-2%, and the treatment temperature is 20
~80°C, preferably about 30-50°C, and a current density of 1-200 A/dm ² , preferably about 20-50 A/d
m ² for 10 seconds to 5 minutes, preferably about 30 to 60 seconds.

In addition, chemical etching performed after electrolytic etching is performed by washing with water after electrolytic etching, and hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid aqueous solutions with a concentration of 0.5 to 30%, preferably about 5 to 15% are used. The amount of chemical etching after removing the smut is approximately 5 to 350 degrees Celsius.
mg/dm ² and also so that unoxidized micropores with a diameter of about 500 to 1000 Å generated after electrolytic etching are not lost.

In addition, the anodizing treatment performed after chemical etching uses a sulfuric acid aqueous solution to which a small amount of electrolyte or metal salt is added as necessary, and uses direct current, alternating current, AC/DC superimposed current, or pulsed current at a current density of about 1 to 1.
At 30A/dm ² , the electrolytic bath temperature is room temperature or about 50℃.
An anodized film with a thickness of approximately 2 to 30 μm is formed by performing the anodization between the following steps.

Also, the coating conditions are thermosetting, such as acrylic, acrylic melamine, urethane, acrylic urethane, or polyvinyl chloride, which is transparent and does not lose the color tone adjusted by chemical etching treatment. A paint consisting of a vehicle mainly composed of a thermoplastic resin is applied by spraying, dipping, electrodeposition, roll coating, etc., and is cured after setting or washing with water as necessary.

The composite film of aluminum or aluminum alloy obtained in this way has an excellent gray to pale grayish-white color depending on the amount of chemical etching, has an extremely excellent decorative effect, and has excellent corrosion resistance. It has extremely excellent weather resistance and adhesion between the paint film and the aluminum surface film, making it suitable for interior decoration or building materials.

In the following, specific examples of the present invention will be described.

Example 1 JIS standard pure aluminum 1050 rolled material (0.5×
⁵⁰⁰ Flow for 60 seconds at 50℃ for electrolytic etching, then rinse with water.

Then, it is immersed in a 13% sulfuric acid aqueous solution at 85°C to color it from gray to pale gray-white.

After washing with water, apply a transparent acrylic/melamine paint (manufactured by Kansai Paint Co., Ltd., product name AM4100) to the aluminum surface with a film of any color from gray to pale gray-white.
Painted by gravure roll coating and heated at 210℃
Bake harden for 60 seconds.

Example 2 A JIS standard 5052 rolled material (0.3 x 500 x 500) was degreased in a conventional manner, washed with caustic, and carbon plates were placed on both sides of the material as counter electrodes using a 1% aqueous hydrochloric acid solution. A direct current with a current density of 50 A/dm ² is applied to the material for 30 seconds at a bath temperature of 50° C. to perform electrolytic etching, followed by washing with water.

Next, it is immersed in a 13% sulfuric acid aqueous solution at 45°C for 20 seconds to color it grayish.

After washing with water, a transparent urethane paint (manufactured by Dainippon Toyo Co., Ltd., trade name: V Top) is spray-painted, and then dried at room temperature.

Comparative Example Instead of the chemical etching with the sulfuric acid aqueous solution in Example 1, chemical etching is performed using a 5% caustic soda aqueous solution to form a milky white film, and then a coating film is formed in the same manner.

The corrosion resistance of the aluminum films obtained in the above examples and comparative examples was evaluated by CASS according to JIS H8602.
When determined by the rating number display method after 72 hours of the test, it was 10 in the example, whereas
The comparative example had a poor corrosion resistance of 8, while the one according to the present example had excellent corrosion resistance.

Furthermore, when measuring the adhesion strength of the coating film, the coating film according to this example was extremely excellent, with a width of approximately 20 to 26 kg/15 mm, whereas that of the comparative example was approximately 16 kg/15 mm width. As small as I said,
Furthermore, the coating film according to this example has much better adhesion than the chemical conversion coating film formed by chromate treatment or boehmite treatment. That is, FIG. 1 shows the amount of chemical etching and adhesion strength, L, in the case of Example 1.
As shown in the diagram, the chemical etching amount is 5 to 350 mg/dm ² (measured according to JIS Z8729).
Therefore, the adhesive strength with the paint film is superior to that of a boehmite film or a chromate film, and is particularly excellent when the amount of chemical etching is 5 to 100 mg/dm ² . The color tone of the film varies from gray to pale grayish white depending on the amount of chemical etching.

This improvement in the adhesion strength between the paint film and the colored film is due to chemical etching performed with acid rather than alkali.
And especially if the amount of this chemical etching is about 5 to 350 mg/
dm ² , and as shown in scanning electron micrographs of the film in Figures 2 and 3, chemical etching was carried out to ensure that the micropores with a pore diameter of 500 to 1000 Å generated by electrolytic etching were not eliminated. This seems to be due to what was done.

In the above example, electrolytic etching was carried out using an aqueous hydrochloric acid solution, but the same applies to an aqueous nitric acid solution.

As mentioned above, the method for forming a film on aluminum or aluminum alloy according to the present invention involves electrolytically etching aluminum or aluminum alloy in an aqueous solution containing hydrochloric acid or nitric acid, and then immersing it in an acidic solution to remove smut and chemically etching it. Therefore, depending on the amount of chemical etching, it is possible to produce a film with any color tone ranging from gray to pale gray-white.Moreover, this film has excellent corrosion resistance and weather resistance, and also has excellent adhesion to the paint film. Furthermore, after electrolytically etching aluminum or aluminum alloy in an aqueous solution containing hydrochloric acid or nitric acid, it can be immersed in an acidic solution to remove the smut and leave pores generated by the electrolytic etching. Since it is chemically etched and then forms a transparent paint film, it is possible to form a film in any color tone from gray to pale gray-white, and since this film has good adhesion to the paint film, it is also durable. It has features such as being excellent.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of chemical etching, the L value, and the adhesion strength of the coating film, Figures 2 and 3
The figures show the respective results in sulfuric acid aqueous solution after electrolytic etching.
Scanning electron micrograph of the film after smut removal and chemical etching by immersion for 20 and 600 seconds (×100,000
times).

Claims (1)

[Claims] 1. Formation of a film on aluminum or aluminum alloy, which is characterized by electrolytically etching aluminum or aluminum alloy in an aqueous solution containing hydrochloric acid or nitric acid, and then immersing it in an acidic solution to remove smut and perform chemical etching. Law. After electrolytically etching aluminum or an aluminum alloy in an aqueous solution containing hydrochloric acid or nitric acid, it is immersed in an acidic solution to remove the smut and chemically etched so that the pores generated by the electrolytic etching remain, and then a transparent coating film is formed. A method for forming a film on aluminum or aluminum alloy, characterized by forming a film.