Corrosion prevention method for aluminum-based metal component, and corrosion-proof aluminum-based metal product produced by the method

Abstract

The present invention provides a corrosion prevention method for an aluminum-based metal component, which makes it easy to check the presence or absence of, a chemically formed film on the component, and a corrosion-proof aluminum-based metal product produced by the method. In the method, the following treatment agent (A) is brought into contact with a surface of the aluminum-based metal component to chemically form the following film (B) on the surface of the aluminum-based metal component: (A) a treatment agent comprising an aqueous solution of zinc sulfate, an aqueous solution of ammonium fluoride and an aqueous solution of sodium molybdate which are mixed together; (B) a chemically formed film (B) comprising zinc fluoride, molybdenum and aluminum fluoride. Thus, the corrosion-proof aluminum-based metal product is produced.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a corrosion prevention method for an aluminum-based metal component made of aluminum or an aluminum alloy, and a corrosion-proof aluminum-based metal product produced by the method.

2. Description of the Related Art

In the automotive field, for example, an attempt is currently made at weight reduction of motor vehicles for reduction of fuel costs (or for improvement in ecological performance). For the weight reduction, aluminum or an aluminum alloy is employed instead of iron as a metal material for automotive components such as suspension arms, bushings, engines, engine mounts and body plates.

Under such circumstances, it is initially a common practice to chemically form films on aluminum and aluminum alloy components by a chemical treatment (surface treatment) such as a chromate treatment for prevention of corrosion of the components. A treatment agent for the chromate treatment contains hexavalent chromium, which adversely affects human bodies and the environment, thereby presenting problems.

Therefore, a recent trend is to refrain from the use of the hexavalent chromium. The applicant of the present invention previously proposed a corrosion prevention method as an alternative to the chromate treatment (see Japanese Unexamined Patent Publication No. 2001-47515) In this method, an aluminum-based metal component made of aluminum or an aluminum alloy is etched with an acid, an alkali or a fluoride, and then chemically formed with a film by a chemical treatment using an alkali metal phosphate.

However, the film chemically formed by the prior art method is pale in color or colorless and transparent, making it difficult to check whether or not the component has been subjected to the chemical treatment (film forming process). This requires improvement from the viewpoint of production control.

In view of the foregoing, it is an object of the present invention to provide a corrosion prevention method for an aluminum-based metal component, which makes it easy to check the presence or absence of a chemically formed film on the component, and to provide a corrosion-proof aluminum-based metal product produced by this method.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention to achieve the aforementioned object, there is provided a method of preventing corrosion of an aluminum-based metal component made of aluminum or an aluminum alloy, the method comprising; bringing a treatment agent (A) into contact with a surface of the aluminum-based metal component to chemically form a film (B) on the surface of the aluminum-based metal component; the treatment agent (A) comprising an aqueous solution of zinc sulfate, an aqueous solution of ammonium fluoride and an aqueous solution of sodium molybdate which are mixed together; the chemically formed film (B) comprising zinc fluoride, molybdenum and aluminum fluoride.

According to a second aspect of the present invention, there is provided a corrosion-proof aluminum-based metal product comprising: an aluminum-based metal component made of aluminum or an aluminum alloy; and a film (B) chemically formed on a surface of the component by the aforementioned corrosion prevention method.

The inventors of the present invention conducted a study on a novel chemical film formation method to make it easy to check the presence or absence of a film chemically formed on a surface of an aluminum-based metal component. As a result, the inventors found that the aforesaid chemically formed film (B) is formed on the surface of the aluminum-based metal component by bringing the aforesaid treatment agent (A) into contact with the surface of the component to impart the component with a sufficient anti-corrosive property and that the presence or absence of the chemically formed film (B) can be easily checked because the film (B) is colored. Thus, the present invention is attained.

According to the inventive corrosion prevention method for the aluminum-based metal component, the film (B) chemically formed by bringing the treatment agent (A) into contact with the surface of the aluminum-based metal component is colored. This makes it easy to check the presence or absence of the film (B) chemically formed on the surface.

Particularly, where the treatment agent (A) comprises the following zinc sulfate aqueous solution (a), the following ammonium fluoride aqueous solution (b) and the following sodium molybdate aqueous solution (c) which are mixed in a mass ratio of (a)/(b)/(c)=7/3/2, the respective aqueous solutions efficiently contribute to a reaction for the formation of the film:

• (a) an aqueous solution containing 15% by weight of zinc sulfate heptahydrate;
• (b) an aqueous solution containing 10% by weight of ammonium fluoride; and
• (c) an aqueous solution containing 5% by weight of sodium molybdate dihydrate Further, zinc of zinc fluoride, molybdenum and aluminum of aluminum fluoride are present in the chemically formed film (B) in a content ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis. Thus, the chemically formed film (B) is optimized from the viewpoint of adhesion and anti-corrosive property.

Where the treatment agent (A) having a mass ratio of (a)/(b)/(c)=7/3/2 comprises 35 to 175 g of the solution (a), 15 to 45 g of the solution (b) and 10 to 50 g of the solution (c), the formability of the film (B) is improved. Further, the film (B) is excellent in anti-corrosive property and adhesion.

Where the treatment agent (A) has a temperature of 45 to 65° C. and the surface of the aluminum-based metal component is kept in contact with the treatment agent (A) for a period of 1 to 10 minutes, the formability of the film (B) is further improved.

The inventive corrosion-proof aluminum-based metal product is produced by the aforementioned corrosion prevention method, and the film (B) chemically formed on the surface of the aluminum-based metal component is colored. This makes it easy to check the presence or absence of the chemically formed film (B) on the surface.

Particularly, where zinc of zinc fluoride, molybdenum and aluminum of aluminum fluoride are present in the chemically formed film (S) in a content ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis, the chemically formed film (B) is excellent in adhesion, and imparts the aluminum-based metal component with excellent anti-corrosive property.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will hereinafter be described in detail.

In a corrosion prevention method according to the present invention, a treatment agent to be described later in detail is brought into contact with a surface of an aluminum-based metal component made of aluminum or an aluminum alloy, whereby a film to be described later in detail is chemically formed on the surface of the component to prevent corrosion of the aluminum-based metal component. The chemically formed film is colored yellow or sunny yellow. The color makes it easy to check the presence or absence of the chemically formed film on the surface of the component.

More specifically, the treatment agent is a solution mixture obtained by mixing an aqueous solution of zinc sulfate, an aqueous solution of ammonium fluoride and an aqueous solution of sodium molybdate. The treatment agent (solution mixture) is prepared in the following manner. For the formation of the film, an aqueous solution (a) containing 15% by weight of zinc sulfate heptahydrate, an aqueous solution (b) containing 10% by weight of ammonium fluoride, and an aqueous solution (c) containing 5% by weight of sodium molybdate dihydrate are used as the zinc sulfate aqueous solution, the ammonium fluoride aqueous solution and the sodium molybdate aqueous solution, respectively. The aqueous solutions (a), (b) and (c) respectively having the aforesaid concentrations are mixed in a mass ratio of (a)/(b)/(c)=7/3/2 for the preparation of the treatment agent.

The film chemically formed by using the treatment agent thus prepared contains zinc fluoride, molybdenum and aluminum fluoride (resulting from reaction between ammonium fluoride and aluminum in the aluminum-based metal component). More specifically, zinc of zinc fluoride, molybdenum and aluminum of aluminum fluoride are present in the chemically formed film in a content ratio of zinc/molybdenum/aluminum=5/4/1 or in a content ratio close to this ratio (4-6/3-6/0.5-2) on a mass basis. The composition (content ratio) of the chemically formed film is determined by ICP (inductively coupled plasma) emission spectrometry.

More specifically, the treatment agent is prepared by mixing 35 to 175 g of the zinc sulfate aqueous solution (a) containing 15% by weight of zinc sulfate heptahydrate (or 5.25 to 26.25 g of zinc sulfate heptahydrate on a solute basis), 15 to 45 g of the ammonium fluoride aqueous solution (b) containing 10% by weight of ammonium fluoride (or 1.5 to 4.5 g of ammonium fluoride on a solute basis) and 10 to 50 g of the sodium molybdate aqueous solution (c) containing 5% by weight of sodium molybdate dihydrate (or 0.5 to 2.5 g of sodium molybdate dihydrate on a solute basis). Thus, the resulting treatment agent contains the aqueous solution (a), the aqueous solution (b), and the aqueous solution (c) in a mass ratio of (a)/(b)/(c)=7/3/2. In order to improve the formability of the film, the temperature of the treatment agent is preferably 45 to 65° C.

The surface of the aluminum-based metal component is generally brought into contact with the treatment agent by immersing the component in the treatment agent. Alternatively, the treatment agent may be sprayed or applied onto the surface of the aluminum-based metal component. Where the treatment agent is maintained at the aforementioned preferred temperature (45 to 65° C.), the aluminum-based metal component is preferably kept in contact with the treatment agent for a period (contact period) of 1 to 10 minutes by the immersion or the like.

After the film is chemically formed on the surface of the aluminum-based metal component by the immersion or the like, the surface of the component is rinsed with water, rinsed with hot water and dried if necessary.

The aluminum-based metal component generally has an oxide film on its surface. In the case of a conventional chemical treatment using a phosphate of an alkali metal, it is necessary to remove the oxide film by etching prior to the chemical treatment. In the present invention, ammonium fluoride, which is useful for etching away an aluminum oxide film, is contained in the treatment agent, so that the oxide film on the surface of the aluminum-based metal component is dissolved by the treatment agent. This eliminates the need for the etching step, which is otherwise required in the conventional method. However, the etching step may be performed if necessary.

The film thus chemically formed is colored yellow or sunny yellow as described above. Therefore, whether or not the film is present on the component can be checked at a glance. This makes it easy to check the presence or absence of the film, thereby improving the efficiency of the production control in the production process for a corrosion-proof aluminum-based metal product.

Aluminum and the aluminum alloy as the material for the aluminum-based metal component are not particularly limited, but examples thereof include all aluminum-based metals including No. 1000 to No. 7000 series, AC series and ADC series. The corrosion-proof aluminum-based metal product is not particularly limited, but examples thereof include automotive components such as engine mounts, electrical components such as camera bodies, constructive components such as aluminum window sashes, office furniture such as desks, and other products which are used in various fields.

Next, examples of the present invention will be described in conjunction with a prior art example. It should be understood that the invention be not limited to these examples.

EXAMPLE 1

Aluminum-Based Metal Component

A plate (25 mm×60 mm×3 mm (thickness)) of an aluminum alloy A6063 was prepared.

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 35.0 g of a zinc sulfate aqueous solution (containing 15wt % of zinc sulfate heptahydrate), 15.0 g of an ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 10.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dehydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 5.25 g/liter, 1.5 g/liter and 0.5 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 75° C. for 20 minutes, whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the resulting component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

EXAMPLE 2

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 52.5 g of a zinc sulfate aqueous solution (containing 15 wt % of zinc sulfate heptahydrate), 22.5 g of a ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 15.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dihydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 7.875 g/liter, 2.25 g/liter and 0.75 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 65° C. for 10 minutes, whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

EXAMPLE 3

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 70.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zinc sulfate heptahydrate), 30.0 g of a ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 20.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dihydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 10.5 g/liter, 3.0 g/liter and 1.0 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 55° C. for 5 minutes, whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

EXAMPLE 4

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 87.5 g of a zinc sulfate aqueous solution (containing 15 wt % of zinc sulfate heptahydrate), 37.5 g of a ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 25.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dihydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 13.125 g/liter, 3.75 g/liter and 1.25 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 45° C. for 1 minute, whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

EXAMPLE 5

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 105.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zinc sulfate heptahydrate), 45.0 g of a ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 30.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dihydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 15.75 g/liter, 4.5 g/liter and 1.5 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 35° C. for 30 seconds (0.5 minute), whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

EXAMPLE 6

Treatment Agent

A treatment agent for chemical film formation was prepared by mixing 175.0 g of a zinc sulfate aqueous solution (containing 15 wt % of zinc sulfate heptahydrate), 45.0 g of a ammonium fluoride aqueous solution (containing 10 wt % of ammonium fluoride) and 50.0 g of a sodium molybdate aqueous solution (containing 5 wt % of sodium molybdate dihydrate), and diluting the resulting mixture so that zinc sulfate heptahydrate, ammonium fluoride and sodium molybdate dihydrate were present in concentrations of 26.25 g/liter, 4.5 g/liter and 2.5 g/liter, respectively, in the treatment agent.

Chemical Film Formation

The aluminum-based metal component was immersed in the treatment agent at 35° C. for 30 seconds (0.5 minute), whereby a film was chemically formed on a surface of the aluminum-based metal component. Thereafter, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

PRIOR ART EXAMPLE 1

The surface of the aluminum-based metal component was subjected to an etching process, and then to a chemical film formation process employing a phosphate of an alkali metal in the following manner.

Etching Process

An acidic etching agent JCB-7030 (available from Japan C. S. Chemical Co., Ltd.). was diluted into a concentration of 200 g/liter with water, and the aluminum-based metal component was immersed in the resulting etching agent at 60° C. for 5 minutes.

Chemical Film Formation Process

CHEMIBONDER 5701 (Japan C. B. Chemical Co., Ltd.) essentially containing sodium dihydrogen phosphate was diluted into a concentration of 120 g/liter with water, and the etched aluminum-based metal component was immersed in the resulting treatment agent at 60° C. for 5 minutes, whereby a film was chemically formed on the surface of the component. Then, the component was taken out of the treatment agent, rinsed with water, rinsed with hot water and dried. Thus, a corrosion-proof aluminum-based metal product was produced.

Check of Chemically Formed Films

The corrosion-proof aluminum-based metal products produced in Examples 1 to 6 and Prior Art Example 1 were each visually checked for the presence or absence of a chemically formed film. As a result, it was found that the surfaces of the products of Examples 1 to 6 were colored yellow or sunny yellow. Therefore, the formation of the films was detected at a glance. On the other hand, the chemically formed film of the corrosion-proof aluminum-based metal product of Prior Art Example 1 was not visually detected, because the surface of the product was colorless and transparent. The surface of the corrosion-proof aluminum-based metal product of Prior Art Example 1 was analyzed by an ICP emission spectrometer (OPTIMA 4300DV available from Perkin Elmer Corporation) and, as a result, it was confirmed that the product had a chemically formed film in a surface thereof. The corrosion-proof aluminum-based metal products of Examples 1 to 6 were also analyzed by the ICP emission spectrometer. The analysis indicates that the products each had a chemically formed yellow or sunny yellow film in a surface thereof. Further, the compositions (content ratio) of the films were analyzed. The results of the analysis are shown in Table 1.

	TABLE 1
	Example	Prior Art
	1	2	3	4	5	6	Example 1
Treatment agent
ZnSO4 aq. solution (g)	35.0	52.5	70.0	87.5	105.0	175.0	NaH2PO4
ZnSO4•7H2O (g/L)	5.25	7.875	10.5	13.125	15.75	26.25	120.0 g/L
NH4F aq. solution	15.0	22.5	30.0	37.5	45.0	45.0
(g)
NH4F (g/L)	1.5	2.25	3.0	3.75	4.5	4.5
Na2MoO4 aq. solution (g)	10.0	15.0	20.0	25.0	30.0	50.0
Na2MoO4•2H2O (g/L)	0.5	0.75	1.0	1.25	1.5	2.5
Temperature (° C.)	75	65	55	46	35	35	60
Immersion period (min)	20	10	5	1	0.5	0.5	5
Chemically formed film
Color	Yellow	Sunny yellow	Colorless
Zn/Mo/Al	5/4/1	—

Claims

1. A method of preventing corrosion of an aluminum-based metal component made of aluminum or an aluminum alloy, the method comprising:

bringing a treatment agent (A) into contact with a surface of the aluminum-based metal component to chemically form a film (B) on the surface of the aluminum-based metal component;

the treatment agent (A) comprising an aqueous solution of zinc sulfate, an aqueous solution of ammonium fluoride and an aqueous solution of sodium molybdate which are mixed together;

the chemically formed film (B) comprising zinc fluoride, molybdenum and aluminum fluoride.

2. A method as set forth in claim 1, wherein the treatment agent (A) contains 5.25 to 26.25 g/liter of zinc sulfate heptahydrate, 1.5 to 4.5 g/liter of ammonium fluoride, and 0.5 to 2.5 g/liter of sodium molybdate dihydrate.

3. A method as set forth in claim 1,

wherein the treatment agent (A) comprises the following zinc sulfate aqueous solution (a), the following ammonium fluoride aqueous solution (b) and the following sodium molybdate aqueous solution (c) which are mixed in a mass ratio of (a)/(b)/(c)=7/3/2:

(a) an aqueous solution containing 15% by weight of zinc sulfate heptahydrate;

(b) an aqueous solution containing 10% by weight of ammonium fluoride; and

(c) an aqueous solution containing 5% by weight of sodium molybdate dihydrate, wherein zinc of zinc fluoride, molybdenum and aluminum of aluminum fluoride are present in the chemically formed film (B) in a content ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis.

4. A method as set forth in claim 3, wherein the treatment agent (A) having a mass ratio of (a)/(b)/(c)=7/3/2 comprises 35 to 175 g of the solution (a), 15 to 45 g of the solution (b) and 10 to 50 g of the solution (c).

5. A method as set forth in claim 1, wherein the treatment agent (A) has a temperature of 45 to 65° C. and the surface of the aluminum-based metal component is kept in contact with the treatment agent (A) for a period of 1 to 10 minutes.

6. A corrosion-proof aluminum-based metal product comprising:

an aluminum-based metal component made of aluminum or an aluminum alloy; and

a film (B) chemically formed on a surface of the aluminum-based metal component by a method as recited in claim 1, the film (B) comprising zinc fluoride, molybdenum and aluminum fluoride.

7. A corrosion-proof aluminum-based metal product as set forth in claim 6, wherein zinc of zinc fluoride, molybdenum and aluminum of aluminum fluoride are present in the chemically formed film (B) in a content ratio of zinc/molybdenum/aluminum=5/4/1 on a mass basis.