TW201510233A - Treated surface aluminum material and manufacturing method therefor - Google Patents

Abstract

To provide a treated surface aluminum material, on a portion of the surface of which a corrosion-resistant oxide film layer is formed and on the other areas of the surface of which a porous oxide film layer of excellent adhesiveness and closeness of adhesion is formed. A treated surface aluminum material and a manufacturing method therefor. The treated surface aluminum material comprises an aluminum material, a monolayer structure corrosion resistant oxide film formed on a portion of the surface of the aluminum material, and a porous oxide film formed on the other areas of the surface. The corrosion resistant oxide film has a 10- 100 nm thickness and has specified FT-IR analysis characteristics. The porous oxide film is obtained from a 20-500 nm thick porous aluminum oxide film layer formed on the surface side and 3-30 nm barrier aluminum oxide film layer formed on the base side. Pores of 5-30 nm diameter are formed in the porous aluminum oxide film layer. The range of fluctuation in thickness in the overall porous oxide film formed on the surface of the aluminum material is within +-50% of the arithmetic mean thereof.

Description

Surface treated aluminum material and method of manufacturing same Field of invention

The present invention relates to a surface-treated pure aluminum material or aluminum alloy material (hereinafter referred to as "aluminum material") and a method for producing the same, and more particularly to a surface-treated aluminum material and a method for producing the same, In the surface-treated aluminum material, a corrosion-resistant oxide film layer having excellent corrosion resistance is formed on a part of the surface, and a porous oxide film excellent in adhesion and adhesion is formed in a portion where the surface of the corrosion-resistant oxide film layer is not formed. Floor.

Background of the invention

Aluminium is quite lightweight and has moderate mechanical properties, and has excellent aesthetics, formability, and corrosion resistance. It can be widely used in various containers, structural materials, mechanical parts, electronic parts, and the like. By surface treatment of one of the aluminum materials, corrosion resistance, wear resistance, resin adhesion, adhesion, hydrophilicity, water repellency, antibacterial property, creativity, infrared radiation, high reflection can be added and improved. Sexuality and other functions.

For example, as a surface treatment method for improving corrosion resistance and wear resistance, anodizing treatment (so-called aluminum anodizing treatment) is often widely used. Specifically, as described in Non-Patent Documents 1 and 2, aluminum is immersed in acidic electrolysis. The bath is subjected to electrolytic treatment with a direct current to form an anodized film, and various treatment methods are proposed depending on the application.

Further, in particular, a surface treatment method for improving resin adhesion is disclosed in Patent Document 1 as an alkali exchange electrolysis method. That is, an alkaline solution having a bath temperature of 35 to 85 ° C is used, and an AC electrolytic treatment is performed at a current density of 4 to 50 A/dm ^{2 for a time} when the amount of electricity exceeds 80 C/dm ² . Thereby, a substrate for a printed wiring in which an oxide film having a film thickness of 500 to 5,000 Å is formed is obtained.

Prior technical literature Non-patent literature

Non-Patent Document 1: Aluminum Guide, 7th Edition, 179~190 pages, 2007, General Association of Japan Aluminum Association

Non-Patent Document 2: Japanese Industrial Standard JIS H8601, "Anodized Film of Aluminum and Aluminum Alloys" (1999)

Patent literature

Patent Document 1: Japanese Patent Publication No. 5-191001

Summary of invention

As in the above-mentioned prior art, the aluminum material itself can be made into an electrode by direct wiring to an aluminum material for electrolytic treatment. Therefore, the surface treatment is performed on the entire area where the aluminum material touches the electrolytic solution.

When a surface treatment for imparting resin adhesion is applied to a part of the aluminum material, the surface portion which is not subjected to the treatment is covered with a mask tape or the like. The uncoated part will be surface treated, thus enhancing the resin density. Sexuality. However, since the portion where the coating is removed after the surface treatment is not subjected to the surface treatment, the portion is not imparted with resin adhesion as it is, and there is a problem that corrosion resistance is not obtained.

At this time, after performing a surface treatment for improving the adhesion of the resin to a part of the surface of the aluminum material, it is necessary to perform a surface treatment for imparting corrosion resistance to the portion not subjected to the surface treatment. In this case, it is necessary to re-cover the portion of the surface treatment which has been subjected to the improvement of the resin adhesion, and perform the second surface treatment. Therefore, the two-time coating not only causes an increase in cost, but also has a problem that the second surface treatment is required to reduce the productivity.

As a result of intensive studies to solve the above problems, the present inventors have found that a surface in which a corrosion-resistant oxide film is formed only in a specific portion and a porous oxide film is formed in a portion where a corrosion-resistant oxide film is not formed is formed. The present invention has been accomplished by a method of producing an aluminum material and a surface-treated aluminum material which simultaneously forms the corrosion-resistant oxide film and the porous oxide film.

That is, the invention of claim 1 is a surface-treated aluminum material characterized by comprising: an aluminum material; a corrosion-resistant oxide film formed on a part of the surface of the aluminum material and having a single-layer structure; and a porous oxide film, It is formed on a portion of the surface of the aluminum material where the corrosion-resistant oxide film is not formed; the corrosion-resistant oxide film has a thickness of 10 to 100 nm, and the peak absorption wave number obtained by FT-IR analysis is b (cm ^-1 ) When the peak absorption rate in the peak absorption wave number b is a (%), the relationship between 1≦a≦95 and b≧3a+710 is satisfied; the porous oxide film is formed on the surface side and has a thickness of 20 to 500 nm. a porous aluminum oxide film layer and a barrier aluminum oxide film layer formed on the substrate side and having a thickness of 3 to 30 nm, wherein the porous aluminum oxide film layer is formed with a small hole having a diameter of 5 to 30 nm and formed on the surface of the aluminum material. In the entire porous oxide film, the fluctuation range of the total thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer is within ±50% of the arithmetic mean of the total thickness.

According to the invention of claim 2, in the request item 1, the peak absorption wave number b (cm ^-1 ) is the wave number of the peak of the strongest stretching vibration from the Al-O, and appears in the range of 720 ≦ b ≦ 995.

According to the invention of claim 3 or 2, the ratio of the total pore area of the pores of the porous aluminum oxide film layer to the apparent surface area is 25 to 75%.

The invention of claim 4 is a method for producing a surface-treated aluminum material, which comprises using an electrode of a surface-treated aluminum material, an opposite electrode, and a conductive material connected to the aluminum electrode, at a pH of 9 to 13, and a liquid temperature An alkaline aqueous solution having a dissolved aluminum concentration of 5 ppm or more and 1000 ppm or less is an electrolytic solution at 35 to 85 ° C, and is exchanged at a frequency of 10 to 100 Hz, a current density of 4 to 50 A/dm ^{2 ,} and an electrolysis time of 5 to 60 seconds. The electrolytic treatment forms a porous oxide film on the surface of the aluminum material facing the counter electrode, and at the same time forms a corrosion-resistant oxide film on the surface of the aluminum material facing the conductive material connected to the aluminum electrode.

According to the invention of claim 5, in the request item 4, the aluminum electrode to be surface-treated and the opposite electrode are in the form of a flat plate, and the area of the conductive material connected to the aluminum electrode in the electrolytic solution is intended to form a corrosion resistant layer. Sexual oxidation The area of the film is 80 to 150%, and the distance between the conductive material and the aluminum material is 1 to 50 mm.

According to the invention of claim 6 or claim 5, the electrically conductive material that has been wired to the aluminum electrode is made of a stainless steel material or a copper material.

The invention of claim 7 is characterized in that, in any one of claims 4 to 6, the conductive material is disposed on one side of the aluminum electrode and faces the surface, and the opposite electrode is disposed on the aluminum material. The other surface side of the electrode faces the surface, whereby a corrosion-resistant oxide film is formed on one surface of the aluminum electrode, and a porous oxide film is formed on the other surface.

The invention of claim 8 is characterized in that, in any one of claims 4 to 6, the opposite electrode is disposed on the other surface side of the aluminum electrode and faces the surface, and the conductive material is disposed on the other surface The opposing electrodes are opposed to a portion of the other surface of the aluminum electrode, and the opposing electrode is disposed to face the other portion complementary to the other portion of the other surface of the aluminum electrode, thereby forming the aforementioned portion of the other surface of the aluminum electrode. A corrosion-resistant oxide film is formed, and a porous oxide film is formed on the other portions of the aluminum electrode.

According to the present invention, there is provided a surface-treated aluminum material characterized in that a surface-treated aluminum material is formed on a part of the surface to form a corrosion-resistant oxide film, and is formed in a portion where the corrosion-resistant oxide film is not formed. A porous porous oxide film.

1‧‧‧Surface treated aluminum

2‧‧‧Aluminum

3‧‧‧Corrosion resistant oxide film

4‧‧‧Porous oxide film

5‧‧‧relative electrode

6‧‧‧Electrical materials

7‧‧‧AC power supply

8‧‧‧Electrolysis solution

41‧‧‧Unobstructed aluminum oxide coating

42‧‧‧Porous aluminum oxide coating

420‧‧‧ hole

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross-sectional view showing a portion of a surface treated aluminum material of the present invention.

Fig. 2 is a schematic cross-sectional view showing another portion of the surface-treated aluminum material of the present invention.

Fig. 3 is a front elevational view showing an embodiment of an electrolytic apparatus for aluminum of the present invention.

Fig. 4 is a front elevational view showing another embodiment of the electrolysis device for aluminum of the present invention.

Form for implementing the invention

The details of the invention are described in order below.

As shown in Fig. 1, a part of the surface of the surface-treated aluminum material 1 of the present invention is formed with a corrosion-resistant oxide film 3. Moreover, as shown in FIG. 2, the porous oxide film 4 is formed in the surface part of the aluminum material 2 in which the corrosion-resistant oxide film 3 was not formed.

A. About aluminum

The aluminum material used in the present invention can utilize pure aluminum or an aluminum alloy. The composition of the aluminum alloy is not particularly limited, and various alloys including the alloy specified in JIS can be used. Although the shape is not particularly limited, it is suitable from the viewpoint of stably forming a treated film.

B. Corrosion resistant oxide film structure on the surface of aluminum

As shown in Fig. 1, a part of the surface of the aluminum material 2 used in the present invention is formed into an amorphous corrosion-resistant oxide film 3 having a thickness of 10 to 100 nm and preferably 20 to 80 nm. The corrosion-resistant oxide film 3 is a film of a single-layer structure. In the case of a multilayer structure having a non-single layer structure, crevice corrosion occurs between the surface layer and the intermediate layer, and sufficient corrosion resistance cannot be exhibited. Also, corrosion resistance can be formed on the entire surface of the aluminum material 2 Oxidation film 3. When the thickness of the corrosion-resistant oxide film 3 is less than 10 nm, sufficient corrosion resistance cannot be obtained. On the other hand, when it exceeds 100 nm, it is difficult to control the thickness of the corrosion-resistant oxide film, and processing unevenness occurs.

The film quality of the corrosion-resistant oxide film 3 is characterized by the following two points: the peak absorption wave b (cm ^-1 ) of the infrared absorption spectrum when analyzed by FT-IR (Fourier transform infrared spectrophotometer). And a peak absorption rate (a%) obtained by absorbing the baseline of the wave number by the peak. Here, the peak absorption wave number b (cm ^-1 ) is the absorption wave number from the peak of the strongest stretching vibration of Al-O. However, the peak absorption wave number b (cm ^-1 ) usually appears in the range of 720 ≦ b ≦ 995.

In the corrosion-resistant oxide film used in the present invention, the peak absorption rate a (%) at the time of analysis by FT-IR is 1 ≦ a ≦ 95 and preferably 2 ≦ a ≦ 75. When the peak absorption rate is less than 1%, the thickness of the corrosion-resistant oxide film is less than 10 nm, and the corrosion resistance is insufficient. When the peak absorption rate exceeds 95%, the thickness of the corrosion-resistant oxide film exceeds 100 nm, and it is difficult to control the thickness of the corrosion-resistant oxide film, which tends to cause uneven processing. Further, in the present invention, the peak absorption rate a (%) and the peak absorption wave number b (cm ^-1 ) satisfy the relationship of b ≧ 3a + 710 and desirably b ≧ 3a + 720. If the relationship is not satisfied, the shape of the film of the corrosion-resistant oxide film becomes a porous shape, so that the corrosion resistance is lowered.

C. Porous oxide film structure on the surface of aluminum

As shown in FIG. 2, a porous oxide film 4 is formed on the surface of the aluminum material where the corrosion-resistant oxide film is not formed. The porous oxide film 4 is composed of a barrier-type aluminum oxide film layer 41 on the substrate side of the aluminum material 2 and a porous aluminum oxide film layer 42 on the surface layer side.

C-1. Porous aluminum oxide film layer

The porous aluminum oxide film layer 42 has a thickness of 20 to 500 nm. When the thickness is less than 20 nm, the thickness is insufficient, so that the formation of the pore structure described later tends to be insufficient, and the adhesion or adhesion is lowered. On the other hand, when it exceeds 500 nm, the porous aluminum oxide film layer itself is liable to be cohesively broken and the adhesion or adhesion is lowered. The thickness of the porous aluminum oxide film layer 42 is desirably 30 to 400 nm.

Further, the porous aluminum oxide film layer 42 has a small hole 420 which penetrates deep in the depth direction from the surface. The small holes 420 have a diameter of 5 to 30 nm and desirably 10 to 20 nm. This small hole has an effect of increasing the contact area between the resin layer, the adhesive, and the like with the aluminum oxide film to increase the adhesion or adhesion. If the pore diameter is less than 5 nm, the contact area is insufficient, so that sufficient adhesion or adhesion cannot be obtained. On the other hand, when the pore diameter exceeds 30 nm, the porous aluminum oxide film layer as a whole becomes brittle and cohesive failure occurs, and the adhesion or adhesion is lowered.

The ratio of the total pore area of the small pores to the surface area of the porous aluminum oxide coating layer is not particularly limited. The ratio of the total pore area of the pores of the porous aluminum oxide film layer to the apparent surface area (the area expressed by the product of the length and the width irrespective of the minute unevenness of the surface) is preferably 25 to 75%. When it is less than 25%, the contact area may be insufficient to obtain sufficient adhesion or adhesion. On the other hand, if it exceeds 75%, the porous aluminum oxide film layer as a whole may become brittle and cause cohesive failure, and the adhesion or adhesion may be lowered.

C-2. Barrier aluminum oxide film layer

Barrier-type aluminum oxide film layer 41 is densely oxidized with a thickness of 3 to 30 nm Membrane. When the thickness is less than 3 nm, the bonding between the porous aluminum oxide film layer 4 and the aluminum matrix 2 cannot be sufficiently provided as an interposer, and the bonding strength in a severe environment such as high temperature and humidity is insufficient. On the other hand, when it exceeds 30 nm, it is easily affected by the denseness to cause cohesive failure of the barrier-type aluminum oxide film layer 3, and conversely, the adhesion or adhesion is lowered. Further, the thickness of the barrier aluminum oxide film layer 41 is preferably 5 to 25 nm.

C-3. Variation range of the overall thickness of the porous oxide film

The total thickness of the porous oxide film 4, that is, the thickness of the porous aluminum oxide film layer 42 described in C-1 and the thickness of the barrier aluminum oxide film layer 41 described in C-2, is the same regardless of the formation of the porous oxide film 4 For any position measurement, the variation must be within ±50% and ideally within ±20%. In other words, the arithmetic mean value of the total thickness of the porous oxide film measured at any of a plurality of positions on the surface of the aluminum material (preferably at 10 or more positions and preferably at 10 or more points at the respective positions) is T. In the case of (nm), the overall thickness of the porous oxide film in all of the plurality of measurement positions must be in the range of (0.5 × T) to (1.5 × T). If there is a position smaller than (0.5 × T), the porous oxide film at its position will be thinner than its surroundings. As a result, in the thin portion of the film, a gap is formed between the adhesive or the resin layer to be adhered to the porous oxide film, and a sufficient contact area cannot be ensured, thereby further bonding the force or The adhesion is reduced.

On the other hand, if there is a position exceeding (1.5 × T), the porous oxide film at the position is thicker than the surrounding circumference. As a result, the stress from the resin layer to be adhered or the like is concentrated at the position of the film thickness, and the cohesive failure in the porous oxide film is caused, and the adhesion or adhesion is lowered.

Further, in the position where the overall thickness of the porous oxide film as described above is a position where the film is thin or the film thickness is different from the surrounding phase, the optical characteristics are different, so that a color such as brownish or white turbid color can be visually observed. Variety.

D. How to manufacture aluminum materials

One of the methods for producing a surface-treated aluminum material having a corrosion-resistant oxide film and a porous oxide film having the above conditions may be a method of using an aluminum electrode to be surface-treated, wiring to an aluminum material, and setting it to an aluminum material. The conductive material in the vicinity of the surface and the electrode as the material of the counter electrode described later are an electrolytic solution having a pH of 9 to 13, a liquid temperature of 35 to 85 ° C, and an dissolved aluminum concentration of 5 ppm or more and 1000 ppm or less as an electrolytic solution at a frequency of 10~. AC electrolysis treatment is carried out under the conditions of 100 Hz, current density of 4 to 50 A/dm ² and electrolysis time of 5 to 60 seconds, thereby forming a corrosion-resistant oxide film on the surface of the aluminum material opposite to the conductive material, and The electrode forms a porous oxide film with respect to a portion of the surface of the aluminum material. Further, by making the total surface of the aluminum material face the conductive material, only the corrosion-resistant oxide film is formed on the surface.

The surface-treated aluminum material of the present invention is subjected to an AC electrolytic treatment using an aluminum electrode to be surface-treated, a conductive material which is connected to the electrode of the aluminum, and a counter electrode. The electrically conductive material that has been wired to the surface of the aluminum electrode to be surface-treated is disposed in the vicinity of the surface portion of the aluminum material to be formed into the corrosion-resistant oxide film in the electrolytic solution, and is opposed to the surface portion. A porous oxide film is formed on the surface portion of the aluminum material that is not opposed to the conductive material. Further, in the electrolytic solution, the area of the conductive material opposed to the aluminum material is 80 to 150%, and preferably 90 to 130%, of the area of the aluminum material forming the corrosion-resistant oxide film. also, The distance from the conductive material opposite to the aluminum material is 1 to 50 mm. When the area of the conductive material is less than 80% of the area of the aluminum material to form the corrosion-resistant oxide film, a porous oxide film may be formed in a portion where the corrosion-resistant oxide film is to be formed; if it exceeds 150%, it may also be A portion of the porous oxide film is formed to form a corrosion-resistant oxide film. When the distance from the conductive material to the aluminum material is shorter than 1 mm, it is difficult to cause convection of the electrolytic solution between the conductive material and the opposite aluminum material, and the uneven treatment of the corrosion-resistant oxide film is likely to occur. On the other hand, if the distance exceeds 50 mm, the interval between the conductive material and the aluminum material is too wide to form a porous oxide film on the entire surface of the aluminum material.

In the alternating electrolytic treatment step, as the alkaline aqueous solution used as the electrolytic solution, a phosphate such as sodium phosphate, potassium hydrogen phosphate, sodium pyrophosphate, potassium pyrophosphate or sodium metaphosphate; an alkali metal hydrogen such as sodium hydroxide or potassium hydroxide can be used. An oxide; a carbonate such as sodium carbonate, sodium hydrogencarbonate or potassium carbonate; ammonium hydroxide; or an aqueous solution of such a mixture. As will be described later, from the viewpoint that the pH of the electrolytic solution must be maintained within a specific range, it is preferred to use an aqueous alkali solution containing a phosphate-based substance having a desired buffering effect. The concentration of the alkali component contained in the alkaline aqueous solution can be appropriately adjusted so that the pH of the electrolytic solution becomes a desired value, and is usually 1 × 10 ^-4 to 1 mol / liter and desirably 1 × 10 ^-3 to 0.8 mol / liter. Further, in such an alkaline aqueous solution, a surfactant may be added in order to enhance the detergency.

The pH of the electrolytic solution must be set to 9 to 13, and it is preferably set to 9.5 to 12. When the pH is less than 9, the alkali etching force of the electrolytic solution is insufficient, and the porous oxide film becomes an amorphous film, and the formation of the predetermined porous aluminum oxide film layer and the barrier aluminum oxide film layer is incomplete. Also, it will be difficult if the pH is below 9. In order to control the thickness of the corrosion-resistant oxide film, uneven processing is likely to occur. On the other hand, when the pH exceeds 13, the alkali etching force is excessive and the oxide film layer is hard to grow, which hinders the formation of a desired porous oxide film.

The temperature of the electrolytic solution must be set to 35 to 85 ° C, and it is preferably set to 40 to 70 ° C. When the temperature of the electrolytic bath is lower than 35 ° C, the alkali etching force is insufficient to cause the formation of the porous oxide film to be incomplete. On the other hand, when it exceeds 85 ° C, the alkali etching force is excessive, and the formation of the porous oxide film and the corrosion-resistant oxide film is inhibited.

The concentration of dissolved aluminum contained in the electrolytic solution must be 5 ppm or more and 1000 ppm or less, and preferably 10 ppm or more and 500 ppm or less. When the concentration of the dissolved aluminum is less than 5 ppm, the formation reaction of the oxide film is violently generated at the initial stage of the electrolysis reaction, and thus a porous oxide film having a thick film may be locally formed to cause a difference in the corrosion-resistant oxide film. On the other hand, if the concentration of dissolved aluminum exceeds 1000 ppm, the viscosity of the electrolytic solution increases to impede uniform convection in the vicinity of the surface of the aluminum material in the electrolysis step, and the dissolved aluminum acts in the direction of suppressing the formation of the porous oxide film. As a result, a thin porous oxide film is locally formed. When the concentration of the dissolved aluminum is not in the above range, it is difficult to change the total thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer in the entire porous oxide film formed on the surface of the aluminum material. Within ±50% of the arithmetic mean of the total thickness. As a result, the adhesion force ‧ the adhesion of the obtained porous oxide film is lowered.

The frequency used is 10~100Hz. When the temperature is lower than 10 Hz, the element of direct current is increased in terms of electrolysis, and as a result, the formation of the porous aluminum oxide film layer cannot be performed to form a dense structure. On the other hand, if it exceeds 100 Hz, The reversal of the anode and the cathode is too fast, and the formation of the entire oxide film is extremely slow, and the porous oxide film and the corrosion-resistant oxide film become extremely long in order to obtain a predetermined thickness. However, the frequency used is preferably 20 to 80 Hz.

The current density must be set to 4~50A/dm ² . When the current density is less than 4 A/dm ² , only the barrier aluminum oxide film layer is preferentially formed in the porous oxide film, so that the porous aluminum oxide film layer cannot be obtained. On the other hand, when it exceeds 50 A/dm ² , the current is too large, and it is difficult to control the thickness of the porous oxide film and the corrosion-resistant oxide film, and the processing unevenness is likely to occur. However, it is preferable that the current density is set to 5 to 30 A/dm ² .

The electrolysis time must be set to 5 to 60 seconds. At a treatment time of less than 5 seconds, the formation of the porous oxide film is excessively rapid, and the porous aluminum oxide film layer cannot be sufficiently formed, and an oxide film composed of an amorphous aluminum oxide is formed. On the other hand, if it exceeds 60 seconds, the oxide film is redissolved and the corrosion-resistant oxide film layer cannot be sufficiently formed, and the productivity is also lowered, which is not preferable. However, the electrolysis time is preferably set to 10 to 50 seconds.

One of the electrodes used in one of the alternating current electrolytic treatments is an aluminum material which is subjected to surface treatment by electrolytic treatment. As the other counter electrode, for example, a known electrode such as a black lead, aluminum or titanium electrode can be used, but it is necessary to use a material which does not deteriorate the alkali component or temperature of the electrolytic solution, has good conductivity, and does not cause an electrochemical reaction itself. . From this point of view, it is preferable to use a black lead electrode for the counter electrode. This is because the black lead electrode is chemically stable and can be easily started at a low price, and the power line can be moderately diffused in the alternating current electrolysis step by the action of most of the pores present in the black lead electrode. Therefore, it is easy to make the porous oxide film and the corrosion-resistant oxide film uniform together.

The electrode material is connected to the conductive material which forms part of the corrosion-resistant oxide film on the aluminum material by electrolytic treatment, and the electrode potential must be a noble potential as compared with the aluminum material. By electrolyzing a conductive material that is at a noble potential to the electrode potential of the aluminum electrode and placing it in the vicinity of the aluminum material for electrolysis, a cathodic reaction of alternating current electrolysis can be caused on the surface of the conductive material, and is caused on the surface of the aluminum material and the conductive material. Anode reaction. Therefore, a corrosion-resistant oxide film caused by an anodic reaction can be formed on the surface of the aluminum material. The conductive material having a noble potential with respect to the electrode potential of the aluminum material is, for example, gold, platinum, copper, iron, stainless steel, nickel, or the like. In the present invention, stainless steel or copper is suitably used. This is because these are excellent in corrosion resistance in an alkali solution, low in cost, and easy to process.

In the present invention, the aluminum material to be electrolytically treated and the counter electrode should preferably be used together with the flat plate, and the opposite faces of the opposing aluminum material and the opposite electrode should be substantially the same size, and the two electrodes are carried out in a stationary state. Electrolysis operation. As shown in FIG. 3, the counter electrode 5 is prepared, and the surface of the surface-treated aluminum material 2 is preferably disposed in parallel with the surface of the counter electrode 5 in a manner opposed to the counter electrode plate 5. In the figure, 7 is an AC power source, and 8 in the figure is an electrolytic solution.

The shape of the electrically conductive material 6 connected to the flat aluminum material 2 to be electrolytically treated is not necessarily a flat plate shape, and may be a mesh shape or a porous shape. As shown in FIG. 3, for example, on one side (the side on the left side in the drawing) of the aluminum material 2 to be surface-treated, the conductive material 6 is disposed so as to face the surface, and is on the other side of the aluminum material 2 (in the figure) The side of the right side is disposed opposite to the surface so as to face the opposite electrode 5 By performing alternating current electrolysis, a corrosion-resistant oxide film can be formed on one surface of the aluminum material 2, and a porous oxide film can be formed on the other surface.

As shown in FIG. 3, as shown in FIG. 4, the conductive material 6 may be disposed between the other surface of the aluminum material 2 to be surface-treated (the surface on the right side in the drawing) and the counter electrode 5 corresponding to the surface. At this time, a part of the other side of the aluminum material 2 (the upper half of the figure is opposed to the conductive material 6), and the other part of the other side of the aluminum material 2 (the lower half of the figure) is not The conductive material 6 faces the opposite side and faces the opposite electrode 5. By performing alternating current electrolysis in the above-described arrangement, a corrosion-resistant oxide film can be formed on a part of the other surface of the aluminum material facing the conductive material 6, and the aluminum material can be opposed to the opposite electrode 5 instead of the conductive material 6. On the other surface of the other surface, a porous oxide film is formed, and a corrosion-resistant oxide film and a porous oxide film can be formed on the same surface of the aluminum material 2. Further, in Fig. 4, a part of the other side of the aluminum material 2 (the lower half of the figure) may be opposed to the conductive material 6, and the other side of the other side of the aluminum material 2 (the upper side in the drawing) The half portion is disposed so as not to face the conductive material 6 but to face the counter electrode 5, whereby a corrosion-resistant oxide film is formed on a part of the other surface, and a porous oxide film is formed on the other surface of the other surface.

Further, a portion that is a part of the other surface of the aluminum material 2 and that faces the conductive material 6 can be set to any shape, size, and position. Further, another portion which is the other surface of the aluminum material 2 and a portion facing the opposite electrode 5 may be provided on the other surface in a portion complementary to the aforementioned portion.

The structure observation and thickness measurement of the porous oxide film and the corrosion-resistant oxide film of the present invention are suitable for use using a penetrating electron microscope (TEM) cross section observation. Specifically, the thickness of the porous oxide film and the corrosion-resistant oxide film and the small pore diameter of the porous oxide film layer can be measured by TEM observation by processing into a thin sheet by an ultramicrotome or the like.

Example

Hereinafter, appropriate embodiments of the present invention will be described based on examples and comparative examples.

The aluminum material to be electrolytically treated is a flat plate of JIS 5052 having a length of 500 mm × a width of 500 mm × a plate thickness of 1.0 mm. The aluminum plate was used as one of the electrodes, and a black lead plate or a titanium plate having a length of 500 mm × a width of 550 mm × a plate thickness of 2.0 mm was used for the counter electrode. In Examples 1 to 23 and Comparative Examples 1 to 13, as shown in FIG. 3, the counter electrode 5 and the aluminum material 2 were disposed in parallel with each other. The conductive material 6 is disposed in the vicinity of the surface of the aluminum alloy plate 2 opposite to the counter electrode 5, and a corrosion-resistant oxide film is formed on one surface of the aluminum alloy plate 2 on the side of the conductive material 6, and a porous surface is formed on the entire surface of the opposite electrode side. Oxidation film. Further, in the embodiment 24, as shown in Fig. 4, a part (the upper half of the figure in the figure) on the other surface (the side on the right side in the drawing) of the surface of the aluminum material 2 is disposed opposite to the conductive material, and The other portion on the other surface of the surface of the aluminum material 2 (the lower half portion in the drawing) does not face the conductive material 6 and opposes the opposite electrode 5, so that the corrosion-resistant oxide film and the porous oxide film are formed on the aluminum material 2 The same side.

For the conductive materials, in Examples 1 to 14, 18 to 21, and Comparative Examples 1 to 13, SUS304 stainless steel sheets having a length of 500 mm × a width of 500 mm × a thickness of 0.5 mm were used. In Example 15, a copper plate of 500 mm in length × 500 mm in width × 0.5 mm in thickness was used. In Example 16, a vertical 500 mm × a width of 500 mm, a wire diameter of 0.5 mm, a porosity of 64.5%, and a 10-mesh SUS304 stainless steel were used. Steel gold mesh. In Example 17, a SUS304 stainless steel punched metal having a length of 500 mm × a width of 500 mm, a hole diameter of 6 mm, and a hole-to-hole distance (pitch) of 8 mm was used. In Example 22, a SUS304 stainless steel plate having a length of 480 mm × a width of 480 mm × a thickness of 0.5 mm was used. In Example 23, a SUS304 stainless steel plate having a length of 570 mm × a width of 570 mm × a thickness of 0.5 mm was used. In Example 24, a SUS304 stainless steel plate having a length of 250 mm × a width of 500 mm × a thickness of 0.5 mm was used. Further, the distance between the conductive material and the aluminum alloy plate was set to 5 mm in Examples 1 to 17, 22 to 24 and Comparative Examples 1 to 13. In Example 18, it was set to 2 mm, in Example 19, it was set to 45 mm, in Example 20, it was set to 0.5 mm, and in Example 21, it was set to 55 mm. On the other hand, in the area ratio of the conductive material shown in Table 1 to the area of the aluminum material, the area of the conductive material was determined by ignoring the longitudinal and lateral lengths of the void portion.

As the electrolytic solution, an alkaline aqueous solution having a pH, a temperature, and a dissolved aluminum concentration as shown in Table 1 and containing sodium pyrophosphate as a main component was used. On the other hand, the pH of the electrolytic solution was adjusted with a 0.1 mol/liter NaOH aqueous solution. The alkali component concentration of the alkaline aqueous solution was set to 0.1 mol/liter. Further, electrolytic treatment was carried out under the conditions of the alternating current electrolytic treatment shown in Table 1, and a surface-treated aluminum material having a corrosion-resistant oxide film formed on one surface of the aluminum plate and a porous oxide film formed on the other surface was prepared. Test materials.

[Table 1]

[Measurement of thickness of corrosion resistant oxide film]

The cross-section of the corrosion-resistant oxide film was carried out by TEM on the test material prepared by the above method. Specifically, the thickness of the corrosion-resistant oxide film was measured, and the structure of the corrosion-resistant oxide film (whether the film layer was a single-layer structure) was observed. In order to measure the thickness of the corrosion-resistant oxide film and the structure of the corrosion-resistant oxide film, a cross-sectional observation sheet sample was prepared from the test material using an ultramicrotome. Then, any 100 points of the observation field (1 μm × 1 μm) were selected in the sheet sample, and the thickness of the corrosion-resistant oxide film was measured by TEM cross-section observation and whether the corrosion-resistant oxide film was observed as a single-layer structure (single layer structure was regarded as ○) Non-single-layer structure is treated as ×). When the thickness of the corrosion-resistant oxide film is 10 to 100 nm and the corrosion-resistant oxide film is a single-layer structure, the evaluation is acceptable (○), the thickness of the corrosion-resistant oxide film is not in the range of 10 to 100 nm, and the corrosion-resistant oxide film is not a single-layer structure. At least one of the results is evaluated as unacceptable (x). The above results are shown in Table 2.

[Table 2]

[Measurement of film thickness of porous oxide film]

In the porous oxide film layer, cross-sectional observation was also carried out by TEM. Specifically, the thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer in the porous oxide film layer, and the pore diameter of the porous oxide film layer are measured. In order to measure these, a cross-sectional observation sheet sample was produced from the test material using an ultramicrotome. Then, any 100 points of the observation field (1 μm × 1 μm) were selected in the sheet sample, and the porous aluminum oxide film layer and the barrier aluminum oxide film in the porous oxide film layer were measured at various points by TEM cross-section observation. The thickness of the layer and the pore diameter of the porous oxide film layer. The maximum value, the minimum value, and the arithmetic mean of the total thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer measured at 100 points as described above were determined. Further, it was also examined whether or not the fluctuation range of the total thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer was within ±50% of the arithmetic mean value. Specifically, when the arithmetic mean value is T (nm), all the total thickness including the maximum value and the minimum value are regarded as qualified (○) in the range of (0.5×T) to (1.5×T), and when not in the range It is considered as unqualified (×). However, in the embodiment 24, the side opposite to the opposite electrode was used. The above results are shown in Table 2.

[Evaluation of Corrosion Resistance of Corrosion Oxide Film]

Ten pieces of test pieces each having a length of 50 mm and a width of 50 mm were prepared from each test piece. The corrosion resistance test was carried out in accordance with the CASS test described in the salt spray test method (JIS Z 2371). The obtained anodized product was taken out after 3 hours of CASS test, and the corrosion area of the corrosion-resistant oxide film was measured and the corrosion area ratio was evaluated. Here, the corrosion area ratio (%) is defined as [(corrosion area) / (total area of corrosion-resistant oxide film)] × 100.

○: Corrosion area ratio is less than 10%

△: The corrosion area ratio is 10% or more and less than 50%

×: The corrosion area ratio is 50% or more

The results are shown in Table 3. In the table, the number of the above-mentioned ○, △, and × in the ten test pieces was shown, and when all the results were ○, it was judged as pass, and the other results were judged as unacceptable.

[table 3]

[Evaluation of Membrane Quality of Corrosion Oxide Film]

The peak absorption wave number b (cm ^-1 ) of the corrosion-resistant oxide film and the peak absorption rate a (%) in the peak absorption wave number b were measured by FT-IR. The detector of the FT-IR uses a wavenumber that can measure a wide range (400 to 4000 cm ^-1 ), and the sample area is set to 30 mm × 50 mm. The results are shown in Table 3. In the table, the relationship between the peak absorption rate a (%) and the peak absorption wave number b (cm ^-1 ) satisfying 1≦a ≦ 95 and the peak absorption rate a (%) and the peak absorption wave number b (cm ^-1 ) are satisfied. The result of b≧3a+710 is regarded as qualified (○), and the result of not being ranged is regarded as unqualified (×).

[Adhesive adhesion evaluation of porous oxide film]

Two pieces of the test piece were cut into test pieces of 50 mm in length and 25 mm in width. The two test pieces were overlapped with each other along the total width direction by a width of 10 mm, and the surface of the porous oxide film was overlapped with each other, and a commercially available two-liquid epoxy adhesive (main agent = modified epoxy) Resin, hardener = modified polyimine, weight mixing ratio = main agent 100 / hardener 100) The overlapping portion was adhered to prepare a shear test piece. The end portions of the two test materials in the longitudinal direction were stretched in the longitudinal direction at a speed of 10 mm/min by a tensile tester, and their loads (converted to shear stress) and peeling state were used. The benchmark evaluates the adhesion of the adhesive. On the other hand, 10 test pieces were prepared by shearing test pieces and evaluated for each group. However, in the embodiment 24, the side opposite to the opposite electrode was used.

○: the shear stress is 20 N/mm ² or more and the adhesive layer itself is in a state of cohesive failure.

△: Although the shear stress is 20 N/mm ² or more, the adhesive layer and the test material are interfacially peeled off.

×: the shear stress is lower than 20 N/mm ² and the adhesive layer is interfacially peeled off from the test material.

The results are shown in Table 3. In the table, the number of the above-mentioned groups of ○, △, and × in the ten test pieces was shown, and when all the results were ○, it was judged as pass, and the other results were judged as unacceptable.

[Evaluation of Coating Adhesion of Porous Oxide Film]

On the surface of the porous oxide film side of the test material, "V Flon #2000" manufactured by Nippon Paint Co., Ltd. was applied and dried (160 ° C, 20 minutes) to prepare a resin coating film having a thickness of 30 μm. Adhesive test piece. A 1 mm checkerboard-like score was cut out from the resin coating film of the adhesion test piece by a cutting knife according to the method of JIS-K5600-5-6. Next, after the test piece was subjected to a distiller immersion treatment at 125 ° C for 30 minutes, it was taken out from the treatment liquid immediately and the water was removed. The test piece was subjected to a peeling test using a transparent pressure-sensitive adhesive tape. The adhesion was evaluated by the following criteria on the basis of the coating film residual ratio. On the other hand, in the adhesion test piece, 10 test pieces were produced from the same test piece and evaluated for each test piece. However, in the embodiment 24, the side opposite to the opposite electrode was used.

○: The residual rate of the coating film is 100%.

△: The residual rate of the coating film is 75% or more and less than 100%.

×: The residual rate of the coating film is less than 75%

The results are shown in Table 3. In the table, the number of the above-mentioned ○, △, and × in the ten test pieces was shown, and when all the results were ○, it was judged as pass, and the other results were judged as unacceptable.

[Comprehensive Evaluation]

Corrosion resistance evaluation and membrance evaluation of corrosion resistant oxide film, and porous The adhesion evaluation of the adhesion of the oxide film and the evaluation of the adhesion of the film are all qualified, and the comprehensive evaluation is regarded as qualified, and the comprehensive evaluation of at least one of the evaluations is considered as unqualified. .

As shown in Tables 2 and 3, in Examples 1 to 24, since the main conditions of the present invention were satisfied, the corrosion resistance and the film quality of the corrosion-resistant oxide film were good, and the adhesiveness of the porous oxide film and the adhesion of the coating film were good. Good sex, comprehensive assessment is qualified. On the other hand, in Comparative Examples 1 to 13, since the main conditions of the present invention were not satisfied, at least one of the above evaluations was unacceptable, and the comprehensive evaluation was unacceptable.

Specifically, in Comparative Example 1, since the pH of the electrolytic solution in the alternating current electrolysis was too low, the alkali etching force was insufficient. Therefore, the diameter of the small pores in the porous aluminum oxide film layer is insufficient, and the thickness of the barrier aluminum oxide film layer becomes too thick, and the variation of the thickness of the porous oxide film is excessively large. As a result, the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 2, since the pH of the electrolytic solution in the alternating current electrolysis was too high, the alkali etching was excessively caused. Therefore, the thickness of the corrosion-resistant oxide film is insufficient and the thickness of the porous aluminum oxide film layer is insufficient, the pore diameter is too large, and the thickness of the barrier aluminum oxide film layer is insufficient. As a result, the corrosion resistance of the corrosion-resistant oxide film was unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 3, since the temperature of the electrolytic solution in the alternating current electrolysis was too low, the alkali etching force was insufficient. Therefore, the thickness of the barrier aluminum oxide film layer becomes thick. Further, the pore diameter of the porous aluminum oxide film layer becomes small. Knot As a result, the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 4, since the temperature of the electrolytic solution in the alternating current electrolysis was too high, the alkali etching was excessively caused. Therefore, the thickness of the corrosion-resistant oxide film and the thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer are insufficient. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 5, the dissolved aluminum was not present in the electrolytic solution of the alternating current electrolysis. Therefore, the initial reaction of the electrolytic reaction rapidly produces a formation reaction of the porous oxide film, and the fluctuation range of the thickness of the porous oxide film is increased. Further, the formation of the corrosion-resistant oxide film is also uneven and the thickness is insufficient. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 6, a large amount of dissolved aluminum was present in the electrolytic solution of the alternating current electrolysis. Therefore, the formation of the barrier-type aluminum oxide film layer is uneven and the film thickness is locally formed, and the pore diameter of the porous aluminum oxide film layer becomes small. Further, the variation range of the thickness of the porous oxide film becomes large. As a result, the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 7, since the frequency of the alternating current electrolysis was too low, the direct current component of the current was enhanced. Therefore, the formation of the porous aluminum oxide film layer is suppressed and the thickness is insufficient. As a result, the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 8, since the frequency of the alternating current electrolysis was too high, the formation of the corrosion-resistant oxide film and the porous aluminum oxide film layer was suppressed, and the respective thicknesses were insufficient. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 9, since the current density of the alternating current electrolysis was too low, the formation of the corrosion-resistant oxide film and the porous aluminum oxide film layer was suppressed, and the respective thicknesses were insufficient. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 10, since the current density of the alternating current electrolysis was too high, the formation of the porous oxide film and the corrosion-resistant oxide film was uneven. Therefore, the thickness of the corrosion-resistant oxide film and the thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer become thick. Moreover, the fluctuation range of the thickness of the porous oxide film becomes large. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 11, since the electrolysis time of the alternating current electrolysis was too short, the thickness of the corrosion-resistant oxide film and the thickness of the porous aluminum oxide film layer were insufficient. Moreover, the fluctuation range of the thickness of the porous oxide film becomes large. As a result, the corrosion resistance and the film quality of the corrosion-resistant oxide film were unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 12, since the electrolysis time of the alternating current electrolysis was too long, The formation of the porous oxide film is excessively advanced. Therefore, the thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer becomes thick. As a result, the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

In Comparative Example 13, the dissolved aluminum in the electrolytic solution of the alternating current electrolysis was less than 5 ppm. Therefore, the initial reaction of the electrolysis reaction rapidly causes the formation reaction of the porous oxide film to increase the variation range of the thickness of the porous oxide film. Further, the formation of the corrosion-resistant oxide film is not uniform and the thickness is insufficient. As a result, the corrosion resistance of the corrosion-resistant oxide film was unacceptable, and the adhesiveness of the adhesive of the porous oxide film and the adhesion of the coating film were unacceptable, and the overall evaluation was unacceptable.

Industrial availability

According to the present invention, it is possible to obtain a surface-treated aluminum material which is excellent in corrosion resistance of a part of the surface of the aluminum material and which has good adhesion and adhesion to other parts of the surface of the aluminum material. Therefore, the surface-treated aluminum material of the present invention is suitably used for an aluminum-resin joint member, a printed wiring board, and the like which require adhesiveness and adhesion only in one part of the aluminum material.

1‧‧‧Surface treated aluminum

2‧‧‧Aluminum

3‧‧‧Corrosion resistant oxide film

Claims (8)

A surface-treated aluminum material comprising: an aluminum material; a corrosion-resistant oxide film formed on a part of the surface of the aluminum material and having a single-layer structure; and a porous oxide film formed on the surface of the aluminum material a portion of the corrosion-resistant oxide film; the corrosion-resistant oxide film has a thickness of 10 to 100 nm, and the peak absorption wave number obtained by FT-IR analysis is b (cm ^-1 ), and the peak absorption in the peak absorption wave number b When the ratio is a (%), the relationship between 1≦a≦95 and b≧3a+710 is satisfied; the porous oxide film is formed of a porous aluminum oxide film layer formed on the surface side and having a thickness of 20 to 500 nm, and formed thereon. a barrier-type aluminum oxide film layer having a thickness of 3 to 30 nm on the side of the substrate, and a pore having a diameter of 5 to 30 nm formed on the porous aluminum oxide film layer and formed on the entire porous oxide film on the surface of the aluminum material, The fluctuation range of the total thickness of the porous aluminum oxide film layer and the barrier aluminum oxide film layer is within ±50% of the arithmetic mean of the total thickness. The surface-treated aluminum material of claim 1, wherein the peak absorption wave number b (cm ^-1 ) is a wave number from a peak of the strongest stretching vibration of Al-O, and appears in the range of 720 ≦ b ≦ 995. The surface-treated aluminum material of claim 1 or 2, wherein the ratio of the total pore area of the pores of the porous aluminum oxide film layer to the apparent surface area It is 25~75%. A method for manufacturing a surface-treated aluminum material, which comprises using an electrode of a surface-treated aluminum material, a counter electrode, and a conductive material connected to the aluminum electrode, at a pH of 9 to 13, a liquid temperature of 35 to 85 ° C, and dissolved An alkaline aqueous solution having an aluminum concentration of 5 ppm or more and 1000 ppm or less is an electrolytic solution, and is subjected to an alternating current electrolysis treatment at a frequency of 10 to 100 Hz, a current density of 4 to 50 A/dm ^{2 ,} and an electrolysis time of 5 to 60 seconds. A porous oxide film is formed on the surface of the aluminum material facing the opposite electrode, and at the same time, a corrosion-resistant oxide film is formed on the surface of the aluminum material facing the conductive material connected to the aluminum electrode. The method for producing a surface-treated aluminum material according to claim 4, wherein the aluminum electrode to be surface-treated and the opposite electrode are in the form of a flat plate, and the area of the conductive material in the electrolytic solution which has been connected to the electrode of the aluminum is intended to be formed. The area of the corrosion-resistant oxide film is 80 to 150%, and the distance between the conductive material and the aluminum material is 1 to 50 mm. A method of producing a surface-treated aluminum material according to claim 4, wherein the electrically conductive material that has been wired to the aluminum electrode is made of stainless steel or copper. The method for producing a surface-treated aluminum material according to any one of claims 4 to 6, wherein the conductive material is disposed on one side of the aluminum electrode and faces the surface, and the opposite electrode is disposed on the aluminum electrode. The other surface side faces the surface, whereby a corrosion-resistant oxide film is formed on one surface of the aluminum electrode, and a porous oxide film is formed on the other surface. The method for producing a surface-treated aluminum material according to any one of claims 4 to 6, wherein the opposite electrode is disposed on the other side of the aluminum electrode and is opposite to the surface In the opposing direction, the conductive material is disposed between the other surface and the opposite electrode and faces a portion of the other surface of the aluminum electrode, and the opposing electrode is disposed to face the other portion complementary to the other portion of the other surface of the aluminum electrode Thereby, a corrosion-resistant oxide film is formed on the other portion of the other surface of the aluminum electrode, and a porous oxide film is formed on the other portion of the aluminum electrode.