JP2008174764A - Mirror surface aluminum material - Google Patents

Abstract

A specular aluminum material having good specular reflection and corrosion resistance and low manufacturing cost is provided.
An aluminum substrate containing 99.5% by weight or more of Al and having a center line average roughness Ra of 0.1 μm or less on the surface and an unevenness average interval Sm of 10 μm or more, and formed on the surface thereof. A mirror surface aluminum material comprising a barrier type anodic oxide film having a thickness of 100 to 500 nm. The thickness of the barrier type anodic oxide film is preferably 150 ± 30 nm or 300 ± 20 nm.
[Selection] Figure 3

Description

  The present invention is a mirror surface aluminum that is excellent in corrosion resistance and regular reflection, and is low in production cost, used for a reflector plate that requires high specularity and a building material, an electronic device, and a housing that require a design with high luster. Regarding materials.

  Materials for automobile parts such as reflectors and wheels such as illuminators have a highly glossy reflecting surface on the surface from the viewpoint of regular reflection and design. In order to protect this reflective surface, conventionally, a pore type anodized film is formed on the surface of a base material of an aluminum plate by sulfuric acid anodizing treatment, or an organic coating film by coating is formed.

  When forming a pore type anodic oxide film, if the corrosion resistance is given priority, the film thickness is increased (usually 1000 nm or more), and if the regular reflection is given priority, the film thickness needs to be reduced. There was an incompatible problem. In addition, in order to reduce the decrease in specular reflectivity due to the absorption of visible light by the pore type anodic oxide film, the pore type anode is improved after the surface of the aluminum substrate is electropolished, smoothed and cleaned to slightly improve the specular reflectivity. An oxidation treatment may be applied. However, there has been a problem that manufacturing cost increases due to such electropolishing.

On the other hand, when directly coating an aluminum plate, the adhesion of the organic coating film may be inferior. Therefore, a method of painting after pore type anodizing treatment (for example, see Patent Document 1), a method of painting after barrier type anodizing treatment (for example, see Patent Document 2), etc. in order to achieve both corrosion resistance and brightness are proposed. Has been.
JP 62-83497 A Japanese Patent Laid-Open No. 10-81997

  As described in Patent Document 1, when an organic coating film is applied to an aluminum plate that has been subjected to pore-type anodizing treatment, the corrosion resistance is good due to the function of the organic coating film as a protective film. However, even if the pore-type anodic oxide film is made 1000 nm or less and specular reflection is prioritized, absorption of visible light by the organic coating film cannot be avoided and the specular reflectivity is poor.

  As described in Patent Document 2, when an acrylic clear coating is applied to an aluminum plate having a coating thickness of 50 nm or less and a barrier type anodizing treatment with a coating thickness of 20 μm, the corrosion resistance is good due to the function of the organic coating. . However, visible light absorption by the organic coating film cannot be avoided and the problem of poor regular reflection remains.

  An object of this invention is to provide the mirror surface aluminum material which is excellent in corrosion resistance and specular reflection property, and whose manufacturing cost is cheap.

  The present inventors use a high purity, smooth and highly specular aluminum material, paying attention to the type and thickness of the anodized film, and controlling the film thickness of the barrier type anodized film within a predetermined range. The inventors have found that a mirror-finished aluminum material can be obtained that has both corrosion resistance and regular reflection properties and is inexpensive to manufacture.

  The present invention is the aluminum substrate according to claim 1, which contains 99.5% by weight or more of Al, has a center line average roughness Ra of 0.1 μm or less on the surface, and an unevenness average interval Sm of 10 μm or more, A mirror-finished aluminum material comprising a barrier type anodic oxide film formed on the surface and having a thickness of 100 to 500 nm. Furthermore, in claim 2, the thickness of the barrier type anodic oxide film is set to 150 ± 30 nm or 300 ± 20 nm.

  The mirror surface aluminum material according to the present invention is excellent in corrosion resistance and specular reflection, and is low in manufacturing cost. Therefore, in particular, a reflector for a solar lighting system, various reflectors for lighting, interior building materials that emphasize design, It is suitably used as a housing material for digital home appliances.

A. Aluminum substrate The aluminum substrate used in the present invention is a high-purity aluminum material having an Al content of 99.5% by weight or more. When the Al content is less than 99.5% by weight, the absorption of visible light by impurities increases, so that even if a high degree of surface smoothness is achieved, the regular reflectance is poor.

  As parameters representing the degree of surface roughness of the aluminum substrate, centerline average roughness Ra and unevenness average interval Sm defined by JIS B 0601 are adopted. Ra needs to be 0.1 μm or less and Sm should be 10 μm or more. When Ra exceeds 0.1 μm or Sm is less than 10 μm, the diffuse reflectivity increases, and as a result, the regular reflectivity is inferior.

  Conventionally, as a parameter of surface roughness, only Ra is often used as an index. As a result of detailed investigations by the present inventors, it has been found that Sm, which is a parameter representing the average interval of unevenness on the surface, is also an important index. In order to make such a smooth surface using Ra and Sm as an index, the roll surface roughness during cold rolling to the product thickness is reduced, or a small rolling roll having a hemispherical contact area is used, etc. It is necessary to appropriately select bright rolling conditions. At least one of the surfaces of the aluminum substrate is defined as such a surface roughness surface.

B. Before forming the anodic oxide film on the surface of the pretreated aluminum substrate, degreasing treatment is performed according to a conventional method. An oxide layer by natural oxidation is usually formed on the surface of the aluminum substrate. Such an oxide layer is often contaminated with residual rolling lubricating oil, foreign matter pushed in by rolling, and the like. Such residual lubricating oil and foreign matters are washed and removed with an alkaline degreasing agent. Further, when a strong alkaline degreasing agent is used, a cleaning deposit is often formed, and it is also necessary to remove this by post-treatment with an acid. In the present invention, specular reflectivity is very important, and therefore the degreasing conditions are appropriately selected within a range that does not reduce specular reflectivity.

C. Barrier type anodized film The anodized film provided on the surface of the aluminum substrate is a barrier type anodized film. There are two types of anodized films: porous pore-type films formed using a sulfuric acid bath as the electrolyte, and non-porous barrier-type films formed using an ammonium tartrate bath or a sodium phosphate bath. . In the case of a pore type anodic oxide film, since many holes exist in the film, corrosive substances such as moisture and chlorine tend to enter the holes. As a result, the corrosion resistance is inferior to the barrier type anodic oxide film, the diffuse reflectance is increased, and the regular reflectance is also inferior. Therefore, a barrier type film is used as the anodized film instead of the pore type film. When only one surface of the aluminum substrate is the above surface roughness surface, a barrier type anodic oxide film is usually formed on the surface roughness surface and the other surface of the aluminum substrate. It is not necessary to form a barrier type anodic oxide film on the other surface of the substrate. Moreover, when making both surfaces of an aluminum base material into the said surface roughness surface, a barrier type anodic oxide film is formed on each of the surface roughness surfaces.

C-1. As an electrolytic solution used for forming an electrolytic solution barrier type anodic oxide film, it is necessary to use a solution in which the generated barrier type oxide film is difficult to dissolve. The electrolyte is selected from the group consisting of tartrate, phosphate, boric acid, borate, adipate, citrate, malonate, silicate, maleate, benzoate, phthalate At least one of the above is preferably used. Examples of the alkali component of each acid salt include alkali metals such as sodium, potassium and lithium and alkaline earth metals such as magnesium, calcium, barium and strontium. Moreover, it is preferable to use the aqueous solution which melt | dissolved these electrolytes as electrolyte solution. The electrolyte concentration of the electrolytic solution is appropriately selected depending on the type and electrolysis conditions, but is usually 2 to 150 g / L. If the electrolyte concentration is less than 2 g / L, unevenness is likely to occur in the formed film, and if it exceeds 150 g / L, precipitation may occur due to difficulty in dissolving in the solvent.

C-2. A direct current constant current electrolysis method is used for electrolysis electrolysis. The electrolysis temperature is usually 2 to 60 ° C. If the electrolysis temperature is less than 2 ° C., the solubility of the electrolyte is low, and the voltage loss due to the liquid resistance becomes large, which is uneconomical. On the other hand, when the electrolysis temperature exceeds 60 ° C., not only is it difficult to form a dense anodic oxide film, but an increase in heating cost is unavoidable.

A certain relationship is established between the electrolytic voltage and the film thickness. Therefore, once the target film thickness is determined, the film is formed by the constant current electrolysis method using the corresponding voltage as the ultimate voltage. Although it depends on the film thickness, a voltage in the range of 30 to 450 V is usually used. In this case, since the electrolytic resistance increases as the electrolysis operation proceeds, it is necessary to increase the applied voltage to obtain a desired current density. In the constant current electrolysis method, a current density of 0.2 to 10 A / dm ² is usually used. When the current density is less than 0.2 / dm ² , the production rate of the formed electrolytic film is slow and productivity is lowered. When the current density exceeds 10 A / dm ² , the temperature of the electrolytic surface also rises, and it is difficult to form a dense anodic oxide film and the barrier property is lowered. The time required for electrolysis is usually 10 seconds to 4 minutes.

C-3. The thickness of the barrier type anodic oxide film formed is required to be 100 to 500 nm. If the thickness is less than 100 nm, the thickness as a protective film is insufficient and the corrosion resistance is poor. When the thickness exceeds 500 nm, the influence of absorption of visible light by the anodized film becomes large, and the regular reflection property is inferior.

  According to Patent Document 2, it is shown that the glitter is deteriorated when the film thickness exceeds 50 nm. The inventors experimentally obtained a detailed relationship between the film thickness up to 550 nm and the regular reflectance. Here, the regular reflectance is obtained by subtracting the regular reflected light removal reflectance (SCE) from the regular reflected light including reflectance (SCI) having a wavelength of 550 nm as described later. The results are shown in FIG. In FIG. 1, the horizontal axis indicates the thickness of the barrier type anodic oxide film, and the vertical axis indicates the regular reflectance. As is apparent from FIG. 1, when the film thickness is 40 to 150 nm and 210 to 300 nm, the regular reflectance increases as the film thickness increases, and the maximum value of the regular reflectance is obtained at 150 nm and 300 nm. ing. This means that the specular reflectance does not decrease monotonously with the coating thickness, as is generally accepted, but surprisingly the specular reflectance increases with increasing coating thickness in a given region of the coating thickness. It clearly shows an increase. From the result of FIG. 1, the thickness of the barrier type anodic oxide film is preferably 150 nm ± 30 nm or 300 nm ± 20 nm. This is because the specular reflectivity is remarkably reduced except in these thickness ranges.

  In FIG. 1, the spectral reflectance spectra in the visible light region were compared at the thicknesses of 150 nm and 300 nm of the barrier type anodic oxide film where the maximum value of regular reflectance was obtained. The results are shown in FIG. In FIG. 2, the horizontal axis indicates the wavelength of visible light, and the vertical axis indicates the spectral regular reflectance at each wavelength. As is apparent from FIG. 2, a high regular reflectance is obtained in the visible light region at a film thickness of 150 nm. However, a peak is observed at a wavelength of 550 nm at a film thickness of 300 nm, but the short wavelength side and the long wavelength side of the visible light region are long. On the wavelength side, there was a tendency for the spectral regular reflectance to decrease. Therefore, the thickness of the barrier type anodic oxide film is preferably 150 nm ± 30 nm rather than 300 nm ± 20 nm.

The reason why the specular reflectance does not decrease monotonously with the film thickness but the specular reflectance increases with the increase of the film thickness in a predetermined region of the film thickness is not clear at present.
For example, according to optical specialist books on reflectivity, it is known that the following relationship holds between optical thickness (refractive index x film thickness) and visible light (wavelength) based on the multilayer system theory of dielectric layers. It has been. That is, the reflectance is maximized when the optical thickness is an odd multiple of (wavelength / 4). Since the wavelength of visible light used is 550 nm, the odd multiples of (wavelength / 4) are 138 nm (1 time), 413 nm (3 times), 688 nm (5 times), and 963 nm (7 times). When the refractive index of the barrier type anodic oxide film (Al ₂ O ₃ ) is about 1.6, the film thickness that maximizes the reflectance is 86 nm (1 time), 258 nm (3 times), 430 nm (5 times). 602 nm (seven times), which is not 150 nm or 300 nm obtained in FIG. Thus, although the results of FIG. 1 cannot be explained by the multilayer system theory of dielectric layers, it is considered that some property of light has an influence.

Hereinafter, the present invention will be described in detail by way of examples and comparative examples.
Examples 1-12 and Comparative Examples 1-7
An aluminum plate having a predetermined Al content was subjected to conventional bright rolling to prepare a mirror aluminum substrate (plate thickness: 0.5 mm) having a predetermined surface roughness (Ra and Sm) on one side. Next, degreasing treatment was performed on both surfaces of the aluminum base material with a commercially available degreasing agent for aluminum. Further, both degreased surfaces were subjected to anodization treatment by a constant current density direct current electrolytic method to form barrier type anodized films (Examples 1 to 12 and Comparative Examples 3 to 7). The electrolyte contains
An aqueous sodium phosphate solution having a concentration of 20 g / L was used. The temperature of the electrolytic solution was room temperature, and current density of 1A / dm ^2. Electrolysis was performed until a predetermined film thickness was obtained. On the other hand, in Comparative Examples 1 and 2, a pore type anodic oxide film was formed. The pore-type anodic oxide film was also formed using a constant current density direct current electrolysis method. A conventional sulfuric acid bath (concentration: 15% by weight) was used as the electrolytic bath, and a sulfuric acid anodic oxide film having a predetermined thickness was formed at an electrolysis temperature of room temperature and a current density of 1 A / dm ² . Electrolysis was performed until a predetermined film thickness was obtained.

  FIG. 3 shows a schematic cross-sectional view of the mirror surface aluminum material produced in this way. In FIG. 3, 2 is an aluminum substrate and 1 is an anodized film.

  Table 1 shows the Al content, surface roughness (Ra, Sm), type of anodized film, and film thickness of the mirror-finished aluminum material produced as described above.

  Next, the specular reflectivity and corrosion resistance of the produced mirror-finished aluminum material were evaluated as follows. The evaluation results are also shown in Table 1.

The specular reflectivity is determined by using a spectrocolorimeter CM-2600d manufactured by Konica Minolta Co., Ltd., a SCI having a wavelength of 550 nm under the conditions of a D65 light source—2 ° field of view as a measurement condition, and 8 ° received diffused light illumination as an optical condition. (Specular reflection light included) The reflectance and SCE (regular reflection light removal) reflectance were measured and evaluated by regular reflectance (SCI-SCE). In addition, regular reflectance (SCI-SCE) made 60% or more the pass.

Corrosion resistance durability evaluated the external appearance after the CASS test 16 hours (JISH8681-2 conformity) with the following references | standards. ○: No abnormality, ○ △: Corrosion is minor and good, △: Corrosion is available, x: Corrosion is too great, and cannot be used. ○, ○ △ and △ were accepted, and x was rejected.

As is clear from the results shown in Table 1, in Examples 1 to 12 that satisfy the constituent requirements of the present invention, both regular reflection and corrosion resistance were acceptable.
On the other hand, in Comparative Example 1, since the pore type film was used as the anodic oxide film, the corrosion resistance was not satisfactory. In Comparative Example 2, a pore-type film was used as the anodized film, and the film thickness was too thick, so that neither regular reflection nor corrosion resistance could be satisfied. In Comparative Example 3, the Al content of the used aluminum substrate was low, and regular reflection was not satisfactory due to absorption and scattering of visible light by impurities. In Comparative Example 4, since Ra was too large, the diffuse reflectivity increased and the regular reflectivity was not satisfactory. In Comparative Example 5, since Sm was too small, the diffuse reflectivity increased and the regular reflectivity could not be satisfied. In Comparative Example 6, the corrosion resistance was not satisfactory because the thickness of the barrier type anodic oxide film was too thin. In Comparative Example 7, the regular reflection property in which the barrier type anodic oxide film was too thick was not satisfactory.

  According to the present invention, a mirror surface aluminum material that is excellent in specular reflection and corrosion resistance and is inexpensive to manufacture is provided. This mirror surface aluminum material can be suitably used as a reflector for solar lighting systems, various reflectors for lighting, interior building materials that emphasize design, and housing materials for recent digital home appliances.

FIG. 1 is a graph showing the relationship between the anodized film thickness and the regular reflectance. FIG. 2 is a graph showing the relationship between the visible light wavelength and the specular specular reflectance when the anodized film thickness is 150 nm and 300 nm. FIG. 3 is a cross-sectional view of a mirror-finished aluminum material according to the present invention.

Explanation of symbols

1 ... Anodized film 2 ... Aluminum base

Claims (2)

  An aluminum substrate containing 99.5% by weight or more of Al and having a center line average roughness Ra of 0.1 μm or less and an unevenness average interval Sm of 10 μm or more on the surface; A mirror surface aluminum material comprising a barrier type anodic oxide film having a thickness.   The mirror surface aluminum material of Claim 1 whose thickness of the said barrier type | mold anodized film is 150 +/- 30nm or 300 +/- 20nm.

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