JP5614671B2 - Oxide film and method for forming the same - Google Patents

Description

  The present invention relates to an oxide film formed on an aluminum substrate and a method for forming the same.

  Conventionally, when coating aluminum substrates such as cast aluminum, in order to ensure adhesion and corrosion resistance with the coating film, cationic electrodeposition coating is performed as a base treatment for coating, and then solvent coating is performed. Yes.

  In recent years, due to amendments to the Air Pollution Control Law, there has been a demand for suppression of volatile organic compound (VOC) emissions, and powder coating that does not use organic solvents instead of solvent coating that uses organic solvents has been studied. Yes. On the other hand, even in the case of powder coating, adhesion and corrosion resistance are insufficient with only the coating film, and since VOC is discharged in cationic electrodeposition coating, investigations are also being made on a ground treatment that does not discharge VOC. ing.

  As such a base treatment, it has been proposed to anodize an aluminum substrate to form an alumite (anodized) film (for example, see Patent Document 1). In this technology, after applying semi-sealing treatment to the anodized coating, it is difficult to get oil and other contaminants into the hole, and the anchor effect by allowing the paint for coating to enter the hole In addition, the adhesion to the coating film and the corrosion resistance are improved by the activation of the surface chemical reaction.

JP 2001-348696 A

  However, in the conventional alumite coating, the thickness of the barrier layer having an effect on the corrosion resistance cannot be formed thick, so that the corrosion resistance may be insufficient depending on the use conditions. Therefore, in order to improve the corrosion resistance, further sealing treatment is required.

  In the process of forming the anodized film, a degreasing process, a sealing process, etc. are required before and after the anodizing process, and the film forming speed in the anodizing process itself is slow, so it takes time to form the anodized film. The manufacturing cost is also high.

  This invention is made | formed in view of the said subject, It aims at providing the oxide film which is cheap, and is excellent in adhesiveness and corrosion resistance with a coating film, and its formation method.

In order to achieve the above object, a first characteristic configuration of an oxide film according to the present invention is an oxide film formed on an aluminum base material and used as a base of the film, the amorphous barrier layer, and the barrier layer. And a porous layer composed of a plurality of hollow columnar cells, the porous layer is a mixture of γ phase and α phase, and the average cell diameter of the cells is 200 to 800 nm. And the thickness is 1 to 4 μm.

The barrier layer is composed of a dense amorphous layer. For this reason, the corrosion resistance of an aluminum base material can be improved by providing a barrier layer like this structure.
Moreover, when coating an oxide film, the surface of a porous layer chemically reacts and couple | bonds with a coating film. According to this configuration, the average cell diameter of the porous layer is 200 to 800 nm, which is larger than that of the conventional alumite coating, and the chemical reaction with the coating occurs uniformly over the entire surface of the oxide coating cell. The adhesion is improved.

  The second characteristic configuration of the oxide film according to the present invention is that the thickness of the barrier layer is 100 to 300 nm.

According to this structure, since sufficient thickness as a barrier layer can be ensured, corrosion resistance can be improved more.
A third characteristic configuration of the oxide film according to the present invention is that the porous layer has micropores formed in the cell, and the micropores branch as they approach the barrier layer.

  The characteristic means of the method for forming an oxide film according to the present invention is a method for forming an oxide film formed on an aluminum base material and used as a base of the film, in an electrolyte having a pH of 5 to 6 containing potassium titanium oxalate. By performing plasma electrolysis treatment by alternately applying a voltage of 250 to 450 V using the aluminum base material as an anode and a voltage of 40 to 100 V using the aluminum base material as a cathode, An oxide film including an amorphous barrier layer and a porous layer laminated on the barrier layer and containing a γ phase and an α phase is formed on the surface.

  According to this means, an oxide film comprising a barrier layer and a porous layer can be easily formed on an aluminum substrate simply by performing a plasma electrolysis treatment using an electrolytic solution having a pH of 5-6. Further, since the corrosion resistance is ensured by the barrier layer, it is not necessary to separately perform a sealing treatment or the like on the porous layer, and an oxide film can be easily formed at a low cost.

When the porous layer of the oxide film grows to some extent, it becomes difficult for current to flow.
If the positive voltage with the aluminum base as the anode and the negative voltage with the aluminum base as the cathode are alternately applied as in this means, the porous layer is dissolved when the negative voltage is applied, Since the current flows continuously, the porous layer can be continuously grown.

It is a schematic diagram explaining the oxide film of this invention. It is a SEM photograph of the section of the oxide film of the present invention. It is a figure explaining the cell diameter and micropore diameter of the oxide film of this invention. 2 is a SEM photograph of a cross section of the oxide film of Example 1. It is the SEM photograph which expanded the cross section of the oxide film of FIG. It is the SEM photograph which expanded the cross section of the boundary part of the oxide film of FIG. 2 is a SEM photograph of the surface of the oxide film of Example 1. It is the SEM photograph which expanded the surface of the oxide film of FIG. 4 is a SEM photograph of a cross section of an oxide film of Example 2. FIG. 10 is an SEM photograph in which a cross section of the boundary portion of the oxide film in FIG. 9 is enlarged. 3 is a SEM photograph of the surface of the oxide film of Example 2. It is the SEM photograph which expanded the surface of the oxide film of FIG. 4 is a SEM photograph of a cross section of an oxide film of Example 3. 4 is a SEM photograph of the surface of the oxide film of Example 3. It is the SEM photograph which expanded the cross section of the boundary part of the oxide film of Example 4. FIG. 6 is an SEM photograph in which a cross section of a boundary portion of an oxide film of Example 5 is enlarged. It is the SEM photograph which expanded the cross section of the boundary part of the oxide film of a reference example. 4 is a SEM photograph of a cross section of an oxide film of Comparative Example 1. 4 is a SEM photograph of a cross section of an oxide film of Comparative Example 2. 6 is a photograph showing the peel test result of the oxide film of Example 3. FIG. 6 is a photograph showing a peel test result of an oxide film of Comparative Example 3. 6 is a photograph showing the results of a peel test of an oxide film of Comparative Example 4. 4 is a SEM photograph of a cross section of an oxide film of Comparative Example 3. 4 is a SEM photograph of the surface of an oxide film of Comparative Example 3. 6 is a SEM photograph of a cross section of an oxide film of Comparative Example 4. It is a SEM photograph of the section and surface of the conventional alumite film.

Hereinafter, an embodiment of an oxide film according to the present invention will be described with reference to the drawings.
As shown in FIGS. 1 and 2, the oxide film according to the present embodiment is formed on the surface of an aluminum substrate, and includes a barrier layer and a porous layer laminated on the surface of the barrier layer. ing. The porous layer is composed of a plurality of hollow columnar cells, and the average cell diameter of the porous layer is considerably larger than the average cell diameter of the conventional alumite coating, which is 200 to 800 nm. The average cell diameter of the porous layer is, as shown in FIG. 3, the cell width observed on the surface of the porous layer as the cell diameter, which is averaged by sampling from a plurality of cells. . At the time of sampling, only cells having a clear outline were measured, and cells that did not retain the shape of the cells and cells that had an extremely small cell diameter compared to other cells were excluded. The fine pore diameter is a value obtained by measuring the longest diameter among the diameters of the holes formed in the cell. The average fine pore diameter of the porous layer is not particularly limited, but is, for example, about 100 to 160 nm.

  In such an oxide film, the barrier layer is formed of a dense amorphous layer, and in the cell of the porous layer, a γ phase and an α phase are mixedly formed. This barrier layer improves the corrosion resistance of the aluminum substrate. In addition, the adhesiveness is improved by the property that the surface chemical reaction of the γ phase mixed in the porous layer is active. Moreover, the strength of the coating itself is improved due to the high strength of the α phase. Although the film thickness of a barrier layer is not specifically limited, It is preferable to have a certain amount of thickness, for example, 100-300 nm is preferable. The barrier layer may be thicker than 300 nm, but the corrosion resistance does not change much. The thickness of the barrier layer can be controlled by, for example, the magnitude of the voltage applied in the plasma electrolysis process. Thus, in the oxide film according to the present invention, since the corrosion resistance is ensured by the barrier layer, it is not necessary to perform a sealing treatment or the like on the porous layer.

The porous layer affects the adhesion with the coating film when the coating is applied. When the coating is baked, heat treatment is performed at 180 ° C. for about 30 minutes. At this time, unstable Al and Al ₂ O ₃ γ phases in the porous layer react with OH groups contained in the paint components. As a result, boehmite is formed and a chemical reaction occurs between the coating film and the surface of the porous layer, thereby ensuring adhesion. In the case of the oxide film according to the present invention, the porous layer has a large average cell diameter and is an aggregate of uniform cells. Therefore, the chemical reaction with the film occurs on the entire surface of the cell formed by the γ phase of the oxide film. It occurs uniformly and the adhesion is further improved. For this reason, the film thickness of a porous layer should just have the porous layer which reacts with a coating film, and is not specifically limited. What is necessary is just to set the film thickness of a porous layer to about 1-4 micrometers, for example. The film thickness of the porous layer can be controlled by, for example, the voltage application time in plasma electrolysis.

In addition, in the oxide film which concerns on this embodiment, the hydrate is formed in the surface of a film, and it has a structure where the micropore of a porous layer is plugged up near the film surface. For this reason, it can suppress that corrosive liquid etc. approach into a micropore itself. When applying the coating, the contact area with the coating film can be increased on the surface of the porous layer, and the unstable γ phase of Al and Al ₂ O ₃ during coating baking is contained in the coating component. Since boehmite is formed by reaction with the contained OH group, it can be reacted with the coating film.
Moreover, the micropores of the porous layer have branch portions (a part of them can be observed as horizontal holes in FIG. 2). This branching portion branches as it approaches the barrier layer, and even when a corrosive liquid or the like enters the micropores, permeation into the barrier layer itself can be suppressed.

  Therefore, if it is the oxide film which concerns on this invention, the corrosion resistance of an aluminum base material can be improved. Moreover, when apply | coating to an aluminum base material, adhesiveness with a coating film can be improved as a foundation | substrate.

  The aluminum substrate is not particularly limited, and for example, an aluminum extruded material, an aluminum cast material, an aluminum forged material, or the like can be used. As aluminum, pure aluminum, aluminum alloy or the like can be applied, and there is no particular limitation. Examples of the aluminum alloy include alloys with one or more of copper, manganese, silicon, magnesium, zinc, nickel, tin, lead, titanium, chromium, zirconium and the like. The aluminum substrate may contain any additives, impurities, etc.

  An oxide film can be formed on the surface of an aluminum substrate by placing the aluminum substrate as an anode together with a cathode in an electrolyte having a pH of 4 to 9, and performing plasma electrolysis treatment. When plasma electrolytic treatment is performed on an aluminum substrate, the treatment before and after that is not necessary and can be easily performed, so that the manufacturing cost can be reduced.

  As the electrolytic solution, one having a pH in the range of 4 to 9 is used. Whereas the conventional plasma electrolytic treatment uses an alkaline electrolytic solution having a pH of 10 or more, the barrier layer and the porous layer are formed by performing the plasma electrolytic treatment using an acidic to weak alkaline, preferably a weakly acidic electrolytic solution having a pH of 5 to 6. An oxide film comprising a quality layer can be formed. As such an electrolytic solution, for example, in plasma electrolytic treatment, a solution of pyrophosphate generally used to pH 4 to 9 can be mentioned, specifically, sodium pyrophosphate and potassium potassium oxalate, What was made to contain at 5-30 g / L in water, respectively is illustrated. Titanium potassium oxalate may affect the formation of the porous layer depending on the plasma electrolytic treatment conditions, as shown in Examples described later. For this reason, it is particularly preferable to use potassium potassium oxalate as the electrolyte. The temperature of the electrolytic solution is not particularly limited. However, in the case of plasma electrolytic treatment, the applied energy is large and accompanied by heat generation.

  Although a cathode is not specifically limited, For example, a lead plate, a SUS plate, etc. are used and it arrange | positions so that it may oppose the aluminum base material as an anode. And plasma electrolysis can be performed by energizing between both poles by constant voltage control, constant current control, etc.

  Plasma electrolysis treatment can form an oxide film on the aluminum substrate by energizing the aluminum substrate as an anode. Although there is no particular limitation, a positive voltage using the aluminum substrate as an anode and an aluminum substrate It is preferable to carry out while alternately applying a negative voltage as a cathode. When the porous layer of the oxide film grows to some extent, it becomes difficult for the current to flow, so by applying a negative voltage in the middle and melting the porous layer, the current continues to flow and grow the porous layer Can do. The applied positive voltage contributes to the film thickness of the barrier layer and the formation of the porous layer. The barrier layer is thin when the voltage value is low, and the porous layer is difficult to be formed if the voltage value is too low or too high. For this reason, the positive voltage is preferably 250 to 450V. Since the negative voltage dissolves the porous layer and assists film growth, it is preferably 40 to 100V. The application conditions of the positive voltage and the negative voltage are not particularly limited as long as they are applied alternately, and a pulse voltage with a frequency of 50 to 60 Hz is exemplified.

  Hereinafter, examples using the oxide film according to the present invention will be shown to explain the present invention in more detail. However, the present invention is not limited to these examples.

Table 1 shows the pulse voltage of an aluminum extruded material (A6063-T5, 150 mm × 50 mm × 12 mm, surface area 1.5 dm ² ) as an aluminum base, and a positive voltage using this as an anode and a negative voltage using a cathode. Under the conditions shown, plasma electrolysis was applied to form an oxide film.

  When the cross section and surface of the obtained oxide film were observed with a scanning electron microscope (SEM), the oxide film of Example 1 was composed of a barrier layer and a porous layer as shown in FIGS. It was found that the layer was formed of a plurality of hollow columnar cells as shown in FIGS. In addition, as for the oxide films of Examples 2 and 3, the same film as that of Example 1 was formed as shown in FIGS. About the average film thickness of the oxide film of Examples 1-3, and the film-forming speed | rate, it was as showing in Table 2.

  About the oxide film of Examples 1-3, the average cell diameter and the average fine pore diameter were measured based on FIG. As a result, as shown in Table 3, it was confirmed that the average cell diameter was in the range of 200 to 800 nm.

When only the applied voltage was changed under the same conditions as in Example 1 (Examples 2, 4 and Reference Examples), and in Example 1, the concentration of sodium pyrophosphate in the electrolyte was doubled (20 g / L). 6 (Example 1), FIG. 10 (Example 2), FIG. 15 (Example 4), FIG. 16 (Example 5), It investigated based on FIG. 17 (reference example). As a result, as shown in Table 4, it was found that the thickness of the barrier layer was in the range of 100 to 300 nm. Further, when the positive voltage was 200 V, the porous layer was not formed, and the barrier layer could not be confirmed.
Therefore, in the case of the present embodiment, it is preferable that the applied positive voltage is greater than 200V.

  Under the conditions of Examples 1 and 2, the oxide film formed when only the electrolyte was changed was examined. As the electrolytic solution, a solution in which potassium potassium oxalate was not added, sodium pyrophosphate 20 g / L was added, and the pH was 10.23 and the temperature was 4.1 ° C. was used. As a result, as shown in FIG. 18 (Comparative Example 1), FIG. 19 (Comparative Example 2), and Table 5, when plasma electrolytic treatment was performed without using titanium potassium oxalate, the film formation rate decreased, and the film It was found that the thickness was reduced and no porous layer was formed. Therefore, in the case of the present Example, it is preferable to contain potassium titanium oxalate in electrolyte solution.

About the oxide film of Example 3, the boiling process generally performed as a sealing process of an alumite film was performed, and the oxide analysis of the oxide film surface layer when not boiling and when boiling was performed. As shown in Table 6, it was found that AlO (OH) (boehmite) was formed by boiling. That is, it is known that aluminum generally has an unstable γ phase boehmite when the temperature exceeds 70 ° C., and an oxide film formed by plasma electrolytic treatment has a stable α phase and an unstable γ phase. Presumed to be mixed. For this reason, when powder coating is applied, boehmite formation occurs during baking (180 ° C.) during coating, a chemical reaction with hydroxyl groups occurs on the surface, and adhesion with the coating film is ensured. Conceivable. Therefore, when coating an oxide film, if it is boiled, boehmite formation proceeds, and the ratio of the unstable γ phase of Al and Al ₂ O ₃ may decrease, resulting in decreased adhesion to the coating film. For this reason, it is preferable not to boil.

  Powder coating (paint: Nippon Paint Bilucia PL5600, 5 parts gloss black) is applied to the oxide film of Example 1, and after baking for 30 minutes at an ambient temperature of 180 ° C., initial adhesion test, water resistance adhesion test, corrosion resistance test Went. Further, as a comparative example, the same aluminum base material as in Example 1 was used, and an electrolytic solution (pH 12.55, 5.0 ° C.) containing 3 g / L of sodium hydroxide was used (Comparative Example 3), and When using an electrolytic solution (pH 10.17, 5.0 ° C.) containing sodium pyrophosphate (0.1 mol / L in terms of phosphorus) and a Zr-based metal ion (0.5 mol / L in terms of zirconium) (Comparative Example) For 4), an oxide film was formed under the conditions shown in Table 7, powder coating and baking were performed, and the same test was performed.

In the initial adhesion test, a cross-cut test (JIS Z 8703) was performed to examine the peeling of the coating film in each square.
In the water-resistant adhesion test, a 40 ° C. water resistance test method was used. Distilled water or deionized water was placed in a constant temperature water bath and kept at 40 + 1 ° C., and after 240 hours, the occurrence of wrinkles, cracks, blisters, and peeling was examined.
In the corrosion resistance test, a salt spray test (using a test machine specified in JIS Z 2371) was performed, and the state after 240 hours was examined.

  As a result, in the oxide film of Example 1, peeling did not occur in both the initial adhesion test (FIG. 20A) and the water resistance adhesion test (FIG. 20B), and the adhesion was good. . Further, there was no problem with the corrosion resistance after 240 hours, and after 480 hours were examined, there was no problem. In the oxide film of Comparative Example 3, peeling did not occur in the initial adhesion test (FIG. 21A), but peeling occurred in the water resistance adhesion test (FIG. 22B), and the adhesion was insufficient. It was. Even in the oxide film of Comparative Example 4, as in Comparative Example 3, peeling did not occur in the initial adhesion test (FIG. 23 (a)), but peeling occurred in the water resistance adhesion test (FIG. 23 (b)). Adhesion was insufficient. As for corrosion resistance, there was no problem after 240 hours in any of the oxide films of Comparative Examples 3 and 4.

When the cross section and the surface of the oxide film of Comparative Example 3 were observed with an SEM, as shown in FIGS. 23 and 24, unlike the oxide film of Example 1, only the barrier layer was formed, and the surface was uniform with no cells. It wasn't. The oxide film of Comparative Example 4 was the same as shown in FIG. Although these oxide coatings have irregularities, the paint does not flow into the details of the irregularities during paint baking, and the air trapped in the porous formed inside pushes up the coating, resulting in an anchor effect. It can be assumed that it will not be possible. Furthermore, since these oxide films have been subjected to conventional plasma electrolytic treatment with high energy, there are many stable α phases, no chemical reaction with the paint film, and insufficient adhesion. It is thought that there was.
In addition, when the cross section and the surface of the conventional anodized film were observed with an SEM, the average cell diameter was small as shown in FIG. 26 (b) although it had a plurality of cells as shown in FIG. 26 (a). It was confirmed that the oxide film of Example 1 was significantly different.

  Since the oxide film according to the present invention is excellent in adhesion and corrosion resistance with the paint film, it can be applied, for example, to a base before coating of an aluminum base material used for automobile parts, building materials and the like.

Claims (4)

An oxide film formed on an aluminum substrate and used as a base of a coating film,
An amorphous barrier layer;
A porous layer laminated on the barrier layer and composed of a plurality of hollow columnar cells;
The porous layer is an oxide film in which a γ phase and an α phase are mixed, the average cell diameter of the cells is 200 to 800 nm, and the thickness is 1 to 4 μm.   The oxide film according to claim 1, wherein the barrier layer has a thickness of 100 to 300 nm. The porous layer has micropores formed in the cell,
3. The oxide film according to claim 1, wherein the micropores branch as they approach the barrier layer. A method of forming an oxide film formed on an aluminum substrate and used as a base of a coating film,
In an electrolyte having a pH of 5 to 6 containing potassium potassium oxalate, a voltage of 250 to 450 V using the aluminum base as an anode and a voltage of 40 to 100 V using the aluminum base as a cathode are alternately applied. A method for forming an oxide film, comprising: performing an electrolytic plasma treatment to form an oxide film comprising an amorphous barrier layer and a porous layer laminated on the barrier layer in which a γ phase and an α phase are mixed.