JP3212754B2 - Water-based inorganic paint coating pretreatment method for aluminum-based metal surfaces - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel pretreatment method for coating an aluminum-based metal surface with a water-based inorganic paint, for example, an alkali silicate-based inorganic paint. More specifically, the present invention relates to a method of forming a film of a chromium compound containing metal chromium and trivalent chromium as main components (hereinafter referred to as a chromate film) on the surface of the material by a cathodic electrolysis method, A water-based inorganic paint is applied to the finished surface, and it is possible to stably form a high-quality coating film having no coating defects such as bumps, blisters, cracks, and peeling, and having excellent adhesion after coating and excellent corrosion resistance. An object of the present invention is to provide a novel water-based inorganic paint pretreatment method.

[0002]

2. Description of the Related Art Water-based inorganic paints such as alkali silicate-based ceramic coating agents have high corrosion resistance, chemical resistance, abrasion resistance, weather resistance, heat resistance, mold resistance, antibacterial properties, and conventional organic coatings. Due to its design, which is different from the appearance of resin-based paint films, it has attracted great attention in the field of building materials and structural materials. Especially similar weather resistance, wear resistance, corrosion resistance,
It is inexpensive and has the same performance as fluororesin paint, which has attracted attention for the purpose of chemical resistance. Moreover, since the formed coating film has a significantly higher coating hardness than the organic resin coating film because it is glassy,
By modifying a relatively soft metal surface to have a high hardness and significantly improving scratch resistance, permanent design can be provided.

However, since these aqueous inorganic paints have a high pH and exhibit strong alkalinity, when they are applied to a material that is easily immersed in an alkali such as aluminum, which is an amphoteric metal, metal elution starts immediately after the application, and the subsequent baking process. In this case, paint defects such as bumps, blisters, and peeling are caused. In order to avoid this, a reaction type or coating type chromate treatment or an anodizing treatment (alumite treatment) is applied to the aluminum or aluminum alloy material as a pre-coating treatment.

[0004] These conventional pretreatment methods are not problematic when the treatment is carried out very carefully and completely. However, if a slight defect expected in the treatment or a scratch at the time of transportation is made, the portion is intensively attacked. This causes the above-mentioned poor paint appearance. In addition, in the chromate treatment, the film itself does not have a sufficiently satisfactory alkali resistance.
It is necessary to bake at a temperature of 0 ° C. or more for about 20 to 30 minutes, and thus there is a problem that the production efficiency is reduced.
On the other hand, the anodizing treatment performed on the aluminum material is less likely to cause these defects due to a larger amount of film adhesion compared to the chromate treatment or the like, but compared to the chromate treatment which can be usually performed within 60 seconds at most,
A long processing time of about several tens of minutes is required, and this method also has poor production efficiency. In addition, since the anodic oxidation treatment is usually porous as it is, sealing treatment is applied.However, unlike the case of coating with an organic resin-based coating, the water-based inorganic coating film after this sealing treatment is reversed. Coating defects such as peeling are likely to occur. Therefore, in order to apply the water-based inorganic paint to the anodic oxide film, it must be applied in an unsealed state.However, since the unsealed anodic oxide film is unstable, the elapsed time after the treatment is long. A coating defect similar to that in the case where the sealing treatment is performed due to a change in the surface state is likely to occur, and there is a process restriction that the coating must be performed immediately after the anodic oxidation treatment.

Although the prior art relating to the pretreatment method for coating a water-based inorganic paint on the surface of an aluminum-based metal has been outlined above, depending on the type of metal material, even if the above-described method is applied with care, the conventional technique can be used. In some cases, a good coating cannot be formed by the method. For example, aluminum alloy ADC widely used for aluminum die casting
Series (JIS), which contain more alloying elements than other aluminum alloys, and are generally not considered to have good corrosion resistance. Especially ADC1
With any of the two types of materials, any of the above-mentioned coating pretreatment methods may cause blisters, bumps, or peeling from the cavity formed during casting during baking of the coating film, making it impossible to perform good coating. Have been. Although the reaction-type and coating-type chromate films can provide a uniform film, the amount of the film is relatively small as about 50 mg / m ² as Cr, and the alkali resistance is inferior. The anodized film is as thick as about 5 μm. The drawbacks are that the scratches and the like are rather intensively attacked, and furthermore, since no sealing treatment is performed, the stability over time is inferior. Therefore, the pretreatment film for solving these problems is excellent in alkali resistance, and even if the film has defects such as scratches, the film does not intensively corrode but works in a direction of relaxing. It is desired that

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to coat a water-based inorganic paint on the surface of an aluminum-based metal material to form a coating film having excellent coating appearance and excellent adhesion and corrosion resistance. An object of the present invention is to provide a novel water-based inorganic paint pretreatment method.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on a surface treatment method for imparting the above-described functions, and as a result, it has been found that a solution containing hexavalent chromium ions and / or trivalent chromium ions can be used. The present inventors have found that a film obtained by subjecting an object to be treated to a cathodic electrolytic treatment is most suitable as a coating base for a water-based inorganic paint, and completed the present invention.

That is, according to the present invention, in the case of applying a water-based inorganic paint to an aluminum-based metal surface, the metal surface is preliminarily treated with hexavalent chromium ions and / or trivalent chromium ions in the pretreatment for coating.
Disclosed is a method for pretreating a water-based inorganic paint on an aluminum-based metal surface, which comprises performing a cathodic electrolysis treatment with an acidic aqueous solution containing valent chromium ions.

Hereinafter, the configuration of the present invention will be described in detail. The metal targeted in the present invention is an aluminum-based metal. More specifically, it is aluminum, an aluminum alloy, an aluminum plating material, and an aluminum alloy plating material.

First, the case of performing cathodic electrolytic treatment with a solution containing hexavalent chromium ions will be described. With such an electrolytic solution, if the treatment is continued, a part of the chromate film mainly composed of trivalent chromium formed as a film is dissolved and trivalent chromium ions accumulate in the bath.
The case where both trivalent and trivalent chromium ions are mixed will be discussed at the same time.

The electrolyte to be used may be an ordinary chromium plating bath containing hexavalent chromium with some modifications, and there is no particular limitation. The most typical chromium plating bath is a surge bath (chromic anhydride: 200 to 300 g).
/ L, sulfuric acid: 2 to 3 g / L) and a part of sulfuric acid in the surge bath include a silicon fluoride bath. In the present invention, a continuous metal chromium film having a thickness of several μm to several tens μm is used as in the case of ordinary chromium plating. It is not necessary to completely cover the surface of the material with a high concentration, so that it is not necessary to use a high-concentration material as in a normal chromium plating bath. From experience, it is sufficient to use 2 g / L or more in terms of hexavalent chromium using chromic anhydride, and it is industrially more advantageous to use a dilute solution especially considering the problem of wastewater treatment of hexavalent chromium. is there. However, although the ratio of chromic anhydride / sulfuric acid is 100/1 in the Sargent bath, the addition of sulfuric acid at this ratio even in a diluted solution has a preferable result in terms of film formation efficiency.

On the other hand, the electrolysis conditions are not particularly limited, and the temperature can be at room temperature. However, it is preferable to control the temperature at about 40 ° C. in order to keep the electrolysis conditions constant. Regarding the current density, 0.5 to 60 A / dm ²
Although the required total amount of chromium depends on the current density and the electrolysis time, the current density is about 1 to ^{2 A} / dm ² and the electrolysis time is 30 to Should be around 100 seconds. As described above, such low-current-density and short-time electrolysis is considered to be very advantageous in terms of power supply equipment costs and running costs as compared with ordinary chromium plating and anodizing of aluminum. The anode (counter electrode) to be used is preferably an insoluble electrode such as lead, lead oxide (catalyst electrode) or carbon. However, iron or stainless steel can be used. / L is acceptable. When stainless steel is used as the anode, elution of chromium (trivalent) may occur, but trivalent chromium ions are generated during the deposition of metallic chromium at the cathode and accumulate in the bath. On the other hand, nitrate ions, which are oxidizing acids as anions, and phosphate ions, which have a small solubility product with trivalent chromium ions, should be avoided from being mixed as impurities in order to reduce the efficiency of film formation. Also,
Incorporation of chlorine ions or the like is not preferred because it not only inhibits the formation of a film but also attacks the anode material.

Next, a case where an electrolytic solution containing only trivalent chromium ions is used as a chromium supply source will be described. Trivalent chromium ions form relatively stable aquocomplexes and are more difficult to deposit chromium than hexavalent chromium baths. However, various types of baths have been proposed because they are advantageous for wastewater treatment and the like. . Basically, chromium sulfate (Cr ₂ (SO ₄ )
₃ ) or chromium chloride (CrCl ₃ ) as a chromium source,
A bath obtained by complexing them with an organic complexing agent such as oxalic acid, formic acid, glycine, or oxycarboxylic acid may be used. By appropriately controlling the electrolysis current density and electrolysis time, the same film deposition as in the hexavalent chromium bath is possible, but the deposition efficiency is inferior to the hexavalent chromium bath. Requires a long electrolysis time. In the case of a trivalent chromium bath, if lead or lead oxide is used as an anode, trivalent chromium in the bath is oxidized to hexavalent chromium due to their catalytic properties. Therefore, a carbon electrode should be used.

The film according to the present invention will be described in more detail with reference to the amount of chromium deposited. It is industrially advantageous to view the amount of the cathode electrolytic film in the present invention in terms of the total amount of chromium deposited. This means the total amount of chromium including metallic chromium, and can be easily measured by fluorescent X-ray analysis or the like. The optimum total chromium deposition amount as a pretreatment film of the aqueous inorganic paint targeted by the present invention is slightly different depending on the material, so it is difficult to clearly define it.
A good range is 0 to 300 mg / m ² . In general, if the amount of the coating is too small, the function as a pretreatment coating is reduced, and coating defects and performance deterioration after coating as seen in the prior art are likely to occur. On the other hand, if the coating amount is too large, for example, if it exceeds 1000 mg / m ² , it is not preferable because the adhesion after coating is deteriorated and the processing time is prolonged to lower the productivity.

The operability of the cathodic electrolysis according to the present invention will be described. Generally, when electrodepositing metallic chromium or a chromium compound from a treatment solution containing chromium ions as represented by chromium plating, current efficiency is generally very poor. Also in the cathodic electrolysis treatment according to the present invention, the ratio of the amount of electricity supplied to the film formation is at most about 20%, which is not necessarily good. However, the total amount of chromium required for the above-mentioned coating pretreatment film is at most 300 mg / m2.
^Since it is ^2, it is judged that the deposition efficiency is hardly required to be a problem. The electrolysis condition may be a low current density of about 2 A / dm ² as described above, and even under this condition, for example, 30 to 30 mg / m ² may be required to form a film.
60 seconds of electrolysis is sufficient. Therefore, at a current density of ³ to 5 A / dm ² as in the anodic oxidation of aluminum,
It can be said that this method is extremely advantageous in terms of workability, equipment, and production efficiency as compared with the case where the electrolysis time is required for one minute.

The film formed by the cathodic electrolysis treatment does not require the baking treatment which is indispensable after the treatment in the reaction type and coating type chromate treatments described above, and must be applied immediately after the anodizing treatment. There are no process restrictions. The film formed by this method is a composite type chromate film mainly composed of non-uniform metal chromium and a hydrated oxide of trivalent chromium covering the metal chromium, and may further contain a trace amount of hexavalent chromium in some cases. It turns out that. Of these, a chromate film composed of trivalent chromium is attacked by a strong alkali, and it is unclear why the trivalent chromium-based film is good as a coating base for such a strongly alkaline type aqueous inorganic paint. Considering the reason, the following two reasons can be considered. The thickness of the chromate film is extremely thin, several tens of angstroms, compared to chromium metal, and the cathodic electrolytic treatment makes the natural potential distribution on the material surface flatter, making it difficult to form a local battery that is the starting point of corrosion by alkali. It is estimated that

The method for pre-coating an aqueous inorganic paint according to the present invention can have further secondary advantages in the case of coating an aluminum-based metal surface. This is obtained in the formation of a film having a total chromium deposition amount of 150 mg / m ² or less. With the film amount limited as described above, the material appearance hardly changes and it is as if untreated.
The purpose is to exhibit an appearance having a beautiful metallic luster caused by the aluminum base. As a result, it can be said that the pre-coating film according to the present invention is most suitable as a clear coating base. Further, since the water-based inorganic paint itself exhibits strong alkalinity, there are some restrictions on the pigments that can be used, and at present, those having low opacity are usually used. This is unique in color compared to organic resin-based paints, which has attracted particular attention in the field of construction materials.On the other hand, the delicate color tone of pre-coating films due to the low opacity of water-based inorganic paints There is a problem that even if the same water-based inorganic paint is applied, the finished appearance is different due to various changes. However, the pretreatment for coating according to the present invention has a secondary advantage that there is almost no need to worry about such a problem.

The above-described cathodic electrolysis method can be applied not only to a coating base of a water-based inorganic coating targeted by the present invention but also to a sol-gel type inorganic coating and a general organic resin-based coating. Post-coating performance such as coating adhesion and corrosion resistance is better than conventional coating and reactive chromate treatments. When the material is aluminum, it is not inferior to anodic oxidation, which is said to be the best pretreatment for painting. Therefore, the equipment for the cathodic electrolytic treatment can be applied not only to the coating with the water-based inorganic paint, which is the object of the present invention, but also to a general coating system, and the investment for that purpose is not wasted.

[0019]

EXAMPLES The present invention will be described more specifically with reference to examples and comparative examples. Hereinafter, the materials of the test pieces and the method of preparing them will be described. The test pieces in the respective examples and comparative examples were prepared in the following steps (1) to (6).
In Comparative Examples 6 and 13, the pretreatment of the coating was only degreasing, and thus the steps (3), (3 ') or (3 "), and (4) were omitted. ~
For 8, 9 to 12, reaction-type chromate treatment or anodic oxidation treatment was performed instead of cathodic electrolytic treatment.
Therefore, the step (3 ′) or (3 ″) was adopted instead of the step (3).

1. Test material ・ Aluminum cast alloy (JIS AC4C) milling material ・ Aluminum cast alloy (JIS ADC12) ・ JIS A5052 sheet

2. Test piece preparation process (1) Alkaline degreasing Using a 2% aqueous solution of a non-etching type alkaline degreasing agent (Fine Cleaner 4327, manufactured by Nippon Parkerizing Co., Ltd.), spray degreasing was performed at a temperature of 60 ° C for 1 minute. (2) Cleaning Spray cleaning was performed for 20 seconds using deionized water. (3) Cathodic electrolysis Cathodic electrolysis was performed under the electrolysis conditions shown in Table 2 using the predetermined chromium-containing electrolytic solution shown in Table 1. However, in Table 1, a stainless steel electrode (SUS304) was used as the anode in the case of the hexavalent chromium-containing bath (Nos. 1 and 2), and a carbon electrode was used in the case of the trivalent chromium bath (No. 3). Further, the coating in the step (6) was basically performed immediately after the completion of the steps (4) and (5). However, a level at which the coating was performed after a lapse of 5 days as necessary was set. (3 ′) Reactive Chromate Treatment Using a 7% aqueous solution of Alchrome 713 manufactured by Nippon Parkerizing Co., Ltd., a spray treatment was performed at a temperature of 50 ° C. for 20 seconds to form a reactive chromate film. After performing the above-mentioned processes, if necessary, a baking process at 230 ° C. for 20 minutes was performed. (3 ″) Anodizing treatment 30 ° C. current density 3A in 15% H ₂ SO ₄ aqueous solution
Anode electrolysis was performed under the condition of / dm ² . At this time, AC4
The anodic oxide film thickness was controlled to about 3 μm by setting the electrolysis time to 20 minutes for the C material, 30 minutes for the ADC12 material, and 12 minutes for the A5052 material. Further, the coating in the step (6) was basically performed immediately after the step (5) was completed. Separately, after the lapse of 3 days after the completion of the step (5), a coating was prepared. In addition, after anodizing treatment, add 20% in boiling deionized water.
A sample that had been immersed for a minute and sealed was also prepared. (4) Cleaning Spray cleaning was performed for 20 seconds using deionized water. (5) Drying Draining and drying were performed in an oven set at 100 ° C. for 2 minutes. (6) Painting Nippon Parkerizing water-based inorganic paint Pulcera coat 4
Spray coating was performed using 00S (black) with a target film thickness of 20 μm. Baking after painting was performed by holding at 230 ° C. for 30 minutes.

Hereinafter, a method for evaluating a test piece will be described. (A) Amount of chromium adhering The total amount of chromium adhering to the surface of the test piece was measured using a fluorescent X-ray analyzer. (B) Observation of coating film appearance The coating film after coating and baking was observed, and the occurrence of peeling, bumps, blisters, and cracks was visually observed. (C) Cross-cut adhesion test 100 cross-cuts of 1 mm square are drawn on the painted surface with an NT cutter so as to reach the substrate, and peeled off with a cellophane tape. The following ranks were evaluated according to the number of crosses remaining after the tape was peeled off. ：: No abnormality △: Number of remaining crosses 95/100 or more ×: Number of remaining crosses less than 95/100 (d) Salt spray test After applying a cut to the coated test piece using an NT cutter to reach the substrate, Based on JIS-Z2371, the maximum width of rust and swelling from cuts generated after spraying with salt water for 1000 hours was measured. ：: Maximum rust / swelling width 1 mm or less △: Maximum rust / swelling width 2 mm or less ×: Maximum rust / swelling width 2 mm or more

The test results in the respective Examples and Comparative Examples are shown in Table 3 for the AC4C material, Table 4 for the ADC12 material, and Table 5 for the A5052 material.

The following can be said from the present embodiment and the comparative example. In the aluminum material and aluminum alloy material subjected to the cathodic electrolysis treatment of the present invention, coating defects such as peeling and blisters do not occur in the appearance observation after the application of the aqueous inorganic paint as shown in Examples 1 to 6, and The results are also extremely excellent in adhesion and corrosion resistance. In addition, there is no need to pay attention to how to handle the process up to the post-treatment coating, such as the baking treatment or the treatment such as the reaction chromate or anodic oxidation treatment, which is an industrially very advantageous method. On the other hand, as shown in Comparative Examples 1-2 and 9-10, in the reactive chromate treatment, a high temperature
If a baking process that requires a long time treatment is not added, paint defects will occur, and even after that, adhesion and corrosion resistance after painting will be slightly inferior to those subjected to cathodic electrolytic treatment Was. In the anodic oxidation treatments shown in Comparative Examples 3 to 5 and 11 to 12, coating defects occur if time elapses after the treatment, so that careful process control is required. In the case of performing the hole treatment, the coating defects became rather conspicuous. Moreover, even if this problem was sufficiently noted, the corrosion resistance after coating was slightly inferior to that of the cathodic electrolytic treatment. On the other hand, as shown in Comparative Examples 7 and 8, the target material was ADC1.
In the case of two materials, painting was impossible at all even after the above-mentioned post-treatment and process control. Furthermore, Comparative Examples 6 and 13 include examples in which pre-coating treatment was not performed, but in these cases, of course, no coating was possible.

[0025]

Effect of the Invention When a water-based inorganic paint is applied to an aluminum-based metal surface, the cathodic electrolytic treatment of the present invention is performed as a pre-coating treatment, so that a complicated process is not required and the process is specially performed. Not only are there any restrictions, but the workability is good, and beautiful coating without coating film defects can be achieved, and extremely high adhesion and corrosion resistance can be imparted to the coating film of the water-based inorganic paint. It becomes possible.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-54-13431 (JP, A) JP-A-62-278297 (JP, A) JP-A 2-118099 (JP, A) 179486 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C25D 11/38

Claims (1)

(57) [Claims] In the pretreatment for coating an aqueous inorganic paint on the surface of an aluminum-based metal, the metal surface is subjected to cathodic electrolysis in an acidic aqueous solution containing hexavalent chromium ions and / or trivalent chromium ions in advance. A method for pre-treating a water-based inorganic paint on an aluminum-based metal surface, comprising performing a treatment.