JPH0765234B2 - Pretreatment method for aluminum plate before painting - Google Patents

Description

TECHNICAL FIELD The present invention relates to an aluminum plate (including both a pure aluminum plate and an aluminum alloy plate) used for fins for heat exchangers, body sheets for automobiles, building materials, etc.
The present invention relates to a method of subjecting the surface of an aluminum plate to a surface treatment for improving corrosion resistance and the like before forming a coating film such as an organic coating film.

2. Description of the Related Art Generally, when an aluminum plate is used as a fin for a heat exchanger, a body sheet for an automobile, a building material, etc., a final coating film such as an organic resin or various paints is often formed. Prior to the formation, the surface of the aluminum plate is usually preliminarily treated to improve the corrosion resistance.

Conventionally, as an aluminum plate undercoating method for the purpose of improving corrosion resistance, (a) a method of degreasing and desmutting with an acidic or alkaline solution, and then anodizing in an acidic solution in another tank, (B) A method in which electrolytic cleaning is performed after degreasing with a solution, and then anodizing treatment is performed in another electrolytic bath, or (c) only degreasing and desmutting are performed as a base treatment, and immediately thereafter an organic coating film. A typical method is to form. In the method (a) or (b), the anodic oxidation treatment is usually carried out by using a direct current so that the anodic oxide film has a thickness of 9 μm or more. In some cases, an acidic solution was usually used.

Problems to be Solved by the Invention Among the conventional methods for treating the surface of an aluminum plate as described above, the method (c) involves only degreasing and desmutting, so that sufficient corrosion resistance has not been obtained.

On the other hand, in the methods (a) and (b), since the anodic oxide film is formed, the corrosion resistance can be improved as compared with the method (c), but there are the following problems.

That is, first of all, these methods require a large number of processing tanks such as a degreasing tank, a desmut processing tank, and an electrolytic processing tank, so that the manufacturing line becomes long, the processing time becomes long, and the equipment cost also increases. There was a problem. Further, as described above, since the whole process requires a long time, dirt is likely to be attached in the middle of the process and an inactive film or the like is easily generated, and several kinds of treatment liquids must be controlled separately, so that the quality is high. In many cases, it became unstable and could not stably improve the corrosion resistance. Further (a),
Since the anodic oxide film in the method (b) requires a film thickness of 9 μm or more, the treatment time is long and the quality tends to be unstable. Further, since it is a thick film, cracks are likely to occur during the molding process, and the moldability is improved. There was a disadvantage that was inferior to.

The present invention effectively solves the above problems, can stably ensure excellent corrosion resistance, does not require a long time for processing, and requires a low cost in terms of equipment, and further deteriorates moldability. It is an object of the present invention to provide a method for treating the surface of an aluminum plate.

Means for Solving the Problems The inventors of the present invention have conducted extensive experiments and studies in order to find a method for solving the above-mentioned problems, and as a result, electrolytic treatment under conditions that have not been conventionally applied to the base treatment of aluminum plates. That is, by applying a large amount of electricity electrolysis for a short time at a predetermined current density from an AC waveform in a high temperature alkaline solution (however, a general alkaline solution excluding an aqueous solution containing an alkali metal silicate) The inventors have found that the above-mentioned various problems can be solved by simultaneously performing the degreasing treatment and the anodizing treatment, and have completed the present invention.

Specifically, the pretreatment method for pre-painting an aluminum plate of the present invention uses an alkaline solution having a bath temperature of 40 to 90 ° C. (however, an aqueous solution containing an alkali metal silicate is desired) and using a current density of 3
〜35A / dm ² , electricity amount 2〜80C / dm ²
Performs electrolytic treatment for 10 seconds, which degreases the surface
It is characterized in that a dense barrier type anodic oxide film having a film thickness of 0.01 to 0.05 μm is formed together with cleaning.

Action In the substrate treatment method of the present invention, electrolytic treatment is performed in an alkaline solution at a high temperature of 40 to 90 ° C. by using an AC waveform.
As described in detail below, such electrolytic treatment strongly degreases and cleans the surface of the aluminum plate, and at the same time, forms a dense barrier type anodic oxide film with a thickness of 0.01 to 0.05 μm. It

That is, first, regarding the degreasing / cleaning action, in addition to the alkaline solution having a degreasing property by itself, the degreasing property is stronger because of the high temperature. In addition, in electrolysis using an AC waveform, oxygen gas is generated during the anode reaction, while hydrogen gas is generated during the cathode reaction, so during the anode reaction, degreasing and cleaning actions are performed by the oxidation of organic substances adhering to the plate surface, and the cathode reaction Occasionally, the mechanical cleaning action is caused by the expansion of hydrogen bubbles on the plate surface. Therefore, according to the AC electrolysis treatment in a high temperature alkaline solution, each of the above-mentioned actions synergistically exerts a strong degreasing / cleaning effect, and the plate surface derived from rolling oil or the like is produced in an extremely short time. Oil and various dirt are removed.

Also, in the above electrolytic treatment, an anodized film is formed on the surface of the aluminum plate at the same time as degreasing and cleaning, but the high temperature alkaline solution used as the electrolytic solution at this time has a relatively high solubility. The anodic oxide film is an extremely dense and thin barrier type film. Such a dense barrier-type coating has superior corrosion resistance compared to conventional porous coatings by general anodic oxidation treatment, and since it is dense and thin, it does not crack during molding. Is less likely to occur, and therefore the moldability is also excellent. Furthermore, since the electrolysis treatment is performed with an AC waveform, rough skin is unlikely to occur, which also contributes to the improvement of corrosion resistance. Further, as the AC waveform is used as described above, it is possible to apply a large amount of electric current in a short time without causing rough skin because the rough skin is unlikely to occur, thus significantly reducing the processing time and improving workability. Can be improved. That is, in the conventional general electrolytic treatment with a direct current, when a large amount of electricity is applied in a short time (and therefore electrolysis is performed at a high current density), the bath voltage is apt to rise rapidly, resulting in uneven electrolysis. However, this is not the case with electrolysis using an AC waveform, and electrolysis with a large amount of electricity can be performed in a short time without causing skin roughening, and therefore electrolysis with high current density, and a film with excellent corrosion resistance can be efficiently generated. It can be done.

Further, in the present invention, as described above, since degreasing / cleaning and generation of an anodized film having excellent corrosion resistance are performed by the same treatment in the same tank, unlike the case where anodization is performed in another tank after degreasing. Since the anodic oxide film is formed with the plate surface being activated without stains adhering between the steps, a film of uniform quality can be obtained and stable and excellent corrosion resistance can be exhibited.

Also, degreasing / cleaning and anodized film formation are performed by the same electrolytic treatment in the same tank, and the electrolysis time is also short, so the overall work time is significantly shortened compared to conventional production. And the facility cost will be significantly reduced.

Here, the reason for limiting each process condition in the method of the present invention will be described.

The bath temperature of the alkaline solution during the electrolytic treatment needs to be in the range of 40 to 90 ° C. If the temperature is lower than 40 ° C, the degreasing / cleaning effect cannot be sufficiently obtained, while if it exceeds 90 ° C, the solubility is too strong and the anodic oxide film of the required thickness cannot be obtained. Even within the range of 40 to 90 ° C, the range of 60 to 80 ° C is particularly preferable.

The amount of electricity in the electrolytic treatment must be within the range of ^{2 to} 80 c / dm ² . If it is less than ² c / dm ² , sufficient degreasing and cleaning effects cannot be obtained, and an anodized film with the required thickness cannot be obtained.
On the other hand, even if a large amount of electricity exceeding 80 c / dm ² is applied, the barrier type anodic oxide film does not grow any more, which is not only economically wasteful, but rather makes the film porous. It is desirable that the amount of electricity be within this range, particularly within the range of 8 to 80 c / dm ² .

The current density in the electrolytic treatment is within the range of 3 to 35 A / dm ² . If the current density is too small, the reaction speed of the film forming becomes slower, so that not only the treatment time becomes longer, but also the erosion rate (dissolution rate) of the film by the alkaline solution becomes faster, resulting in a dense barrier type anodization. It becomes impossible to form a film. In particular, if the current density is less than 3 A / dm ² , it may be difficult to form an anodized film, or even if it is formed, a dense barrier type film may not be formed,
On the other hand, if it exceeds 35 A / dm ² , the surface of the aluminum plate will become rough due to partial heat generation. Therefore, the current density needs to be within the range of 3 to 35 A / dm ² .

The electrolysis time during electrolysis is within the range of 1 to 10 _sec . When the electrolysis time is less than 1 _sec , when the quantity of electricity is close to the lower limit of the above range, sufficient degreasing and cleaning effects cannot be obtained and the required film thickness cannot be secured, while the quantity of electricity is close to the upper limit of the above range. In this case, the current density becomes excessively large, which may cause the surface of the aluminum plate to become rough due to partial heat generation. Further, even if the electrolysis time exceeds 10 _sec , the barrier type anodic oxide film does not grow any more, which is not only economically wasteful, but rather the film becomes porous. Note that the electrolysis time is 1 to 5 s, especially considering productivity.
It is preferable that the ec is short.

Furthermore, the thickness of the anodic oxide film formed is 0.01 to 0.05 μm.
Within the range of. If it is less than 0.01 μm, it is too thin to obtain sufficient corrosion resistance. On the other hand, if it exceeds 0.05 μm and becomes a thick film, the film becomes porous.

Specific Description for Carrying Out the Invention Hereinafter, preferable conditions and the like for actually carrying out the substrate processing method of the present invention will be described step by step.

The aluminum plate to which the surface treatment method of the present invention is applied is not particularly limited, but a pure JIS 1100, 1050, etc.
Al-based, 3003, 3004, etc. Al-Mn-based, 5052, 5083, etc. Al-Mg
System, 2017--2024, etc., Al-Cu system, 6061, etc., Al-Mg-Si system rolled sheet can be suitably applied, and can be applied to both the sheet (single sheet) and coil.

The aluminum plate as described above is subjected to electrolytic treatment in an electrolytic cell in which a suitable counter electrode is arranged. The electrolytic solution used for this electrolytic treatment is not particularly limited as long as it is an alkaline solution other than an aqueous solution of an alkali metal silicate, but is usually sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, sodium phosphate. It is desirable to use an aqueous solution such as the above, or a mixed solution of two or more thereof, and a surfactant may be contained in order to improve the degreasing property. Also, the desired hydrogen ion index (pH) of the electrolyte
Is 9 to 13. When the pH is less than 9, the degreasing property is poor, and the bath voltage is increased, which is not preferable because uneven electrolysis is likely to occur. On the other hand, when the pH is more than 13, the solubility is too strong and an anodized film having a required thickness cannot be obtained, which is preferable. Absent. The bath temperature is 40 to 90 ° C, preferably 60 to 80 ° C as described above.
And Furthermore, an aluminum plate, an iron plate, a graphite electrode, or the like may be used as the counter electrode used in the electrolytic treatment.

As mentioned above, the current waveform in the electrolytic treatment must be an AC waveform. By using an AC waveform, a strong degreasing / cleaning effect can be obtained by an alternating anodic reaction and cathodic reaction, and at the same time rough skin occurs. It is possible to carry out electrolysis for a short time at a large current density without any problem, and the corrosion resistance is superior to that in the case of direct current electrolysis. The use of the AC waveform is also advantageous in that rectification is unnecessary and the facility cost is low. However, the AC waveform need only be a waveform that alternately changes from side to side, and does not necessarily have to be a sine waveform. The alternating current may be either single-phase or three-phase.

The amount of electricity in AC electrolysis (total amount of electricity from the start to the end of electrolysis) is ² to 80 c / dm ² , preferably 8 to 80 c / dm ² , as already described, and the electrolysis time is 1 to ² as already described. It is set to 10 _sec , preferably 1 to 5 _sec . The current density may be set within the range of ³ to 35 A / dm ² as described above, but it is necessary to properly determine the current density within the range so as to satisfy the above-mentioned electric quantity range and electrolysis time range.

The anodic oxide film formed by performing the electrolytic treatment as described above is a dense and thin barrier type film as described above, can stably exhibit excellent corrosion resistance, and has a good formability. It is good. Here, the thickness of the anodized film is 0.01 to 0.05 μm, which is remarkably thinner than the film of about 9 μm or more obtained by the anodizing treatment in the conventional undercoating method, but since it is a dense barrier type film, Even if it is thin, it can exhibit sufficiently excellent corrosion resistance.

After performing the electrolytic treatment as the base treatment as described above, appropriate washing and drying may be performed. Then, after that, a coating process such as application of an organic coating film may be performed. As the organic coating film, for example, water-soluble or water-insoluble epoxy resin, acrylic resin, melamine resin, cellulose resin or the like can be used. By applying such an organic coating film, moisture absorption or moisture adhesion is unlikely to occur. Therefore, in combination with the stable and excellent corrosion resistance of the anodized film as the undercoating tank, high corrosion resistance can be stably exhibited. It should be noted that such an organic resin may contain chromic acid or the like, and the thickness thereof is usually 0.1 to 1
The thickness is about 0 μm, preferably 0.5 to 2 μm.

Examples Hereinafter, examples and comparative examples of the present invention will be described together with the test results thereof. In each of the following examples, an example in which an organic resin is further applied and baked after performing a base treatment according to the method of the present invention or the comparative method is shown.

[Example 1] An aluminum plate having a thickness of 0.115 mm and equivalent to JIS 3003 was prepared by a single-phase alternating current (sinusoidal wave) using an aqueous solution (pH 10.8) containing 1 wt% each of sodium carbonate and potassium hydroxide as an electrolytic solution. Electrolysis was performed for 5 _seconds under the conditions shown in Table 1. A graphite electrode was used as the counter electrode.
After electrolytic treatment, the aluminum plate immediately washed with water and dried, after which the water-soluble acrylic resin is applied by coating weight 1 g / m ² (thickness of about 1 [mu] m), and 100 _sec baked at 250 ° C..

[Comparative Example 1] For the same aluminum plate as that used in Example 1, the same electrolytic solution as in Example 1 was used, and under the conditions shown in Table 1, 5 _sec.
During that period, DC electrolysis was performed. Then, as in Example 1, a water-soluble acrylic resin was applied and baked.

[Comparative Example 2] For the same aluminum plate as that used in Example 1, the same electrolytic solution as in Example 1 was used, and under the conditions shown in Table 1, 5 _sec.
During that period, direct current cathode electrolysis was performed. Then, as in Example 1, a water-soluble acrylic resin was applied and baked.

Regarding the aluminum plates respectively treated in Example 1, Comparative Example 1 and Comparative Example 2 described above, degreasing property (degreasing property before resin application), uniformity of anodized film) uniformity before resin application), and corrosion resistance (Corrosion resistance after resin coating and baking) was examined as follows. The results are shown in Table 1.

Here, the degreasing property is a plate after electrolysis-washing-drying, that is, the water contact angle of the surface of the plate before acrylic resin application, measured by Kyowa Chemical Co., Ltd. mini line 600, the water contact angle is 10 ° or less. It was evaluated as excellent (⊚) if any, good (∘) at 10 to 30 °, and bad (×) at 30 ° or more. Regarding the uniformity of the anodic oxide film, the anodic oxide film in the electrolyzed, water-washed, and dried state is visually observed, and if the skin is rough or uneven due to "burn", it is bad (x), and if there is no rough or uneven skin, it is good. It was judged as (◯). Corrosion resistance is further evaluated by spraying acetic acid salt water containing CuCl ₂ for 7 hours continuously on the plate after coating and baking of acrylic resin according to the Cass test method specified in JIS H8681-1980. The case where there was no corrosion was judged to be good (◯), and the case where corrosion was generated even partially was judged to be bad (x).

Example 1 is the same as Comparative Example 1 and Comparative Example 2 except that the current waveforms during the electrolytic treatment were different and the other conditions were the same, but as is clear from Table 1, direct current anode electrolysis was used. In Comparative Example 1, the uniformity of the coating was poor, and in Comparative Example 2 using DC cathodic electrolysis, the corrosion resistance was poor, whereas in Example 1 using the method of the present invention using an AC waveform, degreasing and coating uniformity were obtained. The corrosion resistance was good.

[Example 2] An aluminum plate similar to that used in Example 1 was subjected to electrolytic treatment under the same conditions as in Example 1 except that the bath temperature at the time of electrolytic treatment was 40 ° C to obtain a treated plate A. A treated plate B was obtained by electrolytic treatment under the same conditions as in Example 1 except that the bath temperature during treatment was 90 ° C. Further, each of the treated plates A and B was coated and baked with a water-soluble acrylic resin under the same conditions as in Example 1. Details of each electrolytic treatment condition are shown in Table 2.

Comparative Example 3 An aluminum plate similar to that used in Example 1 was subjected to electrolytic treatment under the same conditions as in Example 1 except that the bath temperature during the electrolytic treatment was 30 ° C. to obtain a treated plate C. A treated plate D was obtained by electrolytic treatment under the same conditions as in Example 1 except that the bath temperature during treatment was 95 ° C. Further, each of the treated plates C and D was coated with a water-soluble acrylic resin and baked under the same conditions as in Example 1. Details of each electrolytic treatment condition are shown in Table 2.

With respect to the treated plates A and B according to Example 2 and the treated plates C and D according to Comparative Example 3 described above, the degreasing property, the uniformity of the anodized film, and the corrosion resistance were examined by the same method as described above. The results are shown in Table 2.

As is clear from Table 2, the treated plate C, which had a bath temperature too low as 30 ° C. even after the same AC electrolysis, was inferior in degreasing property, film uniformity and corrosion resistance, and the bath temperature was low. 95 ° C
When the treated plate D was too high, the uniformity of the film and the corrosion resistance were poor. On the other hand, the treated plates A and B of Example 2 in which the bath temperature was in the range of 40 to 90 ° C. of the present invention had good degreasing properties, film uniformity, and corrosion resistance.

[Example 3] An aluminum plate similar to that used in Example 1 was subjected to electrolytic treatment with a single-phase alternating current using an electrolyte solution having the same components and the same bath temperature as in Example 1, and the amount of electricity was 4 C / dm. change as constant at ² electrolysis time 3sec, varied in two stages of 1sec processing plate E, to give the F, also the quantity of electricity in two steps of ^{40C / dm 2, 80C / dm} 2 as a constant electrolysis time 8sec Then, treated plates G and H were obtained. Further, on each of the treated plates E to H, a water-soluble acrylic resin was applied and baked in the same manner as in Example 1. The electrolysis conditions are shown in Table 3.

[Comparative Example 4] An aluminum plate similar to that used in Example 1 was subjected to electrolytic treatment by a single-phase alternating current using an electrolytic solution having the same components and the same bath temperature as in Example 1, and an electric quantity of 1 C / dm ² , Electrolysis time 1sec to obtain the processing plate I, and the electric quantity 100C / d
A treated plate J was obtained by treating with m ² and electrolysis time of 10 sec. Further, on each of the treated plates I and J, a water-soluble acrylic resin was applied and baked in the same manner as in Example 1. The electrolysis conditions are shown in Table 3.

With respect to the treated plates E to H according to Example 3 and the treated plates I and J according to Comparative Example 4 described above, the results of examining the degreasing property, the uniformity of the coating, and the corrosion resistance by the same method as described above are shown in Table 3. Shown in.

As is clear from Table 3, the treated plate I, which had an insufficient amount of electricity of 1 C / dm ² , was inferior in degreasing property and corrosion resistance. Further, the treated plate J, which had an excessive amount of electricity of 100 C / dm ² , was slightly inferior in degreasing performance, but inferior in efficiency. On the other hand, the treated sheets E to H of Example 3 all had good degreasing properties, film uniformity, and corrosion resistance, and were treated efficiently with the amount of electricity, electrolysis time, and current density being within the scope of the present invention. did it.

EFFECTS OF THE INVENTION According to the surface treatment method of the present invention, it is possible to obtain an aluminum plate whose corrosion resistance is remarkably superior and stable as compared with the case where it is treated by the conventional method, and the moldability is not impaired, and further degreasing is performed. Since the formation of an anodized film and the formation of an anodized film are performed at the same time in a very short time by electrolytic treatment in the same tank, it is possible to obtain various effects such as the overall work time being extremely short and the equipment cost being low. To be

Claims (1)

[Claims] With 1. A bath temperature 40 to 90 ° C. in an alkaline solution (except an aqueous solution containing an alkali metal silicate), alternating current density 3~35A / dm ^2, in an electric quantity 2~80C / dm ² Electrolytic treatment for 1 to 10 seconds by corrugation, degreasing and cleaning the surface by this, and forming a dense barrier type anodic oxide film with a film thickness of 0.01 to 0.05 μm. Processing method.