JPH0559998B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to an Al alloy plate for painting used for automobile body panels, etc., and particularly to an Al alloy plate subjected to alkali degreasing treatment as a pre-painting treatment. Conventional technology In the past, steel plates were generally used for automobile body panels, but recently there has been a demand for improved corrosion resistance to prevent corrosion caused by spraying anti-freezing agents in cold regions, and for weight reduction to improve fuel efficiency. The demands are becoming stronger and stronger, so for example
Progress is being made in applying surface-treated steel sheets with metal plating, such as Zn plating or Zn alloy plating, and thin, high-strength high-tensile steel sheets to automobile body panels. However, the reality is that as long as steel is used as a material, there are limits to improving corrosion resistance and reducing weight. Therefore, instead of steel plates, it has been considered to use aluminum alloys, which have better corrosion resistance and are lighter in material properties than steel, for automobile body panels, etc., and practical use has begun in some cases. By the way, automobile body panels are usually painted before use, and when aluminum alloy plates are used for body panels, they are first degreased as a pretreatment to remove rolling oil and fats and oils that adhere to the plate surface. After treatment and washing with water, a chemical conversion treatment is performed to improve the adhesion of the paint to the base and the corrosion resistance of the paint base to form a chemical film, followed by electrodeposition, followed by intermediate coating and finishing. It is common to apply a coat of paint.
Conventionally, sodium silicate (Na ₂
SiO ₃ ) is generally used, and as a chemical conversion treatment, a chromate treatment and/or a zinc phosphate treatment is generally performed to form a chemical conversion film. Problems to be Solved by the Invention Aluminum alloy itself has better corrosion resistance than steel, and therefore even as a painted plate, an aluminum alloy coated plate has better corrosion resistance than a painted steel plate, but it is not suitable for automobiles. When considered as a vehicle body panel, aluminum alloy coated plates cannot yet be said to have satisfactory corrosion resistance. In other words, when used as an automobile body panel, if it is used for a long time in an extremely corrosive environment such as on roads sprayed with anti-freezing agents or in coastal areas, even aluminum alloy coated plates can easily become rusty. In particular, the corrosion resistance is still not sufficient for automobile body panels, where aesthetic appearance is highly sought after. Therefore, there is a strong need for the development of technology to improve the corrosion resistance of aluminum alloy coated plates, especially the thread rust resistance. Attempts have also been made to improve painting techniques and paints, but all have had their effectiveness limited. By the way, in the past, degreasing as a pre-painting treatment was thought to only be enough to remove the oils and fats such as anti-rust oil and rolling oil that adhered to the surface of the aluminum alloy plate, and the oils and fats on the surface could be completely removed. It was thought that such a degreasing treatment would not cause any particular factors that would degrade the corrosion resistance of a painted plate. However, when the present inventors conducted detailed experiments and studies on the degreasing of aluminum alloy plates, they found that the surface condition of the plate immediately after the degreasing treatment greatly influenced the occurrence of thread rust on painted plates. This led to his invention. Therefore, an object of the present invention is to provide an aluminum alloy degreased plate for painting, which can exhibit excellent thread rust resistance when used as a painted plate for use in automobile body panels, etc. It is something. Means for Solving the Problems The aluminum alloy degreased plate for painting of the present invention is an Al alloy plate for painting that has been subjected to alkali degreasing treatment, and the silicate remaining on the surface is SiO ₂
It is characterized by an amount of 50 mg/m ² or less in terms of conversion. Function First, the findings newly discovered by the present inventors regarding the relationship between thread rust resistance of an aluminum alloy coated plate and the surface condition of the aluminum alloy plate after degreasing treatment will be explained. As already mentioned, sodium silicate is used as an alkaline degreasing agent in the conventional degreasing treatment of aluminum alloy plates, but in this case, silicate derived from the treatment agent (xSiO ₂ yM ₂ O (represented by: x, y are integers, M is a metal such as Na, K, etc.) is generated or adsorbed on the plate surface, and a considerable amount of silicate remains attached to the plate surface even after degreasing treatment. It was discovered that this was the case. On the other hand, we evaluated the string rust resistance of aluminum alloy coated plates that had been degreased with sodium silicate, chemically treated, and then electrocoated, intermediate coated, and finished coated. When we investigated the relationship between the amount of silicate remaining attached and the thread rust resistance of painted boards, we found that the greater the amount of silicate remaining after degreasing, the lower the thread rust resistance. It has been newly discovered that when the amount exceeds 50 mg/m ² per side in terms of SiO ₂ , thread rust resistance is poor. From this, we found that by performing alkaline degreasing treatment such that the residual amount of silicate on the surface of the aluminum alloy plate after degreasing is less than 50 mg/m ² per side in terms of SiO ₂ , it is possible to improve the thread rust resistance of painted plates. They discovered that it is possible to improve the performance of the invention, and have completed the present invention. As mentioned above, the reason why the thread rust resistance of a painted board can be improved by reducing the residual amount of silicate on the board surface after alkaline degreasing to 50 mg/m ² or less in terms of SiO ₂ is as follows. It can be thought of as follows. In the painting process of aluminum alloy plates,
The chemical conversion film formed after degreasing plays a major role in improving the adhesion between the aluminum alloy plate substrate and the coating film. If the chemical conversion coating is formed evenly and densely, the coating will be formed uniformly and with high adhesion in the next process of electrodeposition coating, and in this case, the coated plate will have good corrosion resistance and will be free from thread rust. The risk of occurrence is reduced. On the other hand, if the chemical conversion coating is uneven, there will be areas with poor adhesion or defects in the coating, and such areas with poor adhesion or defects in the coating will cause threads to form. Rust occurs. However, if a large amount of silicate remains and adheres to the surface of the aluminum alloy plate in the degreased state before chemical conversion treatment, the silicate will inhibit the formation of a uniform film in the next chemical conversion process. This results in the formation of an uneven and non-uniform chemical conversion coating, which causes areas of poor adhesion and defects in the coating film, making thread rust more likely to occur in those areas. The amount of silicate deposited after degreasing is
If it is less than 50 mg/m ² in terms of SiO ₂ , a chemical conversion film can be formed almost uniformly,
Therefore, the adhesion of the coating film is uniform and good, and there are fewer defects in the coating film, and there is less risk of thread rust. Note that since Al is amphoteric, both acidic and alkaline degreasing agents can be applied to aluminum alloy plates; however, in the present invention, alkaline degreasing is performed. In other words, the alkaline processing agent is
Not only does it remove oils and fats such as rust preventive oil and rolling oil that exist on the material surface through saponification, but it also removes surface oxide layers, oil-baked layers, and other contaminant layers through etching and dissolution. By exposing the active surface on the surface of the aluminum alloy plate, a chemical conversion film can be formed uniformly and evenly in the next step, which in turn makes the adhesion of the paint film uniform and good, which contributes to preventing the occurrence of thread rust. In addition, when an alkaline degreasing solution is used, fine irregularities occur on the surface due to dissolution etching, and these irregularities not only increase the surface area but also serve as an anchor effect in the subsequent electrodeposition coating process. This also contributes to improving the adhesion of the coating film and, ultimately, to improving the thread rust resistance. However, as mentioned above, even in alkaline degreasing, if a large amount of silicate remains on the surface after degreasing, the silicate will have a negative effect on the formation of a uniform chemical conversion film. The residual amount of silicate must be regulated to 50 mg/m ² or less in terms of SiO ₂ . Here, as mentioned above, etching and dissolving by alkaline degreasing treatment and controlling the amount of silicate remaining after degreasing treatment to a small amount are mutually related. In other words, in a degreasing treatment in which a large amount of silicate remains attached, the progress of etching dissolution is inhibited by the silicate on the surface even during the degreasing treatment, and active surfaces and unevenness due to etching dissolution are less likely to occur. Although it becomes difficult to obtain the effect of improving thread rust resistance due to the active surface and unevenness, if the degreasing treatment reduces the residual amount of silicate to less than 50mg/ ^m2 , etching dissolution will progress and the activation will be improved. As a result, a smooth surface and unevenness can be obtained, and the above-mentioned effect of improving thread rust resistance can be obtained. Figures 1 and 2 show JIS 5182 alloy, which is an Al-Mg-based aluminum alloy, and Al-Mg-Si.
JIS 6061 alloy, which is an aluminum alloy, is degreased in such a way that a large amount of silicate remains after degreasing, that is, when a sodium silicate-based treatment agent is used as the degreasing agent, and when the degreasing treatment The amount of etching dissolution on the plate surface is shown as a function of treatment time when a degreasing treatment is performed so that almost no silicate remains afterward, that is, when a sodium phosphate treatment agent is used as the degreasing agent. Here, the sodium silicate treatment agent is a solution containing 30% sodium silicate at a temperature of 45%.
The sodium phosphate treatment agent used was a solution containing 2% sodium phosphate at a temperature of 70°C. As shown in Figures 1 and 2, when a sodium silicate treatment agent was used, etching dissolution progressed only slightly even if the treatment time was increased, whereas the sodium phosphate treatment When the etching agent was used, the amount of etching dissolution increased significantly as the treatment time became longer. By the way, when a sodium silicate treatment agent is used, the amount of silicate remaining on the board surface (in terms of SiO ₂ ) is usually 60 mg/m ² to several times as shown in the examples below. 100mg/
^m2 , while when using a phosphate treatment agent,
It is about 10 mg/m ² or less, and from this it is clear that the effect on etching dissolution of the silicate film is large. As already mentioned, in the case of this invention, the residual amount of silicate on the board surface after alkaline degreasing is 50mg/
If it is less than m ² , good thread rust resistance can be obtained on the coated board, but when the present inventors separately investigated the relationship between the amount of etching dissolved in degreasing treatment and the thread rust resistance on the coated board, it was found that the etching It has been found that when the dissolved amount (average) is 0.05 μm or more, excellent thread rust resistance can be obtained in the coated plate. Specific explanation for carrying out the invention This invention is applicable to all aluminum alloys, regardless of the type of aluminum alloy to which it is applied, but Al -Mg alloy (JIS
5000 series alloy) and Al-Mg-Si series alloy (JIS
6000 series alloys), the decrease in thread rust resistance due to residual silicate is particularly noticeable in the case of Al-Mg-Si alloys, and therefore Al-Mg-Si alloys are the most This invention is effective. Alkaline degreasing should be carried out so that the amount of silicate remaining on the board surface after degreasing is 50 mg/m ² or less in terms of SiO ₂ , and in order to do so, the alkaline degreasing solution must contain virtually no silicic acid. It is best to use something that does not include the Specifically, sodium phosphate, sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium hydroxide, sodium sulfate (Na ₂ SO ₄・10H ₂ O), sodium sesquisulfate (Na ₂ CO ₃・NaHCO ₃・2H ₂ O) or a solution containing two or more of these may be used as the treatment liquid. Note that even if a treatment liquid other than sodium silicate is used, some industrial water for dilution may be used.
Normally, SiO ₂ is included, and even in that case, a small amount of silicate often remains on the surface after treatment, but as already mentioned, the amount is calculated in terms of SiO ₂ . There is no problem if it is less than 50mg/ ^m2 . However, use of water containing a large amount of SiO ₂ as dilution water should be avoided. The pH of the treatment liquid in the alkaline degreasing treatment may be 8 or higher, and its concentration is not particularly limited, but it is usually desirable to use it at a concentration of 1 to 3%. In addition, when actually painting the degreased board according to the present invention, after performing the degreasing treatment as described above, it is washed with water, and then chemical conversion treatment is performed by chromate treatment and/or zinc phosphate treatment. After applying the coating and washing with water, it is usual to perform electrodeposition coating and, if necessary, to perform one or more layers of baking coating. Example JIS 5182 alloy, which is an Al-Mg alloy, and Al
- Test pieces of JIS 6061 alloy, which is a Mg-Si alloy, were subjected to alkaline degreasing under various conditions as shown in Table 1. After degreasing, washing with water, chromate treatment as a chemical conversion treatment, washing with water and pure water, and then applying cationic electrodeposition coating (180°C x 20 minutes) of epoxy resin to a thickness of 20 μm.
Then, as an intermediate coating, melamine alkyd resin baking coating (140℃ x 30 minutes) was applied to a thickness of 35μm.
After that, apply a baking coat of melamine alkyd resin (140℃ x 30 minutes) to a thickness of 35μm as a top coat.
I did it at The painted test pieces thus obtained were tested under the following conditions to evaluate their thread rust resistance. In other words, first make cross cuts in the paint film,
A salt spray test in accordance with JIS Z2371 was conducted for 24 hours, followed by a humidity test at a temperature of 25°C and a humidity of 85%.
It was left in a humid atmosphere of RH for 42 days, and the thread rust resistance was evaluated based on the length of thread rust generated on the surface. The results are shown in Table 1. In addition, in the thread rust resistance evaluation, if the length of thread rust is 1.0 mm or less, it is considered to be good and marked with ◎.
A value of 1.0 to 2.0 mm was marked as somewhat good, a mark of ◯, a value of 2.0 to 4.0 mm was marked as △, and a value of 4 mm or more was marked as poor.

【table】

[Table] As shown in Table 1, No. 13 and No. 14 where the residual amount of silicate on the degreased plate exceeds 50 mg/m ²
In the comparative example, the thread rust resistance on the painted plate was poor, especially in the case of the Al-Mg-Si alloy. In contrast, the residual amount of silicate in the degreased board is 50 mg/m ²
In the following examples of the present invention (No. 1 to No. 12), it was found that all coated plates exhibited good thread rust resistance. Effects of the Invention The degreased plate for painting of this invention has silicate remaining on the plate surface after alkaline degreasing treatment is 50 mg/m ² or less in terms of SiO ₂ . By regulating this, it is possible to obtain extremely excellent thread rust resistance in the state of a painted board. Therefore, painted plates using the degreased plate of the present invention have little risk of rusting even when used for long periods of time in harsh corrosive environments such as roads sprayed with antifreeze or beach areas, and are suitable for automobiles. It is ideal for painted plates used for vehicle body panels and various automobile parts.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the treatment time and the amount of alloy dissolved in etching when an Al-Mg alloy (JIS 5182 alloy) is degreased using a sodium phosphate treatment agent or a sodium silicate treatment agent. ,
Figure 2 shows the relationship between the treatment time and the amount of etched alloy dissolved when Al-Mg-Si alloy (JIS 6061 alloy) is degreased using a sodium phosphate treatment agent or a sodium silicate treatment agent. This is a graph showing.

Claims (1)

[Claims] 1 This is an Al alloy plate for painting that has been subjected to alkali degreasing treatment, and the silicate remaining on the surface is SiO ₂
An Al alloy degreased plate for painting, characterized in that it has a content of 50 mg/m ² or less in terms of conversion.