JP2842700B2 - Pretreatment method for coating Al-based plate for automobile body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for pre-coating an Al-based plate made of Al or an Al alloy used as an outer plate of an automobile body.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of a vehicle body, an outer panel of an automobile body has been made from a conventional cold-rolled steel sheet to a lightweight material such as Al.
Practical research on the practical use of substituting a base plate has been actively pursued. Normally, cold-rolled steel sheets commonly used for automobile bodies are subjected to a chemical conversion treatment with phosphate for imparting corrosion resistance before painting, but the same treatment is required when using Al-based sheet materials. Become. However, when the phosphate treatment is directly applied to the Al-based plate, the dense oxide film formed on the surface exerts an obstructive effect, thereby significantly reducing the treatment efficiency, and the effect of improving the rust resistance is also small. Become. In addition, since the Al ions are eluted into the phosphating solution to lower the efficacy, when the steel plate parts are simultaneously processed, the formation of the entire phosphate film does not proceed smoothly.

In order to solve such a problem, there has been proposed an Al-based plate in which a metal plating layer of Zn or the like is formed on the surface before a phosphate treatment is performed (Japanese Patent Laid-Open No. 61-15 / 1986).
No. 7693, JP-A-63-153262, JP-A-63-16669
No. 64). In these prior arts, a Z
The main purpose of forming the n-plated layer, etc. is to enable simultaneous phosphate treatment of Al-based sheets and steel sheets and surface-treated steel sheets, and to prevent the elution and accumulation of Al ions to improve the efficiency of chemical conversion treatment. The point is to increase. It is an important requirement that the formed Zn plating layer and the like be interposed as an intermediate layer between the Al-based plate and the phosphate film after the phosphate treatment. For this reason, the Zn plating layer and the like must be formed to a sufficient thickness so as not to be completely dissolved by the chemical conversion treatment, and the amount of the Zn film is limited to 0.4 g / m ² or more.

[0004]

However, when the inventors examined in detail, they found that the coating amount was 0.4 g /
When applying an Al-based plate having a Zn plating layer of m ² or more to an automobile body material, Zn remaining after the chemical conversion treatment is used.
It was clarified that the layer caused blistering in a corrosion test after exterior coating.

[0005] The present invention has been developed after confirming that dissolving a Zn plating layer formed in advance at the time of chemical conversion treatment is effective for improving the quality after coating when the target is an Al-based plate for an automobile body. It is an object of the present invention to provide a method for pre-painting an Al-based plate for an automobile body, which can improve the rust resistance after painting and effectively prevent the blistering phenomenon.

[0006]

According to the present invention, there is provided a method for pre-painting an Al-based plate for an automobile body according to the present invention, in which the surface of an Al or Al alloy plate is cleaned and then plated with Zn by electroplating. Alternatively, a structural feature comprises a first step of forming a Zn alloy plating layer and a second step of performing a chemical conversion treatment with zinc phosphate under conditions in which the Zn or Zn alloy plating layer formed in the first step is dissolved. And

The Al-based plate used as the material of the present invention may be any wrought aluminum or Al alloy.
There are no particular restrictions on the material, temper, and the like.

In the first step, the surface of an Al-based plate is cleaned, for example, by immersion in dilute sulfuric acid, and then Z-plated by electroplating.
This is performed in a process of forming a plating layer of n or Zn alloy. Electroplating conditions suitable for the purpose of the present invention are:
It is as follows. Zinc sulfate (heptahydrate): 300 g / l Sodium sulfate: 50 g / l pH: 1.6-1.8 Temperature: 50 ° C Current density: 5 A / dm ² Current supply time: 3-16 seconds

The amount of the Zn or Zn alloy plating layer formed in the first step must be equal to or less than the amount of etching dissolved in the zinc phosphate treatment in the second step. If this amount exceeds the etching amount, the Zn or Zn alloy film remains after the zinc phosphate treatment, causing swelling defects. The amount of etching of the Zn or Zn alloy plating layer varies depending on the type of zinc phosphate treating agent used and the processing conditions. It is preferable to set the upper limit of the amount to 0.38 g / m ² . However, when a processing agent having high activity and processing conditions are applied, Zn or Zn alloy can be dissolved up to an adhesion amount of 1.0 g / m ² . On the other hand, if the adhesion amount of the Zn or Zn alloy plating layer is less than 0.05 g / m ² , a uniform zinc phosphate film is not formed in the chemical conversion treatment in the second step, and yarn rust easily occurs. Therefore, the deposition amount of the Zn or Zn alloy plating layer formed in the first step is 0.05 to 1.0 g /
It is desirable to set the value in the range of m ² , more preferably 0.05 to 0.38 g / m ² .

The second step is a process in which a chemical conversion treatment with zinc phosphate is performed under the condition that the Zn or Zn alloy plating layer formed in the first step is dissolved. As the zinc phosphate treating agent, a commercially available one can be used, but the chemical conversion treatment is performed by appropriately adjusting the activity and the treatment conditions according to the amount of the Zn or Zn alloy plating layer formed in the first step. It needs to be done. The zinc phosphate film formed in the second step has a proper film amount range of 0.5 to 3 g / m ² .

The Al-based sheet provided with the zinc phosphate single film layer formed through the first step and the second step is transferred to a normal automobile body coating step, where the outer surface is coated.

[0012]

According to the present invention, in the first step, a Zn or Zn alloy plating layer is formed on the surface of a clean Al-based plate by using an electroplating method in which the weight per unit area can be easily controlled, thereby increasing the efficiency of the chemical conversion treatment. Prevent the generation of thread rust. Next, in the second step, the surface of the Zn or Zn alloy plating layer formed in the first step is subjected to a chemical conversion treatment with zinc phosphate, whereby Zn or Zn
The alloy plating layer elutes into the processing solution and raises the pH at the interface, and this action causes the deposition of a zinc phosphate film. Through this process, the Zn dissolution mechanism is utilized to the utmost to completely dissolve and remove the once formed Zn or Zn alloy plating layer to finally form a coating layer of zinc phosphate alone which does not cause a blistering phenomenon. Form.

Through the first and second processing mechanisms, the occurrence of thread rust and swelling after the outer surface coating is completely prevented, and an effective pre-coating process as an Al-based plate for an automobile body is completed. I do.

[0014]

Hereinafter, examples of the present invention will be described in comparison with comparative examples. Examples 1 and 2 and Comparative Examples 1 to 3 (1) First Step A test piece of an Al alloy plate containing 4.5% of Mg (width 70 mm, length 15
(0 mm, thickness 1 mm) was immersed in a 2% sulfuric acid solution heated to 70 ° C. for 1 minute to perform a surface cleaning treatment. Then
300 g / zinc sulfate (heptahydrate)
pH 1.6-1 consisting of l and sodium sulfate 50 g / l.
8 in a plating bath (50 ° C.) with a current density of 5 A / dm ^{2 and an} energization time in the range of 3 to 16 seconds to reduce the amount of Zn deposition to 0.3.
To form an electroplating layer of to 1.5 g / m ^2.

(2) Second step After the Al alloy test piece on which the Zn plating layer was formed in the first step was degreased and surface-adjusted by a conventional method, a commercially available zinc phosphate treating agent [manufactured by Nippon Parkerizing Co., Ltd .;
L-3020], and immersion treatment was performed under prescribed treatment conditions to form a zinc phosphate film.

(3) Evaluation after coating The surface of the test piece on which the zinc phosphate film had been formed was subjected to cationic electrodeposition coating, intermediate coating and top coating sequentially in the usual manner to form a coating film having a thickness of 100 μm. A yarn rust test and a swelling test were performed on the obtained coated plate, and the results were obtained. The Zn adhesion amount of the electric Zn plating layer formed in the first step, Zn
Are shown in Table 1 in comparison with the remaining state of and the state of the zinc phosphate film surface in the second step (visual observation, SEM observation). In addition, the thread rust test and the swelling test were performed as follows.
Yarn rust test: After spraying a test piece put in a cross cut with salt water for 24 hours, it is transferred to a constant temperature bath at a temperature of 50 ° C and a relative humidity (RH) of 80% and held for 1000 hours. Evaluation was made based on the total length of rust. Swelling test: The test piece placed in the cross cut was subjected to a 60-cycle deterioration test under the following processing conditions, and the state (number) of swelling was observed. Salt spray 6 hours Drying 50 ° C, 8 hours Constant temperature and humidity 50 ° C, relative humidity (RH) 80%, 10 hours

[0017]

As can be seen from the results in Table 1, Examples 1-2
As a result, the coating surface of zinc phosphate alone formed by dissolving the Zn plating layer of the first step in the second step has a very uniform structure, and defect phenomena such as thread rust and swelling after coating are not recognized at all. Not. On the other hand, in Comparative Examples 1 and 2 in which the Zn plating layer remains after the second step, swelling occurs and Z
In Comparative Example 3 in which the n-plated layer was not formed, the zinc phosphate coating structure was not uniform, and the generation of thread rust was observed.

[0019]

As described above, when the treatment of the present invention is applied, the occurrence of thread rust and swelling after the outer surface coating can be prevented at the same time. The effect is exhibited. In addition, since the formation of the Zn or Zn alloy plating layer in the first step is performed by the electroplating method, the control of the basis weight becomes easy, and the advantage of the processing operation surface that enables the fine adjustment of the adhesion amount is brought.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification code FI C25D 11/36 C25D 11/36 Z (72) Inventor Tatsumi Takahashi 3-1-1-12 Minato-ku, Minato-ku, Nagoya-shi, Aichi Sumitomo Light Metal Industries, Ltd. (56) References JP-A-61-157693 (JP, A) JP-A-63-166964 (JP, A) JP-A-53-102840 (JP, A) JP-A-4-160181 ( JP, A) JP-A-3-146693 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C25D 5/00-7/12 C25D 11/36 C23C 22/12 C23C 28 / 00

Claims (2)

(57) [Claims] 1. A first step of forming a Zn or Zn alloy plating layer by electroplating after cleaning the surface of an Al or Al alloy plate, and then forming the Zn or Zn alloy plating layer in the first step.
Alternatively, the method comprises a second step of performing a chemical conversion treatment with zinc phosphate under conditions in which the Zn alloy plating layer dissolves. 2. The method according to claim 1, wherein the amount of the Zn or Zn alloy plating layer in the first step is set in a range of 0.05 to 1.0 g / m ² .