JPH08281208A - Pretreatment for coating of ground part of aluminum alloy - Google Patents

Abstract

PURPOSE: To lessen the generation of filiform rust after coating by subjecting the surface of a ground part to finish grinding to a smooth surface where many fissure-like recessed parts do not exist when this surface is observed with an optical microscope of a specific magnification, then subjecting the surface to a zinc phosphate treatment. CONSTITUTION: The ground surface of an aluminum alloy material is subjected to finish grinding to form the smooth surface where the many fissure-like recessed parts do not exist when the surface is observed with the optical microscope of the magnification <=400 times, and thereafter, the surface is subjected to the zinc phosphate treatment. The ground part is subjected to finish grinding by buffing or scraping. An Al-Mg alloy or Al-Si-Mg alloy is used for the aluminum alloy. As a result, the filiform rust resistance of the ground part after coating is improved to equal to or better than the filiform rust resistance of the non-ground parts. The deterioration in the corrosion resistance of the ground part after the coating is averted when the aluminum alloy particularly for panels for automobile bodies is subjected to surface grinding after forming.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for pretreatment of coating of an aluminum alloy grinded portion, and more particularly, when a surface of an aluminum alloy for an automobile body panel is ground after molding, a grinding portion of a grinded portion which does not deteriorate corrosion resistance after coating The pretreatment method for painting.

[0002]

2. Description of the Related Art Aluminum alloy sheets for automobile body panels are usually formed by press forming into a predetermined shape, and then, together with a steel sheet and a surface-treated steel sheet, a zinc phosphate treatment solution is used as a pretreatment for coating. A coating base treatment is performed.

As an aluminum alloy for automobile body panels, 5000 series Al containing Mg as a main alloying component.
-Mg-based alloys, 6000-based Al-Si-Mg-based alloys containing Si and Mg as essential alloying components are often used, but the zinc phosphate treatability of aluminum alloys generally tends to vary depending on the alloying components. Even when the zinc phosphate treatment is performed under the same conditions, depending on the alloy composition, the processability may be poor and a sufficient amount of zinc phosphate coating may not be formed.

In particular, the influence of Cu is large, and for example, Al
Taking the effect of Cu content as an example of a -5% Mg alloy, as shown in FIG. 1, the film formation amount of the zinc phosphate film increases as the Cu content increases. (Refer to the graph for the non-abrasive material in FIG. 1) When the Cu content is less than 0.10%, the zinc phosphate coating amount becomes insufficient, so
Poor adhesion and swelling resistance at the metal / coating interface after painting,
Corrosion resistance such as thread rust resistance will be poor.

On the other hand, in plate materials of aluminum alloys for automobile body panels such as Al-Mg alloys, in order to repair surface defects such as scratches generated during press forming,
It is not uncommon for the surface of the entire surface to be locally or entirely ground after the molding process using abrasive paper. However, when the ground portion is treated with zinc phosphate, the zinc phosphate treatment is inferior to that in the non-ground portion, and particularly Al-Mg having a large amount of Cu is used.
As shown in FIG. 1, in the alloy-based alloy material, there is a large difference in the amount of zinc phosphate film formed between the non-ground portion and the ground portion.
(See the graph for the abrasive material in Figure 1)

That is, since the zinc phosphate coating amount is reduced in the grinding portion, the adhesion of the coating film is poor and the corrosion resistance tends to deteriorate after coating. Various measures have been proposed in order to improve the zinc phosphate processability of the ground portion, and the inventors have also
In order to improve the phosphoric acid treatment property of the grinding part of the Al-Mg alloy, the grinding part is washed with an inorganic acid or a solution containing an inorganic acid as a main component and then treated with zinc phosphate (Japanese Patent Application No. 6-25
1284), and a method (Japanese Patent Application No. 6-259247) in which grinding is performed in the presence of at least one of metals nobler than aluminum and oxides of those metals.

However, in the subsequent research, even if the zinc phosphate treatment property of the ground portion was improved, the corrosion resistance, particularly the thread rust resistance was not improved, and the thread rust resistance equivalent to that of the non-ground portion was obtained. It has been found to be difficult in some cases. The following is a partial outline of the experiments conducted by the inventors. For Al-5% Mg alloy plates, a non-abrasive material without surface grinding and an abrasive material that was ground with abrasive paper were prepared and After the zinc phosphate treatment is performed under different conditions, the amount of zinc phosphate coating is measured for each test material, and after coating, the coating is cross-cut to reach the base with a cutter knife, and a salt spray test is performed to start the test. The length of the thread rust that occurred 4 weeks and 8 weeks thereafter was measured.

As a result, as shown in FIG. 2, the non-abrasive material was treated with zinc phosphate for 2 minutes as compared with the test material (non-abrasive material (0 min)) which was not treated with zinc phosphate. The test material has improved yarn rust resistance. On the other hand, comparing the non-abrasive material treated with zinc phosphate and the abrasive, when zinc phosphate treatment for 2 minutes was performed, the amount of zinc phosphate coating decreased in the abrasive and the thread rust resistance Is significantly deteriorated.

From these results, it is considered that the deterioration of the thread rust resistance is due to the decrease of the zinc phosphate coating amount.
Zinc phosphate treatment time increased to 5 minutes for non-abrasive material (2 min)
The rust resistance of the abrasive material (5 min) with a larger zinc phosphate coating amount is inferior to that of the non-abrasive material (2 min).
It is recognized that even if the zinc phosphate coating amount is increased above the non-ground portion, sufficient thread rust resistance equivalent to that of the non-ground portion may not be obtained.

Studies have been conducted on the rust resistance after coating of the surface-ground portion of an Al-Mg alloy and the surface shape of the material. In the ground portion, the larger the surface roughness, the more the zinc phosphate coating is formed. It has been reported that the precipitation becomes non-uniform and the zinc phosphate treatment becomes poor, and the thread rust resistance after coating is lowered. (Summary of the 81st Autumn Meeting of the Japan Institute of Light Metals 19th
(Pp. 5 to 196)

With respect to the Al-Mg alloy material, the present inventors have confirmed the influence of the surface roughness of the ground portion on the surface rust resistance of the ground portion after coating in order to confirm the effect of Al-5% Mg.
A test material in which the alloy plate was ground with abrasive paper and a test material in which the alloy plate was ground with a scraper were prepared, and the same amount of zinc phosphate coating was formed and the thread rust resistance after coating was evaluated. As a result, even though the surface roughness of the scraper abrasive is larger than the surface roughness of the abrasive paper abrasive, the scraper abrasive has significantly better yarn rust resistance than the abrasive paper abrasive,
It had the same rust resistance as the non-ground portion. Therefore, it was found that the surface roughness of the ground portion does not necessarily affect the yarn rust resistance, but according to the observations of the inventors, the direction in which the yarn rust occurs corresponds to the rolling direction in the non-ground portion, and Since the part has a good agreement with the grinding direction regardless of the rolling direction, it was predicted that the surface properties of the ground part other than the surface roughness were involved in the occurrence of thread rust.

[0012]

The present invention is directed to Al-Mg
In order to improve the thread rust resistance after coating in the grinding part of aluminum alloy materials such as series alloys, factors other than surface roughness in the surface shape of the grinding part, zinc phosphate treatment and thread rust resistance after coating The result was the result of further experiments and investigations regarding the relationship with the toughness, and the purpose was to improve the thread rust resistance after painting in the ground part to the same level as or better than the non-ground part. An object of the present invention is to provide a method for pretreatment of coating of an alloy grinding part.

[0013]

In order to achieve the above object, a method of pretreatment for coating an aluminum alloy ground portion according to the present invention is a grinding surface of an aluminum alloy material having a magnification of 40.
The basic feature of the constitution is that after observing with an optical microscope at a magnification of 0 or less, finish grinding is performed on a smooth surface without a large number of crack-shaped recesses, and then zinc phosphate treatment is performed.

The second and third characteristics of the construction are that the grinding portion is finish-ground by buffing and that the grinding portion is finish-grinding by scraper grinding. The aluminum alloy is Al--Mg. System alloy or A
The fourth characteristic is that the alloy is an l-Si-Mg-based alloy.

In the present invention, a large number of crack-shaped recesses are observed with an optical microscope having a magnification of 100 to 400 times, and the base material of the material is turned up and down by unevenness including recesses having an opening of less than 90 ° and grinding. , Or swelling, which covers the substrate and the substrate is double-folded. It is considered that the corrosive liquid easily penetrates into the portions having these surface textures and deteriorates the thread rust resistance.

The above-mentioned surface shape is formed, for example, when it is polished with abrasive paper. After the grinding with the abrasive paper, the finish is ground by buffing or scraper grinding so that these recesses are removed and smooth. The surface can be obtained. Regarding surface grinding, a smooth surface can be formed even if grinding is performed with a scraper or the like from the beginning.

[0017]

In the present invention, when the aluminum alloy material is locally machined or entirely ground after forming and zinc phosphate treatment is applied, the surface of the ground portion is observed by an optical microscope with a magnification of 400 times or less. At this time, by finishing grinding to a smooth surface without many crack-shaped recesses, the recesses where corrosive liquid easily enters are removed, and zinc phosphate treatment,
Less occurrence of thread rust after painting.

[0018]

Hereinafter, examples of the present invention will be described in comparison with comparative examples. Example 1 An annealed material (O material) of a rolled plate (thickness 1 mm) of an Al-Mg alloy containing 4.5% Mg and 0.3% Cu was used as a test material (dimensions:
70 mm × 150 mm), each test material was ground and treated with zinc phosphate under the following conditions, and the amount of zinc phosphate film was measured by a fluorescent X-ray analysis method. The surface of the test material was dull finished, and the surface roughness was an average roughness (Ra) of 1.07.
The maximum roughness (Rt) was 8.5 μm.

Grinding 1. Grinding with abrasive paper Polish the surface of the test material in a certain direction with abrasive paper using No. 180 alumina abrasive. 2. Buff polishing Buff polishing is performed until the grinding marks on the polishing paper disappear.
As a concrete polishing method, the number of the polishing paper is 180, then
400, then 1200, and the like, by gradually finely grinding and polishing, and finally by a buffing cloth containing a fine-grained polishing agent to obtain a surface close to a mirror surface. 3. Scraper Scraper grinding is performed until the grinding marks by the above-mentioned abrasive paper disappear.

Zinc phosphate treatment 1. ~ 3. It was done in order. 1. Degreasing treatment Commercially available alkaline degreasing agent (Nippon Parkerizing Co., Ltd.
FC-L4460) for 2 minutes at 43 ° C. 2. Surface conditioning Soak for 30 seconds at room temperature using a commercially available titanium-based surface conditioning agent (PL-4040 manufactured by Nippon Parkerizing Co., Ltd.). 3. Chemical conversion treatment Commercially available zinc phosphate treatment agent (Nihon Parkerizing Co., Ltd.)
Soaked for 2 minutes at a temperature of 43 ° C using PB-L3020 manufactured by K.K.

Each test material treated with zinc phosphate was subjected to a three-coat coating for automobiles (cationic electrodeposition, intermediate coating and top coating) to form a coating film having a total film thickness of 90 μm. After forming the coating film, after cross-cutting to reach the substrate with a cutter knife on the surface of each test material, after spraying salt water (5% NaCl, 35 ° C × 24 h),
A salt spray test was carried out under the conditions of washing with water and then wetting (RH 80%, 50 ° C. × 1000 h), and the length of thread rust generated 4 weeks and 8 weeks after the start of the test was measured. FIG. 3 shows the measurement results of the amount of zinc phosphate film formed and the length of thread rust of each test material.

As shown in FIG. 3, the test material subjected to finish grinding by scraper grinding and the test material subjected to finish grinding by buffing, compared with the test material ground only with abrasive paper, were zinc phosphate. Although the amount of film formed was about the same, the yarn rust resistance was remarkably excellent, and the yarn rust resistance was equal to or higher than that of the non-abrasive material.

In order to investigate the surface properties of each test material, the surface roughness was measured and the cross section of the test material was observed with an optical microscope at a magnification of 400 times. As a result, regarding the surface roughness, the test material ground with only abrasive paper had Ra 0.8 μm, Rt 11.7 μm,
Ra 0.1 μ is the test material that was finish ground by buffing.
m, Rt 0.6 μm, and the test materials that were finished ground by scraper grinding were Ra 0.8 μm and Rt 20.7 μm, the buff abrasive had the smallest surface roughness, and the scraper abrasive was more abrasive than the abrasive paper abrasive. However, the surface roughness is rather large.

On the other hand, the optical microscope shown in FIGS.
According to the observation result of the cross section of the test material by 400 times), the test material finished by buffing has the flattest surface texture. The scraper abrasive material is inferior in flatness to the buff abrasive material, but exhibits a smooth surface shape similar to that of the buff abrasive material. On the other hand, on the surface of the test material ground only with abrasive paper, many irregularities including crack-shaped recesses were observed, and the surface texture was significantly different from that of the buff abrasive and scraper abrasive. There is.

Example 2 A rolled plate of an Al-Si-Mg based alloy containing 0.75% Si, 0.53% Mg, 0.30% Cu and 0.24% Fe was annealed, and this annealed plate (O material, thickness 1 mm) as a test material, grinding, zinc phosphate treatment and coating were carried out in the same manner as in Example 1, and the same corrosion test by salt spray as in Example 1 was carried out. The length of thread rust generated in the test material and the test material finish-ground by scraper grinding was significantly smaller than that of the test material ground only with abrasive paper, and the thread rust resistance was equal to or higher than that of the non-ground material.

[0026]

As described above, according to the present invention, when the surface defect portion is ground after the molding process, and zinc phosphate treatment and coating are performed, the ground portion has a thread rust resistance equal to or higher than that of the non-ground portion. A method of pre-painting an aluminum alloy grind is shown. The method of the present invention is particularly effective for Al-Mg-based alloys and Al-Si-Mg-based alloys, and can be effectively used when these alloy plates are applied for automobile body panels.

That is, when the above aluminum alloy material is used as an automobile body panel, after a molding process such as a pressing process, a surface defect portion generated by the molding is removed by grinding and then a zinc phosphate treatment is performed in an automobile assembly line. In such a case, there is a problem that the corrosion resistance after coating in the grinding portion, particularly the thread rust resistance is significantly deteriorated, but this problem is avoided by the present invention.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Cu content and the zinc phosphate coating amount in an Al—Mg-based alloy.

FIG. 2 is a graph showing the amount of zinc phosphate film formed and the length of thread rust in non-abrasive materials and abrasive materials made of abrasive paper in Al—Mg alloys.

FIG. 3 is a graph showing the amount of zinc phosphate film formed and the length of thread rust of non-abrasive materials and various abrasive materials in Al—Mg alloys.

FIG. 4 is a cross-sectional view showing a surface shape when an Al—Mg-based alloy material is ground only with abrasive paper.

FIG. 5 is a cross-sectional view showing a surface shape when an Al—Mg alloy material is finish-ground by buffing.

FIG. 6 is a cross-sectional view showing a surface shape when an Al—Mg alloy material is finish ground by scraper grinding.

Claims (4)

[Claims] 1. A surface of a ground portion of an aluminum alloy material,
A pretreatment method for coating an aluminum alloy ground portion, which comprises performing a finish grinding on a smooth surface free from a large number of crack-shaped recesses when observed under an optical microscope with a magnification of 400 times or less, and then performing a zinc phosphate treatment. 2. The pretreatment method for coating an aluminum alloy ground portion according to claim 1, wherein the ground portion is finish-ground by buffing. 3. The pretreatment method for coating an aluminum alloy ground portion according to claim 1, wherein the ground portion is finish ground by scraper grinding. 4. The pretreatment method for coating the aluminum alloy ground portion according to claim 1, wherein the aluminum alloy is an Al—Mg based alloy or an Al—Si—Mg based alloy.