JPH06280085A - Galvanized aluminum and aluminum alloy excellent in adhesion and production thereof - Google Patents

Abstract

PURPOSE:To provide a galvanized aluminum and aluminum alloy excellent in adhesion and to provide the producing method thereof. CONSTITUTION:On the surface of aluminum and an aluminum alloy 1, an iron- zinc flash plating 3 in which the ratio of iron is 50 to 100% so as to regulate its plating thickness into 0.01 to 7g/m<2>, and a galvanizing 4 are applied. In the case where anode treatment is executed in a plating soln. before the iron- zinc flash plating 3 and a thin oxidized film 2 is formed, the optimum operating range of the iron-galvanizing can be widened. In this way, on the surface of the aluminum and aluminum alloy, galvanizing excellent in adhesion can be applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to zinc-plated aluminum and aluminum alloy plates and strips having excellent adhesion, which are used in automobiles, vehicles, building materials, ships, aircraft and the like.

[0002]

2. Description of the Related Art In recent years, aluminum and aluminum alloys have come to be used in automobiles and the like for the purpose of weight reduction. However, in terms of cost and material strength, all of them cannot be replaced with conventional materials, and they are thin. It is often shared with steel sheets or zinc-based plated steel sheets. In this case, the pre-coating treatment such as phosphate treatment, electrodeposition coating, and topcoat are processed in exactly the same process as the steel plate, but the process suitability and the properties after coating are equivalent to those of the steel plate. Or even higher characteristics are required.

It has been found that zinc-based plating is preferably applied when simultaneously treating aluminum or an aluminum alloy with a steel sheet as described above, but it is difficult to apply zinc-based plating to aluminum and aluminum alloys, and the latest surface treatment is required. The displacement plating method as described in Technical Guide (published by the Industrial Technology Service Center in April 1988), or JP-A-51-1332 and JP-A-51-4754.
As disclosed in Japanese Patent Laid-Open No. 9-35095 or Japanese Patent Laid-Open No. 4-350195, there have been attempts such as a method in which an oxide film is formed by anodizing and then zinc-based plating is performed thereon.
In particular, the method of plating after the anode treatment was excellent in productivity, and it was possible to treat aluminum and aluminum alloys continuously in a short time.

[0004]

In comparison with such a conventional technique, it is possible to perform processing at a higher productivity and at a higher speed and at a lower cost. It is desired that the quality characteristics are suitable as a coating base, and that the characteristics after coating are the same as or superior to those of a steel sheet or a galvanized steel sheet even if they are treated simultaneously with the steel sheet or the galvanized steel sheet. For that purpose, aluminum and aluminum alloys having a zinc-based plating layer with more excellent adhesion than ever are required.

[0005]

Therefore, as a result of continuous studies, the present inventor has found that in order to improve the adhesion between zinc-based plating and aluminum and aluminum alloys, the iron content on the surface of aluminum and aluminum alloys should be improved. It was found that excellent adhesion can be obtained by performing iron-zinc flash plating, which is often used, and then performing a predetermined plating thereon, and has reached the present invention. That is, the present invention has an iron-zinc flash plating layer having a ratio of iron of 50 to 100% on the lower layer surface in a thickness of 0.01 to 7 g / m ^{2 and} having a zinc-based plating layer thereon, which is excellent in adhesion. Aluminum and aluminum alloys. Alternatively, an aluminum oxide film is formed on the surface, and an iron-zinc flash plating layer having an iron ratio of 50 to 100% is formed on the surface of 0.01 to 7 g / m ^{2 in} thickness,
Aluminum and aluminum alloys having a zinc-based plating layer thereon and excellent in adhesion.

Further, in order to obtain such aluminum and aluminum alloys, ferrous ions are added in an amount of 20 to 10
0 g / l, zinc ion 50 g / l or less, free acid 5-8
Current density of 100-700 A / with plating solution containing 0 g / l
Flash plating with dm ² followed by zinc-based electroplating. Alternatively, the ferrous ion is 20 to 10
0 g / l, zinc ion 50 g / l or less, free acid 5-8
Electricity of 10 to 200 c / dm with a plating solution containing 0 g / l using aluminum and aluminum alloy as an anode
Electrolyzed at ² , current density 50 as cathode
The gist is a method of performing flash plating at up to 600 A / dm ^{2 and} then performing zinc-based electroplating.

[0007]

The present invention will be described in detail below. A configuration diagram of the plating film of the present invention is shown in FIG. (A) shows an iron-zinc flash plating 3 having a high iron content excluding the oxide film on the surface of aluminum and aluminum alloy 1, and zinc-based plating 4 thereon, and (b) shows anodization. By depositing an oxide film 2 on the surface of aluminum and aluminum alloy 1, and iron with a high iron content on the oxide film 2.
Zinc flash plating 3 is applied, and then zinc-based plating 4 is applied, and (c) is an enlarged schematic view of 2000 times viewed from the surface at the stage of applying iron-zinc flash plating of (b).

As the aluminum and aluminum alloy of the present invention, in addition to high-purity aluminum having a plate thickness of 2.0 mm or less, those containing 10% or less of magnesium such as 5000 series or 6000 series and those alloyed with copper can be used. it can. From the viewpoint of surface treatment, it is desirable that the amount of alloying metal is as small as possible. The surface may be as-rolled or subjected to heat treatment, but it is desirable that the oxide film be as thin as possible and the rust preventive oil be as small as possible. It is necessary to determine the processing conditions for alkaline degreasing and pickling depending on the state of adhesion of oxide film, oil and dirt to obtain an appropriate plating surface.

Since alkaline degreasing and / or pickling of aluminum and aluminum alloys also serves as surface adjustment, it is necessary to select treatment conditions fairly carefully. In addition, even if the oxide film on the surface is removed in these steps, an oxide film will start to be formed again immediately after the treatment and the plating property will be impaired, so it is necessary to move to the plating step as early as possible. Ordinary surfaces should be degreased with aluminum or aluminum alloy plates at room temperature or after alkaline degreasing at room temperature.
It was obtained by dipping and pickling in% sulfuric acid. In addition, a good surface condition that is smooth and completely free of oxide film can be obtained by a method of plating with alkali degreasing or after alkali degreasing followed by pickling, dipping in a plating solution for at least 1 to 30 seconds, and plating in that solution. Met.

Regarding the adhesion of iron-zinc flash plating, first, the result of an investigation on the relationship between the plating composition and the adhesion revealed that sufficient adhesion can be obtained if the iron content is 50% or more under normal pretreatment conditions. did. The required minimum iron ratio of iron-zinc plating could be determined by the time of contact with the plating solution after plating, the free acid concentration of the plating solution, the immersion time of the next plating in the plating solution, and the free acid concentration. In the normal case, the proportion of iron is 50 to 100
% Was sufficient. The amount of such iron-zinc flash plating having an iron composition ratio of 50% or more may be 3 g / m ² or less in the case of a smooth surface,
Further, if the surface condition is smooth and the oxide film is completely removed, 0.01 g / m ² is sufficient.

The composition of the plating bath is ferrous ion,
Zinc ion and free acid are indispensable. The ferrous ion concentration was required to be 20 g / l or more in order to cause iron to be contained in the plating layer in an amount of 50% or more and to be efficiently deposited. On the other hand, the ferrous ion concentration of 100 g / l or more was difficult to dissolve.
Further, the composition ratio of iron in the iron-zinc plating is primarily determined by the ratio of zinc ions and ferrous ions in the plating bath. Therefore, when performing plating containing 50% or more of iron, the ratio of ferrous ions to ferrous ions and zinc ions needs to be 60% or more. For this purpose, the zinc ion content is 67 g / l or less, preferably 50 g / l.
It was necessary to make it 1 or less.

Regarding the free acid concentration, if it is too low, hydroxides of iron and zinc, etc. will adhere and the plating will turn black.
It was necessary to make it more than 1 / l. However, if it is too high, the composition ratio of iron will be low, and the current efficiency will be reduced.
It was necessary to make it g / l or less. Further, as a result of investigating the current density of the plating, the zinc content of the plating becomes high when the current density is low, and the adhesion becomes poor. Therefore, 100 A / dm ² or more was usually desired. But,
If the current density becomes too high, not only the control of the plating amount becomes difficult, but also the adhesiveness deteriorates, so it was necessary to set the upper limit to 700 A / dm ² . As the plating on the iron-zinc flash plating having a high iron content, it is possible to perform iron-zinc plating having a high zinc content or zinc plating to a required thickness.

Next, a method of performing anode electrolysis will be described. In the present invention, an aluminum oxide film is formed on the surface by anodic treatment. In general, an oxide film has a high electric resistance and therefore cannot be directly plated. However, when performing the iron-zinc flash plating of the present invention, as a result of an experiment, iron having a good adhesion of a composition ratio of iron of 50% or more-
It has been found to be effective in facilitating zinc flash plating. As shown in FIG. 1 (c), if the area of the aluminum oxide film seen from the surface at the time of iron-zinc flash plating is 60% or less of the plating area, sufficient adhesion can be obtained. It was

The anode treatment was carried out by alkaline degreasing or alkaline degreasing, followed by electrolysis in a plating solution using aluminum and aluminum alloy as an anode to form an oxide film. After the anode treatment, the plating is performed before the surface is changed, so it is better to switch the polarity to the cathode as it is in the plating solution. The thickness of the oxide film formed by the anode treatment can be controlled by the amount of electricity supplied, but if it is too thick, many unplated parts remain, so the amount of electricity supplied is 200 c / d.
m ² or less, preferably 150 c / dm ² or less. It was desired that the average film thickness of the oxide film be 100 Å or less. On the other hand, if the energization amount is too low, the oxide film is too small, so it is necessary to energize at least 10 c / dm ² . The generated oxide film may partially disappear during the passage of the plating solution or in the early stage of iron-zinc flash plating with a high iron content, and plating may be generated in that part. There was also a side where it was not possible to determine the optimum plating conditions.

The conditions of the plating solution after the anode treatment may be the same as the above-mentioned conditions. However, regarding the current density, if the anode treatment is performed before plating, 50 A / d
Plating with good adhesion was possible from a relatively low current density of m ² or more, preferably 70 A / dm ² or more. This is because a substantial part of the surface is covered with an oxide film having a large electric resistance by anodizing, so the actual plating area at the time of initial electrodeposition is reduced, so compared with the case where anodizing is not performed, It is considered that the electrodeposition was substantially carried out at a high current density even when the current was small, and the iron-zinc plating having a high iron content was generated in the plating near the interface with aluminum and the aluminum alloy. As a result of the experiment, when the proportion of zinc in the iron-zinc plating is high, the zinc is preferentially dissolved by the plating solution during the contact with the plating solution after energization and further plating on the plating solution. It was also found that the adhesiveness of was impaired. Even when the anode treatment was performed, the current density had an upper limit, and when the current density was 600 A / dm ² or more, there was a problem in adhesion.

The amount of iron-zinc plating deposited depends on the thickness of the oxide film when an oxide film is formed by anodic treatment, but in order to make the aluminum underlayer invisible under a SEM of 2000 times. In some cases, 7 g / m ^{2 was} required.
However, when the plating of the lower layer does not cover the entire surface of the aluminum or aluminum alloy due to the anode treatment, it is difficult to perform plating on it. If the unplated area is extremely small, the current density should be as small as possible when plating on it, eg 5 A / dm
^In some cases, it could be covered by setting it to ² or less.

The method shown in FIG. 2, for example, can be considered as the step of performing the plating of the present invention. FIG. 2A shows the first tank 1 while the aluminum and aluminum alloy strips 1 are being conveyed through the conveying roll 12 and the sink roll 11.
In the iron-zinc plating solution 13 of No. 0, the counter electrode 6 was used to conduct high-current density energization, the iron-zinc flash plating with a large iron content was performed, and the zinc-based plating solution 14 in the next tank 10 was used.
In this method, zinc-based plating is performed with the counter electrode 7. Further, FIG. 2B shows the first iron-zinc plating solution 1 in the tank 10.
Anode treatment is performed using the counter electrode 5 on the inlet side of No. 3, iron-zinc flash plating having a high current density and high iron content is performed using the counter electrode 6 on the outlet side, and zinc is applied using the counter electrode 7 of the next tank 10. This is a method of plating the system.

When iron-zinc plating is performed on the upper layer, the fact that the iron content depends on the plating current density is utilized, and for example, as shown in FIG. 2 (c), the band 1 of aluminum and aluminum alloy is first counter electrode. 6 and 6 are flash-plated with a high current density, and a separator 8 of an insulator is sandwiched between the counter electrode 8
It is also conceivable to perform two-layer plating by providing an electric current having a current density capable of obtaining iron-zinc plating having a predetermined zinc content. Further, as shown in FIG. 2D, it is also conceivable to provide the counter electrode 5 in the first pass and perform the anode treatment, and then perform the same treatment as the treatment of FIG. 2C. It is also possible to change the plating composition in the cross-sectional direction by disposing the anode obliquely with respect to the strip, and making the gap between the anode and the strip small at the initial stage of plating and increasing it at the plating outlet side. However, it is difficult for these methods to perform plating with a composition greatly different.

[0019]

【Example】

Example 1 A cold-rolled strip of an aluminum alloy (Al-5.5% Mg) having a plate thickness of 1.0 mm used as a member for automobiles was manufactured at a temperature of 70 ° C. of 60 g / l FC315 (Nippon Parkerizing Co., Ltd., non-etching type). 1 for weak alkaline cleaner)
It was immersed for 20 minutes per minute, washed with water, then pickled with 40 g / l sulfuric acid at 60 ° C., and washed with water. Using this material as a cathode, at a temperature of 60 ° C., a ferrous ion 95 g / l, a zinc ion 10 g / l, a free acid 45 g / l, and a current density of 200 in a solution containing additives.
Electrolyze at A / dm ^{2 for} 0.3 seconds and deposit plating amount 1 g /
m ² and iron-zinc flash plating with an iron content of 70% in the plating, followed by electrolysis for 4 seconds at a current density of 100 A / dm ^{2 with} a solution of zinc ions of 110 g / l and free acid of 50 g / l at 50 ° C. It was washed with water and dried. Stick cellophane tape on the plating of the aluminum alloy plate thus obtained,
After rubbing it with an eraser and pushing out the air inside, the cellophane tape was rapidly peeled off. The adhesion of the plating was examined by checking whether the plating adhered to the side of the cellophane tape, but there was no peeling at all.

Example 2 A cold-rolled strip of an aluminum alloy (Al-5.5% Mg) having a plate thickness of 1.0 mm used as a member for automobiles was manufactured at a temperature of 70 ° C. of 60 g / l FC315 (manufactured by Nippon Parkerizing Co., Ltd.). 1 for non-etching type weak alkaline cleaner)
It was immersed for 20 seconds per minute and washed with water. This material is ferrous iron at 60 ° C. 95 g / l, zinc ion 10 g / l, free acid 45 g / l
First, in a solution containing an additive in 1 liter, an electrolytic treatment was first performed as an anode at a current density of 3 A / dm ^{2 for} 18 seconds, and an aluminum alloy was used as a cathode in the same solution at a current density of 100 A.
Electrolysis at 0.8 psi / dm ^{2 for} 0.8 seconds, coating weight 1g / m
^2. After iron-zinc flash plating with an iron content of 75% in the plating, electrolysis was performed for 1 second at a current density of 100 A / dm ^{2 with} a solution of zinc ions of 110 g / l and free acid of 50 g / l at 50 ° C. It was washed with water and dried. Stick cellophane tape on the plating of the aluminum alloy plate thus obtained,
After rubbing it with an eraser and pushing out the air inside, the cellophane tape was rapidly peeled off. The adhesion of the plating was examined by checking whether the plating adhered to the side of the cellophane tape, but there was no peeling at all.

Example 3 A cold-rolled strip of an aluminum alloy (Al-5.5% Mg) having a plate thickness of 1.0 mm used as a member for automobiles was treated with 60 g / l of fine cleaner 315 (Nihon Parkerizing ( Co., Ltd., non-etching type weak alkaline cleaner) for 1 minute 20 seconds, washed with water, and then at 60 ° C for 4 minutes.
It was pickled with 0 g / l sulfuric acid and washed with water. Using this material as a cathode, electrolysis is carried out at 60 ° C. in a solution containing 81 g / l of ferrous ions, 23 g / l of zinc ions, 52 g / l of free acid and an additive at a current density of 50 A / dm ^{2 for} 1.3 seconds, After the iron-zinc flash plating with a coating weight of 1 g / m ² and an iron content rate of 40% in the plating, zinc ions at 50 ° C. 11
Current density of 100 with a solution of 0 g / l and free acid 50 g / l
It was electrolyzed at A / dm ^{2 for} 4 seconds, washed with water and dried. In this way, the condition of the current density of the iron-zinc flash plating is treated outside the range of 100 A / dm ² or more of the present invention, and the cellophane tape is applied to the plating of the aluminum alloy plate in which the iron content in the plating deviates from 50% or more. After sticking, rubbing with an eraser from above and pushing out the air inside, the cellophane tape was rapidly peeled off. When the adhesion of the plating was examined by checking whether the plating adhered to the side of the cellophane tape, it was completely peeled off.

Example 4 A cold-rolled strip of an aluminum alloy (Al-5.5% Mg) having a plate thickness of 1.0 mm used as a member for automobiles was treated with a fine cleaner 315 (Nihon Parkerizing Co., Ltd., non-etching type weak alkaline cleaner) was immersed for 1 minute 20 seconds and washed with water. 81 g / l of ferrous ion, zinc ion 23 at 60 ° C
g / l, free acid 52 g / l in a solution containing an additive, first pass through a path in which a lead electrode is installed as a counter electrode, current density 3 A
/ Dm ^{2 for} 18 seconds and then led to a path in the same liquid in which a lead electrode was installed as a counter electrode, and an aluminum alloy was used as a cathode at a current density of 40 A / dm ² .
Electrolyze for 5 seconds, deposit 1g / m ² of plating, iron-zinc flash plating with an iron content of 40% in the plating, then 5
Electrolysis was performed for 4 seconds at a current density of 100 A / dm ^{2 with} a solution of zinc ions of 110 g / l and free acid of 50 g / l at 0 ° C., followed by washing with water and drying. In this way, when the anode treatment is performed in the iron-zinc flash plating solution and the iron-zinc flash plating is performed as the cathode as it is, the current density condition is outside the range of 50 A / dm ² or more of the present invention, and the iron in the plating is applied. Stick a cellophane tape on the plating of the aluminum alloy plate whose content is out of 50% or more, rub it with an eraser from it, push out the air inside, and then suddenly peel off the cellophane tape, and the plating will stick to the cellophane tape side When the adhesion of the plating was examined by whether or not it was completely peeled off.

[0023]

EFFECTS OF THE INVENTION By plating the coating composition of the present invention, zinc-based plating with excellent adhesion to aluminum and aluminum alloy, which has been difficult in the past, can be easily performed.

[Brief description of drawings]

FIG. 1 (a) is an example of iron-zinc flash plating having a high iron content on aluminum and an aluminum alloy, and FIG. 1 (b) is anodizing aluminum and an aluminum alloy to form an oxide film on the surface. An example in which iron-zinc flash plating with a high iron content is applied on top, and further zinc-based plating is applied, (c) is a 2000 times enlarged view of the surface where iron-zinc flash plating in (b) is applied. Figure,

FIG. 2 is an example of an apparatus for performing iron-zinc alloy flash plating having a high iron content and excellent zinc-based plating having excellent adhesion to aluminum and aluminum alloy strips of the present invention, (a) is an iron having a high iron content -Example of an apparatus for performing zinc alloy flash plating and zinc-based plating, (b) is an example of an apparatus for performing iron-zinc alloy flash plating and zinc-based plating after anodization, and (c) is direct iron-containing High amount of iron
An example of an apparatus for performing zinc alloy flash plating and zinc-based plating with a higher zinc content, (d) shows iron-zinc alloy flash plating with a higher iron content and zinc-based plating with a higher zinc content after anodizing It is a figure which shows the example of an apparatus.

[Explanation of symbols]

1 Aluminum, aluminum alloy band 2 Oxide film 3 Iron-zinc flash plating with high iron content 4 Zinc-based plating 5 Counter electrode for anode treatment 6 Counter electrode for iron-zinc alloy flash plating with high iron content 7 Zinc-based plating counter electrode 8 Counter electrode on which iron-zinc system plating having a higher zinc content is applied 9 Insulating partition wall 10 Processing tank 11 Sink roll 12 Conveying roll 13 Iron-zinc plating bath 14 Zinc system plating bath

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[Procedure amendment]

[Submission date] May 25, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

It has been found that zinc-based plating is preferably applied when simultaneously treating aluminum or an aluminum alloy with a steel sheet as described above, but it is difficult to apply zinc-based plating to aluminum and aluminum alloys, and the latest surface treatment is required. The displacement plating method as described in Technical Guide (published by the Industrial Technology Service Center in April 1988), or JP-A-51-1332 and JP-A-51-4754.
No. 9, or JP-A-4-35019 , there have been attempts such as a method in which an oxide film is formed by anodic treatment and then zinc-based plating is performed thereon. In particular, the method of plating after the anode treatment was excellent in productivity, and it was possible to treat aluminum and aluminum alloys continuously in a short time.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

[0004]

In comparison with such a conventional technique, it is possible to perform processing at a higher productivity and at a higher speed and at a lower cost. It is desired that the quality characteristics are appropriate as a coating base and that the characteristics after coating are the same as or superior to those of a steel sheet or a galvanized steel sheet even when treated simultaneously with a steel sheet or a galvanized steel sheet. For that purpose, aluminum and aluminum alloys having a zinc-based plating layer with more excellent adhesion than ever are required.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

As the aluminum and aluminum alloy of the present invention, in addition to high-purity aluminum having a plate thickness of 2.0 mm or less, those containing 10% or less of magnesium such as 5000 series or 6000 series and those alloyed with copper can be used. it can. From the viewpoint of surface treatment, it is desirable that the amount of alloying metal is as small as possible. The surface may be as-rolled or subjected to heat treatment, but it is desirable that the oxide film be as thin as possible and the rust preventive oil be as small as possible. It is necessary to determine the processing conditions for alkaline degreasing and pickling depending on the state of adhesion of oxide film, oil and dirt to obtain an appropriate plating surface.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

Next, a method of performing anode electrolysis will be described. In the present invention, an aluminum oxide film is formed on the surface by anodic treatment. In general, an oxide film has a high electric resistance and therefore cannot be directly plated. However, when performing the iron-zinc flash plating of the present invention,
As a result of the experiment, good adhesion iron composition ratio of 50% or more of iron - has been found to be effective to facilitate the zinc flash plating. As shown in FIG. 1 (c), iron-
If the area of the aluminum oxide film viewed from the surface at the stage of zinc flash plating was 60% or less of the area of plating, sufficient adhesion could be obtained.

Claims (4)

[Claims] 1. An iron-zinc flash plating layer having an iron content of 50 to 100% (hereinafter referred to as "weight%") having a thickness of 0.
Aluminum and aluminum alloy having an excellent adhesiveness, which has a zinc-based plating layer on the surface of the aluminum alloy and has an amount of 01 to 7 g / m ² . 2. An aluminum oxide film is formed on the surface of aluminum and an aluminum alloy, and the ratio of iron is 5 on the aluminum oxide film.
Aluminum and aluminum alloys having a 0 to 100% iron-zinc flash plating layer with a thickness of 0.01 to 7 g / m ^{2 and} having a zinc-based plating layer thereon and having excellent adhesion. 3. To produce the aluminum and aluminum alloys according to claim 1, ferrous ions are added to 20
~ 100 g / l, zinc ion 50 g / l or less, current density 100-70 with plating solution containing 5-80 g / l free acid
Flash plating at 0 A / dm ² or higher, followed by zinc-based electroplating. 4. Ferrous ions are added in an amount of 20 to produce the aluminum and aluminum alloys of claim 2.
~ 100 g / l, zinc ion 50 g / l or less, a plating solution containing 5-80 g / l of a free acid, and an electric quantity of 10-200 c using aluminum and an aluminum alloy as an anode.
/ Dm ² , electrolysis treatment is performed, and then the cathode is directly subjected to flash plating at a current density of 50 to 600 A / dm ² or more, and then zinc-based electroplating.