JP2509940B2 - Method for producing Zn-Ni alloy plated steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a surface-treated steel sheet having excellent corrosion resistance, chemical conversion treatability and paint adhesion, and the product obtained by the present invention is The use of excellent corrosion resistance, chemical conversion treatability and paint adhesion for anti-corrosion steel sheets for automobiles, home appliances, building materials, etc. produces a great economic advantage.

(Prior Art) Conventionally, electrogalvanized steel sheets, hot-dip galvanized steel sheets, or various alloy-plated steel sheets have been produced and widely used for home appliances, automobile body anticorrosion steel sheets, building materials, and the like. Under these circumstances, in recent years, there has been a strong demand for a surface treatment material having particularly excellent corrosion resistance, and the number tends to increase in the future.

For example, in the automotive industry, recent changes in the environment, such as corrosion due to rock salt to prevent freezing of roads in winter in North America and Northern Europe, and corrosion due to acid rain caused by SO ₂ gas generation in industrial areas There is a strong demand for a surface-treated steel sheet that is exposed to a severe corrosive environment and has excellent corrosion resistance from the viewpoint of safety. In addition, in the home electric appliance industry, there is a demand for a steel sheet having excellent corrosion resistance that can be used without a coating, from the viewpoint of process saving and cost saving.

Various studies have been made to solve these problems, and many products have been developed.

In particular, in the case of electrogalvanized steel sheets, it is already known as Japanese Patent Publication No. 56-1400 that plating is performed in an electrogalvanizing bath in which lead is suppressed to 3 ppm or less in order to improve the chemical conversion treatability among these drawbacks. This indicates that the galvanized steel sheet obtained by electroplating in a plating bath in which lead is suppressed to 3 ppm or less suppresses the occurrence rate of abnormal film growth during phosphate treatment when the chemical conversion treatment is performed.

Further, JP-B-48-69627 and JP-B-53-18170 disclose a method for removing impurities such as Pb ⁺⁺ from an electrogalvanizing bath.

Thus, in electrogalvanized steel sheets, Pb ⁺⁺ in the bath
Although a method of suppressing the concentration to improve the partial chemical conversion treatment property is known, corrosion resistance, paint adhesion (particularly water resistance adhesion), etc. are not improved at all.

Various alloy-plated steel sheets have been developed as a method for improving the corrosion resistance of such galvanized steel sheets. As these alloy-plated steel sheets, for example, Zn-Ni containing Zn as a main component relative to Zn, Zn-Ni such as Zn-Ni-Co and Zn-Ni-Cr.
An example is a system alloy plated steel sheet. These Zn-Ni
It is recognized that the base alloy plating improves the bare corrosion resistance by about 3 to 5 times as compared with the ordinary electrogalvanized steel sheet.

There is also a method of performing chromate treatment after plating in order to further improve the corrosion resistance, which is quite effective, but a chemical conversion film is not formed on the chromate film. Further, generally in the Zn-Ni alloy plated steel sheet with improved corrosion resistance, the chemical conversion film is difficult to form, the coating adhesion is correspondingly reduced, and excellent corrosion resistance, chemical conversion treatment and coating adhesion are simultaneously satisfied. There is no type alloy plated steel sheet.

(Problems to be solved by the invention) On the other hand, the present invention is a method for significantly improving the corrosion resistance, the chemical conversion treatment property and the paint adhesion of the formed plated steel sheet in producing the Zn-Ni alloy plated steel sheet. Yes, the plated steel sheet obtained by the present invention has ultra-high corrosion resistance, and when a chemical conversion treatment is performed thereon, it easily forms a fine and dense chemical conversion film in any commercially available chemical conversion treatment bath. It is also possible to secure excellent adhesion for the coating film after coating.

(Means for Solving Problems) The inventors of the present invention investigated in detail the behavior of trace elements contained in the bath when producing a Zn—Ni alloy plated steel sheet. As a result, it was found that among the trace elements, Pb ⁺⁺ has a particularly large effect on the properties.

For example, when manufacturing a Zn-Ni-Co alloy plated steel sheet, as an impurity of a reagent or an electrode (generally Pb-Sn alloy,
(Pb-Ag alloys are often used as electrodes)
A trace amount of Pb ⁺⁺ is mixed in or eluted in the plating bath, and a small amount of co-deposited in the Zn-Ni-Co alloy plating layer during electrodeposition. While other elements have almost no effect, Pb ⁺⁺ , when co-deposited in the plating layer, causes strain in the crystal lattice of the Zn-Ni-Co alloy, even if it is very slight, and weakens the interatomic bond energy. I understood. As a result, the formed Zn-Ni-
It was found that the Co alloy is likely to be eluted as ions in a corrosive environment, and therefore the corrosion resistance is reduced.

As a result of further detailed study by the present inventors, Pb ^{++ in} the bath
It was found that the concentration of s was significantly different at the boundary of 1 ppm.
That is, if the Pb ⁺⁺ concentration in the bath is 1 ppm or less, Pb during electrodeposition
^It was found that even if ⁺⁺ was co-deposited, no strain was generated in the crystal lattice of the Zn-Ni-Co alloy, and thus the formed Zn-Ni-Co alloy had extremely excellent corrosion resistance. On the other hand, Pb
Zn-Ni-Co alloy formed from a bath containing ⁺⁺ 1 ppm or more,
The strain generated in the crystal lattice gradually increases depending on the concentration of Pb ⁺⁺ , and the corrosion resistance decreases accordingly.

On the other hand, Zn deposited from the bath with Pb ⁺⁺ concentration of 1 ppm or less
When a conversion treatment is performed on a -Ni-Co alloy, a conversion coating is hardly formed. This is because the formed Zn-Ni-Co alloy plating is extremely excellent in corrosion resistance, so that ions are difficult to elute from the plating layer during the chemical conversion treatment, and thus a chemical conversion film crystal is less likely to be formed.

On the other hand, as a result of further studies by the present inventors, the Pb ^{+ +} concentration in the bath was 1 ppm or less, and the Zn-Ni-Co alloy electrodeposited from the bath was subjected to electrolytic reduction treatment in a specific bath, the plating layer surface was It has been found that it is possible to obtain a plated steel sheet that becomes active and easily forms a chemical conversion film crystal, and therefore has excellent paint adhesion.

As shown above, the present inventors have high corrosion resistance, chemical conversion treatability,
It was found that the following conditions were obtained in order to obtain a Zn-Ni alloy-plated steel sheet that sufficiently satisfies paint adhesion and paint corrosion resistance. That is, the method is characterized in that electrolytic plating is performed from a Zn-Ni alloy plating bath having a Pb ⁺⁺ concentration of 1 ppm or less, and further electrolytic reduction treatment is performed with a specific solution.

It was confirmed that by satisfying the above conditions, it is possible to produce a plated steel sheet having excellent corrosion resistance, chemical conversion treatability, paint adhesion, and coating corrosion resistance. The method for greatly improving the corrosion resistance, chemical conversion treatment property, paint adhesion and coating corrosion resistance of the present invention will be specifically described below.

Table 1 shows the production of Zn-Ni alloy plated steel sheets.
Electrolytically in a Zn-Ni alloy plating bath with a Pb ⁺⁺ concentration of 0 to 1 ppm
Zn-Ni alloy plating (deposition amount = 30g / m ² , Ni = 12.0%),
After that, a cathodic electrolysis treatment (C treatment) was performed in a weakly acidic (pH = 5.0) sodium dihydrogen phosphate bath (NaH ₂ PO ₄ .2H ₂ O = 100 g /) for D _k = 10 A / dm ² × 1.5 seconds. The results of corrosion resistance, chemical conversion treatability, paint adhesion, and coating corrosion resistance are shown with and without.

Here, the corrosion resistance was examined by a salt spray test according to JIS-Z-2371 standard (saline concentration 5%, bath temperature 35 ° C, spray pressure 20 psi), and the rust condition after 800 hours was investigated, and ◎, ○, △ ，
The evaluation was made in 5 grades of x and xx, and ⊚ is the best.

◎: Red rust generation 0% ○: 〃 0 to 1% △: 〃 1 to 10% ×: 〃 10 to 50% XX: 〃 50% or more Chemical conversion treatment was performed using a commercially available chemical conversion treatment bath, and the appearance was observed after the treatment. Also, evaluation was performed by observing with a 1000 times SEM image. The evaluation was made in five grades of ⊚, ○, Δ, ×, and XX, and ⊚ is the best.

⊚: Fine and dense chemical conversion crystals are uniformly formed ○: Chemical conversion crystals are slightly large, but uniform conversion crystals are formed Δ: Partially coarsened crystals are formed, lacking uniformity ×: Coarsely formed crystals are formed , Lack of uniformity, some scale is generated XX: Almost all scale is generated Also, the water-resistant adhesion of the coating film is obtained by using a commercially available chemical conversion treatment bath.
After the treatment, the commercially available ED coating is also applied to 20μ, and the melamine alkyd coating is applied 30μ each in the middle coating and the top coating, and it is diluted with distilled water at 40 ° C to 10μ.
Immersion was carried out for a day, and immediately after the completion of the immersion, the film was cut on a 2 mm goggle and the tape was peeled off.

◎: Peeling area 0% ○: 〃 0 to 1% △: 〃 1 to 10% ×: 〃 10 to 50% XX: 〃 50% or more Moreover, the corrosion resistance after coating is the same as the above 3 coated materials A cross-cut was put into the sample and exposed to the atmosphere sprayed with salt water (for 6 months). The cellophane tape was peeled off, and the peeled width (one side) was obtained and evaluated.

◎: Peeling width 0 to 0.2 mm ○: 〃 0.2 to 1 mm △: 〃 1 to 3 mm ×: 〃 3 to 5 mm XX: 〃 5 mm or more As is clear from Table 1, by performing the cathodic electrolysis treatment (C treatment), excellent corrosion resistance is ensured as it is, and the chemical conversion treatment property, the water-resistant adhesion of the coating film and the corrosion resistance after coating are greatly improved.

It is considered that the characteristics including the chemical conversion treatment property are improved by performing the cathode electrolysis treatment (C treatment) as described above.

Generally, in electroplating, when plating is completed, the surface is coated with an extremely thin oxide such as zinc oxide, but when cathodic electrolysis is performed, zinc oxide is removed and the vicinity of the surface becomes active, forming a chemical conversion film. Easily formed. It is also believed that the formation of an excellent chemical conversion film ensures the water resistant adhesion of the coating film.

Also, with the formation of an excellent chemical conversion film, the Pb ⁺⁺ concentration is 0-
It is considered that the plating layer formed from the bath adjusted to 1 ppm exhibits extremely excellent corrosion resistance in a corrosive environment, and the elution of ions from the plating layer is suppressed, so that excellent coating corrosion resistance is secured.

The treatment bath for activating the plated layer after plating in the present invention may be any of an acidic bath, a neutral bath and an alkaline bath.

In addition, the above results are almost the same whether the C-A treatment (cathode-anode electrolysis treatment), A-C treatment (anode-cathode electrolysis treatment) or A treatment (anode treatment) is performed under specific conditions. Was obtained.

The above results were explained for Zn-Ni alloy plating.
The same was true for the -Ni-Co and Zn-Ni-Cr alloy plating.

As a result, in the present invention, in producing a Zn-Ni-based alloy plated steel sheet, after electroplating from a Zn-Ni-based alloy plating bath having a Pb ⁺⁺ concentration of 0 to 1 ppm, the plated steel sheet is acidic, neutral, or alkaline. C treatment (cathodic electrolysis treatment), CA treatment (cathode-anode electrolysis treatment), A in either liquid
-By performing either C treatment (anode-cathode electrolysis treatment) or A treatment (anode electrolysis treatment), corrosion resistance is maintained as it is, chemical conversion treatment property and paint adhesion are improved, and coating corrosion resistance is greatly improved. An excellent zn-Ni alloy alloy plated steel sheet can be obtained.

(Example) The following describes the example.

Example 1 Using a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration was adjusted to 0.2 ppm, the plating adhesion amount was 20 g / m ² by electroplating, Ni concentration =
11.0% Zn-Ni alloy plating, immediately followed by H ₂ SO ₄ 30g
/ D _k = 10 A / dm ² × 2 sec cathodic electrolysis treatment was performed in a bath.

Example 2 Using a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration was adjusted to 0.1 ppm, the amount of plating adhered by electroplating was 20 g / m ² , Ni concentration =
11.2% Zn-Ni alloy plating, immediately followed by H ₂ SO ₄ 30g
/ CA treatment in the bath (cathode-anode electrolytic treatment: 10
A / dm ² × 1 sec) was performed.

Example 3 A Zn-Ni alloy plating bath whose Pb ⁺⁺ concentration was adjusted to 0.8 ppm was used, and the amount of plating adhered by electroplating was 20 g / m ² , Ni concentration =
10.5% Zn-Ni alloy plating, immediately followed by H ₂ SO ₄ 20g
/ AC treatment in the bath (cathode-anode electrolytic treatment: 10
A / dm ² × 1.5 sec) was performed.

Example 4 A Zn-Ni alloy plating bath having a Pb ⁺⁺ concentration adjusted to 0.05 ppm was used to perform Zn-Ni alloy plating with a coating weight of 20 g / m ² and a Ni concentration of 11.1% by electroplating, followed immediately by NaH. ₂ PO
₄ · 2H ₂ O150g / bath at A processing (anodization process: D _k = 1
0 A / dm ² × 0.5 sec) was performed.

Using Example 5 Pb ⁺⁺ concentration Zn-Ni-Co alloy plating bath was adjusted to 0.9ppm the coating weight by electroplating 20 g / m ^2, Ni concentration = 10.2%, Co concentration = 2.5% Zn- Ni-Co alloy plating was performed, and immediately thereafter, C treatment (cathodic electrolysis treatment: D _k = 10 A / dm ² × 1.5 sec) was performed in a bath of NaH ₂ PO ₄ · 2H ₂ O 100 g /.

Comparative Example 1 Using a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration was adjusted to 0.7 ppm, the amount of plating adhered by electroplating was 20 g / m ² , Ni concentration =
A 10.2% Zn-Ni alloy plated steel sheet was produced.

Comparative Example 2 Using a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration was adjusted to 8 ppm, the plating deposition amount by electroplating was 20 g / m ² , Ni concentration =
A 10.9% Zn-Ni alloy plated steel sheet was produced.

Comparative Example 3 Using a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration was adjusted to 12 ppm, the amount of plating adhered by electroplating was 20 g / m ² , Ni concentration =
11.2% of and Zn-Ni alloy plating, followed immediately H ₂ SO ₄ 40 g
/ C treatment in the bath (cathode electrolysis treatment: 10A / dm ² × 1.6se
c) was performed.

Table 2 shows the results of various tests performed on the plated steel sheets obtained in Examples 1, 2, 3, 4, and 5 and Comparative Examples 1, 2, 3. The evaluation method conforms to Table 1.

(Effect of the Invention) Conventionally, there has not been a plated steel sheet that simultaneously sufficiently satisfies corrosion resistance, chemical conversion treatability, paint adhesion and post-coating corrosion resistance. On the other hand, the present invention, when producing a Zn-Ni alloy plated steel sheet, after electroplating from a Zn-Ni alloy plating bath having a Pb ⁺⁺ concentration adjusted to 0 to 1 ppm, is acidic, neutral, or alkaline. In the solution of C (cathode electrolysis), C
-A treatment (cathode-anode electrolysis treatment), AC treatment (anode-cathode electrolysis treatment), and A treatment (anode electrolysis treatment) are performed to obtain corrosion resistance, chemical conversion treatment property, paint adhesion and It is possible to manufacture a plated steel sheet having excellent corrosion resistance after painting, and the economical effect thereof is extremely large by applying the present invention.

Claims (1)

(57) [Claims] 1. When manufacturing a Zn-Ni alloy plated steel sheet, electroplating is performed from a Zn-Ni alloy plating bath in which the Pb ⁺⁺ concentration is adjusted to 0 to 1 ppm, and the plated steel sheet is then acid, neutral,
C treatment (cathode electrolysis treatment), C-A treatment (cathode-anode electrolysis treatment), A in either alkaline solution
Zn-Ni alloy excellent in corrosion resistance, chemical conversion treatability, paint adhesion and coating corrosion resistance, characterized by performing either C treatment (anode-cathode electrolysis treatment) or A treatment (anode electrolysis treatment) Manufacturing method of plated steel sheet.