JP2636589B2 - Zinc-nickel-chromium alloy electroplated steel sheet with excellent corrosion resistance, plating adhesion and chemical conversion treatment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc-nickel-chromium alloy electroplated steel sheet excellent in bare corrosion resistance, corrosion resistance after coating, plating adhesion and chemical conversion treatment, which is suitable as a steel sheet for automobiles.

[0002]

BACKGROUND OF THE INVENTION At least one surface has zinc-
BACKGROUND ART A zinc-nickel alloy electroplated steel sheet having a nickel alloy electroplated film formed thereon is widely used as a steel sheet for automobiles because of its excellent corrosion resistance. Recently,
In the automobile industry, the weight of automobile bodies has been actively reduced in order to save energy. For this reason, zinc-nickel with excellent corrosion resistance with a much thinner plate thickness and a smaller plating amount has been requested by automobile manufacturers. There is a demand for the development of base alloy electroplated steel sheets.

As a zinc-nickel alloy electroplated steel sheet having more excellent corrosion resistance, for example, JP-A-55-50484, JP-A-58-6796 and JP-B-59-38313 (hereinafter referred to as prior art 1). ) Has on at least one surface thereof
There is disclosed a zinc-nickel-chromium alloy electroplated steel sheet having a zinc-nickel-chromium alloy electroplated film formed thereon. According to the zinc-nickel-chromium alloy electroplated steel sheet of Prior Art 1, the corrosion resistance is improved as compared with the conventional zinc-nickel alloy electroplated steel sheet, but the chromium content in the plating film is 0.005 to 1.0 wt. It is about.% And extremely small. Therefore, the main effect of improving corrosion resistance still depends on the nickel content in the plating film. For example, in Japanese Patent Publication No. 59-38313 in Prior Art 1, the nickel content in the plating film is 11 to 20 wt. .%.

[0004]

However, as in the prior art 1, when the nickel content in the plating film is large, the surface properties and structure of the plating film become non-uniform,
In addition, the adhesion of the plating film is poor, and therefore, the adhesion of the coating film formed on the plating film is also poor. Therefore,
Such a zinc-nickel-chromium alloy electroplated steel sheet is not suitable as a steel sheet for automobiles.

On the other hand, JP-A-1-55397 (hereinafter referred to as Prior Art 2) discloses that at least one surface has
There is disclosed a zinc-chromium alloy electroplated steel sheet on which a zinc-chromium alloy electroplating coating composed of 40 wt.% Chromium and the balance of zinc is formed. According to Prior Art 2, an electroplated steel sheet having a zinc-chromium alloy electroplated film having a uniform surface property can be obtained.

However, in the zinc-chromium alloy electroplated steel sheet of prior art 2, when forming a phosphate film by chemical conversion treatment, which is important as a pre-coating treatment, on the plated film, crystals of the phosphate film are formed. There is a problem of coarsening and incomplete formation of a phosphate film. As described above, the zinc-chromium alloy electroplated steel sheet of the prior art 2 has a fatal effect on the chemical conversion treatment at the time of performing the chemical conversion treatment necessary for improving the adhesion of the coating film and improving the corrosion resistance after coating. There are defects.

Accordingly, it is an object of the present invention to provide a zinc-nickel-chromium alloy electroplated steel sheet which solves the above-mentioned problems and is excellent in bare corrosion resistance, corrosion resistance after coating, plating adhesion and chemical conversion treatment.

[0008]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, the following findings were obtained. In a zinc-nickel alloy electroplated steel sheet, if the nickel content in the zinc-nickel alloy electroplated film is about 11 wt.
^- and, the corrosion products formed by the Zn in the plating film, Zn- Ni alloy phase and a γ phase results to be deposited in the crystal, zinc - to impart good corrosion resistance to the nickel alloy electroplating film It has been known.

Therefore, a zinc-nickel-chromium alloy electroplated film containing about 1 wt.% Chromium in addition to about 11 wt.
When formed on one surface, corrosion resistance is further improved, for example,
Red rust does not occur even after a salt spray test of about 10,000 hours. It is considered that the reason why the corrosion resistance is improved by the addition of chromium is that chromium in the plating film forms a hardly soluble and very stable protective film containing chromium in the surface layer of the plating film.

In the above-mentioned plated steel sheet having a zinc-nickel-chromium alloy electroplated film containing nickel in an amount of about 11 wt.% Or more, when the chromium content in the plated film is increased, the corrosion resistance is further improved. However, when a zinc-nickel-chromium alloy electroplated film having a high chromium content is formed on at least one surface of a steel sheet by ordinary electrolysis conditions, a so-called plating burn occurs and a plated film having an appropriate surface property is formed. It cannot be obtained, and the adhesion of the plating film also deteriorates.

On the other hand, in a zinc-chromium alloy electroplated steel sheet having a zinc-chromium alloy electroplating film containing no nickel, the chromium content in the plating film is 5 watts.
t. %, The phosphate coating becomes coarse and the phosphate coating becomes unhealthy in a chemical conversion treatment, which is important as a pretreatment for coating a plated steel sheet. Therefore, in addition to such chromium, 1 wt. % Of a zinc-nickel-chromium alloy electroplated coating containing at least
When formed on one surface, the phosphate film formed by the chemical conversion treatment becomes finer, and a sound phosphate film is formed. As a result, the adhesion of the film formed thereon is improved.

In view of the above, in a zinc-nickel-chromium alloy electroplated steel sheet having a zinc-nickel-chromium alloy electroplated film formed on at least one surface thereof, the nickel content in the plated film is reduced to 1
From 5 wt. % And the chromium content is 2 to 30 wt. %, It was found that a zinc-nickel-chromium alloy electroplated coating having excellent plating adhesion, chemical conversion properties, bare corrosion resistance and post-coating corrosion resistance was obtained.

The present invention has been made based on the above-described findings, and the present invention provides a zinc-nickel alloy having a zinc-nickel-chromium alloy electroplated film formed on at least one surface thereof. In the chromium alloy electroplated steel sheet, the zinc-nickel-chromium alloy electroplated coating is formed of nickel: 1 to 5 wt. %, Chromium: 2 to 30 wt. %, Balance: It is characterized by being composed of zinc and unavoidable impurities.

[0014]

Next, the reason why the composition of the plating film is limited as described above in the present invention will be described. (1) Nickel The content of nickel in the zinc-nickel-chromium alloy electroplated coating is 1 to 5 wt. It should be limited to the range of%. The nickel content in the plating film is 1 wt. %, The effect of improving the chemical conversion property cannot be obtained. On the other hand, when the nickel content is 5 wt. %, The surface properties of the plating film deteriorate, and the adhesion of the plating film deteriorates.

(2) Chromium The chromium content in the zinc-nickel-chromium alloy electroplated coating should be limited to the range of 2 to 30 wt.%. If the chromium content in the plating film is less than 2 wt.%, There is no effect of improving the chemical conversion treatment. On the other hand, the chromium content is 30w
If the content exceeds t.%, the phosphate film formed on the plating film by the chemical conversion treatment becomes coarse, and the phosphate film becomes unhealthy.

FIG. 2 shows a conventional zinc-chromium alloy electroplating film containing 11 wt.
It is an electron micrograph (750 times magnification) of a phosphate film when a phosphate film is formed by a chemical conversion treatment on a plating film of a chromium alloy electroplated steel sheet. From FIG. 2, it can be seen that the crystals of the phosphate coating are coarser than those of the plated steel sheet of the present invention shown in FIG. 1 described below.

FIG. 1 shows that 1 wt. % Nickel and 11 wt. % Chromium containing zinc-
On the zinc-nickel-chromium alloy electroplated steel sheet of the present invention, on which the nickel-chromium alloy electroplated film was formed, phosphoric acid was formed by a chemical conversion treatment under the same conditions as for the conventional plated steel sheet of FIG. Electron micrograph of the phosphate film when the salt film was formed (750 times magnification)
It is. As is clear from FIG. 1, the crystals of the phosphate film do not become coarse as in the case of the above-described plated steel sheet in which the zinc-chromium alloy electroplated film not containing nickel shown in FIG. It can be seen that the size has been reduced to the same level as that of the system-coated steel sheet.

The formation of the phosphate film on the surface of the plating film is carried out by using a known phosphating solution and performing a chemical conversion treatment by a known spray type or immersion type.

The preferred range of the thickness of the plating film, that is, the adhesion amount is 10 to 100 g / m ² per one side of the steel sheet. If the coating weight is less than 10 g / m ² per one side of the steel sheet, the corrosion resistance deteriorates. On the other hand, the coating weight of the plating film was 10
If it exceeds 0 g / m ² , weldability and workability deteriorate. The preferred coating weight is 20 to 50 g / m ² per side of the steel sheet.
Is within the range.

[0020] In addition to zinc, nickel and chromium, trace amounts of Co, Fe, Mn, Ti, F, B, N, C,
P and the like may be included.

Next, a method for producing the zinc-nickel-chromium alloy electroplated steel sheet of the present invention will be described. As the plating solution, an acidic plating solution containing predetermined amounts of Zn ²⁺ , Ni ²⁺ and Cr ³⁺ is used. In order to increase the conductivity of the plating solution, an alkali metal such as Na ⁺ , K ⁺ , NH ₄ ⁺ may be added to the plating solution, and polymers acting as a chromium precipitation promoter may be added. May be. As the anion, any of SO ₄ ²⁻ , Cl ⁻ , and BF ₄ ⁻ can be used.

The preferred pH value of the plating solution is 0.5 to 3.0. If the pH value of the plating solution is less than 0.5, the electrolysis efficiency decreases. On the other hand, when the pH value of the plating solution exceeds 3.0, a precipitate is easily generated in the plating solution. The preferred temperature of the plating solution is 30 to 70 ° C. When the temperature of the plating solution is lower than 30 ° C., undissolved substances are easily generated in the plating solution, and
The electrodeposition efficiency decreases. On the other hand, when the temperature of the plating solution exceeds 70 ° C., problems occur in terms of facilities such as a plating tank. The plating current density is not particularly limited because a plating film having a desired composition can be formed by changing the plating current density.However, as the current density is increased, the chromium content increases, and , Nickel content is not as high as chromium, but also tends to increase.

Instead of forming a phosphate film on the plating film by a chemical conversion treatment, a chromate film may be formed on the plating film by a chromate treatment. In the plating film of the plated steel sheet of the present invention, since the reactivity with the acidic treatment solution composed of Cr ³⁺ and Cr ⁶⁺ is relatively high, the formation of the chromate film is performed by a conventional reaction type, coating type, or electrolytic type. It can be performed by any of the chromate treatment methods.
The preferred amount of the chromate film is 5 to 100 m as chromium.
g / m ² . In addition, 0.5 to 2.5
An organic resin film in an amount of μm may be formed. As such an organic resin film, any conventionally used organic resin can be used. In particular, an organic resin containing a rust preventive additive such as SiO ₂ is effective.

[0024]

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples. The following A,
Three types of plating solutions B and C were used. Plating solution _{A ZnSO 4 7H 2 0: 100} ~500 g / l, NiSO 4 6H 2 0: 100 ~500 g / l, Cr 2 (SO 4) 3: 100 ~500 g / l, Na 2 SO 4: 60 g / l, pH value: 1.2

Plating solution B Zn (BF ₄ ) ₂ : 10 to 100 g / l, Ni (BF ₄ ) ₂ : 10 to 100 g / l, H ₃ BO ₃ : 60 g / l, Cr (BF ₄ ) ₃ : 10 to 100 g / l, pH value: 2.0

Plating solution C: ZnCl ₂ : 10 to 300 g / l, NiCl ₂ 6H ₂ O: 10 to 300 g / l, NaCl: 20 g / l, CrCl ₃ : 10 to 200 g / l, pH value: 2.5

Using a plating solution having a component composition and a pH value of any one of the above-mentioned A, B and C and having a temperature of 50 to 60 ° C., changing the current density within the range of 30 to 150 A / dm ^2. Then, a zinc-nickel-chromium alloy electroplating coating in an amount of 20 g / m ² was formed on one surface of a steel plate having a thickness of 0.7 mm, and zinc-nickel within the scope of the present invention shown in Table 1 was obtained. Specimen having a chromium alloy electroplated coating (hereinafter referred to as the specimen of the present invention) 1 to 8 were prepared. For comparison, a test piece having a zinc-nickel-chromium alloy electroplating coating out of the range of the present invention (hereinafter referred to as a comparative test piece) No. 2 shown in Table 2 was used. 1 to 8 were prepared.

[0028]

[Table 1]

[0029]

For each specimen thus prepared,
The tests described below were performed. (1) Adhesion test of plating film This test is based on JIS H 8504, and each specimen is bent at a predetermined angle, and then an adhesive tape is applied to the alloy plating film of the bent specimen and then peeled off. The peeling amount of the plating film was visually inspected and evaluated based on the peeling amount as described below. ：: good, △: slightly bad, ×: bad

(2) Chemical conversion test Each specimen was subjected to a immersion type phosphate treatment with a phosphating solution to form a phosphate film on the surface of the specimen. The crystal state of the obtained phosphate film was observed by an electron microscope and evaluated as follows. ：: dense crystal, ×: coarse or unhealthy crystal

(3) Bare corrosion resistance test A test in accordance with JIS-Z-2371, in which each specimen was subjected to a salt spray test, and the time until red rust was generated was examined and evaluated.

(4) Corrosion resistance test after painting Each specimen was subjected to a immersion type phosphate treatment with a phosphating solution to form a phosphate film on the surface of the specimen, and then a cationic type electrode was formed. Applying a coating process,
An undercoating film having a thickness of 20 μm was formed on the phosphate film. Then, a 30 μm thick intermediate coat and a 30 μm thick top coat were formed on the surface of the undercoat.
A cross-cut is made on the coating film of the specimen having the three-layer coating film obtained in this manner, and the specimen with the cross-cut is subjected to one cycle of salt spray, drying, wetting, and drying per day. After 120 cycles of the corrosion promotion test, the blister width generated at the cross cut portion was examined and evaluated according to the following. ◎: blister width 0 to less than 1 mm, ○: blister width 1 mm to less than 3 mm, Δ: blister width 3 mm to less than 5 mm, ×: blister width 5 mm or more

(5) Appearance The surface appearance of the specimen was evaluated by visual observation.

Table 3 shows the test results of the above-mentioned specimens.

[0036]

[Table 3]

As is clear from Tables 1 to 3, comparative nickel specimens having a large nickel content in the plating film beyond the scope of the present invention and a small chromium content outside the scope of the present invention.
In No. 1, the adhesion of the plating film and the corrosion resistance after painting were slightly poor and the surface appearance was poor. Comparative Sample Nos. 2 to 5 in which the amount of nickel in the plating film was very large beyond the range of the present invention, the adhesion of the plating film was poor, the corrosion resistance after painting was slightly poor, and the surface appearance was poor. .

The comparative sample No. 6 having a very large nickel content exceeding the range of the present invention and a small chromium content outside the range of the present invention has excellent adhesion of the plating film and corrosion resistance after coating. However, it was somewhat poor and the surface appearance was poor. Comparative sample No. 7 having a chromium content exceeding the range of the present invention has a poor chemical conversion property and corrosion resistance after painting,
The adhesion of the plating film was also slightly poor. Comparative specimen No. 8 having a low nickel content outside the scope of the present invention
Was poor in chemical conversion treatment and corrosion resistance after painting.

On the other hand, the specimen No. 1-8
Was excellent in all of the adhesion of the plating film, the chemical conversion treatment, the bare corrosion resistance, the corrosion resistance after painting, and the surface appearance.

[0040]

As described above, according to the present invention,
Industrially useful effects can be obtained, whereby a zinc-nickel-chromium alloy electroplated steel sheet excellent in plating adhesion, chemical conversion treatment, bare corrosion resistance, corrosion resistance after painting, and surface appearance can be obtained.

[Brief description of the drawings]

FIG. 1 is an electron micrograph (750-fold magnification) showing a crystal structure of a phosphate film formed on a plated film of a zinc-nickel-chromium alloy electroplated steel sheet of the present invention.

FIG. 2 is an electric micrograph (magnification: 750 times) showing a crystal structure of a phosphate film formed on a plating film of a conventional zinc-chromium alloy electroplated steel sheet.

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein the at least one surface comprises zinc-
In a zinc-nickel-chromium alloy electroplated steel sheet having a nickel-chromium alloy electroplated film formed thereon, the zinc-nickel-chromium alloy electroplated film has a nickel content of 1 to 5 wt. %, Chromium: 2 to 30 wt. %, Balance: Zinc-nickel-chromium alloy electroplated steel sheet excellent in corrosion resistance, plating adhesion and chemical conversion treatment, comprising zinc and unavoidable impurities.