JP2569993B2 - Method for producing chromate-treated galvanized steel sheet with excellent corrosion resistance, fingerprint resistance and paintability - 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 producing a chromate-treated zinc or zinc alloy plated steel sheet having excellent corrosion resistance, fingerprint resistance and paintability.

[0002]

2. Description of the Related Art A zinc-coated steel sheet or a zinc-alloy-coated steel sheet (hereinafter, referred to as a "zinc-based plating steel sheet") having a zinc-plated layer or a zinc-alloy-plated layer (hereinafter referred to as "zinc-based plating layer") formed on at least one surface thereof. Coated steel sheet)
In order to improve its corrosion resistance and paintability,
A chromate treatment is performed to form a chromate film on the zinc-based plating layer.

[0003] A chromate treatment method includes a reaction type chromate treatment comprising reacting a plating layer with a chromate treatment solution by spraying or dipping to form a chromate film on the surface of the plating layer. There are a coating-type chromate treatment in which a chromate treatment solution is applied on the top and drying without washing with water, and an electrolytic chromate treatment in which a chromate film is electrochemically formed on the surface of the plating layer.

[0004] In the reaction type chromate treatment and the coating type chromate treatment, unevenness in adhesion occurs in the width direction of the steel sheet, resulting in poor appearance, and when operating conditions such as line speed are changed, it becomes difficult to control the amount of chromate film. The resulting chromate film varies depending on drying conditions and the like, and has a problem that its performance is lacking in stability.

On the other hand, in the electrolytic chromate treatment, the surface of a plating layer is formed by performing cathodic electrolysis using a plated steel sheet as a cathode in an aqueous solution containing Cr ⁶⁺ as a main component and anions such as SO ₄ ²⁺ and F ^−. A chromate film is formed on the top, the amount of adhesion does not depend on the type of plated steel sheet, and depending on the current density at the time of cathodic electrolytic treatment,
There are advantages such that the amount of chromium adhered can be easily controlled and uneven adhesion does not occur.

Recently, the demand for corrosion resistance of zinc-based plated steel sheets has become much higher. Therefore, it is impossible for a chromate film formed on a zinc-based plating layer by conventional chromate treatment to satisfy the demand for corrosion resistance. Can not. Further, when a plated steel sheet is handled, fingerprints attached to the surface of the steel sheet greatly affect the commercial value of the final product, and therefore, a demand for fingerprint resistance of a zinc-based plated steel sheet has increased.

As a means for improving the corrosion resistance of a chromate-treated galvanized steel sheet, for example, the following techniques are disclosed. JP-B-61-54880: In a solution containing cationic colloidal silica as silica, a cathodic electrolytic treatment is performed with an electric quantity of 30 coulomb / dm ² or more, and Cr and S are formed on the surface of the zinc-based plating layer.
A chromate film containing i is formed (hereinafter referred to as Prior Art 1).

JP-A-62-107096: Chromate treatment liquid containing NO ₃ ⁻ in addition to silica, and the ratio of Cr ³⁺ / Cr ⁶⁺ is maintained in the range of 1/50 to 1/3. And subjecting the galvanized steel sheet to cathodic electrolytic treatment (hereinafter referred to as prior art 2).

JP-A-62-278298: Cathodic electrolytic treatment is applied to a zinc-based plated steel sheet using a chromate treatment solution containing fluoride in addition to silica (hereinafter referred to as prior art 3).

JP-A-62-278297: Cr ⁶⁺ as a main component, containing silica and silicate.
Using a chromate treatment solution containing at least one of n, Ni, Co, Al, Mg, Sn, Pb, and Mn ions, subjecting the zinc-based plated steel sheet to a cathodic electrolytic treatment to form a chromate on the zinc-based plating layer After forming a film, an anodic electrolytic treatment is immediately performed (hereinafter referred to as prior art 4).

[0011]

SUMMARY OF THE INVENTION
The third method has the following problems. That is, although silica is contained, some improvement in corrosion resistance is recognized, but no satisfactory improvement in coatability is recognized. In addition, when special consideration is given to improvement in paintability, desired corrosion resistance cannot be obtained. In the case of the method of the prior art 4, since the amount of the coating is significantly larger than that of the chromate coating formed only by the cathodic electrolytic treatment, the method is excellent in corrosion resistance by a salt spray test or the like. This is considered to be because the zinc-based plating layer was dissolved by the anodic electrolytic treatment, and a new chromate film containing an oxide of the base metal was deposited on the zinc-based plating layer, thereby performing a sealing action. However, according to the prior art 4, the coatability is hardly improved.

Accordingly, an object of the present invention is to provide a chromate-treated galvanized steel sheet having an excellent corrosion resistance, fingerprint resistance and paintability, having a stable chromate film, and having an industrially stable solution. It is an object of the present invention to provide a method for producing a chromate-treated galvanized steel sheet which can be produced by a method.

[0013]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, a chromate film composed of a predetermined amount of a metal layer made of Cr and Ni and a predetermined amount of an oxide layer made of Cr oxide, Ni oxide and Si oxide is formed on the zinc-based plating layer. It has been found that when formed, a chromate-treated galvanized steel sheet having a stable chromate film and having excellent corrosion resistance, fingerprint resistance and paintability can be obtained.

The present invention has been made based on the above-mentioned findings, and the present invention relates to a zinc or zinc alloy plated steel sheet having a zinc or zinc alloy plated layer formed on at least one surface thereof. Forming a chromate film on the zinc or zinc alloy plating layer by performing a cathodic electrolytic treatment in a chromate treatment solution, wherein the chromate treatment solution comprises 5 ~ 75g / l
Weight ratio of Cr ⁶⁺ to total Cr (Ni / Cr)
And ^{Ni 2+} but is within the range of 0.05 to 0.8, total Cr
Silica having a weight ratio (SiO ₂ / Cr) to the silica in the range of 0.3 to 3.0, and H _{3 in} the range of 5 to 50 g / l.
It contains BO ₃ and at least one of SO ₄ ²⁻ , Cl ⁻ , NO ₃ ⁻ and F ⁻ , and has a pH of 1.
It is characterized in that a treatment liquid in the range of 8-4.2 is used, and thereby a chromate film is formed on the zinc plating layer or the zinc alloy plating layer.

[0015]

According to the method of the present invention, at least one of the steel sheets
A chromate-treated galvanized steel sheet in which a chromate coating composed of a metal layer and an oxide layer is formed on a zinc or zinc alloy plating layer formed on two surfaces, wherein the metal layer is made of Cr and Ni. Cr and Ni
Is in the range of 3 to 1,000 mg / m ² , and the oxide layer mainly has a content of 3 to 100 in terms of Cr.
mg / m ² of Cr oxide and 1 to 50 mg / Ni equivalent.
m ² Ni oxide and 1 to 100 mg / m ^{2 in} terms of Si
A chromate-treated galvanized steel sheet having excellent corrosion resistance, fingerprint resistance and paintability, which is made of a composite oxide with Si oxide, is produced. In addition, the said metal layer means the coating layer which was not melt | dissolved when the chromate-treated zinc-alloy-plated steel plate was immersed in a 5% aqueous sodium hydroxide solution at a temperature of 70 [deg.] C. for 10 minutes. .

(1) Cr and Ni The total metal content of the metal Cr and metal Ni constituting the metal layer should be limited within the range of 3 to 1,000 mg / m ² . Total metal content of metal Cr and metal Ni is 3 mg / m
^If it is less than ² , no improvement in corrosion resistance can be obtained. On the other hand, metal C
r and the total metal content of metal Ni is 1,000 mg / m ²
Even if it exceeds, further improvement in corrosion resistance cannot be expected, and it becomes uneconomical.

(2) Cr oxide The amount of Cr oxide constituting the oxide layer is 3 to 3 in terms of Cr.
It should be limited to the range of 100 mg / ^{m 2.} If the amount of Cr oxide is less than 3 mg / m ^{2 in} terms of Cr, no improvement in corrosion resistance and paintability can be obtained. On the other hand, if the amount of the Cr oxide exceeds 100 mg / m ^{2 in} terms of Cr, the coatability deteriorates.

(3) Ni oxide The amount of Ni oxide constituting the oxide layer is 1 to 1 in terms of Ni.
It should be limited to the range of 50 mg / ^{m 2.} If the amount of Ni oxide is less than 1 mg / m ^{2 in} terms of Ni, no improvement in corrosion resistance and fingerprint resistance can be obtained. On the other hand, when the amount of the Ni oxide exceeds 50 mg / m ^{2 in} terms of Ni, the paintability deteriorates.

(4) Si oxide The amount of Si oxide constituting the oxide layer is 1 to 1 in terms of Si.
It should be limited to the range of 100 mg / ^{m 2.} When the amount of the Si oxide is less than 1 mg / m ^{2 in} terms of Si, improvement in corrosion resistance and paintability cannot be obtained. On the other hand, when the amount of the Si oxide exceeds 100 mg / m ^{2 in} terms of Si, the corrosion resistance deteriorates.

Next, in the present invention, the chromate treatment
For the reason that the component composition of the liquid was limited as described above,
This will be described below. (1) Cr⁶⁺  Cr⁶⁺Is limited to the range of 5 to 75 g / l.
Should. Cr⁶⁺If the content of is less than 5 g / l,
Uniform and stable formation of a chromate film over a long period of time
Can not be. On the other hand, Cr⁶⁺Is 75 g /
l, zinc on the plating surface is easily eluted,
The concentration balance in the chromate treatment solution is lost,
In addition, the paintability cannot be stably improved. this
Like Cr⁶⁺Chromic anhydride, dichromic acid
Of alkali metal salts of ammonium and dichromic acid
One or more of these can be used arbitrarily.

(2) Ni²⁺  Ni²⁺Is determined by the weight ratio to the total Cr (Ni / C
r) should be limited to the range of 0.05 to 0.8.
Ni²⁺Is 0.05% by weight based on the total amount of Cr.
If less than or more than 0.8, Ni oxidation in the chromate film
The amount of the product is 1 to 50 mg / m in terms of Ni.²Within the range of
To improve corrosion resistance and fingerprint resistance.
I can't do that. Such Ni²⁺As NiSO
₄, NiCl₂, NiCO₃And Ni (NO₃)₂of
One or more of these can be used arbitrarily
You.

(3) Silica The silica content is determined by the weight ratio (SiO ₂ /
Cr) should be limited to the range of 0.3 to 3.0.
When the content of silica is less than 0.3 or more than 3.0 in terms of weight ratio to total Cr, the amount of Si oxide in the chromate coating may be in the range of ² to 200 mg / m ^{2 in} terms of Si. It is not possible to improve corrosion resistance and paintability.

(4) H ₃ BO ₃ (boric acid) H ₃ BO ₃ (boric acid) has a buffering action at the interface of the cathode coating, and prevents the precipitation of silica in the chromate coating to prevent the plating steel sheet from being treated. It has the effect of making it more uniform in the width direction. The content of H ₃ BO ₃ should be limited within the range of 5 to 50 g / l. When the content of H ₃ BO ₃ is 5 g /
If it is less than 1, the desired effect cannot be obtained. On the other hand, H ₃ BO
_{Even if} the content of ₃ exceeds 50 g / l, the above effects cannot be further improved.

(5) SO₄ ^2-, Cl⁻, NO₃ ⁻, F
⁻  SO₄ ^2-, Cl⁻, NO₃ ⁻And F⁻Few of
At least one anion dissolves weak areas of the coating
And as an etchant to grow the film
It has all the functions. The anion content is
Affects the quality of the mate coating, too little or too much,
Corrosion resistance, fingerprint resistance and paintability can be improved.
Absent. The preferred content of the anion is 0.005 to 80.
g / l.

(6) Impurities In the chromate treatment liquid, there are metals mixed from the plated steel sheet to be treated, metals due to oxidation and reduction of the component elements in the chromate treatment liquid, and metals mixed from the electrodes.
Impurities such as Zn ²⁺ , Cr ³⁺ , Fe ²⁺ , Fe ³⁺ , and Pb ²⁺ are inevitably mixed. There is no problem with such impurities that are inevitably mixed into the chromate treatment solution, provided that their respective ion concentrations are 6 g / l or less.

The pH of the chromate treatment liquid is 1.8 to 4.
It should be limited to the range of 2. Chromate treatment liquid p
When H is less than 1.8, zinc on the plating surface elutes, and the balance of the concentration of Cr ^{6+ in} the chromate treatment solution is lost,
In addition, since the precipitation efficiency of silica decreases, the corrosion resistance and the coating property cannot be improved. On the other hand, when the pH of the chromate treatment liquid exceeds 4.2, it becomes difficult to stabilize the chromate treatment liquid for a long period of time, a precipitate is generated in the liquid, and the corrosion resistance cannot be improved.

The current density of the cathodic electrolytic treatment for forming a chromate film is preferably in the range of ^{2 to} 40 A / dm ² . When the current density is less than 2 A / dm ² , the metal layer is not formed uniformly, and the amount of chromate coating increases, which is not preferable in appearance. On the other hand, if the current density exceeds 40 A / dm ² , the amount of Cr and Ni in the composite oxide of the chromate film is too small, and sufficient corrosion resistance cannot be obtained.

By adjusting the cathodic electrolysis time, the amounts of Ni, Cr and Si can be maintained at desired values. That is, the content of each component of the chromate treatment liquid and the cathodic electrolysis current density are set to preferable conditions, and the amount of chromate film can be controlled to a desired value by changing the cathodic electrolysis treatment time, or By setting the processing time to a predetermined time and changing the current density, the chromate coating amount can be controlled to a desired value.

The galvanized steel sheet subjected to the chromate treatment of the present invention includes electrogalvanized steel sheet, Zn-F
e, an electro-galvanized steel sheet such as Zn-Ni, a hot-dip galvanized steel sheet, a heated zinc alloy-plated steel sheet, or the like can be used.

The chromate-treated galvanized steel sheet having a chromate film formed thereon according to the method of the present invention can be used as a corrosion-resistant steel sheet after being washed with water and dried, or used as a steel sheet for coating undercoat. You can also. Further, the chromate film formed by the method of the present invention may be subjected to post-treatment with a generally used aqueous chromate solution or an organic composite aqueous solution, if necessary. Next, the present invention will be described in more detail with reference to examples and comparative examples.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a known manner, 20 g / m²No
For electrogalvanized steel sheet with lead plating film formed,
According to the method of the present invention, cathodic electrolysis is performed under the following conditions,
Form a chromate film on the galvanized layer,
After washing with water and drying, the steel sheet No. of the present invention was dried. Prepare 1
Was. Chromate treatment liquid: Cr⁶⁺: 10.5 g / l (Na₂Cr₂O₇・ 2H₂O) Ni²⁺: 3.4 g / l (NiSO₄・ 6H₂O) SiO₂: 10.0 g / l (using Snowtex-O) H₃BO₃: 30.0 g / l SO₄ ^2-: 5.5 g / l (NiSO₄・ 6H₂O) pH: 3.4 Cathodic electrolysis conditions Current density: 4 A / dm²  Processing time: 1.0 second

According to the present invention, there is provided an electrogalvanized steel sheet as described above.
The cathodic electrolysis is performed under the following conditions by the method of
Board No. 2 was prepared. Chromate treatment liquid: Cr⁶⁺: 7.8 g / l (CrO₃Use) Ni²⁺: 3.7 g / l (NiCl₂・ 6H₂O) SiO₂: 4.0 g / l (using Snowtex-OL) Cl⁻: 4.5 g / l (NiCl₂・ 6H₂O) H₃BO₃: 20.0 g / l pH: 2.8 Cathodic electrolysis conditions Current density: 16 A / dm²  Processing time: 2.2 seconds

For comparison, the galvanized steel described above
For a plate, by a method outside the scope of the present invention, under the following conditions
After performing cathodic electrolysis, the comparative steel sheet no. 1 was prepared. Chromate treatment liquid: Cr⁶⁺: 23.4 g / l (CrO₃Use) SO₄ ^2-: 0.2 g / l (Na₂SO₄PH: 3.4 Cathodic electrolysis conditions Current density: 5 A / dm²  Processing time: 1.0 second

For comparison, the galvanized steel described above was used.
For a plate, by a method outside the scope of the present invention, under the following conditions
After performing cathodic electrolysis, the comparative steel sheet no. 2 was prepared. Chromate treatment liquid: Cr⁶⁺: 10.5 g / l (Na₂Cr₂O₇・ 2H₂O) SiO₂: 10.0g / l (using Snowtex-O) SO₄ ^2-: 0.3 g / l (Na₂SO₄PH: 3.0 Cathodic electrolysis conditions Current density: 10 A / dm²  Processing time: 1.8 seconds

For comparison, the galvanized steel described above was used.
For a plate, by a method outside the scope of the present invention, under the following conditions
After performing cathodic electrolysis, the comparative steel sheet no. 3 was prepared. Chromate treatment liquid: Cr⁶⁺: 15.7 g / l (Na₂Cr₂O₇・ 2H₂O) Ni²⁺: 3.4 g / l (NiSO₄・ 6H₂O) SO₄ ^2-: 5.5 g / l (NiSO₄・ 6H₂O) H₃BO₃: 20.0 g / l pH: 2.5 Cathodic electrolysis conditions Current density: 10 A / dm₂  Processing time: 1.6 seconds

For comparison, the galvanized steel described above was used.
For a plate, by a method outside the scope of the present invention,
After performing cathodic electrolysis, the comparative steel sheet no. 4 was prepared . Chromate treatment ^{_{solution: Cr 6+: 12 g / l}} (Na 2 Cr 2 using _{O 7 · 2H 2 O) Ni} 2+: 3.6g / l (NiSO 4 · 6H 2 O to use) SiO _2: 10.8g / 1 (using Snowtex O ) SO ₄ ²⁻ : 1.3 g / l (using NiSO ₄ .6H ₂ O ) pH: 2.5 Cathodic electrolysis condition Current density: 12 A / dm ² Processing time: 2 seconds

[0037] For comparison, the galvanized steel described above
For a plate, by a method outside the scope of the present invention, under the following conditions
After performing cathodic electrolysis, the comparative steel sheet no. 5 was prepared. Chromate treatment solution: Cr ⁶⁺ : 17 g / l (Na ₂ Cr ₂ O ₇ ・ 2H ₂ Use O) Ni ²⁺ : 11.1 g / l (NiSO ₄ ・ 6H ₂ Use O) SiO ₂ : 22.1 g / l (using Snowtex O) SO ₄ ^2- : 4.1 g / l (NiSO ₄ ・ 6H ₂ Use O) H ₃ BO ₃ : 3 g / l pH: 2.9 Cathodic electrolysis conditions Current density: 15 A / dm ² Processing time: 1.5 seconds

The specimens thus prepared were subjected to the tests described below. (1) Corrosion resistance test The test specimen was subjected to a salt spray test specified in JIS-Z-2371, and the area where white rust was generated after 168 hours had elapsed was measured.

[0039] (2) anti-fingerprint test specimen surface, touch by hand leaving sweat, was visually observed a state of the fingerprint adhering, the good was judged poor.

[0040] (3) painted test specimen on the surface to form a coating film with about 30μm painted white paint commercial alkyd melamine. Next, 100 cross-cut notches were formed at intervals of 1 mm in the coating film immediately after the coating, extruded 7 mm with an Erichsen tester, and then an adhesive tape was stuck on the surface of the extruded portion and peeled off. Evaluation was made based on the peeling state of the coating film at this time.
Table 1 shows the compositions of the chromate films of the steel sheet of the present invention and the comparative steel sheet, and the test results.

[0041]

As is clear from Table 1, the comparative steel sheet No. 1 in which both the metal layer and the oxide layer of the chromate film consisted of only Cr was used. Sample No. 1 was inferior in corrosion resistance, fingerprint resistance and paintability. The comparative steel sheet No. 1 in which the metal layer of the chromate film was composed of only Cr and the oxide layer was composed of Cr and Si
No. 2 has no corrosion resistance because no precipitation of nickel occurs.
The fingerprint resistance was poor. In addition, the comparative steel sheet No. 2 is the width
Direction uniformity of silica is low and the amount of precipitated silica is small.
Parts with poor paint adhesion coexist
There is a problem in terms of stability. Comparative steel sheet N in which both the metal layer and the oxide layer of the chromate coating are made of Ni and Cr
o. No. 3 was inferior in corrosion resistance and paintability.

[0043] H ₃ BO ₃ Chromate-free
The comparative steel sheet No. 4 and H ₃ BO ₃ Including
Use a chromate treatment solution whose amount is outside the scope of the present invention.
Comparative steel sheet No. 5 is the amount of silica as a whole
Despite the desired range, the precipitation of silica in the width direction is uneven
And as shown in this comparative example,
There is a small part, white rust occurs and the coating film peels off at that part
And corrosion resistance and paintability were both inferior.

[0044] On the other hand, the steel sheet of the present invention No. 1, 2
All were excellent in corrosion resistance, fingerprint resistance and paintability, and were also excellent in uniformizing the silica and chromate films in the width direction of the steel sheet.

[0045]

As described above, according to the present invention,
This provides an industrially useful effect that allows a chromate-treated galvanized steel sheet having a chromate film having excellent corrosion resistance, fingerprint resistance, and paintability to be stably manufactured industrially.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toyofumi Watanabe 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-2-270994 (JP, A) JP-A-2 JP-A-88799 (JP, A) JP-A-62-278297 (JP, A) JP-A-63-250496 (JP, A) JP-B-52-3983 (JP, B2)

Claims (3)

(57) [Claims] A zinc or zinc alloy plated steel sheet having a zinc plated layer or a zinc alloy plated layer formed on at least one surface thereof is subjected to a cathodic electrolysis treatment in a chromate treatment solution, whereby the zinc or zinc alloy plated steel sheet is treated. A method for producing a chromate-treated galvanized steel sheet, comprising forming a chromate film on a zinc alloy plating layer, wherein the chromate treatment solution contains 5 to 75 g / l of Cr ⁶⁺
And the weight ratio (Ni / Cr) to total Cr is 0.05 to
Ni ²⁺ in the range of 0.8, silica with a weight ratio (SiO ₂ / Cr) to total Cr in the range of 0.3 to 3.0, and H _{3 in} the range of 5 to 50 g / l. BO ₃ and S
_{^{O 4 2-, Cl -, NO}} 3 - and F ^- contain at least one of, and, pH is used the processing liquid is in the range of 1.8 to 4.2, thereby, the A method for producing a chromate-treated galvanized steel sheet having excellent corrosion resistance, fingerprint resistance and paintability, characterized by forming a chromate film on a zinc or zinc alloy plating layer. 2. The content of at least one of SO ₄ ²⁻ , Cl ⁻ , NO ₃ ⁻ and F ⁻ is from 0.05 to 0.05.
The method according to claim 1, which is in the range of 80 g / l. 3. The method according to claim 1, wherein a current density of the cathodic electrolysis in the chromate treatment liquid is in a range of ^{2 to} 40 A / dm ² .