JPH0696794B2 - AC electrolytic chromate treatment plated steel sheet manufacturing method - 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 an AC electrolytic chromate-treated plated steel sheet for the purpose of improving surface properties such as corrosion resistance and paintability of various plated steel sheets.

[0002]

2. Description of the Related Art Chromate is generally widely used for zinc, aluminum and alloy-plated steel sheets mainly containing these metals. Chromate treatment is a coating chromate method in which a chromate treatment solution is applied to the surface of a metal plating and then dried or baked, a reaction chromate method in which the metal plating is immersed in or sprayed with the chromate treatment solution, and then washed with water and dried, and plating in the chromate treatment solution. It can be roughly classified into an electrolytic chromate method in which a steel sheet is immersed and electrolyzed. Each method has merits and demerits, and is selected and used according to the desired quality, processing equipment and plating metal.

The present invention is an electrolytic chromate method of treating with alternating current, and a known technique related to the present invention will be described.

Conventional electrolytic chromating is basically a cathodic electrolysis process based on a Sargent bath, as described in Metal Surface Technical Handbook 347-359.

There are few examples of electrolytic chromating using an alternating current, and as a disclosed technique, an example of treatment with alkaline chromate and phosphoric acid (Japanese Patent Laid-Open No. 288099/1986) is used.
No.).

[0006]

In the conventional electrolytic chromate method, the chromate film formed is mainly composed of metallic chromium (hereinafter Cr ⁰ ) and trivalent chromium (Cr ³⁺ ), so that the corrosion resistance depends on Cr ^6+. Is not enough. Almost no electrolytic chromate mainly composed of Cr ⁶⁺ is found, and much less electrolytic chromate in which Cr ³⁺ / Cr ⁶⁺ is controlled in a coating film is known. In addition, since it is a strongly acidic solution with a pH of 2 or less that contains sulfate ions, fluorine ions, silicofluoride ions, etc. in the chromate treatment liquid, the chemical reaction with the plating metal occurs at the same time as or before and after the electrolytic reaction, resulting in an appearance. It is difficult to finish it uniformly. Further, it is significantly affected by the surface condition of the plated metal, alloy composition, etc., and several treatment baths and treatment conditions must be provided according to the plated metal.

In the present invention, the conventional electrolytic chromate coating is Cr
^A method for efficiently obtaining electrolytic chromate composed of Cr ⁶⁺ and Cr ³⁺ , which is excellent in corrosion resistance due to a chromate film mainly composed of ⁰ , Cr ³⁺ and has excellent corrosion resistance and adhesion, is provided. is there.

[0008]

According to the method of the present invention, after the cold-rolled steel sheet is plated, the total Cr content of Cr ³⁺ / (Cr ³⁺ + Cr ⁶⁺ ) is 0.1 to 0.7. A method for producing an AC electrolytic chromate-treated plated steel sheet, which comprises performing AC electrolysis in an acidic aqueous solution having a concentration of 5-100 g / l and containing reduced chromic acid and an inorganic sol and having a pH of 5 or less.

The acidic aqueous solution may further contain divalent or higher valent metal ions.

[0010]

The function of the present invention will be described below.

The acidic aqueous solution used in the present invention is Cr ³⁺ , C
r ⁶⁺ , and optionally an inorganic anion ([A
^m- ]) and additives are added.

Cr ³⁺ and Cr ⁶⁺ are chromium reduction ratios (Cr
^{3 +} / (calculated as Cr ³⁺ + Cr ⁶⁺ )) is 0.1
It is 0.7. The concentration of total chromium (Cr ³⁺ + Cr ⁶⁺ ) is 5
-100 g / l is preferable, and 10-30 g / l is optimal. The chromium reduction ratio is the most important factor in the present invention.
If it is less than 0.1, the chromate film cannot be efficiently deposited as described below, and corrosion resistance cannot be obtained in terms of quality. Also 0.
If it is 7 or more, only a chromate film having poor corrosion resistance can be obtained. When the chromium reduction ratio is 0.2 to 0.4, it is possible to most efficiently obtain a chromate film having excellent quality. The chromium reduction ratio is prepared by adding a reducing agent to an aqueous solution of chromic anhydride. Although an organic reducing agent can be applied as the reducing agent, it is desirable to use an organic reducing agent such as saccharides, alcohols, phenols, oxycarboxylic acids, carboxylic acids or the like, which does not leave a residue.

The acidic aqueous solution of the present invention contains, as an essential compound, an inorganic sol having a particle size of 100 nm or less, for example, a silica sol, a zirconia sol, a titanium oxide sol, and a sol partially cationized with an alumina sol. These sols eutectoid or thicken the surface of the chromate film to improve corrosion resistance and paint adhesion.

If necessary, the acidic liquid used in the present invention may contain metal ions having a valence of 2 or more ( ^{denoted as} Me ^{n +} ).

Bivalent or higher valent metal ions (Me^{n +}) Is Mg
²⁺, Ca²⁺, Zn²⁺, Al³⁺, Fe ²⁺, Co²⁺, N
i²⁺, Sn²⁺Salt and MoO_Four ^2-, WO_Four ^2-, MnO
_Four ^2-Say These metal ions are deposited during chromate deposition.
Deposited as chromate, dichromate or oxide
To fix hexavalent chromium and improve corrosion resistance. Addition
The amount of each valence is indicated by n and m, and the anion is A^m-When
Shown, Me^{n +}[Cr₂O₇]^2-(N / 2) + Me^{n +}・
[A^m-] (N / m) Me^{n +}Is added as the upper limit
Get

The pH of the acidic aqueous solution is preferably in the range of 1-5. If the pH is less than 1, a chemical reaction occurs due to the dissolution of the plating, and good quality chromate cannot be obtained. On the other hand, if the pH exceeds 5, the chromate deposition efficiency will be significantly deteriorated. The temperature of the acidic aqueous solution is 2
The treatment can be carried out at a temperature at which a usual electrolytic treatment is carried out, which is 0 to 60 ° C.

The electrolysis method is a method of performing alternating current electrolysis.

The present invention is basically based on a method of electrolyzing in an acidic aqueous solution, followed by washing with water and drying, but a method of squeezing with anhydrous washing can also be employed if necessary.

FIG. 1 shows a 12% Ni-Zn alloy-plated steel sheet subjected to AC electrolysis in an acidic aqueous solution having a chromium reduction ratio of 0.4, a total chromium concentration of 25 g / l, colloidal silica of 50 g / l and a pH of 1.8. It shows the relationship between the amount of energization and the amount of deposited Cr. Chromate film is deposited with good current efficiency. The coating has a brown uniform appearance, and a coating having a high Cr ^{6+ content} is uniformly obtained as compared with the known cathode electrolysis method.

[0020]

EXAMPLES Examples of the present invention will be shown below.

The evaluation methods in the examples are shown below.

The total amount of deposited chromium was dissolved in 5% HCl, analyzed by thick spectrophotometry, and expressed in mg / m ² .

The corrosion resistance of unpainted was measured by a salt spray test, and the evaluation was shown by the time until the occurrence of white rust.

[0024]

[Example 1] A 12% Ni-Zn alloy plated steel sheet was Cr3 ^+.
/ (Cr ⁶⁺ + Cr ³⁺ ) = 0.4 Partially reduced chromic anhydride 25 g / l and silica sol 50 g / l aqueous solution (pH
= 2), AC electrolysis was performed. Electrolysis was carried out at a current density of 6 A / dm ² and an energization amount of 20, 40, 60 coulombs / dm ² . The amount of deposited chromium is shown in FIG.

The resulting chromate-treated coating has good adhesion, and the corrosion resistance of the unpainted sheet as determined by a salt spray test is T. Cr
The treatment with 200 mg / m ² was carried out for 172 hours, but no white rust was observed.

[0026]

Example 2 A cold-rolled steel sheet was galvanized at a rate of 20 g / m ² containing 12% Ni by a known method, and then the grain size was 20.
nm silica sol at 15 g / l, Cr reduction rate (Cr ³⁺ /
[Cr ³⁺ + Cr ⁶⁺ ]) is 0 (Comparative Example No. 1), 0.
1 (No. 2), 0.2 (No. 3), 0.3 (No.
4), using lead as a counter electrode in a reduced chromic acid bath of 15 g / l as total chromium changed to 0.5 (No. 5) 5
A sample was prepared by electrolyzing for 5 seconds at a current density of 6 A / dm ² with an alternating current of 0 cycle, washing with water and drying. No. 1
A sample (No. 6) subjected to cathodic electrolysis for 5 seconds in a bath at a current density of 6 A / dm ² was prepared as a comparative example.

In the comparative example (No. 1), the amount of deposited Cr is 10 m.
g / m²As for the corrosion resistance, white rust was generated on the entire surface in 24 hours. Return
The amount of deposited Cr increases with the increase of the original ratio, and 2 is
50 mg / m²Corrosion resistance for 72 hours, No. 3 is 100m
g / m²Corrosion resistance for 120 hours, No. 3 is 150 mg /
m²Corrosion resistance for 144 hours, No. 4 is 200 mg / m ²
Corrosion resistance for 168 hours, No. 5 is 220 mg / m²Withstand
Good corrosion resistance test results of 168 hours were obtained. Comparative example
(No. 6) has a Cr deposition amount of 30 mg / m²There is unevenness
In addition, white corrosion occurred on the entire surface in 24 hours.

[0028]

Example 3 Under the conditions of Example 1, primary zinc phosphate [Zn
(H ₂ PO ₄ ) ₂ ], primary cobalt phosphate [Co (H ₂
PO ₄ ) ₂ ] and added zinc ion to the bath at 2 g / l (N
o. 7) Prepare a sample by galvanizing electrogalvanized steel sheet with a plating amount of 20 g / m ^{2 at} a current density of 6 A / dm ² for 5 seconds in a bath containing cobalt ions of 2 g / l (No. 8), washing with water and drying. did. No. 7: Cr adhesion amount 150 mg / m
No. ² corrosion resistance for 120 hours without white rust No. 8 had a Cr deposit of 150 mg / m ² and showed no corrosion resistance for 168 hours.

[0029]

INDUSTRIAL APPLICABILITY The present invention has a merit that it does not require an expensive rectifier because it is a treatment by AC electrolysis.

Further, according to the present invention, the dependence of metal surface species on chromate precipitation is low, and other metals such as aluminum, iron,
It is also a chromate treatment method that is sufficiently effective when applied to steel and the like.

[Brief description of drawings]

FIG. 1 12 in a chromate solution composed of chromic acid of Cr ³⁺ / (Cr ⁶⁺ + Cr ³⁺ ) = 0.4 and silica sol.
The figure which shows the example which chromate-treated the% Ni-Zn alloy plated steel plate by the alternating current electrolysis method.

Claims (2)

[Claims] 1. After plating a cold-rolled steel sheet, Cr
^{3 +} / (Cr ³⁺ + Cr ⁶⁺ ) ratio of 0.1-0.7 total Cr
A method for producing an AC electrolytic chromate-treated plated steel sheet, which comprises performing AC electrolysis in an acidic aqueous solution having a concentration of 5-100 g / l and containing reduced chromic acid and an inorganic sol and having a pH of 5 or less. 2. After plating a cold-rolled steel sheet, Cr
^{3 +} / (Cr ³⁺ + Cr ⁶⁺ ) ratio of 0.1-0.7 total Cr
PH 5 containing reduced chromic acid with a concentration of 5-100 g / l and an inorganic sol, and further containing divalent or higher valent metal ions
The following is a method for producing an alternating current electrolytic chromate-treated plated steel sheet, which comprises performing alternating current electrolysis in an acidic aqueous solution.