JPH10158856A - Electrolytic chromate treated galvanized steel sheet excellent in blackening resistance and surface appearance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an electrolytic chromate treated galvanized steel sheet excellent in blackening resistance and having a uniform and beautiful surface appearance and to provide a method for producing the same. SOLUTION: This electrolytic chromate treated galvanized steel sheet excellent in blackening resistance and surface appearance is the one in which the surface of galvanizing coating of a galvanized steel sheet in which the galvanizing coating has been formed is applied with an oxide layer contg. Al oxide by 5 to 40mg/m<2> as metallic Al, and the surface of the oxide layer has chromate coating by the coating weight of 5 to 80mg/m<2> expressed in terms of metallic chromium formed by electrolytic chromate treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic chromate-treated galvanized steel sheet having excellent resistance to blackening and surface appearance, and a method for producing the same.

[0002]

2. Description of the Related Art Generally, a hot-dip galvanized steel sheet subjected to chromate treatment has excellent corrosion resistance, and is therefore used in a wide range of fields such as building materials, constituent materials of household electric appliances, and covers of electric motors. The hot-dip galvanized steel sheet is subjected to skin pass rolling in order to improve appearance and surface shape.
The chromate-treated galvanized steel sheet subjected to skin pass rolling and subjected to chromate treatment has a problem of so-called black discoloration in which a part or the entire surface of a plating surface turns gray black during storage in a warehouse or the like. This blackening phenomenon is a phenomenon that appears in the early stage of corrosion of the galvanized steel sheet, and is often observed in a chromate-treated galvanized steel sheet that has been accelerated in a high-temperature and high-humidity atmosphere and has been subjected to skin pass rolling.

As a method for improving the blackening resistance of this chromate-treated galvanized steel sheet, Japanese Patent Application Laid-Open No.
In step 1151, a hot-dip galvanized steel sheet is prepared by controlling the composition of the hot-dip galvanized bath, and if necessary, a skin pass treatment with an elongation of 0.3 to 1.5% is performed on the hot-dip galvanized steel sheet. A method for producing a chromate-treated hot-dip galvanized steel sheet that performs a chromate treatment is disclosed (hereinafter referred to as Prior Art 1). Further, Japanese Patent Application Laid-Open No. 5-816
No. 73 has an elongation of 0.3 to 0.3 mm on the surface of the hot-dip galvanized steel sheet.
A method for producing a chromate-treated hot-dip galvanized steel sheet is disclosed in which a 1.5% skin pass treatment is performed, a treatment is performed with a solution of one or both of Ni and Co ions, and then a chromate treatment is performed. 2).
Further, JP-A-6-57392 discloses a method for producing a chromate-treated galvanized steel sheet, comprising treating a hot-dip galvanized (alloy) -coated steel sheet with a molybdate aqueous solution and then performing a chromate treatment (hereinafter, referred to as a chromate-treated hot-dip galvanized steel sheet). , Referred to as Prior Art 3).

[0004]

Prior arts 1 to 3 have the following problems. That is, the prior art 1 attempts to improve the plating film by controlling the composition of the plating bath. However, according to the method of Prior Art 1, the effect of improving blackening resistance is insufficient. In the prior art 2, the surface oxide layer is partially destroyed by subjecting the surface of the hot-dip galvanized steel sheet to a skin pass treatment with an elongation of 0.3 to 1.5%. Concentrates and precipitates at the fracture of the layer. It is estimated that blackening is prevented by the precipitated Co and Ni. However, according to the prior art 2, although the blackening resistance and the uniformity of the surface appearance are partially improved as compared with the prior art 1, also in the prior art 2, the improvement effect is not always sufficient, and Large variation occurs in blackening resistance.

In Prior Art 3, molybdenum in sodium molybdate and ammonium molybdate is
It is presumed that it plays a role similar to that of the deposited metals Co and Ni in Prior Art 2, but its effect is slightly inferior to Co and Ni. Large variations occur. As mentioned above,
The methods of the prior arts 1 to 3 control the plating bath composition, and are an improvement measure by applying a skin pass or a post-plating treatment, and the effect of improving the blackening resistance is partially observed. There is a problem that the improvement effect is not always sufficient.

[0006] In addition, regarding the surface appearance, conventionally, when the amount of the chromate film adhered becomes large, the non-uniformity of the chromate film has been increased, and the appearance tends to be uneven. Further, in hot-dip galvanizing, thick and thin portions are unevenly formed on the attached chromate due to the influence of cracks inevitably generated on the surface of the surface oxide due to skin pass or the like. Further, since such a surface oxide has poor wettability of the chromate treatment solution, thick and thin portions of chromate coexist. It has been found that the uniformity of the surface appearance is reduced by the chromate treatment unevenness thus generated. It was also found that the blackening resistance also decreased due to the non-uniformity of the chromate. That is, it can be seen that in hot-dip galvanizing, the surface oxide layer has a very important effect on blackening resistance and surface appearance.

Accordingly, an object of the present invention is to provide a galvanized steel sheet treated with electrolytic chromate which is excellent in blackening resistance, has a uniform surface appearance, and is beautiful, and a method for producing the same.

[0008]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, as a metal Al, 5
Forming an oxide layer containing ４０40 mg / m ² Al oxide;
Then, a chromate film having an adhesion amount of 5 to 80 mg / m ^{2 in} terms of metallic chromium is formed on the surface of the oxide layer by electrolytic chromate treatment, so that electrolytic chromate-treated molten zinc excellent in blackening resistance and surface appearance is formed. It has been found that a plated steel sheet can be obtained.

The chromated hot-dip galvanized steel sheet according to the present invention, which is excellent in blackening resistance and surface appearance, has been made based on the above-mentioned findings. On the surface of the hot-dip galvanized film, there is an oxide layer containing 5 to 40 mg / m ² of Al oxide as metal Al, and in terms of metal chromium formed on the surface of the oxide layer by electrolytic chromate treatment. It has a chromate film having an adhesion amount of 5 to 80 mg / m ² .

Further, the method for producing a chromated hot-dip galvanized steel sheet excellent in blackening resistance and surface appearance according to the present invention is to provide a hot-dip galvanized steel sheet by subjecting the surface of the steel sheet to hot-dip galvanizing. 5-40m as metal Al on the surface of hot-dip galvanized film of hot-dip galvanized steel sheet
forming an oxide layer comprising g / m ² Al oxide; and
Next, the hot-dip galvanized steel sheet having the oxide layer is subjected to electrolytic chromate treatment using a chromate treatment solution having a pH of 2 to 6.

[0011]

Next, the chromate-treated galvanized steel sheet having excellent blackening resistance and surface appearance according to the present invention and a method for producing the same will be described in detail. The hot-dip galvanized steel sheet used in the present invention may be a hot-dip galvanized steel sheet, and there is no particular limitation on the material, thickness, and the like.

The amount of aluminum metal oxide in the oxide layer containing Al oxide formed on the surface of the galvanized steel sheet of the present invention is 5 to 5 in terms of metal.
When it is in the range of 40 mg / m ² , excellent blackening resistance can be obtained. When the amount of the aluminum (metal) oxide is less than 5 mg / m ^{2 in} terms of metal, the blackening resistance is deteriorated, and no remarkable effect is obtained. On the other hand, when the amount of the aluminum oxide exceeds 40 mg / m ² in terms of metal, the adhesion of the oxide layer is reduced, the oxide layer is detached, and the corrosion resistance, uniform appearance, and blackening resistance are deteriorated. Therefore, the amount of the aluminum oxide should be limited to the range of 5 to 40 mg / m ^{2 in} terms of metal.

The thickness of the oxide layer depends on the oxygen concentration in the atmosphere gas when the hot-dip plating solidifies, the bath temperature of the hot-dip plating,
Also, the plating is controlled by dipping or spraying an alkaline solution into an alkaline solution after plating, but in the present invention, the thickness of the oxide layer may be controlled by another method, and is limited to this. is not.

In the method of the present invention, the electrolytic chromate treatment may be any one capable of forming a chromate film on the surface of the oxide layer formed on the hot-dip galvanized layer by cathodic electrolytic treatment. Is not particularly limited. When the pH of the chromate treatment solution is 2 to 6, excellent blackening resistance and surface appearance can be obtained. If the pH is less than 2, the etching power is too strong.
The metal zinc layer (hot-dip galvanized film) was dissolved, and as a result, the oxide layer formed on it was desorbed,
Black discoloration resistance and uniform appearance decrease. On the other hand, if the pH exceeds 6, the rate of formation of the chromate film decreases, or
A dense chromate film is hardly formed, and the surface appearance and corrosion resistance are greatly reduced. Therefore, p of the chromate treatment liquid
H should be limited to the range of 2-6.

When the chromate film formed by the electrolytic chromate treatment has an adhesion amount in the range of 5 to 80 mg / m ^{2 in} terms of chromium metal, excellent blackening resistance and surface appearance can be obtained. When the chromate film has an adhesion amount of less than 5 mg / m ^{2 in} terms of chromium metal, there is no problem in appearance uniformity, but there is a problem in corrosion resistance. On the other hand, when the chromate film has an adhesion amount of more than 80 mg / m ^{2 in} terms of metallic chromium, interference colors are very conspicuous in the chromate film, and the uniformity of appearance is greatly reduced. Therefore, the adhesion amount of the chromate film is 5 to 80 mg / metal chromium equivalent.
It should be limited to a range of m ^2.

[0016]

The chromate-treated hot-dip galvanized steel sheet having excellent blackening resistance and surface appearance according to the present invention and a method for producing the same will be described with reference to Examples. Al: 0.13%, Pb:
A hot-dip galvanizing bath containing 0.01% and the balance consisting of zinc and unavoidable impurities has a zinc adhesion amount of 90 g /
A hot-dip galvanized steel sheet of m ² was prepared. Next, on the surface of the galvanized steel sheet thus prepared,
Depending on the heat treatment conditions and the like, an oxide layer composed of an aluminum oxide was formed in an amount in the range of 5 to 40 mg / m ² within the range of the present invention in terms of metal in terms of metal. Next, the galvanized steel sheet having the oxide layer formed on its surface was subjected to electrolytic chromate treatment under the following conditions of the present invention.

The conditions for the electrolytic chromate treatment were as follows. That is, the electrolytic chromate treatment solution contains CrO ₃ as a chromium component, contains hexavalent chromic acid as a main component, and adds sodium hydroxide to adjust the pH to 2 to 6 within the range of the present invention. A bath was used. The hot-dip galvanized steel sheet is immersed in the electrolytic chromate bath, and the hot-dip galvanized steel sheet is used as a cathode and subjected to electrolytic treatment at a temperature of 50 ° C. and a current density of 10 A / dm ² so that the chromium deposition amount is within the range of the present invention. Sample No. 5 of the electrolytic chromate-treated hot-dip galvanized steel sheet within a certain range of 5 to 80 mg / m ² . 1 to
9 was prepared. The desired amount of chromium was obtained by changing the energization time.

Next, for comparison, Al: 0.13
%, Pb: 0.01%, and the balance is 90 g / m ² by the hot-dip galvanizing bath containing zinc and unavoidable impurities. The same hot-dip galvanized steel sheet as used was prepared. Then, the surface of the plating film of the molten zinc plated steel sheet, the heat treatment conditions, etc., in terms of a metal, oxide of an oxide of range of the amount of aluminum in 5 mg / m ² or less than 40 mg / m ² greater than the present invention A layer was formed. Next, the galvanized steel sheet having the oxide layer formed on its surface was subjected to electrolytic chromate treatment under the following conditions outside the scope of the present invention.

The conditions for the electrolytic chromate treatment were as follows. That is, as an electrolytic chromate treatment liquid, CrO ₃ is contained as a chromium component, hexavalent chromic acid is used as a main component, and sodium hydroxide is added to reduce the amount to less than 2 or 6
A comparative electrolytic chromate bath having a pH outside the range of the present invention was used. The hot-dip galvanized steel sheet is immersed in the electrolytic chromate bath, and the hot-dip galvanized steel sheet is used as a cathode and subjected to electrolytic treatment at a temperature of 50 ° C. and a current density of 10 A / dm ² so that the chromium adhesion amount is 5 mg / m ² or 80 mg. / M ²
The sample of the electrolytic chromate-treated hot-dip galvanized steel sheet for comparison out of the scope of the present invention exceeds the sample No. 1-6 were prepared. The desired amount of chromium was obtained by changing the energization time. Table 1 shows the results.

[0020]

[Table 1]

The amount of the oxide formed on the surface of the hot-dip coating was measured by Auger electron spectroscopy (AES). A standard sample (Al ₂ O ₃ ) was prepared by a vacuum deposition method, and Al present as Al ₂ O ₃ was quantified by chemical analysis and used. The standard sample (Al ₂ O ₃ ) is sputtered by AES until Al existing as Al ₂ O ₃ is eliminated,
Assuming that the measured Al intensity corresponds to the oxide amount, a calibration curve for the oxide amount was created. In the sample measurement, the sample was sputtered in the depth direction, the sputtering was continued until the Al peak as Al ₂ O ₃ was not observed, and the amount of oxide as Al ₂ O ₃ was calculated from the comparison with the calibration curve. .

The blackening resistance, surface appearance, and corrosion resistance of the above-described chromate-treated hot-dip galvanized steel sheet of the present invention and comparative examples were examined. The blackening resistance of the chromate-treated hot-dip galvanized steel sheet was evaluated in a constant temperature and humidity environment of 90% relative humidity and 50 ° C. for 24 hours, and the appearance was evaluated according to the following criteria. ○ No blackening is observed by visual observation △ Blackening is slightly observed by visual observation × Blackening is observed by visual observation The uniformity of appearance of the chromated galvanized steel sheet is evaluated according to the following criteria. did. ○ Non-uniform part is not observed by visual observation △ Slightly non-uniform part is observed by visual observation × Clearly non-uniform part is observed by visual observation Corrosion resistance of chromate-treated galvanized steel sheet Using a salt spray test (JIS C2550), evaluation was made according to the following criteria based on the white rust generation area after 48 hours. ○ No rust at all △ Rust generation area is 10% or less × Rust generation area exceeds 10% The results are shown in Table 2.

[0023]

[Table 2]

As is clear from Table 2, the sample No. of the chromated hot-dip galvanized steel sheet of the present invention prepared by the method of the present invention. All of Nos. 1 to 9 have excellent blackening resistance, surface appearance and corrosion resistance. On the other hand, the comparative sample No. Sample No. 1 was inferior in blackening resistance due to the fact that the amount of aluminum oxide was less than 5 mg / m ^{2 in} terms of metal. Sample No. for comparison. 2
Means that the amount of aluminum oxide is 40 m
The blackening resistance and the surface appearance were inferior due to exceeding g / m ² . Sample No. for comparison. Sample No. 3 was inferior in blackening resistance and surface appearance due to the pH of the chromate treatment solution being less than 2. Sample No. for comparison.
Sample No. 4 had poor corrosion resistance due to the pH of the chromate treatment solution exceeding 6, and the amount of chromium deposited was less than 5 mg / m ^{2 in} terms of metal chromium. Sample No. for comparison. 5 means that the amount of chromium deposited is 5 mg in terms of metal chrome.
/ M ² , the corrosion resistance was poor.
Sample No. for comparison. No. 6 was inferior in surface appearance due to the chromium adhesion amount exceeding 80 mg / m ^{2 in} terms of chromium metal.

[0025]

According to the present invention, there is provided an electrolytic chromate-treated hot-dip galvanized steel sheet which is excellent in blackening resistance, has a uniform and beautiful surface appearance, and a method for producing the same, and has industrially useful effects.

Claims (2)

[Claims] 1. A hot-dip galvanized steel sheet having a hot-dip galvanized film having an oxide layer containing 5 to 40 mg / m ² of Al oxide as metal Al on the surface of the hot-dip galvanized steel sheet; 5 to 80 mg in terms of metal chromium formed on the surface of the oxide layer by electrolytic chromate treatment
An electrolytic chromate-treated galvanized steel sheet having excellent resistance to blackening and surface appearance, characterized by having a chromate film with an adhesion amount of / m ² . 2. A hot-dip galvanized steel sheet is prepared by subjecting the surface of the steel sheet to hot-dip galvanizing, and 5 to 40 mg / m ² of Al as metal Al is formed on the hot-dip galvanized film surface of the hot-dip galvanized steel sheet. Forming an oxide layer containing an oxide, and then subjecting the galvanized steel sheet having the oxide layer to electrolytic chromate treatment using a chromate treatment solution having a pH of 2 to 6, followed by washing with water A method for producing an electrolytic chromate-treated hot-dip galvanized steel sheet having excellent blackening resistance and surface appearance.