JPH04176874A - Production of chromated steel sheet - Google Patents

Abstract

PURPOSE:To produce a steel sheet having a Cr-SiO2 composite coating film excellent in corrosion resistance and coating suitability by applying a Zn-SiO2 composite plating, in which SiO2 grains are uniformly dispersed, on the surface of a steel sheet, anodizing the plating to dissolve and remove Zn and then chromating the steel sheet. CONSTITUTION:A steel sheet is dipped in a Zn plating soln. contg. fine granular SinO2, and a current is applied with the sheet as a cathode to form a Zn-SiO2 composite plating layer 2, in which SiO2 grains are uniformly dispersed, on the surface of the steel sheet 1. The Zn-SiO2 composite plated steel sheet A' is anodized in an electrolyte in which NaCl is dissolved to selectively dissolve and remove the whole or a part of the Zn in the layer 2, and an SiO2 grain layer 3 is left. A chromating soln. contg. Cr ion is then applied on the layer 3 and dried, or a current is applied with the sheet as a cathode in the chromating soln. to form a Cr-SiO2 composite layer 4 in which fine SiO2 grains are uniformly dispersed, and a chromated steel sheet A excellent in corrosion resistance and coating suitability is produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a method for producing a chromate-treated steel sheet having a uniform ROM-silica composite coating on its surface, which has excellent corrosion resistance and paintability. .

[Conventional technology]

Chromate-treated steel sheets are known in which a chromium-silica composite film in which silica particles are uniformly dispersed is formed on the surface of the steel sheet in order to improve the corrosion resistance and paintability of the steel sheet.

Methods for manufacturing such romate-treated steel sheets can be broadly classified into the following two methods.

(1) Coating method For example, Japanese Patent Publication No. 42-14504, Japanese Patent Publication No. 45-38
As disclosed in No. 891, JP-A No. 52-17340, etc., this method consists of the following steps. That is, by applying a chromate treatment liquid containing chromium ions and silica particles onto the surface of a steel plate, and then drying the surface of the steel plate coated with the chromate treatment liquid, a chromium-silica composite is formed on the surface of the steel plate. Forms a film.

(2) Cathode electrolytic treatment method For example, Japanese Patent Publication No. 47-44417, Japanese Patent Publication No. 48-43
As disclosed in No. 019, JP-A No. 62-146295, etc., this method consists of the following. That is,
A chromium-silica composite film is formed on the surface of a steel plate by subjecting the steel plate to cathodic electrolysis treatment in an electrolytic chromate treatment solution containing chromium ions and silica particles.

C Problems to be Solved by the Invention] However, the above-mentioned coating method has the following problems. In other words, in order to improve corrosion resistance, when the amount of chromate treatment liquid applied to the surface of a steel sheet is increased, the amount of silica deposited increases, resulting in a decrease in the bond between the steel sheet and the chromium-silica composite coating. Adhesion decreases and paintability deteriorates.

Further, the cathodic electrolytic treatment method has the following problems. That is, although the chromium-silica composite coating formed by this method has good paintability, its corrosion resistance is insufficient.

Therefore, it is an object of the present invention to provide a method for producing a chromate-treated steel sheet having a uniform ROM-silica composite coating on its surface that has excellent corrosion resistance and paintability.

[Means to solve the problem]

The present inventors solved the above-mentioned problem, and on the surface,
We conducted extensive research in order to develop a method for producing a chromate-treated steel sheet with a uniform ROM-silica composite coating that has excellent corrosion resistance and paintability.

As a result, the present inventors obtained the following knowledge. That is, a zinc-silica composite plating film is formed on the surface of a steel sheet by cathodic electrolysis treatment, and then the zinc in the composite plating film is dissolved by anodic electrolysis treatment, thereby forming a silica particle layer on the surface of the steel sheet. If a chromate treatment is applied to a steel plate with a silica particle layer formed on its surface and the silica particle layer is embedded in the formed chromate film, a uniform ROM with excellent corrosion resistance and paintability can be obtained. - A silica composite coating can be formed on the surface of a steel plate.

This invention was made based on the above findings, and
A steel plate is subjected to cathodic electrolysis treatment in an acidic zinc electroplating solution containing silica particles, and the surface of the steel plate is coated with
A zinc-silica composite plating film in which silica particles are uniformly dispersed is formed, and then the steel plate on which the dumbbell-silica composite plating film is formed is subjected to an anodic electrolytic treatment in an electrolytic solution. - By selectively dissolving all or part of the zinc in the silica composite plating film, a desired amount of silica particle layer is formed on the surface of the steel sheet, and then the silica particle layer is formed on the surface of the steel sheet. The formed steel plate is subjected to chromate treatment, a chromate film is formed on the surface of the steel plate on which the silica particle layer is formed, the silica particle layer is embedded in the chromate film, and thus the silica particle layer is formed on the surface of the steel plate. , is characterized in that it forms a chromium-silica composite film in which silica particles are uniformly dispersed.

In this invention, first, using an acidic zinc electroplating solution containing silica particles and using a steel plate or a zinc electroplated steel plate as a cathode, the steel plate is subjected to cathodic electrolytic treatment, as shown in FIG. , has a zinc-silica composite plating film 2 on the surface of a steel plate 1 in which silica particles are uniformly dispersed. A dumbbell-silica composite electroplated steel sheet A' is prepared.

Next, using the zinc-silica composite electroplated steel sheet A' as an anode, the zinc-silica composite electroplated steel sheet A' is subjected to anodic electrolytic treatment in an electrolytic solution. As a result, zinc-
Zinc in the zinc-silica composite plating film 2 on the silica composite electroplated steel sheet A' is dissolved, and a silica particle layer 3 is formed on the surface of the steel sheet 1, as shown in FIG.

Next, the steel plate 1 on which the silica particle layer 3 has been formed as described above is subjected to a known chromate treatment method, that is, a chromate treatment solution containing chromium ions is applied to the surface of the silica particle layer 3 of the steel plate. Chromate treatment is performed by a coating method that involves drying, or a cathode electrolytic treatment method that involves cathodic electrolyzing the steel plate 1 on which the silica particles Wi3 are formed in an electrolytic chromate treatment solution containing chromium ions. , a chromate film is formed on the surface of the steel plate 1. As a result, the silica particle layer 3 is embedded in the chromate film, and as shown in FIG. Treated steel plate A
is obtained.

When dissolving the zinc in the zinc-silica composite plating film 2 by anodizing the zinc-silica composite electroplated steel sheet A' shown in FIG. For example, a part of the zinc-silica composite plating film 2 remains on the surface of the steel plate 1, and the thickness of the silica particle layer 3 shown in FIG. 1(b) is adjusted.

Further, the thickness of the chromate film formed on the surface of the steel plate 1 by the chromate treatment can be adjusted as desired, whether the treatment is performed by a coating method or by a cathodic electrolytic treatment method.

As described above, according to the present invention, the thickness of the silica particle layer and the chromate coating can be adjusted independently and arbitrarily, so that a suitable thickness of chromium with excellent corrosion resistance and paintability can be formed on the surface of the steel plate. - A silica composite coating can be easily formed.

When preparing the zinc-silica composite electroplated steel sheet A' shown in FIG. It is preferable to use

What is the particle size of silica particles?

It is preferable to limit the thickness to 1 μm or less. If the particle size exceeds 1 μm, silica will peel off from the surface of the steel sheet along with zinc during the above-mentioned anodic electrolytic treatment, making it impossible to form a silica particle layer with a uniform thickness on the surface of the steel sheet. The eutectoid rate of silica in the zinc-silica composite plating film 2 is 0, 1
Wt,% or more is preferable. The eutectoid rate of silica is 0,
If it is less than I Ilt, %, silica will peel off from the surface of the steel sheet along with zinc during the anodic electrolytic treatment, making it impossible to form a silica particle layer with a uniform thickness on the surface of the steel sheet.

The electrolytic solution used when subjecting the zinc-silica composite electroplated steel sheet A' shown in FIG. 1(a) to anodic electrolytic treatment may be an aqueous solution or a non-aqueous solution as long as it has conductivity. Further, the current density during the anodic electrolytic treatment can be arbitrarily selected depending on the electrolytic solution used within a range that does not generate bubbles.

Next, the present invention will be explained by examples.

〔Example〕

Acidic dumbbell electroplating bath consisting of the following: Zinc sulfate: 300g/F! Sodium sulfate: 30 g/Q Sodium acetate: 15 g/Q is used as a basic bath, and colloidal silica containing silica particles with a particle size of 1 μm or less is added to this basic bath with a silica particle content of 0.1 to 200 g IQ. An acidic zinc electroplating bath was prepared by adding sodium nitrate so that the content of nitrate ions was within the range of 0.1 to 20 gIQ. Using the acidic zinc electroplating bath prepared in this way, the wall thickness was 0.7wn.
A cold-rolled steel sheet was electrogalvanized in an amount of 40 g/m)N, and then cathodic electrolytic treatment was applied at a current density of 50 A/d♂ to uniformly distribute silica particles on the surface of the steel sheet. A zinc-silica composite electroplated steel sheet having a zinc-silica composite plating film dispersed in zinc-silica was prepared.

Then, using a 5% sodium chloride solution,
The silica composite electroplated steel sheet was subjected to anodic electrolysis treatment at a current density of IA/di for 30 seconds to dissolve all the zinc in the zinc-silica composite plating film, and a 0.2 μm thick silica was applied to the surface of the steel sheet. A particle layer was formed.

Next, a chromate treatment solution containing 50 g/Ω of chromic acid (Cry3) was applied onto the surface of the silica particle layer in a total amount of chromium of 10 to 150 cm/rn', and then placed in a drying oven. By drying at a temperature of 100° C. for 1 minute, specimens Nα1 to 3 of chromate-treated steel sheets according to the method of the present invention (hereinafter referred to as "specimens of the present invention") were prepared.

In addition, the above-mentioned steel sheet with a silica particle layer formed on its surface was subjected to cathodic electrolytic treatment at a current density of 10 A/drrr in an electrolytic chromate treatment solution containing 30 g/[ of chromic acid (Cr○,)]. In this way, test specimens of the present invention Na 4 to 6 were prepared. The amount of chromate deposited was adjusted by the amount of coulombs.

For comparison, chromic acid (Cry): 50g/Q silica powder:
A chromate-treated steel sheet specimen for comparison was prepared by applying a chromate treatment solution containing 100 g IQ onto the surface of a galvanized steel sheet and then drying it in a drying oven at a temperature of 100'C for 1 minute. No. 2 (hereinafter referred to as "comparative specimen") was prepared.

Furthermore.

Chromium 1 (CrO2): 30g/Q Colloidal Silica: 100g10 sulfur! l! :
Comparative specimen NG3.4 was prepared by performing cathodic electrolytic treatment at a current density of 10 A/dm2 in an electrolytic chromate treatment solution containing 0.2 g/Q.

In addition, a comparison specimen Na5 made of a pure galvanized steel plate was prepared.

Table 1 shows the amount of chromium and silica deposited in the chromate coating and the corrosion resistance of the present invention specimens Na] to 6 and comparative specimens Na 1 to 5.

Note that the corrosion resistance was evaluated by the method described below.

That is, the specimen was immersed in 5% NaCΩ salt water at a temperature of 40°C for 10 minutes, then dried at a temperature of 60°C for 10 minutes, and then immersed in an atmosphere at a temperature of 50°C and a relative humidity of 95% for 10 minutes. After 300 cycles of treatment, each cycle consisting of wetting, the amount of reduction in the thickness of the specimen was measured, and the following evaluation was made based on the amount of reduction in the obtained thickness.

◎: Plate thickness reduction 0.1wn or less, O: Plate thickness reduction more than 0.1m+~0.2+m, △:
Amount of plate thickness reduction exceeding 0.2m to 0.3mn, ×: Amount of plate thickness reduction exceeding 0.3+m+.

Table 1 As is clear from Table 1, a chromate treatment solution containing silica powder is applied to the surface of the zinc-plated steel sheet and then dried to form a chromium-silica composite coating on the surface of the zinc-plated steel sheet. Comparative specimen Nn 1 formed
and 2, the corrosion resistance was slightly inferior, and
Although not shown in the table, the paintability was poor. In addition, comparative specimen N was prepared with a chromium-silica composite coating formed on the surface of a dumbbell-plated steel sheet by cathodic electrolytic treatment in an electrolytic chromate treatment solution containing silica powder.
In a 3 and 4, the paintability was good, but
Corrosion resistance was poor. Comparative specimen Na 5, which was made of a pure galvanized steel sheet, had poor corrosion resistance and paintability.

On the other hand, specimens Nos. 1 to 6 of the present invention had excellent corrosion resistance and paintability.

〔Effect of the invention〕

As described above, according to the present invention, on the surface,
This provides an industrially useful effect in that a chromate-treated steel sheet having a uniform ROM-silica composite coating with excellent corrosion resistance and paintability can be easily produced.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the manufacturing method of the present invention. In the drawings, 1. steel plate, 2... dumbbell-silica composite plating film, 3. silica particle layer, 4. chromium-silica composite film.

Claims (3)

[Claims] 1. A steel plate is subjected to cathodic electrolysis treatment in an acidic zinc electroplating solution containing silica particles to form a zinc-silica composite plating film in which silica particles are uniformly dispersed on the surface of the steel plate, and then the electrolytic solution In the process, a steel plate on which the zinc-silica composite plating film is formed is subjected to anodic electrolysis treatment to selectively dissolve all or part of the zinc in the zinc-silica composite plating film. , forming a desired amount of a silica particle layer on the surface of the steel plate, and then subjecting the steel plate on which the silica particle layer is formed to a chromate treatment, to obtain a surface of the steel plate on which the silica particle layer is formed. A chromate film is formed on the steel plate, and the silica particle layer is embedded in the chromate film, thereby forming a chromium-silica composite film in which silica particles are uniformly dispersed on the surface of the steel plate. , a method for producing chromate-treated steel sheets. 2. 2. The method according to claim 1, wherein the chromate treatment is performed by a coating method comprising applying a chromate treatment solution containing chromium ions to the surface of the silica particle layer of the steel plate and then drying. 3. 2. The method according to claim 1, wherein the chromate treatment is performed by a cathodic electrolytic treatment method comprising subjecting the steel plate on which the silica particle layer is formed to cathodic electrolytic treatment in an electrolytic chromate treatment solution containing chromium ions.