JPS63157900A - Production of zinc-chromium composite plated steel sheet - Google Patents

Abstract

PURPOSE:To inexpensively produce the title Zn-Cr composite plated steel sheet having excellent corrosion resistance and paint adhesion after coating, by adding an appropriate amt. of the positively charged alumina sol having a specified Cr<3+> ion adsorption to an acidic galvanizing bath, and carrying out electrolysis at an appropriate pH. CONSTITUTION:Cr<3+> ions are adsorbed on the Al2O3 particles dispersed in an aq. soln. to obtain a positively charged alumina sol wherein the Cr<3+> ion adsorption is controlled to 0.3-2 molar ratio of Al2O3/Cr. The alumina sol is added to the acidic galvanizing bath to the extent of 0.2-20g/l, expressed in terms of Al2O3. The plating bath is used, and electrolysis is carried out at >=1pH with the steel sheet as the cathode to deposit eutectoid Zn-Al2O3-Cr on the steel sheet surface. When the electrolysis is carried out at high current density, >=10g/l pH buffer soln. consisting of >=1 kind among boric acid, an amino acid such as glycin, an org. carboxylic acid such as citric acid, and their salts if preferably added to stabilize the dispersibility of the alumina sol. By this method, the corrosion resistance and coated film adhesion of the Zn-Cr composite plated steel sheet after coating can be improved without an increase in the production cost.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a composite plated steel sheet, and more particularly to a method for manufacturing a zinc-chromium composite plated steel sheet that has excellent corrosion resistance and coating adhesion after painting.

<Prior art and its problems> Galvanized steel sheets are widely used in materials that require corrosion resistance, such as automobiles, home appliances, and building materials. This is because the pure galvanized layer is base against the iron of the steel sheet, so the zinc is corroded first against plating defects such as pin poles and exposed areas of the base metal due to the sacrificial corrosion protection effect. This is because it has the effect of preventing red rust from forming on the base steel.

However, pure zinc produces conductive corrosion products in a corrosive environment dominated by a humid atmosphere, resulting in a significantly increased corrosion rate. Further, under the painted film, corrosion products of zinc cause the paint film to blister, leading to peeling of the paint film. In addition, after painting with 3 coats of phosphate treatment, the chemical conversion film was dissolved in a water resistant secondary adhesion test (immersed in warm pure water for 10 days).
The paint film peels off.

In order to improve the corrosion resistance of zinc plating,
In order to suppress the activity of the galvanized layer, a method has been considered in which a metal having a higher potential than zinc is deposited as an alloy.

For example, ■ A method of plating in an acidic Zn electroplating bath in which trivalent chromium salt is dissolved at a concentration of 3 g/It (including C3+) to have uniform and good surface color and gloss, and improve corrosion resistance ( Special Publication No. 58-56039).

(2) A method of containing Go, Mo, W and Fe in a Zn plating bath (Japanese Patent Publication No. 16522/1983).

■7. Co, Cr3+, Cr6+, In in n plating bath
, A method of plating with the addition of Zr to improve the bare corrosion resistance of the Zn plating film itself and at the same time to improve the chromate treatability (Japanese Patent Publication No. 56-517).

However, although the steel sheets obtained by these methods have better bare corrosion resistance than pure Zn plating, the corrosion resistance after painting is such that if cationic electrodeposition is applied after phosphate treatment, the paint film will blister. There were some easy drawbacks.

In addition, in the method (■), 3 Cr3+ is added to the acidic Zn plating bath.
g/I1. Plating is performed with the above additions, but despite the high concentration of Cr3+ in the plating bath, C
The amount of r eutectoid is 0.0001 to 0.001 wt %, which means that only a small amount of eutectoid occurs, and a sufficient effect of Cr cannot be obtained.

In method (2), Cr3+ that can be added to the plating bath
The concentration is 0.7g/f1. is the upper limit, and if it exceeds it, it will be difficult to dissolve in the plating bath, and Cr3
It was not possible to add more than 0.7 g/JZ of +.

<Objective of the Invention> The object of the present invention is to solve the drawbacks of the prior art and to improve the corrosion resistance and coating adhesion of zinc-chromium composite plated steel sheets after painting without increasing production costs. The purpose is to provide a manufacturing method.

<Structure of the Invention> The present inventors have aimed to improve the corrosion resistance and coating adhesion after painting without significantly changing the manufacturing conditions of electrogalvanized steel sheets, minimizing the increase in manufacturing costs, and in particular improving the corrosion resistance and coating adhesion after painting. Based on this, we conducted extensive research.

As a result, C.31 ions were adsorbed in positively charged alumina sol in advance, added to an acidic Zn plating bath with a pH of 1 or higher, and plating was carried out.The alumina and chromium in the plating enabled plating of corroded areas and adhesion of the coating film. The present inventors have discovered that it is possible to improve properties such as water resistance and secondary adhesion, leading to the present invention.

That is, in the first aspect of the present invention, a positively charged alumina sol having an adsorption amount of trivalent chromium ions of 0.3 to 2 in IL203/Cr molar ratio is 0.2 to 20 g/in terms of alumina.
To provide a method for producing a zinc-chromium composite plated steel sheet, which is characterized in that zinc-alumina-chromium is eutectoided on the surface of the steel sheet by adding it to an acidic zinc plating bath and electrolyzing the steel sheet as a cathode at pH 1 or higher.

The second aspect of the present invention is that the adsorption amount of trivalent chromium ions is A
IL□0. /Cr molar ratio of 0.3 to 2 positively charged alumina sol to 0.2 to 20 g/R in terms of alumina, boric acid,
Zinc-alumina-chromium Provided is a method for manufacturing a zinc-chromium composite plated steel sheet, characterized by eutectoiding the zinc-chromium composite plated steel sheet.

The present invention will be explained in detail below.

Positively charged alumina sol with three chromium ions adsorbed is one in which Cr3+ ions are adsorbed onto Aft203 particles, and the following production method can be cited as a representative example, but the present invention is limited to this. It is not something that will be done.

(2) Disperse IL203 particles in an aqueous solution by stirring, adjust the pH of the aqueous solution to less than 3.5 (:, r 3
A method of gradually adding + ions.

■After adding C'34 ions to the alumina sol solution,
A method of powerful stirring using a high-speed stirrer such as a disper.

The adsorption amount of Cr'+ ions on alumina sol is AI! ,2
The molar ratio of 03/Cr is preferably 0.3 to 2.

If this value is less than 0.3, Cr into the plating layer,
The amount of eutectoid AJI203 is small and does not contribute to improving corrosion resistance, water resistant secondary adhesion, etc.

Moreover, when it exceeds 2, the viscosity of the plating solution increases, making plating difficult and eutectoiding of chromium and alumina into the plating layer becomes uneven.

Further, the amount of the positively charged alumina sol added to the plating solution is preferably 0.2 to 20 g/ji in terms of alumina. 0.
If it is less than 2 g/l, the effect of improving plating on corroded areas and coating adhesion will be insufficient, and if it exceeds 20 g/4, the viscosity of the plating solution will increase, making plating difficult.

Further, the zinc plating bath used in the present invention is an acidic zinc plating bath having a pH of 1 or more.

If the pH is less than 1, the amount of Cr eutectoided into the plating layer will be significantly reduced, and the dispersibility of the colloidal alumina sol will be poor, which is not preferable.

In addition, when one or more pH buffers selected from the group consisting of amino acids such as boric acid and glycine, organic carboxylic acids such as citric acid, and salts thereof, are added at least 71 g, it is possible to increase the suppresses the rapid increase in p)I at the plating bath and cathode interface, and stabilizes the dispersibility of the alumina sol. If it is less than 10g74, the pH buffering property will not be sufficient.

The reason why a composite plating containing alumina and chromium in Zn suppresses the blistering of the paint film in a corrosive environment and exhibits excellent performance in water-resistant secondary adhesion is not necessarily clear, but it is as follows. It can be considered.

The 3-coat material with phosphate treatment is resistant to corrosion in corrosive environments.
As the corrosion of the plating layer progresses, the defective parts and scratches in the coating film become the anode, and the area under the coating film becomes the cathode.
The reduction of 02 shown in 1) occurs and alkali (OH-) is produced. By this OH-, the coating film and the chemical conversion coating under the coating film (in the case of Zn-based plating, Hopite (Zn3
(PO4)2-4820) is dissolved, Zn(OH)2 as shown in formula (2) is produced, and further Zn(OH)2 as shown in formula (3) is dissolved.
) etc. lead to ZnO.

(Reduction reaction of 02) 1/202 + H2O+ 2e →20H-(1)(
Hopite dissolution reaction) Zn3(PO4)2・4H20+20H--I-37n
(OH)2+ 282PO4-(2) (decomposition of zinc hydroxide) Zn(OH)2→ZnO+ H2O(3) Formed Z
Since nO is brittle and conductive, it reduces the adhesion of the coating and accelerates corrosion.

This action reduces the adhesion of the paint film, and 820 penetrates due to the osmotic pressure action, pushing the paint film upward and causing blistering.

Zn added with alumina sol adsorbing Cr3+ ions
When plating is performed in a plating bath, it is thought that C r 3 + ions adsorbed to the eutectoid alumina exist in the Zn plating layer as hydroxides or oxides around the alumina. The chemical conversion coating is dissolved and Zn
If Cr3+ ions are present with alumina when (01()2) is formed, Cr3+ will be incorporated into the corrosion products, stabilizing the less conductive Zn(OH)2,
Furthermore, since Zn (OH) 2 is retained by alumina, generation of ZnO can be suppressed and destruction of the corrosion product film can be prevented. This is thought to suppress blistering of the paint film.

<Examples> Next, the present invention will be specifically described with reference to Examples and Comparative Examples.

A cold rolled steel plate (SPCD) was subjected to alkaline electrolytic degreasing, pickled with 5% hydrochloric acid, washed with water, and plated under the following conditions.

The plating solution is stirred by a pump, and the liquid flow rate is approximately 60 m/
A pure zinc plate is used for the anode, and the distance between the electrodes is 20 mm.
, and the liquid temperature was 50°C. The date amount was 20 g/rn'. Note that, unless otherwise specified, the alumina sol used was an alumina sol with an alumina content of 20 wt% in which Or'' ions were adsorbed by a high-speed stirring method.

(Example 1) Bath composition Zinc chloride 200g/l Potassium chloride 300g/l Alumina sol 3g/l (alumina equivalent value) A j2203/Cr"=1.2 pH2,5 Current density 90A/dm' Obtained plating layer When examined by glow discharge emission spectroscopy, it was found that alumina particles and chromium were eutectoid uniformly in the depth direction of the plating, and the content was 1wt alumina.
%, Cr: 0.7 wt%.

(Example 2) Bath composition Zinc chloride 300g/4 Sodium chloride 200 gel Alumina sol 10g/l (alumina equivalent value) A 120. /Cr3"-0,5 pH H2,0 Current density 120A/dm" In the resulting plating layer, alumina particles and chromium are eutectoid uniformly in the depth direction of the plating, and the content is alumina: 0.3 wt%, chromium: 0.4 wt%.

(Example 3) Bath composition Zinc chloride 200 g/I/.

Ammonium chloride 300g/l Alumina sol 20g/R.

(Alumina equivalent value) A It 20310r3”-1,9 pH1 Current density 150A/d In the resulting plating layer, alumina particles and chromium were eutectoid uniformly in the depth direction of the plating, and the content were alumina: 2 wt% and chromium: 0.9 wt%.

(Example 4) Bath composition Zinc sulfate 200g/Il.

Magnesium chloride 80g/4 Glycine 20g/l Alumina sol 0.3g/l (alumina equivalent value) A 1203/Cr"-0,8 p H2,0 Current density 150A/dThe resulting plating layer contains alumina particles and chromium were uniformly eutectoid in the depth direction of the plating, and the content was 0.2 wt% for alumina and 0.4 wt% for chromium.

(Example 5) Bath composition Zinc sulfate 300 g/I.

Sodium sulfide 40 g/JL Aluminum sulfate Log/l Sodium citrate 10 g/l Alumina sol 5 g/It (alumina equivalent value) A IL 203/CS"-1,0 p) (1,5 Current density 150 A/d In the plating layer, alumina particles and chromium were eutectoid uniformly in the depth direction of the plating, and the content was 0.7 wt% alumina and 0.7 wt% chromium.

(Comparative Example 6) Bath composition Zinc chloride zoog/It Sodium chloride 100g/fL Alumina sol 4g/l (alumina equivalent value) A 11.2037Cr”-0.1 pH3 Current density 80A/d , alumina particles and chromium were eutectoid uniformly in the depth direction of the plating, and the content was 2 wt % for alumina and 0.4 wt % for chromium.

(Comparative Example 7) Bath composition Zinc sulfate 200 g/l Potassium sulfate 30 g/4 Boric acid 5 g/2 Alumina sol 6 g/II.

(Alumina equivalent value) A l 20. , /Cr''-3,0 pH H1,0 Current band 40A/dThe content of alumina and chromium in the obtained plating layer was 0.3wt% and 0.8wt%, respectively. The eutectoid was non-uniformly eutectoid in the depth direction of the plating.

(Comparative Example 8) Bath composition Zinc chloride 200 g/basic potassium chloride 350 gait Alumina sol 10 g/l (alumina equivalent value) (No Cr adsorbed) pH 4.5 Current density 80 A/d obtained In the plating layer, alumina particles were eutectoid uniformly in the depth direction, and the content thereof was 0.8 wt%.

(Comparative Example 9) Bath composition Zinc chloride 200g/l Potassium chloride 350g/l Alumina sol 5g/2 (alumina conversion value) A 11 . 03/Cr"-1,8 p H0,5 Current band 100A/dm' The contents of alumina and chromium in the resulting plating layer are as low as 0.005wt% and 0.005wt%, respectively, and the plating depth It was uniformly eutectoid in the horizontal direction.

The composite plated steel sheets obtained in the above Examples and Comparative Examples were subjected to phosphate treatment (Granozin SD manufactured by Nippon Paint Co., Ltd.).
-2000) at a coating amount of 2 to 3 g/ml,
Cationic electrodeposition coating (Nippon Paint Group U-30) film thickness 2
Apply 0 to 22μ, then apply intermediate coat and top coat.
The total film thickness was 90 to 100 μm. The following tests were conducted on these coated plates.

(1) Water resistance secondary adhesion test After immersing in pure water (50°C), scratches are made on the coating film in the shape of a 2 mm square substrate, and the degree of coating film peeling is measured by peeling off with tape.

The results are shown in Figure 1.

(2) Corrosion resistance after 3 pieces As shown in Figure 2, a perforated material with 2510111φ punched holes 2 was made before painting, painted with 3 coats to form painted board 1, and the punched holes were sealed with seal tape 3. A cyclic corrosion test was conducted under the following conditions, covering one side and the surrounding area of the plate.

Salt spray test (JIS Z 2371) 2 hours, dry (70°C) 2 hours, wet (50°C, 95% relative humidity) 2
Time, freezing (-23℃) 2 hours is one cycle, 80
After the cycle, the swelling width of the coating around the perforated area was evaluated.

The results are shown in Figure 3.

<Effects of the Invention> According to the present invention, trivalent chromium ions are
By adding 0.2 to 20 g/l of positively charged aluminum sol adsorbed at a molar ratio of 0.3 to 2 into an acidic zinc plating bath and plating at a pH of 1 or higher, efficiency can be improved (Z
Zn that can eutectoid n-8n□03-Or, improves water resistant secondary adhesion, and suppresses paint film blistering in corroded areas.
-Cr composite plated steel sheets can be manufactured at low cost.

Further, in the method of the present invention described above, which uses a plating solution containing a predetermined amount of pH buffer, a uniform composite plated steel sheet with stable alumina sol dispersibility even under high current density can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of a waterproof secondary adhesion test. FIG. 2 is a perspective view of the corrosion test material. FIG. 3 is a graph showing the average coating film blistering size (mm) after 80 cycles of the cyclic corrosion test. Explanation of symbols 1...Painted plate, 2---25 +n+nφ punched hole, 3...Seal table

Claims (2)

[Claims] (1) Adsorption amount of trivalent chromium ions is Al_2O_3/C
A positively charged alumina sol with an r molar ratio of 0.3 to 2 is added to an acidic zinc plating bath of 0.2 to 20 g/l in terms of alumina, and electrolyzed at a pH of 1 or higher using the steel plate as a cathode to coat the surface of the steel plate with zinc-alumina. - A method for manufacturing a zinc-chromium composite plated steel sheet, characterized by eutectoiding chromium. (2) Adsorption amount of trivalent chromium ions is Al_2O_3/C
Positively charged alumina sol with a molar ratio of 0.3 to 2 and 0.2 to 20 g/l in terms of alumina, and 1 selected from the group consisting of boric acid, amino acids, organic carboxylic acids, and salts thereof.
Zinc-chromium composite plating characterized by eutectoiding zinc-alumina-chromium by electrolyzing a steel plate as a cathode at a pH of 1 or more in an acidic zinc plating bath to which 10 g/l or more of pH buffering agent or more is added. Method of manufacturing steel plates.