JPH0142356B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Field of Application> The present invention relates to a composite electroplated steel sheet having excellent properties such as bare corrosion resistance, post-painting performance (corrosion resistance and paint adhesion), and weldability, and a method for manufacturing the same. <Prior art and its problems> Galvanized steel sheets are widely used as rust-prevention treated steel sheets for automobiles, home appliances, building materials, etc. that require corrosion resistance. This is because the pure zinc layer is base against the iron of the steel sheet, so if there are plating defects such as pinholes or exposed areas of the base steel due to processing, the zinc will corrode first, which is sacrificial corrosion protection. This is because it is effective and has the effect of preventing the occurrence of red rust on steel plates. However, pure zinc produces conductive corrosion products when exposed to salt spray or in a humid environment, resulting in extremely high corrosion rates.Furthermore, under the painted coating, the zinc corrosion products cause the coating to blister and eventually peel. There are drawbacks to pure Zn due to its active nature, such as . On the other hand, in order to improve the corrosion resistance of Zn plating,
In order to suppress the activity of the Zn-plated layer, a method has been considered to precipitate an alloy of metals that are potentially nobler than Zn, such as Co, Ni, Cr, Fe, etc., and there are many documents and patents on this method. . For example, a method in which Co, Mo, W, and Fe are added to a Zn plating bath. (Japanese Patent Publication No. 47-16522) A method of containing oxides of Mo, W, and Co and/or Ni, Sn, Pb, and Fe in a plating layer. (Special Publication No. 49-19979) Plating is carried out by adding Co, Cr ³⁺ , Cr ⁶⁺ , In, and Zr to the Zn plating bath, and at the same time improves the bare corrosion resistance of the Zn plating film itself. A method for improving chromate treatment properties. (Tokuko Showa 56-
517) A method of plating in an acidic Zn electroplating bath in which 3 g/or more of trivalent chromium salt as Cr ³⁺ is dissolved to provide uniform and good surface color and gloss and improve corrosion resistance. (Tokuko Showa 58-
56039) etc. However, the steel sheets obtained by these methods are pure
Although bare corrosion resistance is better than that of Zn plating, the corrosion resistance after coating has the disadvantage that the coating film tends to blister when cationic electrodeposition coating is applied after phosphate treatment. Also, the method is during the plating bath.
By plating with the addition of Cr ³⁺ and Cr ⁶⁺ , the purpose is to improve the chromate treatment properties of galvanized steel sheets and improve the corrosion resistance after chromate treatment. There is no improvement in corrosion resistance when cationic electrodeposition coating is applied after phosphate treatment. <Object of the Invention> The object of the present invention is to provide a composite plated steel sheet that eliminates the drawbacks of the prior art, has excellent corrosion resistance both unpainted and after painting, and has excellent workability, paint adhesion, and weldability, and its production. The purpose is to provide a method. <Structure of the Invention> Such an object is achieved by the following present invention. In other words, the present invention includes metal in the electroplated Zn layer.
Co and Co oxide 0.1 to 10 wt% as total Co amount,
Cr oxide and Cr hydroxide as total Cr amount 0.05
This is a highly corrosion-resistant composite plated steel sheet characterized by containing Al oxide and Al hydroxide in an amount of 1.6 to 8 wt% based on the total amount of Al. In addition, the present invention provides Co ²⁺ in an acidic electroplating bath.
0.3 to 60 g of one or more water-soluble compounds as metal cobalt, and 0.2 to 0.2 g of one or more water-soluble Cr ³⁺ compounds as metal chromium.
This is a method for producing a highly corrosion-resistant composite plated steel sheet, which is characterized in that plating is performed by adding ~2.5g/pseudo-boehmite-like alumina sol/0.5~20g/alumina equivalent. The highly corrosion-resistant composite galvanized steel sheet of the present invention and its manufacturing method will be explained in detail below. Including Co in the Zn plating layer improves bare corrosion resistance. This is because Co ²⁺ is generated during the process of corrosion of Zn and Co in the plating layer, contributing to the generation of corrosion products with excellent protective effects and their stability. According to ESCA (high electron spectroscopy), Co is in a metal and oxide state, and the total amount of Co is preferably 0.1 to 10 wt%. In the composite galvanized steel sheet of the present invention, the Co content is set at 0.1 to 10.0wt%.The reason is that if the Co content is less than 0.1wt%, the effect of improving bare corrosion resistance will not be exhibited, This is because if it exceeds the above effect, it becomes saturated and it is economically undesirable to contain more than that amount. Furthermore, the appearance of the fitting becomes black,
This is because the commercial value decreases, and as the alloy content increases, the hardness of the plated layer increases, resulting in a decrease in workability. When Cr coexists with Co and Al, it has the effect of improving the bare corrosion resistance of the plating film itself, and this effect is particularly noticeable in the early stages of corrosion. Furthermore, Cr has a great effect of improving paint film adhesion. In the composite plated steel sheet of the present invention, the above Cr content is 0.05 to 5wt.
The reason for this is that if Cr is less than 0.05wt%, no improvement in bare corrosion resistance will be seen even if it coexists with Co and Al, and if it exceeds 5wt%, the above effect will be saturated and the plating will be reduced. This is because the adhesion is also slightly reduced. Al is thought to be eutectoid in the form of oxide or hydroxide in the plating layer, and
The eutectoid promoting effect of Cr and the formation of a dense and stable corrosion product film together with Co and Cr in a corrosive environment, and Zn
It has the effect of suppressing the elution of. In the composite plated steel sheet of the present invention, the above Al content is 1.6 to
8wt% is preferred. The reason is that the Al content is
When the content is 0.05wt% or more, corrosion resistance is improved, but when the content is 1.6wt% or more, a sufficient corrosion resistance improvement effect is obtained, and when it exceeds 8wt%, the plating adhesion slightly decreases. The method for manufacturing a composite plated steel sheet of the present invention includes adding one or more water-soluble Co ²⁺ compounds as metallic cobalt in an amount of 0.3 to 60% in an acidic electrolytic Zn plating bath.
g/, 0.2 to 2.5 g of one or more water-soluble compounds of Cr ³⁺ as metallic chromium, and 0.5 to 20 g of pseudoboehmite-like alumina sol in terms of Al ₂ O ₃
Added. This plating liquid has a pH of 1 or higher and a current density of 40A/dm ²
or more, preferably PH2 to 3.5, current density
It is better to set it to 60A/dm ² or more. Here, water-soluble compounds of Co ²⁺ include cobalt chloride, cobalt sulfate, cobalt nitrate, etc., or compounds that can be dissolved in a known acidic zinc plating bath. Water-soluble compounds of Cr ³⁺ include chromium chloride, chromium nitrate, chromium sulfate, chromium potassium sulfate, and the like. Next, the effects of the components added to the electric Zn plating bath in the method for producing a composite plated steel sheet of the present invention will be explained. (1) Co ²⁺ eutectoids with Zn during plating, passivates the plating film, and suppresses dissolution of the plating layer, thereby improving bare corrosion resistance. The amount of Co added to the plating bath is 0.3 to 60 g/.
The reason for this is that if it is less than 0.3g/, the amount of Co eutectoid in the plating layer will be small and sufficient corrosion resistance will not be obtained, and if it exceeds 60g/, the plating appearance will be black and the plating adhesion will be poor. This is because it is economically unfavorable. (2) Cr ³⁺ eutectoids as chromium oxide and hydroxide in the plating layer, and together with Co and Al oxide (estimated to be AlOOH), improves the corrosion resistance (bare corrosion resistance) of the plating layer itself. Furthermore, it improves adhesion with the paint film. Alumina sol (pseudo-boehmite-like) eutectoids as Al oxide (AlOOH) in the plating layer. The alumina sol is preferably pseudo-boehmite-like with a particle size of 5 to 30 nm. Amorphous alumina sol (particle size of 100 to 200 nm) is not preferred because Al is difficult to eutectoid in the plating layer and thickening is severe. This pseudoboehmite-like (AlO _x (OH) _y form)
By adding this alumina sol, Cr, which is generally difficult to co-deposit uniformly and in large amounts, can be precipitated uniformly and in large amounts together with Al oxide. The reason for this is that trivalent chromium is adsorbed to the negatively charged alumina sol, and both co-deposit simultaneously. The precipitated Cr and Al oxides, together with Co, further improve the bare corrosion resistance of the plating layer itself, and also form and maintain stable corrosion products (zinc hydroxide) on the plating surface. Furthermore, the corrosion resistance after cationic electrodeposition coating after phosphate treatment is also dramatically improved when alumina sol is added compared to Zn-Co-Cr based plating. Although the reason for this is not clear, it is thought that appropriate sacrificial corrosion protection and good paint film adhesion combine to suppress blistering of the paint film and elution of the base metal. The amount of Cr ³⁺ added to the plating bath is 0.2 to 2.5 g/metal chromium. In this case, the amount of alumina sol in the bath is required to be 0.5 g/(in terms of Al ₂ O ₃ ) or more. The above reason is that if the amount is less than 0.2 g/l, sufficient effects in improving coating film adhesion and corrosion resistance cannot be obtained;
This is because if it exceeds 2.5 g/l, the adhesion of the plating decreases and green oxides adhere to the plating surface, which deteriorates the appearance, which is not preferable. Also, the amount of alumina sol added is 0.5 to 20g/
(in terms of Al ₂ O ₃ ). The reason for this is that if it is less than 0.5g/, the amount of Cr and Al eutectoid in the plating layer is insufficient and sufficient effects in improving corrosion resistance and coating adhesion cannot be obtained, and if it exceeds 20g/, plating will occur. This is because the viscosity of the liquid increases, making it difficult to tighten. Note that when using atomic absorption spectroscopy, only acid-soluble Al can be analyzed for quantitative determination of Al element.
It is preferable to quantitatively analyze the total amount of Al using a calibration curve using EPMA (X-ray microanalyzer). FIG. 1 is a schematic cross-sectional view of a steel plate coated with Zn-Co-Cr-Al system plating. In the figure, 1 shows a plating layer, 2 shows a steel plate, and 4 shows an Al oxide (AlOOH). As shown in Figure 1, the presence of Al oxide on the plating surface increases the insulation resistance of the plating film, shifting the appropriate welding current range to the lower side, and therefore increasing heat generation with a small welding current. be able to. In other words, the appropriate welding current range is 6.5 to 13KA for Zn-Co-Cr system, while Zn-Co-Cr-Al
The welding current is 6 to 12.5 KA in the system, and since welding can be performed with a low welding current, the number of continuous welding points in spot welding increases. In the method for producing a composite plated steel sheet of the present invention, electrolytic Zn plating is performed in an acidic bath such as a chloride bath or a sulfuric acid bath. In the bath composition mentioned above, the pH of the plating solution is 1.0.
Above, it is preferably 2 to 3.5. The reason for this is that if the pH is less than 1.0, Cr is difficult to eutectoid in the plating layer (see Figure 2). Also, the PH
If it exceeds 3.5, Cr hydroxide is generated in the plating bath,
Since there are disadvantages such as instability of the plating bath during operation in a continuous plating line, it is desirable to keep the pH at 3.5 or less. The current density is 40A/dm2 or ^more , preferably 60A/dm2 or more.
It is better to set it to dm ² or more. The reason for this is that when the current density is less than 40 A/dm ² , the plating appearance becomes blackish gray and the plating adhesion deteriorates. <Example> Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. Alkaline electrolytic degreasing of cold rolled steel plate (SPCC), 5
After pickling with % hydrochloric acid, washing with water, and plating under the following conditions. Stirring is performed by a pump, the liquid flow rate is approximately 60 m/min, and a pure zinc plate is used as the anode.
The distance between the electrodes was 10 mm, and the liquid temperature was 50°C. The basis weight is
It was set to 20g/ ^m2 . Furthermore, for Al, alumina sol #520 manufactured by Nissan Chemical, which is a water-dispersible colloidal sol, was added to the plating solution. The conditions of the plating baths of Examples and Comparative Examples are described below. Examples 1 to 9 Chloride bath ZnCl ₂ 200 g/KCl 350 g/ CoCl ₂ ×6H ₂ O (0.3 to 5.9 g/as metal Co) CrCl ₃ 6H ₂ O (0.2 to 2.5 g/as metal Cr) Alumina sol (pseudo boehmite-like/particle size 15nm)
(No thixotropy) 0.5-20g/ (Al ₂ O ₃ equivalent) PH3, bath temperature 50℃, current density 100A/dm ² Examples 10-13 Sulfuric acid bath ZnSO ₄ 350g/ Na ₂ SO ₄ 40g/ CoSO ₄・7H ₂ O (1.2 to 59.2 g/as metal Co) CrCl ₃・6H ₂ O 12 g/ (2.5 g/ as metal Cr) Alumina sol (pseudo-boehmite-like, particle size 15 nm)
0.5-20g/PH3, bath temperature 50℃, current density 80A/dm ^2Comparative Examples 1-2 Chloride bath Same as Examples 1-9 except that 2g/amorphous alumina sol with a particle size of 100nm was added. did. Comparative Examples 3 to 5 Chloride bath CoCl ₂・6H ₂ O (0.2 g/as metal Co) CrCl ₃・6H ₂ O (0.1 to 2.6 g/ as metal Cr) Alumina sol (pseudo-boehmite-like, particle size 15 nm)
2 g/(in terms of Al ₂ O ₃ ) ZnCl ₂ , KCl, and plating conditions were the same as in Examples 1 to 9. Comparative Examples 6-7 Chloride bath Plating conditions: PH3, bath temperature 50℃, current density 30A/
The procedure was the same as in Examples 1 to 9 except that ^dm2 was used. Comparative Examples 8 to 10 Sulfuric acid bath CoSO ₄・7H ₂ O (10.5 to 70 g/as metal Co) K ₂ Cr ₂ (SO ₄ ) ₄・24H ₂ O (1.0 to 2.6 g/ as metal Cr) Alumina sol (pseudo-boehmite-like・Particle size 15nm)
0.1-30g/ (Al ₂ O ₃ equivalent) ZnSO ₄ , Na ₂ SO ₄ , plating conditions are as in Examples 10-13
The same is true. The following tests were conducted on the tempered steel plates obtained in the above Examples and Comparative Examples. The results are shown in Table 1 (Example) and Table 2 (Comparative Example). Incidentally, Co and Cr in the plating layer were determined using atomic absorption spectrometry, and Al was determined using EPMA. (1) Plating adhesion evaluation method Dupont impact test (center of impact diameter 1/4 inch, weight 1
The peeling state of the plating layer was evaluated after the test was carried out. ◎: No peeling 〇: Slightly peeling △: Slightly peeling ×: Peeling (2) Workability evaluation method As shown in Fig. 3, cup drawing is applied to the plated steel plate 5, and tape peeling is performed on the drawn surface.
The weight loss was measured and evaluated. ◎: 0 to 2 mg ○: 2 to 5 mg △: 5 mg or more ×: Unable to squeeze (3) Corrosion resistance evaluation method Bare corrosion resistance and corrosion resistance after painting were comprehensively evaluated. (3-1) Bare corrosion resistance A salt spray test (JIS Z2371) was conducted and 720
The plate thickness reduction value after time was measured and evaluated. (3-2) Corrosion resistance after painting Samples were treated with phosphate (Bonderite #3030, manufactured by Nippon Parkerizing Co., Ltd.), and then cationic electrodeposition coating (Power Top U-30 Gray, manufactured by Nippon Paint Co., Ltd.) was applied to a thickness of 20 μm. A cross-cut was made until reaching 1, and the blister width was evaluated after 340 hours of salt spray (JIS Z2371). Plate thickness reduction value Bulge width (bare corrosion resistance) (corrosion resistance after painting) ◎: 0.2mm or less ◎: 0~2mm 〇: 0.2~0.4mm 〇: 2~4mm △: 0.4~0.6mm △: 4~6mm ×: 0.6 mm or more ×: 6 mm or more

【table】

[Table] <Effects of the Invention> The steel sheet obtained by the present invention has excellent corrosion resistance in the plating layer itself as well as corrosion resistance after coating, and can further improve workability, paint adhesion, and weldability. Furthermore, since it is only necessary to specify the bath composition, composite plated steel sheets can be easily produced.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a steel plate coated with Zn-Co-Cr-Al system plating. Figure 2 shows ZnCl ₂ 200
g/, KCl350g/, CoCl ₂・6H ₂ O12g/,
CrCl ₂・6H ₂ O13.5g/, alumina sol 2g/
2 is a graph showing the relationship between the pH of the plating solution and the amount of Cr eutectoided into the plating layer at a bath temperature of 50° C. and a current density of 150 A/dm ² . FIG. 3 is a cross-sectional view of a tamed steel plate subjected to cup drawing in a workability evaluation test. Explanation of symbols, 1...Plated layer, 2...Steel plate, 4
...Al oxide (AlOOH), 5...Plated steel plate.

Claims (1)

[Claims] 1. Metallic Co and Co oxide are contained in the electrolytic Zn plating layer in an amount of 0.1 to 10 wt% based on the total amount of Co, Cr oxide and Cr
A highly corrosion-resistant composite plated steel sheet, characterized in that it contains hydroxide in a total Cr content of 0.05 to 5 wt%, and Al oxide and Al hydroxide in a total Al content of 1.6 to 8 wt%. 2 Adding one or more water-soluble compounds of Co ²⁺ as metallic cobalt in an acidic electrolytic Zn plating bath.
0.3 to 60 g/, 0.2 to 2.5 g/of one or more Cr ³⁺ water-soluble compounds as metal chromium,
Pseudo-boehmite alumina sol in terms of alumina
A method for producing a highly corrosion-resistant composite plated steel sheet, which comprises adding 0.5 to 20 g/plating.