JPH0242917B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a highly corrosion-resistant electrical composite zinc-plated steel sheet with excellent metallic luster. (Conventional technology) Zinc-plated steel sheets have a wide range of uses, including automobiles, home appliances, and building materials.In recent years, there has been a desire to improve the durability of products from the perspective of saving resources and energy, and the improvement in corrosion resistance of zinc-plated steel sheets has been strongly desired. It has been demanded.
In particular, this tendency is becoming stronger for plated steel sheets for automobiles. This is because in cold winter regions such as North America and Europe, rock salt and calcium chloride are being sprayed to prevent roads from freezing, and the environment in which cars are used is becoming increasingly harsh. . Automotive steel sheets used in such environments are required to have bare rust resistance, perforation resistance, and the like. (Problems to be solved by the invention) In response to the above requirements, for example,
-Zn-Fe alloy two-layer plated steel sheet of Publication No. 133488,
The Zn--Ni alloy plated steel sheet disclosed in Japanese Patent Publication No. 50-29821 has been developed and commercialized. However, as the characteristics required by users become more sophisticated, surface-treated steel sheets with even higher corrosion resistance are required. Therefore, recently, the possibility of producing a plated steel sheet in which fine particles having corrosion-inhibiting properties are dispersed and co-deposited in the plated layer, so-called a highly corrosion-resistant composite plated steel sheet, has been studied. For example, in JP-A-60-141898, SiO ₂ , TiO ₂ ,
Composite plated steel sheet with dispersed ZrO ₂ etc. has higher JIS Z2371 than alloy plated steel plate which does not contain fine particles.
It is said that the resistance to red rust is improved by about 1.5 times in salt spray tests. However, the above-mentioned composite plated steel sheet has not yet reached the point of satisfying the future goal of ``no external red rust for 5 years and no holes for 10 years''. Therefore, the present inventors have developed a composite galvanized steel sheet with even better corrosion resistance by adding chromate, which has effective anti-corrosion properties, or SiO ₂ , Al ₂ O ₃ , TiO ₂ , _{ZrO2 , Cr2O3}
It has been found that an electrical composite plated steel plate with excellent corrosion resistance can be obtained by cathodic electrolyzing a steel plate in an acidic Zn-based plating solution containing one or more of the following. However, although it has excellent corrosion resistance, in the production of composite plated steel sheets, the amount of fine particles dispersed and eutectoid in the plated layer is unstable, resulting in a problem of deterioration of the plated appearance and metallic luster. (Means for Solving the Problems) The present invention provides chromate or even SiO ₂ ,
In the plating method, in which a steel plate is electrolytically treated in an acidic Zn-based plating solution containing one or more of Al ₂ O ₃ , TiO ₂ , ZrO ₂ , and Cr ₂ O ₃ , the acidic Zn-based plating solution is Plating in which Cr 6+ is reduced to Cr 3+ by adding metal particles with a high ionization tendency into the plating solution in a relative amount commensurate with the concentration ^{of Cr 6+ eluted} into the plating solution while adjusting the pH to ^{4 or} less. This is a method for producing a highly corrosion-resistant electrical composite zinc-plated steel sheet with excellent metallic luster, which is characterized by electroplating in a liquid. (Function) The present inventors discovered that the cause of deterioration of the plating appearance and metallic luster is Cr ⁶⁺ generated from the dissolution of a small amount of chromate contained in the plating solution. A method for reducing the amount of ⁶⁺ was established. FIG. 1 is a schematic cross-sectional view of a composite plated steel sheet obtained by the present invention. The plated original plate (iron original plate) 1 is a thin steel plate whose surface has been cleaned through a normal surface treatment steel plate production process. The zinc-based plating layer 2 is made of chromate or further SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ ,
Contains one or more types of corrosion inhibiting fine particles 3 of Cr ₂ O ₃ . 4 is an additive having a positive charging effect, such as nickel or alumina. Here, the zinc-based plating layer refers to Zn, Zn-Ni, Zn-Fe, Zn-Co, Zn
-Mn, Zn-Ni-Fe, Zn-Ni-Co, Zn-Ni-
Cr, Zn-Fe-Cr, Zn-Fe-Co, etc., and chromates include BaCrO ₄ , SrCrO ₄ , PbCrO ₄ , ZnCrO ₄
etc. By the way, the reason why the corrosion resistance is significantly improved by dispersing and eutectoiding chromate in the plating layer is considered to be as follows. The chromate dispersed and eutectoid in the plating layer is partially dissolved in a corrosive environment, and Cr ⁶⁺
emit. This Cr ⁶⁺ reacts with the metal in the plating layer to form a chromate film with excellent corrosion resistance. Furthermore, even if this chromate film is destroyed, the chromate is uniformly dispersed throughout the plating layer (surface and cross-sectional direction), so the trace amount of eluted Cr ⁶⁺ reacts with the plating layer metal and destroys the chromate film. Because it is repeatedly formed, corrosion resistance can be significantly improved compared to conventional zinc-plated steel sheets. Along with chromic acid, SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ ,
Even when one or more of Cr ₂ O ₃ is contained,
The above chromate film formation occurs, but further
A synergistic effect can be expected due to the inclusion of SiO ₂ etc. That is, SiO ₂ and the like have a barrier effect against attack by external corrosion factors, and therefore the progress of corrosion is suppressed. As the chromate, we use chromates such as BaCrO ₄ that are generally said to be poorly soluble, but in an acidic plating solution (a solution with a pH of 4 or less), the following behavior can be considered. MCrO ₄ M ²⁺ +CrO ₄ ^2- (=Cr ⁶⁺ )...(1) Therefore, it is expected that Cr ⁶⁺ will accumulate in the plating solution. Figure 2 shows the change over time in the Cr ⁶⁺ concentration due to the dissolution of BaCrO ₄ in a plating solution with an actual pH of 4 or less. As time passes, an increase in the Cr ⁶⁺ concentration in the Metsuki liquid is observed. i.e. reaction (1)
moves to the right. This accumulation of Cr ⁶⁺ has an adverse effect on plating properties (plating appearance, amount of fine particle dispersion, eutectoid, etc.). If Cr ⁶⁺ is present in the plating solution, a chromium oxide film (chromate film) will form on the cathode (iron base) surface in the early stages of electrolytic treatment, and plating metal and fine particles will attempt to precipitate on it. , cause burnt skin (the appearance of skin becomes black and powdery),
A phenomenon is observed in which the amount of fine particles precipitated decreases. Figure 3 shows the plating solution when BaCrO ₄ is used.
The relationship between Cr ⁶⁺ concentration and BaCrO ₄ precipitation into the plating layer is shown. As the Cr ⁶⁺ concentration in the liquid increases,
The precipitation of BaCrO ₄ decreases. In other words, the corrosion resistance of the plating layer deteriorates. It was confirmed that Cr ⁶⁺ eluted from chromates such as BaCrO ₄ had a negative effect on plating properties. Therefore, in order to have a beautiful appearance and to ensure the amount of chromate, SiO ₂ , etc. deposited in the plating layer, it is necessary to
Suppression of Cr ⁶⁺ is required. Therefore, the present inventors attempted to suppress Cr ⁶⁺ in the plating solution by changing Cr ⁶⁺ to Cr ³⁺ by redox action using metal particles. 3M+nCr ⁶⁺ →3Mn ⁺ +nCr ³⁺ ...(2) Here, the metal particles refer to metals with a high ionization tendency, such as Al, Zn, Mg, Mn, and Fe. Metal grains and Cr ⁶⁺
The reaction is thought to be as shown in reaction formula (2). This reaction takes place in an acidic plating solution, that is, when the pH is 4.
It tends to occur in the following areas. If the plating liquid pH exceeds 4, a metal ion hydroxide film will be formed on the surface of the metal particles, or the oxide film that has already formed in the air will be less likely to be destroyed, so the reaction between the metal particles and Cr ⁶⁺ will occur. This is because it becomes difficult to proceed. Figure 4 shows the change in the concentration of Cr ⁶⁺ in the plating solution when Zn particles are used as the metal particles. The reduction of Cr ⁶⁺ differs depending on the PH in the liquid, but if the PH is maintained at 4 or less, the Cr ⁶⁺ in the liquid tends to decrease. PH is 4
If it exceeds Cr 6+ , the reducing action of Cr ⁶⁺ by Zn particles will not occur, and the Cr ⁶⁺ concentration in the plating solution cannot be reduced. In addition, as for the method of using the metal particles, instead of putting the metal particles into the plating cell section as shown in FIG. The plating solution is introduced from below into a cylinder 10 (reduction tank) in which metal particles 9 are enclosed, and as the plating solution circulates through the area clogged with metal particles, Cr ⁶⁺ is converted to Cr ³ by the reaction (2). ⁺
It is better to reduce it to In addition, when metal particles are added to the plating cell section, not only is it difficult for the Cr ⁶⁺ → Cr ³⁺ reaction to occur, but the finer metal particles are suspended in the plating solution, which is then used to produce coil materials. It may also cause press scratches due to getting caught in the roll. Therefore, the above-mentioned reduction tank method is optimal for effectively and stably causing the oxidation-reduction action of metal particles and Cr ⁶⁺ . Plating liquid is prepared using the above method using metallic Zn particles.
Figure 6 shows the results of cathodic electrolytic treatment with reduced Cr ⁶⁺ content. Chromate (BaCrO ₄ ) in the plating layer by reducing Cr ⁶⁺ in the plating liquid
It can be seen that the amount of precipitation increases and high current density electrolytic treatment becomes possible. (Example) Next, the present invention will be described based on an example. The glazing liquid is based on ZnSO ₄ - NiSO ₄ based acidic liquid.
Chromates or even SiO ₂ , Al ₂ O ₃ , TiO ₂
After one or more of these were mixed and added, the Cr ⁶⁺ concentration in the plating solution was reduced by redox action using metal Zn particles according to the present invention. A composite plated steel plate with excellent metallic luster and corrosion resistance was produced by cathodic electrolysis treatment in the above plating solution on a cold rolled steel plate whose surface had been cleaned by alkaline degreasing → water washing → sulfuric acid pickling → water washing. The following evaluation tests were conducted on various composite plated steel sheets manufactured under the above conditions. (1) Plating finish (plating appearance, metallic luster) Evaluation: 〇...Good plating appearance and good metallic luster △...Good plating appearance but slightly poor metallic luster ×...Poor both plating appearance and metallic luster ( 2) Evaluation of the precipitation of BaCrO ₄ in the plating layer: 〇…Large amount of BaCrO ₄ precipitation (>BaCrO ₄ 0.5wt)
%) △...The amount of BaCrO ₄ precipitated is slightly small (BaCrO ₄ = 0.1 to 0.5wt%) ×...Almost no BaCrO ₄ precipitated (<BaCrO ₄ 0.1wt%) (3) Resistance of bare material (as plated) Rust property evaluation: Measure red rust incidence after 20 days of salt spray test ◎… Red rust incidence 5% or less 〃… 〃 5-20% △… 〃 20-35% ×… 〃 35-50% ××… 〃 50% The evaluation results are shown in Table 1. As is clear from this, the composite plated steel sheet produced by the method of the present invention has sufficient metallic luster and excellent corrosion resistance.

[Table] (Effects of the invention) As described above, by stably depositing corrosion-inhibiting particles such as chromate in the plating layer according to the present invention, a highly corrosion-resistant composite plated steel sheet with excellent metallic luster is manufactured. It becomes possible to do so.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of a composite plated steel sheet obtained by the present invention, Figure 2 is a diagram showing the change in Cr ⁶⁺ concentration over time due to dissolution of BaCrO ₄ in a plating solution with a pH of 4 or less, and Figure 3 is Figure 4 shows the relationship between the Cr ⁶⁺ concentration in the plating solution and the precipitation of BaCrO ₄ into the plating layer when BaCrO ₄ is used.
A diagram showing the change in the concentration of Cr ⁶⁺ in the plating solution when Zn particles are used, Figure 5 is a diagram showing how to use metal particles, and Figure 6 is a diagram showing the change in the concentration of Cr 6+ in the plating solution using the metal Zn particles using the reduction tank method. FIG. 3 is a diagram showing the results when cathodic electrolytic treatment was performed with reduced Cr ⁶⁺ . 1... Plating original plate, 2... Zinc-based plating layer, 3... Corrosion inhibiting fine particles, 4... Additive, 5... Tank, 6... Plating liquid, 7... Chemical pump, 8... Filter,
9...Metal grain, 10...Cylinder.

Claims (1)

[Claims] 1 chromate or further SiO ₂ , Al ₂ O ₃ ,
In the plating method, in which electrolytic treatment is performed using a steel plate as a cathode in an acidic Zn-based plating solution containing one or more of TiO ₂ , ZrO ₂ , and Cr ₂ O ₃ , the pH of the acidic Zn-based plating solution is reduced to 4 or less. By adding metal particles with a high ionization tendency into the plating solution in a relative amount commensurate with the Cr ⁶⁺ concentration eluted into the plating solution while adjusting the
A method for producing a highly corrosion-resistant electrical composite zinc-plated steel sheet with excellent metallic luster, which is characterized by electroplating in a plating solution in which Cr ⁶⁺ is reduced to Cr ³⁺ .