JPH07268679A - Production of zn-cr alloyed electrogalvanized steel sheet having excellent appearance quality - Google Patents

Abstract

PURPOSE:To deposit chromium without limiting the plating conditions even at the time of using a low current density and to improve the appearance quality in the production of the alloyed electrogalvanized steel sheet through immersing a steel sheet in an acidic bath contg. Zn<2+> and Cr<3+> ions. CONSTITUTION:This electroplating is performed through adding 0.01 to 20g/l polyvinyl ether to the acidic bath. As the polyvinyl ether, preferably, and only of polyvinyl methyl ether, polyvinyl ethyl ether and polyvinyl propyl ether is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Zn-Cr alloy electroplated steel sheet for use in automobiles, home appliances, building materials, etc., which has excellent corrosion resistance and corrosion resistance after painting and has excellent appearance quality for rust prevention. Is.

[0002]

2. Description of the Related Art As a method for producing an electroplated steel sheet containing chromium in zinc or a zinc-based alloy plating, for example, Japanese Patent Laid-Open No. 1-191797 and Japanese Laid-Open Patent Publication No. 1-16271.
No. 95, a method of using a high molecular weight organic additive in a plating bath, or an organic additive that forms a complex with chromium ions as described in JP-A-3-120393. The method is common. Although the corrosion resistance of the Zn-Cr alloy electroplated steel sheet produced in this way is very high, in recent years users of rust-preventing steel sheets such as automobile manufacturers have demanded not only high corrosion resistance but also improvement in appearance quality after painting. is doing. In order to meet this need, in JP-A-64-55398, a polymer organic additive is used as a chromium depositing agent, and a current density of 50 A / dm.
It is described that plating with ² or more improves the corrosion resistance and the appearance quality.

[0003]

However, when the current density is limited as in the above method, the operating range is narrowed, which may reduce the production efficiency, and it is complicated to manage the current density. This is not preferable, and no measures have been taken so far to solve this problem. This problem is due to the fact that chromium does not precipitate at low current densities.

In view of these problems, the present invention does not limit the plating conditions to deposit chromium even at a low current density,
Z with excellent appearance quality that can improve appearance quality
A method for manufacturing an n-Cr alloy electroplated steel sheet is provided.

[0005]

The present invention relates to a method for producing a Zn-Cr alloy electroplated steel sheet in which a steel sheet is immersed in an acidic bath containing Zn ²⁺ ions and Cr ³⁺ ions, wherein polyvinyl chloride is used in the acidic bath. 0.01 to 20 g / l of ether
Zn-Cr excellent in appearance quality characterized by being added
It is a method for manufacturing an alloy electroplated steel sheet. As the polyvinyl ether, it is preferable to use any one of polyvinyl methyl ether, polyvinyl ethyl ether, and polyvinyl propyl ether.

[0006]

[Function] A Zn-Cr alloy electroplated steel sheet shows the ability to precipitate chromium, and as a result of various studies on additives that improve the plating appearance quality, the results show that the appearance quality of the plating is not affected by the surface active action in order to improve the glossiness. Was thought to be important. Since a brightening agent has been often used in plating and a surfactant is used as the brightening agent, it was considered that the glossiness is improved by adsorbing the brightening agent on the plating surface. On the other hand, regarding the precipitation ability of chromium, it is presumed that the interaction of coordination adsorption with chromium ions has an influence, and attention was paid to facilitating the coordination adsorption. Based on such an inference, as a result of investigating an additive having excellent surface active action and coordinate adsorption ability, it was revealed that polyvinyl ether is suitable.

Polyvinyl ether has a structure as shown in Chemical formula 1, in which the alcohol hydroxyl group of polyvinyl alcohol is replaced with an ether group.

[0008]

[Chemical 1]

Here, if the ether group is an alkyl chain,
Since the alkyl group is an electron-donating functional group, the electron density of oxygen is higher than the electron density of the hydroxyl group. By the way, since chromium ions are cations, the higher the electron density of oxygen, the easier it becomes for coordinate adsorption. It is considered that this action causes the polyvinyl ether to have a chromium deposition action. At the same time, this polyvinyl ether has a basic structure of polyvinyl alcohol, and since it has characteristics as a surfactant, it also acts as a brightening agent for plating. Therefore, it contributes to the chromium precipitation action and at the same time improves the appearance quality.

Any kind of polyvinyl ether may be used as long as it is an ether having an electron donating functional group such as methyl ether, ethyl ether, propyl ether. However, in order to maximize its effect as a brightener,
Since the additive needs to be uniformly adsorbed on the surface, a water-soluble one is preferable. When an alkyl chain is used, a methyl group,
Ethyl group and propyl group are preferred.

In the present invention, polyvinyl ether exerts a chromium depositing ability and a function as a brightening agent in the immediate vicinity of the cathode electrode, so that it is not necessary to add more than 20 g / l. Rather, a large amount of addition deteriorates the plating physical properties such as plating adhesion and the appearance quality, and may increase the viscosity and decrease the electric conductivity. Therefore, the addition amount is 20 g / l or less. However, if it is less than 0.01 g / l, the chromium precipitation effect disappears, so 0.01 g / l or more must be added.

A second additive may coexist in addition to polyvinyl ether. Also, if necessary, Ni ²⁺ , C
0.1 ^% of metal ions such as o ²⁺ , Fe ³⁺ , Mn ²⁺ , Pb ²⁺
By adding mol / l or less, electrodeposition can be performed as a third component in the plating film.

In the present invention, the plating composition can be changed by changing the plating bath composition and the current density. The chromium content in the plating film is preferably 5% by weight or more in consideration of corrosion resistance, while 40% by weight
If the above content is contained, the so-called powdering property that the plating becomes powdery during processing is deteriorated, so less than that is desirable. The plating bath may be either a sulfuric acid acidic bath or a hydrochloric acid acidic bath. Moreover, the base steel sheet for plating is not limited, and the steel composition and the manufacturing method of the steel sheet are not limited.

[0014]

EXAMPLES Examples of the present invention will be described.

A cold-rolled steel sheet having a plate thickness of 0.8 mm was used as a base steel sheet, and an acidic bath having a pH of 1.5 containing Zn ²⁺ ions and Cr ³⁺ ions at 0.5 mol / l was used as a plating bath. The additives shown in Tables 1 to 5 were added to this acidic bath, and the plating was performed at a bath temperature of 50 ° C. while changing the current density as shown in Tables 1 to 5. With respect to the obtained plated steel sheet, the weight of chromium in the plating film was measured, and the appearance quality and workability of the plating were evaluated. The appearance quality of the plating is JI
It was evaluated by 60 ° / 60 ° gloss according to S Z 8741. The evaluation was performed in three grades of ⊚ (glossiness of 90 or more), ◯ (80 or more), and Δ (less than 80). In addition, the workability of plating was evaluated from the plating peeling condition of the bent portion after performing the contact bending. Evaluation is ◎ (no plating peeling), ○
It was performed in three stages of (small plating peeling) and Δ (large plating peeling). These results are also shown in Tables 1-5.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

[0019]

[Table 4]

[0020]

[Table 5]

In the examples of the present invention, Zn-Cr alloy electroplated steel sheets were obtained in which the chromium weight percentage in the plating film was 5.1 to 38.2%. The plating appearance and plating workability were also excellent. On the other hand, if the addition amount was too small, the effect was not exhibited and chromium was hardly deposited in the plating film. Further, when the addition amount was too large, deterioration of glossiness and peeling of plating were observed.

[0022]

Industrial Applicability According to the present invention, Zn-
A Cr alloy electroplated steel sheet can be manufactured in a wide operating range from low current density to high current density.

Claims (2)

[Claims] 1. A method for producing a Zn—Cr alloy electroplated steel sheet in which a steel sheet is immersed in an acidic bath containing Zn ²⁺ ions and Cr ³⁺ ions, wherein 0.01 to 20 g of polyvinyl ether is added to the acidic bath. A method for producing a Zn-Cr alloy electroplated steel sheet excellent in appearance quality, which is characterized by adding 1 l. 2. The method for producing a Zn—Cr alloy electroplated steel sheet excellent in appearance quality according to claim 1, wherein the polyvinyl ether is any one of polyvinyl methyl ether, polyvinyl ethyl ether, and polyvinyl propyl ether.