JPH0421797A - Zn-ni-cr alloy-electroplated steel sheet having fine appearance and its production - Google Patents

Abstract

PURPOSE:To obtain an electroplated steel sheet maintaining high corrosion resistance and having improved surface smoothness and appearance by using a chloride plating bath contg. PVA besides Zn, Ni and Cr ions. CONSTITUTION:A chloride plating bath contg. 100-300g/l Zn ions in the form of ZnCl2, 10-200gl Ni ions in the form of NiCl2, 0.1-20g/4l trivalent Cr ions, 0.2-20g/l (expressed in terms of alumina) alumina sol and >=0.1g/l PVA as a brightener is prepd. The surface of a steel sheet is electroplated with the plating bath to form a plating layer consisting of 6-16wt.% Ni, 0.1-10.0wt.% Cr, 0.015wt.% Al, and the balance Zn and contg. dispersed alumina on at least one side of the steel sheet. A plated steel sheet having a smooth surface, uniform quality and high corrosion resistance is obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a Zn-Ni-Cr alloy electroplated steel sheet that maintains high corrosion resistance and has excellent surface smoothness and appearance, and a method for manufacturing the same. be.

<Prior Art> Zn-Ni alloy plated steel sheets are widely used in materials that require corrosion resistance, such as automobiles, home appliances, and building materials.

However, the corrosion resistance of Zn-Ni plating is
The corrosion resistance depends on the content of N.
With an i content of 6 to 16% by weight (hereinafter simply referred to as %), N
Even if the i content is increased beyond 16%, corrosion resistance not only does not improve but also tends to deteriorate, and any further increase in the expensive Ni content is considered harmful rather than pointless.

In order to improve the corrosion resistance of Zn-Ni plating,
Japanese Patent No. 9-38313 proposes that in a Zn-Ni-Cr alloy plated steel sheet, 0.01 to 1.0% of Cr is contained in the Zn-Ni plating using a sulfuric acid bath.

However, this method has the disadvantage that a pattern appears on the black part as the Cr content increases in the Zn-Ni plating.
When the surface was analyzed, Cr was concentrated in the black part,
It was found that a layered layer of concentrated Cr appeared.

A method for uniformly eutectoiding Cr is disclosed in Japanese Patent Application Laid-Open No. 63-1
No. 57900, in Zn-Cr double gold plating,
Trivalent Cr ions at AlzOs/Cr molar ratio of 0.3
It has been disclosed that it can be used by being adsorbed onto a positively charged alumina sol having a particle size of 2 to 2. Although this method can be applied to uniform eutectoid deposition of Cr into Zn-Ni plating, it has the disadvantage that the surface becomes uneven with a size of 0.1 to several pm and the gloss level becomes almost zero.

Furthermore, recently, steel sheets with improved corrosion resistance have been developed by forming a thin organic resin film on the upper layer of plating with or without chromate. Although Zn-Ni plating, which has higher corrosion resistance than pure Zn plating, is used as the base plating for this highly corrosion-resistant steel sheet, it is desired to further improve the corrosion resistance of the base plating. Zn Ni-Cr plating, which has higher corrosion resistance than Zn-Ni plating, satisfies the need for such a base plating from the perspective of corrosion resistance, but it is not possible to use thin film type plating with or without chromate on the top layer. When forming an organic resin film, there was a drawback that the uniformity and smoothness of Cr on the surface were insufficient.

<Invention tries to solve! 1! Problem> The present invention solves the drawbacks of the prior art and provides Zn-Ni with excellent surface smoothness and appearance while maintaining high corrosion resistance.
-An object of the present invention is to provide a Cr-based alloy electroplated steel sheet and a method for manufacturing the same.

<Means for solving the problem> The first invention provides Ni: 6 to 1 on at least one side of the W4 board.
6% by weight, Cr: 0.1-10.0% by weight, AZ:
Excellent appearance characterized by having a plating layer containing 0.01 to 5% by weight, the remainder consisting of Zn, in which alumina is dispersed, and an organic substance that smoothes the plating surface is added as a brightening agent. This is a Zn Ni-Cr alloy electroplated steel sheet.

In the second invention, the Zn ions are ZnC1z and 1
00~300 g/j! , Ni ion as N i C1
z is 10 to 200 g/f, trivalent Cr ion is 0
．． Using a chloride plating bath containing 0.2 to 20 g/l of 1-21-2O and alumina sol in terms of alumina, and 0.1 g/2 or more of polyvinyl alcohol as a brightener,
A method for producing Zn-Ni-Cr based alloy electroplating #1IFj, which is characterized by electroplating on the surface of a steel plate and which has an excellent appearance, and preferably contains Cr ions in advance when preparing the plating bath. After dispersing alumina sol in the solution and adsorbing Cr ions to the alumina sol,
It is characterized by being mixed with a solution containing other of the above ions and additives.

<Function> Cr and Ni have the following four effects on improving corrosion resistance.

(2) Depending on the corrosive environment, excessive activity of Zn plating can be suppressed by incorporating elements such as N1 and Cr, which are more harmful than Zn, into the plating layer.

■ Zn plating tends to produce conductive ZnO as a corrosion product, but by adding Ni, non-conductive ZnO can be produced as a corrosion product.
Stabilizes n(Off)z and provides plating protection.

■ Cr plays a major role in improving corrosion resistance, as chromate treatment generally improves corrosion resistance.

In particular, a passive film is formed in the early stages of corrosion, which prevents the plating from leaching out and also prevents oxidation-reduction reactions, greatly contributing to corrosion resistance.

■ By uniformly depositing Ni and Cr on the plating layer, even if the previously formed passive film is torn, the torn area will remain ■
Due to the effects of , the elution rate of the plating is slow, and even if the plating elutes, due to the effects of
Off) t is generated and a passive film is formed again (self-healing action).

The Ni content in the alloy electroplating layer is 6 to 16%. If N1 is less than 6%, no significant improvement in corrosion resistance will be observed even if a normal amount of Cr is co-fractionated, and if it exceeds 16%, not only will there be no improvement in corrosion resistance, but there will be a tendency for the corrosion resistance to deteriorate. be.

Cr in the alloy electroplating layer is 0.1% or more.

If Cr is less than 0.1%, the occurrence of red rust due to SST is 1
000 hours or less, and deterioration of corrosion resistance is observed.

Although a Cr content of 0.1% or more is clearly effective, a higher Cr content further improves corrosion resistance, and a Cr content of 2% or more is more preferable. However, if it exceeds 10%, the plated surface becomes inactive, making it difficult to grow dense zinc phosphate crystals. Therefore, the amount of Cr eutectoid in the plating layer is set to 0.1-10%.

By providing alumina sol in the form of pseudo-boehmite,
Trivalent Cr is adsorbed onto negatively charged alumina sol, and Cr, which is generally said to be difficult to eutectoid, can be deposited stably, uniformly, and in large quantities.

This eliminates the dark streaks that occur when the Cr eutectoid i in the plating is 10.1% or more.

As the alumina sol, it is appropriate to use a pseudo-boehmite-like alumina sol with a particle size of 5 to 30 particles. The amount of alumina sol added is preferably 0.2 to 20 g/e in terms of alumina. The reason for this is that below 0.2 g/l, carbon is absorbed into the plating layer.
Uniform eutectoid of r is insufficient and streaks appear. 20g/
If it exceeds j2, the viscosity of the plating solution increases, making plating difficult. When plating is performed within this range, the AI in the plating layer
The concentration falls within the range of 0.01 to 5%.

However, due to the addition of alumina sol, several irregularities occur on the surface of the steel plate, the appearance loses its metallic luster, and unevenness occurs when chromate or resin is applied, and in some places the plating penetrates through the resin layer. The plated surface is exposed.

For this reason, an organic substance is added to the plating solution to smooth the plating surface. This makes the surface smooth and gives it a metallic luster, and the chromate and resin are evenly applied.
Corrosion resistance is improved. Furthermore, as the plating surface becomes uniform and smooth, the chemical crystal size tends to become smaller.

Next, the composition of the plating bath is set to 100 to 300 g/j2 with Zn ions as ZnClt. ZnCr, is 1
If it is less than 00 g/f, the plating efficiency will decrease and plating burn will occur, and if it exceeds 300 g/f, Z
nCrz is saturated in the liquid.

The Ni ion in the plating bath is 10-2 as NiC4z.
00g7/! Added. NiC1z is 10 g/f
If it is less than that, the amount of Ni eutectoided into the plating layer will be small and sufficient corrosion resistance will not be obtained. Also, 200g/jl! If it exceeds CrC1z, Crt(
SOa)i, KCrC5Oa), etc. are added in an amount of 0.1 to 20 g/P as trivalent Cr ions. If it is less than 0.1 g/1, the amount of Cr eutectoid will be small and sufficient corrosion resistance will not be obtained. 2
If it exceeds 0 g/N, the amount of Cr eutectoid in the plating layer becomes too high and the phosphorization treatment performance deteriorates. Further, Cr ions must be prepared together with alumina sol separately from Ni and Zn, and added after Cr is adsorbed onto the alumina sol.

At that time, pl+ should be 5 or less. When plI exceeds 5, C
Most of the r ions change to chromium oxide and are not adsorbed onto the alumina sol, making the eutectoid of C in the plating unstable.

The organic substance added is used as a brightening agent. For corrosion resistance, select IR that does not affect the clenching performance. For example, polyvinyl alcohol (PVA, degree of polymerization -20 to +000)
7. without adding 0.1-10g/j2. Added amount of PVA is 0
．． If it is less than 1 g/ff, the ability to smooth the plating is weak;
When added in excess of log/ff, the viscosity of the plating solution increases.

By the method of the present invention, N is applied to the surface of one or both sides of the steel plate.
i: 6-16%, Cr: 0.1-10.0%, A
Contains 1001-5% Zn-Ni-Cr with excellent appearance
System electroplating can be applied.

ZnNi-C with alumina dispersed thus produced
R-based electroplated steel sheets are coated with Fe plating or Fe plating on the surface layer for the purpose of improving phosphate chemical conversion treatment and paintability as necessary
-Zn plating and Fe-P plating can be used.

Furthermore, as a method for further improving corrosion resistance, the upper layer of the plating can be coated with an organic resin with or without chromate, if necessary.

In addition, when preparing the above-mentioned plating bath, it is preferable to disperse alumina sol in a Cr ion-containing solution in advance so that the Cr ions are adsorbed to the alumina sol, and then mix it with a solution containing other above-mentioned ions and additives.

<Examples> The present invention will be specifically described below with reference to Examples and Comparative Examples.

The plated steel sheets used in Examples and Comparative Examples were all cold-rolled steel sheets with the same thickness of 0.7 m, and after plating pretreatment of degreasing and pickling, plating was performed under the conditions shown in Table 1 and below.

Plating conditions for Examples and Comparative Examples 1 to 7: Plating bath temperature
60'C Plating bath ρI+ 2.5~3 Plating current density 100A/dj Quantity of electricity 700 C/dd Anode Plating conditions for the sulfuric acid bath used in Zinc plate comparative example 8: Plating bath temperature 50°C Plating Bath pH: 2.0±0.1 Plating current density: 100 A/d, electric power: 700 C/d, anode, lead iron.The composition of the plating layer of the steel plate plated under these conditions is as shown in Table 1. there were. Table 1 also shows the plating efficiency (current efficiency), gloss, streak pattern, roughness, chemical crystal size, and corrosion resistance during plating.

Here, the plating efficiency is the ratio (%) of the amount of electricity actually contributing to plating, which is calculated from the amount of deposited metal to the amount of electricity supplied. In addition, gloss, streaks, and roughness were determined visually. The chemical crystal size was measured by cleaning the plated steel sheet, treating it with a phosphate chemical treatment solution (Bonderite 3020 manufactured by Nippon Parkerizing Co., Ltd.), and observing the chemical crystal size with an SEM (scanning electron microscope). .

Evaluation of corrosion resistance of plating is based on JIS Z 2372-
The evaluation was based on the red rust occurrence area ratio (%) after 1000 hours in the salt spray test (hereinafter abbreviated as SST) specified in 1955.

Table 1 shows that the present invention has excellent plating appearance, significantly improved corrosion resistance, and improved phosphorization performance and smoothness.

<Effects of the Invention> As explained above, according to the present invention, industrially stable and large amounts of Cr and AI are eutectoided into Zn-Ni plating, resulting in high corrosion resistance with a smooth surface and uniform appearance. It is possible to produce plated steel sheets.

In particular, the present invention can significantly improve the corrosion resistance of a steel plate by plating with a thin coating weight. Excellent industrial effects can be obtained by applying it to materials for transportation vehicles such as automobiles, materials for construction, materials for electrical equipment, etc.

Claims (3)

[Claims] (1) Ni: 6 to 16% by weight on at least one side of the steel plate;
Contains Cr: 0.1 to 10.0% by weight, N: 0.01 to 5% by weight, the balance is Zn, and alumina is dispersed, and an organic substance that smoothes the plating surface is added as a brightening agent. A Zn-Ni-Cr alloy electroplated steel sheet with an excellent appearance, characterized by having a plating layer that is transparent. (2) Zn ion as ZnCl_2 100-300
g/l, Ni ion as NiCl_2 10-200
g/l, trivalent Cr ions 0.1 to 20 g/l and alumina sol converted to alumina 0.2 to 20 g/l,
A method for producing a Zn-Ni-Cr alloy electroplated steel sheet with an excellent appearance, characterized by electroplating the surface of the steel sheet using a chloride plating bath containing 0.1 g/l or more of polyvinyl alcohol as a brightening agent. . (3) When preparing the plating bath, the alumina sol is dispersed in advance in a Cr ion-containing solution so that the Cr ions are adsorbed onto the alumina sol, and then mixed with a solution containing the other ions and additives. A method for manufacturing a Zn-Ni-Cr alloy electroplated steel sheet with excellent appearance as claimed in the claims.