JPH0635673B2 - Method for producing zinc-chromium electroplated steel sheet excellent in surface quality and corrosion resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a zinc-chromium electroplated steel sheet having excellent surface quality and corrosion resistance, which is used in automobiles, home appliances, building materials and the like.

(Problems to be Solved by the Prior Art and Invention) As a method for producing an electroplated steel sheet in which chromium is contained in zinc or a zinc alloy, for example, Japanese Patent Publication No.
-36078, JP-B-58-56039, JP-A-61-
270398 and the like are known, but the eutectoid ratio of Cr is 0.005 to
At very low amounts up to 5%, the effect of Cr on corrosion resistance can only be incidental. For the purpose of improving the corrosion resistance, it is strongly desired to co-deposit a higher content of chromium.

However, there has been no Zn-Cr electroplating technology that can increase the Cr eutectoid ratio. That is, even if the concentration of trivalent chromium ions in the plating bath is simply increased, normal adhesion with adhesion cannot be obtained, and current efficiency is sharply reduced, which is extremely difficult industrially. It was

Regarding the appearance of plating, JP-B-58-56039 discloses that pearl-like luster can be obtained by co-depositing 10 to 100 ppm of chromium in the zinc plating layer. However, when the Cr content exceeds 1%, it gradually becomes black gray as the Cr content increases, and a streak-like mottled pattern appears, which significantly deteriorates the commercial value in appearance. In addition, JP-A-61-2
Since 70398 contains Al ₂ O ₃ etc., the plating appearance is further deteriorated. That is, the unevenness on the surface becomes severe, and the unevenness in the form of streaks is promoted. Therefore, there was irreconciliation between the improvement of corrosion resistance and the surface quality of plating.

Since it is generally coated and used in the above applications, not only corrosion resistance but also a beautiful appearance after coating is required. For that purpose, it is desired that the surface of the plated steel plate has a small roughness and a glossy quality. Furthermore, it is actually strongly desired that press defects such as push defects can be easily detected in the press working process.Since the conventional general plated steel plate has coarse plated crystals and absorbs the press oil, it is difficult to remove the oil through the oil film. Defect inspection became difficult, and there was a problem in quality assurance of pressed products.

The present invention intends to provide a method for producing a zinc-chromium electroplated steel sheet having a high Cr content ratio capable of greatly improving the corrosion resistance in order to advantageously solve the conventional problems and having an excellent surface quality. It is a thing.

(Means for Solving the Problem) A feature of the present invention is that an acidic plating bath containing zinc ions, trivalent chromium ions and 0.01 to 20 g / polyoxyalkylene is used to obtain a current density of 50 A / dm ^2. The present invention relates to a method for producing a zinc-chromium electroplated steel sheet excellent in surface quality and corrosion resistance, which is characterized by the above-mentioned plating.

(Function) Originally, the zinc-trivalent chromium plating system is an incompatible plating system. That is, when the proportion of zinc ions in the plating bath is increased, Zn is deposited but Cr is hardly deposited. While 3
If the ratio of valent chromium ions is increased, the precipitation of zinc will be significantly suppressed, and chromium will not normally precipitate.

Such a behavior is caused by other alloy plating systems such as Zn-Fe, Zn-
Not seen in Ni series. This is a phenomenon peculiar to zinc-three-valent chromium system, and is also the reason why the conventional industrialization was hindered.

However, when a polyoxyalkylene derivative is added to this system, both Zn and Cr are electrodeposited with favorable current efficiency.

It is the essence of the present invention that Zn-Cr plating with a high Cr eutectoid ratio can be achieved only by adding a polyoxyalkylene derivative.

Regarding the corrosion resistance of the plating layer, it is preferable to increase the Cr content. When Cr is 1% or more, a slight effect is recognized, and when it is 5% or more, for example, in a salt spray test, the generation of red rust is suppressed, and the effect is remarkably recognized. When it reaches 7% or more,
Red rust will not occur at all, and a revolutionary effect will appear.
Since the corrosion potential also shifts to the noble side, it can be considered that the effect of Cr has appeared almost completely.

If it exceeds 30%, the corrosion resistance improving effect is saturated, and if it exceeds 40%, the corrosion resistance effect is recognized, but since peeling easily occurs during processing, practically, the plating composition range of Cr is 1 to 40%, preferably Cr. It is preferable to apply the present invention to Cr of 5 to 40%.

Such high corrosion resistance is due to the well-known zinc plating or
It can be understood that the plated steel sheet according to the present invention exhibits epoch-making performance, which is a level that cannot be achieved by plating with an alloy such as Zn-Fe or Zn-Ni.

The corrosion potential of the plated steel sheet according to the present invention is about -1.0 to -0.9 V on the basis of the sweet potato electrode, and it can be seen that it has a sufficient sacrificial anticorrosion action. Nevertheless, the durability of the plating layer is thought to be due to the corrosion product of Cr depositing the protective coating.

Next, the surface quality of the plating layer will be described. The surface color of the plating layer of the present invention is silver white, which is bright like stainless steel, has an appearance completely different from the milky white of zinc plating, and has a uniform color. With the rust preventive oil or the press oil applied, the oil film shines and can be easily detected if there are surface flaws. On the other hand, the oily film loses its luster on the milky white surface with zinc-based plating, making it difficult to identify surface defects. Also,
With respect to the appearance after coating, the plated steel sheet according to the present invention can improve the sharpness.

The surface quality of the plating layer is considered to be an effect of the addition of the polyoxyalkylene derivative. If a polyoxyalkylene derivative is not added, a plating layer containing 5% or more of Cr cannot be obtained even if plating is performed under the same conditions, so the effects of polyoxyalkylene derivatives cannot be directly compared and discussed, but other plating conditions Compared with the Zn-Cr plated steel sheet containing 5% or more of Cr obtained in 1., the effect of the polyoxyalkylene derivative is remarkable. That is, without adding a polyoxyalkylene derivative, if the plating is carried out at a high current density of about 300 A / dm ² from an acid plating bath containing zinc ions and trivalent chromium ions, C
Zn-Cr plating having a r content of 5% or more is obtained, but the plating appearance is grayish white to black gray, and streaky unevenness is likely to occur. On the other hand, the plated steel sheet of the present invention has a uniform silver-white appearance throughout the Cr content of 1 to 40%.

Polyoxyalkylene derivative is generally _{R 2 -O- (R 1 -O)} n -H and, or R ₂ - refers to (R _{1 -O)} a compound represented by n -H.

Here, R ₁ : an alkylene group R ₂ : H, an alkyl group, a phenyl group, a naphthyl group and / or a derivative thereof, n = 1 to 2000, specific examples include: polyoxyethylene (polyethylene glycol) HO- ( _{CH 2 -CH 2 -O) n -H} n = 1~2000 · alkyl polyoxyethylene ether _{R-O- (CH 2 -CH 2} -O) n -H n = 1~2000 R: alkyl group C _m H _{2m + 1} , m = 0 to 20 R: alkyl group C _m H _{2m + 1} m: 0 to 20 ・ alkylphenyl polyoxyethylene ether n: 6 to 2000 R: alkyl group C _m H _{2m + 1} m: 0 to 20 ・ Alkylnaphthyl polyoxyethylene ether n: 4 to 2000 R: alkyl group C _m H _{2m + 1} m: 0 to 20 · Polyoxypropylene (polyprene glycol) n: 3 to 2000 ・ Alkyl polyoxypropylene ether n: 1 to 2000 R: Alkyl group C _m H _{2m + 1} m: 0 to 20 Alkylphenyl polyoxypropylene ether n: 6 to 2000 R: alkyl group C _m H _{2m + 1} m: 0 to 20 ・ Alkylnaphthyl polyoxypropylene ether n: 4 to 2000 R: alkyl group C _m H _{2m + 1} m: 0 to 20 Polyoxymethylene derivative R ₁ —O— (CH ₂ —O) _n —H n = 3 to 5000 R ₁ : H, alkyl , Aryl ・ α-ethoxylated naphthol (abbreviated as EN) n = 1 to 20 ・ Ethoxylated-α-naphthol sulfonic acid (abbreviation ENSA) n = 1 to 20 and so on.

The amount of polyoxyalkylene derivative added to the plating bath is 0.
The preferred range is 01 to 20 g /. Almost no effect is observed at less than 0.01 g /. When it exceeds 20 g /, both the effect of promoting Cr precipitation and the effect on the surface quality are saturated, and the solubility in the plating bath is restricted.

The polyoxyalkylene derivative may be used alone or in combination of two or more.

The concentration of zinc ions and trivalent chromium ions is not particularly limited, but it is usually in the range of 10 to 150 g / each, and optimum conditions can be adopted in consideration of the intended plating composition and current efficiency.

The anion of the plating bath can be applied to both the sulfuric acid bath and the chloride bath. The pH of the plating bath is preferably in the range of 3 to 0.5.

_{^{Furthermore, N a +, K +,}} NH 4 +, adding unrelated salts such as Mg ⁺² ions enhances the conductivity of the bath, it is effective for assisting the precipitation effect of adjusting colloidal oxide.

Depending on the purpose, Cr ⁺⁶ , Ni, Co, Fe, Mn, Cu, Sn,
Even if a small amount of ions such as Cd and Pb are added to cause the plating layer to co-deposit a small amount of the third component, the effect of the present invention is essentially unchanged.

Next, the plating condition will be described.

The current density is preferably 50 A / dm ² or more. Fifty
If it is less than A / dm ² , Cr is hardly codeposited. Codeposition of Cr becomes easy in the high current density region, but practically 250 A / dm
It is preferable to operate up to ² . If it exceeds 250 A / dm ₂ , the voltage load becomes excessive and the current load also becomes large, which makes it difficult to apply it to a practical plating cell.

When the polyoxyalkylene derivative is not added, for example, at a high current density of 150 A / dm ² or more, it is possible to deposit 5% or more of Cr depending on the conditions, but the current efficiency decreases, and a black gray appearance is obtained. There are problems such as presenting. By adding the polyoxyalkylene derivative, it is possible to easily deposit in the medium current density region, which is a practically advantageous effect.

The flow velocity of the plating solution is 30 to 200 as a relative velocity with the steel strip.
Appropriate m / min and plating temperature are 40 to 70 ° C.

(Example) Next, the Example of this invention is given with a comparative example.

Weight per unit area of cold rolled steel sheet (0.6mm thickness) 20g / m ^{2 under} the conditions shown in Table 1.
I gave it a notice. The obtained plating plates of Examples 1 to 11 were
It had a glossy and uniform appearance of 80 or more at a 60 ° / 60 ° gloss level in accordance with JIS Z 8741. Comparative Examples 1 and 4 had a milky white appearance similar to that of a galvanized steel sheet, and Comparative Examples 2 and 3 had an uneven gray to black gray appearance.

A salt spray test (according to JIS Z 2371) of the obtained laminated plate was carried out. No red rust was observed after 700 hours for Cr having a Cr content of 7% or more. On the other hand, Comparative Examples 1 and 4
Produced red rust within 24 hours.

(Effects of the Invention) As described above, according to the present invention, a corrosion-resistant steel sheet having high corrosion resistance and excellent surface quality can be produced in a practically easy current density region.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Suzuki 5-3 Tokai-cho, Tokai City, Aichi Pref., Nippon Steel Co., Ltd. Nagoya Steel Works (56) References JP-A-53-26741 (JP, A) JP-A-61-30695 (JP, A) JP-A-61-143590 (JP, A)

Claims (1)

[Claims] 1. Zinc ion, trivalent chromium ion and 0.01
A zinc-chromium electroplated steel sheet excellent in surface quality and corrosion resistance, characterized in that plating is performed at a current density of 50 A / dm ² or more using an acid plating bath containing a polyoxyalkylene derivative of up to 20 g / Manufacturing method