JPH09316686A - Production of zinc-chromium alloy electroplated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the industrially stable production of a zinc-chromium alloy plated steel sheet excellent in corrosion resistance and corrosion resistance after coating without deteriorating its quality by specifying the amt. of hydrogen ions in the surface of the steel sheet to be plated and executing zinc-chromium alloy electroplating while the fluctuation of chromium hydroxide film is suppressed. SOLUTION: The amt. of hydrogen ions in the surface of the steel sheet to be plated is allowed to satisfy hydrogen ion feeding velocity >= hydrogen ion consuming velocity in the vicinity of the surface. For example, it is regulated by the inequality of ([H<+> ] × the distance between the steel sheet and the anode ×the relative flow rate)/(the length of an electrolytic cell)>=(0.5 × current density)/96500. Then, while the fluctuation of chromium hydroxide film is suppressed, a zinc-chromium alloy layer is formed by electroplating. For example, even in the case zinc-chromium alloy electroplating is continuouslly executed by a horizontal electrolytic cell using an acidic plating soln. contg. Zn<2+> ions, Cr<3+> ions and organic additives, by regulating the flow rate of the plating soln. to 0.5 to 2.5m/sec, the purpose can be attained.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art As a manufacturing method or equipment for continuously moving a steel sheet to perform plating, in a horizontal plating cell, the flow of a plating solution is in the same direction as the moving direction of the steel sheet (hereinafter referred to as "parallel flow"). Or the opposite direction (hereinafter referred to as "counterflow"), or a vertical type that utilizes the gravity drop of the plating solution as disclosed in Japanese Patent Publication No. 62-15638. Plating cells and the like are known. In a plating cell in which the flow of the plating solution jets in one direction, either parallel or countercurrent, or a vertical plating cell that uses gravity drop of the plating solution, the distance between the steel plate and the anode is not stable and the current This causes density fluctuations, resulting in quality fluctuations and deterioration, making stable manufacture difficult.

To address this problem, for example, JP-A-61-119
As disclosed in Japanese Patent Publication No. 0094, Japanese Patent Publication No. 61-2040, etc., a plating solution is jetted from the central portion of the electrode to form a static pressure between the electrode and the steel plate, and the steel plate and the anode are separated from each other. It has been found that the distance between the steel plate and the anode can be stabilized and the distance can be shortened by using the fluid cushion type cell capable of shortening the distance. However, both parallel flow and counter flow of plating solution flow in the same cell,
Relative flow velocity fluctuates.

In general, in zinc-based alloy electroplating, the alloy composition in the plating layer also fluctuates as the relative flow velocity fluctuates.
In the plating cell in which both the parallel flow and the counter flow of the plating solution coexist, deterioration in quality such as appearance quality and adhesion was unavoidable. These phenomena are no exception in zinc-chromium alloys. For example, JP-A-1-191798, JP-A-3-120393, and JP-A-6-4178.
No. 1 discloses a method for producing zinc-chromium, but when it is produced by using an electrolytic cell in which a parallel flow and a counter flow coexist, quality deterioration is observed, and these phenomena are caused by zinc-chromium alloy electroplating. It hinders the industrialization of steel sheets.

With respect to the problems of quality deterioration such as variations in alloy composition and deterioration of appearance due to the above-mentioned fluctuations in relative flow velocity, Japanese Patent Publication No. 6-60433 discloses an improvement by limiting the electrode length to a certain range. ing. This is an invention found from the knowledge of mass transfer of zinc ion and iron ion,
Chromium ions, unlike iron ions or other metal ions, form a polynuclear complex in an aqueous solution, so that their mass transfer rate is very low, and the problem is solved by the electrode length regulation based on the mass transfer of the publication. Cannot be achieved. Regarding the production of zinc-chromium alloy electroplated steel sheet,
No solution has been made to the above problems.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a fluid-cushion type horizontal plating cell in which a zinc-chromium alloy electroplated steel sheet is mixed with a parallel flow and a counter flow, and the relative flow velocity fluctuates ( The purpose of the present invention is to provide a method for industrially stable production of a horizontal electrolysis cell) without deteriorating the quality.

[0007]

Means for Solving the Problems The gist of the present invention is to suppress the fluctuation of a chromium hydroxide film by setting the amount of hydrogen ions on the surface of a steel sheet to be plated as hydrogen ion supply rate ≧ hydrogen ion consumption rate in the vicinity of the surface. A zinc-chromium alloy electroplated steel sheet is characterized by forming a zinc-chromium alloy layer by electroplating. Further, the amount of hydrogen ions on the surface of the steel sheet to be plated is adjusted by ([H ⁺ ] × distance between steel sheet and anode × relative flow velocity) / (electrolysis cell length) ≧ (0.5 × current density) / 96500 formula The method for producing a zinc-chromium alloy electroplated steel sheet is characterized in that the zinc-chromium alloy layer is formed by electroplating while suppressing fluctuation of the chromium hydroxide film. In addition, by using an acidic plating solution containing Zn ²⁺ ions, Cr ³⁺ ions, and an organic additive, the steel sheet is placed in a multi-stage electrolytic cell in which the plating solution flows in both the same direction and the opposite direction with respect to the moving direction of the steel sheet. In the method of continuously producing zinc-chromium alloy electroplating by passing through a plate, the flow rate of the plating solution is 0.5 dm / sec to 2.5 m /
sec is preferred.

As a result of various studies, the present inventors have found that
The quality of the alloy composition and adhesion varies due to fluctuations in the chromium hydroxide film called the cathode film formed during electrodeposition.
Is newly found to be affected by. Based on this knowledge, by controlling the relative flow velocity of the plating solution with the steel sheet and controlling the current density to eliminate the fluctuation of the cathode coating,
This enables stable production of zinc-chromium alloy plated steel sheets.

[0009]

DETAILED DESCRIPTION OF THE INVENTION Chromium ions, prior to electrodeposition,
It is known to form a hydroxide called a cathode coating on the surface of the steel sheet that is the cathode, but the present inventors, as described above, the chromium content in the plating layer and the plating adhesion,
It has been found that it is this cathode coating that determines the quality of the appearance and the like, and a manufacturing method with stable quality has been achieved by controlling variations in the coating.

In order to obtain a constant quality, it is necessary to keep the cathode coating itself constant. The cathode coating is greatly affected by pH on the surface of the steel sheet. During the electrolysis, the hydrogen generation reaction proceeds along with the electrodeposition of the plating, and the pH on the surface of the steel sheet rises. The increase in pH promotes excessive formation of hydroxide, and the quality fluctuates or deteriorates, such as an increase in chromium content and deterioration of adhesion due to the inclusion of chromium in the plating film. . In order to always produce stable quality, it is essential that the cathode film is formed in a proper and constant manner, and it is important to keep the pH on the surface of the steel sheet constant. The fluctuation of pH occurs when the amount of H ⁺ ions consumed by hydrogen generation during electrolysis is larger than the amount of supplied H ⁺ ions. Therefore, the pH
To suppress the fluctuation, it is necessary to supply the consumed is H ⁺ ion amount or more H ⁺ ions, which makes it possible to produce stable plated steel sheet quality.

Hereinafter, the present invention will be described in detail. The H ⁺ ion consumption concentration per unit area of the steel sheet is calculated from the current density and the hydrogen generation efficiency. The consumed H ⁺ ion is calculated by multiplying the current density by the hydrogen generation efficiency and then taking the quotient of the Faraday constant (96500 C / electron mol). Hydrogen generation efficiency is controlled and fluctuated by the pH of the plating solution, current density, and other factors.
As a result of examination, since it was 50% or less, 0.5 was used. The value (0.5 × Dk) / 96500 obtained here indicates the amount of H ⁺ ions (mol / dm ² · sec) consumed per unit area and per unit time.

The feed rate can be calculated as follows. H ⁺ ion concentration [H ⁺ ] (mol / dm ³ ) in the solution,
Distance between steel plate and anode (dm), plating solution flow rate (d)
m) and the plate width (dm) respectively, and the H ⁺ ion supply amount per unit time obtained by multiplying the plate width (d)
m) x cell length (dm), the H ⁺ ion concentration (m) supplied per unit area and unit time
ol / dm ² · sec). That is, {[H ⁺ ] (mol / dm ³ ) × plating solution flow rate (dm /
sec) x distance between steel plate and anode (dm) x plate width (d
m)} / {cell length (dm) × plate width (dm)} =
{([H ⁺ ] × plating solution flow rate × distance between steel plate and anode)}
/ Cell length (dm). Here, the plating solution flow rate is replaced by the relative flow rate when the steel sheet moves, and as a result, the supply rate is expressed as ([H ⁺ × distance between steel sheet and anode x relative flow rate) / (cell length)). To be done.

The supply rate and the consumption rate obtained above are substituted into the hydrogen ion (H ⁺ ) supply rate ≧ the hydrogen ion (H ⁺ ) consumption rate, which is the essence of the present invention, to obtain (H ⁺ ] × steel plate The relational expression of (distance between anodes × relative flow velocity) / (electrolytic cell length) ≧ (0.5 × current density) / 96500 is obtained. The upper limit of the relative flow velocity is not particularly defined, but if the relative flow velocity exceeds 7 m / sec, the influence of the physical friction of the plating solution increases and the amount of the electrodeposited plating adheres decreases. The lower limit is not particularly limited as long as it satisfies the above formula. The upper limit of the distance between the steel plate and the anode is not particularly defined, but if it exceeds 20 mm, the voltage load becomes excessive, so that it is preferably less than that. The lower limit is not particularly limited as long as it satisfies the above formula.

Next, the flow rate of the plating solution will be described in detail. If the plating solution flow rate is less than 0.5 m / sec, it is difficult to degas the hydrogen gas generated during electrolysis, which causes an increase in voltage load and further deteriorates plating burnability. The upper limit of the plating solution flow rate is not particularly defined, but if the plating solution flow rate exceeds 2.5 m / sec, the flow of the solution in the plating cell may be severely disturbed, and uneven plating may occur, and 0.5m / sec considering ability level
~ 2.5 m / sec is preferable. The upper limit and the lower limit of the length of the electrolytic cell are not specified. There is no particular problem as long as the above formula is satisfied. The material of the electrolytic cell is Pt-based, Pb-based, Ir
The effect of the present invention does not change even if an electrode made of a system or any other material is used.

The current density is 50 A / dm ^{2 to} 250 A / d
m ² is preferred. When the current density is less than 50 A / dm ² , chromium is hardly deposited, and when it exceeds 250 A / dm ² , plating burn is likely to occur and the voltage load becomes excessive, which is not practical. Zinc ion of plating solution,
Trivalent chromium ions are 10 g / dm ^{3 to} 150 g / d, respectively
m ³ is preferred. Zinc ion, chromium ion 10g /
If it is less than dm ³ , plating burn is likely to occur and
When it exceeds g / dm ³ , the ion concentration has almost reached saturation, and precipitation occurs in the plating solution. The pH of the acidic plating solution is preferably 0.5-3. When the pH is less than 0.5, the rate of hydrogen generation is large and the plating efficiency is significantly reduced. When the pH exceeds 3, ion precipitation occurs. The anion of the plating solution can be applied to both the sulfuric acid bath and the chloride bath.

As the organic additive, maleic acid, ascorbic acid, tartaric acid and the like can be used, but polyoxyalkylene derivatives are particularly preferable in consideration of the stability of chromium electrodeposition. Furthermore, NH ^{4 +} ions and / or L
The addition of one or more supporting salts containing i ⁺ , Na ⁺ , K ⁺ , Mg ^{2 +} , Al ^{3 +} ions, etc. as a conduction aid enhances the electrical conductivity of the plating solution, It is preferable because it has an effect of suppressing burns and uneven plating, and enhancing the electrodeposition action of chromium. These supporting salt is preferably 5g / dm ³ ~40g / dm ³ as cation concentration. If it is less than 5 g / dm ³ , the effect of improving the electric conductivity and suppressing plating burn and uneven plating is small, and it is 40 g / dm ³
If it exceeds, the effect is saturated and a large amount of cations may lead to a decrease in plating efficiency.

Further, depending on the purpose, Cr ^{6 +} , Ni, C
One or two or more kinds of ions such as o, Fe, Cu, Mn, P, Pb, Sn, Sb, and Cd can be added within a range not exceeding the ion concentration of trivalent chromium. Alternatively, when it is inevitably present, the effect of the present invention does not essentially change even if a small amount of eutectoid is added to the plated layer within a range not exceeding the chromium content. Chromium content in plating layer is 5-40
Mass% is preferred. If it is less than 5 mass%, no improvement in corrosion resistance is observed. On the other hand, if it exceeds 40 mass%, the plating becomes powdery during processing and the so-called powdering property deteriorates.

Further, after plating is carried out by the production method of the present invention, post-treatments such as chromate treatment, phosphate treatment and coating of organic thin film coating can be carried out according to the purpose.
The composition of the steel sheet to be plated can be selected from low-carbon aluminum killed steel, ultra-low carbon steel, low-alloy steel, high-alloy steel, etc. according to the required characteristics of the material, and the steel sheet is limited to cold-rolled steel sheet. However, it does not matter even if it is a hot rolled steel sheet.

[0019]

Next, examples of the present invention will be described together with comparative examples. Using the plating solution components and plating conditions shown in Table 1, a 0.7 mm-thick cold rolled steel sheet that had been subjected to alkaline electrolytic degreasing and pickling was plated with an adhesion amount of 20 g / m ² , and the chromium content of the plating layer and the plating The adhesion and plating appearance were evaluated and are shown in Table 2. The chromium content of the plating layer was measured by dissolving the plated plate and using an atomic absorption method. The plating adhesion is evaluated from the plating peeling condition of the adhesion bending part after the adhesion bending, and
(No peeling of the plating or a slight peeling that does not hinder practical use) and x (a large peeling that causes peeling or is practically unusable). The plating appearance is visually
A sample having no burnt and uniform appearance was evaluated as ◯, and a sample that was not practically used due to burning or uneven appearance was evaluated as x. Corrosion resistance after coating is zinc phosphate treated, cationic electrodeposition coating 20
After applying μm, a cross-cut scratch was made, and the coating film blistering width after 600 hours of salt spray test (according to JIS Z 2371) was measured. Evaluation is within 1 mm of swelling ○, 1
A value exceeding 3 mm and within 3 mm was evaluated as Δ, and a value exceeding 3 mm was evaluated as x.

[0020]

[Table 1]

[0021]

[Table 2]

As shown in Table 3, Examples 1 to 10 are examples of the present invention, and all have good adhesion, appearance and corrosion resistance. On the other hand, in Comparative Examples, Comparative Examples 1, 2 and 5
Since the H ⁺ supply rate on the parallel flow side was lower than the H + consumption rate, the adhesion was poor and the appearance was slightly burnt. In Comparative Examples 3 and 4, the H ⁺ supply rate was higher than the H ⁺ consumption rate, but in Comparative Example 3, the plating solution flow rate was too low and degassing of hydrogen gas was not sufficiently performed, resulting in severe uneven appearance. Burns occurred and the corrosion resistance was poor.
Further, in Comparative Example 4, since the flow rate of the plating solution was too high, the flow of the plating solution was disturbed and uneven plating occurred.

[0023]

[Table 3]

[0024]

As described above, the present invention enables industrial stable production of zinc-chromium alloy electroplated steel sheets. In particular, it is possible to provide manufacturing without deteriorating the quality even in an electrolytic cell in which a parallel flow and a counter flow are mixed and the relative flow velocity fluctuates. Therefore, contribution to the industry of the present invention is extremely large.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuya Kanamaru 5-3 Tokai-cho, Tokai-shi, Aichi New Nippon Steel Co., Ltd. Nagoya Steel Works

Claims (3)

[Claims] 1. A zinc-chromium alloy is formed by electroplating while suppressing fluctuation of a chromium hydroxide film by setting the amount of hydrogen ions on the surface of a steel sheet to be plated as hydrogen ion supply rate ≧ hydrogen ion consumption rate near the surface. A method for producing a zinc-chromium alloy electroplated steel sheet, which comprises forming a layer. 2. The amount of hydrogen ions on the surface of a steel plate to be plated is calculated by the following formula: ([H ⁺ × distance between steel plate and anode × relative flow velocity) / (electrolytic cell length) ≧ (0.5 × current density) / 96500 2. The method for producing a zinc-chromium alloy electroplated steel sheet according to claim 1, wherein the zinc-chromium alloy layer is formed by electroplating while controlling the variation of the chromium hydroxide film. 3. Multi-stage electrolysis in which an acidic plating solution containing Zn ²⁺ ions, Cr ³⁺ ions, and an organic additive is used, and both plating solution flows in the same direction and opposite directions with respect to the moving direction of the steel sheet are mixed. A method for continuously producing zinc-chromium alloy electroplating by passing a steel sheet through a cell, wherein the flow rate of the plating solution is 0.5 m / sec to 2.5 m / sec. Or the manufacturing method of the zinc-chromium alloy electroplated steel sheet according to 2.