JPH09316687A - Production of zinc-chromium alloy electroplated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for industrially producing a zinc-chromium alloy electroplated steel sheet excellent in corrosion resistance and corrosion resistance after coating. SOLUTION: A steel sheet is passed through a multistage electrolytic cell jetting a plating soln. in either the same direction or opposite direction to the moving direction of the steel sheet, and furthermore, as for the intermittent energizing pattern, with the energizing time as 0.15 to 4.5sec and the energizing ratio as >=0.10, while the fluctuation of hydroxide film essentially consisting of chromium is suppressed, electroplating is executed to form a zinc-chromium alloy layer.

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"). Plating cell in the opposite direction (hereinafter referred to as “counterflow”), and Japanese Patent Publication No. 62-1.
As disclosed in Japanese Patent No. 5638, a vertical type plating cell that utilizes gravity drop of a plating solution, and further, JP-A-61-100944 and JP-B-61-2204.
As disclosed in Japanese Patent Publication No. 0, etc., there is known a fluid cushion type plating cell or the like in which a plating solution is jetted from a central portion of an electrode to form a static pressure between the electrode and a steel plate.

The vertical plating cell utilizing gravity drop of the plating solution has the advantage that the equipment can be made compact, and the fluid cushion type horizontal cell has the advantage that the distance between the steel plate and the anode can be shortened. In both plating cells, the parallel flow and the counter flow are mixed, and plating is performed at different relative flow rates. In general, in zinc-based alloy electroplating, the alloy composition in the plating layer also fluctuates as the relative flow rate fluctuates.Therefore, in plating cells in which both parallel and counter-current plating solution flows coexist, appearance quality and adhesion The quality was inevitable. These phenomena are no exception in zinc-chromium alloys.

For example, JP-A-1-191798, JP-A-3-120393, and JP-A-6-41781 disclose methods for producing zinc-chromium. However, plating in which a parallel flow and a counter flow coexist. It is difficult to suppress the deterioration of quality and control the variation in manufacturing plated steel sheets using cells, and these phenomena hinder the industrialization of zinc-chromium alloy electroplated 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. This is an invention found from the knowledge of the mass transfer of zinc ion and iron ion, but chromium ion is different from iron ion or other metal ions in that it forms a polynuclear complex in an aqueous solution. The moving speed is very low, and the problem cannot be solved by the electrode length regulation based on the mass transfer of the publication.

On the other hand, although the above-mentioned problems do not exist in the horizontal type electrolytic cell in which the flow of the plating solution is one direction of the parallel flow or the counter flow, when the intermittent current plating is performed in the multi-stage cell, uneven plating is caused. There is a problem that the plating adhesion is deteriorated. As a manufacturing method for manufacturing a zinc-chromium alloy electroplated steel sheet by intermittent energization, there is disclosed in Japanese Unexamined Patent Publication No.
9779, JP-A-6-136593, etc. are known. However, in JP-A-5-9779, no organic additive is used, and if the production is continuously continued and a current load is continuously applied to the plating solution, the chromium composition of the chromium composition is maintained even if ion replenishment and additive replenishment are performed. There is a problem in bath stability due to variations and deterioration in adhesion, and there is concern that the waste liquid treatment cost will increase and the environment will be affected as the plating liquid waste liquid increases. Further, in Japanese Patent Application Laid-Open No. 6-136593,
Not only metallic chromium but also oxide chromium is produced to improve quality such as adhesion. However, oxide chromium tends to cause uneven plating, resulting in a significant decrease in commercial value.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, the present invention is a multi-stage method in which a zinc-chromium alloy electroplated steel sheet is jetted with a plating solution in either the same direction or the opposite direction to the moving direction of the steel sheet. It is an object of the present invention to provide a method for stable production using an electrolysis cell and an energization method different from the conventional intermittent energization.

[0007]

The gist of the present invention is to pass a steel sheet through a multi-stage electrolytic cell in which a plating solution is jetted in either the same direction or the opposite direction to the moving direction of the steel sheet,
The zinc-chromium alloy electroplated steel sheet is characterized in that the zinc-chromium alloy layer is formed by electroplating while intermittently energizing to suppress fluctuation of the chromium-based hydroxide film. Furthermore, the energization pattern of intermittent energization has an energization time of 0.15 seconds to 4.5 seconds and an energization ratio of 0.1.
It is preferably 0 or more.

As a result of various studies on zinc-chromium alloy electroplating, the present inventors have found that a chromium-based hydroxide film (thickness, composition, etc.) called a cathode film formed during electrodeposition varies. As a result, the quality of the alloy composition and adhesion will vary, and the fluctuation of the cathode coating will occur in the plating cell in which parallel flow and counter flow are mixed,
In addition, the inventors have newly found that they are also generated by continuous electrodeposition. Based on this knowledge, the plating solution is jetted either in the same direction or in the opposite direction with respect to the moving direction of the steel sheet to stabilize the relative flow velocity, and by specifying the intermittent energization method, the fluctuation of the cathode coating disappears. Let
It is possible to manufacture a zinc-chromium alloy electroplated steel sheet of stable quality.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. It is generally known that chromium ions form a hydroxide called a cathode coating (a hydroxide mainly consisting of chromium) on the surface of the steel plate that is the cathode prior to electrodeposition, but the chromium composition during plating and plating adhesion The present inventors have found that it is this cathode coating that determines the quality of properties, appearance, etc., and further found a suitable method for suppressing the fluctuation of the cathode coating itself in order to obtain a certain quality. It was

The fluctuation of the cathode coating is the fluctuation of the relative flow velocity,
It is generated by continuous electrodeposition, and the quality changes accordingly. A method that does not change the cathode coating is to stabilize the relative flow rate and always maintain the initial state in which the cathode coating is formed. That is, the relative flow velocity is stabilized by using a plating cell in which the jet direction of the plating solution is one direction,
Further, when the cathode film starts to fluctuate due to electrodeposition, the current is cut off and the current is applied again to maintain the initial state, so that the quality becomes stable. The intermittent energization pattern is preferably 0.15 seconds to 4.5 seconds. If the energization time is less than 0.15 seconds, the efficiency will drop significantly,
If it exceeds 4.5 seconds, the effect of intermittent energization decreases, the cathode coating changes, and the quality of plating adhesion, appearance, etc. tends to deteriorate.

The energization ratio (energization time / (energization time + non-energization time)) is preferably in the range of 0.10. If the energization ratio is less than 0.10, the ratio of the non-energization time to the energization time becomes too large, and the redissolution of the plating that occurs during the non-energization time cannot be ignored and is not efficient. The upper limit is not particularly defined, but it is preferably 0.70 or less in consideration of the plating length and the plating cell interval of the multi-stage cell type production line.
The current density is preferably 50 A / dm ^{2 to} 250 A / dm ² . 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.
Not practical.

Zinc ions and trivalent chromium ions in the plating solution are preferably 10 g / l to 150 g / l, respectively. More preferably, it is 30 g / l to 120 g / l. When the amount of zinc ions and trivalent chromium ions is less than 10 g / l, plating burn is likely to occur, and at 150 g / l, 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. pH is 0.
If it is less than 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.

Further, the organic additive is a hydrocarbon group having 20 or less carbon atoms or having 2 to 10 carbon atoms, which has an aromatic ring of 4 m + 2 (m = 1 or 2) π electrons from the stability of chromium precipitation.
The polyoxyalkylene derivative consisting of the saturated hydrocarbon group and the ethylene oxide group having an addition mole number of 15 or less is preferable. The addition amount is preferably 0.01 g / l to 20 g / l.
If it is less than 0.01 g / l, sufficient chromium precipitation is not performed, and if it exceeds 20 g / l, the effect of chromium precipitation is saturated, which is not a cost effective measure. Flow rate of plating solution is 30m
/ Min or more is preferable. If it is less than 30 m / min, it becomes 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 specified, but the plating solution flow rate is 150 m.
If it exceeds / min, the flow of the liquid in the electrolysis cell will be severely disturbed and uneven plating may occur.
30 m / min to 150 m / min is preferable.

Further NH⁴⁺Ions and / or Li
⁺, Na⁺, K⁺, Mg^{2 +}, Al ^{3 +}Including ions
The addition of one or more supporting salts is a plating solution.
Enhances the electrical conductivity of
And the effect of increasing the eutectoid effect of chromium.
Preferred. These supporting salts have a cation concentration of 5 g
/ L to 40 g / l is preferred. Below 5g / l, electricity
Little effect of improving conductivity, plating burn, uneven plating control
If the amount exceeds 40 g / l, the effect will be saturated and a large amount will be produced.
Cations may reduce the plating efficiency
Not a good idea.

Further, depending on the purpose, Cr ^{6 +} , Ni, C
o, Fe, Cu, Mn, Cu, Pb, Sn, Sb, Cd
One kind or two or more kinds of ions such as 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. The chromium composition in the plating layer is 5-4
0 mass% is preferable. 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 manufacturing 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 a cold-rolled steel sheet. However, the hot rolled steel sheet may be used.

[0017]

Next, examples of the present invention will be described together with comparative examples. Under the plating solution components and plating conditions shown in Tables 1 and 2, 0.7 mm thick cold-rolled steel sheet subjected to alkaline electrolytic degreasing and pickling was plated with a basis weight of 20 g / m ² , plating layer composition and plating adhesion. And the plating appearance were evaluated and are shown in Table 2. The plating layer composition was measured by dissolving the plated plate and using an atomic absorption method. Plating adhesion is evaluated from the plating peeling condition of the adhesion bending part after performing contact bending, and ○ (no plating peeling or slight peeling that is practically unproblematic), × (with peeling or practical specification not possible) The peeling was performed with a large degree). The appearance of the plating is done visually.
A sample having no burn and a uniform appearance was evaluated as ◯, and a sample having a burn or uneven appearance which was not practical was evaluated as x.

Corrosion resistance after coating: zinc phosphate treatment, cationic electrodeposition coating 20 μm, cross-cut scratches, salt spray test (according to JIS Z 2371) 60
The coating film blister width after 0 hours was measured. Evaluation is blister width 1
The range within mm is ◯, the range exceeding 1 mm and within 3 mm is Δ, and the case exceeding 3 mm is x. Further, the stability of the plating solution was evaluated under the condition that the corrosion resistance was good. The bath stability is the stability of the chromium alloy composition when the plating solution is subjected to a current load of 100,000 C / dm ² , ion replenishment and additive replenishment, various concentrations are returned to their initial values, and plating is performed again. ,
The appearance was evaluated. In the evaluation, the composition variation was within ± 3% by weight, and the appearance was not changed, ○, the composition variation was ±.
If more than 3 wt% or deterioration of appearance occurred ×
And

[0019]

[Table 1]

[0020]

[Table 2]

Although Examples 1 to 10 are the present invention, all of them have good adhesion, appearance, corrosion resistance, and bath stability.
On the other hand, in Comparative Example 1, in Comparative Example 1, since the energization time was too short, re-dissolution of the plating occurred during the non-energization time, and the plating could not be stably manufactured. In Comparative Example 2, the bath was lacking in stability because the plating was produced by continuous energization. In Comparative Example 3, since the energization time was too long, the effect of intermittent energization was lost, and the bath stability was poor. In Comparative Example 4, the energization ratio was too small, the plating adhesion was deteriorated and the plating unevenness was caused due to the redissolution of the plating generated during the non-energization time, and the corrosion resistance was slightly deteriorated.

[0022]

INDUSTRIAL APPLICABILITY As described above, the present invention makes it possible to industrially stably produce a zinc-chromium alloy electroplated steel sheet, and by virtue of this, the production stability and the plating solution (EN) It is possible to manufacture the plated steel sheet at an extremely low cost in order to dramatically improve the life. Furthermore, the plating waste liquid is reduced to minimize the influence on the environment. 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 (2)

[Claims] 1. A hydroxide film mainly composed of chromium is obtained by passing a steel sheet through a multi-stage electrolysis cell in which a plating solution is jetted in either the same direction or an opposite direction to the moving direction of the steel sheet and by intermittently energizing the steel sheet. The method for producing a zinc-chromium alloy electroplated steel sheet, which comprises forming a zinc-chromium alloy layer by electroplating while suppressing the fluctuation of 2. The zinc-chromium alloy electricity according to claim 1, wherein an energization pattern of intermittent energization has an energization time of 0.15 seconds to 4.5 seconds and an energization ratio of 0.10 or more. Manufacturing method of plated steel sheet.