JPH09111492A - Method for continuously and electroplating metallic sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the current density in a medium without generating chlorine on an anode at the time of electrogalvanizing a steel sheet as a cathode in an electrolyte contg. chlorides by the use of an insoluble anode by adding a chlorine-reducible compd. into a bath.

SOLUTION: Plural anodes, among which at least the one is insoluble, are arranged in the bath of an electrolyte with a chloride as the base, a current is applied between the anode and cathode, and a steel sheet as the cathode is continuously electroplated with zinc or zinc alloy. In this case, a compd. reducing the chloride in the bath and not reducing the zinc and alloy element close to the cathode in the bath is added as the soln. to the bath during electroplating. The bath is kept at ≤pH5.5, hydrazine, hydroxylamine, hydroxylamine hydrochloride, etc., are preferably used as the compd., and the concn. is kept at 0.03-0.3mol/l. Consequently, the current density is increased to ≥100A/dm² in the insoluble anode region.

COPYRIGHT: (C)1997,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous electroplating method for a metal plate, and more particularly, to use the metal plate as a cathode in an electroplating tank containing an insoluble anode and an aqueous electrolyte solution containing chloride as a main component. The present invention relates to a method for continuously electroplating a steel sheet with zinc or a zinc alloy.

[0002]

2. Description of the Related Art Zinc (Zn
^A method using an electrolytic bath based on a sulfate having ²⁺ ions dissolved therein is already known, and oxygen is generated at the anode at the same time when zinc is deposited on the cathode.

However, chloride-based electrolytes have a high conductivity, a low ohmic loss in the electrolytic cell, a high current density, and in many cases 100 A / dm ² or more. Generally, it is preferable to use a chloride-based electrolyte. In fact, high zinc (Zn ²⁺ ion) concentrations similar to chloride-based baths commonly used in soluble anode electroplating baths, and in some cases zinc alloy concentrations and high current densities, can be used to achieve very high deposition rates. Can be fast. Generally, the zinc concentration in the bath is 100 g or more in terms of zinc amount per liter. An acid bath is preferably used to obtain such a high solution concentration without causing precipitation of hydroxide, and the pH of the bath is usually 4 to 5.5.
However, when an insoluble anode is used in a chloride medium, chlorine is generated from the anode instead of oxygen. This is unacceptable in a factory plant.

The solution to the above-mentioned problems described in EP 0,501,547 uses a porous anode to inject hydrogen from the pores into the electrolyte. As a result, instead of oxidizing chloride ions to chlorine at the anode, hydrogen is oxidized to H ⁺ ions. The disadvantage of the method of this patent is that it cannot be operated at high current densities due to the injection of hydrogen in the vicinity of the porous anode. That is, in other types of industrial processes using chloride-based electrolytes (eg, using a soluble anode cell), the current density is typically 100 A
/ dm ² or more, but the current density of the above method is actually about
Limited to 50 A / dm ² . Therefore, one of the important advantages of using a chloride-based electrolyte solution is lost by the solution to prevent chlorine evolution.

British Patent 1,123,005 describes a chemical and electrochemical process for plating zinc on steel substrates using an insoluble anode bath. The baths described in this patent are, for example, based on sulphate, acetate or formate, no chloride-based baths are described. The zinc concentration in this bath is low.
Since it is 20 g / l, 46 g / l), there is no danger of hydroxide precipitation even if operated at a neutral pH (5-8, preferably 7).
Hydrazine has been proposed as a substance that reduces zinc (Zn ²⁺ ) at the cathode in order to effect chemical deposition. This compound was used at high concentrations (eg 20 cm ³ hydrazine hydrate per liter) and around pH 7 (polarized).
It functions as a substance that reduces zinc near the cathode. In this bath, operating at a current density of about 1.3 A / dm ² , a plating layer of 224 mg / dm ² or a thickness of about 3 μm can be obtained in 5 minutes. That is, the plating rate of this method is quite slow.

[0006] In order to increase the plating rate, it is necessary to operate in a medium containing at least zinc, and for that purpose, an acidic pH is required. However, at acidic pH, the reducing power of hydrazine decreases (this is due to N ₂ H ₄ + Zn ²⁺ → N ₂
The chemical precipitation function of hydrazine is lost (due to the reaction of + Zn + 4H ⁺ ). Therefore, the method described in GB 1,123,005 does not make it appropriate to use an acidic bath with a high zinc concentration, which makes it impossible to operate at high current densities and limits the plating rate.

[0007]

SUMMARY OF THE INVENTION It is an object of the invention to increase the current density in chloride-based media without generating chlorine from the anode.

[0008]

SUMMARY OF THE INVENTION The present invention has a plurality of anodes in a bath containing a chloride-based electrolyte bath, at least one anode being insoluble and a metal plate serving as the cathode.
A method for continuously electroplating zinc or a zinc alloy on a metal plate, especially a steel plate by passing an electric current between at least one anode and the metal plate, which has a reducing action on chlorine in the bath and which is Provided is a method characterized in that a compound having no reducing action on zinc and alloying elements in the vicinity of an inner metal plate is added to the bath in a solution state during electroplating.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION When the pH of the bath is 5.5 or less, the above compound to be added in a solution state is preferably selected from hydrazine, hydroxylamine and hydroxylamine hydrochloride. British Patent No. 1,123,005
The main difference between the method described in No. 1 and the method of the present invention is that when hydrazine is used as an example, the pH of the bath is sufficiently acidic in the present invention and the concentration of hydrazine in the bath is sufficiently high. Low enough that the redox potential of the hydrazine / nitrogen pair in the bath is high enough so that zinc in the bath solution is not reduced and the hydrazine function of preventing chlorine evolution at the anode is fully fulfilled. It is necessary.

Also, the hydrazine concentration must be low enough to prevent or limit the reduction of hydrazine at the cathode (at low pH the reduction reaction of hydrazine takes precedence at the expense of Zn ²⁺ ion reduction). Happens). The compound selected from hydrazine, hydroxylamine and hydroxylamine hydrochloride is preferably added so that the concentration in the bath is maintained at 0.03 to 0.3 mol / liter. Other characteristics of the present invention are as follows: a) The current density in the anode region is 100 A / dm ² or more. b) When the reducing compound is hydroxylamine hydrochloride, the concentration in the bath should always be kept at least 2 g / l.

Another subject of the invention is a plating bath for carrying out the method described above. Hereinafter, the present invention will be described with reference to Examples using hydroxylamine hydrochloride as a substance added to the electrodeposition bath in order to prevent generation of chlorine.
The present invention is not limited to the following examples, the present invention also includes other chlorine reducing compounds, especially hydrazine or hydroxylamine.

Example 1 A first example of the present invention is continuous galvanizing performed using an electrogalvanizing plant with an insoluble anode using a chloride based bath. Other than the compound of the present invention, that is, the bath composition, the anode material, and the plating conditions themselves are known, and therefore detailed description thereof is omitted here. The pH is generally 4-5.5. An electrogalvanizing plant has a series of insoluble anode type electrolyzers, which generally contains, for example, 340 m ³ of a chloride-based electrolytic bath, the total current flowing through the plant being, for example, about 600 kA.

In this case, in order to completely prevent the emission of chlorine, 1400 kg is added to the electrogalvanizing bath of the plant in the present invention.
It is necessary to add hydroxylamine hydrochloride at a rate of / hour. The flow rate of the additive to the bath, in this case hydroxylamine hydrochloride, reduces the chlorine that may be generated at the anode and reduces the hydroxylamine hydrochloride at the cathode and the oxidation of hydroxylamine hydrochloride by Zn ²⁺ ions. Keep concentration low enough to prevent. This method itself is known. The amount of chlorine that can be generated depends on the current flowing through the soluble anode of the plant and the coulombic yield of chlorine production, which depends on the material used for the insoluble anode.

The addition flow rate of hydroxylamine hydrochloride is such that the concentration of this additive in the bath is at least 2 g /
It is preferably adjusted so that it is kept at 1, ie about 0.03 mol / l, which is much lower than the known 20 g / l. This concentration is preferably maintained at a higher level, eg 8 g / l. In the present invention, an insoluble anode can be used in chloride-based electrolytes to plate zinc layers at high current densities, in this case about 100 A / dm ² , without the generation of chlorine from the electrolyte, and thus high speed. Can be plated. The reducing compound of the present invention, hydroxylamine hydrochloride in this example, reacts with "nascent" chlorine (Cl-atomic chlorine) generated at the anode to form nitrogen and hydrochloric acid. Therefore, nitrogen is generated from the electrogalvanizing bath, which is not dangerous unlike chlorine generation. The hydrochloric acid formed is convenient for the dissolution reaction of the zinc block used to resupply zinc ions into the bath.

Example 2 A zinc / nickel alloy was continuously plated according to the present invention using a plant similar to the plant described above. In this second example, a part of the anode was a soluble zinc anode, and the remaining anode was used. Was used as an insoluble anode for controlling the nickel concentration of the zinc / nickel alloy to be plated. As an example, 5 out of 6 anodes were zinc anodes and the rest were insoluble anodes. In the present invention, at a total current of 300 kA, hydroxylamine hydrochloride was added to the electrodeposition bath at a rate of 140 kg / hr. The amount of additive added to the bath also depends on the proportion of insoluble anode in the plant and the proportion of nickel in the zinc / nickel alloy to be plated. The addition flow rate of hydroxylamine hydrochloride is such that the concentration of hydroxylamine hydrochloride in the bath is 0.4 g / l or approximately
It is preferable to adjust it so that it is maintained at 0.006 mol / l.

The minimum concentration is lower than in Example 1, but this is because the amount of chlorine that can be released is small, the current value is low, and only part of the anode is insoluble. According to the present invention, some of the anodes are insoluble in chloride-based electrolytes, high current densities without generating chlorine from the electrolytes,
In this embodiment, a layer of zinc / nickel alloy can be plated at about 100 A / dm ² . Hereinafter, specific test results of the present invention will be described.

The following tests were carried out using a rotating electrode type laboratory plating tank equipped with a platinum anode having a surface area of 0.03 cm ² and a zinc cathode. The aqueous solution of the electrolyte at the start of the test contained the following, pH about 4 and temperature about 60 ° C .: 1) 2 mol / l zinc chloride (ZnCl ₂ l) 2) 4.5 mol / l chloride Put the above electrolytic solution into the potassium (KCl) plating tank and make the current density 50 to 100 A /
A current was passed between the electrodes to reach dm ² . The test lasted 2 hours.

During the test, chlorine released from the electrolytic solution was quantified by the following method. Nitrogen is blown into the electrolytic solution, and chlorine generated at the anode is made to accompany with nitrogen and collected in an attached container. This container contains potassium iodide solution,
Chlorine oxidizes iodide to iodine. The amount of chlorine released from the electrolyte during the test can be determined by measuring the amount of iodine contained in the solution (eg, using thiosulfate) after the test.

Comparative Example 1 For comparison, the procedure was performed without adding any special additive to the electrolytic solution. The potassium iodide solution in the accessory container rapidly colored,
It was proved that chlorine was released in the cell. After the test, measure the amount of iodine produced in the attached container,
The amount of chlorine generated at the anode was calculated from this value, and the Coulomb yield for chlorine formation was calculated from this value, giving a value of 90%.

Test Example 1 This test is for specifically explaining the present invention. This test was also performed under the same conditions as in Comparative Example 1, but in this test, 2 g / l of hydroxylamine hydrochloride (NH ₂ OH.HCl) was added to the electrolytic solution. Under examination,
No color was found in the potassium iodide solution in the accessory container.
It was proved that chlorine was not generated in the cell even when operated at a current density of 100 A / dm ² .

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Claims (5)

[Claims] 1. A plurality of anodes are provided in a bath containing a bath of chloride-based electrolyte, at least one anode being insoluble, a metal plate being the cathode, and at least one anode and a metal plate. In a method of continuously electroplating zinc or a zinc alloy on a metal plate, especially a steel plate by passing an electric current between them, zinc having a reducing action on chlorine in the bath and zinc in the vicinity of the metal plate in the bath A method comprising adding a compound having no reducing action to an alloying element to the bath in a solution state during electroplating. 2. The method according to claim 1, wherein the pH of the bath is 5.5 or less and the compound added to the solution is a compound selected from hydrazine, hydroxylamine and hydroxylamine hydrochloride. 3. The concentration of the above compound during electroplating is 0.03.
The method according to claim 2, wherein the compound is added in the form of a solution so as to maintain the concentration at 0.3 mol / liter. 4. The method according to claim 2, wherein the compound is hydroxylamine hydrochloride, and the concentration in the bath during electroplating is always maintained at least 2 g / liter. 5. The method according to claim 1, wherein the current density in at least one insoluble anode region is 100 A / dm ² or more.