JPH0841695A - Reduction of ferric ion in plating solution - Google Patents

Abstract

PURPOSE:To efficiently reduce without discarding a plating solution by specifying the materials of a cathode and an anode in the case of electrolytically reducing Fe<3+> propduced by the oxidation in the plating solution containing Fe<2+>. CONSTITUTION:The electrolysis is executed by using Pb, a Pb alloy, Sn or Ta as the cathode and Fe or a metal to be plated as the anode. In this case, the current density (A/dm<2>) is controlled to be equal to or lower than the value of {Fe<3+> concn. in the plating solution (g/l)}X{the flow rate of the plating solution (m/sec)}X2.5. Pb, the alloy, Sn or Ta has large hydrogen overvoltage and Fe deposition overvoltage, is more noble than Fe in normal electrode potential and suitable to efficiently reduce Fe<3+>. By this method, Fe<3+> is reduced without using a diaphragm and without accompanying the generation of hydrogen and the deposition of Fe and the concn. of the metallic ion in the plating solution is basically not increased and it is unnecessary to discard the plating solution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is ferric ion (Fe ³⁺⁾ efficiently ferrous ions produced by the anode oxidation and air oxidation of an alloy plating solution containing an iron-based plating and iron (Fe ^{2 +} ) To a reduction method of ferric ion in a plating solution.

[0002]

BACKGROUND ART Zinc - iron, iron - galvanizing has been widely used as rust preventive plated steel plates, Fe ²⁺ in the plating solution
^Has a manufacturing problem in that it is oxidized to Fe ³⁺ by anodic oxidation and air oxidation. Since Fe ³⁺ deteriorates the plating quality and reduces the plating efficiency, it must be controlled to a certain concentration or less.

As a method of managing Fe ³⁺ in the plating solution in this way a certain concentration or less, a method of removing Fe ³⁺ outside plating solution, there is a method of reducing the Fe ^2+. As a method of removing Fe ³⁺ from the plating solution, there is a method of adsorbing Fe ³⁺ on an ion exchange resin, but since the ion exchange resin is expensive in addition to requiring a large-scale apparatus, the initial running cost is low. It is expensive and has many problems in industrial production.

On the other hand, as a method for reducing Fe ³⁺ to Fe ²⁺ , there is a method disclosed in JP-A-58-151489. In this method, Fe ³⁺ is brought into contact with iron or zinc to cause a reaction represented by the formulas (1) to (3) below.

2Fe ³⁺ + Fe → 3Fe ²⁺ (1) 2Fe ³⁺ + Zn → 2Fe ²⁺ + Zn ²⁺ (2) 2H ⁺ + Fe (or Zn) → H ₂ + Fe ²⁺ (Zn ²⁺ ) ... (3) Since this method also serves as replenishment of Fe ²⁺ and Zn ²⁺ reduced by plating, the reactions of the above formulas (1) and (2) occur efficiently. This will be an effective method of reduction. However, in practice, the reduction reaction of hydrogen ions shown in the above formula (3) that occurs at the same time becomes predominant, which is inefficient. Further, according to this publication, the reduction efficiency of Fe ³⁺ is described as 30 to 83%, and when the reduction efficiency is low, an excessive amount of Fe ³⁺ is required to cause the required reduction of Fe ³⁺ . Fe and Zn must be dissolved in the plating solution, and the concentration of Fe ²⁺ and Zn ^{2+ in} the plating solution becomes excessive,
In order to keep these concentrations constant, part of the plating solution must be discarded and water must be added.

Regarding the reduction efficiency of Fe ³⁺ , Japanese Patent Publication No.
No. 11440 discloses a polyoxyethylene derivative compound or a polyoxypropylene derivative compound.
It is said that the addition efficiency of 01 to 10 g / l increases the reduction efficiency to 95% or more, but no consideration is given to the influence on the plating film characteristics. It is sufficiently predicted that these additives will adversely affect the plating film properties, and it is not preferable to mix these additives into the plating solution in order to promote F ³⁺ reduction.

As a technique similar to the above technique, a method of connecting a soluble metal and an insoluble metal with a conductor to form a battery and reducing F ³⁺ on the insoluble electrode is disclosed in JP-B-5-1044.
No. 0 publication. However, the technique disclosed in this publication is intended to remove F ³⁺ mixed in a copper plating solution or a nickel plating solution, as shown in the examples, and the reduction efficiency of F ³⁺ is reduced. Is not considered at all, and it cannot be directly applied to a plating solution containing Fe ²⁺ as a main plating component.

Further, in Japanese Patent Laid-Open No. 59-25991, an ion exchange membrane is used to separate the cathode chamber and the anode chamber, and the cathode potential is the equilibrium potential of Fe {-0.44+.
0.031log [Fe ²⁺ ]} V (standard hydrogen electrode standard)
A more noble electrolysis method has been disclosed, and an electrode having a low hydrogen generation overvoltage such as platinum plating on platinum or platinum-iridium oxide is said to be suitable as the cathode. However, as is well known, ion exchange membranes are not only expensive, but also have a short life and require frequent replacement, and membrane electrolysis has a problem that the electrolysis voltage is high and the cost is high.

[0009]

As described above, the prior art cannot efficiently reduce F ³⁺ in the plating solution containing Fe ²⁺ as the main plating component. The present invention has been made in view of the above circumstances, and it is possible to reduce the increase of the metal ion concentration in the plating solution as much as possible, without discarding the plating solution, and at a low cost, to remove F ³ in the plating solution. and to provide a method for reducing ⁺ to efficiently Fe ^2+.

[0010]

In order to solve the above-mentioned problems, the present invention provides a method of electrolytically reducing ferric ions produced by oxidation in a plating bath containing ferrous ions, which comprises a cathode The present invention provides a method for reducing ferric ion in a plating solution, characterized in that Pb, Pb alloy, Sn, and Ta are used as the electrolyte and iron or a metal to be plated is used as an anode for electrolysis.

In the above method, the current density (A / dm ² ) during the electrolytic reduction is {concentration of ferric iron ion in the plating solution (g / l)} × {plating solution flow rate (m / sec)}. ×
It is characterized in that it is 2.5 or less.

The present invention will be specifically described below.
Considering an optimal Fe ³⁺ reduction system in consideration of economy, a method of electrolytically reducing Fe ³⁺ directly in a plating solution without using a diaphragm is desirable. Therefore, in the present invention, Fe ³⁺ is electrolytically reduced without using a diaphragm.

Here, in order to reduce Fe ³⁺ efficiently, it is necessary to suppress the reduction reaction of H ⁺ . Therefore,
The inventors of the present application noted that the easiness of the H ⁺ reduction reaction is controlled by the hydrogen overvoltage of the electrode where the reduction reaction occurs. The hydrogen overvoltages of the electrodes that have been studied in the past are shown in Table 1 (Institute of Electrical Engineers, University Lecture “Electrochemistry” page 135) and Table 2.
(Electrochemistry Vol.38, No.1,17).

[0014]

[Table 1]

[0015]

[Table 2]

The values of the hydrogen overvoltage of each electrode shown in Tables 1 and 2 are different because of different measurement conditions.
The size relationship is almost the same in each table, and Pt, Ni, Fe
Has a small hydrogen overvoltage, and Sn, Pb, Zn, and Ta have a large hydrogen overvoltage. Therefore, Sn, which has a large hydrogen overvoltage,
It is considered that Pb, Zn, Ta and the like are more suitable for efficiently reducing Fe ³⁺ .

Further, as a reducing electrode for Fe ³⁺ , F is used on the surface.
It is necessary that the reduction reaction of e ²⁺ is difficult to occur. Fe
^This is because the fact that the ²⁺ reduction reaction occurs means that metallic Fe is deposited, so that the hydrogen overvoltage in that portion becomes the same as that of the Fe electrode, and hydrogen generation becomes dominant. Table 1 also shows the standard electrode potential, but in that sense, in the case of an electrode whose standard electrode potential is lower than Fe, the electrode potential must be lower than the deposition potential of Fe in order to make it a cathode. The possibility of precipitation increases, and it is unsuitable as a Fe ³⁺ reduction electrode. However, the standard electrode potentials of Cr and Ta are baser than that of Fe, but since they form a passivation film on the surface, they show a nobler potential than Fe in the plating solution, and at a potential nobler than the Fe deposition potential, the cathode Can be used as Where F
Since the precipitation of e is often accompanied by a relatively large overvoltage (several hundred mV), the actual deposition potential shifts to a base potential due to this deposition overvoltage. Therefore, it is expected that even an electrode having a standard electrode potential lower than that of Fe can be used as an Fe ³⁺ reduction electrode when it has a large Fe precipitation overvoltage. Further, it is desirable that even an electrode having a standard electrode potential nobler than Fe has a large Fe precipitation overvoltage in order to prevent Fe precipitation.

From the above reasons, it is considered that an electrode having a large hydrogen overvoltage and a large Fe precipitation overvoltage is suitable as the Fe ³⁺ reduction electrode, and that the standard electrode potential is more noble than Fe.

Here, there is almost no conventional knowledge about the Fe precipitation overvoltage of various metals and alloys, and Pb, Sn, Ta and their alloys are selected as electrodes satisfying the above conditions from the hydrogen overvoltage and the standard electrode potential. Then Fe ³⁺
The suitability as a reduction electrode was investigated. As a result, it is possible to reduce Fe ³⁺ with almost no hydrogen generation or Fe precipitation by conducting electricity by using these electrodes as cathodes and Fe or a metal to be plated such as Zn as an anode. I found it. In addition, such a method does not increase the metal ion concentration in the plating solution in principle, and there is no need to discard the plating solution.

At this time, in order to maximize the reduction efficiency of Fe ³⁺ , it is desirable that the flow rate of the plating solution is high. This is for the following reasons. Fe industrially
^In order to carry out ³⁺ reduction, it is necessary to increase the reduction rate and make the reactor as small as possible, and for this purpose, the current density must be increased. Meanwhile, since the normal Fe ³⁺ is managed at a low concentration, the reduction reaction of Fe ³⁺ is under diffusion control of Fe ^3+, thereby promoting the diffusion is effective in the reduction reaction promoting Fe ^3+. Therefore, the flow velocity of the plating solution is increased to promote the diffusion of Fe ³⁺ , and thus F
It enhances the reduction efficiency of e ³⁺ .

According to the study by the inventors of the present application, the reduction efficiency is sufficiently high (assuming 90% or more), and the industrially applicable reduction current density upper limit (allowable reduction current density) is Fe ³ in the plating solution. It has been clarified that the relationship between the ⁺ concentration and the flow rate of the plating solution during reduction is expressed by the following formula.

Allowable reduction current density (A / dm ² ) = {Fe
³⁺ concentration (g / l)} × {plating solution flow rate (m / sec)} ×
2.5 Further, the cathode potential in a state where the reduction efficiency was sufficiently high was −550 to −720 mV based on the hydrogen electrode. Considering the equilibrium potential of Fe (Fe ²⁺ / Fe, −440 mV), it is expected that this cathodic electricity causes reduction of Fe.
In fact, precipitation of Fe was not observed, and stable reduction efficiency was obtained over a long period of time. This is considered to be due to the large Fe precipitation overvoltage of the cathode.

The p of the plating solution to which the present invention is applied is
H is preferably 0.5 to 3.0. If the pH is less than 0.5, the electrolysis efficiency during plating is originally low, which is unsuitable for industrial production. In addition, since the hydrogen concentration is high, the hydrogen reduction reaction at the cathode becomes predominant, and the Fe ³⁺ reduction reaction The reaction efficiency becomes low. If the pH exceeds 3.0, Fe ³⁺ becomes hydroxide and easily precipitates, which is not desirable.

[0024]

Embodiments of the present invention will be described below. (Example 1) Pb, a Pb alloy in an Fe-Zn plating solution,
Constant current electrolysis was performed using Sn and Ta as cathodes and a cold-rolled steel sheet as an anode. The electrolysis was performed using a circulating electrolysis cell having a structure in which the plating solution uniformly flows between the opposing anode and cathode at an interval of 20 mm. Plating solution flow rate is 0.5-2
It was set to m / sec.

The composition and conditions of the plating solution used here are as follows. Ferrous sulfate: 380 g / l Zinc sulfate: 20 g / l Sodium sulfate: 30 g / l pH: 2.0 Temperature: 50 ° C. Here, in order to accurately evaluate the Fe ³⁺ reduction rate, a plating solution without electrolysis Is circulated and Fe ²⁺ is converted to Fe ^{3+ by} air oxidation.
The rate of oxidization was calculated. The reduction efficiency was calculated assuming that this air oxidation occurred during the actual electrolytic reduction.
The reduction efficiency was calculated assuming that the reduction of Fe ^{3+ on} the surface of the cold-rolled steel sheet can be ignored. The energization amount was 3500 coulombs / l, but with this amount of electricity, almost 100%
In the case of the reduction efficiency of, the Fe ³⁺ concentration decreases by about 2 g / l. At this time, type of cathode electrode, concentration of Fe ³⁺ in plating solution, flow rate of plating solution, allowable current density, reduction current density, Fe
^{The 3+} reduction efficiency is shown in Table 3. The allowable reduction current density is
The value of Fe ³⁺ concentration × plating solution flow rate × 2.5.

[0026]

[Table 3]

As shown in Table 3, Pb and Pb were added to the cathode.
No. of the example using alloy, Sn, and Ta. 1 to 20 is 5
While the reduction efficiency of 6% or more was obtained, No. 6 of the comparative example using Pt and Fe. 21-28 showed a reduction efficiency of at most 14%. Further, in the examples, No. 1-1
^No. 6 has a reduction current density of Fe ³⁺ concentration × plating solution flow rate × 2.5
Since the allowable current density is less than or equal to, the high reduction efficiency of 90% or more was obtained. No. Since the reduction current densities of 17 to 20 are equal to or higher than the allowable current density, the reduction efficiency is 70%.
Stopped below. Although the anode and the cathode were short-circuited and an attempt was made to carry out reduction without energization, no clear Fe ³⁺ reduction was measured at any of the electrodes. (Example 2) Pb, Pb alloy in Fe-Zn plating solution,
Constant current electrolysis was performed using Sn and Ta as cathodes and a cold-rolled steel sheet as an anode. The electrolysis was performed in the same manner as in Example 1.

The composition and conditions of the plating solution used here are as follows. Ferrous sulphate: 300 g / l Zinc sulphate: 200 g / l Sodium sulphate: 30 g / l pH: 1.5 Temperature: 60 ° C The amount of electricity passed was 7,000 coulomb / l, but at this amount of electricity the reduction efficiency was almost 100%. In the case of, the Fe ³⁺ concentration decreases by about 4 g / l. The type of cathode electrode at this time,
Fe ³⁺ concentration of plating solution, plating solution flow rate, allowable current density,
Table 4 shows the reduction current density and the Fe ³⁺ reduction efficiency.

[0029]

[Table 4]

As shown in Table 4, Pb and Pb were added to the cathode.
No. of the example using alloy, Sn, and Ta. 29-45 obtained reduction efficiency of 59% or more, whereas Pt, Fe
No. of the comparative example using. 46-51 showed a reduction efficiency of at most 18%. Further, in the examples, No. 29
~ 41 is the reduction current density Fe ³⁺ concentration × plating solution flow rate ×
90% because it is less than the allowable current density represented by 2.5
The above high reduction efficiency was obtained. No. Since the reduction current densities of Nos. 42 to 45 were not less than the allowable current density, the reduction efficiency was 70% or less. (Example 3) Pb, Sn, Ta in a Fe-Cr plating solution
Was used as a cathode and a cold-rolled steel sheet as an anode to carry out constant current electrolysis. The electrolysis was performed in the same manner as in Examples 1 and 2.

The composition and conditions of the plating solution used here are as follows. Ferrous sulphate: 200 g / l Chromium sulphate: 300 g / l Ammonium sulphate: 50 g / l pH: 1.8 Temperature 50 ° C The energization amount was 3500 coulombs / l, but in the case of a reduction efficiency of almost 100% at this electricity amount The Fe ³⁺ concentration is reduced by about 2 g / l. The type of cathode electrode at this time,
Fe ³⁺ concentration of plating solution, plating solution flow rate, allowable current density,
Table 5 shows the reduction current density and the Fe ³⁺ reduction efficiency.

[0032]

[Table 5]

As shown in Table 5, Pb and S are added to the cathode.
No. of the example using n and Ta. Nos. 52 to 64 obtained a reduction efficiency of 52% or more, whereas Nos. 52 to 64 of Comparative Examples using Pt and Fe. 46-51 showed a reduction efficiency of at most 12%. Further, in the examples, No. In Nos. 52 to 60, the reduction current density was not more than the allowable current density represented by Fe ³⁺ concentration × plating solution flow rate × 2.5, so that a high reduction efficiency of 90% or more was obtained. No. In Nos. 61 to 64, the reduction current density was equal to or higher than the allowable current density, so that the reduction efficiency was 65% or less.

[0034]

As described above, according to the present invention,
Provided is a method for efficiently reducing F ³⁺ in a plating solution to Fe ²⁺ at a low cost, without discarding the plating solution, by minimizing the increase in the concentration of metal ions in the plating solution. . Further, by controlling the reduction current density to be equal to or lower than the allowable current density, extremely high reduction efficiency can be obtained.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toyofumi Watanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A method for electrolytically reducing ferric ions produced by oxidation in a plating bath containing ferrous ions, wherein Pb, Pb alloy, Sn, Ta is used for the cathode, and iron or iron is used for the anode. A method for reducing ferric ion in a plating solution, which comprises electrolyzing using a metal to be plated. 2. The current density (A / dm) during the electrolytic reduction.
² ) = {trivalent iron ion concentration in plating solution (g / l)} x
The method for reducing ferric ion in the plating solution according to claim 1, wherein {plating solution flow rate (m / sec)} × 2.5 or less.