JPH0428800B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for regenerating a plating bath in iron-zinc alloy electroplating.

In recent years, iron-zinc alloy electroplated steel sheets have come into the spotlight as steel sheets for automobiles, for example, due to their excellent corrosion resistance, and various methods of plating them are being developed. However, as is the case with iron-based plating in general, Fe ²⁺ ions in the plating bath are extremely unstable;
For example, Fe ²⁺ ions are easily oxidized to Fe ³⁺ ions by dissolved oxygen in the plating solution or by electrode reactions on the anode surface. These reactions can be expressed as follows.

Fe ²⁺ +1/4O ₂ +1/2H ₂ O→Fe ³⁺ +OH ^- ...(1) Fe ²⁺ →Fe ³⁺ +e ...(2) The Fe ³⁺ ions generated as above are
In order to change the plating bath composition and adversely affect the plating, always reduce and/or reduce the amount equivalent to the amount produced.
or need to be removed. The oxidation caused by the reaction in equation (1) can be reduced to a considerable extent by improving the Metxel structure, but the oxidation caused by the electrode reaction in equation (2) can be reduced by 50% of the amount of current applied when an insoluble anode is used. ~90% occur. Therefore, especially in the latter case, reduction and/or removal of Fe ³⁺ ions is an essential problem in plating operation.

Therefore, the present inventors first proposed a method for effectively and advantageously reducing Fe ³⁺ ions generated in the plating bath in iron-based electroplating, by adding the metal to be supplied to the plating bath to the plating bath in a stirring tank. They have proposed a method that can reduce Fe ³⁺ and at the same time replenish the plating metal by adding and dissolving it in a liquid (for example, Japanese Patent Application No. 57-52565 (Japanese Patent Application Laid-open No. 58-171593)). )). These methods require that the metals to be replenished, such as Fe powder or powder and Zn powder or grains in the case of iron-zinc alloy electroplating,
Each of these is individually brought into contact with a plating bath containing Fe ³⁺ ions to dissolve them and reduce Fe ³⁺ ions. The reaction between each metal and the plating bath is as follows:
It is as follows.

Reduction of Fe ³⁺ by metal Fe Fe ³⁺ +1/2Fe→3/2Fe ²⁺ ...(3) Acid dissolution of metal Fe 2H ⁺ +Fe→Fe ²⁺ +H ₂ ...(4) Fe ³⁺ by metal Zn Reduction of Fe ³⁺ +1/2Zn→Fe ²⁺ +1/2Zn ²⁺ ...(5) Acid dissolution of metal Zn 2H ⁺ +Zn→Zn ²⁺ +H ₂ ...(6) However, these methods mainly There are the following problems.

(1) In the chemical dissolution reaction between the metals to be replenished, namely Fe and Zn, and the plating liquid, we investigated and tested the reduction efficiency of each metal (=amount dissolved by Fe ³⁺ reduction reaction/total amount of the metal dissolved). As is clear from Figure 1, the reduction efficiency of Zn is extremely low compared to that of Fe. Note that the test shown in Figure 1 was conducted at
Fe powder (200 mesh or less) and Zn (250 mesh) were individually added and dissolved in a plating solution at 60°C and pH 2.0.

For automotive alloy electroplated steel sheets, the lower layer (Fe: 20~30%, Zn: 80~
surface layer (Fe: ⁹⁰ %, Zn: 10%,
It often has a plating film structure consisting of a coating weight of 2 to 5/gm ² ). For example, in the case of lower layer plating, when the plating bath is regenerated by addition and dissolution of metal, the balance of the plating bath concentration cannot be maintained unless a larger amount of Zn, which has low reduction efficiency, is added.

However, the average reduction efficiency of both metals is: Average reduction efficiency = (Fe reduction efficiency) (Fe dissolution amount) + (Zn reduction efficiency) (Zn dissolution amount) / (Fe dissolution amount) + (Zn dissolution amount) , when the amount of Zn added is larger than the amount of Fe added, there is a disadvantage that the average reduction efficiency becomes lower.

(2) Furthermore, since each metal is brought into contact with the plating bath individually, it is necessary to install an Fe reduction reaction tank and a Zn reduction reaction tank in series or in parallel, which has the disadvantage of increasing the scale of the device.

In view of these shortcomings of the conventional technology, the present inventors further investigated the reduction efficiency of the added metal and found that Fe powder or grains and Zn powder or grains were simultaneously added and dissolved in a plating bath while making them contact each other. The present inventors have discovered that by doing so, the average reduction efficiency can be improved due to galvanic interaction, and the apparatus can be made more compact, leading to the present invention.

That is, in the present invention, when reducing Fe ³⁺ ions generated in a plating bath in iron-zinc alloy electroplating, metallic Fe powder or grains and metallic Zn powder or grains are simultaneously mixed in a stirring dissolving tank. Fe ³⁺ ions are reduced by adding it to the plating liquid led from the plating bath in the plating bath and stirring it, and stirring it while repeating mutual solid-solid contact and separation in the stirring dissolution tank.
Due to galvanic interaction due to solid-solid contact
Generating Fe ²⁺ ions and Zn ²⁺ ions,
This reduced plating solution is returned to the plating bath. Next, the present invention will be explained in detail based on the drawings.

FIG. 2 is a schematic diagram of an apparatus implementing the method of the invention. In the figure, 1 is Fe ²⁺ ion and
² is an insoluble anode; 3 is a material to be plated, such as steel strip; 4 is a buffer tank for replenishing plating solution to plating tank 1; 5 is a supply tank; should metal i.e.
Fe powder or granules and Zn powder or granules, 6 is a stirring dissolving layer that performs metal dissolution and reduction treatment with plating liquid extracted from plating tank 1, 7 is a solid-liquid separator for separating sludge, etc., 8 is plating This is a liquid drain off receiver.

Now, when electricity is applied in the device as described above, the reactions of equations (1) and (2) above occur in parallel on the electrode interface of the insoluble anode 2 in the plating bath 1, and Fe ³ is added to the plating bath. ⁺ is generated. In order to regenerate this plating bath, a predetermined amount of plating liquid is extracted from the plating tank 1 to the stirring dissolving tank 6 through the extraction pipe 9, and metal Fe and metal are simultaneously added to the stirring dissolving tank 6.
brought into contact with Zn. The metal Fe and metal Zn to be added are preferably in the form of powder or powder.
The ratio of metallic Fe and metallic Zn added to the reduction supply tank 6 is appropriately selected depending on the desired plating film composition Fe ³⁺ generation amount, plating liquid drain-off amount, and plating bath Fe ³⁺ concentration. is 20-30%, Zn
When obtaining a synthetic film composition of 80-70%,
It is preferable that Fe:Zn=20:80 to 50:50.

Contact between the plating solution and the metal causes the reactions (3) to (6) above, and the metal is dissolved in the plating solution, and Fe ³⁺ ions in the plating solution are reduced. What should be noted here is that according to the present invention, as mentioned above, dissimilar metals, namely metal Fe and metal Zn
Since Fe 3+ is simultaneously added to the stirring dissolution tank 6, solid-solid contact between different metals occurs in the stirring dissolution tank 6, and the reduction of Fe ³⁺ ions is further promoted by galvanic interaction as shown in Fig. 3. , the average reduction efficiency is greatly improved compared to the case where each metal is added separately. The anodic and cathodic reactions in galvanic interaction are as follows, respectively.

Anodic reaction Zn→Zn ²⁺ +2e ^- ...(7) Cathode reaction 2Fe ³⁺ +2e ^- →2Fe ²⁺ ...(8) Plating liquid subjected to metal replenishment and Fe ³⁺ reduction treatment in stirring dissolution tank 6 After the sludge is removed by the solid-liquid separator 7, the sludge is introduced into the buffer tank 4. The buffer tank 4 undergoes a bath composition adjustment process, and the plating solution is recycled to the plating tank 1.

In the present invention, in addition to reducing Fe ³⁺ ions, metal Fe powder or grains and metal
By stirring Zn powder or grains, floating, flowing,
While dispersing, they repeatedly collide (contact) and separate from each other. At this time, when metal Fe and metal Zn are in contact, the reactions (7) and (8) occur, and when they are separated, the reactions (3) to (6) mentioned above occur. occurs. This results in reduction of Fe ³⁺ ions and
in the plating bath, which is deposited on the strip and consumed.
Fe ²⁺ ions and Zn ²⁺ ions can be replenished.

Next, the effects of the present invention will be explained using examples.

Metsuki liquid (composition: Fe ²⁺ 50g/, Zn ²⁺ 20g/
, Na ₂ SO ₄ 100g/, tartaric acid 10g/, ;Fe ³⁺
Fe powder and Zn powder were simultaneously added to the solution (concentration: 1000 ppm; PH 2.0; temperature: 60°C), and the reduction efficiency was investigated for each addition ratio. The results are as shown by the solid line in FIG. Note that the broken line in FIG. 4 indicates the arithmetic mean reduction efficiency calculated from the reduction efficiency when Fe powder and Zn powder were added alone. As is clear from FIG. 4, according to the method of the present invention, Fe powder and Zn
Compared to the case where each powder was added individually, the average reduction efficiency was able to be significantly improved.

Conventionally, in order to regenerate a 500 m ³ /H Fe ³⁺ -containing plating bath, a 250 m ³ stirred tank-type reduction reaction tank for Fe powder and a 250 m ³ reduction reaction tank for Zn powder were installed in parallel. Alternatively, reactors arranged in series were used, but according to the method of the present invention, since Fe powder and Zn powder are added simultaneously, a 250 m ³ stirred tank reactor 1
The process became possible using the formula.

[Brief explanation of drawings]

Fig. 1 is a diagram showing reduction efficiency when Fe and Zn are added alone, Fig. 2 is a schematic diagram of an apparatus for carrying out the present invention, Fig. 3 is an explanatory diagram showing galvanic interaction, and Fig. 4 is a diagram showing the reduction efficiency when Fe and Zn are added alone. FIG. 2 is a diagram showing the average reduction efficiency when Fe and Zn are added simultaneously. 1... Plating tank, 2... Insoluble anode, 3... Steel strip, 4... Buffer tank, 5... Metal Fe, Zn,
6... Stirring dissolution tank, 7... Solid-liquid separator, 8...
Drain-off receiver, 9...extraction pipe.

Claims (1)

[Claims] 1. When reducing Fe ³⁺ ions generated in the plating bath during iron-zinc alloy electroplating,
Metallic Fe powder or grains and metallic Zn powder or grains are simultaneously added to the plating solution drawn from the plating bath in the stirring dissolution tank and stirred to reduce Fe ³⁺ ions and to dissolve them by stirring. The solids are stirred in a tank while repeating mutual contact and separation, and Fe ²⁺ ions and Zn ²⁺ ions are generated by galvanic interaction due to solid-solid contact.
A method for regenerating an iron-zinc alloy electroplating bath, which comprises returning this reduced plating solution to the plating bath.