JPS5940239B2 - How to adjust the pH of an electroplating bath - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for adjusting the pH of an electroplating bath in which free acid in the plating bath is removed using diffusion dialysis in electroplating using an insoluble electrode as the anode of the plating bath.

In recent years, as a measure to prevent corrosion of automobile suspension steel materials caused by various agents sprayed to prevent roads from freezing, electrolytic Zn-plated steel sheets with excellent corrosion resistance, especially Zn, Ni, Sn, etc., have been added to automobile steel sheets. The use of combination plated electroalloy plated steel sheets is increasing.

For example, in electroplating of a single plated metal such as electrogalvanized steel sheet, Zn was generally used as the anode itself, and as Zn ions in the plating bath precipitated onto the object to be plated, Zn corresponding to the amount of precipitation was dissolved from the anode. A soluble anode is used, but in the case of electroalloy plating, the concentration of each ion in the plating alloy components in the bath is maintained by ionizing the plating metal from the anode as it is deposited onto the object to be plated. Since this is impossible, an electroalloy plating method has been adopted in which the plating metal is added to the bath in the form of a salt and the anode is formed from an insoluble electrode, and has already reached the stage of practical use.

However, in electroalloy plating using the above-mentioned insoluble electrode, for example, when Zn is supplemented with zinc sulfate, the reaction in which Zn is electrodeposited on the steel plate of the cathode is expressed by the following formula: Znso4+H2
O → Zn (electrodeposition) + H2SO4 + n02...1, and free sulfuric acid H2S0l is generated, so the pH of the plating bath is
The disadvantage is that the value falls outside the control range.

This decrease in pH can be overcome by neutralizing the plating bath based on the addition of alkali, but adding alkali to the plating bath may result in the formation of lumpy metal hydroxide dross, an increase in the concentration of unnecessary salts, and This is undesirable because it causes a decrease in the concentration of plating components due to an increase in the holding capacity of the plating solution, which adversely affects the plating properties. At present, a technology for efficiently performing electroalloy plating using an electrode as an anode has not yet been established. In view of the above, the present inventors have proposed a method for preventing a decrease in the pH of a plating bath without using an alkali neutralization method in electroalloy plating in which an insoluble electrode is used as an anode and a plating metal is added to the bath in the form of a salt. As a result of extensive experimental research, we have found that the P of the plating bath can be reduced by using diffusion dialysis to remove the free acid newly generated in the bath from electroalloy plating.
We succeeded in preventing the decrease in H and keeping the PH value within the control range. The method of the present invention will be explained in detail below.

First, an example of the method of the present invention is shown in FIG.

In the figure, 1 is a strip, 2 is a plating bath, 3 is a plating bath, 4 is an anode made of an insoluble electrode (for example, a lead alloy), and the strip 1 is immersed in the bath 3 as a cathode.
Electric alloy plating progresses in the direction of arrow a between the two. Reference numeral 5 designates a diffusion dialysis device, which includes pipes 9 and 9 for circulating the plating solution between the plating solution circulation tank 6, the diffusion dialysis tank 7, the dialysing solution circulation tank 8, and the plating solution circulation tank 2 and the plating solution circulation tank 6 by means of a pump P1.
', the pump P between the liquid circulation tank 6 and the diffusion dialysis tank 7.
It is formed of pipes 10 and 10' for circulating the plating solution by means of P2 and pipes 11 and 11' for circulating the circulating fluid between the diffusion dialysis tank 7 and the dialysate circulation tank 8 by means of a pump P3.

12 is a liquid storage tank for sending NaOH etc. to the dialysate circulation tank 8 as needed by a pump P4;
Reference numeral 3 denotes a filter 1 for cleaning the plating liquid by removing foreign matter from the plating liquid drawn out from the plating tank 2.

The diffusion dialysis tank 7 is a known diffusion dialysis tank configured to perform diffusion dialysis on the plating solution using a diffusion dialysis membrane (not shown) comprising an anion exchange membrane provided inside. Next, the method of the present invention will be explained based on FIG.

The plating liquid whose pH has been lowered due to generation of free acid by electroplating in the bath 3 of the plating tank 2 is drawn out from the plating tank 2 through a pipe 9 and purified by a filter 13 to a plating liquid circulation tank. 6. The plating solution in the circulation tank 6 is fed by a pump P2 through a pipe line 10 into a diffusion dialysis membrane (not shown) inside the diffusion dialysis tank 7, and the free acid in the plating solution is removed from the dialysis membrane by the dialysis action. remains in the membrane and the PH
The plating liquid with the increased temperature returns to the circulation tank 6 again through the pipe 10/, and is further circulated by the pump P1 through the pipe 9' and returned to the plating tank 2. In this way, the plating liquid in the plating tank 2 passes through the plating liquid circulation tank 6, reaches the diffusion dialysis tank 7, undergoes diffusion dialysis, and returns to the plating tank 2 via the circulation tank 6. The circulation process is repeated, and during this period, the plating solution in the plating tank The free acid generated in the bath 3 in the plating bath 2 by electroplating is substantially removed to the outside of the plating solution, and a decrease in the pH of the plating bath 3 is prevented. The free acid dialyzed to the outside of the dialysis membrane in the diffusion dialysis tank 7 is passed through a pipe 11 between the outside of the dialysis membrane of the diffusion dialysis tank 7 and the dialysate circulation tank 8.
, 11', into the dialysis-side circulating fluid (usually tap water is used) in the dialysis-side circulating tank, which is circulated by the pump P3, and is sent to the dialysing fluid circulating tank 8 together with the circulating fluid, where it is appropriately discharged from the tank. .

In the diffusion dialysis treatment of the plating solution of the present invention, due to the osmotic pressure based on the difference in salt concentration between the plating solution and the circulating fluid in the dialysate circulation tank, the dialysis side passes from the dialysis side to the plating solution through the diffusion dialysis membrane. A phenomenon has been observed in which the retention capacity of the plating solution circulation tank increases due to the migration of the clean water from the circulating fluid in the dialysate circulation tank, and as the amount of water transferred increases, the component concentration of the plating solution is diluted and the properties of the plating solution increase. Therefore, it is preferable to suppress migration of clean water as much as possible. As a result of various experimental studies on suppressing the transfer of clean water, we found that the transfer of clean water from the dialysis side to the dialysis side can be suppressed by adding an appropriate amount of salt to the circulating fluid in the dialysate circulation tank. It has been found that the most effective and simplest method is to reduce the rate of transfer of the circulating fluid in the dialysate circulation tank to the clean water side.

That is, in FIG. 2, for example, the holding capacity of the dialysate circulation tank and the coating liquid circulation tank are each 31, and the diffusion dialysis membrane area is 40.
A characteristic curve shows the relationship between the water transfer rate from the dialysate circulation tank to the plating liquid circulation tank and the salt concentration in the dialysate circulation tank when an electric alloy plating solution of PH2 is circulated using a diffusion dialysis device with the DD method. This is a chart shown by P. As shown in the figure, in order to minimize the transfer rate of water from the dialysate circulation tank to the dialysis liquid circulation tank, the salt concentration of the circulating fluid in the dialysate circulation tank should be reduced to 1.
It has been found that a range of 0 to 809/l is preferable.

The salt concentration of the circulating fluid in the dialysate circulation tank is controlled by the following formula between free sulfuric acid and NaOH, which have been dialyzed from the dialysis fluid side to the dialysate circulation tank via the diffusion dialysis membrane: H2SO4+2Na0H→N
aSO4+2H20...2 This is carried out by the neutralization treatment shown in 2. In this case, the salt concentration (%) of the circulating fluid in the dialysate circulation tank is calculated by the following formula: Salt concentration (%) of dialysate = NaOH concentration (
%) × Completion... 3. Therefore, the NaOH concentration to achieve the desired salt concentration of the circulating fluid in the dialysate circulation tank is calculated by the above formula 3, and the necessary amount of Na based on the above calculation result is calculated.
This is done by adding OH to the circulating fluid in the dialysate circulating tank.

Furthermore, due to the above, when the salt concentration of the circulating fluid in the dialysing fluid circulation tank increases, a phenomenon generally occurs in which the dialysis rate of the free acid in the dialysis membrane decreases. Since it may adversely affect the pH control of the plating solution, it is preferable that the decrease in the dialysis rate be suppressed within an appropriate range.

As a result of various experimental studies on suppressing the decrease in the dialysis rate, it has been found that the method of suppressing the decrease in the dialysis rate by adjusting the pH of the circulating fluid in the dialysate circulation tank is the simplest and most effective method. In other words, Fig. 3 shows the relationship between the pH of the dialysate circulating fluid and the dialysis rate of free acid in the dialysate solution when the pH of the dialysate solution is 2 and the salt concentration of the circulating fluid in the dialysate circulation tank is 409/l. In the chart shown in Q, when the pH of the circulating fluid in the dialysing fluid circulation tank is lowered as shown in the diagram, the dialysis rate of free acid in the plating solution is reduced.

Therefore, if the pH of the circulating fluid in the dialysing fluid circulation tank is too low, the dialysis of the free acid will be hindered, so at least the pH of the circulating fluid in the dialysing fluid circulating tank and the pH of the plating fluid in the dialysis tank (the control range is 1.75 to 2.0), and if the pH of the circulating fluid in the dialysate circulation tank is set to 65 or higher, metal hydroxide precipitates will form, and the entrance and exit of the dialysate circulation tank will be blocked. It has been found that the pH of the circulating fluid in the dialysing fluid circulation tank is preferably 3 to 6.5 since there is a risk of blockage. Next, the effects of the present invention will be explained with reference to Examples.

Example: 1 internal volume 1ri? A plating tank was filled with a Ni-Zn plating bath (sulfuric acid bath), a Pb-Ag (1%) insoluble electrode was used as an anode, and a strip with a width of 300 TfLm was used as a cathode to pass Ni through the bath at a line speed of 25 m/Hr. - In electroplating for Zn alloy plating, the diffusion method shown in Figure 1 consists of a diffusion dialysis tank equipped with a diffusion dialysis membrane 60 d from the plating bath, a plating liquid circulation tank, and a dialysate circulation tank using tap water as the circulating fluid. For the dialysis machine, 1.5rr1'/
The plating liquid was drawn out from the plating tank at a rate of Hr and treated to remove free sulfuric acid by diffusion dialysis under various conditions.

The results are shown in Table 1. As is clear from this table, the pH of the dialysate is kept constant at 4.0, and the salt concentration of the circulating fluid in the dialysate circulation tank is set to 0, 5, 10, 30, 80, 1609/
When the salt concentration of the circulating fluid in the dialysate circulation tank was O, the pH of the plating bath could be maintained at 1.8, but the rate of increase in the plating solution holding capacity of the plating solution circulation tank was 2851/1. Hr
If the salt concentration increases to 1609/l, the retention capacity of the plating solution decreases, and the dialysis rate of the plating solution also decreases, causing the pH of the plating bath to decrease to 1. I couldn't keep it at .8. In contrast, the salt concentration was 109/1,309/l and 80g/l.
In this case, the pH of the plating bath can be maintained at 1.8, and the plating liquid holding capacity increase rate of the plating liquid circulation tank is also 781/Hr, 48! /Hr, 6l/Hr, which was extremely small, and did not have any adverse effect on the component concentration of the plating solution. In addition, the salt concentration of the circulating fluid in the dialysate circulation tank was set to 4.
09/l, and the pH of the dialysate in the circulation tank was set to 3,
When the pH of the circulating fluid in the dialysate circulation tank was changed to 4, 5, 6, and 8, the pH of the plating bath could be maintained at 1.8 in all cases. If the pH of the circulating fluid in the dialysate circulation tank is 8, metal hydroxide precipitates will form in the dialysate circulation tank, which will block the dialysate circulation tank and make it difficult to perform diffusion dialysis treatment. 3.
Even if it is set to 0, the pH of the plating bath can be maintained at 1.8, but the dialysis rate will decrease a little.

As described above, in the present invention, in electric alloy plating using an insoluble electrode as an anode, a decrease in the pH of the plating bath can be prevented by circulating the plating bath by diffusion dialysis. This method is extremely effective in improving the production efficiency of electric alloy plating, which is performed by adding metals in the form of salts to the bath.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an example of a flowchart for carrying out the method of the present invention, and Fig. 2 is a diagram showing the relationship between the transfer rate of water to the bath circulation tank and the salt concentration of the circulating fluid in the dialysate circulation tank. ,
FIG. 3 is a chart showing the relationship between the PH of the circulating fluid in the dialysing fluid circulation tank and the dialysis rate of free acid in the plating solution. 1... Strip, 2... Plating tank,
3...Metallic bath, 4...Anode, 5...
... Diffusion dialysis device, 6 ... Metsuki liquid circulation tank, 7 ... Diffusion dialysis tank, 8 ... Dialysate circulation tank, 9, 9', 10, 10', 11, 11'...
...Pipe line, 12...Liquid storage tank, 13...
Filter one.

Claims (1)

[Claims] 1. Electroplating using an insoluble electrode as the anode of the plating tank, characterized in that the plating bath used in the plating tank is circulated by diffusion dialysis to remove free acid from the plating bath. How to adjust the pH of the bath. 2. The pH of the electroplating bath according to claim 1, wherein the salt concentration of the circulating fluid in the dialysate circulation tank in the diffusion dialysis device is 10 to 80 g/l and the pH is 6.5 or less. Adjustment method.