JPH0322480B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for replenishing zinc to a Zn-Ni alloy plating bath when producing Zn-Ni alloy plating using an insoluble anode in a continuous electroplating line. It is. (Prior Art) In recent years, various electrogalvanized steel sheets have been developed and put into practical use in response to demands for improved corrosion resistance of automobiles, home appliances, and the like. Among these, demand for Zn-Ni alloy electroplated steel sheets is increasing significantly, mainly for automobile applications, due to their excellent corrosion resistance. In order to cope with this increased demand, a high-speed plating method that performs plating under high current density and high line speed is essential.
In the high-speed plating method, it is more advantageous to use an insoluble anode than a soluble anode because there is no need to replace the anode, but when using an insoluble anode, the metal ions consumed during plating are Needs to be continuously replenished. As plating speed increases, the consumption rate of metal ions in the plating bath increases proportionally, so it is important for quality control to replenish metal ions smoothly and suppress fluctuations in the bath composition as much as possible. , becomes extremely important. In particular, the corrosion resistance of Zn-Ni alloy plating largely depends on the Ni content in the plating layer, so in order to keep the Ni content within a predetermined range, metal ions in the plating bath can be rapidly replenished. Maintaining the concentration within a certain range is an important issue. The following two methods are generally used to replenish metal ions in an acidic electroplating bath using an insoluble anode. (a) A method in which the plating metal is dissolved in the plating solution in the form of a soluble chemical such as a basic carbonate. (b) A method of melting plating metal by bringing the metal into direct contact with a plating liquid. Method (a) has better solubility than method (b) and can easily generate plating metal ions, but has the drawback of high cost. On the other hand, method (b) is low in cost but has the disadvantage of poor solubility. Therefore, when using method (b), in order to increase the contact area between the metal and the plating solution, that is, the melting area, use a shape with as large a surface area as possible, such as a powder, granule, or shot shape. be done. In addition, as a method for promoting the solubility of metallic zinc in an electrogalvanizing bath, there is
32701, method of contacting zinc or zinc alloy with stainless steel, JP-A-61-600 or JP-A-Sho.
No. 61-3900 discloses a method in which shot-shaped metallic zinc with a particle size of 2 to 4 mm is used and a trace amount of Ni or Fe ions is present in the galvanizing bath. These methods are based on the following ideas. When a metal is dissolved in an acidic solution, it is essential that hydrogen be generated by the following reaction as a cathodic reaction. H ⁺ +e ^- = 1/2H ₂ The potential at which this reaction occurs is greatly influenced by the electrode surface where the reaction occurs. Therefore, metals with larger hydrogen overvoltages are more difficult to dissolve. The hydrogen overvoltage of zinc is on the rather large side, and it is therefore thought that the rate of dissolution of zinc into the electrolytic galvanizing bath is low. By the way, cathode current density = 10 ^- ₃ A/
The overpotential of zinc at cm ² , 1N-HCl, and room temperature is 0.85, which is at a very high level along with Sn and Cd. Therefore, by bringing metal zinc into contact with a metal such as stainless steel, Ni, or Fe, which has a nobler potential than zinc and has a small hydrogen overvoltage, the rate of dissolution into the electrolytic galvanizing bath can be increased. By the way, in the case of Zn-Ni alloy plating bath,
Although it is necessary to replenish Zn ions and Ni ions,
In either case, it is difficult to replenish by dissolving metals, and the common method is to replenish by using soluble chemicals. This is due to the following reason. In the case of metallic zinc, Ni is substituted and precipitated on the zinc surface as it dissolves, and the zinc surface is coated with Ni, making it impossible to ensure the solubility of metallic zinc. In the case of metallic nickel, it cannot normally be dissolved chemically, except when it is electrically dissolved as an anode. For this reason, there is currently no prior art for dissolving and replenishing metal into a Zn--Ni alloy plating bath, even in the case of zinc, which seems to be more likely. (Problems to be Solved by the Invention) All conventional techniques related to dissolving metal zinc utilize dissimilar metals to lower the hydrogen overvoltage of zinc and promote hydrogen generation, which is a counter reaction to zinc dissolution. This is an attempt to improve the solubility of zinc. However, in the case of Zn-Ni alloy plating baths, when metallic zinc is dissolved, it contains a large amount of Ni ions that are likely to be precipitated by substitution. is completely different in appearance. The solubility of metallic zinc in a Zn--Ni alloy plating bath is good at the very beginning of dissolution, but gradually decreases, and in extreme cases, dissolution stops altogether. That is, at the very early stage of melting, a trace amount of Ni precipitated on metallic zinc causes
The hydrogen overvoltage of zinc is reduced and good zinc solubility is obtained, but as Ni precipitation progresses, the contact area between metallic zinc and the plating bath decreases, and as a result, dissolution of metallic zinc decreases. Sexuality also decreases. Although it is physically possible to replace metallic zinc whose solubility has decreased due to Ni coating with new metallic zinc and maintain the amount of dissolved metallic zinc,
Requires a large amount of metallic zinc, due to displacement precipitation
Due to the increased consumption of Ni ions, etc., a large amount of cost is required, and the advantage of using inexpensive metallic zinc is completely lost. For these reasons, up until now, zinc replenishment by dissolving metallic zinc into a Zn-Ni alloy plating bath has been difficult.
Although it is extremely advantageous in terms of cost, it has not been realized because solubility cannot be guaranteed. The present invention aims to solve the fundamental problem of dissolution of metallic zinc in a Zn-Ni alloy plating bath, which is interference with dissolution due to substitutional precipitation of Ni. (Means for Solving the Problems) In view of the above-mentioned circumstances, the present inventors have conducted various studies on methods of dissolving metallic zinc into a Zn-Ni alloy plating bath. As a result, by setting the free sulfuric acid concentration in the Zn-Ni alloy plating bath to 10 to 20 g/and the presence of Co ions 0.5 to 5 g/,
By being able to achieve the initial purpose and by adding small amounts of Fe ³⁺ ions and molybtenic acid,
It has been found that even more reliable zinc solubility can be guaranteed. The present inventors first focused on the fact that free sulfuric acid concentration has a large effect on the chemical dissolution of zinc, and
The solubility of metallic zinc was tested by varying the free sulfuric acid concentration in the alloy plating bath. The test conditions were as follows: an insoluble anode was used as the dissolving solution, and a Zn-Ni alloy plating bath with a general composition for high-speed plating was used. 1 Basic conditions of solution Basic composition ZnSO ₄・7H ₂ O 200g/(Zn ion: 46g/) NiSO ₄・6H ₂ O 200g/(Ni ion: 45
g/) Na ₂ SO ₄ (anhydrous) 50g/Free sulfuric acid 5-30g/Liquid temperature 50-60℃ Liquid amount 1 2 Properties of metal zinc Electrolytically refined zinc plate 50mm x 50mm, approx. 30g Chemical components Fe: 15ppm, Pb: 10ppm, Cb:
6ppm 3 Test method Dissolved solution 1 was placed in a beaker, and while it was maintained at 50 to 60°C in a constant temperature water bath, a metal zinc plate was added thereto, and the amount of dissolved metal zinc plate was checked at 30 minute intervals. The dissolution time was 5 hours, and since the dissolution of metal zinc was accompanied by a pH change, sulfuric acid was continuously replenished to maintain the liquid pH at the initial value. Further, stirring was not performed. The results are shown in Figure 1. In the figure, solid lines 1 to 3 indicate the solubility of metallic zinc when the concentration of free sulfuric acid is varied using a Zn-Ni alloy plating bath with the basic composition.
, 2 indicates the case of 10 g/, and 3 indicates the case of 20 g/. Obviously, the higher the concentration of free sulfuric acid, the greater the amount of metal zinc dissolved. However, it gradually decreases with the dissolution time, and especially in the case of 5 g/free sulfuric acid, zinc dissolution stops in 2 to 3 hours. On the other hand, in the case of 10 and 20g/, although the amount dissolved is reduced compared to the initial stage,
The dissolution continues without stopping. These phenomena are caused by a decrease in the dissolution area due to the substitutional precipitation of Ni. Examining the amount of Ni that precipitates by displacement on the metal zinc surface and its form, it is found that the amount of Ni increases until the amount of dissolved Ni begins to decrease, but after that the amount of Ni increases regardless of the concentration of free sulfuric acid.
~3g/ ^m2 , and its form is dense metallic Ni at 5g/m2 of free sulfuric acid,
It was found that when free sulfuric acid was used at 10 g/min or more, it was a mixture of metal Ni and Ni oxide, which was porous and had many cracks. If you look at it in relation to the solubility of metallic zinc,
In the case of free sulfuric acid of 5 g/min, the metal zinc is coated with insoluble dense metallic Ni, so dissolution stops in several hours, but with free sulfuric acid of 10 g/min or more,
Since the Ni precipitates are porous and have many cracks, it is thought that zinc dissolution will continue. Furthermore, the higher the concentration of free sulfuric acid, the more soluble Ni oxide becomes the precipitate of Ni, so the precipitated Ni is redissolved and
It is thought that a new metallic zinc surface appears, and as a result, the amount of zinc dissolved is large. From these results, it was found that the dissolution of metallic zinc into the Zn-Ni alloy plating bath continued when the free sulfuric acid concentration was set to 10 g/min or more;
This method does not solve the problem of decreasing dissolution rate due to precipitation, and is not practical. In order to maintain and maintain the initial solubility, we conducted further research and found that the addition of Co ions was extremely effective. Broken line 4~ in Figure 1
6 shows the amount of metal zinc dissolved when 2 g/2 g of Co ions in the form of CoSO ₄ .7H ₂ O were added to the basic bath. In the case of 5 g of free sulfuric acid/dotted line 4, the effect on the left is not seen, but in the case of broken line 4,
In the case of 10 and 20 g/ of zinc, the amount of dissolved zinc is maintained at a level close to the initial level over time, and the amount of zinc dissolved is
The effect of ion addition is shown by the solid line 2 without Co ions,
This is obvious even when compared with 3. When examining the precipitation form of Ni on metal zinc when Co ions are added, it is found that compared to the case without Co ions,
Furthermore, it was found that the precipitates were porous with many cracks, and the proportion of soluble Ni oxide was also high. It is presumed that the Ni precipitates in this form can no longer become an obstacle to the dissolution of metallic zinc. In this way, by combining the condition of free sulfuric acid of 10g/or more with the addition of Co ions,
The problem of slowing down the dissolution rate of metallic zinc has been solved,
Be practical. Next, to confirm complete dissolution of metallic zinc
Long-term dissolution tests were conducted using a sample bath containing Co ions. When the amount of free sulfuric acid was 10 g or more, about 30 g of metallic zinc completely dissolved and disappeared, but
A very small amount of sedimentary residue remained. This residue was minute pieces of undissolved Ni precipitates that had not been sufficiently oxidized, and gradually decomposed and disappeared by stirring (300 ppm). However, if this were to accumulate during actual operation, there is a risk that the amount of Ni ions in the Zn-Ni alloy plating bath would decrease, creating a new problem of increasing the amount of Ni chemicals to be input. The inventors
It has been found that adding a small amount of Fe ³⁺ ions or molybtenic acid is extremely effective in completely dissolving metallic zinc without leaving any residue. Fe ³⁺ ion or molybutenic acid is Co
It is presumed that like ions, it has the effect of oxidizing Ni precipitates, and if a residue is generated due to insufficient stirring, it is necessary to add these as a measure to prevent the residue. It is also the main component in the Zn-Ni alloy plating bath.
Although higher Zn ion and Ni ion concentrations are preferable from the viewpoint of high-speed plating, it has been found that there is an upper limit to maintaining the solubility of metal zinc. The present invention was constructed based on the above knowledge, and the gist thereof is that when replenishing zinc to a Zn-Ni alloy plating bath,
Zn- characterized in that free sulfuric acid in the plating bath is set at 10 to 20 g/ and Co ion is set at 0.5 to 5 g/ to replenish zinc while dissolving metallic zinc.
How to replenish zinc to a Ni-based alloy plating bath. Zn ions in the Zn-Ni alloy plating bath are 25 to 60
2. The method according to claim 1, wherein the amount of Ni ions is 25 to 60 g/. 0.1 Fe ³⁺ ions were added to the Zn-Ni alloy plating bath.
3. The method according to claim 1 or 2, characterized in that the amount is present at ~2 g/ml. Molybtenic acid is added to the Zn-Ni alloy plating bath.
The method according to claims 1, 2, and 3, characterized in that Mo is present in an amount of 1 to 100 ppm. (Function) In the zinc replenishment method for a Zn-Ni alloy plating bath of the present invention, 10 to 20 g of free sulfuric acid and 0.5 to 5 g of Co ions are contained in the Zn-Ni alloy plating bath. In addition, Zn ions,
It is also effective to use the present invention in combination with the Ni ion concentration of 25 to 60 g/, Fe ³⁺ ion of 0.1 to 2 g/, and molybtenic acid of 1 to 100 ppm as Mo. Next, the reason for limiting the range of each component will be described. As explained in Figure 1, if the free sulfuric acid content is less than 10g/
The solubility of metallic zinc cannot be ensured. Increasing the amount of free sulfuric acid is effective in improving the chemical solubility of metallic zinc, but too high a value will cause problems such as a decrease in the fastening current efficiency, uneven appearance, and fastening problems. For prevention purposes, the upper limit of free sulfuric acid needs to be 20g/. Then Co
If the amount of ions is less than 0.5 g, the important effect of maintaining the initial solubility of metallic zinc as explained in FIG. 1 cannot be achieved. At over 5g/Co ion,
Displacement precipitation of Co ions themselves begins to cause problems such as interference with dissolution of metallic zinc, and the effect of adding Co ions decreases. Due to the addition of Co ions, Co precipitates during Zn-Ni alloy plating, but the amount is only about 0.5% at most even when 5 g of Co ions are added, and it has no effect on the performance of Zn-Ni alloy plating. It has no negative impact. In addition,
The preferred form for adding Co ions is sulfate or chloride. It is desirable that the concentration of Zn ions and Ni ions, which are the main components of the Zn-Ni alloy plating bath, be high from the viewpoint of applying high-speed plating, but from the viewpoint of the solubility of metallic zinc, the concentration is too high. is not desirable. The appropriate concentration range that can achieve both of these is
Both Zn ion and Ni ion are 25 to 60 g/. The addition of small amounts of Fe ³⁺ ions and molybtic acid ensures reliable solubility of metallic zinc through its anti-residue action. Fe ³⁺ less than 0.1g/
This effect is insufficient, and if it exceeds 2 g/g, unevenness tends to occur in the plating appearance. Also in the case of molybutenic acid
When Mo is less than 1 ppm, this effect is insufficient, and when it exceeds 100 ppm, the plating appearance tends to be uneven. Regarding the addition of Fe ³⁺ ions and molybtenic acid, either one or both may be used in combination. In addition, plating baths usually contain Na ₂ SO ₄ ,
(NH ₄ ) ₂ SO ₄ etc. are added as conductive aids,
These do not pose any hindrance to the present invention. As the metallic zinc, it is desirable to use high-purity zinc such as electrolytically purified zinc from the viewpoint of preventing impurity contamination of the plating bath. Metallic zinc can be in any shape such as plate, shot, or powder, but if the present invention is used, there is no need to use metal zinc with a large surface area such as shot or powder. It is plate-shaped and can ensure sufficient solubility. This is also very advantageous in terms of cost. Metallic zinc may be directly charged into the plating tank, but a separate dissolving tank is installed outside the plating tank, metallic zinc is charged into the dissolving tank, and the plating bath is poured from inside the plating tank. A circulation system in which the material is extracted, fed into the dissolution tank, and then sent back into the plating tank is more preferable in terms of concentration control. (Example) A melting tank connected to the plating bath was installed in the electroplating pilot trunk, 10 kg of electrolytically refined metal zinc plate was charged into the melting tank, and Zn-Ni alloy plating baths of various compositions were placed in the melting tank. The solubility and plating properties of metal zinc were simultaneously tested by passing liquid through the test tube. Zn-Ni alloy plating bath is
The bath volume was 1 m ³ and the bath temperature was 50 to 60°C. Table 1 shows Zn
-Ni-based alloy plating bath composition, metal zinc dissolution amount, residue generation status, current density 100A/dm ² , target area 20 after circulating the plating bath in the melting tank for 10 hours
A summary of the plating properties (plating bath current efficiency, plating appearance) when electroplated at g/m ² is shown below.
Note that Zn ²⁺ , Ni ²⁺ , Co ²⁺ , and Fe ³⁺ are sulfates, and Mo
used ammonium molybtenate salt. Test No. 1, 2, 4-7, 9, 13, 14, 16-20,
22 to 25 are examples of the present invention, test Nos. 3, 8, 10 to 12, 15,
21 is a comparative example. Among the comparative examples, No. 3 and 11 are
Since the free sulfuric acid concentration was below the range of the present invention, the solubility of metallic zinc was insufficient, and in Nos. 10 and 21, on the contrary, the solubility of metallic zinc was good because it exceeded the range of the present invention. This resulted in a decrease in current efficiency and poor plating appearance. In Nos. 8, 12, and 15, the solubility of metallic zinc was insufficient because Co ²⁺ was below the range of the present invention. In contrast, all of the examples of the present invention showed good solubility of metallic zinc and also had good plating properties. Among the examples of the present invention, Zn ²⁺ and Ni ²⁺ as in No. 1
If the concentration is as low as 20g/, the current efficiency will be reduced, but if the concentration is as high as 70g/, as in No. 25,
The amount of dissolved metal zinc is somewhat insufficient,

【table】

[Table] When Fe ³⁺ or Mo was not contained or the amount was below the range of the present invention, as in Nos. 6 and 18, a trace amount of residue was generated. Furthermore, in cases where Fe ²⁺ or Mo exceeded the range of the present invention, as in Nos. 14 and 19, the plating appearance was somewhat poor. Other aspects of the present invention include:
There were no problems with either metal zinc solubility or plating properties. (Effects of the Invention) As described above, the present invention provides Zn-
Zn ions can be supplied during the production of Ni-based alloy plated steel sheets using a cheap and simple method of melting metallic zinc, and the demand for Zn-Ni alloys is expected to increase in the future. This makes a useful contribution to the production of galvanized steel sheets.

[Brief explanation of the drawing]

Figure 1 is a graph showing the effect of increasing the concentration of free sulfuric acid and increasing the amount of dissolved zinc metal by adding Co ²⁺ ions, and solid lines 1, 2, and 3 indicate the concentration of free sulfuric acid in the Zn-Ni alloy plating bath. The dashed lines 4, 5, and 6 show cases in which 2 g/Co ²⁺ ions were added and the free sulfuric acid concentration was increased to 5, 10, and 20 g/in order.
The case of 10, 20g/ is shown.

Claims (1)

[Claims] 1. When replenishing zinc to a Zn-Ni alloy plating bath, free sulfuric acid in the plating bath is added at 10 to 20 g/,
It is characterized by supplying zinc while dissolving metal zinc with Co ions at 0.5 to 5 g/
How to replenish zinc to a Zn-Ni alloy plating bath. 2 Zn ions in the Zn-Ni alloy plating bath are 25~
60 g/, and the Ni ion content is 25 to 60 g/. 3 Fe ³⁺ ions were added to the Zn-Ni alloy plating bath.
The method according to claims 1 and 2, characterized in that 0.1 to 2 g/g/m are present. 4. The method according to claims 1, 2, and 3, characterized in that molybdic acid is present in the Zi-Ni alloy plating bath in an amount of 1 to 100 ppm as Mo.