JPH036240B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for dissolving zinc shots in an electrolytic galvanizing bath, and in particular, in order to increase the dissolution rate of metallic zinc, nickel, a metal which has a nobler potential than zinc and has a small hydrogen overvoltage, is predissolved. The present invention relates to a method for dissolving zinc shots characterized by the following. Electrogalvanizing is widely practiced for the purpose of preventing corrosion of steel materials. In recent years, demand for galvanized steel sheets has increased significantly in the fields of automobiles, home appliances, building materials, plant materials, etc., and thick galvanized steel sheets are particularly required for automobile applications. In order to cope with this increased demand, high-speed plating methods that use insoluble anodes to perform plating under high current density are currently attracting attention rather than conventional electrogalvanizing methods that use soluble anodes. High-speed plating methods using insoluble anodes require replenishment of zinc in the plating bath as it is consumed. When supplementing with zinc, zinc dissolution must be sufficiently rapid. When using zinc carbonate as supplementary zinc, rapid dissolution is guaranteed, but due to the increased cost, methods are currently being considered in which zinc shots are dissolved directly into the plating bath. However, the solubility of metallic zinc shots in electrogalvanizing baths is not good. Therefore, in order to suitably implement a high-speed plating method using an insoluble anode, it is necessary to establish measures to improve the solubility of metallic zinc shots. Incurring a considerable increase in cost as a countermeasure for this eliminates the significance of using inexpensive zinc shots. For example, it is possible to promote the dissolution of zinc shots by increasing the temperature of the liquid, but this is not practical due to the cost associated with raising and lowering the temperature of the liquid. Under these circumstances, the present invention aims to establish an inexpensive method for dissolving zinc shots in an electrogalvanizing bath. 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 overpotential of zinc is on the rather large side, and it is therefore thought that the rate of dissolution of the zinc shot into the electrolytic galvanizing bath is low. By the way, cathode current density =
The overpotential of zinc at 10 ^-3 A/cm ² , 1N−HCl, and room temperature is
It is 0.85, which is at a very high level along with Sn and Cd. Therefore, metal zinc shots include Pt, Ni,
It is expected that the rate of dissolution into the electrolytic galvanizing bath can be increased by bringing metals such as Fe, which have a nobler potential than zinc and have a small hydrogen overvoltage, into contact with and coexist with them. Therefore, various metals were investigated. As a result, it was found that the dissolution of zinc shots could be promoted by allowing a trace amount of ions of metals nobler than zinc, especially Ni, to be present in the plating bath. Thus, the present invention provides a method for dissolving zinc shot in an acidic electrogalvanizing bath.
Provided is a method for dissolving zinc shots characterized by the presence of ~1000 ppm of nickel ions. Hereinafter, the present invention will be specifically explained based on experimental examples. At present, the basic bath composition of the high-speed galvanizing method using an insoluble anode is Zn: 100g/Na ₂ SO ₄ : 30g/free-H ₂ SO ₄ : 4.9g/, and in the case of nickel-zinc alloy plating, 100g of nickel is used in addition to the above. The pH in the bath is typically about 0.8 to 3.5, and the bath temperature is typically in the range of 40 to 80°C. Metsuki bath liquid is used in circulation. As the insoluble anode, for example, a lead alloy, a platinum group alloy, or the like is used. The following dissolution test was conducted simulating the electrogalvanizing bath described above. In the test, zinc shot alone (No. 1), one with Fe wire, Ni piece and Pt wire added (No. 2 to 4), and one with FeSO ₄ 6H ₂ O dissolved in advance (No. 5). and NiSo ₄・6H ₂ O dissolved in advance (No. 6) were used: 1 Basic conditions of solution liquid composition ZnSO ₄・7H ₂ O: 450 g/(Zn:
102.4g/) Na ₂ SO ₄ (anhydrous): 30g/ free-H ₂ SO ₄ : 4.9g/(Zn equivalent: 3.27
g/) Liquid temperature 55-60℃ Liquid volume 1l 2 Properties of the purest zinc shot tested Particle size 2-4mm Chemical composition Fe: 15ppmPb: 10ppmCd: 6ppm Input amount 100g 3 Test method Pour 1l of the solution into a beaker and 100g of the purest zinc shot and a predetermined amount of additives (metals or metal salts) were added thereto while maintaining it at 55 to 60℃ using a hot plate, and the liquid pH was adjusted to 55~60℃.
Changes in H ₂ SO ₄ concentration over time and the amount of zinc shot dissolved were investigated. The dissolution time was 3 hours, and the solution was stirred with a stirring blade at 430 rpm. 4 Test Conditions Table 1 summarizes each test condition.

[Table] Shows the results of the dissolution test thus conducted. The following Table 2 shows the actual measured values of the amount of zinc dissolved after 3 hours of dissolution time in Nos. 1-6.

[Table] Attached Figure 1 is a graph showing the change in pH after dissolution over time. The pH after dissolution reflects the amount of zinc dissolved. As can be seen from Table 2 and Figure 1, the dissolution rate is higher in all cases when additives are added than in the case of simple dissolution of purest zinc. The effect is the greatest when Ni is allowed to dissolve in the solution, and the second most effective is when Ni is dissolved in the solution in advance. The dissolution rate is completely different when Ni is added as a metal (No. 3) and when it is added as an ion (No. 6). Whereas Ni shows a much higher dissolution rate when it exists as an ion,
Fe exhibits a higher dissolution rate when added as a metal. As described above, even for the same species, Fe and Ni, the dissolution phenomenon exhibits a complex aspect, making prediction difficult. From the above results, it was found that the dissolution rate of Zn is the highest when Pt is brought into contact with the Zn shot, but when this method is put into actual operation, the initial cost is high, and the wear of Pt It is not practical due to the possibility of wear and tear. Therefore, it shows a high dissolution rate similar to that in contact with Pt.
It is concluded that the Ni pre-melting method is industrially the best zinc shot melting method. Table 3 and FIG. 2 show the amount of Zn dissolved, the Ni concentration in the solution, and the free-H ₂ SO ₄ concentration at each dissolution time. The dissolution rate seems to be extremely slow for about 20 minutes after the start of dissolution, but after that it dissolves almost linearly with the dissolution time until 3 hours.

[Table] According to the test results, there was almost no change in the analytical value of the concentration of Ni added in advance, but it is thought that a trace amount of finely precipitated Ni probably contributed to the promotion of hydrogen generation. In addition, with this method, it is thought that almost no Ni in the solution will precipitate at the current density under normal plating conditions during electrolysis, and even if a small amount of Ni precipitates, it will have a positive effect on the corrosion resistance of the plating film. Since it is thought that
It does not present any problem. It is necessary for the amount of Ni ions added to be 5 ppm or more in order to achieve the effect of increasing the dissolution rate. The upper limit of the amount added is within the range that does not adversely affect the quality of the intended electrogalvanized product (for example, it may cause poor appearance during chemical conversion treatment of Zn-plated plates, poor adhesion after painting, and become uneconomical. ).
It can be as high as 1000ppm. At the Metsuki factory, which performs electrogalvanizing,
Zinc-nickel alloy plating is often carried out in conjunction with this zinc single plating. Zinc-nickel alloy plating baths contain approximately the same amount of nickel as zinc. After the alloy plating tank is discharged, a small amount of nickel always remains in the tank. Therefore, by subsequently plating with zinc alone, the residual nickel can be effectively utilized as a zinc shot dissolution promoter. There is no need to purchase separate means for nickel addition. In an actual plating factory, the alloy plating solution was first dissolved and adjusted, and after it was sent to the next process, a zinc-only plating bath was prepared in the plating tank10.
The presence of ~30ppm of nickel was confirmed, and zinc shots could then be added smoothly. Furthermore, the following test was conducted as another example. Galvanizing solution (ZnSO ₄ 7H ₂ O ^450g /, Na ₂ SO ₄ 30g/
Introduce 2 m ³ of PH liquid temperature 60℃) and add NiSO ₄ 6H ₂ O to Ni
500ppm was added. 7 kg of the purest zinc shot (approximately 3 mmφ) was gradually added thereto and stirred with a stirring blade. After one hour, the zinc shot was completely dissolved. In addition, as a comparison with the above method, the following confirmation test was conducted. In the same manner as in Example 1, add NiSO ₄ to the galvanizing solution.
Zinc shot (approximately 3 mmφ) 5 without adding 6H ₂ O
Kg was gradually added and stirred with a stirring blade. Even after about 3 hours, most of the zinc shots remained undissolved. As described above, the present invention provides a simple and cost-free method for increasing the dissolution rate of zinc shot, and will make a useful contribution to high-speed galvanizing, the demand of which is expected to increase in the future. be.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the change over time in the pH after dissolution in dissolution test Nos. 1 to 6, and FIG. 2 is a graph showing the change over time in the amount of zinc dissolved in No. 6 in which nickel was pre-dissolved.

Claims (1)

[Claims] 1. A method for dissolving zinc shots, which comprises causing 5 to 1000 ppm of nickel ions to be present in the plating bath when dissolving the zinc shots in an acidic electrolytic zinc plating bath.