JPH0125839B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a Ni--Zn alloy electroplated steel sheet. In recent years, with the diversification of plating applications, alloy electroplating using multiple metals has been developed instead of single plating such as Zn, one of which is known as Ni-Zn alloy electroplating. . When manufacturing this Ni-Zn alloy electroplated steel sheet, the ratio of Ni ions to the total amount of Ni and Zn ions in the plating bath (hereinafter referred to as the Ni ratio in the bath)
greatly affects the ratio of Ni to the total amount of Zn and Ni in the film (hereinafter referred to as the Ni ratio in the film). The Ni ratio in the film with excellent corrosion resistance is 10 to 15%, and the Ni ratio in the bath to achieve this is 50 to 75%, so it is necessary to appropriately control the bath concentration during continuous operation. There are various factors that affect the Ni ratio in the film, but the biggest factor is the Ni ratio in the bath. For example, if the flow rate is 1 m/sec or more,
At a current density of 30 to 60 A/dm ² and a pH of 1 to 2, when the Ni ratio in the bath is 50% and 70%, the Ni ratio in the film is 10% and 15%, respectively. A balance of metal ions in the plating bath must be maintained between the amount of metal ions removed by electrodeposition onto the strip and the amount of metal ions fed into the bath. In other words, Ni in the film
In order to maintain the ratio 10-15%, the Ni ratio in the bath should be adjusted to
Must be kept at 50-75%. If all the anodes were Zn self-soluble anodes, the amount of Zn deposited in the film would be less than the amount dissolved in the anode, so the amount of Zn in the bath would be lower.
The amount of Zn ions increases over time. On the other hand, since Ni ions are not replenished, Ni in the bath is deposited by electrodeposition.
The amount decreases, and the Ni ratio in the bath decreases. Therefore, as shown in JP-A-55-110796,
There is a method of maintaining bath balance by appropriately combining Zn anodes and Ni anodes, but
Ni has a higher melting point than Zn, and there is a problem in that large-scale equipment is required to cast anodes from waste anodes. Additionally, the Ni anode dissolves non-uniformly during electrolysis, producing metallic sludge that enters the plating bath and causes scratches on the strip. Using Ni anodes in this way has many problems in terms of cost and product quality. The above-mentioned metallic sludge also occurs when only a zinc anode is used, which poses a problem. That is, when a zinc anode is used, nickel is displace- plated on the anode surface during electrolysis due to the reaction Zn ⁰ + Ni ²⁺ → Zn ²⁺ + Ni ⁰ , which becomes a metallic sludge that enters the plating bath and is deposited on the strip surface. It adheres to the surface and is crushed by the conductor roll, resulting in flaws. This metallic sludge is mainly composed of Ni and is very hard, and scratches caused by this sludge are extremely serious defects in product quality. In order to avoid these drawbacks of self-soluble anodes, it is also possible to use only insoluble anodes and supply metal ions into the bath from the outside. Possible methods for supplying this metal ion include dissolving the metal and replenishing it in the form of a metal salt; however, in the former, the dissolution rate of the metal is slower than the rate at which it is taken out of the system by electrodeposition, so the latter This method is considered practical. However, when replenishing in the form of metal salts such as nickel sulfate or zinc sulfate, only the metal content is carried out by electrodeposition and sulfate radicals are accumulated in the plating bath. Therefore, in order to avoid this, it is necessary to use a metal salt with anions that will not accumulate in the plating bath, and metal salts with carbonate groups such as nickel carbonate and zinc carbonate are effective for this purpose. be. This is because after the metal content is taken out of the system, the carbonate radicals become water and carbon dioxide gas, but since the carbon dioxide gas is discharged outside the system, no anions are accumulated in the plating bath. However, since carbonates are generally more expensive than metals or sulfates, the process of replenishing nickel carbonate and zinc carbonate using such insoluble anodes has the drawback of being expensive. The present invention was made in order to solve the above-mentioned problems, and produces a Ni-Zn alloy electroplated steel sheet that maintains the balance of the amount of metal ions in the bath and has an excellent appearance with no scratches on the strip surface. This is an attempt to provide a possible method. In the present invention, first, a horizontal electrolytic cell is used, and anodes are installed at predetermined intervals from above and below the steel strip, facing the steel strip running inside the cell.
The upper anode is made insoluble, and part or all of the lower anode is made a Zn self-soluble anode, and Zn ions are supplied from this Zn self-soluble anode. Here, the ratio of the Zn self-soluble anode to the insoluble anode is preferably 1:1, that is, the ratio of the Zn self-soluble anode to all the anodes is r=1/2. FIG. 1 is an explanatory diagram thereof, in which X is a steel strip, 1 is a Zn self-soluble anode, and 2 is an insoluble anode. Further, the arrow indicates the flow direction of the plating solution. The reason for the above configuration is as follows. As mentioned above, if Zn ions are supplied through the Zn anode, expensive carbonates may not be used. However, if all the anodes used are Zn, the Zn ions in the bath will increase over time.
This is because the electrolysis efficiency on the cathode side is about 80%, and on the anode side it is more than 100%. Therefore, in order to keep the amount of Zn ions in the bath constant, it is necessary to partially use an insoluble anode to prevent the amount of Zn ions from increasing. Furthermore, when a Zn anode is used, as mentioned above, metallic sludge is generated on the anode surface, which enters the bath during electrolysis and gets caught up in the conductor roll, causing scratches on the strip. As shown in FIG. 2, the metallic sludge 10 is
Generates in the gap between Zn anodes 1. This part is less involved in electrolysis (low current density), and the flow of the plating liquid is weak and stagnant, making it easy for a substitution reaction of Ni ions in the bath to occur. To prevent this, it is effective to mechanically grind the adjacent anode surfaces so that there are no gaps to prevent the plating solution from entering, but if some gaps are created and sludge 10 is generated, However, if the Zn anode is placed below the strip as in the present invention, it is possible to prevent the sludge from falling onto the strip in the bath. This is the reason why, in the present invention, only the anode on the lower side of the strip is a Zn self-soluble anode. The formation of metallic sludge is caused by the stagnation of the liquid, low current density, and the temperature of the plating bath. According to the results of actual line tests conducted by the present inventors, when the bath temperature is 60°C or higher, sludge is formed and scratches are generated, whereas when the bath temperature is 55°C or lower, no scratches are generated. Therefore, it is desirable that the plating bath temperature be 55°C or less. Next, we will discuss the bath balance for Ni-Zn alloy plating. When the Ni ratio in the film is y% and the Ni ratio in the bath is x%, the following equation has been obtained experimentally. y=0.2x (50≦x≦75%, PH 1-2, flow rate 1 m/s or more)... The Ni ratio in the film is determined by the Ni ratio in the bath, as shown in the formula. The Ni ratio in the bath is the ratio of Ni ions to the total amount of Ni and Zn ions in the bath. self-soluble,
Even if the amount of Zn ions lost during electrodeposition on the strip is balanced by a combination of insoluble anodes, the amount of Ni ions in the bath decreases, as Ni ions cannot be replenished from the anode, and at the same time the amount of H in the bath decreases. Bath PH decreases as ⁺ ions increase.
In the present invention, the decrease in Ni ions is compensated for by supplying basic nickel carbonate. At the same time, this also increases the total amount of metal ions (in the bath).
The amount of Ni ions + Zn ions) can also be kept constant, and as a result, the bath pH can also be controlled to a predetermined value. As mentioned above, basic nickel carbonate is advantageous because it does not contain anions and remains in the plating bath. In the present invention, Zn ions in the bath are supplied from the Zn self-soluble anode, but the dissolution efficiency of the self-soluble anode (the ratio of the actual decrease to the theoretical decrease due to the amount of electricity applied) depends on the bath pH. The following relationship has been experimentally obtained: η _A = 4Z ² −64Z + 222 ... η _A : Anode dissolution efficiency % Z : Bath PH Therefore, Zn lost due to electrodeposition on the strip
The amount is supplemented by the anode by controlling the pH as shown in the formula. Next, a method for determining the PH range to maintain the Ni ratio in the film of 10 to 15% when r=1/2 will be described. If the Ni ratio in the film is y% and the amount of electricity applied is Q coulomb, then the amount of Zn dissolved from the anode is η _A /100×r×65.4/2×Q/96500... The amount of Zn electrodeposited on the cathode side (cathode electrolysis efficiency 80 %) 80/100×(1-y/100)×65.4/2×Q/96500…
..., η _A /100×r=0.80×(1−y/100). ∴r=80/η _A × (1-y/100) ... Also, insoluble anode: self-soluble anode = 1:
1, r=1/2..., so η _A = 80×(1-y/100)×2... Also, the bath PHZ to maintain the bath balance at this η _A is given by the following equation. It will be done. Z=8-1/2√34+ _A ... From this, the bath PHZ for maintaining the Ni ratio y in the film is given by the following equation. Z=8-1/2√194-1.6 Table 1 shows the calculated values of the bath PH when the Ni ratio y in the film is 10 to 15% from the formula. Similarly, Table 2 shows the Ni ratio in the film and the bath pH value for maintaining bath balance when the cathode electrolysis efficiency was 85%.

【table】

[Table] Figure 3 shows the results, and shows the relationship between the Ni ratio in the film and the bath pH when the self-soluble anode ratio r = 1/2. PH fluctuations during electrolysis are adjusted with basic nickel carbonate, and at the same time the loss of Ni is compensated for.
Now, suppose the Ni ratio in the bath is 50% (Ni ratio in the film is 10%).
% target, cathode electrolysis efficiency 80%), A with PH of 1.40
When the line starts at point B, the amount of Zn in the bath decreases, and stable operation is achieved at point B on the balance line, that is, the Ni ratio in the bath is 61.5% and the Ni ratio in the film is 12.3%. Conversely, in order to obtain a coating with a Ni ratio of 10% and maintain a Ni ratio of 50% in the bath, the amount of basic nickel carbonate added must be reduced to 1.33.
Stable operation is achieved by lowering the temperature to point C on the balance line. Next, an example of a line test will be shown. In a horizontal line as shown in FIG. 1, anodes are placed above and below the strip, and a jet stream is applied from the side in the direction of travel of the strip. Example 1 Self-soluble zinc anode ratio: r = 1/2 (lower self-soluble zinc anode, upper platinum-clad insoluble anode) PH at start: 1.40 Ni ratio in bath at start: 50% Bath temperature: 50°C Current density: 50A/ ^dm2 Cathode efficiency: 80% and operation was carried out while supplementing basic nickel carbonate to maintain the pH at 1.40 (constant). Figure 4 shows the change over time in the Ni ratio in the bath at that time. Result: No damage caused by sludge on the strip Bath balance Ni ratio in the bath was 61.5% and stable Ni ratio in the film was 12.3% Example 2 Self-soluble zinc anode ratio: r = 1/2 (upper self-soluble zinc anode, Lower lead alloy insoluble anode) PH at start: 1.39 Ni ratio in bath at start: 60% Bath temperature: 60℃ Current density: 50A/dm ² Cathode efficiency: 80% And base to maintain PH at 1.39 (constant) The plant was operated while supplementing with nickel carbonate. In the bath at this time
The change in Ni ratio over time is shown in Figure 4. Result: Stability at 60% Ni ratio in the balance bath (PH
1.39) As described above, the method of the present invention does not require expensive casting equipment and does not use a Ni self-fusing anode that tends to generate sludge due to non-uniform melting, thereby reducing costs and suppressing sludge generation. Moreover, since expensive carbonate is used only for supplying Ni ions, the amount used can be reduced. Furthermore
The problem of scratches caused by metallic sludge generated on the Zn self-fusing anode can be effectively suppressed by installing the Zn self-fusing anode below a horizontally running steel strip.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the method of the present invention, Fig. 2 is an explanatory diagram of the production of metallic sludge, and Fig. 3 is an illustration of the plating bath PH.
Graphs showing the relationship with the Ni ratio, Figures 4 and 5 are graphs showing the change over time in the Ni ratio in the bath. In the figure, 1 is a Zn self-soluble anode, and 2 is an insoluble anode.

Claims (1)

[Claims] 1 A Ni--Zn alloy electroplated steel plate in which anodes are placed at a predetermined interval from above and below a steel strip that is being transported in a horizontal electrolytic cell, and a Ni--Zn alloy is electroplated onto the steel strip. In the manufacturing method, an insoluble anode is arranged from the upper side of the steel strip and a Zn self-soluble anode is arranged from the lower side, and Zn ions in the plating bath in the electrolytic cell are replenished from the Zn self-soluble anode, and the Ni ions are replaced by Ni carbonate. A method for producing a Ni--Zn alloy electroplated steel sheet, characterized by performing electroplating while controlling pH by replenishing with salt.