JPH01132793A - Production of steel plate plated with zn-ni alloy - Google Patents

Abstract

PURPOSE:To carry out plating of a steel plate with a Zn-Ni alloy stably and industrially by specifying each pH of a plating bath, a concn. of contained ion, flow rate, and current density in a plating bath acidified with sulfuric acid contg. Zn ion and Ni ion. CONSTITUTION:In a stage of carrying out Zn-Ni alloy plating of a steel plate using a plating bath acidified with sulfuric acid contg. Zn ion and Ni ion, the pH of the plating soln. is regulated to 1-2.5, the sum of the concn. of Zn<2+> ion, Ni<2+> ion, H<+> ion, and SO4<2-> ion is regulated to 2, 3 mol/liter, and the concn. of at least one ion among Na<+> ion, K<+> ion, and NH4<+> ion is regulated to >=0.1 mol/liter. Further, the anode of each plating bath is divided to plural segments, and the current density of an anode segment at each discharging side of the plating soln. between each anode and the steel plate is held at by 20-30% lower current density than that of the the anode segment at each inlet side of the plating soln. By this constitution, a Zn-Ni alloy plated steel plate having high quality is obtd. stably.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for industrially stably manufacturing Zn-Ni plated steel sheets from low line speeds to high line speeds.

<Prior Art> Since Zn-Ni alloy plating exhibits corrosion resistance several to ten times higher than Zn plating with the same basis weight, its usage has increased significantly in recent years. In order to exhibit this high corrosion resistance, the Ni content in the plating layer must be 10 to 10%.
It is necessary to control the content within a range of 15 wt%.

This is because the corrosion resistance is best in the region of the Zn-Ni alloy phase, which is a solid solution type γ phase called N15Znz+. Exceeding this range of alloy composition is not preferable because the corrosion potential becomes too much of a liability and the sacrificial corrosion protection against steel sheets deteriorates.

In order to stably produce high-quality Zn-Ni plated steel sheets with a Ni content of 10 to 15 wt%, it is generally necessary to keep the plating current density constant, the bath composition constant, and the liquid flow rate and temperature constant. It is necessary to control various plating conditions such as changing the plating temperature, but in order to manufacture it industrially in large quantities at low cost, firstly, it can be manufactured at high line speed, and secondly, it is necessary to control various plating conditions such as changes in line speed and current density. It is necessary to be able to manufacture products so that a constant quality is maintained regardless of changes. In order to perform the desired Zn-Ni alloy plating at a high line speed, it is necessary to perform plating at a high current density because the amount of electroplating depends on the product of current density and plating time. . and,
The higher the current density operation is, the larger the ratio of the voltage due to inter-electrode resistance (plating solution resistance) to the total plating voltage becomes. It is necessary to increase the degree of

Currently, as a technology intended for industrial Zn-Ni alloy plating under high current density, the operating conditions are JP-A-55-1
As described in Japanese Patent Publication No. 52194, the relative speed between the plating solution and the steel plate is maintained at 20 spm or more, and as a plating apparatus, as described in Japanese Patent Publication No. 61-21319, the distance between the electrode and the plated steel plate is 11! A device using a horizontal electrolytic device with a shortened (distance between poles!!I), used as a plating solution in JP-A-61
As described in Japanese Patent Publication No. 133394, a method in which a certain amount of supporting electrolyte is added is disclosed, and as a method for increasing the electrical conductivity of a plating solution, Japanese Patent Publication No. 19719-1987 discloses a method in which Z n S 04 and NiSO4 are added. A method of adding a certain amount is disclosed. Also, regarding technology to maintain constant quality regardless of changes in manufacturing conditions such as line speed or current density,
As described in Japanese Patent No. 106992, a plating bath is disclosed in which current density dependence is reduced by mixing ammonium ions into the solution.

<Problem to be solved by the invention> However, all of these require a high line speed of 100 mpm or more or 100 to 250 A/da.
It cannot be put to practical use under the conditions of high current density of 5 to 10%.
Under the above conditions exceeding A/dm2, the precipitation mechanism becomes an abnormal type in which electrochemically base Zn preferentially precipitates, so the composition of the plating layer differs from the composition of the plating solution, making it difficult to control the composition of the plating layer. In addition to the difficulty of suppressing the increase in power costs due to the increase in plating voltage, the effect of oxygen gas generated during plating due to high current density plating on the composition, and the joule during plating. This is because problems such as the effect of heat generation on the composition had not been resolved.

In addition, the composition of the plating layer also depends on the current density, so in a factory line where the current density is changed depending on the width of the steel sheet to be plated, the coating weight, etc., it is necessary to stably achieve plating with a constant quality. The problem of making things even more difficult has not been resolved.

Here, the effect of oxygen gas generated in the plating solution during plating is that the oxygen gas generated in direct proportion to the increase in current density increases the Ni content in the plating layer, and the Zn- This is a cause that makes it impossible to stably manufacture Ni alloys. In fact, as shown in Figure 5, it can be seen from the relationship between the amount of oxygen gas contained in the plating solution and the rate of increase in the Ni content in the plating layer that the amount of oxygen gas in the plating solution exceeds 10%. and extremely low N in the plating layer.
i content increases. The plating solution used in this measurement had a total concentration of Zn+, Ni+, H" and 304'- ions of 2.9 mol/J!, Na"0,
2 mol/j! , K” 0.2mol/fL, p
H1, 8.

Hereinafter, the influence of such oxygen gas will be explained first.

The effect of oxygen gas changes depending on line speed.
Therefore, Zn-N with Ni content in the range of 10 to 15 wt%
i-alloy plating at line speed of 10 to 300 w+pm
This has caused a problem in that it is difficult to stably manufacture the material within this range.

This means that by arranging 10 radial plating cells,
Using a 2m long anode, the plating solution composition was Zn+, N
The sum of i+, Ho and SO42- is 4 mol/41,
pH 2.0, plating temperature 60℃, liquid flow rate 0.5 m
The third diagram shows the relationship between plating current density, line speed, and Ni content when plating is performed under the condition of /sec.
This is demonstrated from the graph in Figure. As can be seen from Figure 3, when the line speed or plating current density changes, the Ni content in the plating layer also changes significantly.
It can be seen that the content may not fall within the preferred range of 5 wt% for toel.

The influence of such oxygen gas is 100 to 250 A/d
(2) Since plating is carried out under the high current density conditions of (2) while keeping the plating voltage low, this becomes a particular problem when using a plating cell with a small distance between electrodes as described in Japanese Patent Publication No. 61-21319. Since an insoluble electrode is generally used for the anode of a plating cell with a small distance between electrodes, oxygen gas is inevitably generated during plating, and as described later,
This is because the amount of oxygen gas present on the plating surface increases.

On the other hand, the above-mentioned effect of Joule heat generation associated with high current density plating is that the large amount of Joule heat generated at high current density increases the temperature of the plating solution, and the temperature of the plating solution is not maintained constant within the gap between the electrodes. , refers to the inability to stably obtain plating with a constant composition.

This means that 100% of conductive dust in 1m1 of anode length and plating solution
m57cm (the plating solution is the same as the plating solution shown in FIG. 5 above), liquid flow rate 0, 5 m/sec, path between poles w
This is clarified by FIG. 6, which shows the relationship between the plating current density and the temperature rise width of the plating solution (difference between the solution temperature on the outlet side of the plating solution cell and the solution temperature on the inlet side of the plating solution cell) in the case of i10IIIIIIl.

From Figure 6, the plating current density is 100 to 180 A/d”
It can be seen that the plating solution temperature increases by 4 to 13°C in the case of .

As mentioned above, industrially stable high line speed Zn
-In order to perform Ni alloy plating, it is necessary to solve the problems caused by the effects of oxygen gas and Joule heat generation associated with high current density plating, but it is also necessary to use a plating solution with low viscosity. It is said that

This is because when a highly viscous plating solution is used, the plating solution is dragged out of the system, which is called drag-out, which wastes expensive Ni and increases the production cost of plating. This is because retention of oxygen gas in the liquid is promoted.

Therefore, the viscosity of the plating solution is preferably lower, and from the viewpoint of lowering the viscosity of the plating solution, it is preferable to lower the density of the plating solution.

The present invention has been made in view of the above circumstances, and is characterized in that Zn-Ni alloy plating of steel sheets can be carried out industrially and stably in large quantities at low cost from low line speeds to high line speeds. In particular, the present invention provides stable plating at high line speeds when performing high current density plating.
The purpose is to suppress the effects of generated oxygen and Joule heat on the plating composition, thereby ensuring that plating is of constant quality regardless of changes in line speed and current density.

Furthermore, the present invention makes the production line more compact! It is an object of the present invention to make it possible to reduce the number of cells in a plating line by making the anode length per cell sufficiently long.

<Means for Solving the Problems> As a result of intensive research into various problems associated with conventional high current densities, especially the influence of oxygen generated from the anode, the present inventors have found that air bubbles in the plating solution between the anode and the steel sheet As shown in Figures 7(a) to (e), there are three types of fffi: bubbles in the process of floating, bubbles completely dispersing, and bubbles partially floating. The Ni content of the plating layer will increase if 10% of oxygen gas is mixed into the plating solution when the bubbles are completely dispersed, and the Ni content of the plating layer will increase when the bubbles are partially floating. It has been found that the Ni content of the plating layer increases even when the plated layer is used. The difference between the completely dispersed state of the bubbles and the state of partially floated bubbles is the difference in the amount of oxygen gas near the surface of the steel plate to be plated.

It has also been found that these three fffi states occur depending on the liquid flow rate, anode length, and plating current density. −
In other words, when fishing by boat, when the liquid flow rate is high and the anode length is short (for example, less than 30c+n), the generated air bubbles will leave the plating cell before being completely dispersed between the poles, resulting in a state where the air bubbles are floating, and the air bubbles will not completely disappear. There is no possibility of a dispersed state or a partially floating state of bubbles.

On the other hand, when the liquid flow rate is slightly low and the anode length is long, the bubbles are completely dispersed, and when the liquid flow rate is even lower, the bubbles are partially floated.

Such changes in conditions also depend on line speed.

As shown in FIGS. 8a and 8b, when the line speed changes, the thickness of the concentration boundary layer 5, which affects the Ni content in the plating layer, changes. Particularly when operating at high current densities and high line speeds (8a
(see Figure 8), when the line speed is reduced and operation is performed at high current density and low line speed (see Figure 8b), oxygen is present in the concentration boundary layer or near the concentration boundary layer, as shown in Figure 8b. gas will be present, and the influence of oxygen gas will become more pronounced.

As a result of further examination of the existence state of air bubbles in the three types of plating solutions mentioned above with respect to actual lines, the following findings were obtained. In other words, in an actual line, the anode length is long (50 cm or more) and the plating solution is in a turbulent flow state, so air bubbles generated in the anode are dispersed by the influence of turbulent diffusion, and therefore the anode and steel plate are separated. It was found that the number of bubbles in the middle of floating in the plating solution decreased, and most of the bubbles were completely dispersed or partially floated.
After examining this matter in detail, we found that when the anode length of the plating cell exceeds 1 m, there are two types of states in the direction of travel of the plating solution: a fully dispersed state of bubbles and a partially floating state of bubbles. In addition, a region where bubbles are partially floating is formed near the exit side of the plating solution from the cell, but as the flow rate increases and the viscosity of the plating solution decreases, the turbulent flow is promoted. It was found that the length of the partially levitating state becomes shorter.

In addition, even if the flow rate of the plating solution increases and a turbulent state is promoted, and the air bubbles near the outlet side of the plating solution are completely dispersed, which has a smaller effect on the plating layer composition than when the air bubbles are partially floating, the plating It has been found that since the amount of gas in the solution is large, it is not possible to sufficiently reduce the effect on the plating composition by itself, and that the solution to this problem is to reduce the plating current density on the outlet side of the plating solution.

Furthermore, it has been found that the problem of the effects of Joule heat generation can be solved by increasing the conductivity of the plating solution and increasing the flow rate of the plating solution.

Combining the above knowledge, we believe that in order to reduce the effects of oxygen gas and Joule heat generated during plating, it is necessary to use a plating solution with low viscosity and high electrical conductivity, and to improve the The present inventors discovered that it is sufficient to increase the flow rate of the plating solution so that the plating solution becomes sufficiently turbulent, and further reduce the plating current density on the outlet side of the plating solution between the anode and the steel plate, leading to the completion of the present invention. .

That is, in the present invention, when performing Zn-Ni alloy plating on a steel plate using a sulfuric acid acidic plating bath containing Zn and Ni ions, the pH of the plating solution is 1 to 2.5, and the Zn+ in the plating solution is
The sum of N i +, H+ and SO4”- ions is 2~
3 mol/1 and Na+, K" and NH4"
One or more types of ions are 0. 1mol/j! The plating solution has a flow rate of 1 m/sec or more, and the current density on the plating solution outlet side between the anode and the steel plate is lower than the current density on the plating solution inlet side. A method for manufacturing a Zn-Ni alloy plated steel sheet is provided.

Furthermore, in the present invention, the anode of each plating bath is divided into a plurality of segments in the above invention, and the current density of the anode segment on the plating solution outlet side between the anode and the steel plate is changed from the current density of the anode segment on the plating solution inlet side between the anode and the steel plate. Preferably, the current density is 5 to 20% lower than the current density of the anode segment.

In the above invention, Zn--Ni alloy plating is preferably performed in a radial cell or a horizontal cell.

The present invention will be explained in more detail below.

The plating solution used in the present invention has a pH of 1 to 2.
．． 5. The total amount of Zn+, Ni+, Ho and SO42- ions is 2 to 3 mol/fL and Na+, K'' and N
One or more types of H4+ ions at 0.11QO1/
The liquid is adjusted to contain JZ or more.

If the pH of the plating solution is less than 1, Zn-N
i When electroplating is performed, a large amount of iron is leached from the steel sheet and exists as impurity ions in the plating solution, which is undesirable.Also, if the pH exceeds 2.5, the plating line must not be operated continuously. In this case, the pH fluctuation becomes very large and the Ni content in the Zn-Ni plating layer changes, which is not practical.

If the total amount of Zn+, H+, SO4''- ions in the plating solution is less than 2moJ2zξ, it is unsuitable for Zn-Ni alloy plating because it will cause plating burn.
If it exceeds fl/11, it is not preferable because the effect of improving electrical conductivity by the conductive additive is small.

The total amount of conductive aids such as Na+, K'' and NH4'' is 0.
．． 1moj! /J! If it is less than that, there is no point in adding it. Further, there is no need to specifically limit the upper limit, but the solubility limit is determined depending on the type, and addition beyond the solubility limit is meaningless.

Such a plating solution was developed by the present inventors in Japanese Patent Application No. 62-026.
As already disclosed in Japanese Patent No. 011, the viscosity of the plating solution is low and the conductivity of the plating solution is high, so it is suitable. That is, since the viscosity of the plating solution is low, the turbulent state of the plating solution is promoted, and the formation of floating bubbles that tend to affect the Ni content in the plating composition is suppressed. Therefore, it is possible to reduce the influence of oxygen gas generated during line production at a high current density of 100 to 250^/dad''.In addition, since the plating solution has high conductive dust, the plating voltage can be lowered and Joule heat generation can be reduced. It is preferable because it can be sharpened to lower the temperature.

The flow rate of the plating solution during plating is 1 m/sec or more.

If the plating solution flow rate is less than 1 m/sec, sufficient turbulence will not be created between the anode and the steel plate, and the influence of oxygen gas generated during plating on the plating composition will increase. As a result, the Ni content in the plating layer becomes largely dependent on the line speed or current density, which is undesirable.Although there is no particular upper limit to the flow rate of the plating solution, a maximum of about 3 m/sec is practical. Favorable for operation.

This is based on the results shown in FIG. 4 obtained by measuring the difference in Ni content between line speeds of 10 mpm and 300 mpm by changing the plating solution flow rate. From Figure 4, the plating solution flow rate is 1 a+/se.
Below c, the change in Ni content is large;
It can be seen that above c, the change in Ni content is less than 5%. The quenching liquid used for this measurement was Zn+, N
The total concentration of i +, Ho and 5042- ions is 2.
9 mol/JZ, Na” 0.2 mol/fL.

K" 0.2 wol/j2, pH 1.8, and 10 radial type plating cells were used.

In the present invention, it is important to make the current density on the plating solution outlet side between the anode and the steel plate lower than on the plating solution inlet side.

This is to suppress the influence of the high oxygen gas content in the plating solution on the plating solution outlet side as described above. The degree and mode of reducing the current density on the plating solution outlet side depend on the operating conditions of the plating line, such as the plating solution flow rate, the distance between the anode and the steel plate, the length of the anode, and the viscosity of the plating solution, but in general Plating solution inlet side is 100-250
In the case of a current density of A/dm2, it is preferable to reduce the current density on the plating solution outlet side by 5% to 20% compared to the plating solution inlet side.

The manner in which the amount is decreased may be continuous or may be in a stepwise manner such that the amount is suddenly decreased on the plating solution outlet side.

As a method of reducing the current density on the plating solution outlet side, the anode may be divided into two or more parts in the longitudinal direction to create a difference in current density.

If it is less than 5%, the change in the Ni content of the plating due to the change in line speed will be somewhat large, and if it exceeds 20%, the plating will have a layered structure in the depth direction, with each anode having a different Ni content. , undesirable.

Note that the current density on the plating solution inlet side can be set to the current density normally required for high line speed plating, especially 10
In the range of 0 to 250^/dad2, the effects of the present invention are significantly exhibited.

The plating cell used in the present invention is not particularly limited, but radial cells or horizontal cells are preferable. Radial cells have a stable pass line, so there are fewer fluctuations in the plating solution flow rate and less local current concentration, etc. This is because it has very desirable features when plating is performed with a small distance between poles.

The improvement effect of the present invention is also preferably exhibited in horizontal cells.

Here, although the length of the anode within one cell is not particularly limited, the effects of the present invention are significantly exhibited when the anode length is 1 m or more. This is because if the distance is less than in (particularly less than 30 cm), a floating state of air bubbles will not occur between the anode and the steel plate, and the oxygen gas content in the plating solution on the plating solution outlet side will not become high.

<Example> The present invention will be specifically explained below using examples.

(Example 1) In a plating line consisting of 10 radial type plating cells as shown in Fig. 1, the anode was divided into two in the longitudinal direction (each in) and a current density of 1 was applied in the direction of movement of the plating solution.
．． 0510.95 (approximately 10% difference in current density), and the plating solutions were Zn+, Ni "H" and so
The total sum of 4'- is 2.9 mol/JZ, K2 SO40,
2mol/jZ, N at SO40, 2mol/u
% using a plating solution with a composition of pH 1.8, a plating solution flow rate of 2, 0 m/sec, and a plating solution temperature of 60°C.
Zn on steel plate by changing line speed and current density
-Ni alloy plating was performed, and the Ni content of the formed plating layer was measured.

The results are shown in Figure 2.

(Examples 2 to 9) Plating was performed in the same manner as in Example 1, with the plating solution composition, current density distribution, and plating solution flow rate changed as shown in Table 1. The results are shown in Table 1.

(Comparative Example 1.2) Plating was performed in the same manner as in Example 1 under the conditions shown in Table 1 without making any difference in current density in the longitudinal direction of the anode.
The results are also shown in Table 1.

By comparing the results shown in Table 1 and FIG. 2 with FIG. 3, it can be seen that in the examples of the present invention, the Ni content in the plating changes significantly with changes in current density or line speed compared to the comparative examples. It can be seen that the Ni content in the plating is maintained at 10 to 15 wt% regardless of changes in current density or line speed.

In addition, although the above-mentioned example showed the example in a radial type cell, the same effect was confirmed also in the horizontal cell.

<Effects of the Invention> According to the present invention, even if high current density plating is performed, the effects of oxygen gas and Joule heat generation generated during plating can be suppressed, so that the plating composition can be controlled according to the current density, line speed, etc. of the plating line. It remains constant regardless of changes in operating conditions. Therefore, high quality Zn-Ni alloy plating can be produced industrially and stably from low line speeds to high line speeds.

Furthermore, according to the present invention, even if the anode length per cell is long, the influence of oxygen gas in the plating solution can be suppressed by reducing the current density between the anode and the steel plate in the direction of movement of the plating solution, resulting in high quality plating. A stable Zn-Ni alloy plated steel sheet is manufactured. Therefore, the anode length per cell can be made sufficiently long and the number of cells in the plating line can be reduced, so the plating line can be made more compact.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a plating line according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between line speed and current density and Ni content in an example of the present invention. FIG. 3 is a graph showing the relationship between conventional line speed and current density and Ni content. FIG. 4 is a graph showing the relationship between the change in Ni content and the plating solution flow rate at line speeds of 10 mpm and 300 mpm. FIG. 5 is a graph showing the relationship between the gas fraction in the plating solution and the rate of increase in the Ni content. FIG. 6 is a graph showing the relationship between the plating current density and the degree of increase in plating solution temperature due to Joule heat generation (difference between the solution temperature on the outlet side of the plating solution cell and the solution temperature on the inlet side of the plating solution cell). FIGS. 7(a) to (C) show the presence of air bubbles between the anode and the plated steel plate. FIGS. 8a and 8b are diagrams showing the state of the steel plate surface when the line speed changes. Explanation of symbols 1...Plated steel plate, 2...Anode, 3...Bubble, 3a...Bubble in the plating solution dragged by the steel plate, 4...Plating solution, 5... Concentration boundary layer, 50... Mounting cell, 51... Degreasing, pickling 52... Strip, 53... Deflector roll, 54... Conductor roll , 55...Anode FIG, I FIG, 2 Line speed (mpm) NjA-f 100% (%) Ni4'■% cy, > ffi, ″%rj Male fraction during the night (%) Electric Density (A
/dm”)

Claims (4)

[Claims] (1) When performing Zn-Ni alloy plating on a steel plate using a sulfuric acid acidic plating bath containing Zn and Ni ions, the pH of the plating solution is set to 1 to 2.5, and the Zn^2 in the plating solution is
The total amount of ^+, Ni^2^+, H^+ and SO_4^2^- ions is 2 to 3 mol/l, and Na^+, K^+
and 0.1 of one or more types of NH_4^+ ions.
mol/l or more, and the flow rate of the plating solution is 1 m/sec.
c or more, and the method for producing a Zn-Ni alloy plated steel sheet is characterized in that the current density on the plating solution outlet side between the anode and the steel plate is lower than the current density on the plating solution inlet side. (2) When performing Zn-Ni alloy plating on a steel plate using at least one sulfuric acid acidic plating bath containing Zn and Ni ions, the pH of the plating solution is set to 1 to 2.5, and the Zn in the plating solution is ^2
The total amount of ^+, Ni^2^+, H^+ and SO_4^2^- ions is 2 to 3 mol/l, and Na^+, K^+
and 0.1 of one or more types of NH_4^+ ions.
mol/l or more, and the flow rate of the plating solution is 1 m/sec.
c or more, the anode of each plating bath is divided into a plurality of segments, and the current density of the anode segment on the plating solution outlet side between the anode and the steel plate is lower than the current density of the anode segment on the plating solution input side. A method for manufacturing a Zn-Ni alloy plated steel sheet, characterized in that the steel sheet is lowered by 5 to 20%. (3) The method for manufacturing a Zn-Ni alloy plated steel sheet according to claim 1 or 2, wherein the Zn-Ni alloy plating is performed in a radial cell (plating bath). (4) The method for producing a Zn-Ni alloy plated steel sheet according to claim 1 or 2, wherein the Zn-Ni alloy plating is performed in a horizontal cell (plating bath).