JPH0124233B2 - - Google Patents

Description

[Detailed description of the invention]

Technical field There is a strong demand for improving the corrosion resistance of Zn-plated steel sheets, mainly in the use of steel sheets for automobiles.
Research on Zn-based alloy plating or composite plating has become widespread and aggressive. Of the former, the following is particularly relevant to the application of Zn-Ni alloy plating to steel sheets, especially in the field of technology to which so-called surface-treated steel sheets belong.
In particular, it occupies a further subdivision in relation to the production of Zn-based alloy plated steel sheets. Prior art and its problems It is generally believed that steel sheets coated with Zn alloy plating, especially Zn-Ni alloy plating, exhibit excellent surface properties in terms of corrosion resistance, weldability, paintability, etc. , JP-A-55-110791, 55
-152194 The precedent can be seen in each publication. Among them, the former uses zinc sulfate and nickel sulfate, especially in a sulfate bath ^.
Adjust the molar concentration ratio with Zn ²⁺ from 1.5 to 4, thus
The molar concentration percentage of Ni ²⁺ /Ni ²⁺ +Zn ²⁺ is 60
It is taught that a Zn--Ni alloy plated steel sheet having a Ni content of 9 to 20% by weight can be obtained under operating conditions in which the relative speed with the steel sheet in the plating bath is 10 m/min or more at 10% to 80% by weight. On the other hand, for the latter, without touching the molar concentration ratio of Ni ²⁺ and Zn ²⁺ , use an acidic bath with a plating solution flow rate of 20 m/min or more and a current density of 20 A/min.
It discloses that Zn-Ni deposits can be obtained under conditions of dm ² or more, and in the example using chloride, Ni
It is also shown that at a molar percentage of 36%, a precipitation of approximately 12-19% Ni by weight was obtained. However, as a plating anode in a sulfate bath,
Even if soluble Zn and Ni anodes are used, the Ni anode becomes passivated and ionization does not occur.
The Ni ²⁺ concentration in the bath decreases, making it difficult to continuously obtain alloy-plated steel sheets of stable quality, and managing to maintain the liquid composition requires considerable effort. On the other hand, chloride baths are advantageous in that they have a higher electrical conductivity than the sulfuric acid baths, and they are plated at a ratio of Zn and Ni that allows both Zn and Ni to be soluble as anodes to be deposited on the steel sheet surface. Just by arranging them in a tank, they are ionized in an amount commensurate with the amount of precipitation, and have the advantage that the composition of the plating solution is always kept constant. Here, the inventors have specifically investigated the plating solution composition, plating conditions, and precipitated alloy composition regarding plating baths containing zinc chloride and nickel chloride as main components, as well as mixture baths in which sulfate is added. After extensive experiments and studies on the interrelationships, we found that Ni deposits from this type of chloride bath or mixture bath, especially under high current density plating conditions:
Zn-Ni alloy plating of 10 to 20% by weight is especially
At current densities above 20 A/dm ² , undesirably only yellow-brown to blue-violet oxide-containing precipitates were obtained. Precipitates containing such oxides not only have poor surface gloss and lack commercial value, but also generally require pre-painting treatment since Zn-Ni alloy plated steel sheets are primarily used for painting. It is recognized that the surface treatment, that is, phosphate and chromate treatment, is difficult to form uniformly due to the influence of oxides, resulting in poor paint adhesion and post-painting corrosion resistance, which is a decisive drawback. It came to this. Purpose of the Invention The above-mentioned problems can be solved by plating Zn-Ni alloy on the surface of a steel plate using a chloride bath containing zinc chloride and nickel chloride as main components, or a mixture bath to which sulfate is added as a plating solution. When applying this method, especially when operating under high current density, it is possible to effectively suppress the mixed precipitation of yellow-brown or blue-purple oxides that are unavoidably accompanied in the conventional precipitate layer.
Allows arbitrary adjustment of Ni content. It is an object of the present invention to propose a Zn-Ni alloy plating solution for use in manufacturing Zn-Ni alloy plated steel sheets. Structure of the Invention The above object is achieved particularly advantageously under the electrolytic behavior to be described in detail later by embodying a concept which outlines the following matters. In a chloride bath mainly consisting of zinc chloride and nickel chloride or in a mixture bath to which sulfate is added,
Ni content of Zn-Ni alloy plating is 8 to 20% by weight
An amount of ammonium salt sufficient to ensure the precipitation of an alloy plating mainly composed of γ (NiZn ₃ ) phase is added in a range from 0.5 mol/l to reach solubility, and the pH is adjusted to 1.5 to 5.8. adjusted to
In the Zn-Ni alloy plating solution, the Ni ²⁺ /Ni ²⁺ +Zn ²⁺ molar concentration percentage of the plating solution is 10 to 19%.
A Zn-Ni alloy plating liquid characterized by the following. It also directly controls the Ni content in the Zn-Ni alloy plating, and as the alloy phase mainly consists of the γ (NiZn ₃ ) phase, it contributes to suppressing the mixing and precipitation of yellow-brown to blue-purple oxides. A representative example of the ammonium salt is ammonium chloride, which will be explained in more detail later.Other examples include ammonium aluminum chloride, ammonium sulfate, ammonium nickel sulfate, and ammonium hydroxide, each used alone or in combination. Almost the same effect can be obtained by In general, the Ni content in Zn-Ni alloy plating has a significant effect on the durability of the plated steel sheet, which was reconfirmed through experiments conducted by the inventors. In general, the metal ion concentration in the plating solution, pH, solution temperature, current density, stirring conditions, etc. tend to fluctuate depending on the plating conditions, but with the above configuration, industrial Not only is it suitable for operations at high speeds and high current densities that are suitable for large-scale galvanized steel plate manufacturing lines, but it is also possible to stably achieve the target Ni content regardless of fluctuations in operating conditions. can. In other words, the precipitation from the plating bath mentioned above is caused by the change in the Ni content due to fluctuations in the relative speed and current density between the plating solution and the steel sheet, which was explained in the previous example mentioned above and has been generally considered in the past. There is almost no change in the plating phase, and in addition, in a static bath or similar state, under high current density of 20 A/dm ² or more, as in the case of high relative velocity, there is contamination of oxides in the plating phase. A Zn--Ni alloy plated steel sheet consisting mainly of the γ (NiZn ₃ ) phase can be obtained without any Ni content within the range of 8 to 20% by weight. In this regard, JP-A No. 55-110791 and JP-A-Sho
No. 55-152194 discloses that in any case, in order to obtain an alloy composition of 10 to 20% Ni at high current density (for example, 10 A/dm ² or more) and at high speed industrially, it is necessary to use a plating solution and It is essential to set the relative moving speed of the steel plate to 10 m/min or more, and even after detailed studies by the inventors using conventional plating baths, high current densities of 30 A/dm ² or more were found. In order to electrolyze the plating solution, it is impossible to obtain a uniform γ phase (Ni content of 10 to 20%) unless the plating bath is forced to circulate or the plating solution near the steel plate interface is sufficiently stirred. I can't. moreover
At high current densities such as 70 A/dm ² or more, new special stirring equipment is required to enhance the stirring of these plating baths, requiring major changes and modifications to conventional plating equipment. I found out what I needed. However, the plating bath according to the present invention is effective when the relative velocity between the plating solution and the steel plate is relatively small (for example, 20
The shortcomings of conventional plating baths are greatly overcome in that a predetermined alloy composition with no oxides on the surface can be obtained uniformly over a wide current density range of 20 to 250 A/ ^dm2 even when It can be solved advantageously and effectively, and its contribution to the industrial high-speed production of Zn-Ni alloy coated steel sheets is extremely large. In carrying out this invention, usually
Zn ²⁺ concentration 30g/l or more, Ni ²⁺ concentration 10g/l
In the above process, the molar concentration percentage of Ni ²⁺ /Ni ²⁺ +Zn ²⁺ is adjusted to 10 to 19%, and an ammonium salt of 0.5 mol/l or more is added thereto as an additive. If the Zn ²⁺ and Ni ²⁺ concentrations are less than the above concentrations, it becomes difficult to obtain normal alloy plating, especially at high current densities of 50 A/dm ² or more. Here, the molar concentration percentage of Ni ²⁺ /Ni ²⁺ + Zn ²⁺ is
At the end of 10%, it becomes difficult to maintain the Ni content in the plating layer above 10%, and it becomes difficult to expect sufficient corrosion resistance.On the other hand, when the Ni ²⁺ molar concentration percentage exceeds 19%, When the Ni content in the glazing layer exceeds 20%, the sacrificial anticorrosion effect of Zn decreases and becomes dominant, and the glazing adhesion also tends to decrease. Ammonium salts as additives are particularly important, and at less than 0.5 mol/l, current density, flow rate, pH
The Ni content changes depending on plating conditions such as bath temperature, and furthermore, only yellowish brown to bluish-purple precipitates containing oxide or hydroxide impurities are obtained. Further, since the Ni content increases as the concentration of ammonium salt added increases, the Ni content can be arbitrarily set by combining it with the Ni molar concentration percentage. The appropriate pH for the plating solution is 1.5 to 5.8; a pH of less than 1.5 causes chemical dissolution of the plating layer, which is undesirable, and a pH of over 6.0 causes the formation of hydroxide, causing the plating solution to become ineffective. It becomes stable, which is still not desirable. Note that the appropriate temperature for the bath salt is about 35 to 75°C. Next, the appropriate current density is 20 to 250 A/ ^dm2 .
This is because at less than 20 A/dm ² high-speed plating cannot be achieved and it is industrially disadvantageous, and at more than 250 A/dm ² dendrites are formed, which is undesirable. Furthermore, the relative movement speed between the plating liquid and the strip depending on the flow rate of the plating liquid or the line speed of the strip is not particularly limited as one of the features of the present invention, and the plating bath according to the present invention has a flow rate of 20
Even at low speeds of less than m/min, a single γ phase with an excellent surface appearance and no oxides precipitates stably.
Even in plating operations with high current densities exceeding 70A/ ^dm2 , there is no need for a special stirring device as mentioned above, and even in plating systems with uneven liquid flow rates, it is possible to operate satisfactorily. A stable Ni content can be obtained, which is extremely advantageous industrially. In addition, the advantage of chloride baths is that the electrical conductivity is 2-3 times higher than that of sulfuric acid baths, and the power consumption is reduced by 1/2-1/3, thus advantageously meeting the requirements for energy saving. It is also advantageous in that it is easy to maintain a constant plating solution composition. Now, an example of the results of an experiment in which the electrolytic behavior of the ammonium salt-added chloride bath according to the present invention was investigated will be shown below. ZnCl ₂ 136g/l and NiCl _{2.6H 2} O60g/l
(Ni ²⁺ / Ni ²⁺ + Zn ²⁺ molar concentration percentage 20%) NH ₄ Cl 10-400g/l (0.18-7.4mol/l)
The current density was 75 A/dm ² , using a licking bath adjusted to pH 4.0.
Under constant conditions of flow rate 50 m/min and bath temperature 50°C, Zn-
Plating was carried out so that the Ni coating amount was 20 g/m ² . As is clear from Figure 1 showing the results,
It exhibits a peculiar behavior in which the Ni content increases as the amount of NH ₄ Cl added increases. Therefore, by appropriately selecting the amount of NH ₄ Cl added, Ni in the plating layer can be reduced.
By arbitrarily controlling the content within the range of 8 to 20% by weight, it is easy to manufacture a Zn-Ni alloy plated steel sheet that matches the properties. Next, in the case of 200 g/l of NH ₄ Cl among the above, compared with the case without NH 4 Cl, in the Zn-Ni alloy plated steel sheet obtained under the above conditions,
Figure 2 shows the results of X-ray diffraction of the plating film. According to this invention, NiZn ₃ (γ phase) as shown by the solid line in the figure
Single-phase diffraction peaks are obtained, but conventional NH ₄ Cl
When NiZn ₃ is not included, there is an oxide diffraction peak (2θ≒49°) in addition to NiZn ₃ as shown by the broken line in the figure.
This is thought to be the cause of the brown coloring of the precipitate due to the co-separation of other impurities. In accordance with the present invention, similar experiments were conducted using various amounts of NH ₄ Cl, but no impurities were found in the precipitate in any case. In other words, according to the present invention, a plated layer containing any conventionally unavoidable impurities and having any desired Ni content can be easily obtained. Next, the plating flow rate and current density are Zn−
Regarding the effect on Ni content during Ni plating, the electrolytic behavior in this invention is shown in FIGS. 8 and 4 in comparison with that in a conventional bath. The composition of the plating solution is shown in Table 1.

[Table] Figure 3 shows the effect of the flow rate of the plating solution on the Ni content when the current density was set to 75A/ ^dm2 . In the plating liquid composition of the invention,
This shows a peculiar behavior in which a specific Ni content determined according to the NH ₄ Cl concentration can be stably maintained despite a wide range of plating solution flow rates up to 150 m/min. Figure 4 also shows a comparison of the effect of current density on Ni content with the plating solution flow ^rate fixed at 75 m/min. According to the present invention, the Ni content fluctuates significantly during the time when electrolysis becomes impossible due to the occurrence of burns at / ^dm2 .
It is clear that a nearly uniform Ni content is also stably maintained over a wide range of current densities up to 250 A/dm ² . Example The Ni ²⁺ molar concentration percentage of an alloy plating solution consisting of ZnCl ₂ , NiCl ₂ and NiSO ₄ was adjusted to 10, 30, and 50%, respectively, and 150 g/m of ammonium chloride was added to this alloy plating solution. 1 and 25 g/l of ammonium sulfate were added to adjust the pH to 4.0. Using this plating bath, the current density was 50A/
dm ² , flow rate 50m/min, bath temperature 50℃,
Plating was performed so that the Zn-Ni area weight was 40 g/m ² . As is clear from Table 2 showing the results,
From the plating bath containing ammonium chloride and ammonium sulfate, the Ni content in the plating layer was 8.
Alloy plating was obtained in the range of ~17% by weight, and the precipitate had a good glossy surface appearance. On the other hand, when the Ni ²⁺ concentration percentage of the conventional plating bath (comparative bath) to which no ammonium salt is added is below 30%, the Ni content in the plating layer is less than 8% by weight, and the precipitates are yellowish brown or yellowish brown. A blue-purple oxide was mixed into the precipitate.

[Table] Problems also occurred in adhesion.
Effects of the invention The effects of this invention can be summarized as follows. 1 Using the amount of ammonium salt added as a control factor
The Ni content of Zn-Ni plating can be determined as desired. 2. The addition of ammonium eliminates the contamination of yellow-brown to blue-purple oxides in the deposited layer, which is inevitable when using conventional chloride baths. 3. The Ni content in the Zn-Ni based plating layer is freed from dependence on the plating solution flow rate and current density. 4. Therefore, the plating operation can be stabilized at high speed and high efficiency without being affected by fluctuations in the operating conditions of the electroplated steel sheet production line.

[Brief explanation of drawings]

Figure 1 is a graph showing the effect of the amount of NH ₄ Cl added on the Ni content. Figure 2 is the diffraction peak comparing the results of X-ray diffraction of the plated film with and without addition of NH ₄ Cl. Comparison graphs, FIGS. 3 and 4, are graphs showing the effects of plating solution flow rate and current density on Ni content.

Claims (1)

[Claims] 1. Ni content of Zn-Ni alloy plating is in the range of 8 to 20% by weight in a chloride bath mainly composed of zinc chloride and nickel chloride or in a mixture bath to which sulfate is added. An amount of ammonium salt sufficient to ensure the precipitation of an alloy plating mainly composed of γ (NiZn ₃ ) phase was added in a range from 0.5 mol/l to reach solubility, and the pH was adjusted to 1.5 to 5.8. A Zn-Ni based alloy plating solution, characterized in that the plating solution has a Ni ²⁺ /Ni ²⁺ +Zn ²⁺ molar concentration percentage of 10 to 19%.
Alloy plating liquid.