Zn-Ni ALLOY PLATING SOLUTION BASED ON CHLORIDE BATH

Abstract

 A Zn-Ni alloy electroplating solution for obtaining Zn-Ni alloy-electroplated steel with excellent surface appearance, which contains ZnCl₂ and NiCl₂ as major components. In the solution, the percentage molar concentration, x(%), of Ni²⁺/ Ni²⁺ + Zn²⁺ and the concentration of chloride (at least one chloride selected from the group consisting of AlCl₃, NH₄AlCl₄, NH_°Cl, KCI, NaCl, CaCL₂, BaCl₂, and MgCl₂), y (mol/liter), are kept within the following ranges:

Description

TECHNICAL FIELD

[0001] This invention relates to Zn-Ni alloy electroplating solutions for use in plating steel sheets to form Zn-Ni alloy deposits having improved surface appearance.

BACKGROUND ART

[0002] It is well known in the art that Zn-Ni alloy plated steel sheets have improved surface properties such as corrosion resistance, weldability, and paintability, and that Zn-Ni alloy deposits consisting essentially of a gamma-phase and having a nickel content in the range from 10% to 20% by weight exhibit the best corrosion resistance.

[0003] Industrially feasible electroplating baths for Zn-Ni alloy plating are sulfate and chloride baths. The chloride bath has the advantages that liquid flow has less influence on the nickel content of deposits, that it can be operated at higher current density, and that power consumption is lower because of the increased electric conductivity of the solution than is the case with the sulfate bath, but the bath is difficult to form deposits having good appearance.

[0004] Making investigations to improve the appearance of Zn-Ni alloy deposits resulting from plating solutions based on zinc chloride ZnC12 and nickel chloride NiCl₂ in water, the inventors found that the appearance of Zn-Ni alloy deposits can be improved by adding at least one additional chloride selected from the group consisting of AlCl₃, NH₄AlCl₄, NH₄C1, KCl, CaCl2, BaCl₂, NaCl and MgCl₂ to the solution in an amount of 0.2 mols per liter.

[0005] It was, however, found that when the above-described additional chlorides were added, there was the likelihood that the nickel content of deposits would increase and particularly at a high current density of 50 A/dm² or more, it would exceed the optimum range between 10% and 20% by weight.

DISCLOSURE OF INVENTION

[0006] Then the inventors continued investigations on the correlation between plating bath composition and alloy deposit composition in Zn-Ni alloy plating in order to produce Zn-Ni alloy deposits exhibiting improved appearance and having a nickel content within the above-defined optimum range. As a result, the inventors have found that the concentration of an additional chloride added should fall within a specific range in relation to the ratio of molar concentration between nickel and zinc.

[0007] Provided that the ratio of molar concentration between nickel and zinc is expressed as the molar concentration ratio of Ni²⁺/(Ni²⁺ + Zn²⁺), deposits tend to increase their nickel content substantially in proportion to this molar concentration ratio on one hand, and in proportion to the concentration of the additional chloride on the other hand. It has thus been found that the nickel content of deposits will consistently fall within the optimum range of 10% to 20% by weight when the concentration of the additional chloride is controlled within a proper range in relation to the molar concentration ratio of Ni²⁺/(Ni²⁺ + Zn²⁺).

[0008] According to the present invention, there is provided a Zn-Ni alloy electroplating solution, characterized in that the molar concentration ratio X expressed in percent of Ni²⁺/(Ni²⁺ + Zn²⁺) in the solution and the concentration Y expressed in mol/liter of an additional chloride added to the solution are controlled to fall within the range defined by the following equations:

[0009] In the practice of the present invention, the total concentration of nickel and zinc in the plating solution preferably ranges from 1 to 4 mols per liter of the solution. Total concentrations of nickel and zinc of lower than 1 mol/ liter tend to form burnt deposits having unacceptable appearance. At total concentrations of higher than 4 mols/ liter, there are increased drag-outs from the plating bath, undesirably increasing the cost. The plating solution is not particularly limited with respect to pH and temperature while it is practically at pH 1 to 6 and a temperature of 40° to 70°C.

[0010] The machanism by which the additional chloride in the plating solution has an influence on the nickel content of deposits is presumed as follows although the exact mechanism has not been well understood. Since the Zn-Ni alloy plating is codeposition of different metals, a barrier layer of zinc hydroxide Zn(OH)₂ formed at the cathode interface prevents nickel, which is otherwise readily depositable, from depositing so that even when the molar ratio of Zn to Ni is 1:1 in the bath, deposition of zinc preferentially takes place by way of the zinc hydroxide barrier layer. Nevertheless, if a chloride, for example, ammonium chloride NH₄Cl is present in the bath as in the present invention, the following two reactions:



simultaneously take place at the cathode interface, probably reducing the thickness of the zinc hydroxide barrier layer. Tnen, deposition of nickel is correspondingly facilitated so that deposits with increased nickel contents will form.

[0011] Similarly, the additional chloride in the plating bath has an influence on the appearance of Zn-Ni alloy deposits in the following way. There will be readily formed hydrated oxides and analogues on Zn-Ni alloy deposits in the presence of Cl^- ions, to which OH^- ions at the cathode interface contribute. If a chloride such as ammonium chloride NH₄Cl is present in the bath, OH^- ions are removed through the f_Jllowing reaction:

so that formation of colored hydrated oxides is precluded, resulting in aesthetic Zn-Ni alloy deposits having metallic gloss.

BRIEF DESCRIPTION OF DRAWING

[0012] 

Fig. 1 diagramatically shows how the nickel content of Zn-Ni alloy deposits varies with the concentration of ammonium chloride added to the plating solution at varying molar concentration ratios of Ni²⁺/(Ni²⁺ + Zn²⁺); and

Fig. 2 diagramatically shows the concentration of the chloride added vs. the molar concentration ratio of Ni^2;/ (Ni²⁺ + Zn²⁺), demonstrating their limits allowing the nickel content of Zn-Ni alloy deposits to fall within the optimum range.

BEST MODE FOR CARRYING OUT INVENTION

[0013] .Examples of the Zn-Ni alloy electroplating solution according to the present invention are given below by way of illustration.

Example 1

[0014] Plating baths were prepared by adding ammonium chloride NH₄Cl in amounts varying from 0 to 7.0 mols/liter (0 to 375 g/liter) to a solution containing 136 g/liter (1 mol/liter) of zinc chloride ZnCl₂ and 238 g/liter (1 mol/liter) of nickel chloride hexahydrate NiCl₂· 6H₂O and having a molar concentration of Ni²⁺ adjusted to 50%. Using these plating baths, electroplating was carried out at a current density of 75 _A/dm² so that the built-up of Zn-Ni alloy deposited might be 20 g/m².

[0015] The resulting deposits were determined for gloss and nickel content. The gloss and nickel content for each amount of ammonium chloride added are shown in Table 1. The data of Table 1 reveal that colored deposits having poor appearance are formed in the absence of ammonium chloride, but the gloss and appearance of deposits are improved when ammonium chloride is added in amounts of 0.2 mols/liter or more. On the other hand, it is evident that the nickel content of deposits increases with the increasing amounts of ammonium chloride added and goes beyond the optimum range of 10% to 20% by weight when the amount of ammonium chloride exceeds 5 mols/liter.

[0016] It should be noted that the gloss is expressed in the Gs(60) value according to JIS Z 8741, and the appearance is evaluated using the following criterion:

0: metallic gloss, untarnished, uniform

Δ: somewhat tarnished or nonuniform

X: tarnished

Example 2

[0017] Plating baths were prepared by adding ammonium chloride (NH₄Cl) in amounts varying from 0.2 to 7.0 mols/liter to solutions containing varying percentage molar concentrations of zinc chloride ZnCl₂ and nickel chloride hexahydrate NiCl₂·6H₂O. Using these baths, electroplating was effected at a current density of 75 A/dm² so that the build-up of Zn-Ni alloy deposited might be 2₀ g/_m².

[0018] Fig. 1 shows the nickel content of deposits plotted relative to the concentration of ammonium chloride added for different molar concentration ratios of Ni²⁺/(Ni²⁺ + Zn²⁺). The resulting alloy deposits all appeared aesthetically good, but the nickel content increased as the concentration of ammonium chloride added is increased, indicating that the concentration of ammonium chloride should fall within a proper range depending on the molar concentration ratio of Ni²⁺/(_Ni²⁺ _{+ Zn}²⁺). This is more clearly shown in Fig. 2 where X axis represents the molar concentration ratio of Ni²⁺/(Ni²⁺ + Zn²⁺) as expressed in percent and Y axis represents the concentration of the chloride as expressed in mol/liter. Fig. 2 demonstrates that the optimum range is defined by the following three equations:





It should be noted that the upper limit of 7.0 mols/liter was imposed on the concentration of the chloride in this example because it is the saturation level.

Example 3

[0019] Using plating baths containing various chlorides, a Zn-Ni alloy was deposited on a steel sheet to a build-up of 20 g/m². The gloss, appearance, and nickel content of the resulting deposit for each bath composition were shown in Table 2. Run Nos. 1 to 10 and comparative run Nos. 11 and 12 in Table 2 are also plotted in Fig. 2 with the same numerical designation. In all of run Nos. 1 to 10, the deposits exhibited high gloss and satisfactory appearance and their nickel contents fell in the optimum range depicted as a cross-hatched region in Fig. 2.

[0020] As evident from these data, by using the chloride bath of the present invention, Zn-Ni alloy deposits can be readily and consistently formed having improved corrosion resistance, aesthetic appearance and a nickel content within the optimum range between 10% and 20% by weight.

Claims

1. A Zn-Ni alloy electroplating solution based on zinc chloride and nickel chloride, characterized in that the molar concentration ratio X expressed in percent of Ni²⁺/(Ni²⁺ + Zn²⁺) in the solution and the concentration Y expressed in mol/liter of an additional chloride added to the solution are controlled to fall within the range defined by the following equations:

2. The Zn-Ni alloy electroplating solution according to claim 1 wherein the additional chloride is at least one selected from the group consisting of AlCl₃, NH₄AlCl₄ NH₄Cl, KCl, CaCl₂, BaCl₂, NaCl, and MgCl₂.

Drawing