Electroless Nickel-Tin-Phosphorus Plating Solution

Abstract

To provide an electroless nickel-tin-phosphorus plating solution for forming an electroless nickel-tin-phosphorus plated film that contains high-concentration tin with improved plating bath stability.SOLUTION: The present invention adopts an electroless nickel-tin-phosphorus plating solution that contains nickel hydroxide (II) as a nickel source, tin (IV) salts as a tin source, hypophosphorous acid or hypophosphite as a reductant, and organic acids as a complexing agent. The ratio of tin (IV) salts to nickel hydroxide (II) ranges from 1: 1 to 1: 3 in terms of the mass ratio of tin to nickel. The plating bath has a pH of 10.0-12.0. The solution temperature during plating operation ranges from 80°C to 95°C.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an electroless nickel-tin-phosphorus plating solution and an electroless nickel-tin-phosphorus plating film formed using the same. In particular, the present invention relates to an electroless nickel-tin-phosphorus plating solution for forming an electroless nickel-tin-phosphorus plating film containing a high concentration of tin. [Background technology]

[0002] Nickel-tin alloy plating films containing a high concentration of tin have excellent corrosion resistance and a glossy appearance with an elegant color tone, and therefore are used in a wide range of fields such as electronic parts and automobile parts for the purpose of corrosion prevention and decoration. In particular, films containing 50 mass% or more of tin in the film have extremely high corrosion resistance and are industrially useful. However, at present, nickel-tin alloy plating films containing a high concentration of tin are mainly formed by electroplating, and those formed by electroless plating, which is good at forming films on plated objects having complex shapes, have not been put to practical use. The reason for this is that the plating films formed by electroless plating usually have a low tin content in the film and a small film thickness, so sufficient corrosion resistance cannot be obtained.

[0003] In response to this problem, the present applicant has succeeded in developing an electroless nickel-tin-phosphorus plating solution capable of stably obtaining a nickel-tin-phosphorus plating film, which is a nickel-tin alloy plating film containing 40 wt% or more of tin, by controlling the ratio of a water-soluble nickel salt, which is a nickel source, and a water-soluble tin salt, which is a tin source, as the invention of Patent Document 1. Specifically, the electroless nickel-tin-phosphorus plating solution disclosed in Patent Document 1 "contains a water-soluble nickel salt, a water-soluble tin salt, hypophosphorous acid, a first complexing agent, and a second complexing agent, and the ratio of the water-soluble tin salt to the water-soluble nickel salt is 1.5 to 3." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2022-73254 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the electroless nickel-tin-phosphorus plating solution disclosed in Patent Document 1 still has room for improvement in terms of plating bath stability and tin content in the film. Therefore, there has been a demand in the market for an electroless nickel-tin-phosphorus plating solution for forming an electroless nickel-tin-phosphorus plating film containing a high concentration of tin, which has further improved plating bath stability and tin content in the film. [Means for solving the problem]

[0006] As a result of intensive research, the inventors of the present application have achieved the above-mentioned object by adopting an electroless nickel-tin-phosphorus plating solution having the following requirements and an electroless nickel-tin-phosphorus plating film formed using this electroless nickel-tin-phosphorus plating solution.

[0007] A. Electroless nickel-tin-phosphorus plating solution The electroless nickel-tin-phosphorus plating solution according to the present application is for forming an electroless nickel-tin-phosphorus plating film containing tin at a high concentration, and is characterized in that it contains nickel(II) hydroxide as a nickel source, a stannate(IV) as a tin source, hypophosphorous acid or a hypophosphite as a reducing agent, and an organic acid as a complexing agent, the ratio of the stannate(IV) to the nickel(II) hydroxide being 1:1 to 1:3 in terms of the mass ratio of nickel to tin, the pH of the plating bath being 10.0 to 12.0, and the solution temperature during plating operation being 80°C to 95°C.

[0008] In the electroless nickel-tin-phosphorus plating solution according to the present application, the complexing agent is preferably at least one selected from the group consisting of citric acid and its salts, gluconic acid and its salts, acetic acid and its salts, propionic acid and its salts, succinic acid and its salts, glycolic acid and its salts, lactic acid and its salts, and malic acid and its salts.

[0009] B. Electroless nickel-tin-phosphorus plating film The electroless nickel-tin-phosphorus plating film according to the present application is formed using the electroless nickel-tin-phosphorus plating solution according to the present application described above, and is characterized in that it has a tin content of 50 mass% or more.

[0010] The electroless nickel-tin-phosphorus plating film according to the present application preferably has a tin content of 50% by mass to 60% by mass.

[0011] The electroless nickel-tin-phosphorus plating film according to the present application contains a metastable phase having a composition of Ni3Sn2, and the average crystallite size is preferably 10 nm to 20 nm. Effect of the Invention

[0012] The electroless nickel-tin-phosphorus plating solution according to the present application has an extremely high plating bath stability, and can stably and continuously deposit an electroless nickel-tin-phosphorus plating film containing a high concentration of tin on the surface of a plated object. In addition, since the deposition rate of the film is industrially significant, the electroless nickel-tin-phosphorus plating solution according to the present application is a practical solution that satisfies market demands. The electroless nickel-tin-phosphorus plating film formed with the electroless nickel-tin-phosphorus plating solution according to the present application contains 50 mass % or more of tin, has a dense film structure, and is excellent in corrosion resistance. [Brief description of the drawings]

[0013] [Figure 1]1 is a graph showing the tin content in an electroless nickel-tin-phosphorus plating film versus the plating bath pH of an electroless nickel-tin-phosphorus plating solution in Examples and Comparative Examples according to the present application. [Diagram 2] 1 is a graph showing the tin content in an electroless nickel-tin-phosphorus plating film versus the solution temperature of an electroless nickel-tin-phosphorus plating solution in Examples and Comparative Examples according to the present application. [Diagram 3] 1 is a graph showing the tin content in an electroless nickel-tin-phosphorus plating film versus the number of plating runs in an example according to the present application. [Figure 4] 1 is a graph showing the results of electrochemical measurements of an example according to the present application, a comparative example, and an iron substrate used as an evaluation standard. [Diagram 5] (A) and (B) are examples according to the present application, (C) and (D) are comparative examples, and (E) and (F) are photographs of the appearance of an iron substrate, which is the evaluation standard, before and after a nitric acid aeration test. [Figure 6] 1 is a graph showing X-ray diffraction patterns of electroless nickel-tin-phosphorus plating films immediately after deposition on an iron substrate, which is an evaluation standard, and an example and comparative example of the present application. [Figure 7] 1 is a graph showing X-ray diffraction patterns of electroless nickel-tin-phosphorus plating films after heat treatment at 450° C. on an iron substrate used as an evaluation standard and in examples and comparative examples according to the present application. [Figure 8] 1 is a graph showing the results of thermal analysis in an example according to the present application. [Figure 9] 1 is a graph showing the results of thermal analysis in a comparative example according to the present application. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A. Electroless nickel-tin-phosphorus plating solution The electroless nickel-tin-phosphorus plating solution according to the present application is for forming an electroless nickel-tin-phosphorus plating film containing tin at a high concentration. This electroless nickel-tin-phosphorus plating solution contains nickel hydroxide (II) as a nickel source, a stannate (IV) as a tin source, hypophosphorous acid or hypophosphite as a reducing agent, and an organic acid as a complexing agent, and the ratio of the stannate (IV) to the nickel hydroxide (II) is 1:1 to 1:3 in terms of the mass ratio of nickel to tin. The plating bath has a pH of 10.0 to 12.0, and is used at a solution temperature of 80°C to 95°C during plating operation.

[0015] The electroless nickel-tin-phosphorus plating solution according to the present application has an extremely high plating bath stability, and by satisfying the above-mentioned requirements, an electroless nickel-tin-phosphorus plating film containing a high concentration of tin can be stably and continuously deposited on the surface of the plated object. In addition, since the deposition rate of the film is industrially significant, the electroless nickel-tin-phosphorus plating solution according to the present application is a practical solution that satisfies market demands. Each component contained in the electroless nickel-tin-phosphorus plating solution according to the present application will be described below.

[0016] Nickel source and tin source: In the electroless nickel-tin-phosphorus plating solution according to the present application, nickel hydroxide (II) is used as the nickel source, and stannate (IV) is used as the tin source. Examples of stannate (IV) include sodium stannate (IV) (sodium metastannate) and potassium stannate (IV) (potassium metastannate).

[0017] Here, the technical concept of the invention according to the present application will be outlined. In conventional electroless nickel-tin-phosphorus plating solutions, water-soluble nickel salts such as nickel sulfate and nickel chloride are mainly used as nickel sources. Also, water-soluble tin salts such as stannic chloride are mainly used as tin sources. However, sulfate ions (SO4 2- ) or chloride ion (Cl -), except when some of the conditions disclosed in the above-mentioned Patent Document 1 are adopted, the tin content in the film deposited on the surface of the plated object tends to be low, and an electroless nickel-tin-phosphorus plating film with good corrosion resistance cannot be obtained. The reason for this is that in the vicinity of the reaction interface where the electroless nickel-tin-phosphorus plating film is deposited, these anion species react with tin ions and Sn(HSO4) 3+ , Sn(HSO4)4, SnCl 5- It is presumed that this is because, as a result of these forming tin associations consisting of the above-mentioned anions, and these tin associations diffusing (dispersing away from the reaction interface) in the plating bath, the amount of tin source participating in the deposition reaction of the film at the reaction interface where the film is deposited becomes insufficient. Therefore, in the electroless nickel-tin-phosphorus plating solution according to the present application, nickel hydroxide (II) and stannate (IV), which do not contain these anion species, are used as the nickel source and the tin source.

[0018] In the electroless nickel-tin-phosphorus plating solution according to the present application, the preferred ratio of the tin source, stannate (IV) to the nickel source, nickel hydroxide (II), is 1:1 to 1:3 in terms of the mass ratio of nickel to tin. Here, if the ratio of the tin source, stannate (IV) to the nickel source, nickel hydroxide (II), is less than 1:1 in terms of the mass ratio of tin to nickel, the tin content in the electroless nickel-tin-phosphorus plating film deposited on the surface of the plated object tends to be relatively low, less than 50 mass%. On the other hand, if the ratio of the tin source, stannate (IV) to the nickel source, nickel hydroxide (II), is more than 1:3 in terms of the mass ratio of tin to nickel, the nickel content in the electroless nickel-tin-phosphorus plating film deposited on the surface of the plated object tends to be relatively low, and the hardness of the film tends to be low.

[0019] Reducing agent: In the electroless nickel-tin-phosphorus plating solution according to the present application, hypophosphorous acid or a hypophosphite is used as a reducing agent. Examples of the hypophosphite include sodium hypophosphite and potassium hypophosphite.

[0020] Complexing agent: In the electroless nickel-tin-phosphorus plating solution according to the present application, an organic acid is used as a complexing agent. Examples of the organic acid include citric acid and its salts, gluconic acid and its salts, acetic acid and its salts, propionic acid and its salts, succinic acid and its salts, glycolic acid and its salts, lactic acid and its salts, and malic acid and its salts. Nickel (II) hydroxide, which is the nickel source described above, is sparingly soluble and has a low solubility in water, but is easily soluble in an acidic solution. Therefore, it is preferable to use one or more of these organic acids as a complexing agent, form a nickel complex with the nickel source and the complexing agent in water as a solvent, stabilize the nickel complex, and then adjust the plating bath to a predetermined pH, which will be described later. Here, there is no particular restriction on the type of organic acid, but it is more preferable to use citric acid or its salt as a complexing agent for nickel (II) hydroxide, which is the nickel source, and gluconic acid or its salt as a complexing agent for stannate, which is the tin source, because this tends to improve the stability of the electroless nickel-tin-phosphorus plating solution.

[0021] Other components constituting the electroless nickel-tin-phosphorus plating solution: In addition to the above-mentioned components, the electroless nickel-tin-phosphorus plating solution according to the present application may contain a stabilizer such as bismuth and a pH adjuster such as sodium hydroxide. If the electroless nickel-tin-phosphorus plating solution contains a stabilizer such as bismuth, the stability of the plating bath tends to be further improved, which is preferable.

[0022] pH of plating bath: In the electroless nickel-tin-phosphorus plating solution according to the present application, the preferred pH of the plating bath is 10.0 to 12.0. If the pH of the plating bath is less than 10.0, the tin content in the electroless nickel-tin-phosphorus plating film deposited on the surface of the plated object tends to be relatively low, less than 50 mass %. On the other hand, if the pH of the plating bath exceeds 12.0, the stability of the plating bath decreases and bath decomposition tends to occur.

[0023] Solution temperature during plating operation: In the electroless nickel-tin-phosphorus plating solution according to the present application, the suitable solution temperature during plating operation is 80°C to 95°C. Here, if the solution temperature during plating operation is less than 80°C, the tin content in the electroless nickel-tin-phosphorus plating film deposited on the surface of the plated object tends to be relatively low, less than 50 mass %. On the other hand, if the solution temperature during plating operation exceeds 95°C, the stability of the plating bath tends to decrease because it approaches the boiling point of water, which is the solvent.

[0024] B. Electroless nickel-tin-phosphorus plating film The electroless nickel-tin-phosphorus plating film according to the present application is formed using the electroless nickel-tin-phosphorus plating solution according to the present application. This electroless nickel-tin-phosphorus plating film is characterized by having a tin content of 50 mass% or more. By satisfying these requirements, the electroless nickel-tin-phosphorus plating film according to the present application has extremely excellent corrosion resistance.

[0025] The tin content in the electroless nickel-tin-phosphorus plating film is preferably 50% by mass to 60% by mass. If the tin content in the electroless nickel-tin-phosphorus plating film is less than 50% by mass, the corrosion resistance of the film tends to decrease, which is not preferred. On the other hand, even if the tin content in the electroless nickel-tin-phosphorus plating film exceeds 60% by mass, no problems arise in the corrosion resistance of the film, but the deposition of the film becomes unstable and the wear resistance of the film tends to decrease, which is not preferred.

[0026] In addition, the electroless nickel-tin-phosphorus plating film according to the present application contains a metastable phase having a composition of Ni3Sn2, and preferably has an average crystallite size of 10 nm to 20 nm. By satisfying this condition, the electroless nickel-tin-phosphorus plating film according to the present application becomes dense and has excellent corrosion resistance.

[0027] The invention according to the present application will be described in more detail below with reference to examples, but the invention according to the present application is not limited to these examples. EXAMPLES

[0028] In Example 1, first, an iron substrate (Hull Cell cathode iron plate manufactured by Yamamoto Plating Tester Co., Ltd.) having a width of 10 mm, a length of 25 mm, and a thickness of 0.3 mm was prepared as a test substrate. Next, the iron substrate was immersed in a 10% by mass aqueous solution of sodium hydroxide kept at 60° C. for 3 minutes to degrease the surface, and then pre-treated by immersing in a 10% by mass aqueous solution of sulfuric acid at room temperature for 1 minute to activate the surface. This was left for a predetermined time in an electroless nickel-tin-phosphorus plating solution having the composition shown in Table 1, and then washed with water to obtain an iron substrate with an electroless nickel-tin-phosphorus plating film. In each evaluation test described later, the plating bath pH during plating operation was adjusted to 8.0 to 13.0, and the solution temperature was adjusted to 60° C. to 90° C., respectively. In addition, the plating operation time (the time the test substrate was left in the electroless nickel-tin-phosphorus plating solution) was set to 20 minutes to 50 minutes, respectively.

[0029] [Table 1]

[0030] (Evaluation test for the tin content in electroless nickel-tin-phosphorus plating film) Table 2 shows the tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate. In this test, the plating bath pH of the electroless nickel-tin-phosphorus plating solution was adjusted to 10.0, the solution temperature was adjusted to 80°C, and the plating operation was carried out for 20 minutes. The tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate was measured using a field emission electron beam microanalyzer (JXA-8500F manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 1.2 x 10 -7 A, the measurement was performed under the condition of a beam diameter of 200 μm. As shown in Table 2, the tin content in the film on the iron substrate with the electroless nickel-tin-phosphorus plating film obtained by the above-mentioned method was a high concentration of 50.9 mass %.

[0031] (Evaluation test for plating bath pH) FIG. 1 shows a graph of the tin content in the electroless nickel-tin-phosphorus plating film versus the plating bath pH. In this test, the temperature of the electroless nickel-tin-phosphorus plating solution was adjusted to 80° C., and plating was performed for 20 minutes. The tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate was measured under the same conditions using a field emission electron probe microanalyzer as in the above test. As shown in FIG. 1, when the pH value of the electroless nickel-tin-phosphorus plating solution was increased, the tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate increased. In particular, when the plating bath pH was adjusted to 10.0 or more and plating was performed, the tin content in the electroless nickel-tin-phosphorus plating film was highly concentrated at 50 mass% or more. On the other hand, when the electroless nickel-tin-phosphorus plating solution was adjusted so that the pH value of the plating bath was 13.0, bath decomposition occurred, and the electroless nickel-tin-phosphorus plating film was not deposited on the surface of the iron substrate.

[0032] (Evaluation test for liquid temperature) FIG. 2 shows a graph of the tin content in the electroless nickel-tin-phosphorus plating film versus the temperature of the electroless nickel-tin-phosphorus plating solution. In this test, the plating bath pH of the electroless nickel-tin-phosphorus plating solution was adjusted to 10.0, and plating was performed for 20 minutes. The tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate was measured under the same conditions using a field emission electron probe microanalyzer as in the above test. When the temperature of the electroless nickel-tin-phosphorus plating solution was increased, the tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate increased. In particular, when plating was performed with the solution temperature adjusted to 80° C. or higher, the tin content in the electroless nickel-tin-phosphorus plating film reached a high concentration of 50 mass% or more.

[0033] (Evaluation test for plating bath stability) Figure 3 shows a graph of the tin content in the electroless nickel-tin-phosphorus plating film versus the number of plating runs. In this test, the plating bath pH of the electroless nickel-tin-phosphorus plating solution was adjusted to 10.0, the solution temperature was adjusted to 90°C, and each plating run was performed for 20 minutes. The iron substrate, which was the test substrate, was replaced with a new one for each plating run, and the plating bath pH was readjusted to a specified value every five plating runs. The tin content in the electroless nickel-tin-phosphorus plating film formed on the iron substrate was measured under the same conditions using a field emission electron beam microanalyzer as in the above test. The deposition rate of the electroless nickel-tin-phosphorus plating film on the iron substrate was calculated by gravimetric method.

[0034] Here, the method of calculating the deposition rate of the film will be specifically described. In this test, first, the "contents (mass%) of nickel, tin, and phosphorus" in the electroless nickel-tin-phosphorus plating film formed on the iron substrate were measured under the same conditions as those described above using the above-mentioned field emission electron microanalyzer, and the film density was calculated from the specific gravity of these elements. Next, the weight difference of the iron substrate before and after each plating operation was determined using an analytical electronic balance (GR202 manufactured by A&D Co., Ltd.), and the film thickness and deposition rate of the film were calculated from the above-mentioned film density, the surface area of ​​the iron substrate to be plated, and the plating operation time. As shown in FIG. 3, the electroless nickel-tin-phosphorus plating solution according to the present application has extremely high plating bath stability, is capable of continuous plating operation, and has an industrially significant film deposition rate.

[0035] (Evaluation test for the corrosion resistance of the coating) Figure 4 shows the results of electrochemical measurements (LSV measurements) of the electroless nickel-tin-phosphorus plating film formed on the iron substrate. The plating operation conditions for obtaining the iron substrate with the electroless nickel-tin-phosphorus plating film used in this test were a plating bath pH of 10.0, a solution temperature of 80°C, and a plating operation time of 50 minutes. The thickness of the electroless nickel-tin-phosphorus plating film calculated by the gravimetric method was approximately 5 μm. A three-electrode cell was used for this electrochemical measurement, with the working electrode being the "electroless nickel-tin-phosphorus plating film on the iron substrate obtained by the above method, immersed in a 0.5 mol / L sulfuric acid aqueous solution," the counter electrode being a "platinum-coated titanium electrode immersed in a 0.5 mol / L sulfuric acid aqueous solution," and the reference electrode being a "silver / silver chloride electrode immersed in a saturated aqueous solution of potassium chloride." For the measurements, an electrochemical measurement system (HZ-7000 manufactured by Hokuto Denko Corporation) was used, the temperature of each electrolyte was adjusted to 25° C., and the potential was scanned from the natural potential at a sweep rate of 2 mV / sec while the electrolyte was left standing.

[0036] Here, FIG. 4 also shows the results of a similar test using an iron substrate (Hull Cell cathode iron plate manufactured by Yamamoto Plating Tester Co., Ltd., width 10 mm, length 25 mm, thickness 0.3 mm) immersed in a 0.5 mol / L sulfuric acid aqueous solution as the evaluation standard. The iron substrate, which was the reference standard, showed an increase in current density at the same time as the potential scanning started, and dissolution (corrosion) progressed from the surface in the sulfuric acid aqueous solution, which was the electrolyte. On the other hand, the electroless nickel-tin-phosphorus plating film on the iron substrate obtained by the above-mentioned method started to show an increase in current density at a potential of about 0.75 V, then decreased and became passivated at a potential of about 1.5 V to 1.65 V. Therefore, it can be inferred that a passive film was formed on the surface of the electroless nickel-tin-phosphorus plating film at these potentials.

[0037] Next, the iron substrate with the electroless nickel-tin-phosphorus plating film obtained by the above method and the iron substrate as the evaluation standard were subjected to a nitric acid aeration test according to the method specified in JIS H 8620. The plating operation conditions for obtaining the iron substrate with the electroless nickel-tin-phosphorus plating film used in this test were a plating bath pH of 10.0, a liquid temperature of 80°C, and a plating operation time of 50 minutes. Specifically, a 60% by mass aqueous solution of nitric acid was poured into a container in the atmosphere at room temperature of 25°C, and the container was placed in a desiccator, sealed, and left for 1 hour. After that, the iron substrate with the electroless nickel-tin-phosphorus plating film and the iron substrate as the evaluation standard were placed in the desiccator, the desiccator was sealed again, and left for 72 hours. Then, they were taken out of the desiccator, and the presence or absence of corrosion was confirmed by visual inspection and weight method.

[0038] Figure 5 shows photographs of the appearance of the iron substrate with electroless nickel-tin-phosphorus plating film before (A) and after (B) the test, and photographs of the appearance of the iron substrate, which was the evaluation standard, before (E) and after (F) the test. The electroless nickel-tin-phosphorus plating film formed on the iron substrate was hardly corroded by nitric acid gas, and the amount of dissolution of the film calculated from the weight difference of the test piece (iron substrate with electroless nickel-tin-phosphorus plating film) before and after the test was 0.4 mg / cm. 2 On the other hand, the iron substrate, which was the evaluation standard, was significantly corroded on the surface by this test, with the amount of dissolution being 9.5 mg / cm 2 (The average thickness was about 12 μm).

[0039] (Evaluation test for film structure: X-ray diffraction) 6 and 7 show the X-ray diffraction patterns of the electroless nickel-tin-phosphorus plating film formed on the iron substrate and the iron substrate as the evaluation standard immediately after the deposition of the film (i.e., the state in which the electroless nickel-tin-phosphorus plating process was performed) and after the heat treatment at 450 ° C., respectively. The plating operation conditions for obtaining the iron substrate with the electroless nickel-tin-phosphorus plating film used in this test were a plating bath pH of 10.0, a solution temperature of 80 ° C., and a plating operation time of 20 minutes. The thickness of the electroless nickel-tin-phosphorus plating film calculated by the gravimetric method was about 2 μm. Here, in this test, an X-ray diffractometer (RINT-2200 manufactured by Rigaku Corporation) was used, and the X-ray output was set to 40 kV and 20 mA, and the wavelength was CuKα radiation, and the measurement was performed under the conditions of a step width of 0.02 ° and a scan axis 2θ / θ. A programmable electric furnace (SMF-2 manufactured by AS ONE Corporation) was used for the heat treatment. Specifically, the test pieces (iron substrate with electroless nickel-tin-phosphorus plating film and the iron substrate used as the evaluation standard) were heat-treated at 450°C for 1 hour in air, and then, in order to avoid thermal shock, the test pieces were left stationary in the furnace until the temperature inside the furnace dropped to 150°C, after which each test piece was removed from the furnace and allowed to cool.

[0040] As shown in Figure 6, the X-ray diffraction pattern of the electroless nickel-tin-phosphorus plating film on the iron substrate immediately after deposition obtained by the above-mentioned method mainly showed peaks due to the crystal structure consisting of Ni3Sn2. On the other hand, as shown in Figure 7, for the film after heat treatment, the peaks in the X-ray diffraction pattern due to the crystal structure consisting of Ni3Sn2 almost disappeared. Therefore, it can be inferred that the electroless nickel-tin-phosphorus plating film immediately after deposition (i.e., after electroless nickel-tin-phosphorus plating treatment) has a crystal structure consisting of Ni3Sn2, and that this crystal structure consisting of Ni3Sn2 is a metastable phase.

[0041] Next, based on these X-ray diffraction patterns, the average crystallite diameter was calculated immediately after deposition of the film (i.e., in the state as it was after electroless nickel-tin-phosphorus plating treatment) and after heat treatment at 450°C. The results are shown in Table 3. The average crystallite diameter of the electroless nickel-tin-phosphorus plating film obtained by the above-mentioned method was extremely small, generally within the range of 10 nm to 20 nm. Therefore, it can be inferred that the electroless nickel-tin-phosphorus plating film formed using the electroless nickel-tin-phosphorus plating solution according to the present application has a dense structure. EXAMPLES

[0042] In Example 2, the content of potassium stannate (IV) trihydrate, which is the tin source, was changed to 0.10 mol / L (1:2 mass ratio of nickel to tin), and an "evaluation test for the tin content in the electroless nickel-tin-phosphorus plating film" was carried out in the same manner as in Example 1. Note that, since the conditions other than the content of the tin source were the same as those in the evaluation test in Example 1, the plating operation conditions and test method will not be described. The test results of Example 2 are shown in Table 2. EXAMPLES

[0043] In Example 3, the content of potassium stannate (IV) trihydrate, which is the tin source, was changed to 0.15 mol / L (mass ratio of nickel to tin: 1:3), and an "evaluation test for the tin content in the electroless nickel-tin-phosphorus plating film" was carried out in the same manner as in Example 1. Note that, since the conditions other than the content of the tin source were the same as those in the evaluation test in Example 1, the plating operation conditions and the test method will not be described. The test results of Example 3 are shown in Table 2. EXAMPLES

[0044] In Example 4, a copper substrate having a width of 10 mm, a length of 40 mm, and a thickness of 0.3 mm was prepared as a test substrate. The copper substrate was then immersed in a 10% by mass aqueous sodium hydroxide solution kept at 60° C. for 3 minutes to degrease the surface, and then immersed in a 10% by mass aqueous sulfuric acid solution at room temperature for 1 minute to activate the surface, and further immersed in a 0.01% by mass aqueous palladium chloride solution at room temperature for 30 seconds to apply a palladium catalyst to the surface as a pretreatment. The copper substrate was left to stand for 50 minutes in an electroless nickel-tin-phosphorus plating solution having the composition shown in Table 1, and then washed with water to obtain a copper substrate with an electroless nickel-tin-phosphorus plating film. The plating bath pH and solution temperature during plating operation were adjusted to 10.0 and 80° C., respectively.

[0045] (Evaluation test for film structure: thermal analysis) 8 shows the results of thermal analysis (also called TG-DTA, TGA-DTA, thermogravimetry-differential thermal analysis, etc.) performed on a powdered electroless nickel-tin-phosphorus plating film formed on a copper substrate. In this test, the copper substrate with the electroless nickel-tin-phosphorus plating film of Example 3 was first immersed in a 20 mass% nitric acid aqueous solution kept at 60°C, and the copper substrate was dissolved in the nitric acid aqueous solution, thereby peeling the copper substrate from the film. Next, the electroless nickel-tin-phosphorus plating film was recovered from the nitric acid aqueous solution, washed with water, dried, and then pulverized into a powder using a mortar and pestle. 10 mg of this was taken and placed in an aluminum oxide container, and then placed in a thermal analysis system (a differential thermobalance, TG-DTA8122, manufactured by Rigaku Corporation). Thermal analysis of the powdered electroless nickel-tin-phosphorus plating film was performed by heating from room temperature to 1200°C under nitrogen gas flow (0.3 L / min) at a heating rate of 10°C / min.

[0046] As shown in FIG. 8, even when the powdered electroless nickel-tin-phosphorus plating film was heated from room temperature to 1200°C, the mass did not change significantly (see TG in FIG. 8). Meanwhile, the DTA curve rose gradually from 70°C to 350°C (meaning that an exothermic reaction occurred in the powdered film sample), and then fell. A clear downward peak (endothermic peak) appeared at 1171°C in the DTA curve (see DTA in FIG. 8). Here, according to the existing thermal equilibrium diagram, it can be inferred that this endothermic peak at 1171°C is due to the crystal structure of Ni3Sn2 that has transitioned from a metastable phase to an equilibrium stable phase. It can be inferred that this phase transition to the equilibrium stable phase of Ni3Sn2 has occurred due to heating in the thermal analysis. Therefore, it can be inferred that the electroless nickel-tin-phosphorus coating formed on the copper substrate is not a completely amorphous structure, and that a metastable phase of Ni3Sn2 is partially present in the coating. This supports the results of the "Evaluation test for the film structure: X-ray diffraction" in Example 1. Comparative Example

[0047] [Comparative Example 1] In Comparative Example 1, the content of potassium stannate (IV) trihydrate, which is the tin source, was changed to 0.02 mol / L (1:0.4 in terms of the mass ratio of nickel to tin), and an "evaluation test for the tin content in the electroless nickel-tin-phosphorus plating film" was carried out in the same manner as in Example 1. Note that, since the conditions other than the content of the tin source were the same as those in the evaluation test in Example 1, the plating operation conditions and the test method will not be described. The test results of Comparative Example 1 are shown in Table 2.

[0048] [Comparative Example 2] In Comparative Example 2, the content of potassium stannate (IV) trihydrate, which is the tin source, was changed to 0.04 mol / L (1:0.8 in terms of the mass ratio of nickel to tin), and an "evaluation test for the tin content in the electroless nickel-tin-phosphorus plating film" was carried out in the same manner as in Example 1. Note that, since the conditions other than the content of the tin source were the same as those in the evaluation test in Example 1, the plating operation conditions and the test method will not be described. The test results of Comparative Example 2 are shown in Table 2.

[0049] [Comparative Example 3] In Comparative Example 3, the nickel source was changed from nickel hydroxide (II) to nickel sulfate (II) hexahydrate (NiSO4·6H2O), and the following tests were performed in the same manner as in Example 1: "Evaluation test for the tin content in the electroless nickel-tin-phosphorus plating film", "Evaluation test for the plating bath pH", "Evaluation test for the liquid temperature", "Evaluation test for the corrosion resistance of the film", and "Evaluation test for the film structure: X-ray diffraction". Since the conditions other than the components of the nickel source (compounds used as the nickel source) were the same as those in each test in Example 1, the plating operation conditions and each test method are omitted. The test results of Comparative Example 3 are shown in Tables 2 and 3, and in Figures 1, 2, 4, 5 (C) and (D), 6, and 7, respectively. In Figure 5, (C) is a photograph of the appearance of the iron substrate with the electroless nickel-tin-phosphorus plating film before the nitric acid aeration test, and (D) is a photograph of the appearance after the test. As shown in FIG. 5(D), the electroless nickel-tin-phosphorus plating film of Comparative Example 3 formed on an iron substrate was corroded at the end in this test, and the amount of dissolution was 1.5 mg / cm. 2 As shown in Table 3, the average crystallite diameter of the electroless nickel-tin-phosphorus plating film of Comparative Example 3 formed on the iron substrate was 27.6 nm immediately after deposition of the film (i.e., in the state after the electroless nickel-tin-phosphorus plating treatment), which was relatively large.

[0050] [Comparative Example 4] In Comparative Example 4, the nickel source was changed from nickel hydroxide (II) to nickel sulfate (II) hexahydrate (NiSO4·6H2O), and the "Evaluation test for the structure of the coating: thermal analysis" was carried out in the same manner as in Example 4. Since the conditions other than the components of the nickel source (compounds used as the nickel source) were the same as those in the evaluation test in Example 4, the plating operation conditions and the test method are omitted. The test results of this Comparative Example 4 are shown in FIG. 9. As shown in FIG. 9, downward peaks (endothermic peaks) appeared near 911°C and 1176°C in the DTA curve of the powdered electroless nickel-tin-phosphorus plating coating of Comparative Example 4. According to existing thermal equilibrium diagrams, it can be inferred that these endothermic peaks near 911°C and 1176°C are due to the crystal structures of Ni3Sn and Ni3Sn2 that have transitioned from a metastable phase to an equilibrium stable phase, respectively. Here, it is presumed that the phase transition to the equilibrium stable phase of Ni3Sn and Ni3Sn2 occurred due to heating during the thermal analysis. Therefore, the test results of this Comparative Example 4 support the results of the "Evaluation test on the coating structure: X-ray diffraction" in Comparative Example 3.

[0051] [Table 2]

[0052] [Table 3]

[0053] (Comparison between Examples and Comparative Examples) The electroless nickel-tin-phosphorus plating solutions of Examples 1 to 3, which contain nickel hydroxide (II) as a nickel source, stannate (IV) as a tin source, hypophosphorous acid or hypophosphite as a reducing agent, and an organic acid as a complexing agent, and in which the ratio of stannate (IV) to nickel hydroxide (II) is within the range of 1:1 to 1:3 in terms of the mass ratio of nickel to tin, had a high tin content of 50 mass% or more in the electroless nickel-tin-phosphorus plating film formed on an iron substrate using the plating solutions. As can be seen from the test results of Example 1, with regard to the plating bath pH and solution temperature, by performing plating operation under conditions of a plating bath pH of 10.0 to 12.0 and a solution temperature of 80°C to 95°C, an electroless nickel-tin-phosphorus plating film with a tin content of 50 mass% or more could be formed on an iron substrate. In addition, according to the test results of Example 1, the electroless nickel-tin-phosphorus plating film formed using the electroless nickel-tin-phosphorus plating solution satisfying these requirements had a dense structure and excellent corrosion resistance.

[0054] On the other hand, the electroless nickel-tin-phosphorus plating solutions of Comparative Examples 1 and 2, which contained nickel hydroxide (II) as the nickel source, stannate (IV) as the tin source, hypophosphorous acid or hypophosphite as the reducing agent, and an organic acid as a complexing agent, and which had a ratio of stannate (IV) to nickel hydroxide outside the range of 1:1 to 1:3 in terms of the mass ratio of nickel to tin, had a tin content of less than 50 mass% in the electroless nickel-tin-phosphorus plating film formed on an iron substrate using the plating solution, even when the plating bath pH and solution temperature were adjusted to suitable values.

[0055] In addition, even when the ratio of the tin source to the nickel source was within the range of 1:1 to 1:3 as the mass ratio of nickel to tin and the plating bath pH and solution temperature were adjusted to suitable values, the electroless nickel-tin-phosphorus plating solution of Comparative Example 3, which used nickel sulfate (II) hexahydrate instead of nickel hydroxide (II) as the nickel source, had a tin content of less than 50 mass% in the electroless nickel-tin-phosphorus plating film formed on the iron substrate using the plating solution. The electroless nickel-tin-phosphorus plating solution of Comparative Example 3 had a relatively low plating bath stability, and when the solution temperature was adjusted to 90°C, bath decomposition occurred, and the electroless nickel-tin-phosphorus plating film was not deposited on the surface of the iron substrate (see FIG. 2). Furthermore, according to the test results of Comparative Example 3, the electroless nickel-tin-phosphorus plating film formed with the electroless nickel-tin-phosphorus plating solution of Comparative Example 3 was inferior in structure density and corrosion resistance to those of Example 1.

[0056] Here, the corrosion resistance of the films formed with the electroless nickel-tin-phosphorus plating solutions of Example 1 and Comparative Example 3 will be examined in more detail with reference to FIG. 4. In both of these electroless nickel-tin-phosphorus plating films, the current density began to increase at a potential of about 0.75 V, and then decreased and became passivated. However, the rate at which the current density increased was slower in Example 1 than in Comparative Example 3. Therefore, it can be inferred that the electroless nickel-tin-phosphorus plating film obtained in Example 1 corrodes more slowly than that in Comparative Example 3, even in an extremely harsh environment. [Industrial Applicability]

[0057] The electroless nickel-tin-phosphorus plating solution according to the present application has an extremely high plating bath stability and can stably and continuously form an electroless nickel-tin-phosphorus plating film containing a high concentration of tin on the surface of the plated object, and therefore can be applied in the fields of electronic parts, automobile parts, etc. for the purpose of corrosion prevention, decoration, etc. In particular, it can be suitably used for plated objects having a complex surface shape.

Claims

1. An electroless nickel-tin-phosphorus plating solution for forming an electroless nickel-tin-phosphorus plating film containing 50% by mass to 60% by mass of tin, The nickel hydroxide (II) is used as a nickel source, a stannate (IV) is used as a tin source, hypophosphorous acid or a hypophosphite is used as a reducing agent, and an organic acid is used as a complexing agent; The ratio of stannate to nickel(II) hydroxide is 1:1 to 1:3 in terms of the mass ratio of nickel to tin, The pH of the plating bath is 10.0 to 12.0; The electroless nickel-tin-phosphorus plating solution is characterized in that the solution temperature during plating operation is 80°C to 95°C.

2. 2. The electroless nickel-tin-phosphorus plating solution according to claim 1, wherein the complexing agent is at least one selected from the group consisting of citric acid and its salts, gluconic acid and its salts, acetic acid and its salts, propionic acid and its salts, succinic acid and its salts, glycolic acid and its salts, lactic acid and its salts, and malic acid and its salts.

Images