Black rhodium plating solution and method for plating black rhodium-plated film

Abstract

To provide a black rhodium plating solution, which is a plating solution for depositing a black rhodium plating film, can deposit the plating film with no appearance change, and is excellent in stability.SOLUTION: A black rhodium plating solution of the present invention includes: a rhodium salt of 1 g / L or more and 20 g / L or less in metallic rhodium concentration; a sulfuric acid of 10 mL / L or more and 100 mL / L or less; a phosphonic acid derivative of 2 mM or more and 100 mM or less, or salt thereof; and a magnesium salt of 0.1 g / L or more and 10 g / L or less in magnesium metal concentration. The phosphonic acid derivative or salt thereof works as a blackening agent of the rhodium plating film. The blackening agent can form a black rhodium plating film with little appearance change after deposition.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a black rhodium plating solution and a plating method for forming a rhodium-plated film that is made of rhodium and exhibits a black color. More specifically, the invention relates to a plating solution that can stably form a rhodium-plated film exhibiting a clearer black color than conventional methods, and a plating method for forming a suitable black rhodium-plated film using the said plating solution. [Background technology]

[0002] Rhodium (Rh), one of the platinum group metals, is chemically stable, has excellent corrosion and heat resistance, and possesses high hardness and good wear resistance, making it frequently used in the form of plating films. Rhodium plating films have long been used in decorative items and jewelry, and their excellent electrical conductivity has led to their widespread use in components of electrical and electronic equipment. In particular, in recent years, they have been applied to terminals and connectors of electronic devices such as smartphones and tablet terminals. Many rhodium plating solutions have been conventionally known for forming rhodium plating films used in these various applications (for example, Patent Document 1 by the applicant of this application).

[0003] Rhodium plating films generally exhibit the silvery-white color inherent to rhodium. Patent Document 1, mentioned above, also aims to form a lustrous, silvery-white rhodium plating film. However, depending on the composition of the rhodium plating solution, it is also possible to form a black rhodium plating film. While black rhodium plating films have been used in some applications, such as decorative purposes, their use in electronic device terminals is now being considered. Many recent electronic devices, such as tablets, feature black casings, which remain popular. For these black casings, matching the appearance of connectors and terminals to black creates a more unified and aesthetically pleasing design. Therefore, the demand for black rhodium plating films as electronic materials for connectors and other components is increasing.

[0004] Examples of the plating solution capable of forming a black rhodium plating film include, for example, the rhodium plating solution described in Patent Document 2. This rhodium plating solution contains a rhodium salt such as rhodium sulfate salt which is a metal source, an inorganic acid such as sulfuric acid which is a conductive salt, and a hypophosphite. Hypophosphorous acid acts as an additive (blackening agent) for blackening the rhodium plating film. And according to this rhodium plating solution, a black rhodium plating film can be formed according to the film thickness. Incidentally, Patent Document 1 also mentions that the rhodium plating film is blackened by the presence of hypophosphorous acid or hypophosphite in the rhodium plating solution.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In order to meet the increasing demand for black rhodium plating films, a plating solution capable of stably forming a rhodium plating film exhibiting a more "blackish black" is required. According to the inventors of the present application, it has been confirmed that the black rhodium plating solution described in Patent Document 2 can form a suitable black rhodium plating film by adjusting the addition amount of hypophosphite.

[0007] However, according to the inventors' studies, it has been confirmed that the black rhodium plating film obtained by the above-mentioned conventional black rhodium plating solution exhibits a suitable black color immediately after deposition, but undergoes changes in appearance such as color unevenness over time. Such black rhodium plating films that can undergo such appearance changes are difficult to apply to the various uses mentioned above. Furthermore, the above-mentioned conventional black rhodium plating solution does not have good stability, and repeated plating treatments may result in the deposition of rhodium plating films with varying degrees of blackness.

[0008] This invention was made against the aforementioned background and relates to a suitable rhodium plating solution for forming a black rhodium plating film. The present invention provides a black rhodium plating solution that can form a black rhodium plating film without any change in appearance and also exhibits excellent stability of the plating bath. Furthermore, considering its applications as an electrical material such as connectors and a decorative material such as jewelry, this invention also clarifies a suitable plating method for forming a black rhodium plating film. [Means for solving the problem]

[0009] The present invention, which solves the above problems, is a black rhodium plating solution comprising a rhodium salt with a metallic rhodium concentration of 1 g / L to 20 g / L, sulfuric acid with a concentration of 10 mL / L to 100 mL / L, a phosphonic acid derivative or its salt with a concentration of 2 mM to 100 mM, and a magnesium salt with a metallic magnesium concentration of 0.1 g / L to 10 g / L.

[0010] The black rhodium plating solution according to the present invention contains, as essential components, a rhodium salt as a metal source and sulfuric acid as a conductive salt, similar to a normal rhodium plating solution. Here, a phosphonic acid derivative or its salt (hereinafter sometimes referred to as a phosphonic acid derivative, etc.) and a magnesium salt are included as characteristic essential components. The phosphonic acid derivative, etc. acts as a blackening agent for blackening the rhodium plating film. Further, the magnesium salt has an action for continuously depositing a desired black rhodium plating film on the plating solution. Hereinafter, the composition of the black rhodium plating solution according to the present invention and the plating method of the black rhodium plating film using the same will be described in detail.

[0011] In the present invention, as an objective index for determining whether or not the black rhodium plating film exhibits a suitable "black" color, the L * a * b * color space (CIELab color space) is applied. The L * a * b * In the color space, the L * value indicates the lightness of the color (white, black), and the a * value and the b * value indicate the direction of the color (a * : red direction, green direction, b * : yellow direction, blue direction). In the present invention, the L * value is applied as the determination criterion for the black color of the black Rh plating film. More specifically, for the plating film formed on the surface of gold (Au), a film having an L * value of 70 or less was determined to be a suitable black color.

[0012] (A) Composition of the black rhodium plating solution according to the present invention As described above, the black rhodium plating solution according to the present invention contains, as essential components, a rhodium salt as a metal source and sulfuric acid as a conductive salt, and a phosphonic acid derivative, etc. as a blackening agent and a magnesium salt as essential constituent components.

[0013] (A-1) Rhodium salt Suitable rhodium salts for the metal source of the black rhodium plating film include rhodium sulfate, rhodium phosphate, and rhodium sulfite. In the black rhodium plating solution according to the present invention, these rhodium salts are contained in a metallic rhodium concentration of 1 g / L to 20 g / L. If the metallic rhodium concentration is less than 1 g / L, the deposition efficiency of the rhodium plating film becomes too low. Also, plating solutions with a metallic rhodium concentration exceeding 20 g / L have a large amount of rhodium that does not contribute to the deposition of the rhodium plating film, which increases costs. The metallic rhodium concentration is preferably 2 g / L to 10 g / L, and more preferably 4 g / L to 7 g / L.

[0014] (A-2) Sulfuric acid Sulfuric acid is an essential component that acts as a conductive salt in the plating solution. The sulfuric acid concentration in the plating solution should be between 10 mL / L and 100 mL / L. If the sulfuric acid concentration is less than 10 mL / L, the rhodium salt may undergo hydrolysis. If the sulfuric acid concentration exceeds 100 mL / L, it may inhibit the movement of rhodium (ions) in the solution. A sulfuric acid concentration of 20 mL / L to 100 mL / L is preferred, and 20 mL / L to 80 mL / L is more preferred.

[0015] (A-3) Phosphonic acid derivatives or salts thereof (blackening agents) Phosphonic acid derivatives and their salts act as blackening agents for rhodium plating films in the present invention and are one of the characteristic components of the present invention. A phosphonic acid derivative is a compound in which the hydrogen atom (H) bonded to the phosphorus atom (P) of phosphonic acid (H3PO3) is replaced by an organic group. A salt of a phosphonic acid derivative is a compound in which the hydrogen atom bonded to the oxygen atom (O) of a phosphonic acid derivative is replaced by another element such as a metal.

[0016] In the present invention, phosphonic acid derivatives, etc. are used as blackening agents because applying phosphonic acid derivatives, etc. results in L *This is because it is possible to form a rhodium plating film with a value of 70 or less. In addition, the black rhodium plating film formed by applying phosphonic acid derivatives, etc., can maintain its appearance (black) after film formation. Regarding these effects, conventional technologies such as hypophosphorous acid also act as blackening agents for rhodium plating films, L * It is possible to form a black rhodium plating film with a value of 70 or less. However, black rhodium films made with hypophosphorous acid, etc., have a low L value immediately after deposition. * Even if the values ​​are good, changes in appearance such as color unevenness may occur over time. Therefore, in this invention, a phosphonic acid derivative or the like is an essential component from the viewpoint of the stability of the appearance of the black rhodium plating film.

[0017] Specific examples of phosphonic acid derivatives used in the black rhodium plating solution according to the present invention include amino(methylenephosphonic acid) derivatives such as aminomethylphosphonic acid and aminotri(methylenephosphonic acid), alkyleneamine(methylenephosphonic acid) derivatives such as ethylenediaminetetra(methylenephosphonic acid), hexamethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid), and hydroxyalkylidenediphosphonic acids such as 1-hydroxyethylidene-1,1-diphosphonic acid, 1-hydroxybutylidene-1,1-diphosphonic acid, 1-hydroxyhexylidene-1,1-diphosphonic acid, 1-hydroxydodecylidene-1,1-diphosphonic acid, and 1-hydroxyhexadecylidene-1,1-diphosphonic acid. Examples of salts of phosphonic acid derivatives include alkali metal salts such as sodium salts of the above compounds. Furthermore, among the phosphonic acid derivatives mentioned above, particularly preferred are diethylenetriaminepentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and their salts. These phosphonic acid derivatives produce a rhodium plating film with a stronger black color, specifically L * A black rhodium plating film with a value of 65 or less can be formed.

[0018] The content of phosphonic acid derivatives, etc., in the black rhodium plating solution according to the present invention shall be 2 mM or more and 100 mM or less. Below 2 mM, the effect will be similar to that when no additive is used. Furthermore, even if the content of phosphonic acid derivatives, etc., increases, L * The values ​​do not change significantly. Furthermore, since exceeding 100 mM results in an uneven film appearance, this content should be set as the upper limit. To achieve a particularly good film appearance, it is preferable to have a phosphonic acid derivative content of 5 mM to 60 mM.

[0019] (A-4) Magnesium salts The reason why the black rhodium plating solution according to the present invention includes a magnesium salt as an essential component is that, during repeated plating processes, the desired L * This is to stably form a black rhodium plating film with a specific value. In other words, magnesium salts contribute to improving the lifespan of the black rhodium plating solution. Applicable magnesium salts include magnesium sulfate, magnesium oxide, magnesium chloride, and magnesium bromide, and the solution can contain one or more of these magnesium salts.

[0020] The magnesium salt content should be between 0.1 g / L and 10 g / L in terms of metallic magnesium concentration. This is because concentrations below 0.1 g / L have no effect, while concentrations above 10 g / L tend to decrease the precipitation rate. A metallic magnesium concentration of 0.25 g / L to 5 g / L is preferred, and 0.5 g / L to 2 g / L is more preferred.

[0021] It is well known that conventional rhodium plating solutions contain alkaline earth metals such as magnesium as additives. For example, the rhodium plating solution described in Patent Document 1 above has at least one compound selected from the group consisting of phosphorous acid, alkali metal phosphite, alkaline earth metal phosphite, or ammonium phosphite as an essential component, and therefore may contain an alkaline earth metal salt (such as magnesium phosphite). However, the rhodium plating solution described in Patent Document 1 requires a compound that generates phosphorous acid (ions) to form an amorphous rhodium plating film, and the alkaline earth metal is merely a countercation of that compound. In other words, in conventional rhodium plating solutions such as those described in Reference Document 1, magnesium is not an essential additive. In contrast, in the black rhodium plating solution according to the present invention, magnesium is an essential component that has the unique effect of stably forming a black rhodium plating film in cooperation with phosphonic acid derivatives and other blackening agents.

[0022] (A-5) Other configurations The black rhodium plating solution according to the present invention has magnesium salts such as rhodium salt, sulfuric acid, and phosphonic acid derivatives as essential components, but may also contain other components such as other additives. It may also contain known additives used in electroplating solutions. For example, it may contain pH buffers, complexing agents, stabilizers, etc.

[0023] Furthermore, for the black rhodium plating solution according to the present invention described above, known analytical methods can be applied to measure the concentration and content of each component. The rhodium concentration, sulfuric acid concentration, and magnesium concentration can be analyzed and measured in the plating solution state by inductively coupled plasma atomic emission spectrometry (ICP) and ion chromatography (IC). In addition, analytical instruments such as high-performance liquid chromatography (HPLC), high-performance liquid chromatogram mass spectrometer (LC-MS, LC-MS / MS), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR) can be appropriately selected and used. The content of phosphonic acid derivatives, etc., can be measured by titration using an appropriate indicator. For titration, commercially available test kits containing indicators corresponding to the type of phosphonic acid derivative are available and can be used.

[0024] (B) Plating method for a black rhodium-plated film according to the present invention The method for plating a black rhodium film using the black rhodium plating solution according to the present invention, as described above, is basically the same as a conventional electroplating method, but it is preferable to set the following plating conditions.

[0025] (B-1) Plating solution pH The pH of the plating solution during the plating process should be between 1.8 and 2.5. Below a pH of 1.8, the rhodium plating film will not blacken sufficiently. Above a pH of 2.5, the solution stability decreases, and rhodium may precipitate. (L of the rhodium plating film) * To obtain a more favorable value, it is preferable to set the pH of the plating solution to 2.0 or higher.

[0026] (B-2) Plating bath temperature In the plating method using the black rhodium plating solution of the present invention, the L of the rhodium plating film increases with increasing bath temperature. *A decrease in the value is observed. On the other hand, since the bath temperature is related to the deposition efficiency of the rhodium plating film, the bath temperature of the plating solution in this invention should be 50°C or higher and 60°C or lower. Below 50°C, the deposition efficiency is low and efficient film formation becomes difficult. Furthermore, bath temperatures exceeding 60°C reduce the stability of the plating solution and cause problems such as deterioration of the jigs. The bath temperature should preferably be 50°C or higher and 55°C or lower. It is also preferable to stir the plating solution during the plating process.

[0027] (B-3) Current density According to the inventors' studies, the rhodium plating film produced by the black rhodium plating solution of the present invention exhibits a decrease in current density during electrolysis, along with the L of the rhodium plating film. * The value decreases. On the other hand, the resistance (contact resistance) of the rhodium plating film increases as the current density decreases. * Considering the balance between the value and the contact resistance, the current density during electrolysis is 0.1 A / dm 2 More than 2.5A / dm 2 The following applies. In particular, when applying the black rhodium plating film according to the present invention to electrical components such as connectors, the current density is set to 1.0 A / dm² in order to suppress the contact resistance. 2 More than 2.5A / dm 2 The following is preferable. On the other hand, when applying a black rhodium plating film for decorative purposes, there is no need to worry about contact resistance, so the current density should be 0.1 A / dm 2 More than 2.0A / dm 2 The following is preferable:

[0028] (C) Black rhodium plated film formed by the present invention According to the black rhodium plating solution and plating method of the present invention described above, a suitable black rhodium plating film can be formed. When this black rhodium plating film is formed on a gold (thin film or bulk gold) surface, L * a * b * L in color space * The value is 70 or less. Furthermore, by optimizing the composition of the black rhodium plating solution and the plating conditions within the scope of the present invention, L *A more preferable black rhodium plating film is obtained, with a value of 60 or more and 65 or less. Furthermore, regarding the black rhodium plating according to the present invention, a * Value and b * There are no particular restrictions on the value.

[0029] Furthermore, since the black rhodium plating solution of the present invention requires phosphonic acid derivatives, etc., the resulting black rhodium plating film contains a small amount of P. Specifically, it contains 4% by mass or less of P. Therefore, the black rhodium plating film produced by the black rhodium plating solution of the present invention contains the above-mentioned L * It is possible to specify the value (70 or less) and the P concentration (4% by mass or less). The thickness of the black rhodium plating film according to the present invention is preferably 0.05 μm to 2.0 μm, and more preferably 1.0 μm or less. This is because excessively thick film may contain cracks. The black rhodium plating solution according to the present invention can impart a suitable black color to rhodium plating films with thicknesses within the above range. [Effects of the Invention]

[0030] As described above, the black rhodium plating solution according to the present invention allows for the stable formation of a rhodium plating film exhibiting a suitable black color. Furthermore, the black rhodium plating film formed by the present invention maintains a good appearance even over time. In addition, the black rhodium plating solution according to the present invention can form a black rhodium plating film with good deposition efficiency and exhibits good stability as a plating solution. [Brief explanation of the drawing]

[0031] [Figure 1] In the first embodiment, this photograph shows the change in appearance of the black rhodium-plated film formed with the rhodium-plated solutions of Example 1 and Comparative Example 2. [Figure 2] In the third embodiment, a table shows a graph illustrating the deposition efficiency and the L* value of the black rhodium plating film when the plating bath temperature is adjusted during the plating process. [Figure 3]In the third embodiment, a table shows a graph illustrating the deposition efficiency and the L* value of the black rhodium plating film when the plating bath pH is adjusted and the plating process is performed. [Figure 4] In the third embodiment, a table shows a graph illustrating the deposition efficiency, contact resistance, and L* value of the black rhodium plating film when the plating process is performed by adjusting the current density. [Modes for carrying out the invention]

[0032] First Embodiment The embodiments of the present invention will be described below. In this embodiment, a black rhodium plating solution was prepared by applying multiple types of phosphonic acid derivatives as blackening agents, and a black rhodium plating film was formed.

[0033] A black rhodium plating bath was prepared by adding rhodium sulfate and sulfuric acid as rhodium salts to pure water, and further adding a phosphonic acid derivative salt and magnesium sulfate as a magnesium salt as blackening agents. In this embodiment, several types of plating solutions were prepared with a metallic rhodium concentration of 5.0 g / L, a sulfuric acid concentration of 20 mL / L, and a metallic magnesium concentration of 1.0 g / L, while the phosphonic acid derivative content was in the range of 2 mM to 100 mM. Three types of phosphonic acid derivative salts were used as blackening agents: diethylenetriaminepenta(methylenephosphonic acid) 7-sodium (Example 1), ethylenediaminetetra(methylenephosphonic acid) 5-sodium (Example 2), and 1-hydroxyethylidene-1,1-diphosphonic acid 4-sodium (Example 3). An aqueous solution of the above phosphonic acid derivative salt was used to prepare the plating bath.

[0034] Furthermore, as a comparative example, a rhodium plating solution was prepared using hypophosphate (sodium hypophosphite) as a blackening agent. The metallic rhodium concentration and sulfuric acid concentration of the comparative plating solution were the same as in the above examples, but it differed in that it did not contain a blackening agent or magnesium salt.

[0035] Then, rhodium plating films were deposited using each of the prepared black rhodium plating solutions. A test panel (20 mm x 20 mm) with a 0.1 μm gold film deposited on a 5 μm nickel base was used as the substrate, and the plating treatment was performed under the following conditions. Plating bath pH: 2.0~2.3 ·Bath temperature: 50℃ ·Current density: 2A / dm 2 Plating time: 2 min

[0036] After the plating process, the substrate was removed and the amount of rhodium deposited was measured from the change in mass to calculate the thickness of the plating film and the deposition efficiency. Subsequently, the obtained rhodium plating film was subjected to L * a * b * L in color space * value, a * value, b * Each value was measured. This measurement was performed using the SCI method, which includes specular reflection, with a spectrophotometer (CM-3600d: manufactured by Konica Minolta Japan, Inc.).

[0037] Furthermore, in the visual evaluation of the rhodium plating film, we assessed whether there were any changes in appearance after the plating process. The appearance of the rhodium plating film was observed 24 hours after the plating process to check for any changes such as uneven coloring of the rhodium plating film (black).

[0038] Table 1 shows the evaluation results for the rhodium plating films obtained using the black rhodium plating solutions of Examples 1 to 3 and the Comparative Example of this embodiment.

[0039] [Table 1]

[0040] From Table 1, the rhodium-plated films formed by the rhodium-plated solutions using phosphonic acid derivative salts as blackening agents in Examples 1 to 3 of this embodiment are all L *The value is 70 or less, indicating a good black color. Comparative Example 1 was the same rhodium plating solution as Examples 1 to 3 except for the presence or absence of a blackening agent, and the formed rhodium plating film was close to black to the naked eye, but L * The value exceeded 70. It was confirmed that a blackening agent is necessary to obtain a black rhodium plating film.

[0041] Furthermore, in the rhodium plating solution using hypophosphorous acid as a blackening agent in Comparative Example 2, L * A black rhodium plating film with a value of 70 or less was obtained. However, the black rhodium plating film from Comparative Example 2 showed color unevenness after 24 hours. In contrast, no such appearance change was observed in the black Rh plating films from Examples 1 to 3. In other words, it was confirmed that Comparative Example 2 was clearly inferior in terms of the stability of the appearance (black) of the black rhodium plating film. Figure 1 shows photographs of the appearance of rhodium plating films from Example 1 (blackening agent: diethylenetriaminepenta(methylenephosphonic acid) heptodium salt (20 mM)) and Comparative Example 2 (blackening agent: sodium hypophosphite (2 mM)) immediately after plating and after 24 hours.

[0042] Furthermore, the black rhodium plating solutions of Examples 1 to 3 are superior to the black rhodium plating solution of Comparative Example 2 in terms of deposition efficiency. From these results, it was confirmed that phosphinic acid derivatives or their salts are suitable as blackening agents for black rhodium plating solutions. Note that the rhodium plating films produced by Example 1 (blackening agent: diethylenetriaminepenta(methylenephosphonic acid) 7-sodium salt) and Example 3 (blackening agent: 1-hydroxyethylidene-1,1-diphosphonic acid 4-sodium salt) are L * Since the values ​​are 65 or less, these can be considered particularly preferred blackening agents among phosphinic acid derivatives.

[0043] Second EmbodimentIn this embodiment, the stability of the black rhodium plating film obtained by repeatedly plating with a black rhodium plating solution was evaluated. The black rhodium plating solution used in this embodiment is the same as that of Example 1 of the first embodiment, with 20 mM of diethylenetriaminepenta(methylenephosphonic acid) 7-sodium salt added as a blackening agent (metallic rhodium concentration 5 g / L, sulfuric acid concentration 20 mL / L, metallic magnesium concentration 1 g / L). In addition, in this embodiment, as a reference example to confirm the effect of magnesium in the black rhodium plating solution, an evaluation of a plating solution without the addition of magnesium salt (magnesium sulfate) was also performed.

[0044] In this embodiment, the plating conditions were the same as in the first embodiment (plating bath temperature: 50°C, pH: 2.0). In this embodiment, the 0.1 MTO plating process was repeated five times. MTO stands for "Metal Turn Over," and the 1 MTO plating process is a process in which the same amount of rhodium as the rhodium content in the plating solution at the time of bath preparation is deposited. In this embodiment, the substrate was replaced after each 0.1 MTO plating process, and the consumed aqueous solution of rhodium salt was replenished. The deposition efficiency and the L of the formed rhodium plating film were then measured. * value, a * value, b * The values ​​were measured. The test results are shown in Table 2.

[0045] [Table 2]

[0046] Table 2 shows that even in black rhodium plating solutions using phosphonic acid derivatives as blackening agents, the magnesium-free example plating solution shows L in plating treatments from 0.3 MTO onwards. * A rhodium-plated film with a value exceeding 70 is formed. That is, L *It can be confirmed that the addition of magnesium is necessary for the stable formation of black rhodium plating films with a value of 70 or less. Furthermore, black rhodium plating solutions containing magnesium along with phosphonic acid derivatives as blackening agents can withstand multiple plating treatments. * It was confirmed that black rhodium plating films with a value of 70 or less could be stably formed, and that the deposition efficiency was also stable.

[0047] Third Embodiment In this embodiment, the plating process was performed while changing the plating conditions, and the appearance of the resulting black rhodium plating film was evaluated. In this embodiment, the plating solution of Example 1 of the first embodiment (blackening agent: diethylenetriaminepenta(methylenephosphonic acid) heptodium salt (20 mM)) was used.

[0048] The standard plating conditions were then set as follows. In the evaluation test of this embodiment, one of the following conditions—pH, bath temperature, or current density—was adjusted, while the other conditions were kept the same as the standard plating conditions for the plating process. Standard plating conditions Plating bath pH: 2.2 (adjustment: 1.5~2.5) ·Bath temperature: 50℃ (during adjustment: 50℃~60℃) ·Current density: 2A / dm 2 (When adjusting: 0.1A / dm 2 ~3.0A / dm 2 ) Plating time: 2 min

[0049] After plating under each condition, the deposition efficiency and L of the rhodium plating film were determined in the same manner as in the first embodiment. * value, a * value, b * The values ​​were measured. In addition, the contact resistance of the rhodium plating film was also measured for tests when the current density was changed. As for the evaluation results of this embodiment, Figure 2 shows the results when the bath temperature of the plating solution was adjusted, and Figure 3 shows the results when the pH of the plating solution was adjusted. These figures also show the results using a plating solution that does not contain a blackening agent (phosphonic acid derivative, etc.) (plating treatment under the above standard conditions). Figure 4 shows the results when the current density was adjusted.

[0050] From Figure 2, the L of the rhodium plating film in the plating bath temperature range of 50°C to 60°C. * The value was 65 or less, indicating a good appearance.

[0051] Furthermore, from Figure 3, at a pH of 1.5 in the plating solution, the L of the rhodium plating film is * It can be seen that the value exceeds 70. * To obtain a value of 70 or less, the pH of the plating solution must be 1.8 or higher, and by setting the pH to 2.0 or higher, L * A more suitable black rhodium plating film with a value of 65 or less can be obtained.

[0052] Furthermore, looking at the characteristics of the rhodium plating film when the current density of the plating process is adjusted, Figure 4 shows that as the current density decreases, the L of the rhodium plating film... * It can be seen that the value decreases. However, it was also confirmed that the decrease in current density leads to an increase in the contact resistance of the rhodium-plated film.

[0053] Here, considering the application of the black rhodium plating film, the current density of the plating process and the criteria for appearance (L * The relationship between the value (70 or less) and contact resistance will be examined. When a black rhodium plating film is applied to conductive materials such as connectors, if the reference value for contact resistance is 30 mΩ or less, then the current density for forming the black rhodium plating film for the above application should be 1.0 A / dm 2 More than 2.5A / dm 2 The following is preferable. On the other hand, when a black rhodium-plated film is used for decorative purposes, the level of contact resistance is not a concern, so a current density lower than the above range is also acceptable.

[0054] Furthermore, regarding the deposition efficiency, it was confirmed that it tends to decrease with increasing current density. However, the black rhodium plating solution according to the present invention basically exhibits good and stable deposition efficiency, showing a good deposition efficiency of 20 mg / A·min or more under all conditions.

[0055] Fourth Embodiment In this embodiment, the running performance of the black rhodium plating solution according to the present invention was investigated. Using the plating solution of Example 1 of the first embodiment (diethylenetriaminepenta(methylenephosphonic acid) 7-sodium salt: 20 mM, metallic rhodium concentration 5 g / L, sulfuric acid concentration 20 mL / L, metallic magnesium concentration 1 g / L), the plating process was carried out while adding an amount of plating solution corresponding to the amount of metallic rhodium consumed as a replenishment solution.

[0056] First, plating was performed up to 1 MTO. During this process, the rhodium plating film was evaluated visually and its deposition efficiency measured every 0.25 MTO, and a replenishment solution was added. The results are shown in Table 3. Plating bath pH: 2.2 ·Bath temperature: 50℃ ·Current density: 2A / dm 2

[0057] [Table 3]

[0058] Table 3 shows that in plating processes up to 1MTO, the L of the rhodium plating film * Since the value remained below 65, it can be said that a good black rhodium plating film was stably deposited. On the other hand, when referring to the deposition efficiency, a slight decrease in deposition efficiency is observed from 0.75 MTO onwards.

[0059] Therefore, we decided to investigate the feasibility of performing plating processes longer than 1 MTO. In this investigation, we considered cases where plating processes exceeding 1 MTO up to 2 MTO were performed, and examined whether it was possible to accommodate this by changing the plating conditions at predetermined stages. Specifically, we increased the bath temperature by 5°C in the plating process after 1 MTO, and in addition, reduced the current density to 1 A / dm in the 2 MTO plating process. 2 This was the result of the evaluation of the plating process from 1MTO onwards. The results of this study are shown in Table 4.

[0060] [Table 4]

[0061] Table 4 shows that with the black rhodium plating solution according to the present invention, the plating process can be continued with stable deposition efficiency by slightly adjusting the bath temperature and current density during operation. The rhodium plating film formed is L * A suitable black color is obtained when the value is 65 or less. Furthermore, when the rhodium plating film after plating with 2MTO was observed for appearance after 24 hours, no changes in appearance such as color unevenness were observed. [Industrial applicability]

[0062] The black rhodium plating solution according to the present invention can stably form a rhodium plating film having a suitable black color. Furthermore, the black rhodium plating solution according to the present invention also has excellent stability and can form a black rhodium film with good deposition efficiency. The black rhodium plating solution and the plating method for a black rhodium plating film according to the present invention are expected to be applied to terminals and connectors of electronic devices such as smartphones and tablet terminals. They are also useful for decorative items and jewelry.

Claims

1. Rhodium salts with a metallic rhodium concentration of 1 g / L or more and 20 g / L or less, Sulfuric acid with a concentration of 10 mL / L or more and 100 mL / L or less, A phosphonic acid derivative or salt thereof in a concentration of 2 mM to 100 mM, It contains a magnesium salt with a metallic magnesium concentration of 0.1 g / L or more and 10 g / L or less. The phosphonic acid derivative or salt thereof includes at least one of amino(methylenephosphonic acid) derivatives or salts thereof, alkyleneamine(methylenephosphonic acid) derivatives or salts thereof, hydroxyalkylidene diphosphonic acid derivatives or salts thereof, The magnesium salt includes at least one of magnesium sulfate, magnesium oxide, magnesium chloride, and magnesium bromide. Black rhodium plating solution.

2. The black rhodium plating solution according to claim 1, wherein the rhodium salt is one of a sulfate, a phosphate, or a sulfite.

3. The phosphonic acid derivative or salt thereof is diethylenetriaminepentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and salts thereof, as described in claim 1 or claim 2.

4. The black rhodium plating solution according to claim 1 or claim 2, wherein the pH is 1.8 or higher and 2.5 or lower.

5. A method for plating a black rhodium-plated film using the black rhodium plating solution described in claim 1 or claim 2, Plating bath temperature 50°C to 60°C, current density 0.1 A / dm ２ 2.5A / dm or more ２ The following is a plating method for performing the plating process.

6. The method for plating a black rhodium-plated film according to claim 5, wherein the pH of the black rhodium plating solution is set to 1.8 or higher and 2.5 or lower before performing the plating treatment.

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