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

Abstract

The present invention relates to a plating solution for forming a black rhodium plating film. A black rhodium plating solution according to the present invention includes: a rhodium salt that has a rhodium metal concentration of 1 g / L to 20 g / L inclusive; 10 mL / L to 100 mL / L inclusive of sulfuric acid; 2 mM to 100 mM inclusive of a phosphonic acid derivative or a salt thereof; and a magnesium salt that has a magnesium metal concentration of 0.1 g / L to 10 g / L inclusive. According to the present invention, the phosphonic acid derivative or the salt thereof acts as a blackening agent for a rhodium plating film. This blackening agent makes it possible to obtain the black rhodium plating film that has little change in appearance even after film formation.

Description

Black rhodium plating solution and method for producing black rhodium plating film

[0001] The present invention relates to a black rhodium plating solution and a plating method using the same, which are used to form a black rhodium plating film made of rhodium. More specifically, the present invention relates to a plating solution that can stably form a rhodium plating film that is blacker than conventional rhodium plating films, and a plating method that uses the plating solution to form a suitable black rhodium plating film.

[0002] Rhodium (Rh), a platinum group metal, is chemically stable, has excellent corrosion resistance and heat resistance, and is hard and wear-resistant. Therefore, it is often used in the form of a plating film. Rhodium plating films have long been used in ornaments and jewelry, and their excellent electrical conductivity has led to their widespread use in components of electrical and electronic devices. 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 for forming rhodium plating films for such various applications have been known (for example, Patent Document 1, filed by the present applicant).

[0003] Rhodium plating films generally exhibit the inherent silver-white color of rhodium metal. Patent Document 1 also aims to form a lustrous silver-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. Until now, black rhodium plating films have been used for decorative purposes and other limited applications, but recently, their use in terminals of electronic devices has also been considered. Many electronic devices, such as tablets, have recently adopted black housings, which have become extremely popular. For such black housings, the appearance of connectors and terminals can also be made black to create a unified look, thereby enhancing the design. Therefore, demand for black rhodium plating films as electronic materials for connectors and other devices is also increasing.

[0004] An example of a plating solution capable of forming a black rhodium plating film is the rhodium plating solution described in Patent Document 2. This rhodium plating solution contains a rhodium salt such as rhodium sulfate as a metal source, an inorganic acid such as sulfuric acid as a conductive salt, and a hypophosphite. The hypophosphorous acid acts as an additive (blackening agent) for blackening the rhodium plating film. This rhodium plating solution can form a black rhodium plating film depending on the film thickness. Patent Document 1 also mentions that the presence of hypophosphorous acid or hypophosphite in the rhodium plating solution causes the rhodium plating film to turn black.

[0005] Patent No. 6474536 Specification JP-A-58-048688

[0006] In order to meet the growing demand for black rhodium plating films, a plating solution that can stably form rhodium plating films that exhibit a more "true black" color is needed. The inventors of the present application have confirmed that the black rhodium plating solution described in Patent Document 2 can form a suitable black rhodium plating film by adjusting the amount of hypophosphite added.

[0007] However, according to the studies of the present inventors, it has been confirmed that the black rhodium plating film obtained using the above-mentioned conventional black rhodium plating solution exhibits a desirable black color immediately after the film is formed, but changes in appearance such as color unevenness occur over time. Black rhodium plating films that are susceptible to such changes in appearance are difficult to apply to the various applications mentioned above. Furthermore, the stability of the above-mentioned conventional black rhodium plating solution itself is not particularly good, and repeated plating processes can result in the formation of rhodium plating films with different degrees of blackness.

[0008] The present invention was made in light of the above 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 changing its appearance and has excellent plating bath stability. The present invention also clarifies a plating method for forming a suitable black rhodium plating film, taking into account applications such as the above-mentioned electrical materials (e.g., connectors) and decorative materials (e.g., jewelry).

[0009] The present invention, which solves the above-mentioned problems, provides a black rhodium plating solution containing a rhodium salt having a metallic rhodium concentration of 1 g / L or more and 20 g / L or less, sulfuric acid of 10 mL / L or more and 100 mL / L or less, a phosphonic acid derivative or a salt thereof of 2 mM or more and 100 mM or less, and a magnesium salt having a metallic magnesium concentration of 0.1 g / L or more and 10 g / L or less.

[0010] The black rhodium plating solution of the present invention, like a typical rhodium plating solution, contains a rhodium salt as a metal source and sulfuric acid as a conductive salt as essential components, and also contains a phosphonic acid derivative or its salt (hereinafter sometimes referred to as a phosphonic acid derivative, etc.) and a magnesium salt as characteristic essential components. The phosphonic acid derivative, etc., acts as a blackening agent for blackening the rhodium plating film. The magnesium salt also functions to enable the plating solution to continuously deposit the desired black rhodium plating film. The composition of the black rhodium plating solution of the present invention and the plating method for producing a black rhodium plating film using the same are described in detail below.

[0011] In the present invention, L is used as an objective index for determining whether a black rhodium plating film exhibits a suitable "black" color. ＊ a ＊ b ＊ Apply color space (CIELab color space). ＊ a ＊ b ＊ In color space, L ＊ The value indicates the color brightness (white, black), and ＊ value, b ＊ The value is the color direction (a ＊ : Red direction, green direction, b ＊ In the present invention, the criteria for determining the black color of the black Rh plating film are L ＊ More specifically, for a plating film formed on a gold (Au) surface, L ＊ A value of 70 or less was judged to be a suitable black color.

[0012] (A) Composition of the Black Rhodium Plating Solution of the Present Invention As described above, the black rhodium plating solution of the present invention contains, as essential components, a rhodium salt as a metal source, sulfuric acid as a conductive salt, and a phosphonic acid derivative or the like and a magnesium salt as blackening agents.

[0013] (A-1) Rhodium Salts Rhodium sulfate, rhodium phosphate, and rhodium sulfite can be used as rhodium salts serving as the metal source for the black rhodium plating film. The black rhodium plating solution according to the present invention contains these rhodium salts at a metallic rhodium concentration of 1 g / L or more and 20 g / L or less. If the metallic rhodium concentration is less than 1 g / L, the deposition efficiency of the rhodium plating film becomes too low. Furthermore, if the metallic rhodium concentration exceeds 20 g / L, the plating solution contains 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 or more and 10 g / L or less, and more preferably 4 g / L or more and 7 g / L or less.

[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 is 10 mL / L or more and 100 mL / L or less. If the sulfuric acid concentration is less than 10 mL / L, the rhodium salt may be hydrolyzed. If the sulfuric acid concentration exceeds 100 mL / L, the migration of rhodium (ions) in the solution may be inhibited. The sulfuric acid concentration is preferably 20 mL / L or more and 100 mL / L or less, and more preferably 20 mL / L or more and 80 mL / L or less.

[0015] (A-3) Phosphonic acid derivative or its salt (blackening agent) In the present invention, the phosphonic acid derivative and its salt act as a blackening agent that blackens the rhodium plating film, and are one of the characteristic features of the present invention. ３ P.O. ３ ) in which the hydrogen atom (H) bonded to the phosphorus atom (P) is substituted with an organic group. Also, 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 substituted with another element such as a metal.

[0016] In the present invention, the reason why a phosphonic acid derivative or the like is used as a blackening agent is that by applying a phosphonic acid derivative or the like, 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 a phosphonic acid derivative or the like can maintain its appearance (black) after film formation. Regarding these effects, hypophosphorous acid or the like used in the prior art also acts as a blackening agent for rhodium plating films, and ＊ 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 or the like have a high L value immediately after film formation. ＊ Even if the value is good, changes in appearance such as color unevenness occur over time. Therefore, in the present 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 the phosphonic acid derivative used in the black rhodium plating solution according to the present invention include amino(methylene phosphonic acids) such as aminomethylphosphonic acid and aminotri(methylene phosphonic acid), alkyleneamine(methylene phosphonic acids) such as ethylenediaminetetra(methylene phosphonic acid), hexamethylenediaminetetra(methylene phosphonic acid), and diethylenetriaminepenta(methylene phosphonic acid), and hydroxyalkylidene diphosphonic 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. Salts of the phosphonic acid derivatives include alkali metal salts such as sodium salts of the above compounds. Among the above-mentioned phosphonic acid derivatives, diethylenetriaminepentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and salts thereof are particularly preferred. These phosphonic acid derivatives are effective in producing rhodium plating films with a stronger black color, specifically, L ＊ A black rhodium plating film having a value of 65 or less can be formed.

[0018] The content of the phosphonic acid derivative or the like in the black rhodium plating solution according to the present invention is set to 2 mM or more and 100 mM or less. If it is less than 2 mM, the effect will be the same as when no addition is made. ＊ The value does not change significantly. Furthermore, since a content exceeding 100 mM results in a non-uniform coating appearance, this content is set as the upper limit. To obtain particularly good coating appearance, the content of the phosphonic acid derivative or the like is preferably 5 mM or more and 60 mM or less.

[0019] (A-4) Magnesium Salt The reason why magnesium salt is an essential component of the black rhodium plating solution according to the present invention is that it is necessary to obtain the desired L ＊ This is to stably form a black rhodium plating film having a high plating value. That is, the magnesium salt contributes to improving the life of the black rhodium plating solution. As the magnesium salt, magnesium sulfate, magnesium oxide, magnesium chloride, and magnesium bromide can be used, and the plating solution can contain one or more of these magnesium salts.

[0020] The content of the magnesium salt is set to 0.1 g / L or more and 10 g / L or less in terms of metallic magnesium concentration. A concentration of less than 0.1 g / L is ineffective, while a concentration of more than 10 g / L tends to decrease the deposition rate. The metallic magnesium concentration is preferably 0.25 g / L or more and 5 g / L or less, and more preferably 0.5 g / L or more and 2 g / L or less.

[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, mentioned above, contains at least one compound selected from the group consisting of phosphorous acid, alkali metal phosphites, alkaline earth metal phosphites, and ammonium phosphites, and therefore may contain alkaline earth metal salts (e.g., 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 serves merely as a countercation for this compound. In other words, magnesium is not an essential additive in conventional rhodium plating solutions such as those described in Patent Document 1. In contrast, in the black rhodium plating solution of the present invention, magnesium is an essential component that has the unique effect of synergizing with a blackening agent such as a phosphonic acid derivative to stably form a black rhodium plating film.

[0022] (A-5) Other Components The black rhodium plating solution according to the present invention essentially comprises the above-described rhodium salt, sulfuric acid, phosphonic acid derivative, magnesium salt, and the like, 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 a pH buffer, a complexing agent, a stabilizer, etc.

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

[0024] (B) Plating Method for Producing a Black Rhodium Plating Film According to the Present Invention The plating method for producing a black rhodium plating film using the black rhodium plating solution according to the present invention as described above basically conforms to a normal electrolytic plating method, but it is preferable to set the following plating treatment conditions.

[0025] (B-1) pH of plating solution The pH of the plating solution during plating is set to 1.8 or more and 2.5 or less. If the pH is less than 1.8, the blackening of the rhodium plating film is insufficient. If the pH is more than 2.5, the solution stability decreases and rhodium may precipitate. ＊ To obtain a more suitable value, the pH of the plating solution is preferably 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 an increase in the 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 the present invention is set to 50°C or higher and 60°C or lower. If the bath temperature is lower than 50°C, the deposition efficiency is low and efficient film formation becomes difficult. Furthermore, if the bath temperature exceeds 60°C, the stability of the plating solution decreases and problems such as deterioration of the jig occur. The bath temperature is preferably set to 50°C or higher and 55°C or lower. It is preferable to stir the plating solution during the plating process.

[0027] (B-3) Current Density According to the investigations of the present inventors, the rhodium plating film obtained by the black rhodium plating solution of the present invention has a low current density during electrolysis and a low L of the rhodium plating film. ＊ On the other hand, the resistance value (contact resistance value) of the rhodium plating film increases with the decrease in current density. ＊ Considering the balance between the value and the contact resistance, the current density during electrolysis is 0.1 A / dm ２ 2.5A / dm or more ２ In particular, when the black rhodium plating film of the present invention is applied to electrical components such as connectors, the current density is set to 1.0 A / dm or less in order to suppress the contact resistance value. ２ 2.5A / dm or more ２On the other hand, when the black rhodium plating film is used for decorative purposes, there is no need to worry about contact resistance, so the current density is preferably set to 0.1 A / dm ２ 2.0A / dm or more ２ It is preferable that:

[0028] (C) Black rhodium plating 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 the surface of gold (thin film or bulk gold), it has the following characteristics: ＊ a ＊ b ＊ L in color space ＊ The value is 70 or less. In addition, by optimizing the composition of the black rhodium plating solution and plating conditions within the scope of the present invention, the L ＊ The black rhodium plating film has a more preferable value of 60 to 65. ＊ value and b ＊ There is no particular limitation on the value.

[0029] Furthermore, since the black rhodium plating solution of the present invention essentially contains a phosphonic acid derivative or the like, the black rhodium plating film formed contains a small amount of P. Specifically, it contains 4 mass % or less of P. Therefore, the black rhodium plating film formed using the black rhodium plating solution of the present invention does not contain the above-mentioned L ＊ The black rhodium plating film according to the present invention can be specified by the value (70 or less) and the P concentration (4 mass% or less). The film thickness of the black rhodium plating film according to the present invention is preferably 0.05 μm or more and 2.0 μm or less, and more preferably 1.0 μm or less. This is because an excessively thick film may contain cracks. The black rhodium plating solution according to the present invention can impart a suitable black color to a rhodium plating film having a film thickness within the above range.

[0030] As described above, the black rhodium plating solution according to the present invention can stably form a rhodium plating film exhibiting a suitable black color. Furthermore, the black rhodium plating film formed by the present invention can maintain a good appearance over time. Furthermore, the black rhodium plating solution according to the present invention can form a black rhodium plating film with good deposition efficiency and has good stability as a plating solution.

[0031] In the first embodiment, photographs showing the change in appearance of the black rhodium plating film formed using the rhodium plating solutions of Example 1 and Comparative Example 2. In the third embodiment, the deposition efficiency and the L of the black rhodium plating film when the plating treatment was performed by adjusting the plating bath temperature were measured. ＊ 10 is a table showing a graph showing the deposition efficiency and the L value of the black rhodium plating film when the plating bath pH is adjusted in the third embodiment. ＊ 10 is a table showing a graph showing the deposition efficiency and contact resistance of the black rhodium plating film when the plating process is performed by adjusting the current density in the third embodiment, and L ＊ A table showing a graph showing the values.

[0032] First Embodiment: Hereinafter, an embodiment of the present invention will be described. In this embodiment, a black rhodium plating solution was produced using a plurality of types of phosphonic acid derivatives as blackening agents, and a black rhodium plating film was formed using the solution.

[0033] A black rhodium plating bath was prepared by adding rhodium sulfate and sulfuric acid as rhodium salts to pure water, and then adding a salt of a phosphonic acid derivative as a blackening agent and magnesium sulfate as a magnesium salt. In this embodiment, the metal rhodium concentration was set to 5.0 g / L, the sulfuric acid concentration to 20 mL / L, and the metal magnesium concentration to 1.0 g / L, and multiple plating solutions were prepared with phosphonic acid derivative contents ranging from 2 mM to 100 mM. Three types of phosphonic acid derivative salts were used as the blackening agent: diethylenetriaminepenta(methylenephosphonic acid) hexasodium (Example 1), ethylenediaminetetra(methylenephosphonic acid) pentasodium (Example 2), and 1-hydroxyethylidene-1,1-diphosphonic acid tetrasodium (Example 3). An aqueous solution of the phosphonic acid derivative salt was used to prepare the plating bath.

[0034] A rhodium plating solution was also prepared as a comparative example, using hypophosphite (sodium hypophosphite) as a blackening agent. The metallic rhodium concentration and sulfuric acid concentration of the comparative plating solution were the same as those of the above-mentioned examples, but the difference was that the comparative plating solution did not contain a blackening agent or magnesium salt.

[0035] Then, a rhodium plating film was formed using each of the prepared black rhodium plating solutions. A test panel (20 mm x 20 mm) with a 0.1 μm gold film formed on a 5 μm nickel substrate was used as the substrate, and plating was carried out under the following conditions: Plating bath pH: 2.0 to 2.3, Bath temperature: 50°C, Current density: 2 A / dm ２ ・Plating time: 2 min

[0036] After the plating treatment, the substrate was removed and the amount of rhodium precipitated was measured from the change in mass, and the thickness of the plating film and the deposition efficiency were calculated. ＊ a ＊ b ＊ L in color space ＊ Value, a ＊ value, b ＊ The values ​​were measured using a spectrophotometer (CM-3600d, manufactured by Konica Minolta Japan, Inc.) in an SCI system including specular reflection light.

[0037] The appearance of the rhodium plating film was evaluated to see if there was any change in appearance after the plating process. 24 hours after the plating process, the appearance of the rhodium plating film was observed to check for any changes, such as color unevenness, in the color (black) of the rhodium plating film.

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

[0039]

[0040] As can be seen from Table 1, the rhodium plating films formed using the rhodium plating solutions containing the phosphonic acid derivative salts as blackening agents in Examples 1 to 3 of this embodiment all had a blackening effect of L ＊ The value of the rhodium plating solution in Comparative Example 1 was the same as that in Examples 1 to 3 except for the presence or absence of a blackening agent, and the rhodium plating film formed was nearly black to the naked eye. ＊ The value exceeded 70. It was confirmed that a blackening agent is necessary to obtain a black rhodium plating film.

[0041] The rhodium plating solution of Comparative Example 2, which uses hypophosphorous acid as a blackening agent, also showed a blackening effect. ＊ A black rhodium plating film with a value of 70 or less could be obtained. However, the black rhodium plating film of Comparative Example 2 showed color unevenness 24 hours after film formation. In contrast, no such change in appearance was observed in the black Rh plating films of 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 color) of the black rhodium plating film. Figure 1 shows photographs of the appearance of the rhodium plating films of Example 1 (blackening agent: diethylenetriaminepenta(methylenephosphonic acid) hexasodium 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 in deposition efficiency to the black rhodium plating solution of Comparative Example 2. From the above results, it was confirmed that the application of a phosphinic acid derivative or a salt thereof is suitable as a blackening agent for black rhodium plating solutions. The rhodium plating films obtained with Example 1 (blackening agent: diethylenetriaminepenta(methylenephosphonic acid) hexasodium salt) and Example 3 (blackening agent: 1-hydroxyethylidene-1,1-diphosphonic acid tetrasodium salt) were superior in deposition efficiency to the black rhodium plating solution of Comparative Example 2. ＊ Since the values ​​are 65 or less, these can be said to be particularly preferable blackening agents among the phosphinic acid derivatives.

[0043] Second Embodiment: In this embodiment, the stability of the black rhodium plating film obtained by repeatedly performing plating treatment on a black rhodium plating solution was evaluated. The black rhodium plating solution used in this embodiment was the plating solution of Example 1 of the first embodiment, to which 20 mM of diethylenetriaminepenta(methylenephosphonic acid) hexasodium salt was added as a blackening agent (metallic rhodium concentration: 5 g / L, sulfuric acid concentration: 20 mL / L, metallic magnesium concentration: 1 g / L). In this embodiment, a plating solution to which no magnesium salt (magnesium sulfate) was added was also evaluated as a reference example to confirm the effect of magnesium in the black rhodium plating solution.

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

[0045]

[0046] From Table 2, it can be seen that the black rhodium plating solution containing a phosphonic acid derivative or the like as a blackening agent, but not containing magnesium, exhibited L ＊ A rhodium plating film having a value of more than 70 is formed. ＊ It can be confirmed that the addition of magnesium is necessary to stably form a black rhodium plating film with a value of 70 or less. Furthermore, the black rhodium plating solution containing magnesium and a phosphonic acid derivative as a blackening agent maintains the L value even after multiple plating processes. ＊ It was confirmed that a black rhodium plating film 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, plating was performed while varying 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) hexasodium salt (20 mM)) was used.

[0048] The reference plating conditions were set as follows. In the evaluation test of this embodiment, one of the conditions of pH, bath temperature, and current density was adjusted, and the other conditions were plating treatment under the reference plating conditions. Reference plating conditions: Plating bath pH: 2.2 (adjusted: 1.5 to 2.5); Bath temperature: 50°C (adjusted: 50°C to 60°C); Current density: 2 A / dm ２ (During adjustment: 0.1A / dm ２ ~3.0 A / dm ２ ) Plating time: 2 min

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

[0050] From FIG. 2, it can be seen that the L of the rhodium plating film is increased when the plating bath temperature is in the range of 50°C to 60°C. ＊ The value was 65 or less, and the appearance was good.

[0051] Furthermore, as can be seen from FIG. 3, when the pH of the plating solution is 1.5, the L ＊ 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 more, and a pH of 2.0 or more will ＊ Therefore, a more preferable black rhodium plating film having a value of 65 or less can be obtained.

[0052] When the current density of the plating process is adjusted, the characteristics of the rhodium plating film are examined. As shown in FIG. 4, the L of the rhodium plating film decreases with decreasing current density. ＊ However, it was also confirmed that a decrease in current density leads to an increase in the contact resistance of the rhodium-plated film.

[0053] Here, taking into consideration the application of the black rhodium plating film, the current density of the plating process and the standard for appearance (L ＊ When black rhodium plating is applied to conductive materials such as connectors, if the reference value for contact resistance is set to 30 mΩ or less, the current density for forming the black rhodium plating film for the above-mentioned purpose should be set to 1.0 A / dm ２ 2.5A / dm or more ２ On the other hand, when the black rhodium plating film is used for decorative purposes, the contact resistance does not matter, so a current density lower than the above range may be used.

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

[0055] Fourth Embodiment: In this embodiment, the running property of the black rhodium plating solution according to the present invention was investigated. The plating solution of Example 1 of the first embodiment (diethylenetriaminepenta(methylenephosphonic acid) hexasodium salt: 20 mM, metallic rhodium concentration: 5 g / L, sulfuric acid concentration: 20 mL / L, metallic magnesium concentration: 1 g / L) was used, and plating was carried out while adding a plating solution as a replenisher in an amount corresponding to the consumed metallic rhodium.

[0056] First, plating was performed up to 1 MTO. Here, plating was performed under the following conditions, and the appearance of the formed rhodium plating film and the deposition efficiency were evaluated every 0.25 MTO, and replenisher solution was added. The results are shown in Table 3. Plating bath pH: 2.2, bath temperature: 50°C, current density: 2 A / dm ２

[0057]

[0058] From Table 3, in the plating process 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 formed. On the other hand, when looking at the deposition efficiency, a slight decrease was observed in the deposition efficiency after 0.75 MTO.

[0059] Therefore, we decided to examine whether or not it is possible to carry out plating treatment for a period longer than 1MTO. In this examination, we considered the case of plating treatment beyond 1MTO up to 2MTO, and examined whether it is possible to cope with this by changing the plating conditions at a predetermined stage. Specifically, the bath temperature was raised by 5°C in plating treatment after 1MTO, and in addition, the current density was reduced to 1A / dm in plating treatment at 2MTO. ２ Regarding this study, the evaluation results of plating treatment after 1 MTO are shown in Table 4.

[0060]

[0061] From Table 4, it is confirmed that the black rhodium plating solution of the present invention can continue plating with stable deposition efficiency by slightly adjusting the bath temperature and current density during operation. ＊ The rhodium plating film after plating with 2MTO exhibited a suitable black color with a value of 65 or less. When the appearance of the rhodium plating film was observed 24 hours later, no changes in appearance such as color unevenness were observed.

[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 is excellent in stability and can form a black rhodium film with good deposition efficiency. The black rhodium plating solution according to the present invention and the plating method for forming a black rhodium plating film using the same are expected to be applied to terminals and connectors of electronic devices such as smartphones and tablet terminals. They are also useful for ornaments, jewelry, and the like.

Claims

1. A black rhodium plating solution comprising: a rhodium salt having a metallic rhodium concentration of 1 g / L or more and 20 g / L or less; sulfuric acid having a concentration of 10 mL / L or more and 100 mL / L or less; a phosphonic acid derivative or a salt thereof having a concentration of 2 mM or more and 100 mM or less; and a magnesium salt having a metallic magnesium concentration of 0.1 g / L or more and 10 g / L or less.

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

3. The black rhodium plating solution according to claim 1 or 2, wherein the phosphonic acid derivative or its salt is an amino(methylene phosphonic acid), an alkyleneamine(methylene phosphonic acid), a hydroxyalkylidene diphosphonic acid, or a salt thereof.

4. The black rhodium plating solution according to claim 3, wherein the phosphonic acid derivative or its salt is diethylenetriaminepentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, or a salt thereof.

5. The black rhodium plating solution according to claim 1 or 2, having a pH of 1.8 to 2.

5.

6. A method for plating a black rhodium plating film using the black rhodium plating solution according to claim 1 or 2, comprising the steps of: a plating bath temperature of 50°C to 60°C and a current density of 0.1 A / dm ２ 2.5A / dm or more ２ A plating method in which plating is carried out as follows.

7. The method for producing a black rhodium plating film according to claim 6, wherein the plating process is carried out with the pH of the black rhodium plating solution being 1.8 to 2.5.

Images