Black rhodium plating solution, and plating method of black rhodium plating 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 for forming a rhodium plating film made of rhodium and exhibiting black, and a plating method thereof. Specifically, it relates to a plating solution capable of stably forming a rhodium plating film exhibiting a clearer black than conventional ones, and a plating method for forming a suitable black rhodium plating film using the plating solution.

Background Art

[0002] Rhodium (Rh), which is one of the platinum group metals, is chemically stable, has excellent corrosion resistance and heat resistance, has high hardness and good wear resistance, and is often used in the form of a plating film. Rhodium plating films have been used for ornaments and jewelry since ancient times, and their use in members of electric and electronic devices has been expanding due to their excellent electrical conductivity. 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 these various applications have been conventionally known (for example, Patent Document 1 by the applicant of the present application).

[0003] Rhodium plating films generally exhibit the original silver-white color of rhodium ingots. Also in the above Patent Document 1, the formation of a rhodium plating film with a silver-white luster is aimed at. On the other hand, depending on the composition of the rhodium plating solution, it is also possible to form a black rhodium plating film. So far, black rhodium plating films have been used in some applications such as decorative uses, but recently their use in terminals of electronic devices and the like has also been studied. Many recent electronic devices such as tablets have a strong popularity with black adopted for the housing. For such black housings, a higher design can be imparted by making the appearance of connectors and terminals black to give a sense of unity. Therefore, the demand for black rhodium plating films as electronic materials such as connectors is also 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 "black 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 study by the present inventors, the black rhodium plating film obtained by the above conventional black rhodium plating solution exhibits a suitable black color immediately after film formation, but it has been confirmed that appearance changes such as color unevenness occur over time. The black rhodium plating film that can cause such appearance changes is difficult to be applied to the various uses described above. In addition, it is hard to say that the above conventional black rhodium plating solution has good stability of the solution itself, and rhodium plating films with different degrees of blackness may be formed by repeated plating processes.

[0008] The present invention has been made under such a 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 capable of forming a black rhodium plating film without appearance change and having excellent stability of the plating bath. In addition, while considering the uses as electrical materials such as the above-described connectors and decorative materials such as jewelry, a plating method for forming a suitable black rhodium plating film is also clarified.

Means for Solving the Problems

[0009] The present invention for solving the above problems is a black rhodium plating solution containing a rhodium salt with a metal 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 its salt with a concentration of 2 mM or more and 100 mM or less, and a magnesium salt with a metal magnesium concentration of 0.1 g / L or more and 10 g / L or less.

[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 or the like), and a magnesium salt are included as characteristic essential components. The phosphonic acid derivative or the like acts as a blackening agent for blackening the rhodium plating film. Further, the magnesium salt has an action for the plating solution to continuously deposit a desired black rhodium plating film. Hereinafter, the configuration 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 addition, in the present invention, as an objective index for determining whether the black rhodium plating film exhibits a suitable "black" color, L * a * b * the color space (CIELab color space) is applied. L * a * b * In the color space, L * the value indicates the lightness (white, black) of the color, and a * the value, b * the value indicates the direction of the color (a * : red direction, green direction, b * : yellow direction, blue direction). And 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) Configuration 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 or the like as a blackening agent and a magnesium salt as essential constituent components.

[0013] (A-1) Rhodium salt As the rhodium salt serving as the metal source of the black rhodium plating film, rhodium sulfate, rhodium phosphate, and rhodium sulfite can be applied. In the black rhodium plating solution according to the present invention, these rhodium salts are contained at a metal rhodium concentration of 1 g / L or more and 20 g / L or less. If the metal rhodium concentration is less than 1 g / L, the deposition efficiency of the rhodium plating film becomes too low. In addition, a plating solution with a metal rhodium concentration exceeding 20 g / L has a large content of rhodium that does not contribute to the deposition of the rhodium plating film, which causes an increase in cost. The metal 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, there is a risk that the rhodium salt will hydrolyze. In addition, if the sulfuric acid concentration exceeds 100 mL / L, there is a risk of inhibiting the movement of rhodium (ions) in the solution. 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) The phosphonic acid derivative and its salt act as a blackening agent for blackening the rhodium plating film in the present invention, and are one of the characteristic configurations of the present invention. The phosphonic acid derivative is a compound in which the hydrogen atom (H) bonded to the phosphorus atom (P) of phosphonic acid (H 3 PO 3 ) is substituted by an organic group. The salt of the phosphonic acid derivative is a compound in which the hydrogen atom bonded to the oxygen atom (O) of the phosphonic acid derivative is substituted by another element such as a metal.

[0016] In the present invention, using a phosphonic acid derivative or the like as a blackening agent is because by applying a phosphonic acid derivative or the like, L *This is because a rhodium plating film with a value of 70 or less can be formed. In addition, a black rhodium plating film formed by applying a phosphonic acid derivative or the like can maintain the appearance (black color) after film formation. Regarding these effects, hypophosphorous acid or the like, which is a conventional technique, also acts as a blackening agent for the rhodium plating film, and L * It is possible to form a black rhodium plating film with a value of 70 or less. However, a black rhodium film formed by hypophosphorous acid or the like has L * Even if the value is good immediately after film formation, appearance changes 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(methylenephosphonic acid) such as aminomethylphosphonic acid and aminotri(methylenephosphonic acid), alkyleneamine(methylenephosphonic acid) such as ethylenediaminetetra(methylenephosphonic acid), hexamethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic 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. Examples of the salt of the phosphonic acid derivative include alkali metal salts such as the sodium salt of the above compound. Among the above phosphonic acid derivatives and the like, particularly preferred are diethylenetriaminepentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and salts thereof. These phosphonic acid derivatives and the like can form 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] In the black rhodium plating solution according to the present invention, the content of the phosphonic acid derivative or the like is set to 2 mM or more and 100 mM or less. If it is less than 2 mM, the effect is the same as when there is no addition. Also, even if the content of the phosphonic acid derivative or the like increases, the L * value does not change significantly. And when it exceeds 100 mM, the film appearance becomes non-uniform, so this content is set as the upper limit. In order to make the film appearance particularly good, it is preferable that the content of the phosphonic acid derivative or the like is 5 mM or more and 60 mM or less.

[0019] (A-4) Magnesium salt The reason for making the magnesium salt an essential component in the black rhodium plating solution according to the present invention is to stably form a black rhodium plating film having a desired L * value during repeated plating processes. That is, the magnesium salt contributes to improving the life of the black rhodium plating solution. As this magnesium salt, magnesium sulfate, magnesium oxide, magnesium chloride, and magnesium bromide are applicable, and one or more of these magnesium salts can be included.

[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 metal magnesium concentration. This is because if it is less than 0.1 g / L, there is no effect, while if it exceeds 10 g / L, the deposition rate tends to decrease. The metal 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 already known that in conventional rhodium plating solutions, alkaline earth metals such as magnesium are included as additives. For example, the rhodium plating solution described in Patent Document 1 mentioned above essentially comprises at least one compound selected from the group consisting of phosphorous acid, alkali metal phosphites, alkaline earth metal phosphites, or ammonium phosphites. Therefore, there is a possibility that it contains an alkaline earth metal salt (such as magnesium phosphite). However, in the rhodium plating solution described in Patent Document 1, a compound that generates phosphorous acid (ions) is essential for forming an amorphous rhodium plating film, and the alkaline earth metal is merely a counter cation of the compound. That is, in conventional rhodium plating solutions such as Cited Document 1, magnesium is not an essentially required additive. On the contrary, in the black rhodium plating solution according to the present invention, magnesium is an essential component having a specific action of stably forming a black rhodium plating film in cooperation with a phosphonic acid derivative or the like which is a blackening agent.

[0022] (A-5) Other configurations The black rhodium plating solution according to the present invention essentially comprises the above-mentioned rhodium salt, sulfuric acid, phosphonic acid derivative, etc., and a magnesium salt. However, it can also contain other configurations such as other additives. It can contain known additives used in electrolytic plating solutions. For example, it is acceptable to contain a pH buffer, a complexing agent, a stabilizer, etc.

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

[0024] (B) Plating method of the black rhodium plating film according to the present invention The plating method of the black rhodium plating film with the black rhodium plating solution according to the present invention described above basically conforms to the ordinary electrolytic plating method, but it is preferable to set the following plating treatment conditions.

[0025] (B-1) Plating solution pH The pH of the plating solution during the plating treatment is set to be 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. Also, if it exceeds 2.5, the liquid stability decreases and rhodium may precipitate. To make the L value of the rhodium plating film * a more suitable value, the pH of the plating solution is preferably 2.0 or more.

[0026] (B-2) Plating bath temperature In the plating method using the black rhodium plating solution of the present invention, as the bath temperature rises, the L of the rhodium plating film *A decrease in 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 be 50°C or higher and 60°C or lower. If it is less than 50°C, the deposition efficiency is low and efficient film formation becomes difficult. Also, a bath temperature exceeding 60°C has problems such as a decrease in the stability of the plating solution and deterioration of the jig. The bath temperature is preferably 50°C or higher and 55°C or lower. Incidentally, it is preferable to stir the plating solution during the plating process.

[0027] (B-3) Current density According to the studies by the present inventors, the rhodium plating film formed by the black rhodium plating solution of the present invention has a decrease in the L value of the rhodium plating film as the current density during electrolysis decreases. * On the other hand, the resistance value (contact resistance value) of the rhodium plating film increases as the current density decreases. Considering the balance between the L value and the contact resistance value of the rhodium plating film, the current density during electrolysis is 0.1 A / dm or more and 2.5 A / dm or less. * In particular, when applying the black rhodium plating film according to the present invention to an electrical member such as a connector, in order to suppress the contact resistance value, the current density is preferably 1.0 A / dm or more and 2.5 A / dm or less. 2 On the other hand, when applying the black rhodium plating film for decorative purposes, since there is no need to worry about the contact resistance, the current density is preferably 0.1 A / dm or more and 2.0 A / dm or less. 2 2 2 2 2 * * * * *

[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), the L value in the Lab color space is 70 or less. Also, by optimizing the composition of the black rhodium plating solution and the plating conditions within the scope of the present invention, the L * a * b * value in the color space is * 70 or less. Also, by optimizing the composition of the black rhodium plating solution and the plating conditions within the scope of the present invention, the L *It becomes a more preferable black rhodium plating film with a value of 60 or more and 65 or less. Regarding the black rhodium plating according to the present invention, a * value and b * value are not particularly limited.

[0029] In addition, since the black rhodium plating solution of the present invention requires a phosphonic acid derivative or the like, a small amount of P is contained in the formed black rhodium plating film. Specifically, it contains 4% by mass or less of P. Therefore, the black rhodium plating film formed by the black rhodium plating solution of the present invention can be specified by the above-mentioned L * value (70 or less) and P concentration (4% by 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 if the film thickness becomes excessively thick, there is a possibility of including cracks. According to the black rhodium plating solution according to the present invention, it is possible to impart a suitable black color to a rhodium plating film having a film thickness within the above range.

Advantages of the Invention

[0030] As described above, according to the black rhodium plating solution according to the present invention, it is possible to stably form a rhodium plating film exhibiting a suitable black color. And the black rhodium plating film formed by the present invention can maintain 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 also has good stability as a plating solution.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0032] First Embodiment : Hereinafter, embodiments of the present invention will be described. In this embodiment, a black rhodium plating solution applying a plurality of types of phosphonic acid derivatives as a blackening agent was produced to form a black rhodium plating film.

[0033] Rhodium sulfate as a rhodium salt and sulfuric acid were added to pure water, and further, a salt of a phosphonic acid derivative as a blackening agent and magnesium sulfate as a magnesium salt were added to prepare a black rhodium plating solution. In this embodiment, while setting the metal rhodium concentration at 5.0 g / L, the sulfuric acid concentration at 20 mL / L, and the metal magnesium concentration at 1.0 g / L, a plurality of types of plating solutions were produced within the range where the content of the phosphonic acid derivative was 2 mM to 100 mM. As the phosphonic acid derivative as the blackening agent, three types of phosphonic acid derivative salts, namely, heptasodium diethylenetriaminepenta(methylenephosphonic acid) (Example 1), pentasodium ethylenediaminetetra(methylenephosphonic acid) (Example 2), and tetrasodium 1-hydroxyethylidene-1,1-diphosphonic acid (Example 3), were used. An aqueous solution of the above-mentioned phosphonic acid derivative salt was used for preparing the plating solution.

[0034] Also, as a comparative example, a rhodium plating solution applying hypophosphite (sodium hypophosphite) as a blackening agent was also produced. The metal rhodium concentration and sulfuric acid concentration of the plating solution in the comparative example are the same as those in the above embodiment, and the difference lies in that it does not contain a blackening agent and a magnesium salt.

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

[0036] After the plating process, the substrate was taken out, and the rhodium deposition amount was measured from the mass change to calculate the film thickness and deposition efficiency of the plating film. Then, for the obtained rhodium plating film, the L * a * b * values in the color space of L * value, a * value, and b * value were measured. This measurement was performed in the SCI mode including regular reflection light using a spectrocolorimeter (CM-3600d: manufactured by Konica Minolta Japan Inc.).

[0037] In the appearance evaluation of the rhodium plating film, the presence or absence of appearance changes after the plating process was evaluated. After the plating process, the appearance of the rhodium plating film was observed 24 hours later to confirm the presence or absence of changes such as color unevenness in the color (black) of the rhodium plating film.

[0038] Table 1 shows the evaluation results for the rhodium plating films obtained with the black rhodium plating solutions of Examples 1 - 3 and the comparative example of this embodiment.

[0039]

Table 1

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

[0041] And, even with the rhodium plating solution using hypophosphorous acid as the blackening agent in Comparative Example 2, an L * black rhodium plating film with a value of 70 or less could be obtained. However, the black rhodium plating film according to Comparative Example 2 had color unevenness 24 hours after film formation. On the other hand, such an appearance change was not observed in the black Rh plating films according to Examples 1 to 3. That is, it was confirmed that Comparative Example 2 was clearly inferior with respect to the stability of the appearance (black color) of the black rhodium plating film. FIG. 1 is a photograph of the appearance of the rhodium plating films immediately after plating and 24 hours after plating according to Example 1 (blackening agent: sodium diethylenetriaminepenta(methylenephosphonic acid) heptasodium salt (20 mM)) and Comparative Example 2 (blackening agent: sodium hypophosphite (2 mM)).

[0042] In addition, the black rhodium plating solutions of Examples 1 to 3 are also 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 its salt is suitable as the blackening agent for the black rhodium plating solution. Incidentally, the rhodium plating films according to Example 1 (blackening agent: sodium diethylenetriaminepenta(methylenephosphonic acid) heptasodium salt) and Example 3 (blackening agent: sodium 1-hydroxyethylidene-1,1-diphosphonate) have an L * value of 65 or less, so these can be said to be particularly preferable blackening agents among phosphinic acid derivatives.

[0043] Second Embodiment: In this embodiment, the stability of the black rhodium plating film obtained by performing repeated plating treatment on the 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, and 20 mM of sodium diethylenetriaminepenta(methylenephosphonic acid) 7 sodium salt was added as a blackening agent (metal rhodium concentration 5 g / L, sulfuric acid concentration 20 mL / L, metal magnesium concentration 1 g / L). Further, in this embodiment, as a reference example for confirming the effect of magnesium in the black rhodium plating solution, evaluation was also performed on a plating solution to which no magnesium salt (magnesium sulfate) was added.

[0044] In this embodiment, the conditions of the plating treatment were the same as those in the first embodiment (plating bath temperature: 50 °C, pH: 2.0). In this embodiment, the plating treatment of 0.1 MTO was repeated 5 times. Note that MTO is an abbreviation for "Metal Turn Over", and the plating treatment of 1 MTO is a treatment for depositing the same amount of rhodium as the rhodium content in the plating solution at the time of bath formation. Further, in this embodiment, the substrate was replaced for each plating treatment of 0.1 MTO, and an aqueous solution of the consumed rhodium salt was replenished. Then, the deposition efficiency in each plating treatment and the L * value, a * value, b * value were measured. The test results are shown in Table 2.

[0045]

Table 2

[0046] From Table 2, even for a black rhodium plating solution using a phosphonic acid derivative or the like as a blackening agent, a rhodium plating film with an L * value exceeding 70 is formed in the plating treatment after 0.3 MTO. That is, L *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. And a black rhodium plating solution containing magnesium while using a phosphonic acid derivative or the like as a blackening agent can be used for multiple plating treatments without the L * It was confirmed that a black rhodium plating film with a value of 70 or less could be stably formed, and the deposition efficiency was also stabilized.

[0047] Third Embodiment : In this embodiment, the plating treatment was performed while changing the plating treatment conditions, and the appearance of the obtained black rhodium plating film was evaluated. In this embodiment, the plating solution of Example 1 of the first embodiment (blackening agent: sodium salt of diethylenetriaminepenta(methylenephosphonic acid) 7 sodium salts (20 mM)) was used.

[0048] And 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 plated under the reference plating conditions. Reference Plating Conditions · Plating bath pH: 2.2 (during adjustment: 1.5 - 2.5) · Bath temperature: 50 °C (during adjustment: 50 °C - 60 °C) · Current density: 2 A / dm 2 (during adjustment: 0.1 A / dm 2 ~3.0 A / dm 2 ) · Plating time: 2 min

[0049] After the plating treatment under each condition, the deposition efficiency and the L value of the rhodium plating film were measured in the same manner as in the first embodiment. * value, a * value, b * value were measured. Also, for the test when the current density was changed, the contact resistance of the rhodium plating film was 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 of the plating solution without a blackening agent (phosphonic acid derivative, etc.) (plating treatment under the above reference conditions). Also, Fig. 4 shows the results when the current density was adjusted.

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

[0051] Also, from Figure 3, when the pH of the plating solution was 1.5, it can be seen that the L value of the rhodium plating film exceeded 70. * value of the L value exceeds 70. * In order to obtain an L value of 70 or less, it is necessary to set the pH of the plating solution to 1.8 or more, and by setting it to 2.0 or more, * a more suitable black rhodium plating film with an L value of 65 or less can be obtained.

[0052] Then, regarding the characteristics of the rhodium plating film when adjusting the current density of the plating process, from Figure 4, it can be seen that the L value of the rhodium plating film decreases as the current density decreases. However, it was also confirmed that the decrease in the current density leads to an increase in the contact resistance of the rhodium plating film. * value decreases. However, it was also confirmed that the decrease in the current density leads to an increase in the contact resistance of the rhodium plating film.

[0053] Here, while considering the use of the black rhodium plating film, the relationship between the current density of the plating process, the standard regarding the appearance (L * value of 70 or less) and the contact resistance is examined. When applying the black rhodium plating film to a conductive material such as a connector, if the reference value of the contact resistance is set to 30 mΩ or less, in forming the black rhodium plating film for the above use, the current density is preferably 1.0 A / dm 2 or more and 2.5 A / dm 2 or less. On the other hand, when using the black rhodium plating film for decorative purposes, since the level of the contact resistance is not a problem, it can be said that a lower current density than the above range may be used.

[0054] In addition, regarding the deposition efficiency, it was confirmed that it tends to decrease as the current density increases. However, the black rhodium plating solution according to the present invention basically has good deposition efficiency and is stable, and shows a good deposition efficiency of 20 mg / A·min or more under any conditions.

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

[0056] First, the plating treatment was carried out up to 1 MTO. Here, the plating treatment was carried out under the following conditions, and the appearance evaluation and deposition efficiency measurement of the rhodium plating film formed every 0.25 MTO were carried out, and the replenishing solution was added. The results at this time are shown in Table 3. · Plating bath pH: 2.2 · Bath temperature: 50 °C · Current density: 2 A / dm 2

[0057]

Table 3

[0058] From Table 3, in the plating treatment up to 1 MTO, since the L value of the rhodium plating film is maintained at 65 or less, it can be said that a good black rhodium plating film was stably formed. On the other hand, referring to the deposition efficiency, a slight decrease is observed in the deposition efficiency after 0.75 MTO. * Therefore, it was decided to examine whether it is possible to carry out the plating treatment for a longer period than 1 MTO. In this examination, considering the case of carrying out the plating treatment exceeding 1 MTO up to 2 MTO, it was examined whether it is possible to cope by changing the plating conditions at a predetermined stage. Specifically, the bath temperature was raised by 5 °C in the plating treatment after 1 MTO, and in addition, the current density was decreased to 1 A / dm in the plating treatment at 2 MTO.

[0059] For this examination, the evaluation results in the plating treatment after 1 MTO are shown in Table 4. 2 For this examination, the evaluation results in the plating treatment after 1 MTO are shown in Table 4.

[0060]

Table 4

[0061] From Table 4, it is confirmed that in the black rhodium plating solution according to the present invention, by slightly adjusting the bath temperature and current density during operation, the plating process can be continued with a stable deposition efficiency. And all the rhodium plating films formed are those that exhibit a suitable black color with an L * value of 65 or less. In addition, when the appearance of the rhodium plating film after the plating treatment at 2 MTO was observed after 24 hours, no appearance change such as color unevenness was observed here either.

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

Claims

1. A rhodium salt with a rhodium metal concentration of 1 g / L or more and 20 g / L or less, sulfuric acid at 10 mL / L or more and 100 mL / L or less, a phosphonic acid derivative or its salt at 2 mM or more and 100 mM or less, and a magnesium salt with a magnesium metal concentration of 0.1 g / L or more and 10 g / L or less, a black rhodium plating solution containing the same.

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 Claim 2, wherein the phosphonic acid derivative or its salt is an amino(methylenephosphonic acid) type, an alkyleneamine(methylenephosphonic acid) type, a hydroxyalkylidene diphosphonic acid type, or a salt thereof.

4. The black rhodium plating solution according to Claim 3, wherein the phosphonic acid derivative or its salt is diethylenetriamine pentamethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and salts thereof.

5. The black rhodium plating solution according to Claim 1 or Claim 2, having a pH of 1.8 or more and 2.5 or less.

6. A plating method for a black rhodium plating film using the black rhodium plating solution according to Claim 1 or Claim 2, A plating method in which plating is performed at a plating bath temperature of 50°C or higher and 60°C or lower and a current density of 0.1 A / dm ２ or higher and 2.5 A / dm ２ or lower.

7. The plating method for a black rhodium plating film according to Claim 6, wherein plating treatment is performed with the pH of the black rhodium plating solution being 1.8 or more and 2.5 or less.

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