Composite additive for palladium-rhodium alloy plating and palladium-rhodium alloy plating solution

Abstract

This invention belongs to the field of palladium-rhodium alloy plating technology, specifically relating to a composite additive and plating solution for palladium-rhodium alloy plating. It comprises a polysulfonic acid aromatic compound and an amino polycarboxylic acid ligand compound; the polysulfonic acid aromatic compound refers to a compound with two or more sulfonic acid groups attached to an aromatic ring, wherein the aromatic ring is selected from a benzene ring, a naphthalene ring, or a pyridine ring; the amino polycarboxylic acid ligand compound refers to a compound containing one or more amine groups and two or more carboxyl groups, wherein the amine groups are secondary or tertiary amines. The technical solution provided by this invention solves the problem of component segregation in palladium-rhodium electroplating through a composite additive, resulting in a stable and uniform palladium-rhodium alloy plating layer that is bright, dense, has high micro-Vickers hardness, low stress, and is corrosion resistant, perfectly replacing nickel plating.

Description

Technical Field

[0001] This invention belongs to the field of palladium-rhodium alloy coating technology, specifically relating to a composite additive for palladium-rhodium alloy coating and a palladium-rhodium alloy plating solution. Background Technology

[0002] With increasingly stringent environmental regulations (such as the RHS directive) and consumer product safety requirements, the demand for nickel-free electroplating technology is becoming increasingly urgent in fields such as electronic components, precision connectors, and high-end jewelry. Nickel is a common allergenic metal, and long-term contact with products containing nickel plating can easily cause skin allergies. Traditional nickel-alternative platings, such as copper-tin alloys, have solved the nickel-free problem to some extent, but they still cannot meet the requirements for use in extreme environments in terms of corrosion resistance, hardness, and high-temperature stability.

[0003] Chinese patent CN101096769A provides a nickel substitute metal, which is selected from ruthenium, rhodium, palladium, or any 2-4 alloys of palladium, rhodium, ruthenium, and cobalt. Palladium-rhodium alloys have a lower cost than rhodium plating and better corrosion resistance and wear resistance than other rhodium alloys, making them an ideal choice for nickel plating. However, this patent only provides practical preferred examples of ruthenium, rhodium, palladium, ruthenium-rhodium alloys, or palladium-cobalt alloys.

[0004] In the existing technology, the palladium-rhodium alloy exhibits a stepwise deposition phenomenon during electroplating due to the actual deposition potential difference between palladium and rhodium ions. This means that the composition will segregate, and it is not a palladium-rhodium alloy, but more like a composite electroplating layer formed by layered electroplating. The instability of the deposited composition will also affect its overall performance, and ultimately it cannot achieve the effect of replacing nickel plating. Summary of the Invention

[0005] This invention provides a composite additive for palladium-rhodium alloy coatings and a palladium-rhodium alloy plating solution to solve the technical problem of component segregation in current palladium-rhodium alloy electroplating deposition.

[0006] To solve the above-mentioned technical problems, the present invention provides a composite additive for palladium-rhodium alloy coatings, which includes polysulfonic acid aromatic compounds and amino polycarboxylic acid ligand compounds;

[0007] The polysulfonic acid aromatic compound refers to a compound on which two or more sulfonic acid groups are attached to an aromatic ring, wherein the aromatic ring is selected from a benzene ring, a naphthalene ring, or a pyridine ring;

[0008] The aminopolycarboxylic acid ligand compound refers to a compound containing one or more amino groups and two or more carboxyl groups, wherein the amino groups are secondary or tertiary amines.

[0009] The polysulfonic acid aromatic compounds contain multiple sulfonic acid functional groups in their molecules, which can undergo weak secondary coordination with palladium and rhodium ions, forming a synergistic coordination system with the main complexing agent in the plating bath. This precisely regulates the dissociation balance and free ion activity of palladium and rhodium complex ions, specifically narrowing the actual deposition potential difference between palladium and rhodium ions. Meanwhile, the aminopolycarboxylic acid ligand compounds can form stable and soluble complexes with trace impurity metal ions in the plating bath, preventing impurity ions from interfering with the deposition potential of palladium and rhodium ions, ensuring the stability and consistency of potential control. Furthermore, they can embed into the interstices of the palladium-rhodium alloy lattice, relaxing the internal stress caused by lattice distortion, neutralizing the tensile stress during the plating deposition process, and preventing problems such as cracking, peeling, and flaking of thick plating layers.

[0010] Optionally, the aromatic ring is a naphthalene ring.

[0011] Optionally, the polysulfonic acid aromatic compound is selected from one or more of 1,5-naphthalenedisulfonic acid, 1,6-naphthalenedisulfonic acid, 2,7-naphthalenedisulfonic acid, 1,3,6-naphthaltrisulfonic acid, 1,3,5-naphthaltrisulfonic acid, and salts of the above compounds, preferably sodium 1,3,6-naphthaltrisulfonate or sodium 1,3,5-naphthaltrisulfonate.

[0012] Optionally, the amino group in the amino polycarboxylic acid ligand compound is a tertiary amine, and the substituents on the amino group include at least one hydroxyethyl (-CH2CH2OH) and at least two acetic acid groups (-CH2COOH).

[0013] Optionally, the aminopolycarboxylic acid ligand compound is selected from HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid) or HEIDA (N-hydroxyethylimine diacetic acid).

[0014] Optionally, the polysulfonic acid aromatic compound and the amino polycarboxylic acid ligand compound are mixed in a mass ratio of 1:(2-5).

[0015] The present invention also provides a palladium-rhodium alloy plating solution, which includes the above-mentioned composite additives for palladium-rhodium alloy plating, palladium salts, rhodium salts and wetting agents.

[0016] Optionally, the wetting agent shown is selected from one or more of isooctanol polyoxyethylene ether phosphate, octanol polyoxyethylene ether phosphate, and sec-octanol polyoxyethylene ether phosphate.

[0017] Optionally, the palladium salt is an ammonia-coordinated palladium salt, and the rhodium salt is a sulfate-based rhodium salt.

[0018] Using palladium salts coordinated with ammonia and rhodium salts in a sulfuric acid system helps to reduce the deposition potential difference between the two.

[0019] Optionally, the palladium salt is selected from one or more of tetraamminepalladium sulfate, diamminepalladium dichlorosulfate, tetraamminepalladium nitrate, and tetraamminepalladium chloride, and the rhodium salt is selected from one or more of rhodium sulfate pentadecyl hydrate, rhodium sulfate tetradecyl hydrate, rhodium sulfate tetrahydrate, and rhodium sulfate hexahydrate.

[0020] Optionally, the concentrations of each component in the palladium-rhodium alloy plating bath are as follows: palladium ions 5-30 g / L, rhodium ions 1-10 g / L, polysulfonic acid aromatic compounds 0.5-3 g / L, amino polycarboxylic acid ligand compounds 2-8 g / L, and wetting agent 0.1-1 mL / L.

[0021] Optionally, the palladium-rhodium alloy plating solution further includes a conductive salt, a pH buffer, and a complexing agent.

[0022] The technical solution provided by this invention solves the problem of component segregation in palladium-rhodium electroplating by using composite additives, and can obtain a stable and uniform palladium-rhodium alloy coating. The coating is bright, dense, has high micro Vickers hardness, low stress and corrosion resistance, and can perfectly replace nickel plating. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Examples 1-4

[0025] The palladium-rhodium alloy plating solution includes palladium salt (component A), rhodium salt (component B), polysulfonic acid aromatic compound (component C), amino polycarboxylic acid ligand compound (component D), wetting agent (component E), conductive salt (component F), pH buffer (component G), and complexing agent (component H). The specific composition and amount of each component in 1L of plating solution in Examples 1-4 are shown in Table 1. The weights of palladium salt and rhodium salt are calculated based on the molar amounts of palladium ions and rhodium ions.

[0026] Table 1

[0027]

[0028] Plating solution preparation process: After measuring each component, dissolve it in water, adjust the pH value to 8.5-9.5 with ammonia water, and finally bring the volume to 1L.

[0029] The conductive salt (component F), pH buffer (component G), and complexing agent (component H) of Comparative Examples 1-6 remained unchanged from those of Example 1. The main changes were to other components, as shown in Table 2.

[0030] Table 2

[0031]

[0032] Effect Experiment

[0033] The plating solutions of Examples 1-4 and Comparative Examples 1-6 were used to electroplate the surface of brass sheets using the same electroplating process parameters, as follows:

[0034] Plating bath temperature: 45℃; Current density: 2 A / dm²; Plating time: 20 minutes;

[0035] Anode: Platinum-plated titanium mesh

[0036] Substrate: Brass sheet (2cm×5cm), degreased and activated with 10% sulfuric acid.

[0037] The coating was tested using the following methods, and all testing methods comply with national or industry standards.

[0038] Test 1. Appearance Quality Inspection

[0039] Testing method: Under natural light or D65 standard light source, visually observe the color, uniformity, and presence of defects such as pinholes, pits, cracks, and bubbles on the coating surface. A qualified coating is characterized by a bright silver-white color, fine crystals, and no defects.

[0040] Test 2. Uniformity of coating surface composition

[0041] Detection method: Five test points were randomly selected on the coating surface by X-ray energy dispersive spectroscopy (EDS) to perform point scanning elemental analysis, determine the percentage of Pd and Rh atoms, and calculate the relative standard deviation (RSD) of Pd and Rh atomic contents.

[0042] Test 3. Coating composition analysis

[0043] Detection method: Elemental analysis of the coating surface was performed using energy dispersive X-ray spectroscopy (EDS) to determine the mass fraction and atomic percentage of palladium and rhodium.

[0044] Test 4. Porosity Test

[0045] Detection method: Phosphomolybdic acid (PMA) colorimetric method was used. Filter paper was immersed in the PMA test solution, attached to the coating surface, and left for 5 minutes before being removed. The number of blue spots on the filter paper was observed. The number of pores per unit area (1 cm²) was counted. The fewer the pores, the better the coating density.

[0046] Test 5. Micro Vickers Hardness Test

[0047] Testing method: A micro Vickers hardness tester was used, with a load of 50g and a holding time of 15 seconds. Five points were randomly selected on the coating surface for measurement, and the average value was taken.

[0048] Calculation formula: HV = 0.1891 × F / d² (where F is the load force and d is the length of the indentation diagonal)

[0049] 6. Corrosion resistance test

[0050] Test 6.1 Neutral Salt Spray Test (NSS)

[0051] Test conditions: 5% sodium chloride solution, pH 6.5-7.2, spray chamber temperature 35℃±2℃, spray pressure 0.8-1.2 bar, continuous spraying.

[0052] Test method: Place the sample at an angle of 15-30° to the vertical direction, observe it every 24 hours, and record the time when the first corrosion point appears.

[0053] Test 6.2 Nitric Acid Vapor Test

[0054] Test conditions: Place the sample in a sealed container with concentrated nitric acid (68%) at the bottom and expose it at 25℃±2℃ for 48 hours. Observe the oxidation and discoloration of the coating surface and measure the number of corrosion spots or the change in oxide layer thickness.

[0055] Test 6.3 Electrochemical Test

[0056] Test method: A three-electrode system was used, with the coated sample as the working electrode (exposed area 1 cm²), a platinum electrode as the auxiliary electrode, and a saturated calomel electrode as the reference electrode. The corrosive medium was 3.5% NaCl solution, and the scan rate was 1 mV / s. The self-corrosion potential (Ecorr) and self-corrosion current density (Icorr) were recorded, and the polarization resistance (Rp) was calculated using the Tafel extrapolation method. The higher the polarization resistance, the better the corrosion resistance.

[0057] Performance test results analysis:

[0058] Table 3 shows the results of appearance quality inspection, coating surface composition uniformity test, coating composition analysis, and porosity test for Examples 1, 1, and 2. Comparative Example 1 used an ammonia-coordinated palladium salt and a sulfuric acid-based rhodium salt, along with necessary conventional components. Based on this, it was found that the appearance was dull, the surface was rough with cracks, the surface composition was uneven, segregation reduced coating adhesion, and the rhodium content in the coating composition was too low, indicating deposition difficulties and high porosity. Therefore, its micro strength and corrosion resistance would inevitably be substandard. To address these issues, Comparative Example 2 added commonly used brighteners, stress relievers, and wetting agents. However, the test results showed that while the appearance improved, fine cracks appeared in high current density areas. Although the rhodium deposition in the coating composition significantly increased, the surface composition uniformity remained poor, the adhesion was low, and although the porosity decreased significantly, a considerable amount still remained. Based on these effects, it can be inferred that its micro strength and corrosion resistance cannot meet the requirements.

[0059] Table 3

[0060] Test Project Example 1 Comparative Example 1 Comparative Example 2 Test 1 qualified Grayish-white, rough surface, with microscopic cracks. It is shiny, but there are fine cracks in the high current density area. Test 2 (RSD%) 4 23 20 Test 3 (Pd / Rh mass ratio) 72 / 28 85 / 15 78 / 22 Test 4 (pieces / cm²) 0-1 15-25 5-10

[0061] Compared with Example 1, some components of Comparative Examples 3-6 were replaced. The test results of Examples 1-4 and Comparative Examples 3-6 are shown in Table 4. It can be seen that the solution provided by the present invention is specific and has practical significance.

[0062] Table 4

[0063] Test 1 Test 2 (RSD%) Test 3 (Pd / Rh mass ratio) Test 4 (pieces / cm²) Test 5 (HV 0.05) Test 6.1 (h) Test 6.2 Test 6.3 (μA / cm²) Example 1 qualified 4 72 / 28 0-1 685 120 Slight discoloration, no corrosion spots 0.32 Example 2 qualified 5 75 / 25 0-2 658 120 Slight discoloration, no corrosion spots 0.38 Example 3 qualified 5 70 / 30 1-3 642 96 Slight discoloration, a few tiny spots 0.45 Example 4 qualified 6 68 / 32 0-1 712 144 It hardly changes color and has no corrosion spots. 0.28 Comparative Example 3 Semi-gloss, rough surface, with visible microcracks 15 80 / 20 8-15 502 48 Noticeable discoloration, multiple corrosion spots 1.58 Comparative Example 4 Grayish-white, rough surface, no cracks 16 82 / 18 10-15 480 24 Severe discoloration, multiple corrosion spots 1.71 Comparative Example 5 The coating is bright, with a few pinholes on the surface. 5 75 / 25 12-14 630 48 Noticeable discoloration, multiple corrosion spots 1.22 Comparative Example 6 qualified 11 85 / 15 5-10 522 48 Noticeable discoloration, multiple corrosion spots 0.97

[0064] Results analysis:

[0065] The hardness of the coating is affected by the grain size, Pd / Rh ratio, and degree of segregation, while the corrosion resistance is affected by the density of the coating, the presence of pinholes on the surface, and the degree of segregation.

[0066] 1. Comparative Examples 3 and 4 reveal poor uniformity of coating composition and high segregation. This non-uniformity affects grain size and growth rate, leading to microscopic voids, reduced coating density, and increased porosity. Comparative Examples 5 and 6, containing both components C and D, show improved coating composition uniformity, indicating a synergistic effect between components C and D on coating composition uniformity and rhodium deposition in co-deposition. Furthermore, component C refines grains and improves surface gloss, while component D regulates stress and reduces surface cracks.

[0067] 2. In Comparative Example 6, the uniformity of the coating composition was improved due to the presence of components C and D. However, due to the unsuitable selection of rhodium and palladium salts, the Pd / Rh ratio was still relatively high after co-deposition, making rhodium deposition difficult. This indicates that the selection of rhodium and palladium salts has a significant impact on the Pd / Rh ratio, which in turn directly affects the hardness and corrosion resistance of the coating.

[0068] 3. The E component wetting agent reduces the surface tension of the plating solution, reduces pinholes and pits, and improves density. Therefore, the main problem of Comparative Example 5 is its high porosity and poor density, resulting in a significant difference in its corrosion resistance compared to Example 1.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite additive for palladium-rhodium alloy coatings, characterized in that, Including polysulfonic acid aromatic compounds and amino polycarboxylic acid ligand compounds; The polysulfonic acid aromatic compound refers to a compound on which two or more sulfonic acid groups are attached to an aromatic ring, wherein the aromatic ring is selected from a benzene ring, a naphthalene ring, or a pyridine ring; The aminopolycarboxylic acid ligand compound refers to a compound containing one or more amino groups and two or more carboxyl groups, wherein the amino groups are secondary or tertiary amines.

2. The composite additive for palladium-rhodium alloy plating according to claim 1, characterized by comprising: The aromatic ring is a naphthalene ring.

3. The composite additive for palladium-rhodium alloy coating according to claim 2, characterized in that, The polysulfonic acid aromatic compound is selected from one or more of 1,5-naphthalenedisulfonic acid, 1,6-naphthalenedisulfonic acid, 2,7-naphthalenedisulfonic acid, 1,3,6-naphthaltrisulfonic acid, 1,3,5-naphthaltrisulfonic acid, and salts of the above compounds, preferably sodium 1,3,6-naphthaltrisulfonate or sodium 1,3,5-naphthaltrisulfonate.

4. The composite additive for palladium-rhodium alloy coating according to claim 1, characterized in that, The amino group in the aminopolycarboxylic acid ligand compound is a tertiary amine, and the substituents on the amino group include at least one hydroxyethyl group and at least two acetic acid groups.

5. The composite additive for palladium-rhodium alloy coating according to claim 4, characterized in that, The aminopolycarboxylic acid ligand compound is selected from HEDTA or HEIDA.

6. The composite additive for palladium-rhodium alloy coating according to claim 1, characterized in that, The polysulfonic acid aromatic compounds and amino polycarboxylic acid ligand compounds are mixed in a mass ratio of 1:(2-5).

7. A palladium-rhodium alloy plating solution, characterized in that, The composite additive, palladium salt, rhodium salt, and wetting agent are included in any of the palladium-rhodium alloy coatings described in claims 1-6.

8. The palladium-rhodium alloy plating solution according to claim 7, characterized in that, The wetting agent shown is selected from one or more of isooctanol polyoxyethylene ether phosphate, octanol polyoxyethylene ether phosphate, and sec-octanol polyoxyethylene ether phosphate.

9. The palladium-rhodium alloy plating solution according to claim 7, characterized in that, The palladium salt is an ammonia-coordinated palladium salt, and the rhodium salt is a sulfate-based rhodium salt.

10. The palladium-rhodium alloy plating solution according to claim 9, characterized in that, The palladium salt is selected from one or more of tetraamminepalladium sulfate, dichlorodiamminepalladium, tetraamminepalladium nitrate, and tetraamminepalladium chloride, and the rhodium salt is selected from one or more of rhodium sulfate pentadecyl hydrate, rhodium sulfate tetradecyl hydrate, and rhodium sulfate hexahydrate.

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