A functional cyanide-free silver plating method and plated layer structure

Abstract

This invention provides a functional cyanide-free silver plating method and coating structure, belonging to the field of metal surface treatment technology. This invention adds a trivalent chromium plating layer between the cyanide-free copper plating layer and the cyanide-free silver plating layer prepared by existing technology. The trivalent chromium plating layer provides electrochemical protection to the underlying copper plating layer, and the resulting coating structure effectively prevents corrosive media from eroding towards the substrate. The trivalent chromium plating layer has excellent corrosion resistance, effectively resisting corrosion from corrosive media, overcoming the defect of copper rust easily appearing on the surface of parts plated directly on copper plating layers. Simultaneously, by controlling the composition of the plating solution and adding lanthanum chloride heptahydrate and praseodymium chloride heptahydrate, the two have a synergistic effect, further improving the corrosion resistance of the plated parts.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a functional cyanide-free silver plating method and plating structure. Background Technology

[0002] Silver plating possesses excellent electrical conductivity and weldability, making it widely used in the manufacturing of electrical appliances, electronics, communication equipment, and instruments. Due to the high toxicity of cyanide, the industry has conducted extensive research on replacing cyanide silver plating with environmentally friendly cyanide-free silver plating processes. However, cyanide-free silver plating is one of the most challenging plating processes among cyanide-free technologies. The corrosion resistance and discoloration resistance of cyanide-free silver plating layers still lag behind those of cyanide silver plating, and cyanide-free silver plating systems typically suffer from poor plating solution stability.

[0003] The typical process for cyanide-free silver plating of aluminum alloy parts in the electrical manufacturing industry involves sequentially performing chemical zinc plating, chemical nickel plating, cyanide-free copper plating, cyanide-free silver plating, and electrolytic protection on the aluminum alloy substrate. This plating structure has two main drawbacks: First, it lacks electrochemical protection. Second, the copper plating layer in this structure has low corrosion resistance; corrosive media can penetrate the voids in the silver plating layer and corrode the copper plating layer, resulting in reddish-brown copper rust on the surface of the plated part. Furthermore, the corrosive media can penetrate the copper plating layer and corrode the aluminum alloy substrate, leading to extensive white rust on the surface of the plated part. Summary of the Invention

[0004] The purpose of this invention is to provide a functional cyanide-free silver plating method and a plating structure. The plating structure obtained by the method provided by this invention has excellent corrosion resistance.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a functional cyanide-free silver plating method, which prepares a cyanide-free silver plating layer by performing cyanide-free silver plating on a trivalent chromium plating layer; The trivalent chromium plating layer was prepared using a rare-earth modified trivalent chromium plating method. The plating solution comprises: 80-140 g / L chromium chloride hexahydrate, 90-160 g / L potassium chloride, 90-160 g / L ammonium chloride, 16-30 g / L ammonium bromide, 40-60 g / L ammonium formate, 45-65 g / L boric acid, 8-12 mL / L rare earth additives, 2-4 mL / L leveling agent, 1-3 mL / L accelerator, and water; The rare earth additives include 30-80 g / L lanthanum chloride heptahydrate, 30-80 g / L praseodymium chloride heptahydrate, and water; The pH value of the plating solution is 2.5–3.2; The temperature of the plating bath during chromium plating is 25–35°C, and the cathode current density is 8–16 A / dm². 2 An inert graphite rod is used as the anode, and the air is stirred at a moderate temperature.

[0006] Preferably, the leveling agent comprises 30-90 g / L vanadium oxysulfate, 100-160 g / L ammonium formate, and water.

[0007] Preferably, the accelerator comprises 10-40 g / L of acidic ethoxylated alcohol phosphate and water.

[0008] Preferably, it includes the following steps: (1) The aluminum alloy substrate is pretreated to obtain a pretreated aluminum alloy substrate; (2) A chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1).

[0009] Preferably, the zinc precipitate solution used in step (2) for preparing the chemical zinc precipitate layer comprises: 80-120 g / L sodium hydroxide, 18-36 g / L zinc sulfate heptahydrate, 3-7 g / L ferric chloride, 6-16 g / L manganese sulfate, 0.5-1.5 g / L ammonium molybdate, 55-85 g / L disodium ethylenediaminetetraacetate, 15-35 g / L triethanolamine, and water; the temperature of the bath used for preparing the chemical zinc precipitate layer is 20-35°C; and the zinc precipitate time used for preparing the chemical zinc precipitate layer is 8-12 min.

[0010] Preferably, the plating bath used in step (2) to prepare the polythiocyanate copper plating layer comprises: 18-24 g / L of polycuprous thiocyanate, 130-170 g / L of polysodium thiocyanate, 20-30 g / L of disodium hydroxyethylidene diphosphonate, 8-12 mL / L of CB-101 brightener, and water; the pH value of the plating bath is 12-13; the plating bath temperature for preparing the polythiocyanate copper plating layer is 30-40°C, and the cathode current density is 0.5-1.5 A / dm³. 2 The cathode moves at a speed of 3-5 m / min, and the anode is oxygen-free electrolytic copper particles. The oxygen-free electrolytic copper particles are loaded into a titanium anode basket. The area ratio of the anode to the cathode is (3-4):1. The anode moves at a speed of 3-5 m / min.

[0011] Preferably, the plating solution used in step (2) to prepare the cyanide-free silver plating layer comprises: 20-30 g / L silver nitrate, 100-150 g / L potassium aminosulfonate, 40-60 g / L potassium metabisulfite, 0.5-0.8 g / L imidazole propoxy condensate, 0.1-0.2 g / L p-anisaldehyde, and water; the pH value of the plating solution is 9-10; and the cathode current density used to prepare the cyanide-free silver plating layer is 0.3-0.8 A / dm³. 2 The cathode moves at a speed of 4–6 m / min.

[0012] Preferably, the electrolyte used in step (2) to prepare the anti-discoloration electrolytic protective film comprises: 30-40 mL / L of ANTITAR 1127 MUP starter, 70-90 mL / L of ANTITAR 1127 ADDITIVE C additive, and water; the pH value of the electrolyte is 3.3-4.0; the temperature for preparing the anti-discoloration electrolytic protective film is 55-65℃, and the cathode current density is 0.05-0.1 A / dm³. 2 The electrolysis time is 4 to 10 minutes.

[0013] The present invention also provides a functional cyanide-free silver plating structure, which, from bottom to top, includes an aluminum alloy substrate, a chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film.

[0014] Preferably, the thickness of the polymeric thiocyanate copper plating layer is 3–8 μm; the thickness of the pyrophosphate copper plating layer is 6–16 μm; the thickness of the trivalent chromium plating layer is 1–2 μm; and the thickness of the cyanide-free silver plating layer is 10–50 μm.

[0015] This invention provides a functional cyanide-free silver plating method, which prepares a cyanide-free silver plating layer by performing cyanide-free silver plating on a trivalent chromium plating layer. The trivalent chromium plating layer is prepared using a rare earth modified trivalent chromium plating method. The plating solution comprises: 80-140 g / L chromium chloride hexahydrate, 90-160 g / L potassium chloride, 90-160 g / L ammonium chloride, 16-30 g / L ammonium bromide, 40-60 g / L ammonium formate, 45-65 g / L boric acid, 8-12 mL / L rare earth additives, 2-4 mL / L leveling agent, 1-3 mL / L accelerator, and water. The rare earth additives include 30-80 g / L lanthanum chloride heptahydrate, 30-80 g / L praseodymium chloride heptahydrate, and water. The pH value of the plating solution is 2.5-3.2. The temperature of the plating bath during chromium plating is 25-35℃, and the cathode current density is 8-16 A / dm³. 2 An inert graphite rod is used as the anode, and moderate air stirring is employed. This invention adds a trivalent chromium plating layer between the cyanide-free copper plating layer and the cyanide-free silver plating layer prepared by existing technologies. The trivalent chromium plating layer provides electrochemical protection to the underlying copper plating layer, and the resulting plating structure effectively prevents corrosive media from eroding the substrate. The trivalent chromium plating layer exhibits excellent corrosion resistance, effectively resisting corrosion from corrosive media and overcoming the defect of copper rust easily appearing on the surface of parts plated directly on a copper plating layer. Simultaneously, the composition of the plating solution is controlled, with the addition of lanthanum chloride heptahydrate and praseodymium chloride heptahydrate. These two components have a synergistic effect, further improving the corrosion resistance of the plated parts. The results of the examples show that the plated parts prepared by this invention showed no surface rust after 148 hours of salt spray testing. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the coating structure provided by the present invention, wherein 1 is an aluminum alloy substrate, 2 is a chemical zinc plating layer, 3 is a polymeric thiocyanate copper plating layer, 4 is a pyrophosphate copper plating layer, 5 is a trivalent chromium plating layer, 6 is a cyanide-free silver plating layer, and 7 is an anti-discoloration electrolytic protective film. Detailed Implementation

[0017] This invention provides a functional cyanide-free silver plating method, which prepares a cyanide-free silver plating layer by performing cyanide-free silver plating on a trivalent chromium plating layer; The trivalent chromium plating layer is prepared using a rare earth modified trivalent chromium plating method.

[0018] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0019] In this invention, the functional cyanide-free silver plating method preferably includes the following steps: (1) The aluminum alloy substrate is pretreated to obtain a pretreated aluminum alloy substrate; (2) A chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1).

[0020] The present invention preferably pre-treats the aluminum alloy substrate to obtain a pre-treated aluminum alloy substrate.

[0021] The present invention does not impose any special limitations on the specific composition and source of the aluminum alloy matrix, and any aluminum alloy matrix well known to those skilled in the art can be used. As one embodiment, the grade of the aluminum alloy matrix may specifically be 6063, 6A02, 6061, 2A12, 7A04, 5A02, ZL101, or ZL104.

[0022] In this invention, the pretreatment preferably includes the following steps performed sequentially: chemical degreasing, first water washing, ultrasonic degreasing, second water washing, film removal, third water washing, brightening, fourth water washing, micro-corrosion, and fifth water washing.

[0023] The present invention preferably places the aluminum alloy substrate in a degreasing agent solution for chemical degreasing.

[0024] In one embodiment, the degreasing agent in the degreasing agent solution is BIOWAS 102 powerful degreasing powder from Chaobang Chemical Co., Ltd.; the concentration of BIOWAS 102 powerful degreasing powder in the degreasing agent solution is 60-75 g / L; and the solvent in the degreasing agent solution is water.

[0025] In this invention, the preferred temperature for chemical degreasing is 15–80°C; the preferred time for chemical degreasing is 4–10 min; and the chemical degreasing is preferably carried out under stirring conditions. This invention does not impose any particular limitation on the stirring method and rate; stirring methods and rates well known to those skilled in the art can be used.

[0026] The present invention does not impose any special limitations on the operation of the first, second, third, fourth, and fifth water washes; any technical solution known to those skilled in the art can be used.

[0027] In this invention, the aluminum alloy substrate after the first water wash is preferably placed in a degreasing agent solution for ultrasonic degreasing.

[0028] In one embodiment, the degreasing agent in the degreasing agent solution is BIOWAS 103 powerful degreasing powder from Chaobang Chemical Co., Ltd.; the concentration of BIOWAS 103 powerful degreasing powder in the degreasing agent solution is 60-75 g / L; and the solvent in the degreasing agent solution is water.

[0029] In this invention, the preferred temperature for ultrasonic degreasing is 40–80°C; the preferred time for ultrasonic degreasing is 4–10 minutes. This invention does not impose any specific limitation on the ultrasonic power used for ultrasonic degreasing; any technical solution well-known to those skilled in the art can be employed.

[0030] In this invention, the membrane removal is preferably performed using an aqueous sodium hydroxide solution with a concentration of 50–100 g / L; the membrane removal temperature is preferably 40–60 °C; and the membrane removal time is preferably 1–3 min.

[0031] In this invention, the aluminum alloy substrate after the third water wash is preferably placed in a descaling agent solution for brightening.

[0032] In one embodiment, the descaling agent in the descaling solution is AE-7 aluminum alloy descaling agent from Chaobang Chemical Co., Ltd.; the concentration of AE-7 aluminum alloy descaling agent in the descaling solution is 600-800 mL / L; the solvent in the descaling solution is water; and the pH value of the descaling solution is 1.8-2.8.

[0033] In this invention, the light emission temperature is preferably 20-35°C; the light emission time is preferably 10-120s.

[0034] In this invention, the aluminum alloy substrate after the fourth water wash is preferably placed in a corrosive solution for micro-corrosion.

[0035] In this invention, the corrosive agent in the etchant solution is a weak corrosive agent for AL-18 aluminum alloy from Chaobang Chemical Co., Ltd.; the concentration of the corrosive agent in the etchant solution is 30-50 g / L; and the solvent in the etchant solution is water.

[0036] In this invention, the micro-corrosion time is preferably 0.5 to 1.5 min.

[0037] The present invention pre-treats the aluminum alloy substrate, which can further improve the bonding ability between the aluminum alloy substrate and the chemical zinc coating.

[0038] After obtaining the pretreated aluminum alloy substrate, the present invention preferably prepares a chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film sequentially on the surface of the pretreated aluminum alloy substrate.

[0039] In this invention, the zinc precipitate solution used to prepare the chemical zinc precipitate layer preferably comprises: 80-120 g / L sodium hydroxide, 18-36 g / L zinc sulfate heptahydrate, 3-7 g / L ferric chloride, 6-16 g / L manganese sulfate, 0.5-1.5 g / L ammonium molybdate, 55-85 g / L disodium ethylenediaminetetraacetate, 15-35 g / L triethanolamine, and water; the bath temperature used to prepare the chemical zinc precipitate layer is preferably 20-35°C; and the zinc precipitate time used to prepare the chemical zinc precipitate layer is preferably 8-12 min.

[0040] In one embodiment, the sodium hydroxide content in the zinc precipitation solution can specifically be 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, 105 g / L, 110 g / L, 115 g / L, or 120 g / L; the zinc sulfate heptahydrate content in the zinc precipitation solution can specifically be 18 g / L, 20 g / L, 22 g / L, 25 g / L, 28 g / L, or 30 g / L. The concentrations of ferric chloride in the zinc precipitation solution can be 32 g / L, 35 g / L, or 36 g / L; the concentration of manganese sulfate in the zinc precipitation solution can be 6 g / L, 7 g / L, 8 g / L, 9 g / L, 10 g / L, 11 g / L, 12 g / L, 13 g / L, 14 g / L, 15 g / L, or 16 g / L. The ammonium molybdate content in the zinc precipitation solution can specifically be 0.5 g / L, 0.6 g / L, 0.7 g / L, 0.8 g / L, 0.9 g / L, 1.0 g / L, 1.1 g / L, 1.2 g / L, 1.3 g / L, 1.4 g / L, or 1.5 g / L; the disodium ethylenediaminetetraacetate content in the zinc precipitation solution can specifically be 55 g / L, 60 g / L, 65 g / L, 70 g / L, or 75 g / L. The concentrations of triethanolamine in the zinc immersion solution can be 15 g / L, 20 g / L, 25 g / L, 30 g / L, or 35 g / L; the bath temperature can be 20°C, 22°C, 25°C, 28°C, 30°C, 32°C, or 35°C; and the zinc immersion time can be 8 min, 9 min, 10 min, 11 min, or 12 min. This invention controls the parameters during the preparation of the chemical zinc immersion layer within the above ranges, which can further improve the corrosion resistance of the plated parts.

[0041] In this invention, the plating bath used to prepare the polythiocyanate copper plating layer preferably comprises: 18-24 g / L of polycuprous thiocyanate, 130-170 g / L of polysodium thiocyanate, 20-30 g / L of disodium hydroxyethylidene diphosphonate, 8-12 mL / L of CB-101 brightener, and water; the pH value of the plating bath is preferably 12-13; the plating bath temperature used to prepare the polythiocyanate copper plating layer is preferably 30-40°C, and the cathode current density is preferably 0.5-1.5 A / dm³. 2 The cathode movement is preferably 3-5 m / min, the anode is preferably oxygen-free electrolytic copper particles, the oxygen-free electrolytic copper particles are preferably loaded into a titanium anode basket, the area ratio of the anode to the cathode is preferably (3-4):1, and the anode movement is preferably 3-5 m / min.

[0042] In one embodiment, the content of polycuprous thiocyanate in the plating solution can specifically be 18 g / L, 19 g / L, 20 g / L, 21 g / L, 22 g / L, 23 g / L, or 24 g / L; the content of polysodium thiocyanate in the plating solution can specifically be 130 g / L, 135 g / L, 140 g / L, 145 g / L, 150 g / L, 155 g / L, 160 g / L, 165 g / L, or 170 g / L; and the content of disodium hydroxyethylidene diphosphonate in the plating solution can specifically be 20 g / L, 21 g / L, 22 g / L, 23 g / L, 24 g / L, 25 g / L, or 26 g / L. The concentrations of the CB-101 brightener in the plating solution can be 27 g / L, 28 g / L, 29 g / L, or 30 g / L; the specific concentrations of the CB-101 brightener in the plating solution can be 8 mL / L, 9 mL / L, 10 mL / L, 11 mL / L, or 12 mL / L; the specific pH value of the plating solution can be 12, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, or 13; the specific temperatures of the plating bath can be 30℃, 31℃, 32℃, 33℃, 34℃, 35℃, 36℃, 37℃, 38℃, 39℃, or 40℃; and the specific cathode current density can be 0.5 A / dm³. 2 0.6A / dm 2 0.7A / dm 2 0.8A / dm 2 0.9A / dm 2 1.0A / dm 2 1.1A / dm 2 1.2A / dm 2 1.3A / dm 2 1.4A / dm 2 Or 1.5A / dm 2 The cathode movement can be specifically 3m / min, 4m / min or 5m / min; the area ratio of the anode to the cathode can be specifically 3.2:1, 3.5:1, 3.6:1 or 3.8:1; the anode movement can be specifically 3m / min, 4m / min or 5m / min.

[0043] As one implementation method, the CB-101 brightener was purchased from Chaobang Chemical.

[0044] This invention controls the parameters during the preparation of the polymeric thiocyanate copper plating layer within the above-mentioned range, which can further improve the corrosion resistance of the plated parts.

[0045] In this invention, the plating solution for preparing the pyrophosphate copper plating layer preferably comprises: 60-90 g / L copper pyrophosphate, 230-280 g / L potassium pyrophosphate, 2-5 mL / L ammonia, 1-3 mL / L PC-1289 pyrophosphate copper additive, and water; the pH value of the plating solution is preferably 8.6-9.0; the plating bath temperature for preparing the pyrophosphate copper plating layer is preferably 50-58°C, and the cathode current density is preferably 1-6 A / dm³. 2 The preparation of the copper pyrophosphate plating layer is preferably carried out under air stirring conditions. This invention does not impose any special limitations on the stirring method and rate, and employs stirring techniques well-known to those skilled in the art.

[0046] In one embodiment, the content of copper pyrophosphate in the plating solution can be specifically 60 g / L, 65 g / L, 70 g / L, 75 g / L, 80 g / L, 85 g / L, or 90 g / L; the content of potassium pyrophosphate in the plating solution can be specifically 230 g / L, 240 g / L, 250 g / L, 260 g / L, 270 g / L, or 280 g / L; the mass concentration of ammonia water can be specifically 25-28%; and the content of ammonia water in the plating solution can be specifically 2 mL / L or 3 mL / L. / L, 4mL / L, or 5mL / L; the content of PC-1289 pyrometallurgical additive in the plating solution can be specifically 1mL / L, 1.5mL / L, 2mL / L, 2.5mL / L, or 3mL / L; the pH value of the plating solution can be specifically 8.6, 8.7, 8.8, 8.9, or 9.0; the plating tank temperature can be specifically 50℃, 51℃, 52℃, 53℃, 54℃, 55℃, 56℃, 57℃, or 58℃; the cathode current density can be specifically 1A / dm³. 2 2A / dm 2 3A / dm 2 4A / dm 2 5A / dm 2 Or 6A / dm 2 .

[0047] This invention controls the parameters during the preparation of the pyrophosphate copper plating layer within the above-mentioned range, which can further improve the corrosion resistance of the plated parts.

[0048] In this invention, the trivalent chromium plating layer is prepared using a rare-earth modified trivalent chromium plating method. The plating solution used to prepare the trivalent chromium plating layer comprises: 80-140 g / L chromium chloride hexahydrate, 90-160 g / L potassium chloride, 90-160 g / L ammonium chloride, 16-30 g / L ammonium bromide, 40-60 g / L ammonium formate, 45-65 g / L boric acid, 8-12 mL / L rare earth additives, 2-4 mL / L leveling agent, 1-3 mL / L accelerator, and water. The rare earth additives include 30-80 g / L lanthanum chloride heptahydrate, 30-80 g / L praseodymium chloride heptahydrate, and water. The pH value of the plating solution is 2.5-3.2. The temperature of the plating bath during chromium plating is 25-35°C, and the cathode current density is 8-16 A / dm³. 2 An inert graphite rod is used as the anode, and the air is stirred at a moderate temperature.

[0049] In one embodiment, the content of chromium chloride hexahydrate in the plating solution can specifically be 80 g / L, 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, or 140 g / L; the content of potassium chloride in the plating solution can specifically be 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, or 160 g / L; the content of ammonium chloride in the plating solution can specifically be 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, or 160 g / L; and the content of ammonium bromide in the plating solution can specifically be 16 g / L, 18 g / L, or 160 g / L. / L, 20g / L, 22g / L, 25g / L, 28g / L or 30g / L; the content of ammonium formate in the plating solution can be specifically 40g / L, 45g / L, 50g / L, 55g / L or 60g / L; the content of boric acid in the plating solution can be specifically 45g / L, 50g / L, 55g / L, 60g / L or 65g / L; the content of rare earth additives in the plating solution can be specifically 8mL / L, 9mL / L, 10mL / L, 11mL / L or 12mL / L; the content of leveling agent in the plating solution can be specifically 2mL / L, 3mL / L or 4mL / L; the content of accelerator in the plating solution can be specifically 1mL / L, 2mL / L or 3mL / L.

[0050] In one embodiment, the content of lanthanum chloride heptahydrate in the rare earth additive can be specifically 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, or 80 g / L; the content of praseodymium chloride heptahydrate in the rare earth additive can be specifically 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, or 80 g / L. In another embodiment, the rare earth additive is prepared by dissolving lanthanum chloride heptahydrate and praseodymium chloride heptahydrate in water.

[0051] In one embodiment, the pH value of the plating solution may specifically be 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, or 3.2; the temperature of the plating tank may specifically be 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 31°C, 32°C, 33°C, 34°C, or 35°C; and the cathode current density may specifically be 8 A / dm³. 2 9A / dm 2 10A / dm 2 11A / dm 2 12A / dm 2 13A / dm 2 14A / dm 2 15A / dm 2 Or 16A / dm 2 .

[0052] In this invention, the leveling agent preferably comprises 30-90 g / L vanadyl sulfate, 100-160 g / L ammonium formate, and water. As one embodiment, the content of vanadyl sulfate in the leveling agent can specifically be 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, or 90 g / L; the content of ammonium formate in the leveling agent can specifically be 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, or 160 g / L. In this invention, the ammonium formate is used as a coordinating agent to dissolve vanadyl sulfate in water.

[0053] In one embodiment, the present invention first adds water that accounts for 4 / 5 of the total water mass, then adds ammonium formate and stirs until the ammonium formate dissolves, then adds vanadium oxysulfate and stirs until clear, and finally adds the remaining water.

[0054] In this invention, the accelerator preferably comprises 10-40 g / L of acidic ethoxylated phosphate and water. As one embodiment, the content of acidic ethoxylated phosphate in the accelerator can specifically be 10 g / L, 20 g / L, 30 g / L, or 40 g / L. As one embodiment, the type of acidic ethoxylated phosphate is specifically NORFOX PE-600. In this invention, the acidic ethoxylated phosphate is used to increase the deposition rate of trivalent chromium. As one embodiment, the accelerator is prepared by dissolving the acidic ethoxylated phosphate in water.

[0055] In this invention, the trivalent chromium plating layer has the function of preventing corrosive media from corroding the underlying plating layer and the metal substrate; during the trivalent chromium plating process, lanthanum chloride and praseodymium chloride in the rare earth additives hydrolyze on the cathode surface to form a rare earth salt film, which increases the cathodic polarization of the plating solution, making the trivalent chromium plating layer dense and improving the coverage of the plating solution. Lanthanum chloride and praseodymium chloride have a synergistic effect on increasing the cathodic polarization of the plating solution.

[0056] In this invention, the plating solution used to prepare the cyanide-free silver plating layer preferably comprises: 20-30 g / L silver nitrate, 100-150 g / L potassium aminosulfonate, 40-60 g / L potassium metabisulfite, 0.5-0.8 g / L imidazole propoxy condensate, 0.1-0.2 g / L p-anisaldehyde, and water; the pH value of the plating solution is preferably 9-10; and the cathode current density used to prepare the cyanide-free silver plating layer is preferably 0.3-0.8 A / dm³. 2 The cathode movement is preferably 4 to 6 m / min.

[0057] As one embodiment, the silver nitrate content in the plating solution can specifically be 20 g / L, 21 g / L, 22 g / L, 23 g / L, 24 g / L, 25 g / L, 26 g / L, 27 g / L, 28 g / L, 29 g / L, or 30 g / L; the potassium aminosulfonate content in the plating solution can specifically be 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, or 150 g / L; the metabisulfite content in the plating solution... The potassium content can be specifically 40 g / L, 45 g / L, 50 g / L, 55 g / L, or 60 g / L; the imidazole propoxy condensate content in the plating solution can be specifically 0.5 g / L, 0.6 g / L, 0.7 g / L, or 0.8 g / L; the pH value of the plating solution can be specifically 9, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, or 10; the cathode current density can be specifically 0.3 A / dm³. 2 0.4A / dm 2 0.5A / dm 2 0.6A / dm 2 0.7A / dm 2 Or 0.8A / dm 2 The cathode movement can be specifically 4 m / min, 5 m / min, or 6 m / min.

[0058] This invention controls the parameters during the preparation of cyanide-free silver plating within the above-mentioned range, which can further improve the corrosion resistance of the plated parts.

[0059] In this invention, the electrolyte used in preparing the anti-discoloration electrolytic protective film preferably comprises: 30-40 mL / L of ANTITAR 1127 MUP starter, 70-90 mL / L of ANTITAR 1127 ADDITIVE C additive, and water; the pH value of the electrolyte is preferably 3.3-4.0; the temperature used in preparing the anti-discoloration electrolytic protective film is preferably 55-65°C, and the cathode current density is preferably 0.05-0.1 A / dm³. 2 The preferred electrolysis time is 4 to 10 minutes, and the preferred cathode movement is 3 to 6 m / min.

[0060] As one implementation, the content of ANTITAR 1127 MUP starter in the electrolyte can specifically be 30 mL / L, 31 mL / L, 32 mL / L, 33 mL / L, 34 mL / L, 35 mL / L, 36 mL / L, 37 mL / L, 38 mL / L, 39 mL / L, or 40 mL / L; the content of ANTITAR 1127 ADDITIVE in the electrolyte... The content of additive C can be specifically 70 mL / L, 75 mL / L, 80 mL / L, 85 mL / L, or 90 mL / L; the pH value of the electrolyte can be specifically 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, or 4.0; the temperature can be specifically 55℃, 56℃, 57℃, 58℃, 59℃, 60℃, 61℃, 62℃, 63℃, 64℃, or 65℃; the cathode current density can be specifically 0.05 A / dm³. 2 0.06A / dm 2 0.07A / dm 2 0.08A / dm 2 0.09A / dm 2 Or 0.1A / dm 2 The electrolysis time can be specifically 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0061] This invention controls the parameters during the preparation of the anti-discoloration electrolytic protective film within the above-mentioned range, which can further improve the corrosion resistance of the plated parts.

[0062] After preparing the anti-discoloration electrolytic protective film, the present invention preferably dries the plated part after preparing the anti-discoloration electrolytic protective film.

[0063] In this invention, the drying temperature is preferably 60–80°C. This invention does not impose a specific limitation on the drying time; any technical solution well-known to those skilled in the art can be used.

[0064] The functional cyanide-free silver plating method provided by this invention adds a trivalent chromium plating layer between the cyanide-free copper plating layer and the cyanide-free silver plating layer prepared by existing technology. The trivalent chromium plating layer provides electrochemical protection for the underlying copper plating layer. The prepared plating structure can effectively prevent the corrosion medium from eroding towards the substrate, overcoming the defect of existing cyanide-free silver plating layers prepared on aluminum alloy substrates that do not have electrochemical protection. The trivalent chromium plating layer prepared by this invention has excellent corrosion resistance and can effectively resist the corrosion of corrosive media, overcoming the defect of pitting corrosion that easily occurs on the surface of parts plated directly on copper plating layers. This invention develops a cyanide-free silver plating technology on trivalent chromium plating layers, providing a new technical solution for improving the corrosion resistance of functional silver plating on aluminum alloy parts.

[0065] The present invention also provides a functional cyanide-free silver plating structure, which, from bottom to top, includes an aluminum alloy substrate, a chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film.

[0066] In this invention, the thickness of the polymeric thiocyanate copper plating layer is preferably 3-8 μm; the thickness of the pyrophosphate copper plating layer is preferably 6-16 μm; the thickness of the trivalent chromium plating layer is preferably 1-2 μm; and the thickness of the cyanide-free silver plating layer is preferably 10-50 μm.

[0067] A cyanide-free silver plating layer was prepared on a trivalent chromium plating layer. Although the conductivity of metallic chromium is lower than that of metallic silver, the thickness of the prepared trivalent chromium plating layer is only 1-2 μm, which is close to the thickness of the electroless nickel plating layer prepared on the surface of aluminum alloy in the prior art. The influence of the trivalent chromium plating layer on the conductivity of the prepared plating layer is acceptable.

[0068] The standard electrode potential of copper is 0.337V, and that of chromium is -0.74V. Metallic copper has a positive electrode potential of 1.077V compared to metallic chromium. Therefore, the trivalent chromium plating layer is an anodic plating layer relative to the copper plating layer. In this invention, the trivalent chromium plating layer provides electrochemical protection to the underlying copper plating layer.

[0069] In the industry, it is generally understood that the formation of a dense chromium trioxide film on the surface of chromium plating gives it excellent corrosion resistance. In current technology, chromium plating is only suitable as a topcoat. Traditionally, hexavalent chromium plating layers are oxidized by chromic acid in the plating bath to form a chromium trioxide protective film, a fact well-known in the field. However, trivalent chromium plating solutions are not oxidizing, and trivalent chromium-plated parts do not have a chromium trioxide film on their surface before leaving the bath. Production practice shows that, like nickel plating, trivalent chromium plating layers do not form a passivation film on their surface after three water rinses following removal from the plating bath. Therefore, silver plating on trivalent chromium plating layers can achieve good adhesion.

[0070] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0071] Example 1 A functional cyanide-free silver plating method is as follows: (1) The aluminum alloy substrate (grade 6063) is placed in a degreasing agent solution and chemically degreased for 7 minutes under stirring at 50°C. The degreasing agent in the degreasing agent solution is BIOWAS 102 strong degreasing powder from Chaobang Chemical Co., Ltd., with a concentration of 68 g / L. The solvent is water. Then, the substrate is washed with water and placed in the degreasing agent solution again. Ultrasonic degreasing is performed at 60°C for 7 minutes. The degreasing agent in the degreasing agent solution is BIOWAS 103 strong degreasing powder from Chaobang Chemical Co., Ltd. The concentration of 103 strong degreasing powder is 68 g / L, and the solvent is water. Then, the substrate is washed with water. The aluminum alloy substrate is then placed in a sodium hydroxide aqueous solution with a concentration of 75 g / L and defilmed at 50°C for 2 min. After washing with water, it is placed in a descaling agent solution at 28°C for brightening for 65 s. The descaling agent in the descaling agent solution is AE-7 aluminum alloy descaling agent from Chaobang Chemical Co., Ltd., with a concentration of 700 mL / L, a solvent of water, and a pH value of 2.3. After washing with water, the substrate is placed in a etchant solution for micro-etching for 1 min. The etchant in the etchant solution is AL-18 aluminum alloy weak etchant from Chaobang Chemical Co., Ltd., with a concentration of 40 g / L, a solvent of water. After washing with water, the pretreated aluminum alloy substrate is obtained. (2) Chemical zinc plating layer, polymeric thiocyanate copper plating layer, pyrophosphate copper plating layer, trivalent chromium plating layer, cyanide-free silver plating layer and anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1). The zinc precipitate solution for preparing the chemical zinc precipitate layer is: 100 g / L sodium hydroxide, 27 g / L zinc sulfate heptahydrate, 5 g / L ferric chloride, 11 g / L manganese sulfate, 1 g / L ammonium molybdate, 70 g / L disodium ethylenediaminetetraacetate, 25 g / L triethanolamine, and water; the bath temperature is 28℃, and the zinc precipitate time is 10 min. The plating bath for preparing the poly(dichloroisocyanate) copper plating layer consisted of: 21 g / L poly(cuprous) thiocyanate, 150 g / L poly(sodium) thiocyanate, 25 g / L disodium hydroxyethylidene diphosphonate, 10 mL / L CB-101 brightener, and water; the pH of the plating bath was 12.5, the plating tank temperature was 35℃, and the cathode current density was 1 A / dm³. 2The cathode movement speed is 4 m / min, the anode is oxygen-free electrolytic copper particles, the oxygen-free electrolytic copper particles are loaded into the titanium anode basket, the area ratio of the anode to the cathode is 3.5:1, the anode movement speed is 4 m / min; the thickness of the polymeric thiocyanate copper plating layer is 6 μm; The plating bath for preparing the pyrophosphate copper plating layer consisted of: 75 g / L copper pyrophosphate, 255 g / L potassium pyrophosphate, 3 mL / L ammonia (25-28% by mass), 2 mL / L PC-1289 pyrophosphate copper additive, and water; the pH of the plating bath was 8.8, the plating tank temperature was 54℃, and the cathode current density was 3 A / dm³. 2 Air agitation; the thickness of the pyrophosphate copper plating layer is 11 μm; The trivalent chromium plating layer was prepared using a rare-earth modified trivalent chromium plating method. The plating bath for preparing the trivalent chromium plating layer consisted of: 110 g / L chromium chloride hexahydrate, 125 g / L potassium chloride, 125 g / L ammonium chloride, 23 g / L ammonium bromide, 50 g / L ammonium formate, 55 g / L boric acid, 10 mL / L rare earth additives, 3 mL / L leveling agent, 2 mL / L accelerator, and water. The rare earth additives were 50 g / L lanthanum chloride heptahydrate, 50 g / L praseodymium chloride heptahydrate, and water. The pH of the plating bath was 2.8, the temperature of the plating tank was 30℃, and the cathode current density was 12 A / dm³. 2 An inert graphite rod is used as the anode, and the agitation is moderate. The leveling agent consists of 60 g / L vanadium oxysulfate, 130 g / L ammonium formate, and water. The accelerator consists of 25 g / L acidic ethoxylated alcohol phosphate (model NORFOX PE-600) and water. The thickness of the trivalent chromium plating layer is 1.5 μm. The plating bath for preparing the cyanide-free silver plating layer consisted of: 25 g / L silver nitrate, 125 g / L potassium aminosulfonate, 50 g / L potassium metabisulfite, 0.7 g / L imidazole propoxy condensate, 0.15 g / L p-anisaldehyde, and water; the pH of the plating bath was 9.5, and the cathode current density was 0.5 A / dm³. 2 The cathode movement speed is 5 m / min; the thickness of the cyanide-free silver plating layer is 25 μm. The electrolyte used to prepare the anti-discoloration electrolytic protective film consisted of 36 mL / L ANTITAR 1127 MUP starter, 80 mL / L ANTITAR 1127 ADDITIVE C additive, and water; the pH of the electrolyte was 3.6, the temperature was 60℃, and the cathode current density was 0.08 A / dm³. 2 The electrolysis time is 5 minutes, and the cathode moves at a speed of 4 m / min. After preparing the anti-discoloration electrolytic protective film, the plated parts are dried at 70℃ for 20 minutes.

[0072] Example 2 A functional cyanide-free silver plating method is as follows: (1) Except for replacing the aluminum alloy substrate grade with 6A02, the other steps are the same as in Example 1 to obtain a pretreated aluminum alloy substrate; (2) Chemical zinc plating layer, polymeric thiocyanate copper plating layer, pyrophosphate copper plating layer, trivalent chromium plating layer, cyanide-free silver plating layer and anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1). The zinc precipitate solution for preparing the chemical zinc precipitate layer consisted of: 120 g / L sodium hydroxide, 36 g / L zinc sulfate heptahydrate, 7 g / L ferric chloride, 16 g / L manganese sulfate, 1.5 g / L ammonium molybdate, 85 g / L disodium ethylenediaminetetraacetate, 35 g / L triethanolamine, and water; the bath temperature was 20°C, and the zinc precipitate time was 9 min. The plating bath for preparing the poly(dichlorocyanate) copper plating layer consisted of: 24 g / L poly(cuprous) thiocyanate, 170 g / L poly(sodium) thiocyanate, 30 g / L disodium hydroxyethylidene diphosphonate, 10 mL / L CB-101 brightener, and water; the pH of the plating bath was 12, the plating tank temperature was 35℃, and the cathode current density was 1 A / dm³. 2 The cathode movement speed is 4 m / min, the anode is oxygen-free electrolytic copper particles, the oxygen-free electrolytic copper particles are loaded into the titanium anode basket, the area ratio of the anode to the cathode is 3.2:1, the anode movement speed is 4 m / min; the thickness of the polymeric thiocyanate copper plating layer is 8 μm; The plating bath for preparing the pyrophosphate copper plating layer consisted of: 90 g / L copper pyrophosphate, 280 g / L potassium pyrophosphate, 4 mL / L ammonia (25-28% by mass), 2 mL / L PC-1289 pyrophosphate copper additive, and water; the pH of the plating bath was 8.7, the plating tank temperature was 51℃, and the cathode current density was 3 A / dm³. 2 Air agitation; the thickness of the pyrophosphate copper plating layer is 12μm; The trivalent chromium plating layer was prepared using a rare earth modified trivalent chromium plating method. The plating bath for preparing the trivalent chromium plating layer consisted of: 140 g / L chromium chloride hexahydrate, 90 g / L potassium chloride, 150 g / L ammonium chloride, 25 g / L ammonium bromide, 55 g / L ammonium formate, 60 g / L boric acid, 12 mL / L rare earth additives, 2 mL / L leveling agent, 2 mL / L accelerator, and water. The rare earth additives were 70 g / L lanthanum chloride heptahydrate, 30 g / L praseodymium chloride heptahydrate, and water. The pH of the plating bath was 3, the temperature of the plating tank was 32℃, and the cathode current density was 12 A / dm³. 2 An inert graphite rod is used as the anode, and the agitation is moderate. The leveling agent consists of 70 g / L vanadium oxysulfate, 140 g / L ammonium formate, and water. The accelerator consists of 30 g / L acidic ethoxylated alcohol phosphate (model NORFOX PE-600) and water. The thickness of the trivalent chromium plating layer is 1.4 μm. The plating solution for preparing the cyanide-free silver plating layer consisted of: 30 g / L silver nitrate, 150 g / L potassium aminosulfonate, 60 g / L potassium metabisulfite, 0.8 g / L imidazole propoxy condensate, 0.2 g / L p-anisaldehyde, and water; the pH of the plating solution was 9.2, and the cathode current density was 0.8 A / dm³. 2 The cathode movement speed is 5 m / min; the thickness of the cyanide-free silver plating layer is 22 μm. The electrolyte used to prepare the anti-discoloration electrolytic protective film consisted of 38 mL / L ANTITAR 1127 MUP starter, 85 mL / L ANTITAR 1127 ADDITIVE C additive, and water; the pH of the electrolyte was 3.3, the temperature was 55℃, and the cathode current density was 0.09 A / dm³. 2 The electrolysis time is 4 minutes, and the cathode moves at a speed of 3 m / min. After preparing the anti-discoloration electrolytic protective film, the plated parts are dried at 65℃ for 30 minutes.

[0073] Example 3 A functional cyanide-free silver plating method is as follows: (1) Except for replacing the aluminum alloy substrate grade with 6061, the other steps are the same as in Example 1 to obtain a pretreated aluminum alloy substrate; (2) Chemical zinc plating layer, polymeric thiocyanate copper plating layer, pyrophosphate copper plating layer, trivalent chromium plating layer, cyanide-free silver plating layer and anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1). The zinc precipitate solution for preparing the chemical zinc precipitate layer is: sodium hydroxide 80 g / L, zinc sulfate heptahydrate 18 g / L, ferric chloride 3 g / L, manganese sulfate 6 g / L, ammonium molybdate 0.7 g / L, disodium ethylenediaminetetraacetate 55 g / L, triethanolamine 15 g / L, and water; the bath temperature is 35℃, and the zinc precipitate time is 12 min. The plating bath for preparing the poly(dichloroisocyanate) copper plating layer consisted of: 23 g / L poly(cuprous) thiocyanate, 160 g / L poly(sodium) thiocyanate, 28 g / L disodium hydroxyethylidene diphosphonate, 8 mL / L CB-101 brightener, and water; the pH of the plating bath was 12.3, the plating tank temperature was 37℃, and the cathode current density was 1 A / dm³. 2 The cathode movement speed is 4 m / min, the anode is oxygen-free electrolytic copper particles, the oxygen-free electrolytic copper particles are loaded into the titanium anode basket, the area ratio of the anode to the cathode is 3.6:1, the anode movement speed is 4 m / min; the thickness of the polymeric thiocyanate copper plating layer is 7 μm; The plating bath for preparing the pyrophosphate copper plating layer consisted of: 60 g / L copper pyrophosphate, 230 g / L potassium pyrophosphate, 3 mL / L ammonia (25-28% by mass), 2 mL / L PC-1289 pyrophosphate copper additive, and water; the pH of the plating bath was 8.9, the plating tank temperature was 58℃, and the cathode current density was 3 A / dm³. 2Air agitation; the thickness of the pyrophosphate copper plating layer is 10 μm; The trivalent chromium plating layer was prepared using a rare-earth modified trivalent chromium plating method. The plating bath for preparing the trivalent chromium plating layer consisted of: 120 g / L chromium chloride hexahydrate, 140 g / L potassium chloride, 120 g / L ammonium chloride, 18 g / L ammonium bromide, 55 g / L ammonium formate, 55 g / L boric acid, 8 mL / L rare earth additives, 3 mL / L leveling agent, 1 mL / L accelerator, and water. The rare earth additives were 40 g / L lanthanum chloride heptahydrate, 60 g / L praseodymium chloride heptahydrate, and water. The pH of the plating bath was 2.6, the temperature of the plating tank was 26℃, and the cathode current density was 12 A / dm³. 2 An inert graphite rod is used as the anode, and the agitation is moderate. The leveling agent consists of 65 g / L vanadium oxysulfate, 140 g / L ammonium formate, and water. The accelerator consists of 40 g / L acidic ethoxylated alcohol phosphate (model NORFOX PE-600) and water. The thickness of the trivalent chromium plating layer is 1.6 μm. The plating solution for preparing the cyanide-free silver plating layer consisted of: 20 g / L silver nitrate, 100 g / L potassium aminosulfonate, 40 g / L potassium metabisulfite, 0.5 g / L imidazole propoxy condensate, 0.2 g / L p-anisaldehyde, and water; the pH of the plating solution was 9.7, and the cathode current density was 0.5 A / dm³. 2 The cathode movement speed is 6 m / min; the thickness of the cyanide-free silver plating layer is 30 μm. The electrolyte used to prepare the anti-discoloration electrolytic protective film consisted of 34 mL / L ANTITAR 1127 MUP starter, 75 mL / L ANTITAR 1127 ADDITIVE C additive, and water; the pH of the electrolyte was 3.9, the temperature was 65℃, and the cathode current density was 0.06 A / dm³. 2 The electrolysis time was 7 minutes, and the cathode moved at a speed of 6 m / min. After preparing the anti-discoloration electrolytic protective film, the plated parts are dried at 75℃ for 20 minutes.

[0074] Example 4 A functional cyanide-free silver plating method is as follows: (1) Except for replacing the aluminum alloy substrate grade with 2A12, the other steps are the same as in Example 1 to obtain a pretreated aluminum alloy substrate; (2) Chemical zinc plating layer, polymeric thiocyanate copper plating layer, pyrophosphate copper plating layer, trivalent chromium plating layer, cyanide-free silver plating layer and anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1). The zinc precipitate solution for preparing the chemical zinc precipitate layer consisted of: 115 g / L sodium hydroxide, 30 g / L zinc sulfate heptahydrate, 6 g / L ferric chloride, 13 g / L manganese sulfate, 1.3 g / L ammonium molybdate, 80 g / L disodium ethylenediaminetetraacetate, 32 g / L triethanolamine, and water; the bath temperature was 22℃, and the zinc precipitate time was 9 min. The plating bath for preparing the poly(dichlorocyanate) copper plating layer consisted of: 18 g / L poly(cuprous) thiocyanate, 140 g / L poly(sodium) thiocyanate, 20 g / L disodium hydroxyethylidene diphosphonate, 12 mL / L CB-101 brightener, and water; the pH of the plating bath was 13, the plating tank temperature was 40℃, and the cathode current density was 1 A / dm³. 2 The cathode movement speed is 4 m / min, the anode is oxygen-free electrolytic copper particles, the oxygen-free electrolytic copper particles are loaded into the titanium anode basket, the area ratio of the anode to the cathode is 3.8:1, the anode movement speed is 4 m / min; the thickness of the polymeric thiocyanate copper plating layer is 5 μm; The plating bath for preparing the pyrophosphate copper plating layer consisted of: 85 g / L copper pyrophosphate, 270 g / L potassium pyrophosphate, 3 mL / L ammonia (25-28% by mass), 2 mL / L PC-1289 pyrophosphate copper additive, and water; the pH of the plating bath was 8.8, the plating tank temperature was 53℃, and the cathode current density was 3 A / dm³. 2 Air agitation; the thickness of the pyrophosphate copper plating layer is 9 μm; The trivalent chromium plating layer was prepared using a rare-earth modified trivalent chromium plating method. The plating bath for preparing the trivalent chromium plating layer consisted of: 100 g / L chromium chloride hexahydrate, 100 g / L potassium chloride, 160 g / L ammonium chloride, 28 g / L ammonium bromide, 50 g / L ammonium formate, 55 g / L boric acid, 10 mL / L rare earth additives, 2 mL / L leveling agent, 3 mL / L accelerator, and water. The rare earth additives were 60 g / L lanthanum chloride heptahydrate, 40 g / L praseodymium chloride heptahydrate, and water. The pH of the plating bath was 2.6, the temperature of the plating tank was 34℃, and the cathode current density was 12 A / dm³. 2 The inert graphite rod is used as the anode, and the agitation is moderate. The leveling agent is 90 g / L vanadium oxysulfate, 160 g / L ammonium formate, and water. The accelerator is 15 g / L acidic ethoxylated alcohol phosphate (model NORFOX PE-600) and water. The thickness of the trivalent chromium plating layer is 1.5 μm. The plating bath for preparing the cyanide-free silver plating layer consisted of: 29 g / L silver nitrate, 147 g / L potassium aminosulfonate, 58 g / L potassium metabisulfite, 0.7 g / L imidazole propoxy condensate, 0.2 g / L p-anisaldehyde, and water; the pH of the plating bath was 9.3, and the cathode current density was 0.5 A / dm³. 2 The cathode movement speed is 4 m / min; the thickness of the cyanide-free silver plating layer is 28 μm. The electrolyte used to prepare the anti-discoloration electrolytic protective film consisted of 37 mL / L ANTITAR 1127 MUP starter, 83 mL / L ANTITAR 1127 ADDITIVE C additive, and water; the pH of the electrolyte was 3.4, the temperature was 56℃, and the cathode current density was 0.1 A / dm³. 2 The electrolysis time is 5 minutes, and the cathode moves at a speed of 4 m / min. After preparing the anti-discoloration electrolytic protective film, the plated parts are dried at 80℃ for 15 minutes.

[0075] Test Example 1 According to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", the aluminum alloy functional cyanide-free silver-plated samples prepared in Examples 1, 2, 3 and 4 showed no rust on the surface after 148 hours, and the prepared coatings have excellent corrosion resistance.

[0076] Test Example 2 The functional cyanide-free silver-plated aluminum alloy samples prepared in Examples 1, 2, 3, and 4 were immersed in a 5% (w / w) ammonium thiocyanate solution for 4 minutes, and no discoloration was observed on the surface of the plated parts. This meets the industry requirement that silver-plated parts do not discolor after 3 minutes in this ammonium thiocyanate solution.

[0077] Test Example 3 The functional cyanide-free silver-plated aluminum alloy samples prepared in Examples 1, 2, 3, and 4 were tested for 500 hours at 40°C and 93% relative humidity according to GB / T 2423.3-2016 "Basic Environmental Testing Procedures for Electrical and Electronic Products - Test Ca: Constant Damp Heat Test Method". The coating showed no visible changes, which is far higher than the industry requirement of no change in the coating after 168 hours of constant damp heat testing.

[0078] Test Example 4 According to GB / T 5270-2005 "Review of Test Methods for Adhesion Strength of Electrodeposited and Chemically Deposited Metallic Coatings on Metal Substrates", the adhesion of the coating was tested by thermal shock method. The functional cyanide-free silver-plated aluminum alloy samples prepared in Examples 1, 2, 3 and 4 were heated to 220°C in a heating furnace and held for 60 minutes. After being removed, they were immediately placed in room temperature water to cool. No blistering or peeling of the coating occurred, and the adhesion of the coating met the standard requirements.

[0079] Test Example 5 The hardness of the silver plating layers prepared in Examples 1-4 was tested using a micro Vickers hardness tester, and the results are shown in Table 1.

[0080] Table 1. Hardness test results of the silver plating layers in Examples 1-4

[0081] Test Example 6 The circuit resistance of the functional cyanide-free silver-plated aluminum alloy samples prepared in Examples 1, 2, 3 and 4 was tested, and the results are listed in Table 2.

[0082] Table 2. Circuit resistance of functional aluminum alloy cyanide-free silver-plated samples prepared in Examples 1-4

[0083] Example 5 A functional cyanide-free silver plating method is as follows: (1) Except for replacing the aluminum alloy substrate grade with 7A04, the other steps are the same as in Example 1 to obtain a pretreated aluminum alloy substrate; (2) Chemical zinc plating layer, polymeric thiocyanate copper plating layer, pyrophosphate copper plating layer, trivalent chromium plating layer, cyanide-free silver plating layer and anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1). The parameters for preparing the chemical zinc plating layer, the polymeric thiocyanate copper plating layer, the pyrophosphate copper plating layer, the cyanide-free silver plating layer, and the anti-discoloration electrolytic protective film are the same as in Example 1. The trivalent chromium plating layer was prepared using a rare earth modified trivalent chromium plating method. The plating bath for preparing the trivalent chromium plating layer consisted of: 110 g / L chromium chloride hexahydrate, 125 g / L potassium chloride, 125 g / L ammonium chloride, 21 g / L ammonium bromide, 50 g / L ammonium formate, 60 g / L boric acid, 10 mL / L rare earth additives, 3 mL / L leveling agent, 2 mL / L accelerator, and water. The rare earth additives were 50 g / L lanthanum chloride heptahydrate, 50 g / L praseodymium chloride heptahydrate, and water. The pH of the plating bath was 2.8, the temperature of the plating tank was 30℃, and the cathode current density was 12 A / dm³. 2 An inert graphite rod is used as the anode, and the agitation is moderate. The leveling agent consists of 60 g / L vanadium oxysulfate, 130 g / L ammonium formate, and water. The accelerator consists of 25 g / L acidic ethoxylated alcohol phosphate (model NORFOX PE-600) and water. The thickness of the trivalent chromium plating layer is 1.5 μm. After preparing the anti-discoloration electrolytic protective film, the plated parts are dried at 70℃ for 20 minutes.

[0084] The functional cyanide-free silver-plated aluminum alloy sample prepared in Example 5 was subjected to a neutral salt spray test for 140 hours in accordance with GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". No white rust was found on the surface of the sample.

[0085] Comparative Example 1 The rare earth additive in Example 5 was replaced with 100 g / L lanthanum chloride heptahydrate and water, while all other parameters remained the same as in Example 5.

[0086] The aluminum alloy sample prepared in Comparative Example 1 was subjected to a neutral salt spray test for 122 hours according to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". No white rust was observed on the sample surface. Compared with Example 5, the neutral salt spray test time of the plated parts was reduced by 18 hours by using lanthanum chloride heptahydrate as a rare earth additive alone.

[0087] Comparative Example 2 The rare earth additive in Example 5 was replaced with 100 g / L praseodymium chloride heptahydrate and water, while all other parameters remained the same as in Example 5.

[0088] The aluminum alloy sample prepared in Comparative Example 2 was subjected to a neutral salt spray test for 122 hours according to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". No white rust was found on the sample surface. Compared with Example 5, the neutral salt spray test time of the plated parts was reduced by 18 hours when praseodymium chloride heptahydrate was used alone as a rare earth additive.

[0089] The comparison of Example 5, Comparative Example 1 and Comparative Example 2 shows that the simultaneous addition of lanthanum chloride and praseodymium chloride to the trivalent chromium plating solution has a synergistic effect on improving the salt spray resistance of the coating.

[0090] Comparative Example 3 The rare earth additives in the trivalent chromium plating solution of Example 5 were omitted, and everything else was the same as in Example 5.

[0091] The aluminum alloy sample prepared in Comparative Example 3 was subjected to a neutral salt spray test for 96 hours according to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". No white rust was observed on the sample surface. Compared with Example 5, without the addition of rare earth additives, the neutral salt spray test time for the plated parts was reduced by 44 hours.

[0092] As can be seen from Example 5 and Comparative Example 3, the addition of rare earth additives to the trivalent chromium plating solution significantly improves the salt spray resistance of the trivalent chromium plating layer.

[0093] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A functional cyanide-free silver plating method, characterized in that, A cyanide-free silver plating layer is prepared by performing cyanide-free silver plating on a trivalent chromium plating layer; The trivalent chromium plating layer is prepared using a rare-earth modified trivalent chromium plating method. The plating solution comprises: 80-140 g / L chromium chloride hexahydrate, 90-160 g / L potassium chloride, 90-160 g / L ammonium chloride, 16-30 g / L ammonium bromide, 40-60 g / L ammonium formate, 45-65 g / L boric acid, 8-12 mL / L rare earth additives, 2-4 mL / L leveling agent, 1-3 mL / L accelerator, and water; The rare earth additives include 30-80 g / L lanthanum chloride heptahydrate, 30-80 g / L praseodymium chloride heptahydrate, and water; The pH value of the plating solution is 2.5–3.2; The temperature of the plating bath during chromium plating is 25–35°C, and the cathode current density is 8–16 A / dm². 2 An inert graphite rod is used as the anode, and the air is stirred at a moderate temperature.

2. The method according to claim 1, characterized in that, The leveling agent comprises 30-90 g / L of vanadium oxysulfate, 100-160 g / L of ammonium formate, and water.

3. The method according to claim 1, characterized in that, The accelerator comprises 10–40 g / L of acidic ethoxylated alcohol phosphate and water.

4. The method according to claim 1, characterized in that, Includes the following steps: (1) The aluminum alloy substrate is pretreated to obtain a pretreated aluminum alloy substrate; (2) A chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film are sequentially prepared on the surface of the pretreated aluminum alloy substrate obtained in step (1).

5. The method according to claim 4, characterized in that, The zinc precipitate solution used in step (2) to prepare the chemical zinc precipitate layer includes: sodium hydroxide 80-120 g / L, zinc sulfate heptahydrate 18-36 g / L, ferric chloride 3-7 g / L, manganese sulfate 6-16 g / L, ammonium molybdate 0.5-1.5 g / L, disodium ethylenediaminetetraacetate 55-85 g / L, triethanolamine 15-35 g / L, and water; the temperature of the bath solution used to prepare the chemical zinc precipitate layer is 20-35℃; and the zinc precipitate time used to prepare the chemical zinc precipitate layer is 8-12 min.

6. The method according to claim 4, characterized in that, The plating bath used in step (2) to prepare the polythiocyanate copper plating layer includes: 18-24 g / L of polycuprous thiocyanate, 130-170 g / L of polysodium thiocyanate, 20-30 g / L of disodium hydroxyethylidene diphosphonate, 8-12 mL / L of CB-101 brightener, and water; the pH value of the plating bath is 12-13; the plating bath temperature for preparing the polythiocyanate copper plating layer is 30-40℃, and the cathode current density is 0.5-1.5 A / dm³. 2 The cathode moves at a speed of 3-5 m / min, and the anode is oxygen-free electrolytic copper particles. The oxygen-free electrolytic copper particles are loaded into a titanium anode basket. The area ratio of the anode to the cathode is (3-4):

1. The anode moves at a speed of 3-5 m / min.

7. The method according to claim 4, characterized in that, The plating solution used in step (2) to prepare the cyanide-free silver plating layer comprises: 20-30 g / L silver nitrate, 100-150 g / L potassium aminosulfonate, 40-60 g / L potassium metabisulfite, 0.5-0.8 g / L imidazole propoxy condensate, 0.1-0.2 g / L p-anisaldehyde, and water; the pH value of the plating solution is 9-10; and the cathode current density used to prepare the cyanide-free silver plating layer is 0.3-0.8 A / dm³. 2 The cathode moves at a speed of 4–6 m / min.

8. The method according to claim 4, characterized in that, The electrolyte used in step (2) to prepare the anti-discoloration electrolytic protective film includes: 30-40 mL / L of ANTITAR 1127 MUP starter, 70-90 mL / L of ANTITAR 1127 ADDITIVE C additive, and water; the pH of the electrolyte is 3.3-4.0; the temperature for preparing the anti-discoloration electrolytic protective film is 55-65℃, and the cathode current density is 0.05-0.1 A / dm³. 2 The electrolysis time is 4 to 10 minutes.

9. A functional cyanide-free silver plating structure, characterized in that, From bottom to top, it includes an aluminum alloy substrate, a chemical zinc plating layer, a polymeric thiocyanate copper plating layer, a pyrophosphate copper plating layer, a trivalent chromium plating layer, a cyanide-free silver plating layer, and an anti-discoloration electrolytic protective film.

10. The coating structure according to claim 9, characterized in that, The thickness of the polymeric thiocyanate copper plating layer is 3–8 μm; the thickness of the pyrophosphate copper plating layer is 6–16 μm; the thickness of the trivalent chromium plating layer is 1–2 μm; and the thickness of the cyanide-free silver plating layer is 10–50 μm.

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