A special electroplating primer nickel process for titanium alloy

Abstract

The application relates to the technical field of titanium alloy, in particular to a special electroplating base nickel process for titanium alloy. The process comprises the following steps: firstly, removing oil from a titanium alloy base body through alkaline electrolysis; secondly, activating the titanium alloy base body in an activation solution compounded by hydrochloric acid and ammonium hydrogen fluoride at 20-25 DEG C for 20-45s, and then transferring the titanium alloy base body to an electroplating tank after washing; thirdly, electroplating in a cyanide-free base nickel electroplating solution containing nickel sulfate, nickel chloride, boric acid and modified sodium sulfate, controlling the pH value to be 3.5-4.5, the temperature to be 50-65 DEG C, the cathode current density to be 2-5 A / dm 2 , and adopting two-stage current density power supply. The modified sodium sulfate is prepared through freezing-out crystallization and heterogeneous nucleation technology, takes aluminum nitride as a nucleation core, adds sodium silicate, sodium dihydrogen phosphate and sodium tetraborate decahydrate for regulation and control, and finally obtains high-purity and uniform-particle-size crystals. The process is environment-friendly and cyanide-free, the plating layer has strong adhesion, high compactness, few defects and good batch consistency.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy technology, and in particular to a special electroplating process for nickel undercoating of titanium alloys. Background Technology

[0002] Titanium alloys are widely used in high-end industrial fields such as aerospace and medical devices due to their excellent properties such as high specific strength and corrosion resistance. However, a dense and chemically stable oxide film easily forms on the surface of titanium alloys. This inert film severely hinders the formation of a strong metallurgical bond between subsequent functional electroplating layers (such as decorative coatings and wear-resistant coatings) and the titanium substrate. Direct electroplating on its surface often results in poor coating adhesion and is prone to fatal defects such as blistering and peeling, which greatly limits the application of titanium alloy parts for surface strengthening and functional modification through electroplating.

[0003] To address the adhesion problem, existing technologies typically require either a cyanide-containing pre-plating process or a high-concentration fluoride-based strong etching activation. While cyanide pre-plating can improve adhesion to some extent, its highly toxic nature places extremely high demands on production safety, environmental protection, and waste disposal, contradicting the trend of green manufacturing. High-concentration fluoride activation, although effective in removing oxide films, is a violent process that can easily lead to selective corrosion or excessive roughness on the titanium alloy surface, and even induce hydrogen embrittlement. This introduces new stress concentration points and potential defects into the coating deposition, affecting the density and long-term service reliability of the coating.

[0004] In the exploration of cyanide-free electroplating systems, conventional sulfate systems have attracted attention due to their environmental friendliness. However, when industrial-grade sodium sulfate is directly used in nickel plating solutions for titanium alloys, the inherent impurity content (such as chloride ions and metal ions) fluctuates significantly, and the crystal morphology and dissolution behavior are difficult to maintain batch-to-batch consistency. These factors cause uncontrollable drift in key parameters of the plating solution, such as conductivity and polarization state, making it difficult to precisely control the nucleation and growth process of the initial nickel layer deposition on the sensitive titanium alloy substrate. The result is that the coating is prone to microscopic pinholes and poor uniformity. Furthermore, in humid or corrosive environments, these microscopic defects become channels for corrosion propagation, accelerating coating failure.

[0005] Furthermore, the electroplating process window for titanium alloys is typically narrow, making them extremely sensitive to changes in parameters such as temperature, pH, and current density. Existing processes generally suffer from insufficient stability, with significant variations in plating quality (such as thickness uniformity, adhesion, and corrosion resistance) between different batches of workpieces, making it difficult to meet the stringent requirements for product consistency and reliability in high-end applications. Therefore, developing a dedicated nickel plating process for titanium alloys that balances environmental friendliness, high adhesion, low defect sensitivity, and excellent batch stability has become a critical bottleneck that urgently needs to be overcome in this technological field. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a special electroplating process for nickel undercoating of titanium alloys, so as to overcome the problem of insufficient coating adhesion caused by the oxide film on the surface of titanium alloys, as well as the problems of pinholes, blistering defects and poor batch consistency caused by impurity fluctuations and improper crystallization control in existing cyanide-free electroplating processes.

[0007] To achieve the above objectives, the present invention provides a method for preparing modified sodium sulfate for nickel plating of titanium alloys, comprising the following steps: (1) Dissolve anhydrous sodium sulfate in deionized water and filter to obtain sodium sulfate mother liquor; (2) Aluminum nitride powder is dispersed in deionized water to form a heterogeneous nucleation dispersion, and sodium silicate, sodium dihydrogen phosphate monohydrate and sodium tetraborate decahydrate are added in sequence to obtain a composite heterogeneous nucleation dispersion. (3) After mixing the composite heterogeneous nucleation dispersion with sodium sulfate mother liquor, the mixture is subjected to controlled cooling freeze-drying crystallization to precipitate sodium sulfate crystals; (4) Sodium sulfate crystals are subjected to solid-liquid separation, washing and drying to obtain modified sodium sulfate.

[0008] Preferably, in step (1), the mass ratio of anhydrous sodium sulfate to deionized water is 1:(1.5-2.5).

[0009] Preferably, in step (1), the dissolution temperature is 35-50℃, the stirring speed is 200-500rpm, and the filtration accuracy is 0.5-2μm.

[0010] Preferably, the amount of aluminum nitride powder used in step (2) is 0.4 to 0.9 wt% based on the mass of anhydrous sodium sulfate in step (1).

[0011] Preferably, the amount of deionized water used in step (2) is 15 to 30 times the mass of aluminum nitride powder.

[0012] Preferably, in step (2), the aluminum nitride powder is dispersed by ultrasonic dispersion, with ultrasonic conditions of 35-45 kHz and 8-15 min.

[0013] Preferably, based on the mass of anhydrous sodium sulfate in step (1), the amount of sodium silicate in step (2) is 0.2-0.4 wt%, the amount of sodium dihydrogen phosphate monohydrate is 0.2-0.5 wt%, and the amount of sodium tetraborate decahydrate is 0.6-1.5 wt%.

[0014] Preferably, in step (3), the mixing temperature is 33-37°C, the mixing speed is 250-350 rpm, and the constant temperature homogenization time is 3-10 min.

[0015] Preferably, in step (3), the cooling rate is 0.8 to 1.2 °C / min, the target temperature is -2 to 2 °C, and the holding time for crystallization is 90 to 150 min.

[0016] Preferably, the washing in step (4) includes washing with a 0°C saturated sodium sulfate aqueous solution and then washing with anhydrous ethanol, and the ratio of the mass of the washing solution to the mass of the wet crystals is 0.15 to 0.35; the drying / standing temperature in step (4) is 12 to 18°C; and the particle size is adjusted by sieving so that the crystal particle size distribution is such that it passes through a 100-mesh sieve and is retained in a 200-mesh sieve.

[0017] Furthermore, this invention provides a special electroplating nickel base layer process for titanium alloys, comprising the following steps: (a) Degreasing treatment of the titanium alloy substrate; (ii) The titanium alloy substrate after degreasing was activated by an activation solution composed of hydrochloric acid and ammonium bifluoride, and then rinsed and wet-transferred. (iii) Electroplating an activated titanium alloy substrate in a cyanide-free nickel plating solution to form a base nickel layer, wherein the cyanide-free nickel plating solution comprises nickel sulfate, nickel chloride, boric acid, and modified sodium sulfate. During electroplating, the pH of the plating solution is controlled at 3.5-4.5, the temperature at 50-65℃, and the cathode current density at 2-5 A / dm³. 2 And the electroplating solution is filtered during the electroplating process.

[0018] Preferably, step (i) employs alkaline electrolytic degreasing, wherein the mass fractions of each component in the alkaline electrolytic degreasing solution are: sodium hydroxide 2-6 wt%, sodium carbonate 1-5 wt%, trisodium phosphate dodecahydrate 2-10 wt%, and sodium metasilicate pentahydrate 0.5-5 wt%, and the degreasing process conditions are: temperature 50-70℃ and cathode current density 3-8 A / dm³. 2 Time: 2-10 minutes.

[0019] Preferably, the composition of the activation solution in step (ii) satisfies the following conditions: the mass fraction of 38% hydrochloric acid in the activation solution is 20-26 wt%, the mass fraction of ammonium bifluoride is 8-12 wt%, and the activation process conditions are a temperature of 20-25°C and a soaking time of 20-45 s.

[0020] Preferably, in step (ii), the titanium alloy substrate is not dried after rinsing and is wet-transferred to the electroplating tank within 5-15 seconds.

[0021] Preferably, the cyanide-free nickel plating solution comprises, per liter of plating solution: 220-320g of nickel sulfate hexahydrate, 30-70g of nickel chloride hexahydrate, 25-50g of boric acid, and 180-260g of modified sodium sulfate.

[0022] More preferably, the cyanide-free nickel plating solution comprises, per liter of plating solution: 250-300g of nickel sulfate hexahydrate, 40-60g of nickel chloride hexahydrate, 30-45g of boric acid, and 200-240g of modified sodium sulfate.

[0023] Preferably, the cyanide-free nickel plating solution further includes additives, which, per liter of plating solution, include: 1.0-2.5g of sodium saccharin, 0.05-0.30g of sodium dodecyl sulfate, and 0.5-2.0g of sodium allyl sulfonate.

[0024] Preferably, the additives are added in a fixed sequence after filtration, and the order of addition is sodium saccharin, sodium dodecyl sulfate, and sodium allyl sulfonate.

[0025] Preferably, the electroplating solution is circulated and filtered during the electroplating process, with a filtration accuracy of 0.5 to 2 μm and a circulation filtration time of 5 to 20 min.

[0026] Preferably, the electroplating process employs a two-stage current density energizing regime: after immersion in the plating bath, the current density is initially 0.5–1.5 A / dm². 2 Power on for 10–60 seconds, then switch to 2–5 A / dm 2 Electroplating for 5–20 minutes.

[0027] The nickel plating process for titanium alloys provided by this invention brings significant benefits through systematic material design and process control.

[0028] Firstly, regarding the adhesion and density of the coating, a high-purity sodium sulfate component with consistent physicochemical properties was obtained through specific freeze-drying crystallization and surface modification treatments. This component provides stable and moderate polarization in the electroplating bath, and combined with optimized pre-activation treatment, enables uniform and meticulous nucleation and initial deposition of nickel ions on the activated, fresh titanium alloy surface. This effectively promotes the formation of a continuous and robust interface between the coating and the substrate, significantly reducing the risk of blistering or peeling of the coating under thermal shock or mechanical stress, laying a solid foundation for subsequent functional electroplating.

[0029] Secondly, in terms of reducing coating defect sensitivity, the effective isolation and fixation of impurity ions during the preparation process, and the resulting improvement in concentrated crystal grain size distribution and uniform dissolution behavior, allow for extremely low concentrations of impurity ions in the plating bath and cathode diffusion layer. This significantly reduces microscopic pinholes and porous structures in the coating caused by impurity adsorption or co-deposition. The resulting nickel underlay layer is more dense and uniform, thus significantly enhancing its ability to act as a barrier layer against the intrusion of external corrosive media.

[0030] Finally, regarding process stability and reproducibility, the crystallization process was effectively guided by introducing heterogeneous nucleation cores and nucleation regulators, combined with a controllable cooling crystallization procedure. This not only ensures batch-to-batch quality stability of the modified sodium sulfate product itself, but more importantly, when applied to electroplating systems, it ensures highly repeatable key electrochemical parameters of the electroplating solution (such as conductivity and polarizability). Therefore, even within the specified process parameter window, the process exhibits excellent robustness, stably producing a uniformly thick and consistent nickel underlayer, meeting the high requirements for quality consistency in large-scale industrial production. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0032] This invention provides a special electroplating process for nickel undercoating on titanium alloys. The core of this process lies in introducing modified sodium sulfate obtained through freeze-dip crystallization into a cyanide-free nickel undercoating system, combined with heterogeneous nucleation and closed-loop filtration control, to maintain the electroplating system at a pH of 3.5–4.5, a temperature of 50–65°C, and an A / dm³. 2 Within the process window, a fine and dense base nickel layer can still be obtained stably, and the sensitivity to pinholes / bubbling is reduced, thereby improving batch consistency and adhesion stability.

[0033] The titanium alloy-specific electroplating base nickel process provided by the present invention includes steps a) preparation of modified sodium sulfate; b) degreasing; c) activation; d) electroplating base nickel, wherein the preferred materials, conditions and parameters for each step are as follows.

[0034] a) Preparation of modified sodium sulfate (freeze-crystallization—heterogeneous nucleation—washing and displacement—particle size adjustment) a1) Preparation of clarified sodium sulfate mother liquor In the modified sodium sulfate preparation method provided by the present invention, it is preferable to first prepare a clarified sodium sulfate mother liquor to reduce the risk of entrainment of suspended impurities. The ratio of anhydrous sodium sulfate to deionized water is preferably configured at a mass ratio of 1:(1.5-2.5), more preferably 1:(1.8-2.2), specifically 3000g anhydrous sodium sulfate + 6000g deionized water.

[0035] In the preparation of the mother liquor provided by this invention: The preferred melting temperature is 35–50°C, more preferably 38–45°C, and specifically, it can be 40°C for 10–30 min (e.g., 20 min). The stirring speed is preferably 200-500 rpm, more preferably 250-350 rpm, and specifically 300 rpm; The filtration accuracy is preferably 0.5 to 2 μm, more preferably 1 μm (e.g., 1 μm polypropylene filter membrane for hot filtration).

[0036] a2) Preparation of composite heterogeneous nucleation dispersion (aluminum nitride + inorganic site regulation + sodium tetraborate decahydrate + ultrasound) In the modified sodium sulfate preparation method provided by the present invention, aluminum nitride exogenous particles are preferably used as heterogeneous nucleation cores, and the introduction sequence is fixed to achieve site hierarchical regulation: silicate first, then phosphate, and then sodium tetraborate decahydrate is added to expand the freeze-drying crystallization window and shorten the induction period.

[0037] The preferred amount of aluminum nitride powder is 0.2 to 1.2 wt% of the mass of anhydrous sodium sulfate, more preferably 0.4 to 0.9 wt%, specifically 20 g of aluminum nitride powder (approximately 0.67 wt% relative to 3000 g of anhydrous sodium sulfate). The preferred amount of dispersion medium (deionized water) is 10 to 40 times the mass of aluminum nitride powder, more preferably 15 to 30 times, specifically 500g of deionized water; The preferred ultrasound conditions are 20–60 kHz for 5–20 min, more preferably 35–45 kHz for 8–15 min, and specifically 40 kHz ultrasound for 10 min. The preferred amount of sodium silicate is 0.1–0.6 wt% of the mass of anhydrous sodium sulfate, more preferably 0.2–0.4 wt%, specifically 8 g; the preferred amount of sodium dihydrogen phosphate monohydrate is 0.1–0.8 wt%, more preferably 0.2–0.5 wt%, specifically 10 g; the preferred amount of sodium tetraborate decahydrate is 0.3–2.5 wt%, more preferably 0.6–1.5 wt%, specifically 30 g; and the preferred order of addition is sodium silicate, sodium dihydrogen phosphate monohydrate, and sodium tetraborate decahydrate.

[0038] Aluminum nitride particles provide heterogeneous nucleation sites, which can shorten the induction period and reduce the metastable region, thereby narrowing the crystal size distribution; When silicate (alkaline, polysilicate anion) and phosphate (acidic, monomeric phosphate) are introduced in a fixed order, an adsorption layer followed by a charge regulation layer can be formed on the particle surface. This takes into account the adsorption / co-precipitation control of different metal impurities and reduces the probability of impurities entering the crystal and subsequent electroplating solution. Sodium tetraborate decahydrate can reduce supercooling and enhance nucleation reproducibility; Ultrasonic pulses (in the initial stage of supersaturation) can break up agglomerates, reduce the probability of entrainment of mother liquor, and form a more uniform micronucleus field, achieving the combined effect of purification and particle size control.

[0039] a3) Freeze-crystallization, washing and displacement, and particle size adjustment In the modified sodium sulfate preparation method provided by this invention, it is preferable to mix the above-mentioned clarified mother liquor and the composite heterogeneous nucleation dispersion at a medium temperature and then perform controlled cooling freeze-drying to allow Cl to be released. -Metallic impurities preferentially accumulate in the mother liquor and are discharged with the mother liquor, thus obtaining modified sodium sulfate crystals.

[0040] In the freeze-crystallization step provided by this invention: The mixing temperature is preferably 30-40℃, more preferably 33-37℃, and can be 35℃; the mixing and stirring speed is preferably 200-500 rpm, more preferably 250-350 rpm, and can be 300 rpm; the isothermal homogenization time is preferably 3-10 min, and can be 5 min. The cooling rate is preferably 0.3–2.0 °C / min, more preferably 0.8–1.2 °C / min, and specifically 1 °C / min; the target temperature is preferably -2–2 °C, more preferably 0 °C; the holding time for crystallization is preferably 60–180 min, more preferably 90–150 min, and specifically 120 min. Solid-liquid separation is preferably achieved by suction filtration / vacuum filtration; The preferred washing method is to first wash with a 0°C saturated sodium sulfate aqueous solution at low temperature to remove the mother liquor entrainment, and then replace it with anhydrous ethanol to reduce the residual mother liquor on the surface and shorten the equilibrium time. The washing amount is preferably prepared at a ratio of 0.1 to 0.5 of "washing solution mass / wet crystal mass", more preferably 0.15 to 0.35. Specifically, it can be washed once with a 0°C saturated sodium sulfate aqueous solution and once with anhydrous ethanol. The drying / resting temperature is preferably 10-20℃, more preferably 12-18℃, specifically 15℃ in a sealed drying oven for 30-120 minutes (e.g., 60 minutes). The particle size distribution is preferably controlled by sieving, and the preferred sieve mesh size is "80-120 mesh passing through and 160-240 mesh being retained", more preferably "100 mesh passing through and 200 mesh being retained".

[0041] More preferably, before adding the modified sodium sulfate to the electroplating system, it can be dissolved and the electroplating solution can be filtered to 1μm to thoroughly remove any possible residual foreign particles and reduce the risk of pinholes / roughness caused by particles entering the electroplating interface.

[0042] b) Degreasing (alkaline electrolytic degreasing) In the titanium alloy degreasing step provided by this invention, alkaline electrolytic degreasing is preferably used to obtain a clean and uniformly activated titanium alloy surface. The preferred concentrations (by mass fraction) of each component in the alkaline electrolytic degreasing solution are: sodium hydroxide 2-6 wt% (more preferably 3-5 wt%); sodium carbonate 1-5 wt% (more preferably 2-4 wt%); trisodium phosphate (dodecahydrate) 2-10 wt% (more preferably 3-7 wt%); sodium metasilicate (pentahydrate) 0.5-5 wt% (more preferably 1-3 wt%).

[0043] The preferred degreasing process conditions are: temperature 50–70℃ (more preferably 55–65℃); cathode current density 3–8 A / dm³. 2 (More preferably 4-6 A / dm) 2 Electrolytic degreasing time: 2–10 min (more preferably 3–7 min).

[0044] c) Activation (short-time activation with HCl and ammonium bifluoride + wet transfer) In the activation step provided by the present invention, a compound acidic activation system of hydrochloric acid and ammonium bifluoride is preferably used, and re-oxidation is inhibited by wet transfer after short soaking and rapid rinsing.

[0045] The preferred composition of the activation solution is: hydrochloric acid (calculated as 38% hydrochloric acid) in the activation solution at a mass fraction of 15-30 wt%, more preferably 20-26 wt%; The mass fraction of ammonium bifluoride is 5–15 wt%, more preferably 8–12 wt%; The activation process conditions are preferably: temperature 15-30℃ (more preferably 20-25℃); immersion time 10-60s (more preferably 20-45s); rinsing is preferably a two-stage process of rapid rinsing with running clean water + rinsing with deionized water, and after rinsing, the product is not dried and is immediately transferred to the electroplating tank within 5-15s (more preferably ≤10s).

[0046] Ammonium bifluoride releases fluorine-containing reactive species under acidic conditions, F - It can complex and dissolve TiO2 inert films and generate soluble fluorine-containing complexes, forming fresh active titanium on the surface and producing micro-roughness; short-time rapid water washing and wet transfer can prevent the rebuilding of the re-oxidized film, reduce the risk of hydrogen absorption / hydrogen embrittlement, and provide a depositable interface for direct nickel electroplating.

[0047] d) Electroplating base nickel (cyanide-free system + modified sodium sulfate polarization control + additives to fix timing) In the nickel plating base plating step provided by the present invention, it is preferred to use a nickel plating solution with nickel sulfate / nickel chloride as the main salt, boric acid as the buffer, modified sodium sulfate as the polarization control component, and additives; wherein, the high purity and low impurities of the modified sodium sulfate and its particle size / dissolution consistency can reduce pinhole / bubbling sensitivity and improve the adhesion stability.

[0048] (1) Composition of electroplating solution In the nickel plating solution provided by this invention (in g / L): The preferred concentrations of nickel sulfate hexahydrate are 220–320 g / L, more preferably 250–300 g / L, and specifically 280 g / L; the preferred concentrations of nickel chloride hexahydrate are 30–70 g / L, more preferably 40–60 g / L, and specifically 50 g / L; the preferred concentrations of boric acid are 25–50 g / L, more preferably 30–45 g / L, and specifically 40 g / L; the preferred concentrations of modified sodium sulfate are 180–260 g / L, more preferably 200–240 g / L, and specifically 227 g / L; the preferred concentrations of sodium saccharin are 1.0–2.5 g / L (e.g., 1.5 g / L); the preferred concentrations of sodium dodecyl sulfate are 0.05–0.30 g / L (e.g., 0.10 g / L); and the preferred concentrations of sodium allyl sulfonate are 0.5–2.0 g / L (e.g., 1.0 g / L).

[0049] (2) Key process conditions In the electroplated nickel base provided by this invention: The preferred temperature is 50–65°C, more preferably 55–60°C (e.g., 58°C); The pH is preferably 3.5 to 4.5, more preferably 3.8 to 4.2 (e.g., pH=4, adjusted with a 10% sulfuric acid aqueous solution). The preferred filtration accuracy is 0.5–2 μm (more preferably 1 μm), and the preferred circulation filtration time is 5–20 min (e.g., 10 min). The preferred anode is an electrolytic nickel plate equipped with an anode bag; mechanical stirring is preferred to ensure that there are no obvious dead zones on the surface of the bath solution. The preferred energizing procedure is a two-stage current density: initially 0.5–1.5 A / dm² after immersion in the tank. 2 Power on for 10–60 seconds, then switch to 2–5 A / dm 2 Electroplating for 5–20 minutes; specifically, 1 A / dm 2 ×30s and 4A / dm 2 ×10min.

[0050] Nickel sulfate / nickel chloride provides Ni 2+ And ensure anodic dissolution; boric acid buffers the pH of the cathode diffusion layer to suppress Ni(OH)2 inclusions; Modified sodium sulfate provides appropriate complexation and polarization control in the tank, making it easier to form a fine and continuous nickel core layer and suppress pinholes / bubbling during the initial deposition stage of activated titanium surface; The fixed timing of the additive can reduce its non-functional adsorption on microcrystals / filter residue particles, which is beneficial to the density and bonding stability of the nickel layer. Example

[0051] Preparation of modified sodium sulfate Step 1: Weigh 3000g of anhydrous sodium sulfate and add it to 6000g of deionized water. Heat the solution to 40℃ while stirring at 300rpm and maintain the temperature for 20min. Then, filter the hot solution through a 1μm polypropylene membrane, discard the filter residue, and obtain a clear crude sodium sulfate solution. Let the solution temperature drop to 35℃ for later use. Step 2: Weigh 20g of aluminum nitride powder (Huzhou Yuanqin New Material Co., Ltd., model YQ-A01) and add it to 500g of deionized water. Place it in a 40kHz ultrasonic cleaner and sonicate for 10 minutes to obtain a uniform dispersion. While stirring continuously, add 8g of sodium silicate (Shanghai Aladdin Biochemical Technology Co., Ltd., product number S302432) and 10g of sodium dihydrogen phosphate monohydrate in sequence. Continue stirring for 10 minutes, then weigh 30g of sodium tetraborate decahydrate and add it, stirring for 5 minutes to obtain a composite heterogeneous nucleation dispersion for later use. Step 3: The crude sodium sulfate solution obtained in Step 1 at 35℃ was added to the composite heterogeneous nucleation dispersion obtained in Step 2 at a stirring speed of 300 rpm. After maintaining the temperature at 35℃ for 5 min, the system was cooled to 0℃ at a cooling rate of 1℃ / min and kept at that temperature for 120 min to precipitate. The crystals were then collected by vacuum filtration and washed once with 500 g of saturated sodium sulfate aqueous solution at 0℃ and then once with 200 g of anhydrous ethanol to obtain wet crystals. The wet crystals were placed in a sealed drying oven at 15℃ and allowed to stand for 60 min. Then, they were passed through a 100-mesh sieve and retained on a 200-mesh sieve to complete the particle size determination, thus obtaining the modified sodium sulfate product. Specialized electroplating nickel base process for titanium alloys: (1) Degreasing After mounting the titanium alloy TC4 screw workpiece, prepare an alkaline electrolytic degreasing solution: Add 8000g of deionized water to an alkali-resistant tank, then add 400g of sodium hydroxide, 300g of sodium carbonate, 500g of trisodium phosphate dodecahydrate, and 200g of sodium metasilicate pentahydrate in sequence, stirring until dissolved. Add deionized water to a total mass of 10000g, and heat to 60℃. Use the workpiece as the cathode, and apply a cathode current density of 5A / dm³. 2 Electrolytic degreasing was performed for 5 minutes under the specified conditions, followed by rinsing with 60°C hot water for 30 seconds and then rinsing with running cold water for 60 seconds. (2) Activation Preparation of activation solution: Add 3300g of deionized water to the acid-resistant tank, slowly add 1200g of hydrochloric acid (mass fraction 38%), stir and cool to 25℃, then add 500g of ammonium bifluoride, and add deionized water to a total mass of 5000g; immerse the degreased workpiece in the activation solution at 25℃ for 30s, remove it and rinse quickly with running water for 20s, then rinse with deionized water for 20s, do not dry after rinsing and immediately transfer it to the electroplating tank within 10s; (3) Electroplating of nickel base Preparation of the base nickel plating solution: Add 5000g of deionized water to the electroplating tank and heat to 60℃. Add 2800g of nickel sulfate hexahydrate, 500g of nickel chloride hexahydrate, and 400g of boric acid sequentially. Stir at 300rpm until completely dissolved, then add 2270g of modified sodium sulfate. Continue stirring for 20min and stabilize the solution temperature at 58℃. Adjust the pH to 4 with a 10% sulfuric acid aqueous solution and perform a 1μm circulation filtration for 10min. After filtration, add additives in a fixed sequence: 15g of sodium saccharin, 1g of sodium dodecyl sulfate, and 10g of sodium allyl sulfonate. Add deionized water to a total volume of 10L. During electroplating, use an electrolytic nickel plate with an anode bag. Use mechanical stirring to ensure a smooth flow without dead zones on the surface of the solution. After immersing the activated workpiece in the tank, first apply a cathode current density of 1A / dm³. 2 After energizing for 30 seconds, switch to a cathode current density of 4 A / dm³. 2 Electroplating for 10 minutes, followed by rinsing with running water for 60 seconds, rinsing with deionized water for 30 seconds, and drying with hot air at 80℃ for 10 minutes to obtain a titanium alloy containing a base nickel.

[0052] Example 2 (only the amount of heterogeneous nucleation cores was changed): Compared with Example 1, only the amount of aluminum nitride powder in step 2 of the modified sodium sulfate preparation was adjusted from 20g to 10g; the amounts of other raw materials, the order of addition, the ultrasonic conditions, the cooling program, the washing and sieving conditions, and the degreasing / activation / electroplated nickel conditions were the same as in Example 1.

[0053] Example 3 (only the freezing and crystallization cooling rate and holding time were changed): Compared with Example 1, only the cooling rate in step 3 of the modified sodium sulfate preparation was adjusted from 1℃ / min to 0.5℃ / min, and the crystallization time at 0℃ was adjusted from 120min to 180min; the other conditions were the same as in Example 1.

[0054] Example 4 (only the amount of modified sodium sulfate added to the electroplating solution was changed): Compared with Example 1, only the amount of modified sodium sulfate added to the nickel plating solution for the base plating was adjusted from 2270g to 2000g (finally adjusted to 10L); the composition of the plating solution, pH, filtration, temperature, current density-time regime and post-treatment conditions were the same as in Example 1.

[0055] Example 5 (only the amount of modified sodium sulfate added to the electroplating solution was changed): Compared with Example 1, only the amount of modified sodium sulfate added to the nickel plating solution for the base plating was adjusted from 2270g to 2400g (finally adjusted to 10L); the composition of the plating solution, pH, filtration, temperature, current density-time regime and post-treatment conditions were the same as in Example 1.

[0056] The difference between Comparative Example 1 and Example 1 is that when preparing the electroplating solution in the nickel plating base step (3), 2270g of modified sodium sulfate added in Example 1 is replaced with 1000g of anhydrous sodium sulfate; the other conditions are the same as in Example 1.

[0057] The difference between Comparative Example 2 and Example 1 is that in step 2 of the preparation of modified sodium sulfate, instead of weighing 20g of aluminum nitride powder and adding it to 500g of deionized water for ultrasonic dispersion, only 500g of deionized water is weighed and 8g of sodium silicate and 10g of sodium dihydrogen phosphate monohydrate are added sequentially under continuous stirring and stirred for 10min. Then, 30g of sodium tetraborate decahydrate is weighed and added and stirred for 5min to obtain a dispersion. The other conditions are the same as in Example 1.

[0058] The difference between Comparative Example 3 and Example 1 is that in step 2 of the preparation of modified sodium sulfate, after adding 8g of sodium silicate and 10g of sodium dihydrogen phosphate monohydrate and continuing to stir for 10min, 30g of sodium tetraborate decahydrate is no longer weighed and added for stirring for 5min; the other conditions are the same as in Example 1.

[0059] The difference between Comparative Example 4 and Example 1 is that in step 2 of the preparation of modified sodium sulfate, the order of adding sodium silicate 8g first and then sodium dihydrogen phosphate monohydrate 10g is changed to adding sodium dihydrogen phosphate monohydrate 10g first and then sodium silicate 8g; the other conditions are the same as in Example 1.

[0060] The difference between Comparative Example 5 and Example 1 is that in step 2 of the preparation of modified sodium sulfate, the amount of sodium silicate is adjusted to 18g, and sodium dihydrogen phosphate monohydrate is not added; the other conditions are the same as in Example 1.

[0061] The difference between Comparative Example 6 and Example 1 is that in step 2 of the preparation of modified sodium sulfate, the amount of sodium dihydrogen phosphate monohydrate was adjusted to 18g, and sodium silicate was not added; the other conditions were the same as in Example 1.

[0062] Performance testing: Chloride content (calculated as chloride ions) of sodium sulfate powder: determined by ion chromatography. 2.0000 g (accurate to 0.0001 g) of each of the modified sodium sulfate products from the examples and comparative examples, and the anhydrous sodium sulfate used in Comparative Example 1, were weighed and placed in 250 mL volumetric flasks. Deionized water was added to dissolve and the volume was adjusted to 250.0 mL. After shaking and standing for 2 min, the supernatant was filtered through a 0.22 μm polyethersulfone needle filter and then analyzed using ion chromatography. An AS23 anion exchange column with carbonate eluent was used, and a carbonate / bicarbonate system was used as the eluent. A conductivity detector with a suppressor was used for detection. A series of standard curves were prepared using chloride ion standard solutions (0.05-5.00 mg / L) for external standard quantification. Each sample was measured in triplicate, and the arithmetic mean was taken. The results were converted to mg / kg according to the dilution factor.

[0063] Sodium sulfate powder particle size distribution (laser diffraction method): Take 0.1000 g of each of the modified sodium sulfate products of each example and comparative example and the anhydrous sodium sulfate used in comparative example 1, add 100.0 mL of anhydrous ethanol as the dispersion medium, sonicate in a 40 kHz ultrasonic cleaner for 2 min, and immediately transfer to the wet circulation tank of the laser particle size analyzer. After stabilizing at a stirring speed of 2000 rpm for 30 s, start the test. Set the test occlusion to 10%, measure each sample three times consecutively, and output D10, D50 and D90 and take the average value.

[0064] Thermogravimetric-differential scanning characterization of sodium sulfate powder: 10.0 mg each of the modified sodium sulfate products of each example and comparative example, and 10.0 mg of anhydrous sodium sulfate used in Comparative Example 1, were placed in an alumina crucible and heated from 25 °C to 300 °C under a protective atmosphere of high-purity nitrogen at a rate of 50 mL / min, with a heating rate of 10 °C / min. The thermogravimetric curves were recorded. The weight loss rate (%) in the range of 25 °C-200 °C was used as the quantitative indicator of the release of water of crystallization / adsorbed water. Each sample was measured in triplicate and the average value was taken.

[0065] Nickel plating underlayer thickness: Three nickel-plated workpieces were randomly selected from both the example and the comparative example. The screw head was cut along the axial direction to obtain a cross-sectional sample containing the plating layer. After cold mounting and curing, the sample was polished with 240, 600, 1200, and 2000 grit sandpaper in sequence and polished to a mirror finish with 0.05 μm alumina polishing liquid. The plating thickness was measured at 1000x magnification using a metallographic microscope at 10 uniformly selected points along the plating normal, and the average value was taken as the thickness of the workpiece. The arithmetic mean of the three workpieces was then taken to obtain the average thickness of the group. At the same time, the coefficient of variation of the average thickness of the three workpieces was calculated to evaluate the thickness uniformity.

[0066] Nickel plating undercoat adhesion strength (thermal shock cycling-peeling evaluation): Referring to the adhesion evaluation method listed in GB / T 5270-2024, a combination of thermal shock cycling and tape peeling was used to compare the nickel-plated workpieces of the examples and comparative examples: Five nickel-plated workpieces in each group were placed in a 200℃ forced-air drying oven for 60 minutes, then immersed in 25℃ deionized water for 60 seconds and dried. This constituted one cycle, and a total of 5 cycles were performed. After the cycle, a square grid with a side length of 2mm (3×3, a total of 9 grids, cut through to the substrate but without cutting the workpiece) was cut on the surface that could be contacted by the screw head with a knife. A 20mm wide pressure-sensitive tape was applied and rolled back and forth 3 times with a 2kg rubber roller. After standing for 60 seconds, the coating was peeled off at a 90° angle and a peeling speed of 300mm / min. The area of ​​the coating peeled off was observed using a 10x magnifying glass, and the adhesion level was rated from 0 to 5 according to the proportion of the peeled area (level 0 is no peeling, level 5 is peeling area ≥65%).

[0067] Neutral salt spray corrosion and corrosion rating: Neutral salt spray test was conducted according to GB / T 10125-2021 and rating was conducted according to GB / T6461-2002. For each example and comparative example, 5 nickel-plated workpieces were taken. A spray solution was prepared using a 5% (w / w) sodium chloride solution and the pH of the solution was adjusted to 6.8. The test chamber temperature was 35℃ and the deposition rate was 1.5 mL / 80 cm. 2 / h, the workpieces are placed at a 20° angle to the vertical direction and do not contact each other, and sprayed continuously for 240h; every 24h, the workpieces are taken out to observe and record the first occurrence time of failures such as blistering, cracking, and peeling of the coating. After 240h, the corrosion area and defect morphology are graded according to GB / T 6461 and recorded as corrosion rating.

[0068] Table 1 Key Results of Performance Testing

[0069] Data Analysis: As can be seen from the data in Table 1, the modified sodium sulfate prepared by this invention maintains a low chloride content and its particle size distribution is concentrated within the sieve window, thus reducing batch-to-batch fluctuations in powder dissolution rate and solution composition. The thermogravimetric analysis (TGA) curves show stable crystal water release characteristics in the 25–200℃ and 25–300℃ ranges, indicating that the crystal phase is mainly sodium sulfate decahydrate with controlled entrainment of mother liquor. During the nickel plating process, the consistency of the above-mentioned raw materials makes the conductivity and polarization state of the plating solution more repeatable. The average coating thickness remains stable under the same current density and time conditions, while the thickness variation coefficient is low, indicating that the current distribution and deposition rate are more uniform. At the same time, the adhesion strength level and neutral salt spray corrosion level show synchronous optimization, and the first failure time is delayed. The reason is speculated to be that: the foreign aluminum nitride particles provide heterogeneous nucleation nuclei and promote crystal refinement; sodium silicate and sodium dihydrogen phosphate monohydrate regulate the particle surface in a fixed order to differentiate and fix metal impurities; sodium tetraborate decahydrate reduces supercooling and enhances nucleation repeatability; and ultrasonic pulses inhibit agglomeration and entrainment in the initial stage of supersaturation, thereby achieving a synergistic effect of purification, particle size control and dissolution consistency. Finally, a more continuous and dense nickel core layer is formed in the initial nucleation stage of nickel plating, and the sensitivity to pinholes and blistering is reduced.

[0070] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when anhydrous sodium sulfate is directly replaced with modified sodium sulfate, the thermogravimetric weight loss of the powder is significantly reduced, indicating that the anhydrous material lacks water of crystallization. However, the chloride residue and particle characteristics are closer to those of industrial raw materials, leading to more fluctuations in the impurity level and dissolution behavior of the electroplating solution. The main reason is that industrial anhydrous sodium sulfate has not undergone solid-liquid distribution and low-temperature saturated washing during freeze-drying crystallization. Chloride ions and metallic impurities are difficult to remove with the mother liquor, and after entering the electroplating system, they are more likely to induce pinholes and localized corrosion channels in the coating, causing premature failure under salt spray conditions.

[0071] As can be seen from the data in Example 1 and Comparative Example 2 in Table 1, after removing aluminum nitride, the particle size distribution of modified sodium sulfate tends to become coarser and the chloride content increases, which in turn leads to a decrease in the uniformity of coating thickness, a deterioration in adhesion strength, and earlier failure in the salt spray test. The reason for this is speculated to be that aluminum nitride, as a heterogeneous nucleation core, can shorten the induction period and form a uniform micronucleus field, and, combined with ultrasonic pulses, can suppress agglomeration and entrainment. When this core is lacking, crystallization relies more on local supersaturation, and crystal growth is prone to random aggregation and mother liquor entrainment, resulting in amplified concentration fluctuations during dissolution. This leads to insufficient continuity of the nickel core layer in the initial stage of electrodeposition, and defects are more likely to evolve into blistering or peeling.

[0072] As can be seen from the data in Table 1 for Example 1 and Comparative Example 3, without the addition of sodium tetraborate decahydrate, although the site regulation of aluminum nitride, sodium silicate, and sodium dihydrogen phosphate monohydrate is still retained, the coating uniformity and salt spray resistance still decline. This may be because sodium tetraborate decahydrate has a regulating effect on the supercooling degree and nucleation repeatability of freeze-drying crystallization, which can expand the process window and reduce the impact of ambient temperature fluctuations on the induction period. Without it, batch-to-batch differences are more likely to occur during the crystal growth stage, making it difficult to maintain consistency between dissolution behavior and electroplating polarization state, thus forming micro-defects during the nickel layer densification process.

[0073] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, reversing the order of adding sodium dihydrogen phosphate monohydrate and sodium silicate does not necessarily lead to a synchronous deterioration in chloride content, but it does result in an overall decrease in coating adhesion and salt spray rating, and an earlier first failure time. The reason for this is presumably that the present invention first introduces sodium silicate to form an adsorption layer mainly composed of silicon-oxygen bonds on the aluminum nitride surface, and then introduces phosphate ions to form a charge-regulating layer, thereby achieving differentiated fixation of metal impurities with different valence states. When the order is disrupted, the initial charge state of the particle surface changes, and the subsequent adsorption layer structure becomes unstable, leading to an increased probability of metal impurities entering the crystal and electroplating solution. Even if chloride ion control remains within a reasonable range, localized corrosion caused by metal impurities will reduce corrosion resistance.

[0074] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, when only sodium silicate is added without sodium dihydrogen phosphate monohydrate, or when only sodium dihydrogen phosphate monohydrate is added without sodium silicate, the impurity fixation mechanism of modified sodium sulfate tends to be singular, the thickness uniformity and adhesion strength are further deteriorated, and failure under salt spray is more likely to occur earlier. Among them, the scheme that only retains sodium dihydrogen phosphate monohydrate shows a more significant decline. The main reason is that sodium silicate is more inclined to form an adsorption and fixation layer on the particle and crystal surface, while phosphate is more inclined to charge regulation and complexation co-precipitation control. The absence of either one will lead to the inability to effectively constrain a certain type of impurity. At the same time, when the two are coupled in a predetermined order, they can form a hierarchical structure at the same site, so that adsorption-charge regulation occurs simultaneously, thereby significantly reducing the sensitivity to pinholes and blistering in the initial deposition stage of electroplating.

[0075] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A special electroplating process for nickel undercoating on titanium alloys, characterized in that, Includes the following steps: (a) Degreasing treatment of the titanium alloy substrate; (ii) The titanium alloy substrate after degreasing was activated by an activation solution composed of hydrochloric acid and ammonium bifluoride, and then rinsed and wet-transferred. (iii) Electroplating an activated titanium alloy substrate in a cyanide-free nickel plating solution to form a base nickel layer, wherein the cyanide-free nickel plating solution comprises nickel sulfate, nickel chloride, boric acid, and modified sodium sulfate. During electroplating, the pH of the plating solution is controlled at 3.5-4.5, the temperature at 50-65℃, and the cathode current density at 2-5 A / dm³. 2 And the electroplating solution is filtered during the electroplating process; The preparation of the modified sodium sulfate includes the following steps: (1) Dissolve anhydrous sodium sulfate in deionized water and filter to obtain sodium sulfate mother liquor; (2) Aluminum nitride powder is dispersed in deionized water to form a heterogeneous nucleation dispersion, and sodium silicate, sodium dihydrogen phosphate monohydrate and sodium tetraborate decahydrate are added in sequence to obtain a composite heterogeneous nucleation dispersion. (3) After mixing the composite heterogeneous nucleation dispersion with sodium sulfate mother liquor, the mixture is subjected to controlled cooling freeze-drying crystallization to precipitate sodium sulfate crystals; (4) Sodium sulfate crystals are subjected to solid-liquid separation, washing and drying to obtain modified sodium sulfate.

2. The nickel plating process for titanium alloys according to claim 1, characterized in that, Step (I) involves alkaline electrolysis for degreasing. The mass fractions of each component in the alkaline electrolysis degreasing solution are: sodium hydroxide 2-6 wt%, sodium carbonate 1-5 wt%, trisodium phosphate dodecahydrate 2-10 wt%, and sodium metasilicate pentahydrate 0.5-5 wt%. The degreasing process conditions are: temperature 50-70℃ and cathode current density 3-8 A / dm³. 2 Time: 2-10 minutes.

3. The electroplating nickel undercoat process for titanium alloys according to claim 1, characterized in that, In step (ii), the composition of the activation solution meets the following requirements: the mass fraction of 38% hydrochloric acid in the activation solution is 20-26 wt%, the mass fraction of ammonium bifluoride is 8-12 wt%, and the activation process conditions are a temperature of 20-25℃ and a soaking time of 20-45s.

4. The electroplating nickel undercoat process for titanium alloys according to claim 1, characterized in that, The cyanide-free nickel plating solution comprises, per liter of plating solution: 250-300g of nickel sulfate hexahydrate, 40-60g of nickel chloride hexahydrate, 30-45g of boric acid, and 200-240g of modified sodium sulfate.

5. The electroplating nickel undercoat process for titanium alloys according to claim 1, characterized in that, The cyanide-free nickel plating solution also includes additives, which, per liter of plating solution, include: 1.0-2.5g of sodium saccharin, 0.05-0.30g of sodium dodecyl sulfate, and 0.5-2.0g of sodium allyl sulfonate. The additives are added in a fixed order after filtration, namely sodium saccharin, sodium dodecyl sulfate, and sodium allyl sulfonate.

6. The electroplating nickel base layer process for titanium alloys according to claim 1, characterized in that, The electroplating process employs a two-stage current density energizing regime: after immersion in the plating bath, the current density is initially 0.5–1.5 A / dm². 2 Power on for 10–60 seconds, then switch to 2–5 A / dm 2 Electroplating for 5–20 minutes.

7. The nickel plating process for titanium alloys according to claim 1, characterized in that, Based on the mass of anhydrous sodium sulfate in step (1), the amount of aluminum nitride powder used in step (2) is 0.4 to 0.9 wt%.

8. The electroplating nickel undercoat process for titanium alloys according to claim 1, characterized in that, Based on the mass of anhydrous sodium sulfate in step (1), the amount of sodium silicate used in step (2) is 0.2-0.4 wt%, the amount of sodium dihydrogen phosphate monohydrate is 0.2-0.5 wt%, and the amount of sodium tetraborate decahydrate is 0.6-1.5 wt%.

9. The electroplating nickel undercoat process for titanium alloys according to claim 1, characterized in that, In step (3), the mixing temperature is 33-37℃, the mixing speed is 250-350rpm, and the constant temperature homogenization time is 3-10min.

10. The nickel plating process for titanium alloys according to claim 1, characterized in that, In step (3), the cooling rate is 0.8 to 1.2 °C / min, the target temperature is -2 to 2 °C, and the holding time for crystallization is 90 to 150 min.

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