A low-phosphorus electroless nickel plating solution and a method for plating nickel on an aluminum alloy substrate

Abstract

This invention relates to the field of electroless plating technology, and more particularly to a low-phosphorus electroless nickel plating solution and a method for nickel plating on an aluminum alloy substrate. The low-phosphorus electroless nickel plating solution comprises the following components by mass concentration: a nickel ion source in the form of Ni... 2+ Calculated as 1.0~5.0 g / L; hypophosphite as H2PO2 ‑ The concentrations are calculated as follows: 20–45 g / L; glycine 5–40 g / L, gluconate 5–60 g / L; polyaspartic acid and / or its salts 0.2–5.0 g / L; pH buffer 2–25 g / L; alkyl glycosides 0.05–1.0 g / L; iodate stabilizer, in IO3... ‑ The concentration is calculated to be 0.2~5.0 mg / L. This invention maintains a high deposition rate and significantly improves the tolerance of by-products in the bath solution under low nickel ion concentration conditions. It is also free of heavy metals and sulfur-containing organic stabilizers, reducing the metal load and environmental risk of the waste liquid. It is suitable for various aluminum alloy matrices treated with double zinc replacement.

Description

Technical Field

[0001] This invention relates to the field of electroless plating technology, and in particular to a low-phosphorus electroless nickel plating solution and a method for nickel plating on an aluminum alloy substrate. Background Technology

[0002] Aluminum alloys are widely used in automobiles, electronics, and precision structural components due to their low density, high specific strength, and ease of processing. However, the surface of aluminum alloys easily forms a stable oxide film and has high electrochemical activity, resulting in insufficient adhesion of directly deposited nickel layers. Common defects include blistering, localized non-plating, pinholes, and plating peeling. Industrially, a route of "degreasing-alkaline etching-de-ashing-activation-(primary zinc replacement / zinc stripping / secondary zinc replacement)-electroless nickel plating" is typically used to establish a transition layer between the aluminum substrate and the nickel layer to ensure adhesion.

[0003] Electroless nickel plating uses hypophosphite as a reducing agent to autocatalytically deposit a Ni-P alloy layer on a catalytic surface. Low-phosphorus plating layers are favored for their lower electrical resistance, stronger magnetism, better solderability, and superior subsequent functionality, making them ideal for electromagnetic shielding, conductivity, and precision dimensional compensation. However, low-phosphorus systems often rely on a narrow pH and coordination equilibrium window, making them susceptible to the accumulation of phosphite byproducts, which can lead to decreased deposition rates, roughness, decomposition, or shortened lifespan. Some systems use sulfur-containing organic stabilizers or heavy metal stabilizers to enhance stability, increasing environmental pressure. Furthermore, increasing the nickel ion concentration to maintain the plating rate significantly increases the metal load on the waste plating solution.

[0004] Therefore, there is an urgent need for a chemical nickel plating solution that combines low phosphorus control, long-term stability, low metal ion load, and a green additive system, and to form an industrially feasible integrated process with the aluminum alloy double zinc replacement pretreatment, so as to improve consistency and reduce environmental risks. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a low-phosphorus electroless nickel plating solution and a nickel plating method. This invention maintains a high deposition rate under low nickel ion concentration conditions and significantly improves the tolerance of byproducts in the plating bath. Furthermore, it is free of heavy metal stabilizers such as lead and cadmium, as well as sulfur-containing organic stabilizers, reducing the metal load on waste liquid and environmental risks. It is suitable for various aluminum alloy substrates treated with double zinc replacement.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a low-phosphorus electroless nickel plating solution comprising the following components by mass concentration: a nickel ion source, in accordance with Ni... 2+ Calculated as 1.0~5.0 g / L; hypophosphite, as H2PO2 -The concentrations are calculated as follows: 20–45 g / L; glycine 5–40 g / L, gluconate 5–60 g / L; polyaspartic acid and / or its salts 0.2–5.0 g / L; pH buffer 2–25 g / L; alkyl glycosides 0.05–1.0 g / L; iodate stabilizer, in IO3... - The concentration is calculated to be 0.2~5.0 mg / L; the pH value of the low-phosphorus electroless nickel plating solution is 4.6~5.4; the low-phosphorus electroless nickel plating solution does not contain heavy metal stabilizers or sulfur-containing organic stabilizers.

[0007] Preferably, the gluconate includes one or more of sodium gluconate, potassium gluconate, and ammonium gluconate.

[0008] Preferably, the iodate stabilizer includes one or more of sodium iodate, potassium iodate, and ammonium iodate.

[0009] Preferably, the alkyl glycoside is a C8-C12 alkyl glycoside.

[0010] Preferably, the nickel ion source includes one or more of nickel sulfamate, nickel sulfate, and nickel chloride.

[0011] Preferably, the hypophosphite includes one or more of sodium hypophosphite, potassium hypophosphite, and ammonium hypophosphite.

[0012] Preferably, the number-average molecular weight of the polyaspartic acid is 1000-10000.

[0013] Preferably, the pH buffer is acetic acid, acetate, citric acid, or citrate.

[0014] This invention provides a method for nickel plating on an aluminum alloy substrate, comprising the following steps: placing the aluminum alloy substrate, which has undergone double zinc replacement treatment, in the low-phosphorus electroless nickel plating solution described above for electroless nickel plating to form a Ni-P coating; the electroless nickel plating temperature is 80~92℃ and the pH value is 4.6~5.4.

[0015] Preferably, the aluminum alloy substrate subjected to double zinc replacement treatment is obtained by sequentially degreasing, alkaline etching, deashing, and double zinc replacement of the aluminum alloy substrate.

[0016] This invention uses glycine + gluconate as the main complexing system: glycine is a small molecule complexing agent that can react with Ni. 2+ A suitable complexation equilibrium is formed, ensuring that nickel ions are neither too highly concentrated (causing bath instability) nor too tightly complexed (preventing release to the catalytic surface); gluconate further improves solution stability when nickel salts and byproducts coexist. The combination of these two technologies allows for the development of solutions with low Ni content. 2+This invention maintains a high "effective supply of depositable nickel ions" under total volume. It uses polyaspartic acid (PASP) and / or its salts as dispersing and complexing components: PASP disperses and gently complexes byproducts, particles, and locally high-concentration areas, reducing the risk of roughness, precipitation, and decomposition induced by phosphite accumulation in the bath; it also mitigates localized deposition runaway, improving bath life and tolerance to byproduct accumulation. The invention uses iodate as a stabilizing component: iodate acts as a stabilizer at extremely low doses, which helps suppress spontaneous decomposition and localized abnormal nickel precipitation, thereby reducing bath instability and allowing the system to maintain continuous operation under lower metal loads. The invention uses alkyl glycosides (APG) as a green wetting agent: APG improves substrate surface wetting and hydrogen evolution desorption, reduces localized bubble shielding, and allows for more uniform participation of the catalytic surface in deposition, indirectly improving the effective deposition efficiency per unit time. In summary, this invention improves low-Ni deposition efficiency through a combination of "main complexing system (glycine + gluconate) + PASP dispersion and complexation + trace iodate stabilization + APG interface wetting." 2+ The effective nickel utilization rate and bath stability under the given conditions are balanced, thus taking into account both deposition rate and by-product tolerance.

[0017] This invention provides a method for nickel plating on an aluminum alloy substrate. The method controls the working pH at 4.6–5.4 and the temperature at 80–92°C, placing the hypophosphite reduction reaction within a suitable range. This temperature and pH matching allows for maintaining a high deposition rate even at low nickel concentrations, without relying solely on increasing the nickel salt concentration to artificially inflate the rate. Detailed Implementation

[0018] This invention provides a low-phosphorus electroless nickel plating solution comprising the following components by mass concentration: a nickel ion source, in accordance with Ni... 2+ Calculated as 1.0~5.0 g / L; hypophosphite, as H2PO2 - The concentrations are calculated as follows: 20–45 g / L; glycine 5–40 g / L, gluconate 5–60 g / L; polyaspartic acid and / or its salts 0.2–5.0 g / L; pH buffer 2–25 g / L; alkyl glycosides 0.05–1.0 g / L; iodate stabilizer, in IO3... - The concentration is calculated to be 0.2~5.0 mg / L; the pH value of the low-phosphorus electroless nickel plating solution is 4.6~5.4; the low-phosphorus electroless nickel plating solution does not contain heavy metals, stabilizers, or sulfur-containing organic stabilizers.

[0019] The low-phosphorus electroless nickel plating solution provided by this invention contains a nickel ion source in the form of Ni 2+The concentration is calculated to be 1.0~5.0 g / L, and in specific embodiments it can be 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0 g / L. In this invention, the nickel ion source preferably includes one or more of nickel aminosulfonate, nickel sulfate and nickel chloride, more preferably nickel aminosulfonate. The nickel ion source can be flexibly selected according to the workpiece material, chloride ion sensitivity, deposition rate and bath stability requirements. Among them, nickel aminosulfonate has good solubility, mild anion introduction and is more beneficial to bath stability, so it is preferred; nickel sulfate has a lower cost and is suitable for general systems; nickel chloride can improve conductivity and activation under certain conditions, but the amount should be controlled to avoid adverse effects on the stability of certain substrates or systems.

[0020] The low-phosphorus electroless nickel plating solution provided by this invention contains hypophosphite, in the form of H2PO2. - The concentration is calculated to be 20-45 g / L, and in specific embodiments it can be 20, 25, 30, 35, 40, or 45 g / L. In this invention, the hypophosphite preferably includes one or more of sodium hypophosphite, potassium hypophosphite, and ammonium hypophosphite, more preferably sodium hypophosphite. Sodium hypophosphite has a stable source, good solubility, and mature industrial applications, making it more suitable for the system of this invention. In this invention, the hypophosphite acts as a reducing agent.

[0021] The low-phosphorus electroless nickel plating solution provided by the present invention contains 5~40 g / L of glycine, which can be 5, 10, 15, 20, 25, 30, 35 or 40 g / L in specific embodiments.

[0022] The low-phosphorus electroless nickel plating solution provided by this invention contains 5-60 g / L of gluconate, and in specific embodiments, it can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 g / L. In this invention, the gluconate preferably includes one or more of sodium gluconate, potassium gluconate, and ammonium gluconate, more preferably sodium gluconate. In this invention, the gluconate, in combination with glycine, serves as the main complexing system, jointly regulating the nickel ion complexation balance and improving the system's tolerance to byproduct accumulation.

[0023] The low-phosphorus electroless nickel plating solution provided by this invention contains 0.2~5.0 g / L of polyaspartic acid and / or its salts, and in specific embodiments, the concentration can be 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 g / L. In this invention, the number-average molecular weight of the polyaspartic acid is preferably 1000~10000, and in specific embodiments, it can be 2000, 4000, 6000, 8000, or 10000. In this invention, the polyaspartic acid and / or its salts serve as dispersing and complexing components, which on the one hand can disperse and gently complex byproducts, particles, and locally high-concentration areas, reducing the risk of roughness, precipitation, and decomposition induced by phosphite accumulation in the bath; on the other hand, it can also mitigate localized deposition runaway, improve bath life, and increase tolerance to byproduct accumulation. This invention replaces some of the traditional high-risk complexing / dispersing agents with polyaspartic acid and / or its salts, which helps control turbidity and roughness caused by byproducts.

[0024] The low-phosphorus electroless nickel plating solution provided by this invention contains a pH buffer at a concentration of 2-25 g / L. In this invention, the amount of pH buffer is used to maintain the pH value of the electroless nickel plating solution at 4.6-5.4. In specific embodiments, it can be 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, or 5.4. In this invention, the pH buffer is preferably acetic acid, acetate, citric acid, or citrate.

[0025] The low-phosphorus electroless nickel plating solution provided by this invention contains 0.05~1.0 g / L of alkyl glycosides, and in specific embodiments, it can be 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 g / L. In this invention, the alkyl glycosides are preferably C8~C12 alkyl glycosides, and in specific embodiments, they can be C8, C9, C10, C11, or C12 alkyl glycosides. This invention uses alkyl glycosides as wetting agents, which is more environmentally friendly than some traditional surfactants.

[0026] The low-phosphorus electroless nickel plating solution provided by this invention contains an iodate stabilizer, with IO3 as the concentration. -The concentration is calculated to be 0.2~5.0 mg / L, and in specific embodiments it can be 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0 mg / L. In this invention, the iodate stabilizer preferably includes one or more of sodium iodate, potassium iodate and ammonium iodate, more preferably sodium iodate or potassium iodate. This invention uses iodate as a stabilizing component. Iodate plays a stabilizing role at a very low dose, which is beneficial for inhibiting spontaneous decomposition and local abnormal nickel deposition, thereby reducing bath instability and enabling the system to maintain continuous operation under a low metal load. Compared with the traditional use of heavy metal stabilizers and sulfur-containing organic stabilizers, this invention reduces the metal load of waste liquid and environmental risks.

[0027] The low-phosphorus electroless nickel plating solution provided by this invention uses water as a solvent.

[0028] The low-phosphorus electroless nickel plating solution provided by this invention uses PASP+APG+low Ni 2+ The combination achieves low phosphorus content, long-term stability, and low emission load, significantly improving environmental friendliness.

[0029] The present invention does not have any special requirements for the preparation method of the low-phosphorus electroless nickel plating solution; the components can be directly dissolved in water.

[0030] This invention provides a method for nickel plating on an aluminum alloy substrate, comprising the following steps: placing the aluminum alloy substrate, which has undergone double zinc replacement treatment, into the above-mentioned low-phosphorus electroless nickel plating solution for electroless nickel plating to form a Ni-P coating; the temperature of the electroless nickel plating is 80~92℃, and the pH value is 4.6~5.4.

[0031] In this invention, the aluminum alloy substrate subjected to double zinc replacement treatment is obtained by sequentially degreasing, alkaline etching, deashing, and double zinc replacement of the aluminum alloy substrate.

[0032] In this invention, the degreasing process preferably includes immersing the aluminum alloy substrate in an alkaline degreasing agent, followed by rinsing with running water. In this invention, the concentration of the alkaline degreasing agent is preferably 30-60 g / L; the immersion temperature is preferably 45-65°C, and the immersion time is preferably 2-8 minutes. This invention does not specify the type of alkaline degreasing agent; commercially available alkaline degreasing agents for aluminum alloys, known in the art, can be used. Specifically, it can be an alkaline degreasing agent containing sodium hydroxide, sodium carbonate, phosphate, and a surfactant.

[0033] In this invention, the alkaline etching preferably includes: placing the degreased aluminum alloy substrate in a NaOH solution with a concentration of 20-60 g / L and treating it at 20-45°C for 10-90 s. This invention removes the natural oxide film and minor surface defects through alkaline etching, followed by water washing.

[0034] In this invention, the deashing preferably includes: placing the alkaline-etched aluminum alloy substrate in a 100-400 mL / L nitric acid solution at room temperature for 10-90 s; when the aluminum is high-silicon or die-cast, this invention preferably adds a fluorine-containing component, followed by water washing. In this invention, the fluorine-containing component is preferably provided by ammonium fluoride, ammonium hydrofluoride, sodium fluoride, or hydrofluoric acid, preferably ammonium hydrofluoride or ammonium fluoride. The content of the fluorine-containing component in the deashing solution is calculated according to F... - The preferred concentration is 0.2~5 g / L, more preferably 0.5~2.5 g / L. In this invention, the fluorine-containing component is mainly used to enhance the removal capacity of silicon-containing residues on the surface of high-silicon aluminum alloys or die-cast aluminum alloys. This invention removes insoluble ash residue, intermetallic compound enrichment layer, and surface residual alkali after alkaline etching through deashing, restoring a uniformly activated substrate surface and creating conditions for subsequent zinc replacement.

[0035] In this invention, the double zinc replacement preferably includes sequentially performing a first zinc replacement, zinc stripping, and a second zinc replacement; wherein, the first zinc replacement is preferably: immersing the deashed aluminum alloy substrate in a zinc replacement solution for 10-60 s; the zinc replacement solution preferably comprises the following components at the following mass concentrations: zinc oxide 5-30 g / L and sodium hydroxide 80-180 g / L; wherein zinc oxide is more preferably 10-20 g / L, and sodium hydroxide is more preferably 100-150 g / L; the zinc stripping is preferably: immersing the aluminum alloy substrate that has undergone the first zinc replacement in a nitric acid solution for zinc stripping for 5-30 s; the concentration of the nitric acid solution is preferably 10-40 vol.%, more preferably 15-30 vol.%; the second zinc replacement is preferably: immersing the zinc-stripped aluminum alloy substrate again in the zinc replacement solution for 5-40 s, and finally rinsing with water.

[0036] In this invention, the electroless nickel plating temperature is 80~92℃ and the pH value is 4.6~5.4; in specific embodiments, the electroless nickel plating temperature can be 80, 82, 84, 86, 88, 90 or 92℃; the pH value of the electroless nickel plating can be 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3 or 5.4.

[0037] This invention controls the working pH value within the aforementioned range, combined with temperature control, to keep the hypophosphite reduction reaction within a suitable range. The matching of temperature and pH can maintain a high deposition rate at low nickel concentrations, thus avoiding the need to simply increase the nickel salt concentration to "push" the rate forward.

[0038] In this invention, the thickness of the Ni-P coating is preferably 5~30 μm, and in specific embodiments it can be 5, 10, 15, 20, 25 or 30 μm.

[0039] The following detailed description, in conjunction with embodiments, illustrates the low-phosphorus electroless nickel plating solution and nickel plating method for aluminum alloy substrates provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0040] Example 1 The chemical nickel plating solution formula is as follows: Nickel ion source: Nickel sulfamate, in Ni 2+ It is calculated to be 3.0 g / L; Hypophosphite: Sodium hypophosphite, as H₂PO₂ - The concentration is calculated to be 30 g / L; Glycine: 20 g / L; Sodium gluconate: 25 g / L; Sodium polyaspartate: 1.0 g / L; pH buffer: Sodium acetate 10 g / L; Alkyl glycoside: Decyl glucoside, 0.20 g / L; Sodium iodate, in IO3 - It is calculated to be 1.0 mg / L; pH value: 5.0.

[0041] Comparative Example 1 The only difference from Example 1 is that the plating solution does not contain sodium polyaspartate; otherwise, it is the same as Example 1.

[0042] Comparative Example 2 The only difference from Example 1 is that the plating solution does not contain alkyl glycosides; otherwise, it is the same as Example 1.

[0043] Comparative Example 3 The plating solution uses conventional high-Ni 2+ The system, specifically the formula, is as follows: Nickel ion source: Nickel sulfamate, in Ni 2+ The concentration is 6.5 g / L. Sodium hypophosphite: Sodium hypophosphite, according to H2PO2 - The concentration is 28 g / L; Common organic acid / aminocarboxylic acid complexing agents: lactic acid 20 g / L, citric acid 15 g / L; Dispersant: Sodium succinate, 0.2 g / L; The stabilizer is a conventional stabilizer system: thiourea 1.0 mg / L; The pH value is 5.0.

[0044] Comparative Example 4 The only difference from Example 1 is that it does not contain sodium iodate.

[0045] Performance testing: The aluminum alloy workpieces were treated sequentially as follows: first, degreasing was performed in an alkaline degreasing agent with a concentration of 40 g / L at 55°C for 5 min, followed by rinsing with running water for 30 s; then, alkaline etching was performed in a NaOH solution with a concentration of 40 g / L at 25°C for 20 s, followed by rinsing with running water for 30 s; subsequently, descaling was performed in a 200 mL / L nitric acid solution at room temperature for 20 s, followed by rinsing with running water for 30 s; then, a first zinc replacement, zinc stripping, and a second zinc replacement were performed; finally, after rinsing with water, the workpieces were placed in the corresponding plating solution for electroless nickel plating. Except for the different composition of the plating solution, the nickel plating conditions in the examples and comparative examples were the same: temperature 88°C, pH 5.0, and nickel plating time 1 h.

[0046] The test results are summarized in Tables 1 and 2.

[0047] Table 1 Comparison of plating solution performance between Example 1 and Comparative Examples 1-3

[0048] Table 2. Byproduct tolerance and long-term stability in Examples 1 and Comparative Examples 1-2, 4

[0049] The results in Tables 1 and 2 show that in Comparative Example 1, which does not contain PASP, the lack of dispersion and complexation makes the plating solution more prone to roughness, pinholes, and decreased stability after the accumulation of byproducts. In Comparative Example 2, which only introduces PASP without APG, although the stability of the plating solution is improved, the wettability of the workpiece surface and the uniformity of the coating are still limited. Comparative Example 3 relies on high Ni... 2+ Maintaining deposition rate requires high nickel loading; low Ni 2+ However, without the iodate stabilizer (Comparative Example 4), the system lacks continuous operating stability. In contrast, the system of this invention, under the synergistic effect of PASP, APG, and the iodate stabilizer, exhibits a longer continuous operating time, a lower tendency for bath decomposition, and better workpiece surface wettability and coating uniformity. This indicates that the present invention has a wider process window and is more suitable for stable mass production.

[0050] Performance testing methods in Tables 1 and 2: (1) The deposition rate in Table 1 is the deposition rate after 1 h of nickel plating time; the P content of the coating is determined by inductively coupled plasma atomic emission spectrometry (ICP) and is the P content in the Ni-P coating obtained after 1 h of nickel plating time.

[0051] (2) Long-term stability is evaluated through continuous operation tests: The plating solution is continuously operated under fixed temperature, pH and replenishment regime. During continuous operation, phenomena such as spontaneous decomposition, obvious precipitation, sudden drop in deposition rate, and significant increase in roughness / pinholes are observed. When the plating solution remains clear, without obvious precipitation, with a small decrease in deposition rate and stable coating appearance during continuous operation, it is judged as 'good'; when a certain rate decrease or local roughness occurs during continuous operation, but the basic coating can still be maintained, it is judged as 'moderate'; when precipitation, decomposition, significant decrease in deposition rate or significant deterioration of coating quality occurs early during continuous operation, it is judged as 'moderate deviation' or 'poor'.

[0052] (3) Environmental performance is evaluated based on the following factors: whether the plating solution contains heavy metal stabilizers such as lead and cadmium; whether it contains sulfur-containing organic stabilizers; and the total nickel load in a unit volume of waste plating solution under the same working conditions. If the waste plating solution does not contain heavy metal stabilizers and sulfur-containing organic stabilizers and has a low nickel load, it is judged as "good" or "lower, low nickel, low load"; if the nickel load is significantly higher, it is judged as "average".

[0053] (4) Continuous working time is obtained through continuous plating test: Under fixed temperature, pH and replenishment system, multiple batches of plating are continuously carried out, and the cumulative working time that the plating solution can maintain normal plating before obvious decomposition, precipitation, significant decrease in deposition rate or obvious deterioration of coating quality is recorded.

[0054] (5) The tendency of the plating solution to decompose is judged by whether spontaneous nickel precipitation, obvious turbidity, precipitation or nickel adhering to the tank wall occur in the plating solution during continuous operation test. If obvious turbidity, precipitation or spontaneous nickel precipitation occurs in a short period of time, it is judged as "more obvious"; if only a few batches or occasional slight abnormalities occur in the later period, it is judged as "occasional"; if the above abnormalities do not occur in the evaluation period, it is judged as "low".

[0055] (6) The roughness / pinhole probability is obtained by observing and statistically analyzing the coating surface of no less than 10 samples in each group; if the proportion of samples with obvious roughness or pinhole defects is greater than 30%, it is judged as 'high'; if the proportion is 10%~30%, it is judged as 'medium'; if the proportion is less than 10%, it is judged as 'low'.

[0056] (7) The decrease in deposition rate after by-product accumulation was determined by continuous operation test: The deposition rate at the initial stage of the plating solution was recorded as the initial value. After continuous operation for a certain period of time or after by-products accumulated to a set level, the deposition rate was measured again. The decrease in deposition rate was calculated as '(initial deposition rate - later deposition rate) / initial deposition rate × 100%)'. In this invention, the decrease in deposition rate was calculated based on the accumulation of phosphite to 20 g / L.

[0057] (8) The wettability of the workpiece surface is evaluated by observing the liquid spreading, surface bubble formation, and local liquid repulsion after the workpiece is placed in the tank. If the liquid can spread quickly and evenly and there are no obvious bubbles on the surface, it is judged as "good"; if there are a few bubbles or slight liquid repulsion in some areas, it is judged as "average"; if there are obvious bubbles, liquid repulsion, or uneven wetting in some areas, it is judged as "poor".

[0058] (9) The uniformity of the coating can be evaluated by combining the appearance of the coating, the thickness distribution and the local missing coating. If the sample surface has a consistent color, a relatively uniform thickness distribution and no obvious missing coating or local abnormalities, it is judged as "good"; if the sample surface has a certain degree of local thickness difference, but the overall is acceptable, it is judged as "medium"; if there is local missing coating, roughness or obvious thickness fluctuation, it is judged as "average" or "poor".

[0059] (10) The metal load of waste liquid can be evaluated by comparing the total nickel content in the waste tank liquid per unit volume. Under the same continuous working conditions, if the total nickel content in the waste tank liquid is lower and fluctuates less, it is judged as "lower and more stable"; if the total nickel content is lower but fluctuates more, it is judged as "lower"; if the total nickel content is significantly higher, it is judged as "higher".

[0060] 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 low-phosphorus electroless nickel plating solution, characterized in that, Components containing the following mass concentrations: Nickel ion source, in Ni 2+ Calculated as 1.0~5.0 g / L; hypophosphite, as H2PO2 - The concentrations are calculated as follows: 20–45 g / L; glycine 5–40 g / L, gluconate 5–60 g / L; polyaspartic acid and / or its salts 0.2–5.0 g / L; pH buffer 2–25 g / L; alkyl glycosides 0.05–1.0 g / L; iodate stabilizer, in IO3... - The concentration is calculated to be 0.2~5.0 mg / L; the pH value of the low-phosphorus electroless nickel plating solution is 4.6~5.4; the low-phosphorus electroless nickel plating solution does not contain heavy metal stabilizers or sulfur-containing organic stabilizers.

2. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The gluconate includes one or more of sodium gluconate, potassium gluconate, and ammonium gluconate.

3. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The iodate stabilizer includes one or more of sodium iodate, potassium iodate, and ammonium iodate.

4. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The alkyl glycoside is a C8~C12 alkyl glycoside.

5. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The nickel ion source includes one or more of nickel aminosulfonate, nickel sulfate, and nickel chloride.

6. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The hypophosphite includes one or more of sodium hypophosphite, potassium hypophosphite, and ammonium hypophosphite.

7. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The number-average molecular weight of the polyaspartic acid is 1000~10000.

8. The low-phosphorus electroless nickel plating solution according to claim 1, characterized in that, The pH buffer is acetic acid, acetate, citric acid, or citrate.

9. A method for nickel plating on an aluminum alloy substrate, characterized in that, Includes the following steps: An aluminum alloy substrate treated with double zinc replacement is placed in the low-phosphorus electroless nickel plating solution according to any one of claims 1 to 8 for electroless nickel plating to form a Ni-P coating; the electroless nickel plating temperature is 80 to 92°C and the pH value is 4.6 to 5.

4.

10. The nickel plating method according to claim 9, characterized in that, The aluminum alloy substrate that has undergone double zinc replacement treatment is obtained by sequentially degreasing, alkaline etching, deashing, and double zinc replacement of the aluminum alloy substrate.

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