Hot-dip galvanizing fastener inorganic chrome-free passivation solution and passivation treatment method thereof

Abstract

The application discloses a hot-dip galvanizing fastener inorganic chromium-free passivation solution and a passivation treatment method thereof, and belongs to the technical field of metal surface corrosion prevention treatment. The inorganic chromium-free passivation solution is composed of three main salts of sodium molybdate, cerium nitrate and sodium silicate and does not contain chromate, with water as a solvent. The treatment method comprises the following steps in sequence: fastener cleaning and drying, passivation solution preparation, immersion passivation, draining, standing and drying film formation, so as to form a ternary inorganic passivation film containing molybdenum, cerium and silicon on the surface of the hot-dip galvanizing layer. The molybdate plays an anodic corrosion inhibition role, the cerium salt fills the film layer pores and machining grooves, and the silicate forms a network skeleton to block corrosion medium, and the three are synergistically formed into a film. Test results show that the film layer is continuous and dense, the white rust coverage area of the chromium-free passivation sample is smaller than that of a chromium-containing passivation sample after a neutral salt spray test, the corrosion potential and corrosion current density of the chromium-free passivation sample are basically equivalent to those of the chromium-containing passivation sample, and no hexavalent chromium is used, so that pollution is reduced and batch treatment is adapted.

Description

Technical Field

[0001] This invention relates to the field of metal surface anti-corrosion treatment technology, specifically to an inorganic chromium-free passivation solution for hot-dip galvanized fasteners, a passivation treatment method using the inorganic chromium-free passivation solution, and hot-dip galvanized fasteners obtained by the method. Background Technology

[0002] Fasteners are widely used in automotive manufacturing, aerospace, rail transportation, energy engineering, and other fields, serving as fundamental components for reliable connections in various equipment and engineering structures. Most fasteners are made of steel, which is susceptible to corrosion in service environments. Hot-dip galvanizing, through the sacrificial anode protection and physical isolation of the zinc coating, delays corrosion of the steel substrate and is a widely used and cost-effective anti-corrosion technology in the fastener industry. However, hot-dip galvanized layers are prone to oxidation in humid air, forming loose, porous white corrosion products, known as "white rust," which causes the protective effect of the zinc coating to fail prematurely.

[0003] Corrosion is a more prominent problem for machined fasteners than for ordinary galvanized parts. Machining processes such as turning, milling, and tapping leave numerous micro-grooves on the surface of fasteners. These grooves easily trap corrosive media such as moisture and salt, forming closed corrosion cells and accelerating localized corrosion. This has been a long-standing challenge in the fastener industry.

[0004] Chromate passivation has long been the mainstream method for inhibiting "white rust," characterized by mature technology, low cost, and a certain degree of self-repair capability. However, hexavalent chromium has strong carcinogenic and teratogenic properties, and long-term exposure can cause irreversible damage to the human body. Chromium-containing wastewater can also pollute soil and water sources. Regulations on the use of hexavalent chromium are becoming increasingly strict in various countries, and my country has also listed chromium as one of the five major heavy metal pollutants to be controlled. Developing environmentally friendly chromium-free passivation technology has become a real need for the industry.

[0005] Existing chromium-free passivation methods mainly include inorganic systems such as silicates, molybdates, and rare earth metal salts, as well as organic and organic-inorganic composite systems. Single molybdate films suffer from poor adhesion and insufficient long-term corrosion resistance; single silicate films and single rare earth salt films generally exhibit thin film layers and limited self-healing capabilities; some inorganic films lack toughness, easily generating stress microcracks during film formation, which become channels for corrosive media to penetrate the substrate; the synergistic film formation mechanism among multiple components and the long-term "white rust" inhibition mechanism still need further refinement. Furthermore, current research primarily focuses on flat galvanized sheets, with few dedicated chromium-free passivation solutions for irregularly shaped workpieces such as fasteners with machined grooves. Inorganic passivation solutions and supporting processes that can simultaneously achieve groove filling, dense film formation, and long-term protection are still lacking. Summary of the Invention

[0006] To address the issues of the difficulty in completely covering the machined grooves on the surface of hot-dip galvanized fasteners with a passivation film, the thinness of existing single chromium-free systems and insufficient long-term resistance to "white rust", and the hexavalent chromium contamination associated with chromate passivation, this invention provides an inorganic chromium-free passivation solution for hot-dip galvanized fasteners and its passivation treatment method.

[0007] The technical solution of the present invention is as follows: Firstly, this invention provides an inorganic chromium-free passivation solution for hot-dip galvanized fasteners. This inorganic chromium-free passivation solution uses water as a solvent and sodium molybdate, cerium nitrate, and sodium silicate as the three main salts. The entire system is free of chromates, thus avoiding the use of hexavalent chromium at the source. The three main salts work in a coordinated manner: sodium molybdate is used to form a chemically bonded conversion film on the zinc surface through a chemical reaction; cerium nitrate is used to fill micropores and processing defects during the film formation process with hydrolysis products; and sodium silicate is used to form a continuous silica network framework through a sol-gel process.

[0008] As a further improvement of the present invention, the concentrations of the three main salts were selected at the discrete level of the orthogonal experiment: sodium molybdate and cerium nitrate were 9~12 g / L, and sodium silicate was 29~31 g / L.

[0009] Furthermore, based on the maximum average drip time of each factor in the orthogonal experiment, the values ​​were set as follows: sodium molybdate 11 g / L, cerium nitrate 11 g / L, and sodium silicate 30 g / L.

[0010] As a further improvement of the present invention, acetic acid is added to the passivation solution as a pH adjuster to adjust the pH value to 3.3~3.7.

[0011] Secondly, the present invention provides an inorganic chromium-free passivation treatment method for hot-dip galvanized fasteners, which uses the aforementioned inorganic chromium-free passivation solution to treat hot-dip galvanized machined fasteners, including the following steps: Step 1: Clean and dry the fasteners machined by hot-dip galvanizing. Step 2: Dissolve sodium molybdate, cerium nitrate and sodium silicate in water to prepare the inorganic chromium-free passivation solution, and adjust the pH value with acetic acid; Step 3: Immerse the hot-dip galvanized machined fasteners treated in Step 1 into the inorganic chromium-free passivation solution for passivation treatment, so that the surface of the hot-dip galvanized layer undergoes an interfacial film-forming reaction with the three main salts. Step 4: Remove the passivated hot-dip galvanized machined fasteners and let them drain and stand. Step 5: Dry the hot-dip galvanized machined fasteners that have been drained and allowed to stand, and form a ternary inorganic passivation film containing molybdenum, cerium and silicon on the surface of the hot-dip galvanized layer.

[0012] As a further improvement of the present invention, the passivation solution has a pH value of 3.3~3.7, the passivation treatment temperature is 33~37℃, and the passivation treatment time is 50~70s.

[0013] Furthermore, in the optimized implementation, the passivation solution has a pH value of 3.7, a passivation temperature of 35°C, and a passivation time of 60 seconds.

[0014] As a further improvement of the present invention, the method is applicable to fasteners formed by at least one of turning, milling and tapping processes, and having machined grooves on the surface of the hot-dip galvanized layer. The ternary inorganic passivation film fills the machined grooves and continuously covers the hot-dip galvanized layer during the film formation process. The molybdenum in the film exists in a coexisting valence state of tetravalent molybdenum and hexavalent molybdenum, and the cerium exists in a coexisting valence state of trivalent cerium and tetravalent cerium.

[0015] Thirdly, the present invention provides a hot-dip galvanized fastener, comprising a steel substrate and a hot-dip galvanized layer on the surface of the steel substrate, wherein the surface of the hot-dip galvanized layer has a ternary inorganic passivation film formed by the above-described method, the film containing molybdenum, cerium, and silicon but not chromium. In one embodiment, by atomic percentage, the silicon content on the film surface is 9.14%, the molybdenum content is 5.30%, and the cerium content is 1.42%.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention utilizes a compound of three inorganic main salts—sodium molybdate, cerium nitrate, and sodium silicate—to enable synergistic action across three types of processes: anodic corrosion inhibition, pore filling, and network barrier, all within a single film layer. This allows the film to conformally cover the complex microscopic surfaces of machined fasteners and fill machined grooves. Scanning electron microscopy revealed that the formed ternary inorganic passivation film is continuous and intact, without obvious cracks, pinholes, voids, or peeling defects. The cross-sectional film thickness was measured to be 1.574 μm. These observations are based on surface and cross-sectional images at 5000x and 10000x magnification, and the film thickness is a single cross-sectional measurement and does not represent statistical results from multiple batches.

[0017] This invention utilizes a mixed valence state of molybdenum (tetravalent and hexavalent) and cerium (trivalent and tetravalent), enabling the film layer to undergo valence state transitions and redeposition when locally damaged, thus providing a certain degree of self-repair capability. X-ray photoelectron spectroscopy (XPS) analysis revealed that the atomic percentages of silicon, molybdenum, and cerium on the film surface were 9.14%, 5.30%, and 1.42%, respectively. The coexistence of tetravalent and hexavalent molybdenum, as well as trivalent and tetravalent cerium, was also detected, confirming that all three main salts participate in interfacial film formation.

[0018] The ternary inorganic passivation film of this invention achieves an electrochemical corrosion resistance level similar to that of a chromium-containing passivation film under chromate-free conditions. Electrochemical testing using a three-electrode system showed that the inorganic passivation sample had a corrosion potential of -844 mV and a corrosion current density of 28.2 μA / cm².2 The corrosion potential of the chromium-passivated control sample was -844 mV, and the corrosion current density was 28.3 μA / cm². 2 The two are essentially equivalent; the electrochemical impedance spectra of both groups of samples exhibit a single capacitive arc characteristic, with similar capacitive arc radii and interfacial charge transfer resistances on the same order of magnitude. The above data are based on a working electrode exposure area of ​​1 cm². 2 The test conditions were obtained using a 3.5% sodium chloride solution at room temperature and are not generalized to other corrosive media and temperature conditions.

[0019] This invention extends the diffusion path of corrosive media through a silicate-formed network framework, combined with the anodic corrosion inhibition of molybdate and the pore filling of cerium salt, thereby enhancing the film's resistance to "white rust." Neutral salt spray tests were conducted according to GB / T10125-2021. After 3 hours, no "white rust" appeared on either the inorganic passivated or chromium-containing passivated samples. After 24 hours, the "white rust" coverage area on the inorganic passivated sample was smaller than that on the chromium-containing passivated sample. After 48 hours, the "white rust" on the chromium-containing passivated sample had spread and expanded, while the inorganic passivated sample only showed trace amounts of dotted "white rust" at a very few corner locations, with the main area of ​​the film remaining dense and intact. These results were obtained under neutral salt spray conditions of 35±2℃, sodium chloride solution concentration of 5±0.5%, pH value of 6.5 to 7.2, and continuous spraying for 48 hours. These results are not extrapolated to longer test cycles or other accelerated corrosion conditions.

[0020] The passivation solution of this invention does not contain chromates, and the passivation process does not use hexavalent chromium, which helps to reduce the hazards of chromium-containing wastewater and occupational exposure. The treatment method is mainly based on immersion film formation, with simple steps, which is easy to implement on conventional fastener surface treatment production lines and is beneficial for the batch processing of irregularly shaped fasteners. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the inorganic chromium-free passivation treatment method for hot-dip galvanized fasteners according to the present invention. Figure 2 This is a graph showing the results of the single-factor experiment on the color change time of copper sulfate droplets in this invention. Figure 2 (a) shows the results of the single-factor experiment on the formulation. Figure 2 (b) Results of single-factor process experiments; Figure 3 The image shows the surface morphology of the ternary inorganic passivation film of the present invention using a scanning electron microscope. Figure 3 (a) Surface morphology at 5000x magnification. Figure 3 (b) Surface morphology at 10,000x magnification; Figure 4 This is a cross-sectional scanning electron microscope image of the ternary inorganic passivation film of the present invention; Figure 5This is an energy dispersive spectroscopy (EDS) elemental distribution diagram of the ternary inorganic passivation film of the present invention, wherein, Figure 5 (a) is an electronic image of superimposed elements. Figure 5 (b) is a surface distribution diagram of Ce element. Figure 5 (c) is the surface distribution diagram of Si element. Figure 5 (d) is the surface distribution diagram of element C. Figure 5 (e) is a surface distribution diagram of Zn elements. Figure 5 (f) is a surface distribution diagram of Mo element; Figure 6 The image shows the X-ray photoelectron spectrum of the ternary inorganic passivation film of the present invention, wherein... Figure 6 (a) is the full spectrum scan. Figure 6 (b) is the high-resolution spectrum of Ce3d. Figure 6 (c) is the high-resolution spectrum of Mo3d. Figure 6 (d) is the high-resolution spectrum of Si2p; Figure 7 This is a comparison of the potentiodynamic polarization curves of the inorganic chromium-free passivation sample and the chromium-containing passivation sample of the present invention. Figure 8 The images show the electrochemical impedance spectroscopy of the inorganic chromium-free passivated sample and the chromium-containing passivated sample of this invention. Figure 8 (a) is the Nyquist impedance spectrum. Figure 8 (b) shows the Bode impedance magnitude and phase diagram; Figure 9 This is a comparison chart of neutral salt spray tests according to the present invention, wherein, Figure 9 (a) is a chromium-containing passivation process for 3 hours. Figure 9 (b) Chromium passivation for 24 hours, Figure 9 (c) Chromium passivation for 48 hours, Figure 9 (d) Inorganic chromium-free passivation for 3 hours, Figure 9 (e) Inorganic chromium-free passivation for 24 hours, Figure 9 (f) is inorganic chromium-free passivation for 48 hours. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] The present invention will now be described in further detail with reference to the accompanying drawings: I. Experimental Materials and Equipment The experimental materials used in the embodiments of the present invention are shown in Table 1, and the main experimental equipment used are shown in Table 2.

[0024] Table 1 Table 2 II. Preparation of Inorganic Chromium-Free Passivation Solution The inorganic chromium-free passivation solution of this invention uses water as a solvent, and the solutes are sodium molybdate, cerium nitrate, and sodium silicate, with acetic acid added to adjust the pH value. No chromate is added during the entire preparation process. During preparation, the weighed sodium molybdate, cerium nitrate, and sodium silicate are dissolved in water and stirred until the solution is homogeneous using an LC-MS-HD magnetic stirrer. Acetic acid is then added dropwise to adjust the pH value of the passivation solution to the set value. Reagents are weighed using a YH-10002 electronic analytical balance, and the pH value is measured using a pH-10 pH tester. All sodium molybdate, cerium nitrate, sodium silicate, and acetic acid used are analytical grade reagents. In the optimized embodiment, each liter of passivation solution contains 11g of sodium molybdate, 11g of cerium nitrate, and 30g of sodium silicate, with the pH value adjusted to 3.7. This concentration combination was determined by single-factor experiments and orthogonal experiments, the determination process of which is described in Section III of this part.

[0025] III. Optimization of Passivation Solution Formulation and Process Parameters The discoloration time of the zinc plating surface in the copper sulfate drop test was used as an evaluation index for the corrosion resistance of the film. A longer discoloration time indicated a more favorable level. Three parallel samples were prepared for each level, and the average value was taken as the result. The optimal range was first determined through single-factor experiments. The results of the copper sulfate drop discoloration time in the single-factor experiments are shown below. Figure 2 As shown, the globally optimized formula and process are then determined through orthogonal experiments.

[0026] (1) Single-factor experiment of formulation In the single-factor experiments of the formulation, only the concentration of one component was changed each time, while the concentrations of the other components remained constant. Sodium molybdate was set at three levels: 5 g / L, 10 g / L, and 15 g / L; cerium nitrate was set at three levels: 5 g / L, 10 g / L, and 15 g / L; and sodium silicate was set at three levels: 15 g / L, 25 g / L, and 30 g / L. The factor levels are shown in Table 3. The results of the copper sulfate drop color change time are as follows: Figure 2 As shown in (a).

[0027] Table 3 Depend on Figure 2 (a) It can be seen that when the sodium molybdate is 10 g / L, the cerium nitrate is 10 g / L, and the sodium silicate is 30 g / L, the copper sulfate drop color change time is the longest. Therefore, the formula for the single-factor stage is 10 g / L sodium molybdate, 10 g / L cerium nitrate, and 30 g / L sodium silicate.

[0028] (2) Orthogonal experiment of formula Using the single-factor optimization result as the intermediate level, and taking one level above and below it, a three-factor, three-level orthogonal array was used to arrange nine orthogonal experiments. Sodium molybdate was tested at three levels: 9 g / L, 10 g / L, and 11 g / L; cerium nitrate was tested at three levels: 9 g / L, 10 g / L, and 11 g / L; and sodium silicate was tested at three levels: 29 g / L, 30 g / L, and 31 g / L. Each group had three parallel samples. The orthogonal experiment arrangement and the results of the copper sulfate drop test are shown in Table 4. RSD in the table is the relative standard deviation.

[0029] Table 4 The mean drip times for the nine groups of experiments ranged from 27.39 s to 29.24 s. The mean drip times (K) and ranges (R) for the three levels of each factor were calculated, and the results are shown in Table 5.

[0030] Table 5 As shown in Table 5, the range of cerium nitrate is 1.13, the range of sodium molybdate is 0.43, and the range of sodium silicate is 0.38. Therefore, the order of influence of the three main salts on the corrosion resistance of the film is cerium nitrate, sodium molybdate, and sodium silicate. Based on the maximum average drip time of each factor, the optimized formula is determined to be 11 g / L sodium molybdate, 11 g / L cerium nitrate, and 30 g / L sodium silicate.

[0031] (3) Single-factor process test Based on the optimized single-factor formulation, single-factor process experiments were conducted, changing only one process parameter each time. The passivation solution pH was set at three levels: 3.5, 4.0, and 4.5; the passivation temperature was set at three levels: 30℃, 35℃, and 40℃; and the passivation time was set at three levels: 30s, 1min, and 1.5min. The factor levels are shown in Table 6. The results of the copper sulfate drop color change time are as follows: Figure 2 As shown in (b).

[0032] Table 6 Depend on Figure 2 (b) It can be seen that the copper sulfate droplet color change time is the longest when the pH value is 3.5, the passivation temperature is 35℃, and the passivation time is 1min. Therefore, the process of the single factor stage is pH value 3.5, passivation temperature 35℃, and passivation time 1min.

[0033] (4) Process orthogonal test Near the single-factor optimization results, the pH value was set at three levels: 3.3, 3.5, and 3.7; the passivation temperature was set at three levels: 33℃, 35℃, and 37℃; and the passivation time was set at three levels: 50s, 60s, and 70s. Nine orthogonal experiments were arranged using a three-factor, three-level orthogonal array, with three parallel samples in each group. The results are shown in Table 7.

[0034] Table 7 The mean drop times of the nine groups of experiments ranged from 28.47 s to 31.20 s, and the results of the range analysis are shown in Table 8.

[0035] Table 8 Table 8 shows that the range of passivation temperature is 1.71, the range of pH value is 0.91, and the range of passivation time is 0.75. The main influencing factors are passivation temperature, pH value, and passivation time. The optimized process is determined to be a passivation solution pH of 3.7, a passivation temperature of 35℃, and a passivation time of 60s. It should be noted that all the above levels are discrete experimental points. The results of the single-factor stage and the results of the orthogonal optimization stage correspond to different experimental stages and should not be combined and interpreted as a continuous numerical range.

[0036] IV. Passivation Treatment Process and Examples The inorganic chromium-free passivation solution is prepared according to the optimized formula and then used to treat hot-dip galvanized machined fasteners. The process flow of the inorganic chromium-free passivation treatment method of this invention is as follows: Figure 1 As shown, it includes steps S1 to S5.

[0037] S1. Take hot-dip galvanized machined fasteners as the fasteners to be processed. These fasteners are formed by at least one of the processes of turning, milling and tapping, and their hot-dip galvanized layer has machined grooves on its surface. First, clean the surface of the fasteners with anhydrous ethanol to remove oil and debris, and then dry them.

[0038] S2. Prepare an inorganic chromium-free passivation solution according to the method in Section 2 of this part. Each liter of passivation solution contains 11g of sodium molybdate, 11g of cerium nitrate, and 30g of sodium silicate. Adjust the pH value to 3.7 with acetic acid.

[0039] S3. The fasteners to be treated are completely immersed in the prepared inorganic chromium-free passivation solution for passivation treatment. The passivation temperature is controlled at 35℃ and the immersion time is controlled at 60s. During the immersion, the surface of the hot-dip galvanized layer undergoes an interfacial film-forming reaction with the three main salts.

[0040] S4. After removing the fasteners, drain them and let them stand to allow the film to continue reacting on the surface.

[0041] S5. After draining, the fasteners are dried to obtain a ternary inorganic passivation film on the hot-dip galvanized layer on the steel substrate. A chromium-containing passivation control sample is prepared using a trivalent chromium zinc plating passivation solution and treated in the same manner as a comparison.

[0042] V. Morphological Characterization of Ternary Inorganic Passivation Films The passivated sample was cut into 10mm × 10mm pieces, and the surface and cross-sectional morphology were observed using a FEI Quattro S scanning electron microscope. Figure 3 and Figure 4 As shown, the ternary inorganic passivation film has a uniform and dense surface, is continuous as a whole, and has no obvious cracks, pinholes, voids and peeling defects. The film layer fills and covers the machined grooves. Figure 3 (a) Figure 3 (b) Surface morphology at magnifications of 5000x and 10000x, respectively. Figure 4 The cross-sectional morphology shows a film thickness of 1.574 μm, a value obtained from a single cross-section measurement. This dense and uniform film morphology originates from the synergistic film-forming process of the ternary components: molybdate first rapidly forms an initial passivation layer on the zinc surface, preventing excessive zinc dissolution and providing a smooth substrate for subsequent film formation; silicate forms a continuous SiO2 network through a sol-gel process, encapsulating grain boundaries and eliminating nanopores.

[0043] VI. Compositional Analysis of Ternary Inorganic Passivation Films (1) Energy dispersive spectroscopy (EDS): Elemental surface distribution analysis was performed using an EDS. The accelerating voltage was 20 kV and the working distance was 15 mm. The results are as follows: Figure 5 As shown, five elements—zinc, carbon, molybdenum, cerium, and silicon—were detected on the surface of the ternary inorganic passivation film. Zinc originated from the hot-dip galvanizing layer, while molybdenum, cerium, and silicon came from three main salts, confirming that all three main salts participated in the interfacial film formation. Figure 5 (b) to Figure 5 The elemental distribution in (f) shows that all elements are uniformly distributed on the machined surface, without local enrichment, loss, or discontinuity, and the film layer is completely covered. Among them, Mo is diffusely distributed throughout the film layer, inhibiting the dissolution of the zinc layer by anodic oxidation; Ce fills the pores and processing defects of the film layer, refines the structure, and improves the density; Si forms a gel network skeleton on the surface layer, which acts as a "binder" and builds a physical barrier.

[0044] (2) X-ray photoelectron spectroscopy analysis: The chemical composition and elemental valence states of the film were analyzed using a Thermo Kalpha X-ray photoelectron spectrometer. The results are as follows: Figure 6 As shown. The excitation source is AlKα rays, and the vacuum level in the analysis chamber is better than 1×10⁻⁶. -9At Pa, with an energy level of 20 eV and a step size of 0.1 eV, and the binding energy corrected based on the C1s peak of 284.8 eV, high-resolution scanning was performed on characteristic peaks such as Mo3d, Ce3d, and Si2p. The results showed that the atomic percentages of silicon, molybdenum, and cerium on the film surface were 9.14%, 5.30%, and 1.42%, respectively. The content of these three functional film-forming elements showed a clear gradient: silicates had the highest proportion, dominating the formation of a continuous physical barrier; molybdates were next, mainly playing a role in anodic corrosion inhibition; and cerium salts had a relatively low content, mainly serving to fill pores and densify the film. Figure 6 (c) The high-resolution spectrum of Mo3d shows the coexistence of tetravalent and hexavalent molybdenum, indicating that molybdate forms mixed valence oxides through film formation reaction, and the valence state can be reversibly transformed into the film layer to provide self-healing ability; Figure 6 (b) The Ce3d high-resolution spectrum shows the coexistence of trivalent and tetravalent cerium, indicating that cerium salt hydrolyzes to generate hydroxides and oxides, and achieves automatic repair of micro-defects through valence state transformation; Figure 6 (d) The Si2p spectrum corresponds to the SiO2 network. The simultaneous detection of the three characteristic elements proves that the three core main salts, sodium silicate, sodium molybdate, and cerium nitrate, fully participate in the interfacial film-forming reaction and jointly constitute the ternary inorganic passivation film.

[0045] VII. Corrosion Resistance Test of Ternary Inorganic Passivation Film (1) Electrochemical testing: The electrochemical testing adopted a three-electrode system, with the passivated sample as the working electrode and an exposure area of ​​1 cm². 2 A saturated calomel electrode was used as the reference electrode, and a platinum electrode as the auxiliary electrode. The test medium was a 3.5% sodium chloride solution, and the test temperature was room temperature. Before the test, the working electrode was immersed in the test solution for 30 minutes to allow the open-circuit potential to stabilize before starting the test. The polarization curve scanning range was ±0.3V from the open-circuit potential, and the scan rate was 0.005V / s. The electrochemical impedance spectroscopy test frequency range was 10 Hz. -2 Hz to 10 5 Hz, AC excitation signal amplitude is 10mV.

[0046] For example, potentiodynamic polarization curves Figure 7 As shown. The test results are: the corrosion potential of the chromium-passivated sample is -844mV, and the corrosion current density is 28.3μA / cm. 2 The inorganic passivation sample of this invention has a corrosion potential of -844mV and a corrosion current density of 28.2μA / cm. 2 The electrochemical corrosion characteristics of the two groups of samples showed no significant difference, and the thermodynamic corrosion tendency was basically equivalent to the corrosion kinetic rate. The electrochemical impedance spectroscopy results are as follows: Figure 8As shown, the impedance spectra of both groups of samples exhibit a single capacitive arc characteristic, indicating that the corrosion process of both passivation films is controlled by interfacial charge transfer. The capacitive arc radii are similar in size, and the interfacial charge transfer resistance is on the same order of magnitude. Within the full frequency range, the impedance modulus of the inorganic passivation sample is slightly higher than that of the chromium-containing passivation sample. In the low-frequency region, the low-frequency impedance modulus of the inorganic passivation sample is slightly lower than that of the chromium-containing passivation sample, indicating that the ternary inorganic passivation film possesses similar dielectric barrier capabilities and structural stability as the chromium-containing passivation film. Its mechanism of action is a mixed inhibition type: molybdate, as an anodic corrosion inhibitor, preferentially adsorbs on the active sites of zinc, increasing the dissolution overpotential of zinc; cerium salt inhibits the cathodic oxygen reduction reaction; and the silicate network increases the film resistance. These three factors synergistically achieve simultaneous inhibition of the corrosion processes at both the anode and cathode.

[0047] (2) Neutral salt spray test: The neutral salt spray test was conducted in a YW90A salt spray test chamber according to GB / T10125-2021. The test conditions were: temperature 35±2℃, sodium chloride solution concentration 5±0.5%, pH value 6.5 to 7.2, and mist drop rate 1 to 2 mL / (h·80cm). 2 The surface was continuously sprayed and the surface condition was recorded at 3h, 24h, and 48h. The results are as follows: Figure 9 As shown. At 3 hours, as Figure 9 (a) Figure 9 (d) The surfaces of both the chromium-containing passivated sample and the inorganic passivated sample were smooth and clean, without any "white rust," and both passivation films remained intact; after 24 hours, if Figure 9 (b) Figure 9 (e) The area of ​​"white rust" on the surface of the chromium-containing passivated sample is slightly larger than that of the inorganic passivated sample, while the surface of the inorganic passivated sample remains smooth and dense; after 48 hours, if Figure 9 (c) Figure 9 (f) In the chromium-passivated sample, the "white rust" spread and expanded on the surface, and the damaged area of ​​the film increased. In contrast, the inorganic passivated sample only showed trace amounts of dot-like "white rust" at a very few corners, while the film in the main area remained dense and intact, continuously blocking the intrusion of corrosive media. The excellent anti-"white rust" performance of the film stems from triple protection: in the early stage of film formation, the silicate hydrophobic layer reduces the adhesion of salt spray to the surface; in the middle stage of film formation, the conversion layer formed by molybdate and cerium salt inhibits the interfacial electrochemical reaction; and after local damage to the film, the valence transformation products of molybdenum and cerium are redeposited in the damaged area, inhibiting the spread of local corrosion through a self-healing mechanism.

[0048] The above results demonstrate that the ternary inorganic chromium-free passivation system of sodium molybdate, cerium nitrate, and sodium silicate of this invention does not use hexavalent chromium, exhibits stable film formation, and produces a dense, well-defined ternary inorganic passivation film. Its resistance to "white rust" and long-term corrosion resistance reach the level of traditional chromium-containing passivation processes. This system is suitable for surface passivation treatment of hot-dip galvanized machined fasteners and can meet the corrosion protection requirements during fastener storage, transportation, and service. The reagent concentrations and process parameters described above are discrete test points as recorded in the embodiments of this invention. When adjusting fastener specifications or production line equipment, appropriate selections can be made within the recorded discrete levels, using the results of copper sulfate drop tests and neutral salt spray tests as the basis for selection.

[0049] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An inorganic, chromium-free passivation solution for hot-dip galvanized fasteners, characterized in that, The inorganic chromium-free passivation solution uses water as a solvent and contains three main salts: sodium molybdate, cerium nitrate, and sodium silicate. The inorganic chromium-free passivation solution does not contain chromates.

2. The inorganic chromium-free passivation solution according to claim 1, characterized in that, In the inorganic chromium-free passivation solution, the content of sodium molybdate is 9~12 g / L, the content of cerium nitrate is 9~12 g / L, and the content of sodium silicate is 29~31 g / L.

3. The inorganic chromium-free passivation solution according to claim 2, characterized in that, In the inorganic chromium-free passivation solution, the content of sodium molybdate is 11 g / L, the content of cerium nitrate is 11 g / L, and the content of sodium silicate is 30 g / L.

4. The inorganic chromium-free passivation solution according to claim 1, characterized in that, The inorganic chromium-free passivation solution also contains acetic acid, which is used to adjust the pH value of the inorganic chromium-free passivation solution, and the pH value of the inorganic chromium-free passivation solution is 3.3~3.

7.

5. A method for inorganic chromium-free passivation treatment of hot-dip galvanized fasteners, using the inorganic chromium-free passivation solution according to any one of claims 1 to 4, characterized in that, The inorganic chromium-free passivation treatment method includes the following steps: Step 1: Clean and dry the fasteners machined by hot-dip galvanizing. Step 2: Dissolve sodium molybdate, cerium nitrate and sodium silicate in water to prepare the inorganic chromium-free passivation solution, and adjust the pH value with acetic acid; Step 3: Immerse the hot-dip galvanized machined fasteners treated in Step 1 into the inorganic chromium-free passivation solution for passivation treatment, so that the surface of the hot-dip galvanized layer undergoes an interfacial film-forming reaction with the three main salts. Step 4: Remove the passivated hot-dip galvanized machined fasteners and let them drain and stand. Step 5: Dry the hot-dip galvanized machined fasteners that have been drained and allowed to stand, and form a ternary inorganic passivation film containing molybdenum, cerium and silicon on the surface of the hot-dip galvanized layer.

6. The inorganic chromium-free passivation treatment method according to claim 5, characterized in that, The pH value of the inorganic chromium-free passivation solution prepared in step 2 is 3.3~3.7, and the passivation temperature in step 3 is 33~37℃ and the passivation time is 50s~70s.

7. The inorganic chromium-free passivation treatment method according to claim 6, characterized in that, The inorganic chromium-free passivation solution prepared in step 2 has a pH value of 3.7, and the passivation treatment temperature in step 3 is 35℃ and the passivation treatment time is 60s.

8. The inorganic chromium-free passivation treatment method according to claim 5, characterized in that, The hot-dip galvanized machined fastener is a fastener formed by at least one machining process, namely turning, milling and tapping. The surface of the hot-dip galvanized layer of the hot-dip galvanized machined fastener has machined grooves. The ternary inorganic passivation film formed in step 5 fills the machined grooves and continuously covers the hot-dip galvanized layer.

9. The inorganic chromium-free passivation treatment method according to claim 5, characterized in that, In the ternary inorganic passivation film formed in step 5, molybdenum exists in a coexisting valence state of tetravalent molybdenum and hexavalent molybdenum, and cerium exists in a coexisting valence state of trivalent cerium and tetravalent cerium.

10. A hot-dip galvanized fastener, comprising a steel substrate and a hot-dip galvanized layer located on the surface of the steel substrate, characterized in that, The surface of the hot-dip galvanized layer has a ternary inorganic passivation film formed by the inorganic chromium-free passivation treatment method according to any one of claims 5 to 9, wherein the ternary inorganic passivation film contains molybdenum, cerium and silicon, and does not contain chromium.

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