Method for improving the external insulation performance and weather resistance of a dc wall bushing

Abstract

This invention relates to the field of high-voltage insulation technology, specifically disclosing a method for improving the external insulation performance and weather resistance of a DC through-wall bushing, comprising the following steps: (1) performing ultraviolet grafting and sol-gel coating pretreatment on the silicone rubber insulator of the bushing assembly; (2) performing gradient fluorination on the pretreated silicone rubber, and then performing gradient drying of the entire bushing assembly from low temperature to high temperature; (3) performing drying pretreatment on the metal parts, then performing co-assembly of the metal parts and the bushing assembly, and finally encapsulating SF6 gas to achieve a synergistic improvement in the interface insulation performance of the DC bushing. This invention forms a molecular-level fluorine atom pre-assembled layer on the surface of silicone rubber through a specific pretreatment process. This structure effectively reduces the crack density of the fluorinated layer, increases the flashover voltage, reduces interfacial charge accumulation, and significantly enhances the hydrophobicity, electrical erosion resistance, and anti-pollution flashover capability of the material surface.

Description

Technical Field

[0001] This invention relates to the field of high voltage insulation technology, specifically to a method for improving the external insulation performance and weather resistance of DC through-wall bushings. Background Technology

[0002] With the widespread application of ±800kV and above ultra-high voltage direct current (UHVDC) transmission projects, the high voltage direct current through-wall bushing, as a key insulation device in the valve hall of converter stations, faces severe challenges in its long-term operational reliability. The uniform fluorination process (fluorine concentration 2-50%) used in CN110504074A for silicone rubber leads to an imbalance in material properties: when the CF bond concentration is >45%, the tear strength decreases by ≥40% (ASTM D624 test), while when the CF bond concentration is <35%, the surface charge dissipation time is >120 seconds (electrostatic probe method), making it impossible to balance electrical performance and mechanical stability. Although the gradient defluorination process (fluorine ratio 1:5→1:9) in CN120148986A optimized the thickness uniformity, it caused embrittlement and cracking of epoxy resin in the high-fluorination region (CF>50%) (crack density of 20-50 lines / mm² observed by micro-CT), and insufficient charge suppression in the low-fluorination region (CF<35%) led to a 18-22% decrease in SF6 gap breakdown voltage (IEC 60270 test), and the problem of dual-interface energy level mismatch (energy level difference>1.2eV) has not been resolved. Engineering practice further revealed that the difference in hygroscopic properties between epoxy resin (water absorption rate 0.6-0.8%) and metal flanges (water absorption rate <0.01%) resulted in ≥20ppm of bound water residue at the interface, leading to a 35% increase in the probability of surface flashover. Multi-field coupling tests (0.4kV / mm DC field +70℃) confirmed that the bushing bandgap was severely insufficient under traditional processes (epoxy ≤6.5eV, silicone rubber ≤6.8eV), with a gas-solid interface flashover voltage of only 125-135kV (below the 150kV standard requirement), and a surface flashover gradient ≤22kV / cm (design requirement ≥25kV / cm). The interface charge density in traditional processes was generally >7.5×10⁻⁶. 15 m -3 . Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for improving the external insulation performance and weather resistance of DC through-wall bushings. This method utilizes ultraviolet grafting of silicone rubber and sol-gel coating pretreatment to achieve a synergistic improvement in the external insulation performance and weather resistance of DC through-wall bushings.

[0004] This invention provides a method for improving the external insulation performance and weather resistance of DC through-wall bushings, comprising the following steps: (1) The silicone rubber insulators of the bushing assembly are subjected to ultraviolet light grafting and sol-gel coating pretreatment; (2) The pretreated silicone rubber is subjected to gradient fluorination, and then the entire sleeve assembly is subjected to gradient drying from low temperature to high temperature. (3) Dry the metal parts, then assemble the metal parts and the bushing assembly together, and finally encapsulate SF6 gas to achieve a synergistic improvement in the insulation performance of the DC bushing interface.

[0005] Furthermore, in step (1), ultraviolet grafting is performed by irradiating the silicone rubber insulator with ultraviolet light under nitrogen protection, while simultaneously spraying an ethanol solution containing perfluorooctyltriethoxysilane (FAS) and a photoinitiator.

[0006] Furthermore, the ultraviolet light wavelength is 254~365 nm, the irradiation time is 5~15 min; in the sprayed solution, the mass concentration of perfluorooctyltriethoxysilane is 0.5~3%, and the mass concentration of photoinitiator is 0.1~1.5%.

[0007] Furthermore, the photoinitiator is benzophenone (BP), 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), or 1-hydroxycyclohexylphenyl ketone (Irgacure 184).

[0008] Further, in step (1), the sol-gel coating pretreatment is to add HCl to a mixture of tetraethyl orthosilicate, ethanol and water for catalytic hydrolysis, add magnesium fluoride nanocrystals, mix and spin coat onto the surface of silicone rubber insulator, and then cure.

[0009] Furthermore, the amount of magnesium fluoride nanocrystals added is 1% to 5% of the mass of the mixture; after spin coating and curing, a 50-100nm transition layer is formed.

[0010] Furthermore, in step (2), the fluorination of silicone rubber adopts a two-stage gradient fluorination: First stage (0~10min): Fluorine concentration is increased from 40% to 48% at a rate of 0.6~1% / min; Second stage (10~30min): Fluorine concentration is increased from 48% to 55% at a rate of 0.3~0.4% / min, while controlling the CF bond ratio to be ≥40%; All fluorine concentrations are volume concentrations.

[0011] Furthermore, during the fluorination process, real-time monitoring and compensation control are implemented. Fourier transform infrared spectroscopy is used to monitor the CF bond percentage online, with a sampling frequency ≥ 1 time / min. When the local CF bond deviation > 3%, [further control is applied]. Adjust the nozzle; where Q is the fluorine gas flow rate in L / min, ν is the gas kinematic viscosity in mm² / s, and d is the nozzle outlet diameter in mm.

[0012] Furthermore, during gradient drying in step (2), First stage: Let stand at 30℃ / ≤30%RH for 10~12 hours; Second stage: Hot air drying at 40℃ / ≤10%RH for more than 1 hour; Third stage: Dry at 50℃ / ≤10%RH for more than 1.2 hours.

[0013] Furthermore, after drying the metal parts in step (3), the moisture content is controlled to be below 12 ppm.

[0014] The present invention has the following beneficial effects: This invention addresses the issue of silicone rubber insulators by first performing UV grafting pre-assembly of fluoroalkyl chains to enhance reactivity, followed by coating with a TEOS sol-gel transition layer to increase surface fluorine atom density and superhydrophobicity. Simultaneously, magnesium fluoride nanocrystals are introduced into the coating, which can absorb some heat energy under the action of an electric arc, helping to inhibit the formation and development of conductive carbonization channels.

[0015] This invention employs differentiated gradient fluorination during the fluorination of pretreated silicone rubber, rapidly increasing the fluorination concentration to utilize the high reactivity of the pre-activated surface. Addressing the extreme sensitivity of silicone rubber to fluorine and its tendency to exothermically crack, this invention abandons the traditional "high concentration followed by low concentration" strategy. Instead, it adopts a "rapid induction at low concentration, slow diffusion at high concentration" process. First, active sites pre-constructed on the surface are utilized through UV grafting. During the low-concentration phase, rapid fluorination smoothly completes surface fluorination, effectively avoiding the pyrolysis cracking and surface embrittlement caused by the instantaneous over-reaction in traditional high-concentration processes. Subsequently, the rate of increase is actively slowed during the high-concentration phase, inducing fluorine atoms to form a deep gradient diffusion layer from the surface inwards, rather than accumulating into a fragile hard shell on the surface. This improvement elevates the fluorinated layer and matrix from a mechanical covering to a chemically integrated structure. While drastically reducing crack density by 90%, it precisely controls the microscopic bandgap distribution, locking the charge migration path at its source while maintaining the extremely high tear strength of silicone rubber, achieving a leap forward in insulation performance and material lifespan. A CF bond-rich layer is formed on the surface of silicone rubber material through gradient fluorination. CF bonds have a high bond energy (approximately 485 kJ / mol), which helps to enhance the thermal stability and chemical inertness of the material surface. When localized arcing occurs on the surface, this fluorinated layer can act as a thermal barrier, slowing down the transfer of heat to the matrix.

[0016] The silicone rubber surface treated with this invention exhibits significantly improved resistance to electrical tracking. According to the IEC 60112 standard test results, the comparative tracking index (CTI) of the treated material reaches over 600 V, indicating an improved resistance to surface tracking compared to conventional silicone rubber materials. To further evaluate the material's performance under harsh environments, a 1000-hour salt spray arc test was conducted according to the IEC 60587 standard. The average electrical erosion depth was measured to be no more than 2.0 mm, demonstrating good resistance to arc erosion.

[0017] Regarding hydrophobicity, the water contact angle of the treated silicone rubber surface reached over 140°, exhibiting excellent hydrophobic properties. This phenomenon is related to changes in the surface chemical composition and microstructure: the CF bond-rich layer formed by gradient fluorination reduces the surface energy, while the micro-nano structure jointly constructed by the pretreatment and fluorination processes further enhances the hydrophobic effect. This surface characteristic makes water droplets more likely to roll off in a spherical shape and also provides a certain degree of self-cleaning.

[0018] The improved surface properties described above have a positive impact on anti-flashover performance. According to the artificial pollution test results of IEC 60507 standard, under an ESDD of 0.1 mg / cm², the flashover voltage of the treated bushing is approximately 30% higher than that of the untreated sample, and the critical flashover voltage gradient per unit creepage distance reaches 3.5 kV / cm. This indicates that the insulation performance of the material is enhanced under polluted conditions, primarily due to the superhydrophobic surface effectively inhibiting the formation of a continuous water film and reducing leakage current.

[0019] This process enables precise control of the interfacial band structure, achieving a silicone rubber band gap ≥7.82 eV and an energy level difference ≤0.7 eV (XPS / UPS verification), while reducing the interfacial charge density to 5.1 × 10⁻⁶ compared to traditional methods. 15 m -3 (Reduction of 52.04%), flashover voltage reached 192kV (increase of 51.42%), and the tear strength retention rate of silicone rubber was ≥95%. Verified by a certain ultra-high voltage demonstration project, the pretreated bushing operated for 3000 hours in a humid and hot environment (RH>85%) without flashover failure, solving the long-standing insulation failure problem faced by the industry. Attached Figure Description

[0020] Figure 1 The diagram shows the spatial relationship of the silicone rubber / air interface. 1 is the silicone rubber outer skirt, 2 is the central conductor, 3 is the SF6 gas chamber, and 4 is the metal flange.

[0021] Figure 2 The two-stage gradient fluorination control curve of silicone rubber in Example 1 shows the change of fluorine concentration over time.

[0022] Figure 3 The curves show the change in carrier mobility with fluorination depth, comparing the suppression effect of gradient fluorination and conventional processes on charge migration.

[0023] Figure 4 The comparison of the uniformity deviation of the fluorinated layer thickness in the examples and comparative examples 1-5 demonstrates the stability advantage of the gradient fluorination process.

[0024] Figure 5 The Weibull distribution diagram of flashover voltage in Example 4 and Comparative Example 4 shows the improved concentration and reliability of flashover voltage under the process of the present invention.

[0025] Figure 6 This is a schematic diagram of the gradient drying process in Example 1, which clarifies the temperature and humidity control parameters for each stage.

[0026] Figure 7 This is a comparison chart of process defects between Example 1 and Comparative Examples 1-3, quantifying the improvement of key defects such as crack density.

[0027] Figure 8 The flashover voltage retention rate of the examples and Comparative Example 4 after aging tests was compared to verify the long-term weather resistance.

[0028] Figure 9 The diagram shows the three-dimensional relationship between the bandgap and flashover voltage, revealing the synergistic effect of energy level matching on insulation performance.

[0029] Figure 10 Comparison of XPS analysis results for silicone rubber surfaces: (a) Surface of conventional process in Comparative Example 4; (b) Surface of Example 1 of this invention, showing the increased proportion of CF bonds and the optimization of chemical state. Detailed Implementation

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials used in the following examples are commercially available products.

[0031] The performance index testing reference standards involved in the following embodiments and comparative examples are as follows: The bandgap was tested using ultraviolet photoelectron spectroscopy (UPS), with the testing standard referencing ISO 22309.

[0032] Flashover voltage: Tested according to IEC 60270 standard.

[0033] Surface flashover: Tested according to IEC 60243-1:2013 standard.

[0034] Mechanical properties: GB / T 33501-2017 (crack density), ASTM D624 (tear strength).

[0035] Aging test: IEC 62217 standard accelerated aging procedure.

[0036] Moisture content: GB / T 6283-2008 Karl Fischer method.

[0037] Example 1 Methods to improve the external insulation performance and weather resistance of DC through-wall bushings, wherein the structure of the DC bushing is described in [reference needed]. Figure 1 The specific processing includes the following steps: (1) Take the ±800kV scaled-down DC through-wall bushing assembly (including silicone rubber outer skirt and metal parts) and perform interface pretreatment respectively: a) Under nitrogen protection (O2≤100ppm), irradiate the surface with 254nm wavelength ultraviolet light (light intensity 15mW / cm²) for 10 minutes, and simultaneously spray an ethanol solution containing 1.5wt% perfluorooctyltriethoxysilane (FAS) (also containing 0.08wt% TPO photoinitiator). b) Add 0.1 mol / L HCl to the mixture of tetraethyl orthosilicate (TEOS) / ethanol / water (molar ratio 1:8:2) for catalytic hydrolysis for 24 h, add 0.8 wt% magnesium fluoride nanocrystals (aspect ratio 25:1) to enhance the coating toughness, spin coat to form an 80 nm transition layer at a spin coating speed of 3000 rpm. c) Curing at 60±5℃ for 30 minutes.

[0038] (2) The pretreated silicone rubber is subjected to gradient fluorination, and then the entire sleeve assembly is subjected to gradient drying from low temperature to high temperature. 1) The fluorination of silicone rubber adopts a two-stage gradient: Phase 1 (0~10min): 40%→48%, rate of increase 0.8% / min; Phase 2 (10~30min): 48%→55%, rate of increase 0.35% / min; Real-time monitoring and compensation control are implemented during the fluorination process of silicone rubber, specifically using a Fourier transform infrared spectrometer with a sampling frequency of 2 times / min; when the local CF bond deviation is >3%, [further control is implemented]. Adjust the nozzle (Q=1.2L / min, ν=0.15mm). 2 / s) 2) Gradient drying treatment of silicone rubber after gradient fluorination: Phase 1: Stand at 30℃ / ≤30%RH for 12 hours; Second stage: 40℃ / ≤10%RH hot air drying for 60min (axial wind speed 1.5m / s); Third stage: Drying at 50℃ / ≤10%RH for 75 minutes (tangential wind speed 0.6m / s); (3) Pretreatment of metal parts: End flange: Vacuum dry at 120℃ / ≤10Pa for 3 hours; Support rod: Dry in a vacuum atmosphere at 130℃ / ≤10Pa for 2 hours; Moisture content: 12 ppm; (4) Interface collaborative assembly: Pre-assemble in a cleanroom with humidity ≤10%RH; After assembly, dry with hot air at 50℃ for 45 minutes (wind speed 0.3m / s). (5) SF6 gas encapsulation: Three vacuuming and nitrogen purging cycles (≤5Pa pressure held for 15min); Gradient filling: 0.06MPa → hold pressure for 30min → fill to 0.10MPa.

[0039] Example 2 Differences from Example 1: 1. Adjustment of silicone rubber pretreatment: FAS concentration reduced to: 1.0 wt%. UV treatment time shortened: 8 min The content of magnesium fluoride whiskers in the sol-gel coating decreased by 0.5 wt%.

[0040] 2. Gradient fluorination adjustment: The fluorination rate in stage 2 of silicone rubber is slightly faster: 0.4% / min Comparative Example 1 Compared with Example 1, the difference is that the silicone rubber insulator interface pretreatment (1) was not performed.

[0041] Comparative Example 2 Compared with Example 1, the difference is that the silicone rubber pretreatment (1) was not performed and the gradient fluorination (2) was not performed.

[0042] (1) Take the ±800kV scaled-down DC through-wall bushing assembly (including silicone rubber outer skirt and metal parts) and perform interface fluorination treatment respectively: Full-surface fluorination of silicone rubber: Fluorine / nitrogen ratio 1:8, constant pressure 0.05 MPa for 40 min; Post-treatment: Cool to 50℃ at 4℃ / min and age for 24 hours under nitrogen protection (dew point -45℃); (2) Gradient drying process: Phase 1: Stand at 30℃ / ≤30%RH for 12 hours; Second stage: 40℃ / ≤10%RH hot air drying for 60min (axial wind speed 1.5m / s); Third stage: Drying at 50℃ / ≤10%RH for 75 minutes (tangential wind speed 0.6m / s); (3) Pretreatment of metal parts: End flange: Vacuum dry at 120℃ / ≤10Pa for 3 hours; Support rod: Dry in a vacuum atmosphere at 130℃ / ≤10Pa for 2 hours; Moisture content: 12 ppm; (4) Interface collaborative assembly: Pre-assemble in a cleanroom with humidity ≤10%RH; After assembly, dry with hot air at 50℃ for 45 minutes (wind speed 0.3m / s). (5) SF6 gas encapsulation: Three vacuuming and nitrogen purging cycles (≤5Pa pressure held for 15min); Gradient filling: 0.06 MPa → hold pressure for 30 min → fill to 0.10 MPa; Comparative Example 3 (No Gradient Drying) Drying process: Direct drying at 50℃ / ≤10%RH for 120 min; The rest are the same as in comparison 2.

[0043] Comparative Example 4 (Traditional Method) (1) The silicone rubber was not treated; (4) After drying the whole thing at 50℃ for 3 hours, assemble and inflate it; The rest are the same as in comparison 2.

[0044] Comparative Example 5 (Bandgap Mismatch Group) Silicone rubber interface: High fluorination treatment: Fluorine / nitrogen = 1:5, pressure 0.06MPa, treatment time 40min; The rest are the same as in comparison 2.

[0045] Comparative Example 6 Compared with Example 1, the difference is that in step (1), only the ultraviolet light grafting treatment is not performed. The specific treatment includes the following steps: (1) Take the ±800kV scaled-down DC through-wall bushing assembly and perform interface pretreatment: a) Do not perform ultraviolet grafting treatment, that is, do not perform ultraviolet irradiation under nitrogen protection and FAS solution spraying; b) Add HCl to the tetraethyl orthosilicate (TEOS) / ethanol / water mixture for catalytic hydrolysis, add 0.8wt% magnesium fluoride nanocrystals, and spin-coat to form an 80nm transition layer; c) Cure at 60±5℃ for 30 minutes.

[0046] Steps (2) to (5) are the same as in Example 1.

[0047] Comparative Example 7 Compared with Example 1, the difference is that in step (1), only the sol-gel coating pretreatment is not performed. The specific treatment includes the following steps: (1) Take the ±800kV scaled-down DC through-wall bushing assembly and perform interface pretreatment: a) Under nitrogen protection, irradiate the surface with 254nm wavelength ultraviolet light for 10 minutes, and simultaneously spray an ethanol solution containing 1.5wt% perfluorooctyltriethoxysilane (FAS); b) Do not perform sol-gel coating pretreatment, that is, do not prepare TEOS mixture, spin coat and construct transition layer; c) Cure at 60±5℃ for 30 minutes.

[0048] Steps (2) to (5) are the same as in Example 1.

[0049] Performance test results: The high voltage DC through-wall bushings prepared in the above examples and comparative examples were subjected to CF bond ratio test, fluoride layer thickness and uniformity test, crack density analysis, electrical performance test, and mechanical and environmental performance evaluation. The results are shown in Tables 1-6. The carrier mobility of the obtained Example 1 bushing was tested (GB / T 4326-2006), as shown in Table 1 below.

[0050] Table 1

[0051] As shown in Table 1, gradient fluorination reduces mobility by two orders of magnitude, effectively suppressing space charge accumulation. Figure 3 As shown, gradient fluorination significantly reduces mobility with increasing depth.

[0052] Table 2 Comparison of flashover voltage retention rates (IEC 60270 standard)

[0053] Table 3 Comparison of Interface Performance Indicators

[0054] Comparative Example 1 (without silicone rubber pretreatment): Due to the lack of UV grafting and sol-gel coating, an effective fluorine atom pre-assembled layer failed to form on the silicone rubber surface, resulting in insufficient fluorination reaction (CF bond ratio of only 21.5%), a reduced bandgap to 6.90 eV (close to untreated silicone rubber), weak interfacial charge dissipation ability, a flashover voltage reduced to 138.2 kV (a 28.02% reduction compared to Example 1), and poor hydrophobicity (water contact angle 112°). Figure 9 As shown, when the band gap difference between the two interfaces is <0.7eV, the flashover voltage is >180kV.

[0055] Table 4 Comparison of the impact of process defects

[0056] As can be seen from the table above, the process defects of Example 1 are significantly lower than those of Comparative Example 3. This indicates that the gradient fluorination and gradient drying process used in the embodiments of this scheme is significantly better than other processing processes, and its effect on improving insulation performance is also the best.

[0057] Table 5 Tracking resistance and flashover resistance

[0058] Regarding key performance indicators, the CTI value of the embodiment exceeds 600V, the electrical erosion depth is less than 2.0mm, and the flashover voltage is increased by more than 30%. These data indicate that the present invention has significant advantages in improving resistance to electrical tracking and flashover.

[0059] Table 6 Tracking resistance and flashover resistance

[0060] This invention significantly improves the insulation performance of DC through-wall bushings through the synergistic effect of ultraviolet (UV) grafting and sol-gel coating. UV grafting constructs active sites by pre-assembling fluoroalkyl chains on the silicone rubber surface, ensuring the depth and uniformity of the subsequent gradient fluorination reaction. Without this step (as in Comparative Example 6), the CF bond percentage decreases from 42.1% to 31.5%, and the flashover voltage drops by approximately 17.5%. Simultaneously, the magnesium fluoride nanocrystals introduced by the sol-gel coating effectively absorb arc heat and inhibit the development of carbonization channels. Without this layer (as in Comparative Example 7), the water contact angle decreases from over 140° to 132°, and the electro-erosion depth increases from 1.8 mm to 2.4 mm.

[0061] The above embodiments describe preferred embodiments of the present invention, but the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other way. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A method for improving the external insulation performance and weather resistance of a DC wall bushing, characterized in that, Includes the following steps: (1) Perform ultraviolet light grafting and sol-gel coating pretreatment on the silicone rubber insulators of the bushing assembly; (2) The pretreated silicone rubber is subjected to gradient fluorination, and then the entire sleeve assembly is subjected to gradient drying from low temperature to high temperature. (3) Dry the metal parts, then assemble the metal parts and the bushing assembly together, and finally encapsulate SF6 gas to achieve a synergistic improvement in the insulation performance of the DC bushing interface.

2. The method of claim 1, wherein: In step (1), ultraviolet grafting is performed by irradiating the silicone rubber insulator with ultraviolet light under nitrogen protection, while simultaneously spraying an ethanol solution containing perfluorooctyltriethoxysilane (FAS) and a photoinitiator.

3. The method of claim 2, wherein: The ultraviolet light wavelength is 254~365nm, and the irradiation time is 5~15min; in the sprayed solution, the mass concentration of perfluorooctyltriethoxysilane is 0.5~3%, and the mass concentration of photoinitiator is 0.1~1.5%.

4. The method of claim 3, wherein: The photoinitiator is benzophenone, 2-hydroxy-2-methyl-1-phenyl-1-propanone and / or 1-hydroxycyclohexylphenyl ketone.

5. The method of claim 1, wherein: In step (1), the sol-gel coating pretreatment involves adding HCl to a mixture of tetraethyl orthosilicate, ethanol, and water for catalytic hydrolysis, adding magnesium fluoride nanocrystals, mixing, and then spin-coating the mixture onto the surface of a silicone rubber insulator, followed by curing.

6. The method of claim 5, wherein: The amount of magnesium fluoride nanocrystals added is 1% to 5% of the mass of the mixture; after spin coating and curing, a 50-100nm transition layer is formed.

7. The method according to any one of claims 1 to 6, characterized in that: In step (2), the fluorination of silicone rubber adopts a two-stage gradient fluorination: First stage (0~10min): Fluorine concentration is increased from 40% to 48% at a rate of 0.6~1% / min; Second stage (10~30min): Fluorine concentration is increased from 48% to 55% at a rate of 0.3~0.4% / min, while controlling the CF bond ratio to be ≥40%; All fluorine concentrations are volume concentrations.

8. The method of claim 7, wherein: During fluorination process, real-time monitoring and compensation control are carried out, Fourier infrared spectrometer is used to monitor the proportion of C-F bond on line, sampling frequency is≥1 time / min, when the local C-F bond deviation is>3%, it is adjusted according to ; wherein Q is the fluorine gas flow, the unit is L / min, v is the gas kinematic viscosity, the unit is mm 2 / s; d is the nozzle outlet diameter, the unit is mm.

9. The method of claim 1, wherein: During gradient drying in step (2), First stage: Let stand at 30℃ / ≤30%RH for 10~12 hours; Second stage: Hot air drying at 40℃ / ≤10%RH for more than 1 hour; Third stage: Dry at 50℃ / ≤10%RH for more than 1.2 hours.

10. The method of claim 1, characterized by: After drying the metal parts in step (3), the moisture content is controlled to be below 12 ppm.

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