Fiber bragg grating crevice corrosion early warning system based on chloride ion sensitive hydrogel packaging

Abstract

The invention relates to the technical field of optical fiber sensing and metal corrosion monitoring, and discloses a chloride ion sensitive hydrogel packaged fiber bragg grating crevice corrosion early warning system, which comprises a fiber bragg grating sensing subsystem, a chloride ion sensitive hydrogel packaged fiber bragg grating crevice corrosion early warning subsystem and a chloride ion sensitive hydrogel packaged fiber bragg grating crevice corrosion early warning subsystem, anchoring the fiber bragg grating in an axial pre-stretching state in the rigid packaging structure by using a gel material sensitive to chloride ions; the optical signal demodulation subsystem is used for acquiring a spectral signal carrying environment characteristic information; and a corrosion early warning processing subsystem. According to the invention, the prestress of the fiber bragg grating is released and wavelength blue shift is generated based on modulus decline logic caused by reaction of a gel material and chloride ions invading a gap. According to the system, through dual-mode resonance characteristic decoupling analysis of the inclined grating, modulus attenuation and chemical denaturation of gel are synchronously quantified, the problem that in-situ monitoring is difficult to carry out in a hidden gap area is solved, and high-sensitivity and high-confidence early warning of gap corrosion is achieved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing and metal corrosion monitoring technology, specifically to a fiber optic grating crevice corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation. Background Technology

[0002] Currently, bolted connections, riveted joints, or structural laminate interfaces are widely present in various large metal facilities such as offshore platforms, oil pipelines, and aerospace equipment. These areas inevitably form narrow, semi-enclosed crevice regions. In saline service environments, these tiny crevices easily become traps for corrosive anions, especially chloride ions, leading to localized accumulation. Due to the restricted material transport within the crevice, autocatalytic effects can easily occur, leading to localized acidification and creating a corrosive microenvironment drastically different from the external macroscopic environment. This hidden crevice corrosion often occurs internally before any abnormalities are visible on the structural surface, resulting in material dissolution or stress corrosion cracking, causing sudden structural failure.

[0003] To address the monitoring needs of these concealed areas, existing application technologies primarily employ electrochemical detection methods or fiber optic sensing methods for physical quantities. Electrochemical monitoring typically involves embedding miniature reference electrodes or resistance probes near the connectors, inferring corrosion tendency by capturing changes in potential drift or polarization resistance. Fiber optic sensing solutions mostly utilize the elasto-optic or thermo-optic effects of fiber optic gratings. The general operation involves attaching the grating sensor to the structural surface or adjacent area to record the compressive stress caused by the volume expansion of corrosion products, or to monitor temperature fluctuations caused by heat accumulation accompanying the corrosion chemical reaction, thereby establishing a correlation between optical signals and the structural health status.

[0004] However, existing monitoring methods still have limitations in practical applications. Electrochemical probes, limited by their physical size, cannot truly penetrate the core region of micron-sized gaps, and the implantation of the probe itself alters the original geometry and fluid diffusion dynamics of the gap, leading to distorted measurement data. Physical monitoring methods based on corrosion product expansion suffer from time lag; relying on the accumulation of solid oxides to trigger the sensor implies that the substrate material has already undergone substantial damage, making it impossible to achieve zero-damage early warning in the initial stage of chloride ion intrusion. Furthermore, single fiber optic strain measurements face the problem of cross-sensitivity; the sensor cannot distinguish whether spectral drift is caused by corrosion expansion or by changes in external mechanical loads or environmental temperature fluctuations. This signal ambiguity caused by multi-physics coupling makes it difficult for existing technologies to provide high-confidence corrosion assessments under complex dynamic conditions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a fiber optic grating crevice corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation. This system solves the technical problems of existing technologies being unable to perform in-situ monitoring in hidden crevices of metal structures, and having difficulty distinguishing between mechanical interference and chemical corrosion signals, which leads to delayed early warnings and false alarms.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation, comprising: The fiber grating sensing subsystem is embedded inside the gap region of the metal structure to be monitored. The fiber grating sensing subsystem is filled with a chloride ion-sensitive gel material. The chloride ion-sensitive gel material is used to anchor the fiber grating in an axially pre-stretched state within the rigid encapsulation structure. The fiber grating sensing subsystem is used to respond to changes in the chemical environment and microenvironment within the gap region and generate corresponding optical signals. The optical signal demodulation subsystem is used to inject broadband excitation optical signals into the fiber Bragg grating sensing subsystem and acquire spectral signals carrying environmental feature information reflected back from the fiber Bragg grating sensing subsystem. The corrosion early warning processing subsystem is used to input spectral signals, perform feature decoupling analysis on the spectral signals, and calculate the prestress release amount of the fiber grating based on the modulus reduction logic caused by the reaction between the chloride ion-sensitive gel material and the chloride ions invading the crevice region, and output corrosion early warning results.

[0007] By employing the above technical solution, and through a structural design that pre-stretches and anchors a fiber optic grating within a chloride ion-sensitive gel, the system establishes a chemical-mechanical conversion mechanism. When chloride ions, the key corrosion-inducing factor, invade the crevice, the gel material undergoes a chemical response, leading to a decrease in modulus. The pre-stress previously applied to the fiber is then released, resulting in a measurable wavelength shift in the spectrum. Combined with optical signal demodulation and processing, this system enables in-situ monitoring within narrow, concealed metal crevices, solving the problem of traditional electrochemical probes being difficult to implant into crevices. Simultaneously, this approach converts the presence of chemical ions into a physical strain signal, achieving direct and highly sensitive early warning of the crevice corrosion microenvironment.

[0008] Preferably, the fiber Bragg grating sensing subsystem includes a set of sensing components, which includes: a tilted grating sensing module, which serves as a fiber Bragg grating and contains a single-mode fiber with a Bragg grating structure inscribed on it, wherein the single-mode fiber is subjected to axial tension to store elastic potential energy; a rigid encapsulation module, which serves as a rigid encapsulation structure and is made of a corrosion-resistant rigid material and has microporous channels that allow fluid permeation, for providing a reaction force support against axial tension; a chloride ion-sensitive gel module, which serves as a chloride ion-sensitive gel material and is filled between the tilted grating sensing module and the rigid encapsulation module, using a polymer network structure to lock the axial strain of the tilted grating sensing module; and an environmental response gating module, which covers the microporous channels, for establishing a screening channel for acidic fluids.

[0009] By adopting the above technical solution, a stable mechanical support reference system is provided by a rigid encapsulation module, ensuring that the axial strain change of the fiber grating originates solely from the change in gel modulus, thus eliminating interference from external structural deformation. The introduction of the environmental response gating module enhances the sensor's ability to discriminate against acidic preconditions for corrosion, preventing false alarms caused by non-corrosive neutral fluids and improving the accuracy of early warning.

[0010] Preferably, the tilted grating sensing module includes a dual-mode resonant optical structure. The grating surface of the Bragg grating structure is tilted at an angle of 4 to 10 degrees relative to the fiber axis, so that the core mode resonance peak and the cladding mode resonance peak are excited simultaneously in the optical signal. The core mode resonance peak characterizes the degree of release of elastic potential energy, and the cladding mode resonance peak characterizes the refractive index change of the chloride ion sensitive gel module.

[0011] By employing the aforementioned technical solution, the unique structure of the tilted grating (TFBG) excites multiple modes. The core mode, with its optical field confined within the fiber core, is primarily sensitive to axial strain and can accurately reflect stress release caused by a decrease in gel modulus. In contrast, the evanescent field of the cladding mode penetrates deep into the surrounding gel medium and is highly sensitive to the environmental refractive index. This dual-mode resonance mechanism constructs two independent sensing channels on a single optical fiber, providing a physical basis for the subsequent decoupling of mechanical parameters (stress release) and chemical parameters (medium degradation).

[0012] Preferably, the chloride ion sensitive gel module operates based on a chemical coordination dissociation mechanism: polyacrylic acid is used as the polymer backbone, and the coordination bonds between silver ions and carboxylate ions are used as physical cross-linking points to form a three-dimensional network structure; when in contact with chloride ions, silver ions preferentially combine with chloride ions to form precipitates, causing the coordination bonds to break, which transforms the chloride ion sensitive gel module from a high-modulus solid state to a low-modulus rheotropic state.

[0013] By employing the above technical solution, the high binding constant between silver ions and chloride ions enables the specific recognition of chloride ions. In the polymer network, coordination bonds act as mechanical locks; when chloride ions are present, competitive binding reactions disrupt the cross-linking points, causing the collapse of the gel network topology. This phase transition from a solid-like structure to a fluid-like structure linearly amplifies the microscopic change in ion concentration into a macroscopic decrease in bulk modulus, thereby driving a significant wavelength blue shift in the fiber Bragg grating and improving the sensor's signal-to-noise ratio.

[0014] Preferably, the environmental response gating module has a volume phase change screening mechanism: it uses a pH-sensitive hydrogel material with volume phase change characteristics; it maintains a swollen state when the environmental pH value is higher than the preset acid threshold, thereby blocking the micropore channels; and it undergoes volume shrinkage when the environmental pH value drops below the preset acid threshold, thereby opening the micropore channels.

[0015] By employing the above technical solution, a smart physical valve was constructed. Since metal crevice corrosion is often accompanied by localized environmental acidification (pH decrease), this mechanism utilizes the volumetric phase change properties of a pH-sensitive hydrogel, opening the channel only when the environment acidifies to a dangerous threshold, allowing external fluids to contact the internal sensing gel. This design effectively shields against non-corrosive interference from the external environment, ensuring that monitoring targets only the corrosive microenvironment.

[0016] Preferably, the corrosion early warning processing subsystem executes a characteristic variable extraction process: it separates the resonance peak at the longest wavelength from the spectral signal and extracts the center wavelength value of the resonance peak as the first characteristic variable characterizing the mechanical state; it separates the resonance peak group in the short-wavelength direction from the spectral signal and extracts the spectral envelope shape or cutoff wavelength of the resonance peak group as the second characteristic variable characterizing the chemical state. The corrosion early warning processing subsystem also executes parameter decoupling inversion logic: based on the wavelength blue shift of the first characteristic variable, it inverts and calculates the release of axial strain in the fiber using the fiber elasto-optic effect relationship; based on the spectral distortion characteristics of the second characteristic variable, it inverts and calculates the effective refractive index change of the medium surrounding the fiber using the cladding mode phase matching condition.

[0017] By employing the above technical solution, orthogonal separation of multidimensional information is achieved. Utilizing the photoelastic effect principle of fiber gratings, the wavelength change of the core mode is directly correlated with the mechanical stress state, thereby quantifying the mechanical attenuation of the gel. Simultaneously, by leveraging the dependence of the cladding mode's phase-matching condition on the environmental refractive index, the chemical degradation of the gel matrix is ​​quantified. This decoupled analysis eliminates the problem of cross-sensitivity between strain and refractive index in traditional fiber optic sensing, enabling the differentiation between simple mechanical relaxation and corrosion degradation caused by chemical reactions.

[0018] Preferably, the corrosion early warning processing subsystem executes a dual-threshold decision logic: crevice corrosion is determined to have occurred only when the calculated release of the fiber axial strain exceeds a preset mechanical relaxation threshold and the calculated effective refractive index change exceeds a preset refractive index fluctuation threshold.

[0019] By adopting the above technical solution, a rigorous logic and judgment mechanism was established. An alarm is only issued when a clear collapse of the gel network structure and an accompanying change in the medium composition are detected simultaneously. This reduces the false alarm rate caused by temperature fluctuations, mechanical vibrations, or abnormal single parameters, ensuring high confidence in the corrosion early warning results.

[0020] Preferably, the chloride ion-sensitive gel module has gel synthesis parameters that ensure anchoring strength: the weight-average molecular weight of polyacrylic acid is 400,000 to 550,000; and the molar ratio of silver ions to carboxylate ions on the polyacrylic acid chain is set to 1:2 to 1:4.

[0021] By employing the above technical solution and adjusting the molecular weight of the polymer and the ratio of the crosslinking agent, the initial crosslinking density and mechanical strength of the gel were precisely controlled. The selected molecular weight range and coordination ratio ensured that the gel had sufficient stiffness in the initial state to lock the pre-stretched optical fiber, preventing zero-point drift caused by fiber creep, while also ensuring a sufficiently sensitive dissociation response speed when encountering chloride ions.

[0022] This invention provides a fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation. It has the following beneficial effects: 1. This invention constructs a chemical-mechanical conversion sensing mechanism by anchoring a pre-stretched fiber optic grating within a rigid encapsulation structure using a chloride ion-sensitive gel material. When chloride ions invade the gap, the gel experiences a sudden drop in modulus due to the breakage of chemical coordination bonds, driving the release of prestress in the optical fiber and producing a wavelength blue shift. This solves the problem of not being able to implant traditional probes in narrow, concealed gap areas, and achieves highly sensitive in-situ monitoring of the core causes of corrosion.

[0023] 2. This invention utilizes a tilted grating sensing module to excite the dual-mode resonance characteristics of the core mode and cladding mode. Combined with the feature decoupling analysis of the corrosion early warning processing subsystem, it achieves synchronous and independent demodulation of mechanical strain release and environmental refractive index changes. This dual-parameter orthogonal analysis combined with dual-threshold decision logic effectively eliminates cross-interference caused by temperature fluctuations or simple mechanical deformation, reduces the false alarm rate, and ensures high confidence in the corrosion early warning results.

[0024] 3. This invention integrates an environmental response gating module based on pH-sensitive hydrogel into the microporous channel, utilizing its volume phase change characteristics to construct an intelligent ion screening channel. This mechanism only causes volume contraction to conduct external fluid when the monitored environment reaches acidic conditions that lead to corrosion, effectively shielding against invalid triggering by neutral, non-corrosive liquids and achieving precise targeted sensing of the specific microenvironment of crevice corrosion. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall architecture of the fiber optic grating crevice corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to an embodiment of the present invention. Figure 2 This is a schematic flowchart of a fiber optic grating gap corrosion early warning method based on chloride ion-sensitive hydrogel encapsulation according to an embodiment of the present invention. Figure 3 This is a simulation diagram of the characteristic curve of the fiber core mold center wavelength changing with corrosion time, generated in an embodiment of the present invention.

[0026] Among them, 10. Fiber Bragg grating sensing subsystem; 11. Tilt grating sensing module; 12. Rigid packaging module; 13. Chloride ion sensitive gel module; 14. Environmental response gating module; 20. Optical signal demodulation subsystem; 21. Light source emission module; 22. Spectrum acquisition module; 30. Corrosion early warning processing subsystem; 31. Signal feature extraction module; 32. Corrosion judgment and early warning module. Detailed Implementation

[0027] The technical solutions in 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, and 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.

[0028] See attached document Figure 1 The fiber Bragg grating (FBG) crevice corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation provided by this invention includes a FBG sensing subsystem 10, an optical signal demodulation subsystem 20, and a corrosion early warning processing subsystem 30. The FBG sensing subsystem 10 is configured to be embedded inside the crevice region of the metal structure to be monitored, for sensing changes in the chemical environment and microenvironment within the crevice and converting them into optical signals. The optical signal demodulation subsystem 20 is connected to the FBG sensing subsystem 10 via an optical transmission link, for sending broadband optical signals to the FBG sensing subsystem 10 and receiving reflected spectral signals containing environmental information. The corrosion early warning processing subsystem 30 is connected to the optical signal demodulation subsystem 20, for demodulating, analyzing, and logically determining the received spectral signals to output a corrosion early warning result.

[0029] The fiber Bragg grating sensing subsystem 10 mainly consists of a tilted grating sensing module 11, a rigid encapsulation module 12, a chloride ion-sensitive gel module 13, and an environmental response gating module 14. The tilted grating sensing module 11 is located at the central axis of the fiber Bragg grating sensing subsystem 10 and is the core carrier for signal generation and transmission. The tilted grating sensing module 11 includes a section of single-mode fiber, within which a Bragg grating structure with its grating face tilted relative to the fiber axis is etched. The tilt angle of this Bragg grating structure is set between 4 and 10 degrees, so that the reflection spectrum of the tilted grating sensing module 11 simultaneously contains both the core mode resonance peak that can propagate in the single-mode fiber core and the cladding mode resonance peak that can couple to the cladding for propagation. After encapsulation, the tilted grating sensing module 11 is in an axially pre-stretched state, which gives it initial elastic potential energy and a corresponding wavelength redshift.

[0030] The rigid encapsulation module 12 is disposed outside the tilted grating sensing module 11 and has a coaxial sleeve-like structure. The rigid encapsulation module 12 is made of a rigid material that is chemically resistant and has a high Young's modulus, such as porous ceramic or polyetheretherketone (PEEK). Several through-hole micropore channels are distributed on the sidewalls of the rigid encapsulation module 12, allowing external fluid media to permeate into the internal cavity of the rigid encapsulation module 12. The main function of the rigid encapsulation module 12 is to provide a rigid volumetric constraint boundary, serving as a support frame capable of withstanding internal stress reactions, preventing deformation of the fiber optic grating sensing subsystem 10 under external mechanical pressure, and simultaneously providing a physical protective barrier for the internal filling material.

[0031] A chloride ion-sensitive gel module 13 fills the annular gap between the tilted grating sensing module 11 and the rigid encapsulation module 12. The chloride ion-sensitive gel module 13 is a polymer network structure based on metal ion coordination crosslinking, which is tightly bonded to the surface of the tilted grating sensing module 11 and the inner wall of the rigid encapsulation module 12. In the initial monitoring state, the chloride ion-sensitive gel module 13 exhibits a solid-like state with a high storage modulus, anchoring the tilted grating sensing module 11 inside the rigid encapsulation module 12 through interfacial shear force, thereby maintaining the pre-stretched strain state of the tilted grating sensing module 11. The crosslinked network of the chloride ion-sensitive gel module 13 contains metal ion nodes with specific binding ability to chloride ions. When exposed to a high concentration of chloride ions, these metal ion nodes undergo competitive coordination reactions, leading to the disintegration of the crosslinked network and a step-like decrease in the storage modulus of the chloride ion-sensitive gel module 13.

[0032] An environmental response gating module 14 is attached to the outer surface of the rigid encapsulation module 12 and covers or fills the micropore channels on the sidewalls of the rigid encapsulation module 12. The environmental response gating module 14 is made of a pH-responsive hydrogel material, which exhibits a volume phase change characteristic in response to changes in environmental pH. When the pH value of the external environment is higher than a preset acidity threshold, the environmental response gating module 14 expands in volume, physically blocking the micropore channels on the rigid encapsulation module 12 and preventing external ions from penetrating inward. When the pH value of the external environment drops below the preset acidity threshold, the environmental response gating module 14 contracts in volume, opening the micropore channels on the rigid encapsulation module 12, allowing fluid media from the external environment to contact the internal chloride ion-sensitive gel module 13. Through this pH-controlled permeability change, the environmental response gating module 14 achieves screening and response to the acidification process in the early stages of crevice corrosion.

[0033] See attached document Figure 1 The optical signal demodulation subsystem 20 is located in a safe area away from corrosive environments and establishes an optical connection with the fiber Bragg grating sensing subsystem 10 implanted in the gap area via a single-mode fiber transmission link. The optical signal demodulation subsystem 20 is configured to provide the required broadband excitation optical signal to the fiber Bragg grating sensing subsystem 10 and receive the spectral signal containing environmental characteristic information reflected back after modulation by the fiber Bragg grating sensing subsystem 10. The optical signal demodulation subsystem 20 mainly includes a light source emission module 21 and a spectrum acquisition module 22.

[0034] The light source emitting module 21 is a high-power-density and spectrally flat broadband light source generator, such as an amplified spontaneous emission light source or a superluminescent diode light source. The light source emitting module 21 is configured to continuously output a broadband spectral signal covering the C-band and L-band, with a typical operating wavelength range of 1520 nm to 1610 nm. This wavelength range is set to ensure complete excitation and coverage of the core mode resonance peak of the tilted grating sensing module 11, as well as a series of higher-order cladding mode resonance peaks distributed in the short-wavelength direction. The light source emitting module 21 has high wavelength stability and high power stability to eliminate interference caused by power fluctuations or center wavelength drift of the light source itself on subsequent spectral feature extraction. The optical signal generated by the light source emitting module 21 is transmitted to the input port of the optical circulator in the optical path via an optical fiber patch cord, and then enters the fiber grating sensing subsystem 10 via the common port of the optical circulator.

[0035] The spectral acquisition module 22 is connected to the output port of the optical circulator in the optical path to receive the optical signal reflected from the fiber Bragg grating sensing subsystem 10. The spectral acquisition module 22 is a high-resolution spectral analysis device or a high-speed fiber Bragg grating demodulator. It possesses picometer-level spectral resolution and high dynamic range, enabling precise resolution of dense comb-like cladding mode resonance peak structures and subtle spectral envelope variations in the reflection spectrum of the tilted grating sensing module 11. The spectral acquisition module 22 integrates a photodetector array and a grating beam splitter, converting the received optical signal into a digitized spectral data sequence. It features a high-speed data communication interface, transmitting the acquired raw spectral data, including wavelength position and power intensity information, to the corrosion early warning processing subsystem 30 in real time for subsequent algorithmic processing.

[0036] See attached document Figure 1 The corrosion early warning processing subsystem 30 establishes a communication connection with the optical signal demodulation subsystem 20 through a data transmission interface to receive digitized spectral data streams. The corrosion early warning processing subsystem 30 typically consists of an industrial control computer or embedded processing terminal containing a microprocessor, memory, and a human-machine interface. The corrosion early warning processing subsystem 30 is configured to perform noise reduction processing, feature decoupling analysis, and logical state determination on the received raw spectral data, and output visualized corrosion status information or trigger external alarm devices based on the determination results. The corrosion early warning processing subsystem 30 mainly includes a signal feature extraction module 31 and a corrosion determination and early warning module 32 in its logical architecture.

[0037] The signal feature extraction module 31 is a software algorithm unit residing in the memory of the corrosion early warning processing subsystem 30 and executed by the processor. The signal feature extraction module 31 is configured to perform partitioned analysis of the full-band reflectance spectrum to separate feature parameters representing different physical dimensions. The signal feature extraction module 31 locates the independent resonance peak at the longest wavelength in the spectrum using a peak-finding algorithm, identifies it as a Bragg resonance peak generated by the core mode, and extracts the center wavelength value of this resonance peak as the first feature variable characterizing the stress state of the tilted grating sensing module 11. Simultaneously, the signal feature extraction module 31 performs envelope analysis on a series of comb-shaped resonance peaks located in the short-wavelength direction of the core mode resonance peak in the spectrum, extracting the amplitude distribution characteristics of the cladding mode resonance peak group or the cutoff wavelength of a specific order cladding mode, using these as the second feature variable characterizing the refractive index state of the chloride ion sensitive gel module 13. The signal feature extraction module 31 can track the changes of the aforementioned first and second feature variables over time in real time.

[0038] The corrosion detection and early warning module 32 is connected to the signal feature extraction module 31 and is used to receive the decoupled feature variables and perform multi-level logic verification. The storage unit of the corrosion detection and early warning module 32 contains a set of threshold parameters for determining whether the system is in a safe state, a warning state, or an alarm state. The corrosion detection and early warning module 32 is configured to simultaneously monitor the changes in the first feature variable and the second feature variable. When the corrosion detection and early warning module 32 detects a blue shift abrupt change in the first feature variable exceeding the mechanical relaxation threshold, and simultaneously detects a morphological distortion in the second feature variable exceeding the refractive index fluctuation threshold, the corrosion detection and early warning module 32 determines that substantial chloride ion erosion and gel degradation have occurred in the current crevice environment. Based on this dual verification logic, the corrosion detection and early warning module 32 generates an alarm control signal, which is used to drive the warning icon on the display interface to flash or to activate the audible and visual alarm connected to the external I / O port.

[0039] See attached document Figure 2 This invention provides a method for early warning of fiber optic grating gap corrosion based on chloride ion-sensitive hydrogel encapsulation, which may include the following steps: Step S100 is the preparation and system encapsulation step of the sensitive hydrogel. This step aims to construct a hardware foundation with specific physicochemical response characteristics. Specifically, it includes the chloride ion sensitive gel synthesis sub-step S101, which prepares a chloride ion sensitive gel module 13 through polymer backbone selection and coordination reaction with metal ion crosslinking agent; the environmental response gate material synthesis sub-step S102, which prepares an environmental response gate module 14 through pH sensitive monomer polymerization process; and the prestress loading and in-situ encapsulation sub-step S103, which applies axial prestress to the tilted grating sensing module 11 and uses the chloride ion sensitive gel module 13 for in-situ curing and locking, thus completing the physical construction of the sensing unit.

[0040] Step S200 is the crevice corrosion multidimensional sensing step. This step describes the cascade response process of the system under actual corrosive environment, specifically including three consecutive stages. The first stage is the acid environment gating opening. When the ambient pH value decreases, the environmental response gating module 14 undergoes a contraction phase transition to open the permeation channel. The second stage is ion competition and modulus mutation. The invading chloride ions compete for coordination with the chloride ion sensitive gel module 13, resulting in a step decrease in the gel's energy storage modulus. The third stage is a mechanical and optical dual-field linkage response. The decrease in modulus triggers the release of prestress in the tilted grating sensing module 11, generating a mechanical signal. At the same time, ion loss leads to a decrease in refractive index, generating an optical signal.

[0041] Step S300 is the corrosion early warning demodulation and judgment step. This step is executed by the corrosion early warning processing subsystem 30 on the host computer. Specifically, it includes a spectral feature decoupling model sub-step, which uses the wavelength drift calculation formula and cladding mode response function to analyze the prestress release amount and gel refractive index change from the original spectrum; and a dual threshold judgment logic sub-step, which sets two necessary conditions: mechanical shrinkage criterion and component denaturation criterion, and performs dual verification on the decoupled feature parameters through Boolean logic formulas, outputting the final corrosion early warning signal only when both of the above criteria are met simultaneously.

[0042] See attached document Figure 2 In step S101, the chloride ion-sensitive gel module 13 is synthesized. This step aims to construct a hydrogel matrix with a metal-coordinated supramolecular network structure, which exhibits a high storage modulus to maintain mechanical locking in a chloride-free environment, and undergoes modulus decay due to ion exchange reactions in a chloride-ion environment.

[0043] The preparation process first includes the preparation of a polymeric backbone precursor solution. Polyacrylic acid with a weight-average molecular weight between 400,000 and 550,000 was selected as the polymeric backbone material. Polyacrylic acid powder was slowly added to deionized water, and the mixture was continuously magnetically stirred at room temperature until completely dissolved, forming a polyacrylic acid aqueous solution with a mass fraction of 5% to 8%. Subsequently, sodium hydroxide solution was added dropwise to adjust the pH of the polyacrylic acid aqueous solution, stabilizing the pH value between 6.0 and 7.0. The purpose of this adjustment process is to fully ionize the carboxyl groups on the side chains of polyacrylic acid into carboxylate anions, thereby providing binding sites for subsequent metal ion coordination.

[0044] The metal ion crosslinking agent was then prepared and mixed. Silver nitrate was selected as the metal ion source and dissolved in deionized water to prepare a silver nitrate aqueous solution with a concentration of 0.1 mol to 0.5 mol per liter. Under light-protected conditions, the prepared silver nitrate aqueous solution was slowly added dropwise to the above-mentioned pH-adjusted polyacrylic acid aqueous solution. During this process, the molar ratio of silver ions to carboxylate ions was controlled within the range of 1:2 to 1:4. This molar ratio was set to ensure that the formed coordination crosslinking network has sufficient crosslinking density, so that the initial storage modulus of the synthesized gel can reach the kilopascal level, meeting the mechanical requirements for applying and maintaining prestress on the fiber grating.

[0045] Finally, coordination reactions and sol-gel transitions are performed. Under continuous stirring, the mixed solution undergoes an in-situ coordination reaction, where silver ions in the solution form carboxylate coordination bonds with carboxylate ions on the polyacrylic acid chains. These coordination bonds act as physical cross-linking points between polymer chains, causing a sharp increase in the viscosity of the solution system and ultimately transforming it from a sol state to a gel state. The prepared chloride ion-sensitive gel module 13 exhibits a three-dimensional network structure at the microscopic level. Silver ions within this structure are in a metastable coordination state. Upon contact with externally introduced chloride ions, the silver ions preferentially combine with the chloride ions to form silver chloride precipitate, causing the original coordination cross-linking points to break.

[0046] See attached document Figure 2 In step S102, the chemical synthesis of the environmental response gating module 14 is performed. The core of this step is to prepare a smart polymer material with a specific volume phase transition critical point, which can undergo significant volume shrinkage during the transition of environmental pH from neutral to acidic, thereby realizing the physical switching control of the microporous channels of the rigid encapsulation module 12.

[0047] First, the monomer solution ratio and phase transition threshold were controlled. N-isopropylacrylamide was selected as the structural monomer for constructing the temperature-sensitive main chain framework, and methacrylic acid was selected as the pH-sensitive functional monomer to provide proton response sites. The main chain structural monomer and the pH-sensitive functional monomer were dissolved in deionized water at a molar ratio of 9:1 to 7:3. Precisely setting this molar ratio is a key step in adjusting the phase transition threshold. By adjusting the density of hydrophilic carboxyl groups in the polymer network, the critical pH value at which the material undergoes a volumetric abrupt change was precisely locked within the range of 3.5 to 4.5. This pH range corresponds to the early localized acidification stage of metal crevice corrosion, ensuring that the environmental response gating module 14 is only activated when corrosion characteristics appear.

[0048] Subsequently, crosslinking agents and initiators were added to construct a three-dimensional network. N,N'-methylenebisacrylamide was added to the above mixed monomer solution as a chemical crosslinking agent, with the addition amount set to be one to three percent of the total monomer mass. The role of the crosslinking agent is to connect linear polymer chains to construct a stable three-dimensional network structure, ensuring that the environmental response gating module 14 maintains structural integrity and does not dissolve under water absorption and swelling conditions. At the same time, ammonium persulfate was added as an initiator for free radical polymerization, and tetramethylethylenediamine was added as a reaction accelerator. Before adding the initiator, the mixed solution was subjected to high-purity nitrogen bubbling treatment for no less than 30 minutes to completely replace the dissolved oxygen in the solution and prevent the polymerization inhibition effect caused by oxygen from affecting the polymerization conversion rate.

[0049] Finally, polymerization and post-processing of the material were carried out. The reaction system was placed in a constant-temperature, closed environment at 20 to 25 degrees Celsius for static reaction, allowing monomer molecules to undergo free radical copolymerization under the action of crosslinking agents, forming a random copolymer hydrogel network. After the reaction, the synthesized gel material was subjected to long-term dialysis treatment in a large amount of deionized water to remove unreacted residual monomers and small molecule impurities. The prepared environmentally responsive gating module 14 exhibited clear pH-responsive characteristics: in a neutral environment with a pH greater than 4.5, the carboxyl groups of the gel network side chains dissociated and became negatively charged, and the polymer chains extended due to electrostatic repulsion, causing the material to be in a volume expansion state to block the channels; when the environmental pH dropped below 4.0, the carboxyl groups protonated, causing the electrostatic repulsion to disappear and the intramolecular hydrogen bonding to be enhanced, causing the polymer chains to coil and collapse, causing the material to change to a volume contraction state to open the channels.

[0050] See attached document Figure 2 In step S103, prestress loading and in-situ encapsulation of the tilted grating sensing module 11 are performed. This step aims to establish an initial equilibrium state of mechanical and optical coupling by pre-storing elastic potential energy in the tilted grating sensing module 11, providing the necessary mechanical driving source for subsequent sensing of gel modulus decay.

[0051] First, the tilted grating sensing module 11 is passed through the central axis of the rigid encapsulation module 12, and one end of the tilted grating sensing module 11 is fixed to the end of the rigid encapsulation module 12 using a precision clamping device. The other end of the tilted grating sensing module 11 is connected to a high-precision micro-displacement stretching platform, and simultaneously connected to the optical signal demodulation subsystem 20 to monitor its reflection spectrum in real time. The micro-displacement stretching platform is started, and a tensile load is applied to the tilted grating sensing module 11 along the fiber axis. During the stretching process, the center wavelength position of the core mode resonance peak of the tilted grating sensing module 11 is tracked in real time. The tensile load is continuously increased until the core mode resonance peak produces a spectral redshift of 2 to 5 nanometers. At this point, the stretching is stopped, and the current position of the micro-displacement stretching platform is locked, so that the tilted grating sensing module 11 is in a stable axial pre-stretch strain state. This preset wavelength redshift corresponds to the reference zero point of the system in a safe state.

[0052] Subsequently, while maintaining the axially stretched state of the tilted grating sensing module 11, the precursor liquid of the chloride ion-sensitive gel module 13, prepared in the sol state in step S101, is injected into the annular gap between the rigid encapsulation module 12 and the tilted grating sensing module 11. During the injection process, the flow rate is strictly controlled to avoid bubble generation, ensuring that the precursor liquid completely fills the internal cavity of the rigid encapsulation module 12 and fully wets the surface of the tilted grating sensing module 11. The filled component is placed in a constant-temperature static environment to wait for the chloride ion-sensitive gel module 13 to undergo an in-situ coordination crosslinking reaction. As the reaction proceeds, the precursor liquid gradually transforms into a solid gel network with a high storage modulus, and forms a tight physical bond with the tilted grating sensing module 11 and the rigid encapsulation module 12 through interfacial adhesion.

[0053] After the chloride ion-sensitive gel module 13 has fully cured, the external mechanical constraint applied to the tilted grating sensing module 11 by the micro-displacement stretching platform is released. At this time, the tilted grating sensing module 11 has an elastic retraction tendency to recover its initial length, but this retraction tendency is blocked by the high modulus network of the cured chloride ion-sensitive gel module 13. Through the interfacial shear force applied by the chloride ion-sensitive gel module 13, the tilted grating sensing module 11 is locked in a pre-stretched state, and its residual wavelength redshift remains above 90% of the initial set value, realizing the internal transfer and locking of prestress. Finally, the environmental response gating module 14 prepared in step S102 is coated or filled into the micropore channels on the outer wall of the rigid encapsulation module 12 to complete the overall encapsulation of the system.

[0054] See attached document Figure 2 In stage one of step S200, the system executes the physical response process for opening the acid environment gating. This stage corresponds to the initial stage of metal crevice corrosion, at which point the hydrolysis reaction of metal cations in the crevice microenvironment leads to a continuous increase in hydrogen ion concentration, causing the local environment to change from its original neutral state to an acidic state. As a functional component that directly contacts the external environment, the environmental response gating module 14 utilizes its volume-sensitive characteristic to hydrogen ion concentration to control the on / off state of the material exchange channels inside and outside the rigid encapsulation module 12.

[0055] When the pH value of the external environment decreases and crosses the preset phase transition threshold of the environmental response gating module 14, a drastic microscopic conformational change occurs in the polymer network inside the environmental response gating module 14. Initially, when the pH value is above the threshold, the environmental response gating module 14 is in a swollen state and tightly fills the micropores of the rigid encapsulation module 12. The carboxylate groups on its polymer chains are ionized, and the resulting electrostatic repulsion maintains the expanded volume of the polymer network, thus physically blocking the entry of external ions. As the environmental pH value decreases below the threshold, high concentrations of hydrogen ions permeate into the surface and interior of the environmental response gating module 14, causing the carboxylate anions on the side chains to undergo a protonation reaction to generate electrically neutral carboxyl groups.

[0056] As the protonation process proceeds, the electrostatic repulsion that maintains the extension of the polymer chains disappears. Simultaneously, hydrogen bonding within the polymer chains intensifies, and the interactions of hydrophobic groups become dominant, leading to the coiling and collapse of the polymer chains. This microscopic molecular chain motion manifests macroscopically as a significant volume shrinkage phase transition in the environmental response gating module 14. The shrinkage of the environmental response gating module 14 reduces its volume and causes it to detach from the microporous inner wall of the rigid encapsulation module 12, thus forming a physical gap connecting the external environment and the internal cavity at the shell location of the rigid encapsulation module 12. This physical gap constitutes an ion permeation channel, removing the physical shielding against external chloride ions and allowing chloride ions from the external environment to diffuse inward through this channel and contact the chloride ion-sensitive gel module 13, thereby initiating the subsequent chemical sensing process.

[0057] See attached document Figure 2 In stage two of step S200, a chemical response process involving ion competition and modulus mutation is executed. Following the opening of the ion permeation channel by the environmental response gating module 14 in stage one, high-concentration chloride ions from the external environment diffuse along the concentration gradient through this channel into the rigid encapsulation module 12 and directly contact the chloride ion-sensitive gel module 13, which is in a pre-stressed locked state. At this time, the microenvironment of the chloride ion-sensitive gel module 13 undergoes a significant change in chemical composition, triggering a competitive chemical reaction against specific ions.

[0058] The three-dimensional network structure inside the chloride ion-sensitive gel module 13 relies on the coordination bonds formed between the carboxylate groups of the polymer side chains and silver ions as physical cross-linking points to maintain its high mechanical strength as a solid gel. Since the solubility product constant of silver chloride is orders of magnitude lower than the stability constant of the silver carboxylate coordination compound, the thermodynamic equilibrium of the system is disrupted when chloride ions penetrate into the gel network. The invading chloride ions, acting as strong competing ligands, preferentially capture silver ions from the network nodes, forming in-situ insoluble silver chloride precipitates. This process causes irreversible breakage of the carboxylate coordination bonds that originally connected the polymer chains, allowing the previously bound polymer chain segments to regain their ability to move freely.

[0059] As the coordination crosslinking sites continue to dissociate, the crosslinking density of the chloride ion-sensitive gel module 13 decreases exponentially. At the microscopic level, the polymer network structure transforms from a dense network conformation to a loose linear or branched conformation; at the macroscopic physical property level, this microscopic deconstruction process manifests as a dramatic abrupt change in the material's rheological parameters. Specifically, the storage modulus of the chloride ion-sensitive gel module 13 undergoes a step-like decay, rapidly degenerating from a rigid gel state capable of maintaining kilopascal-level stress before the reaction to a viscous flow state or soft gel state with a lower modulus. This abrupt change in storage modulus causes the chloride ion-sensitive gel module 13 to lose its ability to mechanically lock onto the tilted grating sensing module 11, providing a material basis for the release of prestress in subsequent stages.

[0060] See attached document Figure 2 In stage three of step S200, the system performs a physical conversion process of force-optical dual-field linkage response. This stage is a crucial step in converting the corrosion chemical signal into a quantifiable photoelectric signal. The synchronous response of the tilted grating sensing module 11 to changes in the mechanical and optical fields confirms the crevice corrosion event. As the chloride ion-sensitive gel module 13 undergoes cross-linking network disintegration and a step-like decrease in its storage modulus in stage two, the interfacial shear constraint force originally applied to the surface of the tilted grating sensing module 11 fails. At this point, the elastic potential energy accumulated inside the tilted grating sensing module 11 loses the suppression of the external balancing force, causing the optical fiber to elastically retract axially.

[0061] In terms of mechanical response, the axial retraction of the tilted grating sensing module 11 causes a physical shortening of the grating period of the fiber grating. According to the Bragg grating coupling theory, the reduction in the grating period directly leads to a shift of the resonant wavelength of the core mode's fundamental mode towards shorter wavelengths, i.e., a blue shift in the spectrum. Since the magnitude of prestress release directly depends on the modulus attenuation of the chloride ion-sensitive gel module 13, and the modulus attenuation is positively correlated with the concentration of intruding chloride ions, the blue shift of the core mode wavelength directly characterizes the cumulative dose of corrosive ions. This process transforms the invisible degree of chemical corrosion into a precisely measurable optical wavelength shift.

[0062] In the optical response dimension, the chemical composition of the chloride ion-sensitive gel module 13 changes synchronously, leading to alterations in its optical properties. As highly polarizable silver ions detach from the polymer backbone and precipitate as silver chloride, and as external water molecules infiltrate in large quantities due to the relaxation of the gel network, the effective refractive index of the medium surrounding the tilted grating sensing module 11 decreases significantly. The evanescent field of the cladding mode, unique to the tilted grating sensing module 11, penetrates the fiber cladding and extends into the surrounding medium, exhibiting high sensitivity to changes in the medium's refractive index. The decrease in the surrounding medium's refractive index alters the phase-matching conditions and mode coupling efficiency of the cladding mode, resulting in drastic fluctuations in the amplitude intensity of the cladding mode resonance peak in the reflection spectrum and a shift in the cutoff wavelength of specific modes. This change in optical characteristics is independent of the mechanical blue shift of the core mode, thus constituting a second-dimensional optical fingerprint characteristic for determining corrosion occurrence.

[0063] See attached document Figure 2 In step S300, the system enters the corrosion early warning demodulation and judgment stage, where the spectral feature decoupling model sub-step mainly executes the calculation process of the spectral feature decoupling model. This step is specifically executed by the signal feature extraction module 31 in the corrosion early warning processing subsystem 30, and its core task is to map the raw reflectance spectrum data collected by the optical signal demodulation subsystem 20 into specific physical parameters. Given that the tilted grating sensing module 11 undergoes both mechanical relaxation and chemical denaturation processes during corrosion, the decoupling model establishes independent mathematical response functions for the core mode resonance signal and the cladding mode resonance signal, respectively, to quantitatively analyze the degree of prestress release and the change in the refractive index of the sensitive gel.

[0064] Regarding the fundamental mode resonance signal of the fiber core mode inside the tilted grating sensing module 11, due to the physical shielding effect of the fiber cladding, the fiber core mode is insensitive to changes in the refractive index of the external environment, and its wavelength drift is mainly dominated by changes in the axial strain state of the fiber. When the modulus attenuation of the chloride ion sensitive gel module 13 leads to a decrease in the locking force on the fiber, the fiber releases prestress and generates axial shrinkage strain. The signal feature extraction module 31 uses the wavelength drift calculation formula to inversely calculate this mechanical change process, which is expressed as: in, This parameter represents the drift of the core mode center wavelength. It is obtained by differential calculation between real-time spectral scan data and initial reference spectral data. The effective elastic-optic coefficient of the optical fiber material is a dimensionless constant that depends on the material properties of silica optical fiber. This represents the center wavelength value of the core mode of the tilted grating sensing module 11 in its initial equilibrium state; This represents the change in axial strain of the optical fiber, which directly reflects the degree of loss of mechanical support capacity of the chloride ion-sensitive gel module 13 due to the chemical reaction, i.e., the amount of prestress release. Through this model, the system accurately converts the blue shift characteristics of the spectrum into strain values ​​in the mechanical dimension.

[0065] For the cladding mode resonance signal excited by the tilted grating sensing module 11, its resonance wavelength position and amplitude characteristics directly depend on the boundary conditions of the interface between the fiber cladding and the surrounding medium. Due to the specific tilt angle of the tilted grating introducing a non-circular symmetric coupling mechanism, the effective refractive index of the cladding mode is highly dependent on the refractive index of the external environmental medium, namely the chloride ion-sensitive gel module 13. The signal feature extraction module 31 uses the cladding mode response function to establish a mapping relationship between spectral characteristics and the gel refractive index. This phase-matching condition formula is expressed as: in, Indicates the first The resonant center wavelength of the cladding mode; This represents the effective refractive index of the fiber core mode, which is mainly determined by the fiber's own geometry and doping concentration. Indicates the first The effective refractive index of the cladding mode; this parameter is the refractive index of the external chloride ion-sensitive gel module 13. The function of the gel, as silver chloride precipitates out and water molecules permeate, The changes will directly lead to Changes in numerical values; The nominal grating period represents the fiber optic grating along the fiber axis. This represents the tilt angle between the fiber grating surface normal and the fiber axis. It is achieved by monitoring specific cladding modes. By identifying drift or cutoff phenomena, the system can calculate the refractive index evolution of the external gel medium, thereby achieving optical characterization of the corrosion chemical process.

[0066] See attached document Figure 2 In the dual threshold determination logic sub-step of step S300, the corrosion determination and early warning module 32 executes dual threshold determination logic based on multi-dimensional feature fusion. This logic aims to eliminate false alarms caused by environmental temperature fluctuations or non-corrosive external force interference, ensuring the uniqueness and accuracy of the early warning signal for crevice corrosion events. The corrosion determination and early warning module 32 first sets and verifies two independent necessary conditions: a mechanical retraction criterion characterizing a change in mechanical state and a component denaturation criterion characterizing a change in chemical composition. The system only determines that a corrosion event has indeed occurred when both criteria are simultaneously met.

[0067] For condition A, the mechanical retraction criterion, the corrosion judgment and early warning module 32 calls the signal feature extraction module 31 to calculate the wavelength drift value of the fiber core mode center. Since the decrease in the modulus of the sensitive gel caused by chloride ion corrosion inevitably leads to the release of the axial prestress of the fiber grating, this physical process is spectrally manifested as an irreversible blue shift. The system compares the absolute value of the wavelength drift of the fiber core mode center with a preset wavelength drift threshold, which is set slightly larger than the maximum theoretical wavelength drift caused by system noise and environmental thermal fluctuations. When the absolute value of the measured drift exceeds this threshold, the fiber grating is determined to be in a mechanically relaxed state, indicating that the mechanical support structure of the chloride ion sensitive gel module 13 has failed, thus satisfying the mechanical retraction criterion.

[0068] Regarding condition B, the component denaturation criterion, the corrosion detection and early warning module 32 analyzes the envelope characteristics of the reflection spectrum based on the cladding mode response function. Due to the coordination precipitation reaction between chloride and silver ions, the gel refractive index decreases, causing a specific morphological change in the resonance peak distribution of the cladding mode. The system calculates the degree of spectral distortion of the current cladding mode spectral characteristics relative to the initial state. This parameter characterizes the overall shift in the effective refractive index of the cladding mode and the attenuation of mode coupling efficiency. The system compares this degree of spectral distortion with a preset distortion threshold. When the calculated value exceeds the distortion threshold, it determines that a substantial change in the chemical composition of the medium surrounding the optical fiber has occurred, indicating that chloride ions have successfully invaded and caused a phase transition in the gel, thus satisfying the component denaturation criterion.

[0069] After verifying the above two independent conditions, the corrosion detection and early warning module 32 uses Boolean logic operations to output the final early warning signal. The logical formula for this dual threshold determination is expressed as follows: in, This represents the final output corrosion warning Boolean signal, which triggers a system alarm when its value is true; This represents the center wavelength shift of the fiber core mode calculated using the spectral feature decoupling model. This indicates a preset wavelength drift threshold, used to define the effectiveness of mechanical retraction; This indicates the degree of spectral distortion calculated based on the cladding mode response function, and is used to quantify the magnitude of change in the refractive index of the medium. This indicates a preset spectral distortion threshold, used to define the effectiveness of chemical denaturation; This represents the logical AND operator. Through this logical formula, the system implements logical AND verification of mechanical prestress release signals and optical refractive index change signals. Only when the crevice region simultaneously exhibits stress release characteristics induced by gel dissolution and medium denaturation characteristics induced by ion exchange, does the system confirm the occurrence of crevice corrosion and output an early warning, thus achieving highly reliable corrosion monitoring.

[0070] Specific application examples: This embodiment first constructs an experimental platform including a precision fine-tuning support. Two 316L stainless steel plates are used to simulate a flange connection structure. By controlling the distance between the two plates, a narrow gap region with a width of 0.5 mm and a depth of 20 mm is created. This gap is then placed inside an environmental control box to precisely regulate temperature and medium injection. For the selection of the core sensing element, this embodiment uses a nominal center wavelength value... For a 1550 nm tilted grating (TFBG) sensing module with a grating length of 10 mm, the effective elastic coefficient of its fiber material is... The value is set to 0.22. For the configuration of key chemically sensitive materials, the chloride ion sensitive gel module 13 specifically uses a polyacrylic acid silver-polyvinyl alcohol (PAA-Ag / PVA) interpenetrating network hydrogel, which has been proven to have high chloride ion selectivity; the environmental response gating module 14 uses a pH-sensitive N-isopropylacrylamide (PNIPAM) copolymer, which is set to shrink in volume to open channels in an acidic environment with a pH value below 4.0.

[0071] During the sensor encapsulation and initialization phase, the two ends of the TFBG are fixed in a specially designed mold. A high-precision displacement stage is used to apply axial pre-tension until the spectrometer detects a 3.0 nm redshift in the center wavelength relative to the free state, thus pre-storing high-potential elastic tension within the optical fiber. Subsequently, the prepared gel precursor solution is injected and cured in situ. After the gel forms a high-modulus solid, the mechanical clamps are removed, and the mechanical locking effect of the gel network locks the optical fiber in a pre-stressed state. Finally, the sensor is connected to the optical signal demodulation subsystem 20 with a sampling frequency of 1 Hz. After confirming that the environmental response gating module 14 is in a closed expansion state, the wavelength shift measured at this time is taken as... Figure 3 The zero point of the stable baseline from 0 to 10 hours.

[0072] See attached document Figure 3 In the initial stable phase after the start of the experiment (i.e. Figure 3 During the 0-10 hour period, simulating a normal humid environment outside the flange gap, the monitoring curve shown by the solid line indicates that the center wavelength drift is stable around 0 nanometers with minimal fluctuations. This result fully demonstrates that under non-corrosive conditions, the chloride ion-sensitive gel module 13 can maintain a complete high-modulus solid state, effectively resisting the elastic recoil force corresponding to a redshift of approximately 3.0 nm inside the optical fiber. The system did not generate false alarms due to material creep, exhibiting excellent mechanical stability.

[0073] When the experiment reached the 10th hour, a simulated corrosion solution with a pH of 3.5 and rich in chloride ions (concentration 0.6 mol / L) was injected into the crevice region via a microfluidic pump. At this point, the environmental response gating module 14 sensed the acidic environment and rapidly contracted and opened, allowing chloride ions to invade and complex with silver ions in the gel. Figure 3 As shown, starting from the 10th hour, the monitoring curve began to show a significant negative shift (blue shift), which accurately verified the mechanism that as the silver carboxylic acid coordination bonds in the gel network break, the gel crosslinking density and storage modulus decrease sharply, resulting in the inability to continue resisting the elastic shrinkage of the optical fiber.

[0074] As the corrosion reaction continued to intensify, around the 12th hour of the experiment, the absolute value of the wavelength drift detected by the system exceeded the preset 0.5 nm wavelength drift threshold for the first time. (Right now Figure 3 The warning threshold line is marked by a horizontal dashed line. At this time, the corrosion judgment and warning module 32 determines condition A: the mechanical retraction criterion is met, and the demodulation system synchronously detects that the cladding mode spectrum meets condition B: the compositional denaturation criterion due to the change in refractive index. Based on the dual threshold judgment logic, the system accurately outputs a corrosion warning signal at the 12-hour node. Figure 3 (marked with a circle), which closely matches the actual end time of the corrosion induction period.

[0075] After 20 hours of experimentation, the curve entered the complete relaxation phase. At this point, the chloride ion-sensitive gel module 13 had completely solubilized and flowed away, and the prestress of the tilted grating sensing module 11 had been completely released. Figure 3 As shown, the final stable wavelength shift Approximately -2.5 nanometers. Calculated using the wavelength drift formula. By performing reverse verification calculations, the change in axial strain of the optical fiber can be obtained. The strain is approximately -2067 microstrain. This calculation result is consistent with the preset initial tension release during packaging, quantitatively proving that the system can accurately convert the degree of microscopic chemical corrosion into a macroscopic mechanical strain signal, and has high sensitivity and signal-to-noise ratio.

Claims

1. A fiber optic grating crevice corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation, characterized in that, include: A fiber optic grating sensing subsystem (10) is used to be implanted inside the gap region of the metal structure to be monitored. The interior is filled with a chloride ion-sensitive gel material. The chloride ion-sensitive gel material is used to anchor the fiber optic grating in an axially pre-stretched state within a rigid encapsulation structure. The system responds to changes in the chemical environment and microenvironment within the gap region and generates corresponding optical signals. The optical signal demodulation subsystem (20) is used to inject a broadband excitation optical signal into the fiber Bragg grating sensing subsystem (10) and acquire the spectral signal carrying environmental feature information reflected back by the fiber Bragg grating sensing subsystem (10). The corrosion early warning processing subsystem (30) is used to input the spectral signal, perform feature decoupling analysis on the spectral signal, and calculate the prestress release amount of the fiber grating based on the modulus reduction logic caused by the reaction between the chloride ion-sensitive gel material and the chloride ions invading the crevice region, and output the corrosion early warning result.

2. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 1, characterized in that, The fiber Bragg grating sensing subsystem (10) includes a set of sensing components for performing the anchoring function and responding to changes in the chemical environment, the set of sensing components specifically including: The tilted grating sensing module (11) is a single-mode fiber with a Bragg grating structure inscribed on the fiber grating, and the single-mode fiber is subjected to axial tension to store elastic potential energy. A rigid encapsulation module (12), which is made of a corrosion-resistant rigid material and has micropore channels that allow fluid permeation, is used to provide a reaction force support against the axial tensile force; A chloride ion-sensitive gel module (13) is filled between the tilted grating sensing module (11) and the rigid encapsulation module (12) as the gel material. The axial strain of the tilted grating sensing module (11) is locked by the polymer network structure, thereby realizing the anchoring function in the sensing component assembly. An environmental response gating module (14), which covers the microporous channel, is used to establish a screening channel for acidic fluids.

3. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 2, characterized in that, The tilted grating sensing module (11) includes a dual-mode resonant optical structure for generating the environmental feature information, wherein the dual-mode resonant optical structure is specifically defined as follows: The grating surface of the Bragg grating structure is tilted at an angle of 4 to 10 degrees relative to the fiber axis, so that the core mode resonance peak and the cladding mode resonance peak are excited simultaneously in the optical signal. The core mode resonance peak characterizes the degree of release of elastic potential energy, the cladding mode resonance peak characterizes the refractive index change of the chloride ion sensitive gel module (13), and the core mode resonance peak and the cladding mode resonance peak together constitute the environmental feature information generated by the dual-mode resonance optical structure.

4. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 2, characterized in that, The chloride ion-sensitive gel module (13) operates based on a chemical coordination dissociation mechanism that triggers the modulus decrease, and the chemical coordination dissociation mechanism is specifically defined as follows: Polyacrylic acid is used as the polymer backbone, and the coordination bonds between silver ions and carboxylate ions are used as physical cross-linking points to form a three-dimensional network structure. When in contact with chloride ions, silver ions preferentially combine with chloride ions to form a precipitate, causing the coordination bond to break, and the chloride ion sensitive gel module (13) to change from a high energy storage modulus solid to a low modulus rheotropic state. The transition from the solid state to the rheotropic state causes a decrease in the modulus, thereby releasing the mechanical lock on the tilted grating sensing module (11).

5. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 2, characterized in that, The environmental response gating module (14) has a volume phase change screening mechanism for responding to changes in the microenvironment, and the volume phase change screening mechanism is specifically defined as follows: A pH-sensitive hydrogel material with volume phase change characteristics is used; The micropore channels are blocked by maintaining a swollen state when the ambient pH value is higher than a preset acid threshold. When the ambient pH value drops below the preset acid threshold, volume shrinkage occurs, thereby opening the micropore channels; The pH-sensitive hydrogel material responds to changes in the microenvironment through volume changes, thereby enabling the conduction of the acidic fluid within the gaps.

6. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 1, characterized in that, The optical signal demodulation subsystem (20) includes spectral acquisition hardware for fully acquiring the spectral signal, the spectral acquisition hardware specifically including: The light source emission module (21) outputs a spectrum covering the 1520 nm to 1610 nm band, thereby covering the core mode resonance peak and the high-order cladding mode resonance peak group in the short-wave direction of the fiber grating sensing subsystem (10). The spectral acquisition module (22) has a picometer-level spectral resolution and is used to distinguish the comb-like structure of the higher-order cladding mode resonance peak group; The light source emission module (21) and the spectrum acquisition module (22) work together to acquire the spectral signal containing multimodal resonance information.

7. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 1, characterized in that, The corrosion early warning processing subsystem (30) includes a signal feature extraction module (31) that performs a feature variable extraction process for quantifying the spectral signal features. The feature variable extraction process specifically includes: The resonance peak at the longest wavelength is separated from the spectral signal, and the center wavelength value of the resonance peak is extracted as the first characteristic variable characterizing the mechanical state. The resonance peaks in the short-wavelength direction are separated from the spectral signal, and the spectral envelope morphology or cutoff wavelength of the resonance peaks are extracted as a second characteristic variable to characterize the chemical state.

8. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 7, characterized in that, The corrosion early warning processing subsystem (30) executes parameter decoupling inversion logic to complete the feature decoupling analysis. The parameter decoupling inversion logic specifically includes: Based on the wavelength blue shift of the first characteristic variable, the release of axial strain of the optical fiber is calculated by inverting the optical fiber elasto-optic relationship, thereby quantifying the modulus attenuation of the gel material. Based on the spectral distortion characteristics of the second characteristic variable, the effective refractive index change of the medium surrounding the optical fiber is calculated by inversion using the cladding mode phase matching condition, thereby quantifying the degree of chemical denaturation of the gel material. By calculating the release of the axial strain of the optical fiber and the change in the effective refractive index respectively, the mechanical parameters and chemical parameters are separated, thereby completing the feature decoupling analysis.

9. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 8, characterized in that, The corrosion early warning processing subsystem (30) includes a corrosion judgment and early warning module (32) that executes a dual-threshold decision logic for outputting the corrosion early warning result. The dual-threshold decision logic is specifically defined as follows: Simultaneously monitor the mechanical shrinkage criterion and component denaturation criterion of the parameter decoupling inversion logic output; Crevice corrosion is determined to occur only when the calculated release of the axial strain of the optical fiber exceeds a preset mechanical relaxation threshold and the calculated effective refractive index change exceeds a preset refractive index fluctuation threshold. Based on the dual verification of the mechanical retraction criterion and the component denaturation criterion, the corrosion early warning result is generated and output.

10. The fiber optic grating gap corrosion early warning system based on chloride ion-sensitive hydrogel encapsulation according to claim 4, characterized in that, The chloride ion-sensitive gel module (13) has gel synthesis parameters for ensuring the anchoring strength, and the gel synthesis parameters are specifically defined as follows: The weight-average molecular weight of the polyacrylic acid is between 400,000 and 550,000. The molar ratio of silver ions to carboxylate ions on the polyacrylic acid chain is set to 1:2 to 1:

4. The chloride ion-sensitive gel module (13) prepared using the gel synthesis parameters has the initial crosslinking density required to maintain the pre-stretched state of the fiber grating, thereby ensuring the effectiveness of the anchoring.

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