CN110146436B - Stress corrosion cracking on-line monitoring device and analysis method based on electrochemical noise - Google Patents
Stress corrosion cracking on-line monitoring device and analysis method based on electrochemical noise Download PDFInfo
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Abstract
The invention relates to the field of electrochemical corrosion monitoring, in particular to an online stress corrosion cracking monitoring device and an analysis method based on electrochemical noise, wherein a jacketed electrolytic cell is placed in a stress ring, an SCC_WE1 test electrode is positioned in the center of the jacketed electrolytic cell, SCC_WE2 test electrodes are symmetrically distributed around the SCC_WE1 test electrode, a zero-resistance ammeter is respectively connected with the SCC_WE1 test electrode and the SCC_WE2 test electrode, and a potential follower is respectively connected with the SCC_WE1 test electrode and a reference electrode RE; the zero-resistance ammeter and the potential follower are connected with the signal acquisition and analysis unit. Adopting Matlab to compile electrochemical noise data analysis software, distinguishing different stages of SCC crack expansion through different types of current/potential noise peak characteristics, and calculating single crack expansion length and crack average expansion rate according to noise peak integral electric quantity and occurrence frequency; the sensitivity of SCC is evaluated according to the amplitude of the current noise peak, and the method realizes the dynamic monitoring of the microcrack initiation and growth process.
Description
Technical Field
The invention relates to the field of electrochemical corrosion monitoring, in particular to an online stress corrosion cracking monitoring device and an analysis method based on electrochemical noise.
Background
In the exploitation process of oil and gas fields, the tertiary oil recovery technology such as CO 2 flooding and the like can obviously improve the oil and gas recovery ratio, for example, the oil and gas recovery ratio of a medium-oil Jilin oil field is improved by 5% -10% by adopting the technology, and a good demonstration effect is obtained. However, dissolution of CO 2 in the downhole annulus solution can result in Stress Corrosion Cracking (SCC) of the oil casing under the combined action of stress and corrosive environments. Studies have shown that P110 oil tube steels suffer from reduced toughness in low temperature annular liquid environments containing CO 2, leading to SCC. In the H 2S-CO2-Cl- environment, the SCC sensitivity of the 316L stainless steel is increased along with the increase of the H 2S/CO2 partial pressure ratio. The rate of propagation of stress corrosion cracking is orders of magnitude higher than other forms of corrosion and there are no obvious signs, so SCC failure of oil pipe steels is often sudden and catastrophic, resulting in significant economic losses and casualties. Therefore, the SCC can be diagnosed early by the nondestructive monitoring technology, faults can be found timely, the faults are prevented, and the method has important significance for safe production of oil and gas fields.
The China patent application CN 106568665A discloses a method for evaluating the SCC of high-strength pipeline soil, which is mainly used for calculating the residual strength of the pipeline by the defect of the surface of the pipeline, and does not propose a corresponding SCC monitoring method. CN 106404554a discloses a test device for researching sulfate reducing bacteria SCC, and the influence of sulfate reducing bacteria on metal SCC is researched by periodically observing the conditions of nucleation and expansion of cracks on the surface of a sample, but SCC in-situ nondestructive monitoring cannot be realized. CN 105675481a proposes an electrochemical experimental device and a testing method for corrosion of a tensile stress sample loaded in a high-temperature high-pressure fluid environment, and obtains the action mechanism of tensile stress and corrosion of fluid on a metal material at high temperature and high pressure through electrochemical parameters, but no algorithm for the initiation or growth process of SCC cracks is proposed.
The corrosion of the oil pipe steel in the underground annular solution belongs to electrochemical corrosion, and the uniform corrosion and the environmental corrosiveness of the material can be monitored by adopting a hanging piece method, an electrochemical impedance method, a linear polarization method and the like. . However, these conventional electrochemical methods have difficulty monitoring stress corrosion cracking caused by localized corrosion. The electrochemical noise monitoring technology not only can obtain the local corrosion information of the metal material, but also has no external interference to a test system, and can truly reflect the corrosion state of the material.
The electrochemical noise signal analysis method mainly comprises time domain statistical analysis, frequency domain analysis and wavelet analysis at present. The time domain analysis method is most widely applied, but the statistical analysis method based on noise resistance cannot judge the occurrence time node and the severity of local corrosion; the frequency domain analysis is easy to be influenced by background noise interference and signal drift, and the calculation error is large; the noise analysis algorithm based on wavelet transformation is complex and cannot realize real-time online analysis. The above problems have significantly limited the application of electrochemical noise technology in-situ corrosion monitoring. In chinese patent CN107202755A, an electrochemical noise sensor is used to test the potential and current noise on the outer surface of a metal pipe, and the corrosion state of the metal pipe is estimated according to the noise resistance calculated from the standard deviation of the current potential; in CN103983564A, the corrosion of metal materials is monitored through an atmospheric corrosion electrochemical sensor, and the corrosion rate in the atmospheric corrosion process is reflected by a wavelet noise resistor close to 2 Hz; in CN101017128A, a four-electrode probe electrochemical noise device is adopted to collect noise signals, and then statistical calculation and cluster analysis are carried out on data through software to judge the degree of local corrosion. However, no method for monitoring stress corrosion cracking has been reported.
Disclosure of Invention
The invention aims to overcome the defects of the oil casing steel stress corrosion cracking monitoring technology and provides a nondestructive monitoring method for stress corrosion cracking, which is used for evaluating the stress corrosion cracking, namely SCC sensitivity, of materials. The aim of the invention is achieved by the following technical scheme.
The stress corrosion cracking on-line monitoring device based on electrochemical noise comprises a reference electrode RE, an SCC_WE2 test electrode, an SCC_WE1 test electrode, a rubber sealing ring, a jacketed electrolytic cell, a stress ring, a tension meter, a zero-resistance ammeter, a potential follower and a signal acquisition and analysis unit;
the jacketed electrolytic cell is placed in the stress ring and connected with the tension meter;
The SCC_WE1 test electrode and the SCC_WE2 test electrode are round bar-shaped metal samples with the same material, the SCC_WE1 test electrode is arranged in the center of the jacketed electrolytic cell, four or six SCC_WE2 test electrodes are symmetrically distributed on the periphery of the SCC_WE1 test electrode, the surface of the SCC_WE1 test electrode is uniformly polarized, and the noise capturing accuracy is improved; the SCC_WE1 test electrode and the SCC_WE2 test electrode are sleeved with rubber rings at the bottoms and then are installed in a jacketed electrolytic cell;
The zero-resistance ammeter is respectively connected with the SCC_WE1 test electrode and the SCC_WE2 test electrode, and amplifies noise currents from the SCC_WE1 test electrode and the SCC_WE2 test electrode through the precision current-voltage conversion circuit;
The potential follower is respectively connected with the SCC_WE1 test electrode and the reference electrode RE, and the potential follower carries out impedance change on a noise voltage signal from the SCC_WE1 test electrode relative to the reference electrode RE through a high-resistance voltage follower circuit;
The zero-resistance ammeter and the potential follower are connected with the signal acquisition and analysis unit, and all signals are sent to the signal acquisition and analysis unit.
The signal acquisition and analysis unit mainly comprises two multichannel low-pass filters, two 24bit A/D converters, a display/keyboard interface, a singlechip, a USB communication port and a Flash ROM nonvolatile memory;
The two multichannel low-pass filters are respectively connected with the zero-resistance ammeter and the potential follower, each multichannel low-pass filter is respectively connected with one A/D converter, and the two 24bit A/D converters are both connected with the singlechip; the singlechip is also connected with a US communication port, a nonvolatile memory and a display/keyboard interface.
The current noise from the zero-resistance ammeter and the voltage noise signal from the potential follower are firstly subjected to power frequency interference and aliasing noise elimination through a low-pass filter of 10 Hz-20 HzLPF, and then are sent to an A/D converter, the AD converter further eliminates power frequency and electromagnetic interference by adopting a sigma-delta integral/differential principle, all digital signals are sent to a singlechip for data analysis, noise characteristic parameters in the SCC process are calculated, and test results are stored in a nonvolatile memory, or are uploaded to a PC (personal computer) through a US (universal serial bus) communication port, and can also be interacted with a user through a display/keyboard interface.
The electrochemical noise analysis method of the invention is carried out according to the following steps:
(1) Collecting potential and current noise signals in the stress corrosion cracking process by using the electrochemical noise monitoring device, and calculating parameters of the service life (L c), the amplitude (A c) and the integral electric quantity (q c) of a noise peak by using an analysis program compiled by matlab;
(2) Classifying electrochemical noise peaks with different parameter values, wherein different corrosion events are accompanied by different types of current/potential noise peaks, the noise peaks with short service life (3-5 s), low amplitude (0.01-1 mu A) and small integral electric quantity (0.1-1.0 mu C) represent the occurrence of spinodal state pitting events, and SCC crack events can cause strong current potential noise peaks with long service life (> 30 s), high amplitude (> 1 mu A) and large integral electric quantity (> 30 mu C);
(3) Statistical analysis is carried out on metastable state pitting peaks generated by pitting events, the speed of local corrosion rate is judged according to the nucleation rate l of the metastable state pitting peaks, and the statistical analysis is compared with a noise resistor R n obtained by a potential current standard deviation value, so that the consistency of results can be found; the growth time of the single crack can be obtained through analyzing the service life of the noise peak of the crack event, the growth length of the single crack can be obtained through integrating the electric quantity value, and the stress corrosion cracking sensitivity of the single crack is compared according to the length of the growth time and the length of the crack;
(4) The fixed-length time is taken as a statistical window, and the nucleation rate l and the average integral electric quantity of the electrochemical noise signals are calculated Statistical analysis was performed according to the sum of lThe time-dependent profile can determine the time points of the SCC crack initiation phase, crack propagation phase and rapid cracking phase.
The beneficial effects of the invention are as follows:
(1) The adoption of the high-precision noise signal acquisition and analysis unit circuit can sensitively capture weak potential and current noise signals in the stress corrosion cracking process;
(2) According to the electrochemical noise statistical analysis result, not only the degree and the category of local corrosion can be judged, but also the germination and growth rate of SCC cracks can be calculated, and an online monitoring method is provided for SCC early diagnosis.
Drawings
FIG. 1 is a schematic diagram of a testing apparatus according to the present invention;
FIG. 2 is a circuit diagram of the electrochemical noise signal acquisition and analysis of the present invention;
FIG. 3 is a plot of tensile curve and electrochemical noise for example P110 steel in an annular solution containing CO 2: i: elastic stage, II: yield stage, III: hardening stage, IV: a necking stage;
FIG. 4 is an electrochemical noise signature of the crack initiation stage during the SCC test of the examples;
FIG. 5 is an electrochemical noise signature of the crack growth process during the SCC test of the examples;
FIG. 6 is a plot of nucleation rate (λ) and average integrated power (q c) of electrochemical noise during the SCC test of the example;
FIG. 7 is a cross-sectional profile of the SCC test electrode after the end of the example test.
Detailed Description
The invention and its technical solutions will be further described with reference to the accompanying drawings and specific examples.
As shown in fig. 1, the SCC on-line monitoring device based on electrochemical noise comprises a reference electrode RE 1, an scc_we2 test electrode 2, an scc_we1 test electrode 3, a rubber sealing ring 4, a jacketed electrolytic cell 5, a stress ring 6, a tension meter 7, a zero resistance ammeter 8, a potential follower 9 and a signal acquisition and analysis unit 10;
The jacketed electrolytic cell 5 is placed in the stress ring 6 and is connected with the tension meter 7;
The SCC_WE1 test electrode 3 and the SCC_WE2 test electrode 2 are round bar-shaped metal samples made of the same material, the SCC_WE1 test electrode 3 is arranged in the jacket electrolytic cell 5, four or six SCC_WE2 test electrodes 2 are symmetrically distributed around the SCC_WE1 test electrode 3, the surface of the SCC_WE1 test electrode 3 is guaranteed to be uniformly polarized, and the noise capturing accuracy is improved; the reference electrode RE1 can be a saturated calomel electrode, an Ag/AgCl electrode or a mercurous sulfate electrode, the SCC_WE1 test electrode 3 and the SCC_WE2 test electrode 2 are made of P110 oil pipe steel materials, are processed into round bar-shaped samples according to the GB/T15970-2017 standard, and are sleeved with a rubber ring 4 at the bottom and then are installed in the jacket electrolytic cell 5. Four scc_we2 test electrodes 2 were selected in this example.
The zero-resistance ammeter 8 is connected with the SCC_WE1 test electrode 3 and the SCC_WE2 test electrode 2 respectively, and the zero-resistance ammeter 8 amplifies noise currents from the SCC_WE1 test electrode 3 and the SCC_WE2 test electrode 2 through a precise current-voltage conversion circuit;
the potential follower 9 is respectively connected with the SCC_WE1 test electrode 3 and the reference electrode RE1, and the potential follower 9 carries out impedance change on a noise voltage signal from the SCC_WE1 test electrode 3 relative to the reference electrode RE1 through a high-resistance voltage follower circuit;
The zero-resistance ammeter 8 and the potential follower 9 are connected with the signal acquisition and analysis unit 10, and all signals are sent to the signal acquisition and analysis unit 10.
The temperature of the jacket electrolytic cell 5 is controlled at 35-60 ℃ to simulate the temperature of the underground annular solution, and the jacket electrolytic cell 5 is filled with simulated annular protection liquid, wherein the simulated annular protection liquid comprises 0.2mol/L Na 2CO3、0.5mol/L NaHCO3, 0.5mol/L NaCl and 100mg/L Na 2 S. The SCC_WE1 test electrode 3 adopts organic silicon resin to protect the contact surface of the solution except the surface of a round bar with the length of 5-10 mm in the middle, so as to prevent crevice corrosion. The SCC_WE1 test electrodes 3 are subjected to constant strain or slow strain stretching through a stress ring 6 or a slow stretching tester, 4 SCC_WE2 test electrodes 2 are distributed around the WE13 as counter electrodes, no stress is applied to the counter electrodes, electrochemical potential and current noise are respectively connected to the input ends of the potential follower 9 and the zero-resistance ammeter 8, the feedback resistance Rc of the zero-resistance ammeter 8 is adaptively controlled, and the Rc resistance is automatically switched according to the current.
As shown in fig. 2, the signal acquisition and analysis unit 10 mainly includes two multi-channel low-pass filters 11, two 24bit a/D converters 12, a display/keyboard interface 13, a single-chip microcomputer 14, a USB communication port 15, and a Flash ROM nonvolatile memory 16;
The two multichannel low-pass filters 11 are respectively connected with the zero-resistance ammeter 8 and the potential follower 9, each multichannel low-pass filter 11 is respectively connected with one A/D converter 12, and the two A/D converters 12 are both connected with the singlechip 14; the singlechip 14 is also connected with a US communication port 15, a nonvolatile memory 16 and a display/keyboard interface 13.
Firstly, the potential and current noise are respectively sent to two ADS1210 converters 12 for analog-to-digital conversion after passing through a 20Hz cut-off frequency multichannel low-pass filter 11 for anti-aliasing denoising, and the sampling frequency is 5Hz. The digitized potential and current noise signals are read into the memory by the 32-bit STM32F407 singlechip 14, the noise is statistically analyzed by adopting the following algorithm, and the measured result is stored in the 16M byte Flash ROM nonvolatile memory 16, or is uploaded to the PC computer from the USB communication port 15 by means of interaction of the display/keyboard interface 13 for further analysis.
The SCC monitoring in the invention adopts an electrochemical noise statistical analysis algorithm to calculate the metastable state pitting corrosion and crack initiation and growth length of the SCC_WE1 test electrode 3, and judges the progressive stage and severity of the SCC according to the service life (L c), the amplitude (A c) and the integral electric quantity (q c) of the current and potential noise peak.
And adopting Matlab 2012 to program, and automatically counting the frequency, service life, amplitude, integral electric quantity, nucleation rate, noise resistance and average integral electric quantity of an electrochemical noise peak, wherein the calculation formulas of the integral electric quantity and the average integral electric quantity are shown as formulas (1) and (3), and the calculation formula of the crack length is shown as formula (2).
The noise peak lifetime (L c) is the difference between the start and end times of the noise peak, and the peak amplitude (a c) is the difference between the current values at the highest and lowest positions of the noise peak. The calculation formula of the electrochemical noise peak integral electric quantity (q c) is as follows:
In the formula (1), lambda is nucleation rate, which represents the number of noise peaks in unit time, and the unit is s -1; t is the noise data measurement duration, T n、t'n is the start and end times of the nth noise peak, i n (T) is the function of current corresponding to the nth noise peak and time, and i b is the baseline current of the noise peak, respectively.
The SCC crack growth speed can be judged according to the nucleation rate (lambda), and the larger l is, the more cracks are generated in unit time, which indicates that the SCC process is faster. The single crack growth length can be calculated according to the integral electric quantity (q c) value of the single noise peak, and the metal dissolution volume corresponding to q c can be calculated according to Faraday's law, and the greater the q c is, the greater the corrosion degree is. Assuming that the shape of the front end of the crack is semicircular and the width w of the crack is 500nm, the corresponding crack growth depth l crack can be estimated from q c, and the calculation formula is as follows:
M, ρ and z in the formula (2) are the molar mass (g/mol), density (g/cm 3) and valence number of the metal, respectively; f is Faraday constant; q c is the integrated power.
Fig. 3 shows the potential and current noise data emitted by the scc_we1 test electrode 3 and the scc_we2 test electrode 2 monitored simultaneously in a slow strain tensile test. According to the stretching curve form of the SCC test electrode, the stretching test process can be divided into four deformation stages of an elastic stage, a yield stage, a hardening stage and a necking stage, and the electrochemical noise characteristics of each stage are obviously different, so that the stage of the SCC test electrode can be judged according to the noise peak characteristics.
FIG. 4 is an electrochemical noise curve of an SCC test electrode in simulated annulus protection fluid at a pre-crack initiation stage (mainly metastable pitting event). The occurrence of pitting events is accompanied by a plurality of metastable pitting corrosion peaks with short service life, low amplitude and small integral electric quantity, and the typical metastable pitting corrosion noise peaks are formed on an electrochemical noise curve by the continuous rapid dissolution-repassivation-dissolution processes because electrode potential is negatively shifted due to the occurrence of metastable pitting corrosion after the surface passivation film of the SCC_WE1 test electrode 3 breaks down, and then the potential is positively shifted again by repassivation of the etching point.
FIG. 5 shows electrochemical noise peaks corresponding to the growth of cracks on the surface of the SCC_WE1 test electrode 3 during the slow tensile test, and the crack length l crack was estimated to be 164. Mu.m, from the formula (2). FIG. 7 shows a cross-sectional profile of the 3-port of the SCC_WE1 test electrode, from which FIG. 7 the crack growth depth of about 123 μm was measured, which is closer to the calculation result of equation (2). The larger the crack event noise peak q c, the longer the single crack growth depth l crack, the higher the SCC sensitivity of the metal.
Then taking 1h as unit time, the nucleation rate l and the average integral electric quantity of the whole electrochemical noise spectrumA statistical analysis is performed and the data is collected,The calculation formula of (2) is as follows:
FIG. 6 shows the sum of Curves of variation with SCC test time, l andWhen the values are smaller, the surface of the metal sample has no obvious local corrosion, and l is gradually increasedA slight increase indicates that metastable pitting of the metal surface occurs which promotes microcracking into the initiation phase. When (when)When the trend of obvious increase is shown, the microcrack forms a steady-state crack under the action of tensile stress, an electrochemical noise peak is mainly a long crack event peak with longer service life and larger integral electric quantity, and the SCC electrode enters a crack steady-state growth stage. When (when)When the crack starts to decline, the crack expansion tends to be stable, the crack tip is in a high-activity dissolution state, the crack tip is not easy to passivate, the noise current is no longer declining, the noise peak-shaped nuclear frequency is declining, and the SCC electrode enters the crack expansion stage. When l andGradually tending to 0, the noise peak was shown to be completely disappeared, and the crack tip of the SCC test electrode was in a rapid anodic dissolution state and entered into a rapid tearing stage.
Claims (4)
1. The stress corrosion cracking on-line monitoring device based on electrochemical noise is characterized by comprising a reference electrode RE (1), an SCC_WE2 test electrode (2), an SCC_WE1 test electrode (3), a rubber sealing ring (4), a jacketed electrolytic cell (5), a stress ring (6), a tension meter (7), a zero resistance ammeter (8), a potential follower (9) and a signal acquisition and analysis unit (10);
The jacketed electrolytic cell (5) is arranged in the stress ring (6) and is connected with the tension meter (7);
the SCC_WE1 test electrode (3) and the SCC_WE2 test electrode (2) are round bar-shaped metal samples made of the same material, the SCC_WE1 test electrode (3) is positioned in the center of the jacketed electrolytic cell (5), and four or six SCC_WE2 test electrodes (2) are symmetrically distributed around the SCC WE1 test electrode (3); the SCC_WE1 test electrode (3) and the SCC_WE2 test electrode (2) are sleeved with rubber sealing rings (4) at the bottoms and then are installed in a jacketed electrolytic cell (5);
The temperature of the jacket electrolytic cell (5) is controlled at 35-60 ℃ so as to simulate the temperature of the underground annular solution, and the jacket electrolytic cell (5) is filled with simulated annular protection liquid, wherein the simulated annular protection liquid comprises 0.2mol/L Na 2CO3、0.5mol/L NaHCO3, 0.5mol/L NaCl and 100mg/L Na 2 S; the SCC_WE1 test electrode (3) adopts organic silicon resin to protect the contact surfaces of the rest and the solution except the exposed surface of the round bar with the length of 5-10 mm in the middle;
The zero-resistance ammeter (8) is respectively connected with the SCC_WE1 test electrode (3) and the SCC_WE2 test electrode (2), and the zero-resistance ammeter (8) amplifies noise currents from the SCC_WE1 test electrode (3) and the SCC_WE2 test electrode (2) through the precise current-voltage conversion circuit;
The potential follower (9) is respectively connected with the SCC_WE1 test electrode (3) and the reference electrode RE (1), and the potential follower (9) carries out impedance change on a noise voltage signal from the SCC_WE1 test electrode (3) relative to the reference electrode RE (1) through a high-resistance voltage follower circuit;
the zero-resistance ammeter (8) and the potential follower (9) are connected with the signal acquisition and analysis unit (10), and all signals are sent to the signal acquisition and analysis unit (10).
2. The electrochemical noise-based stress corrosion cracking on-line monitoring device according to claim 1, wherein the signal acquisition and analysis unit (10) mainly comprises two multichannel low-pass filters (11), two a/D converters (12), a display/keyboard interface (13), a singlechip (14), a USB communication port (15) and a Flash ROM nonvolatile memory (16);
The two multichannel low-pass filters (11) are respectively connected with the zero-resistance ammeter (8) and the potential follower (9), each multichannel low-pass filter (11) is respectively connected with one A/D converter (12), and the two A/D converters (12) are both connected with the singlechip (14); the singlechip (14) is also connected with a USB communication port (15), a nonvolatile memory (16) and a display/keyboard interface (13).
3. An electrochemical noise analysis method, characterized by comprising the steps of:
(1) Collecting potential and current noise signals in the stress corrosion cracking process by using the electrochemical noise-based stress corrosion cracking online monitoring device according to claim 1 or 2, and calculating characteristic parameters of the service life L c, the amplitude A c and the integral electric quantity q c of a noise peak by using an analysis program compiled by Matlab;
(2) Classifying electrochemical noise peaks with different characteristic parameter values, wherein different corrosion events are accompanied by different types of current/potential noise peaks, including metastable state pitting events and crack events;
(3) Statistical analysis is carried out on metastable state pitting peaks generated by pitting events, the speed of local corrosion rate is judged according to the nucleation rate l of the metastable state pitting peaks, and the statistical analysis is compared with a noise resistor R n obtained by a potential current standard deviation value, so that the consistency of results can be found; the growth time of the single crack can be obtained through analyzing the service life of the noise peak of the crack event, the growth length of the single crack can be obtained through integrating the electric quantity value, and the stress corrosion cracking sensitivity of the single crack is compared according to the length of the growth time and the length of the crack;
(4) The fixed-length time is taken as a statistical window, and the nucleation rate l and the average integral electric quantity of the electrochemical noise signals are calculated Statistical analysis was performed according to the sum of lThe time-dependent profile can determine the time points of the SCC crack initiation phase, crack propagation phase and rapid cracking phase.
4. The method of claim 3, wherein in step (2), the different corrosion event differentiation criteria are: noise peaks with a 3-5 s short life, 0.01-1 muA low amplitude and 0.1-1.0 mC low integrated electric quantity represent occurrence of a spinodal pitting event, and strong current potential noise peaks with a long life of >30s, a1 muA high amplitude and >30 mC high integrated electric quantity represent occurrence of a crack event.
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