A method for studying the pitting corrosion resistance of stainless steel

Abstract

The present invention discloses a method for studying the pitting corrosion resistance of stainless steel materials, namely preparing a working electrode having an activated anode and a passivated cathode on both sides, constructing a closed pitting pit system that can develop automatically, then immersing the working electrode in a NaCl solution, and studying the development law of the stainless steel pitting pit. The present invention is to coat a coating layer on the surface of an electrode sample, drill a hole, then remove the coating layer of the working electrode part, expose the passivated metal (cathode A-c) and perform constant potential polarization, then coat the cathode A-c and perform constant potential polarization to form micro pitting pits (anode A-a), finally remove the cathode A-c coating layer, inject an acidic corrosive solution into the pitting pit groove of the A-a anode and immerse and develop, so that the pitting pit development process formed is more consistent with the real pitting pit, thereby obtaining the pitting pit closest to the real 2Cr13 material, which is beneficial to studying the pitting development law of 2Cr13 martensitic stainless steel in a real corrosive environment.

Description

Technical Field

[0001] The invention relates to a method for studying the pitting corrosion resistance of a stainless steel material, in particular to a method for simulating the development of a single pit to study the pitting corrosion resistance of a stainless steel material, and belongs to the field of metal corrosion. Background Art

[0002] Pitting corrosion is a form of corrosion that forms small holes or rust spots locally on the surface of stainless steel. It is easy to occur in corrosive environments and can cause extremely harmful accidents through continuous development, resulting in losses to the national economy. The characteristic of pitting corrosion is the randomness of the pitting pits generated on the outer surface of stainless steel, which is specifically manifested in: the randomness of the time when pitting corrosion occurs, the randomness of the location of pitting corrosion, the randomness of the number of pitting corrosion, and the randomness of the transition from metastable pitting corrosion to steady-state corrosion. Therefore, the study of pitting corrosion has always been a difficult point in the field of material corrosion. Currently, there is an urgent need for a new method to study the behavior of pitting corrosion in order to effectively study the development laws of pitting corrosion. This is of great significance for preventing the occurrence of pitting corrosion and reducing the impact of pitting corrosion on the operating safety of metal equipment.

[0003] Common pitting corrosion research methods are mainly divided into two categories. The first category is the study of pitting corrosion sensitivity and factors affecting pitting corrosion, mainly including chemical immersion method, polarization curve measurement method, electrochemical noise method, etc. This type of method generates a large number of pitting pits on the surface of the material, and there is no specific pitting corrosion research object. The second category is the study of the pitting corrosion development mechanism, which is generally the study of the development of a single pitting corrosion pit. The pitting corrosion generated by this type of method is divorced from the actual environment and is quite different from the development process of the real pitting corrosion pit. Currently, there is a lack of a single pitting corrosion research method that is similar to the development process of the real pitting corrosion pit and has a specific pitting corrosion research object. Moreover, there has been no report on the pitting corrosion resistance analysis method for materials such as 2Cr13 martensitic stainless steel (using the pitting corrosion resistance analysis method of a single pitting corrosion pit to study the development mechanism of the material). Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for studying the pitting corrosion resistance of stainless steel materials. This method is a single pitting pit research method with a specific pitting research object, which is similar to the development process of real pitting pits.

[0005] In order to achieve the above object, the technical solution of the present invention is:

[0006] A method for studying the pitting corrosion resistance of stainless steel materials involves preparing a working electrode with an active anode and passive cathodes on both sides to construct a closed pitting system that can develop autocatalytically. The working electrode is then immersed in a NaCl solution, and the development pattern of the stainless steel pitting is studied according to different immersion times. The specific method is:

[0007] (1) First, a stainless steel electrochemical sample is prepared. Liquid epoxy resin is coated on the working surface of the electrochemical sample, and after solidification, it is polished. Then, a hole is drilled on the surface of the sample to the metal layer on the working surface of the sample to prepare a working electrode A.

[0008] (2) For the working electrode A, with the long side as the reference and the drilled hole as the center, remove a portion of the coating layer on both sides to expose a portion of the passivated metal. The exposed passivated metal portion is marked as Ac, i.e., the cathode. Then, the working electrode A is placed in a 3.5% NaCl solution and polarized at a constant potential to obtain a stable passivation layer at the Ac portion.

[0009] (3) Coat the Ac site with vulcanized rubber, then place the working electrode A in a 3.5% NaCl solution and polarize at a constant potential to rupture the passivation film at the drilled site of the working electrode A, exposing the activated metal and forming micro-pitting pits. This site is marked as Aa, i.e., the anode.

[0010] (4) The vulcanized rubber at the Ac site was removed, and then the working electrode A was immersed in a 3.5% NaCl solution again, and the corrosive solution was injected into the pitting groove of the Aa anode;

[0011] (5) Study the development law of stainless steel pitting according to different immersion times.

[0012] Furthermore, the liquid epoxy resin in step (1) comprises epoxy resin, diethyl phthalate and ethylenediamine in a mass ratio of 100:7:10.

[0013] Furthermore, in step (1), the epoxy resin is polished to a thickness of 400-600 μm; and the drilling diameter is 200-1200 μm, preferably 500-1000 μm.

[0014] Furthermore, the method for determining the potential of the constant potential polarization in step (2) is to first prepare a stainless steel wire electrode and perform potentiodynamic polarization to obtain a polarization curve, then determine the self-corrosion potential Ecorr and the pitting potential Eb according to the polarization curve, and finally calculate the constant potential polarization potential Ec according to the formula: Ec = Ecorr + (0.6 ± 0.2) × (Eb - Ecorr) V, and the polarization time is 5 min.

[0015] Furthermore, the method for determining the potential of the constant potential polarization in step (3) is to first prepare a stainless steel wire electrode and perform potentiodynamic polarization to obtain a polarization curve, then determine the pitting potential Eb based on the polarization curve, and finally calculate the potential Ea of the constant potential polarization according to the formula: Ea = Eb + (1 ± 0.2) V, and the polarization time is 5 s.

[0016] Furthermore, the method for preparing the corrosive solution in step (4) is as follows: first, a stainless steel wire electrode is prepared, and then the wire electrode is subjected to potentiodynamic polarization to obtain a polarization curve, and the pitting potential Eb of the wire electrode under the environment is determined, and then the lower limit of the potential of the constant potential polarization is determined based on the pitting potential, and finally, the constant potential polarization is used to prepare a one-dimensional pitting pit and obtain an acidic corrosive solution. The specific method is:

[0017] (1) Take a wire-shaped stainless steel sample, use the top surface as the working surface, weld a wire to the tail of the sample, and then embed it into a PVC tube with insulating epoxy resin to obtain a wire-shaped electrode;

[0018] (2) The working surface of the wire electrode was polished and cleaned, and the experiment was carried out in a five-necked flask using a traditional three-electrode system. A saturated calomel electrode was used as the reference electrode, and a platinum electrode was used as the auxiliary electrode. The Gamry electrochemical workstation was connected to the wire electrode, and potentiodynamic polarization was performed on the wire electrode to obtain a potentiodynamic polarization curve. The pitting potential Eb of the wire electrode under this environment was determined. The potentiostatic polarization potential Ea was then calculated according to the formula: Ea = Eb + (1 ± 0.2) V.

[0019] (3) A wire electrode was prepared by the same method and subjected to constant potential polarization to obtain a one-dimensional pitting pit in which the bottom of the corrosion pit was formed by the overall corrosion of the wire electrode and the epoxy resin formed the pit wall;

[0020] (4) After the constant potential polarization is completed, the position of the wire electrode and the experimental environment are kept unchanged, and the immersion is continued; after a long period of immersion, the one-dimensional pitting pit continuously corrodes downward, and acidic corrosive liquid and corrosion products are obtained in the one-dimensional pitting pit.

[0021] Furthermore, the potentiodynamic polarization was performed at a scan rate of 0.5 mV / s, the initial potential of the potentiodynamic polarization was -1.0 V (SCE), and the termination potential was scanned to the over-passivation region.

[0022] Furthermore, the stainless steel material is 2Cr13 martensitic stainless steel.

[0023] The process diagram of the method for studying the pitting corrosion resistance of stainless steel materials of the present invention is as follows Figure 1 shown.

[0024] Beneficial effects of the present invention:

[0025] The real pitting development environment includes: temperature, the solution environment in which the material itself is located when the pitting pit develops, the acidic solution in the pitting pit, the activated metal pit bottom and pit wall in the pitting pit, and the pit cover on the pitting pit that affects ion diffusion. Therefore, the present invention adopts a specific pitting research method, that is, after coating a coating layer on the surface of the electrode sample, drilling a hole, then removing the coating layer of the working electrode part, exposing part of the passivated metal (cathode Ac) and performing constant potential polarization, then coating the cathode Ac and performing constant potential polarization to form micro pitting (anode Aa), finally removing the cathode Ac coating layer, injecting acidic corrosive solution into the pit groove of the Aa anode pit and soaking it for development, so that the pitting pit development process formed is more consistent with the real pitting pit, thereby obtaining the pitting pit closest to the real 2Cr13 material, which is conducive to studying the pitting development law of 2Cr13 martensitic stainless steel in a real corrosion environment, while most of the existing pitting research methods cannot study the development of pitting in a real natural environment.

[0026] The present invention can produce pitting pits that closely resemble those formed in a real corrosion environment on 2Cr13 martensitic stainless steel. The pitting pits are characterized by a small opening and a large interior. The coating layer above the pitting pits acts as a pit cover, performing the same function as a real pit cover.

[0027] The present invention prepares a working electrode having both an activated anode and a passivated cathode on both sides, and constructs a system B of a closed pitting pit system that can develop automatically catalytically (a simulated pitting pit with a large cathode and a small anode for closed autocatalysis). During the immersion process, the metal under the coating layer continuously corrodes around the drilled hole, and the effect of the pitting pit cover formed by the coating layer becomes more and more obvious, which is very close to the characteristics of a real pitting pit.

[0028] The system of the present invention is a controllable single occluded pitting pit, which can obtain a pitting pit morphology close to that of real 2Cr13 stainless steel and can be used to study the pitting development law of 2Cr13 martensitic stainless steel in this environment to evaluate the corrosion resistance of the stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the process of the present invention.

[0030] Figure 2 A photograph of a wire electrode.

[0031] Figure 3 is the potentiodynamic polarization curve of the wire electrode.

[0032] Figure 4 These are the composition analysis results of the acidic corrosion solution obtained by immersing the one-dimensional pit and the bottom salt film obtained by drying the corrosion products.

[0033] Figure 5 Schematic diagram of the process of preparing acidic corrosive solution and corrosion products for wire electrodes.

[0034] Figure 6 Schematic diagram of preparing pits according to the present invention.

[0035] Figure 7 This is a scanning electron microscope photograph of the longitudinal section along the center of the pitting pit prepared in the present invention (with the coating layer removed).

[0036] Figure 8 This is a scanning electron microscope photo of the longitudinal section of a real pit along the center of the pit.

[0037] Figure 9 The effects of two pit opening sizes on pitting development. DETAILED DESCRIPTION

[0038] The specific embodiments of the present invention are further described in detail below with reference to the examples.

[0039] Example 1. Preparation of acidic corrosive solution

[0040] 1.1 Preparation process of experimental instruments and wire electrodes:

[0041] The instrument used to monitor the potential of the sample in this experiment was a Gamry electrochemical workstation from the United States. The experiment was conducted in a five-necked flask using a traditional three-electrode system, with a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the auxiliary electrode. The sample was in the form of a wire made of 2Cr13, with dimensions of 1mm×1mm×15mm and a working surface of 1mm×1mm. A wire was soldered to the tail of the wire sample, and then it was embedded in a PVC tube with insulating liquid epoxy resin. After cooling and solidification, a wire electrode (such as Figure 2 When preparing liquid epoxy resin, the raw material ratio is: epoxy resin: diethyl phthalate: ethylenediamine = 100:7:10 (mass ratio). Stirring should be performed in one direction at a constant speed to prevent excessive bubbles from forming in the prepared liquid epoxy resin, which could affect observation of the pitting cross-section specimen.

[0042] 1.2 The potential required for measuring the constant potential polarization of the wire electrode into a one-dimensional pit:

[0043] (1) Prepare a 3.5% NaCl solution in a wide-mouth bottle using laboratory chemicals and deionized water.

[0044] The sample was polished before the test: the working surface of the wire electrode was polished in sequence with 180, 360, 600, and 800 sandpaper on a metallographic pre-grinder, and the prepared sample was gently rinsed with alcohol and dried.

[0045] (2) Pour a 3.5% NaCl solution into a five-necked flask, place a saturated calomel electrode and a platinum electrode in appropriate positions in the flask, heat it in a water bath and maintain the temperature at 30°C, and place the wire electrode into the solution.

[0046] (3) Connect the Gamry electrochemical workstation to the wire electrode and conduct potentiodynamic polarization on the wire electrode to obtain the potentiodynamic polarization curve. The entire curve can be divided into four regions: activation dissolution region, activation-passivation transition region, passivation region, and over-passivation region.

[0047] Before performing potentiodynamic polarization on the wire electrode, soak it in a 3.5% NaCl solution for approximately 30 minutes. During this soaking process, monitor the open circuit potential (OCP) of the wire electrode. During this process, the potential of the wire electrode will gradually change from a low potential to a positive value, indicating that a film is gradually forming on the electrode surface. This process ensures the accuracy of the potentiodynamic polarization curve test results. Once the open circuit potential stabilizes, begin the potentiodynamic polarization curve measurement.

[0048] The potentiodynamic polarization curve test adopted a scanning rate of 0.5 mV / s, the initial potential of the potentiodynamic polarization was -1.0 V (SCE), and the termination potential was -0.1 V (SCE).

[0049] Potentiodynamic polarization curves Figure 3 As shown in the figure, the self-corrosion potential of the wire electrode is Ecorr = -0.68V (SCE). In the potential range of -0.5V to -0.2V, the current density fluctuates violently, indicating the rupture and repassivation of the passivation film on the surface of the stainless steel wire electrode. When the potential exceeds -0.2V (SCE), the current density gradually increases and no longer fluctuates, indicating that the passivation film on the electrode surface is gradually and irreversibly ruptured. According to the GB / T17899-1999 stainless steel pitting potential measurement method, the anode current density is set to 100μA / cm 2 The most positive potential value is used as the pitting potential Eb. The pitting potential of 2Cr13 is Eb = -0.18V (SCE). That is, if the pitting potential is exceeded, pitting corrosion will begin to occur on the sample surface. To ensure that the passive film in the anode region is irreversibly ruptured during constant potential polarization and that the reaction is not too violent and difficult to control, the lower limit of the potential Ea for constant potential polarization of the anode cannot be lower than the pitting potential Eb. This potential is usually selected based on the pitting potential plus approximately (1±0.2)V, that is, Ea = Eb + (1±0.2)V. Therefore, the potential Ea for constant potential polarization of the anode in the present invention is selected at +0.82V (SCE).

[0050] According to the polarization curve measured by dynamic potentiodynamic polarization, the potential Ec of the cathode for constant potential polarization, i.e., the passivation potential, is also obtained. The passivation potential is selected within the passivation region of the polarization curve, generally about Ecorr + (0.6 ± 0.2) × (Eb - Ecorr), i.e., Ec = Ecorr + (0.6 ± 0.2) × (Eb - Ecorr) V. The Ec range is -0.48 to -0.28 V (SCE). Therefore, the passivation potential of the present invention is selected at -0.4 V (SCE).

[0051] 1.3 Obtaining acidic corrosive solution

[0052] The sample used is still the above-mentioned 2Cr13 wire electrode of the same material.

[0053] (1) Grinding of the sample before the test: Grind the working surface of the wire electrode with sandpaper of 180, 360, 600, and 800 in sequence on a metallographic pre-grinding machine, and gently rinse the prepared sample with alcohol and dry it.

[0054] (2) Pour a 3.5% NaCl solution into a five-necked flask, place a saturated calomel electrode and a platinum electrode in appropriate positions in the flask, heat it in a water bath and maintain the temperature at 30°C, and place the wire electrode into the solution.

[0055] (3) Connect the Gamry electrochemical workstation to the wire electrode and perform constant potential polarization on the wire electrode at a potential of +0.82 V (SCE) for 15 s. During this polarization process, the working surface of the wire electrode was kept horizontal and facing upward to prevent the corrosion solution from overflowing during the preparation of the acidic corrosion solution and corrosion products. The resulting one-dimensional pitting pit was formed by the overall corrosion of the wire electrode forming the bottom of the corrosion pit and the epoxy resin forming the pit wall.

[0056] (4) After the constant potential polarization is completed, the position of the wire electrode is kept unchanged, and the experimental environment such as temperature and solution is kept unchanged, and the sample is immersed in this environment for 28 days. After a long period of immersion, the one-dimensional pitting pit continues to corrode downward, and acidic corrosion liquid and corrosion products are obtained in the one-dimensional pitting pit. The main components of the acidic corrosion liquid and corrosion products (a mixture of acid and corrosion products) are Fe and Cr cations produced by metal dissolution and a layer of salt film containing Cl at the bottom of the pitting pit. The pH of the acidic corrosion liquid and corrosion products is lower than the pH of the 3.5% NaCl solution in the immersion environment (such as Figure 4 shown).

[0057] Schematic diagram of the process of preparing acidic corrosive solution and corrosion products using wire electrodes Figure 5 shown.

[0058] Example 2: Method for studying the pitting corrosion resistance of stainless steel

[0059] 1. A standard electrochemical specimen was prepared using the epoxy resin encapsulation method described above for a 2Cr13 metal specimen measuring 20 mm × 10 mm × 6 mm. The working surface was a 20 mm × 10 mm cross-section. The specimen was then placed with the working surface facing upward and coated with a second coat of liquid epoxy resin to a thickness of approximately 2–3 mm. Thicker coatings are inconvenient for subsequent polishing. Thinner coatings are acceptable, but achieving a very thin thickness is difficult by hand. After the liquid epoxy resin solidifies, the epoxy coating is polished to approximately 500 μm using 1000-grit sandpaper. A 500 μm coating thickness represents an initial depth of approximately 500 μm for the simulated pitting pits at the anode. This depth serves as a barrier to the diffusion of ions within the pits and ensures sufficient acidic etching solution and corrosion products can be injected into the pits. If the remaining coating thickness is too thin, such as 100 μm, it will be difficult to inject sufficient acidic etching solution and corrosion products, resulting in difficulty in corroding the anode pits in System B. Drill a 500μm diameter circular hole in the center of the specimen. The opening of the pit affects the diffusion of ions within it. An opening that is too large can easily cause pitting to stagnate, while an opening that is too small can hinder the injection of acidic corrosive fluid and make subsequent observation difficult. Drill the hole in the center of the specimen. Avoid touching the working electrode surface during drilling; only expose the metal layer. This step yields Working Electrode A.

[0060] 2. For the working electrode A, with the long side as the reference and the drilling site as the center, remove part of the coating layer on both sides to expose part of the passivated metal. Mark the exposed passivated metal site as Ac (cathode); Figure 1 Taking the working electrode A in the experiment as an example, with the 20mm long side as the reference and the drilled hole as the center, 5mm of coating layer was removed on both sides, exposing 5mm of passivated metal on both sides. The working electrode A was placed in a 3.5% NaCl solution (30°C) and polarized at -0.4V (SCE) for 5 minutes to form a stable passivation layer at the Ac site.

[0061] 3. Use vulcanized rubber to coat the Ac site. The reason for coating is that when the anode site Aa is subjected to constant potential polarization at +0.82V (SCE), if the Ac site is not coated, it will also produce a polarization effect on the site, causing corrosion at the Ac site. After the vulcanized rubber dries, the working electrode A is subjected to constant potential polarization at +0.82V (SCE) in a 3.5% NaCl solution (30°C) for 5 seconds. This causes the passivation film at the drilled site of the working electrode to rupture, exposing the activated metal and forming micro-pitting pits. This site is recorded as Aa (anode). At this time, if the polarization time is too short, the activated metal will not be obtained, while if the time is too long, the micro-pitting pits will easily become too deep. The purpose of the present invention is to obtain pitting pits that are authentic to natural conditions, rather than pitting pits formed by polarization. If the polarization time is too long, the pitting pits obtained will deviate from the morphology of the actual pitting pits. The polarization time of the present invention ensures that the opening size of the micro-pitting pits obtained is approximately equal to the size of the small hole drilled in the coating layer. The diameter of the small hole is the opening size of the pitting pit of system B during immersion autocatalysis.

[0062] 4. Remove the vulcanized rubber at the Ac site and expose the Ac site again, so that the cathode and anode are in contact with the solution at the same time in the 3.5% NaCl solution to form a system. Then place the working electrode A in the 3.5% NaCl solution again, with the working surface of the electrode facing upward to prevent the solution injected into the pitting groove of the Aa anode from overflowing, and inject the acidic corrosive solution into the pitting groove of the Aa anode. At this time, the working electrode A has both an activated anode Aa and passivated cathodes Ac on both sides, which meet the conditions for the self-catalytic development of the occluded pits: the exposed activated metal Aa in the pitting pit, the acidic corrosive solution in the pitting pit that keeps the metal in the pitting pit always in the activated state, the epoxy resin layer around Aa constitutes a diffusion barrier to hinder the migration of ions in the pitting pit, and the passivated metal around the pitting pit serves as the cathode Ac.

[0063] The system at this time is a closed pitting system that can develop automatically, called system B.

[0064] 5. A total of 12 independent systems B were prepared and immersed in a 3.5% NaCl solution in the same experimental environment as above. The immersion cycles were 0 days, 6 days, 12 days and 30 days (3 systems in each cycle). The pitting depth was measured after pickling, and the average value was taken as the average depth of system B immersed for n days, and the development law of the pitting of the drill hole size was obtained.

[0065] The method of the present invention can obtain pitting pits that are closest to the real 2Cr13 martensitic stainless steel material in a 3.5% mass concentration NaCl solution. The pitting pits are characterized by a small opening and a large interior. The coating layer above the pitting pit serves as a pit cover, which plays the same role as the pit cover of the real pit (such as Figure 6As shown). In the system B of the present invention (the simulated pitting pit with a large cathode and a small anode for occluded autocatalysis), during the immersion process, the metal under the coating layer continuously corrodes around the drilled hole, and the role of the pitting pit cover formed by the coating layer becomes more and more obvious, which is very close to the characteristics of the real pitting pit (as shown). Figure 7 and Figure 8 shown).

[0066] Example 3: Method for studying the pitting corrosion resistance of stainless steel

[0067] The method of this embodiment is the same as that of embodiment 2, except that the drilling size is changed to 1000 μm to obtain the pitting pit development law of another drilling size.

[0068] The comparison of the relationship between the depth of the pitting pit and time for the simulated pitting pit system B with two drilling hole sizes is shown in the figure. Figure 9 As can be seen from the figure, under the experimental conditions, the development process of the pits formed by the same material and different pit openings is very similar to that of real pits. The smaller the pit opening, the lower the pH of the solution in the pit, the more corrosive it is, and the faster the pit expands.

[0069] Note: The depth shown in the figure is the depth of the pitting pit of system B developed to that time period minus the average depth before immersion.

Claims

1. A method for studying the pitting corrosion resistance of stainless steel, characterized in that: A working electrode with an active anode and passive cathodes on both sides was prepared to construct a closed pitting system that can develop automatically. The working electrode was then immersed in a NaCl solution. The development pattern of the stainless steel pitting was studied according to different immersion times. The specific method is as follows: (1) First, a stainless steel electrochemical sample was prepared. Liquid epoxy resin was coated on the working surface of the electrochemical sample, and after solidification, it was polished. Then, a hole was drilled on the surface of the sample to the metal layer on the working surface of the sample to prepare a working electrode A. (2) For the working electrode A, with the long side as the reference and the drilling site as the center, remove part of the coating layer on both sides to expose part of the passivated metal. The exposed passivated metal site is marked as Ac, i.e., the cathode. Then, place the working electrode A in a NaCl solution and polarize at a constant potential to obtain a stable passivation layer at the Ac site. (3) Use vulcanized rubber to coat the Ac part, then place the working electrode A in a NaCl solution and polarize it at a constant potential to break the passivation film at the drilled part of the working electrode A, exposing the activated metal and forming micro-pitting pits. This part is marked as Aa, i.e., the anode. (4) Remove the vulcanized rubber at the Ac site, then immerse the working electrode A in the NaCl solution again, and inject the corrosive solution into the pitting groove of the Aa anode; (5) Study the development law of stainless steel pitting according to different immersion times.

2. The method according to claim 1, characterized in that The mass concentration of NaCl solution is 3.5%.

3. The method according to claim 1, characterized in that The liquid epoxy resin in step (1) comprises epoxy resin, diethyl phthalate and ethylenediamine in a mass ratio of 100:7:

10.

4. The method according to claim 1, wherein In step (1), the epoxy resin is polished to a thickness of 400-600 μm; the drilling diameter is 200-1200 μm.

5. The method according to claim 1, wherein The method for determining the potential of the constant potential polarization in step (2) is to first prepare a stainless steel wire electrode and perform potentiodynamic polarization to obtain a polarization curve, then determine the self-corrosion potential Ecorr and the pitting potential Eb based on the polarization curve, and finally calculate the potential Ec of the constant potential polarization according to the formula: Ec = Ecorr + (0.6±0.2) × (Eb - Ecorr) V, and the polarization time is 5 min.

6. The method according to claim 1, characterized in that The method for determining the potential of the constant potential polarization in step (3) is to first prepare a stainless steel wire electrode and perform potentiodynamic polarization to obtain a polarization curve, then determine the pitting potential Eb based on the polarization curve, and finally calculate the potential Ea of the constant potential polarization according to the formula: Ea = Eb + (1±0.2) V, and the polarization time is 5 s.

7. The method according to claim 1, characterized in that The specific method for preparing the corrosive solution in step (4) is: (1) Take a wire-shaped stainless steel sample, use the top surface as the working surface, weld a wire to the tail of the sample, and then embed it into a PVC tube with insulating epoxy resin to obtain a wire electrode; (2) The working surface of the wire electrode was polished and cleaned. The experiment was carried out in a five-necked flask using a traditional three-electrode system. A saturated calomel electrode was used as the reference electrode and a platinum electrode was used as the auxiliary electrode. The wire electrode was connected to a Gamry electrochemical workstation and potentiodynamic polarization was performed on the wire electrode to obtain a potentiodynamic polarization curve. The pitting potential Eb of the wire electrode under the current environment was determined. The potentiostatic polarization potential Ea was then calculated according to the formula: Ea = Eb + (1 ± 0.2) V. (3) The same method was used to prepare the wire electrode, and the wire electrode was subjected to constant potential polarization to obtain a one-dimensional pitting pit in which the bottom of the corrosion pit was formed by the overall corrosion of the wire electrode and the wall of the corrosion pit was formed by the epoxy resin; (4) After the constant potential polarization is completed, the position of the wire electrode and the experimental environment are kept unchanged, and the immersion is continued; after a long period of immersion, the one-dimensional pitting pit continues to corrode downward, and acidic corrosion liquid and corrosion products are obtained in the one-dimensional pitting pit.

8. The method according to claim 7, characterized in that The potentiodynamic polarization was performed at a scan rate of 0.5 mV / s, the initial potential was -1.0 V (SCE), and the termination potential was scanned to the overpassivation region.

9. The method according to any one of claims 1 to 8, characterized in that The stainless steel material is 2Cr13 martensitic stainless steel.

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