A method for determining the critical chloride ion concentration causing steel bar corrosion in simulated concrete pore solution

Abstract

The present invention discloses a method for determining the critical chloride ion concentration that causes steel bar corrosion in simulated concrete pore solution. The present invention combines the one-dimensional pitting electrode technology and the multi-cycle polarization scanning test technology, determines the corrosion initiation behavior of a single pitting pit based on the LSF metal local corrosion theory, and provides a new idea for judging the corrosion initiation of steel bars. The present invention can more effectively and accurately obtain the critical point of steel bar corrosion initiation, significantly reduce the dispersion of the test results of the critical chloride ion concentration, and shorten the test cycle at the same time, so as to obtain a more scientific and reliable critical chloride ion concentration value than the traditional test method.

Description

Technical Field

[0001] The present invention relates to the technical field of research on the durability of reinforced concrete structures, and particularly to a method for determining the critical chloride ion concentration causing steel bar corrosion in simulated concrete pore solution. Background Art

[0002] As one of the most widely used composite materials in modern engineering construction, the long-term service performance of reinforced concrete has a crucial impact on the safety and durability of engineering structures. In high-chloride salt erosion areas such as marine environments, this problem is particularly severe. The corrosion of steel bars caused by high-concentration chloride salt erosion has become an important factor affecting the lifespan and safety of infrastructure and cannot be ignored. The critical chloride ion concentration, that is, the chloride ion concentration threshold reached on the surface of the steel bar when the steel bar begins to corrode, has always been regarded as one of the core parameters in durability design and service life modeling. Therefore, studying the critical chloride ion concentration is of great significance for both the durability design of reinforced concrete structures and the prediction of the remaining service life of existing structures.

[0003] Currently, the methods for determining the critical chloride ion concentration of steel bar corrosion mainly include linear polarization method, alternating current impedance spectroscopy method, weight loss method, etc. Among them, the linear polarization method and the alternating current impedance spectroscopy method use the mutation or exceeding of specific critical values of judgment parameters such as corrosion potential, corrosion current, and polarization resistance as the basis for judging the start of steel bar corrosion. In addition, in the alternating current impedance spectroscopy test, the start of steel bar corrosion is usually judged by observing the sharp decrease in the curve radius of the low-frequency part of the Nyquist diagram. The weight loss method calculates the weight loss of the exposed surface of the steel bar before and after corrosion, and judges the occurrence of steel bar corrosion when the weight loss increases significantly.

[0004] The currently popular testing methods lack unified specifications and may include comprehensive information of multiple pitting corrosion pits, which is also one of the important reasons for the large discreteness of the existing test results of critical chloride ion concentration. Therefore, studying the corrosion initiation behavior of a single pitting corrosion pit is of great significance for accurately determining the initiation conditions of steel bar corrosion and the critical chloride ion concentration.

[0005] Based on the above analysis, there is no unified specification for the determination of the critical chloride ion concentration in the prior art, and the prior art cannot accurately find the critical chloride ion concentration theoretically. Therefore, there is an urgent need for a new method that can explain the mechanism of steel bar corrosion from the corrosion initiation behavior of a single pitting corrosion pit and be applied to the test of the critical chloride ion concentration to obtain a scientific and reliable threshold. Summary of the Invention

[0006] The object of the present invention is to overcome the deficiencies of the prior art and provide a method for determining the critical chloride ion concentration that causes steel bar corrosion in simulated concrete pore solution. The present invention combines the one-dimensional pitting electrode technology (One-dimensional pit test, 1D test technology) and the multiple cyclic potentiodynamic polarization test technology (cyclic potentiodynamic polarization test, CPP test technology), and determines the corrosion initiation behavior of a single pitting pit based on the LSF metal local corrosion theory, providing a new idea for judging the initiation of steel bar corrosion. The present invention can test the critical chloride ion concentration of samples in environments with different concentrations of hydroxide ions (high alkalinity), and can more effectively and accurately obtain the critical point of steel bar corrosion initiation, significantly reducing the dispersion of the test results of the critical chloride ion concentration, while shortening the test cycle, so as to obtain a more scientific and reliable critical chloride ion concentration value than the traditional test method.

[0007] To achieve the above object, the technical solution designed by the present invention is as follows:

[0008] The present invention provides a method for determining the critical chloride ion concentration that causes steel bar corrosion in simulated concrete pore solution, including the following steps:

[0009] (1) Prepare the environmental solution: Prepare simulated concrete pore solutions with chloride ion concentrations of 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol / L respectively.

[0010] (2) Pretreatment: Pretreat the steel wire and the steel block respectively. The steel wire is subjected to one-dimensional pitting electrode test, and the steel block is subjected to multiple cyclic potentiodynamic polarization tests.

[0011] (3) One-dimensional pitting electrode test: Adopt a three-electrode system. Place the steel wire in the simulated concrete pore solutions with the above different chloride ion concentrations respectively as the working electrode, and use a platinum mesh and a saturated calomel electrode as the auxiliary electrode and the reference electrode respectively; conduct one-dimensional pitting electrode tests to induce pitting pits with different depths, and obtain the one-dimensional pitting test curves of the pitting pits with different depths of the steel wire in the simulated concrete pore solutions with different chloride ion concentrations.

[0012] (4) Multiple cyclic potentiodynamic polarization test: Adopt a three-electrode system. Place the steel block in the simulated concrete pore solutions with the above different chloride ion concentrations respectively as the working electrode, and use a platinum mesh and a saturated calomel electrode as the auxiliary electrode and the reference electrode respectively; conduct multiple cyclic potentiodynamic polarization tests to obtain the multiple cyclic potentiodynamic polarization test curves of the steel block in the simulated concrete pore solutions with different chloride ion concentrations.

[0013] (5) Judgment: Judge by the image method, including the following steps:

[0014] ① In the one-dimensional pitting electrode test, when a "fully diffusive section type" curve appears, the minimum chloride ion concentration in all environments presenting a "fully diffusive section type" curve can be determined as the upper limit value of the critical chloride ion concentration. b If a "fully diffusive section type" curve does not appear, the chloride ion concentration is changed to conduct the one-dimensional pitting electrode test until a "fully diffusive section type" curve appears.

[0015] ② In the multiple cyclic polarization scanning tests, except when the chloride ion concentration is 0 mol / L, when a "fully negative hysteresis type" curve appears, the maximum chloride ion concentration in all environments presenting a "fully negative hysteresis type" curve can be determined as the lower limit value of the critical chloride ion concentration. a If, except when the chloride ion concentration is 0 mol / L, a "fully negative hysteresis type" curve does not appear, the chloride ion concentration is changed and the multiple cyclic polarization scanning tests are repeated until a "fully negative hysteresis type" curve appears.

[0016] ③ The range of the critical chloride ion concentration is a ≤ critical chloride ion concentration ≤ b .

[0017] Further, in the judgment of step (5), the judgment is carried out by E rp method, including the following specific steps:

[0018] s1: In the one-dimensional pitting electrode test, when a "fully diffusive section type" curve appears, the saturation potential is calculated E sat , according to the repassivation potential E rp < saturation potential E sat , the repassivation potential E rp is obtained;

[0019] Or in the multiple cyclic polarization scanning tests, when a "fully negative hysteresis type" curve appears, the repassivation potential E rp is obtained;

[0020] If neither a "fully negative hysteresis type" curve nor a "fully diffusive section type" curve appears, the chloride ion concentration is changed in the manner in the image method and the experiment is repeated until a "fully negative hysteresis type" curve or a "fully diffusive section type" curve appears;

[0021] s2: Sort the chloride ion concentrations in the environment from small to large in sequence, and calculate the difference between the repassivation potentials E rp in the environments with different chloride ion concentrations. When the repassivation potentialE rp When the difference between them is first > 500 mV, these two repassivation potentials can be obtained. E rp The corresponding chloride ion concentration, at this time, the higher repassivation potential E rp The corresponding chloride ion concentration is the lower limit value of the critical chloride ion concentration. a , the lower repassivation potential E rp The corresponding chloride ion concentration is the upper limit value of the critical chloride ion concentration. b ;

[0022] If the difference between the repassivation potentials E rp is not > 500 mV, then change the chloride ion concentration, repeat the one-dimensional pitting electrode test and multiple cyclic polarization scan tests until the difference between the repassivation potentials E rp is > 500 mV;

[0023] S3: The range of the critical chloride ion concentration is a ≤ critical chloride ion concentration ≤ b .

[0024] Furthermore, in the step (5),

[0025] In the step ①, the standard for changing the chloride ion concentration is: gradually increase the chloride ion concentration at a chloride ion concentration gradient of 0.05 mo / L;

[0026] In the step ②, the standard for changing the chloride ion concentration is: gradually decrease the chloride ion concentration at a chloride ion concentration gradient of 0.05 mo / L. If the "fully negative hysteresis type" curve has not appeared when the chloride ion concentration is reduced to 0.05 mol / L, then reduce the chloride ion concentration gradient to 0.02 mol / L or 0.01 mol / L;

[0027] In the step s2, the standard for changing the chloride ion concentration is: gradually increase the chloride ion concentration at a chloride ion concentration gradient of 0.05 mo / L.

[0028] Furthermore, the "fully diffusion section type" means that the one-dimensional pitting curves of pitting pits at different depths can all enter the diffusion section control and conform to Fick's first law; the "fully negative hysteresis type" curve means that during the reverse scanning process of the cyclic scanning curve, when the current density reaches 1 mA / cm², the curve retraces along the original path, and the phenomenon of the intersection of the forward scanning curve and the negative scanning curve appears.

[0029] Furthermore, the formulation of the simulated concrete pore solution: a mixture of saturated Ca(OH)2, KOH solution and NaOH solution, the concentration ratio of KOH to NaOH is 4:1, and the total concentration of hydroxide ions in the simulated concrete pore solution is 0.04 - 0.4 mol / L; the source of chloride ions is sodium chloride.

[0030] Furthermore, in step (2), the steel wire is pretreated, including the following specific steps: Connect one end of the steel wire to the copper wire with copper tape, and encapsulate the part where the steel wire is connected to the copper wire and the periphery of the steel wire with epoxy resin, leaving only the other end of the steel wire as the only exposed surface; After the epoxy resin is completely cured, polish and grind the exposed surface of the steel wire sample successively with 240, 400, 600, 800 and 1200 - mesh sandpaper, rinse with deionized water and anhydrous ethanol, dry, encapsulate with kerosene and plastic film for standby, and rinse the steel wire surface with anhydrous ethanol before starting the test.

[0031] Furthermore, in step (2), the steel block is pretreated, including the following specific steps: Connect one side of the steel block to the copper wire with copper tape, and encapsulate the part where the steel block is connected to the copper wire and the periphery of the steel block with epoxy resin, leaving only the only exposed surface; After the epoxy resin is completely cured, polish and grind the exposed surface of the steel block successively with 240, 400, 600 and 800 - mesh sandpaper, rinse with deionized water and anhydrous ethanol, dry, encapsulate with kerosene and plastic film for standby, before the electrochemical test, rinse the steel block surface with anhydrous ethanol, completely immerse it in the simulated concrete pore solution with a chloride ion concentration of 0 mol / L, and carry out pre - passivation for 3 days.

[0032] Furthermore, in step (3), the specific implementation parameters for the one - dimensional pitting electrode test are: Apply a potential of 1.5 V vs SCE for 200 s; Then apply potentials of 1.25 V vs SCE, 1 V vs SCE, 0.75 V vs SCE, 0.5 V vs SCE for 100 s each in sequence; Then, start negative potential scanning from 0.5 V to - 0.8 V, with a scanning rate of 5 mV / s, and monitor and collect current data at a sampling rate of 1 hz; Finally, repeat these implementation parameter steps six to seven cycles.

[0033] Furthermore, in step (4), the specific implementation parameters for the multiple - cycle polarization scan test are: Wait for 300 s for the open - circuit potential to stabilize, with a scanning rate of 0.167 mV / s, the positive potential scanning range starts from the open - circuit potential - 50 mV, and ends at + 1000 mV vs SCE or when the current density reaches 1 mA / cm 2Perform a reverse scan at that time and finally stop at the open-circuit potential; repeat this implementation parameter step four times, with a 1-minute delay between each repetition of the implementation parameter step.

[0034] The present invention also provides an application of the described method in determining the critical chloride ion concentration causing steel bar corrosion.

[0035] The working principle of the present invention:

[0036] The LSF metal local corrosion theory proposes that when the external environment is relatively harsh or the corrosion sensitivity of the material is relatively strong, due to the easier and more frequent breakdown of the passivation film, the breakdown of the passivation film becomes unimportant. For example, in the metastable pitting stage, the control step for the initiation of local corrosion is the stability of pitting growth; when the external environment is relatively mild or the corrosion sensitivity of the material is relatively weak, it is very difficult to break down the passivation film. If a high potential is applied to break down the passivation film, the growth of pitting will be very rapid, and at this time, the control step for the initiation of local corrosion changes to the breakdown of the passivation film. For relatively harsh environments or materials with strong corrosion sensitivity, the theory points out that the necessary condition for maintaining the active dissolution and stable growth of the corrosion pit is that the maximum dissolution current density of pitting ( i diss,max ) is greater than or equal to the critical diffusion current density ( i diff,crit ). If the one-dimensional pitting test curve shows a "full diffusion section type", it indicates that i diss,max ≥ i lim > i diff,crit , that is, a single pitting corrosion pit can grow stably in this environment. For relatively mild environments or materials with weak corrosion sensitivity, if the multiple cyclic polarization scan test curve shows a "full negative hysteresis type", it indicates that the passivation film on the metal surface will not be broken down in this environment, that is, a single pitting corrosion pit will not start. In addition, if the repassivation potential of a single pitting corrosion pit undergoes a first large-amplitude (>500 mV) sudden drop, it indicates that it will be very difficult for the corrosion pit to undergo repassivation, and once pitting starts, it will grow stably.

[0037] The beneficial effects of the present invention:

[0038] 1. This test method combines the one-dimensional pitting electrode test technology and the multiple cyclic polarization scan test technology. Based on the LSF metal local corrosion theory, considering the problem of steel bar corrosion initiation from the perspective of the initiation of a single pitting corrosion pit, the critical chloride ion concentration is further tested, the critical chloride ion concentration is measured more accurately, the problem of large discreteness of test results in traditional test methods is effectively reduced, and the reliability and consistency of the test are improved.

[0039] 2. By combining two testing techniques, the present invention can effectively make up for the limitations of a single technique, thereby obtaining more comprehensive local corrosion parameters under different environmental conditions.

[0040] 3. The test period of the one-dimensional pitting electrode testing technique of the present invention is shorter than that of the current traditional testing techniques, and the test results are more stable with a smaller dispersion. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the full diffusion section type in the one-dimensional pitting test curve;

[0042] In the figure, the black dots on each curve are all point B, corresponding to the saturation potential ( E sat ) and the limiting diffusion current density ( i lim ) of different pitting pit depths;

[0043] Figure 2 It is a schematic diagram of the partial diffusion section type in the one-dimensional pitting test curve;

[0044] Figure 3 It is a schematic diagram of the non-diffusion section type in the one-dimensional pitting test curve;

[0045] Figure 4 It is a schematic diagram of the curve in the diffusion control stage;

[0046] In the figure, the higher black dots on each curve are all point A, the lower black dots are all point B, and the section between point A and point B is the AB section; the AB section is the diffusion control stage;

[0047] Figure 5 It is a schematic diagram of the open-loop type in the multiple cyclic polarization scan test curve;

[0048] Figure 6 It is a schematic diagram of the full negative hysteresis type in the multiple cyclic polarization scan test curve;

[0049] Figure 7 It is a schematic diagram of the mixed type in the multiple cyclic polarization scan test curve;

[0050] Figure 8 It is a schematic diagram of path two in the determination method;

[0051] Figure 9 It is a test result diagram when the pitting pit depth is 53 μm in Example 1;

[0052] Figure 10 It is a test result diagram when the pitting pit depth is 37 μm in Example 1;

[0053] Figure 11For Q235 steel bars in c (OH - ) = 0.3 mol / L, c (Cl - ) = 0.1 mol / L environment after soaking for one month, the surface diagram of the sample observed by optical microscope;

[0054] Figure 12 Is the technical roadmap of the measurement method;

[0055] Figure 13 Is the technical roadmap for judgment by the image method;

[0056] Figure 14 For E rp Method judgment technical roadmap. Specific implementation mode

[0057] The following combines specific embodiments to further describe the present invention in detail for those skilled in the art to understand.

[0058] Example 1

[0059] This example provides a method for measuring the critical chloride ion concentration that causes steel bar corrosion in simulated concrete pore solution. The technical roadmap of the method for measuring the critical chloride ion concentration of the present invention is as Figure 12 Shown.

[0060] Including the following steps:

[0061] Description of test materials involved in the example

[0062] 1. The steel grades of the steel wires and steel blocks used in the test are both Q235.

[0063] 2. Formula of simulated concrete pore solution SCPS: A mixture of saturated Ca(OH)2, KOH solution and NaOH solution (the concentration ratio of KOH and NaOH is 4:1), c(OH - ) = 0.04 - 0.4 mol / L.

[0064] I. Making specimens

[0065] 1. Preparation of samples for one-dimensional pitting electrode test technology:

[0066] S1. The specimen to be tested is a steel wire simulating a steel bar. The diameter of the steel wire specimen is 0.3 mm. One end of it is connected to a copper wire with copper tape, and the part where the steel wire is connected to the copper wire and the periphery of the steel wire are encapsulated with epoxy resin, leaving only the other end of the steel wire as the only exposed surface (that is, the cross-section at the other end of the steel wire is the only exposed surface).

[0067] S2. After the epoxy resin is completely cured, polish and grind the exposed surface of the steel wire sample successively with 240, 400, 600, 800, and 1200 - mesh sandpaper, rinse it with deionized water and anhydrous ethanol, dry it with a tissue, and encapsulate it with kerosene and plastic film for standby. Set two parallel samples for each group of tests.

[0068] 2. Preparation of samples for the multiple - cycle polarization scanning test technique:

[0069] S1. The sample to be tested is a steel block simulating a steel bar with a side length of 1 cm. Connect one side of it to a copper wire with copper tape, and encapsulate the part where the steel block is connected to the copper wire and the four sides of the steel block (i.e., the upper, left, right, front, and back sides of the steel block) with epoxy resin, leaving only one side as the only exposed surface (i.e., the lower side of the steel block is the only exposed surface).

[0070] S2. After the epoxy resin is completely cured, polish and grind the exposed surfaces of all steel block samples successively with 240, 400, 600, and 800 - mesh sandpaper, rinse them with deionized water and anhydrous ethanol, dry them with a tissue, and encapsulate them with kerosene and plastic film for standby. Set two parallel samples for each group of tests.

[0071] 3. Preparation of SCPS: In this embodiment, configure SCPS with c(OH - ) = 0.3 mol / L, and add different concentrations of chloride ions (analytical - grade NaCl) to the solution to configure multiple SCPS solutions with different chloride ion concentrations. The chloride ion concentrations in the solutions are 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5 mol / L respectively, that is, obtain SCPS of 0.3OH0Cl, SCPS of 0.3OH0.1Cl, SCPS of 0.3OH0.15Cl, SCPS of 0.3OH0.2Cl, SCPS of 0.3OH0.25Cl, SCPS of 0.3OH0.3Cl, SCPS of 0.3OH0.4Cl, and SCPS of 0.3OH0.5Cl.

[0072] II. Electrochemical testing of the one - dimensional pitting electrode test technique

[0073] 1. Test steps for the electrochemical testing of the one - dimensional pitting electrode test technique:

[0074] S1. Before the electrochemical test of the one-dimensional pitting electrode test technology, clean the surface of the steel wire with anhydrous ethanol, and place the exposed surfaces vertically upward into the above-mentioned SCPSs with different chloride ion concentrations (i.e., SCPS with 0.3OH0Cl, SCPS with 0.3OH0.1Cl, SCPS with 0.3OH0.15Cl, SCPS with 0.3OH0.2Cl, SCPS with 0.3OH0.25Cl, SCPS with 0.3OH0.3Cl, SCPS with 0.3OH0.4Cl, and SCPS with 0.3OH0.5Cl). The tests are all carried out at 25 °C.

[0075] S2. Adopt a three-electrode system, with the steel wire sample, platinum mesh, and saturated calomel electrode (SCE) as the working electrode, auxiliary electrode, and reference electrode respectively. Since SCPS is highly alkaline, place the reference electrode in a Luggin capillary for protection.

[0076] S3. After starting the electrochemical test, continuously apply a relatively high potential of 1.5 V vs SCE for 200 s to stimulate the occurrence of pitting pits on the metal surface; further apply potentials of 1.25 V vs SCE, 1 V vs SCE, 0.75 V vs SCE, and 0.5 V vs SCE for 100 s each to grow the pitting pits to a certain depth.

[0077] S4. Then, start a negative potential scan from 0.5 V to -0.8 V at a scan rate of 5 mV / s, and monitor and collect current data (current density) at a collection rate of 1 Hz.

[0078] S5. Finally, without replacing the sample and solution, repeat steps S3 and S4 six to seven cycles to induce pitting pits of different depths.

[0079] 2. Deduction process of the conclusion of the one-dimensional pitting electrode test technology

[0080] S1. The depth of the pitting pit is x , calculated by Equation (1):

[0081] (1)

[0082] In the formula, K V is the volume of iron metal dissolved per unit anodic charge, , M alloy is the average molecular weight of iron metal, ρ alloy is the density of iron metal, n is the average oxidation state of the dissolved iron metal cations, F is the Faraday constant;Q is the charge passed; r 1D is the radius of the one-dimensional pitting pit.

[0083] Among them, K V , M alloy , n, ρ alloy , F take values as shown in Table 1.

[0084] Table 1 K V , M alloy , n, ρ alloy , F Value

[0085]

[0086] After the electrochemical test of the one-dimensional pitting electrode test technology, one-dimensional pitting test curves of pitting pits with different depths in different environments can be obtained. According to the different environments, the curves can be divided into the following three types:

[0087] ① The first type is the one-dimensional pitting curves of pitting pits with different depths obtained under very harsh environments (0.3OH0.2Cl SCPS to 0.3OH0.5Cl SCPS in this embodiment), all of which can enter the diffusion section control and conform to Fick's first law, called the "full diffusion section type" (as Figure 1 shown). In this case, the saturation potential ( E sat ) and the limiting diffusion current density ( i lim ) of pitting pits with different depths can be determined from the one-dimensional pitting test curves of pitting pits with different depths;

[0088] ② The second type is the one-dimensional pitting curves of pitting pits with different depths obtained under relatively mild environments (0.3OH0.15Cl SCPS in this embodiment), a part of which can enter the diffusion section control and a part cannot enter the diffusion section control, called the "partial diffusion section type" (as Figure 2 shown);

[0089] ③ The third type is the one-dimensional pitting curves of pitting pits with different depths obtained under very mild environments (0.03OH0Cl SCPS and 0.3OH0.1Cl SCPS in this embodiment), all of which cannot enter the diffusion section control, called the "no diffusion section type" (as Figure 3 shown).

[0090] Note: As the chloride ion concentration increases, the type of one-dimensional pitting test curve gradually evolves from the "no diffusion segment type" to the "partial diffusion segment type" and then to the "full diffusion segment type".

[0091] Among them, diffusion segment control means that when the metal continues to dissolve and the concentration of metal cations on the pitting pit surface continues to increase and reaches its saturation concentration (without considering supersaturation), the current at this time will not change with the change of voltage (or changes very slightly), as shown in the AB segment of Figure 4 .

[0092] Among them, Fick's first law, as shown in Equation (2), means that the limiting diffusion current density decreases with the increase of pit depth.

[0093] (2)

[0094] In the formula, i lim is the limiting diffusion current density, n is the average oxidation state of iron metal cations; F is the Faraday constant; D is the effective diffusion coefficient of iron metal cations; C sat is the saturation concentration of iron metal cations on the pitting pit surface; x is the pitting pit depth.

[0095] Among them, D , C sat The values of are shown in Table 2.

[0096] Table 2 D , C sat The values of

[0097]

[0098] S3. Through the i lim and E sat at different pitting pit depths, the corrosion kinetics equation of the saturated pitting solution in a harsh environment is fitted, as shown in Equation (3).

[0099] (3)

[0100] Among them, the derivation of the corrosion kinetics equation of the saturated pitting solution is as follows:

[0101] The LSF metal local corrosion theory framework points out that the maximum dissolution current density of the pitting pit i diss,maxIndicates the dissolution rate of the active metal controlled by charge transfer in the pitting solution. At a certain temperature, according to the Tafel equation, i diss,max is the electric potential ( E max ), and is a function of the electric potential as shown in Equation (4).

[0102] (4)

[0103] In the formula, i corr is the corrosion current density; E corr is the corrosion potential; β a is the anodic Tafel coefficient.

[0104] Taking the logarithm of both sides of Equation (4) and simplifying and arranging, Equation (5) can be obtained.

[0105] (5)

[0106] In the formula, b a is the anodic Tafel slope; k is related to E corr , i corr and b a and is a parameter related to

[0107] When i diss,max = i lim , E max = E sat From this, Equation (3) can be obtained.

[0108] S4. By shortening the time of applying the stimulating pitting depth voltage, determine the minimum pitting depth of the minimum chloride ion concentration that enables the first curve of the one-dimensional pitting test to enter the diffusion section control at this hydroxide ion concentration. Since this minimum pitting depth decreases as the chloride ion concentration increases, the minimum pitting depth obtained at a low chloride ion concentration will definitely cause the one-dimensional pitting curve to enter the diffusion section control at a high chloride ion concentration. Therefore, take the above minimum pitting depth as the conservative value of the minimum pitting depth at which the one-dimensional pitting curve obtained in a more severe environment enters the diffusion section control.

[0109] According to this minimum pitting depth, the E satAs the conservative value of the saturation potential of a one-dimensional pitting pit in each harsh environment (an environment that can make the one-dimensional pitting curve show a "fully diffused section type").

[0110] For the above-calculated value in this environment E sat For the repassivation potential in this environment E rp Make a prediction, that is E rp < E sat . Since E sat and E rp will decrease as the depth of the pitting pit increases, so the above minimum pitting pit depth of E sat and E rp Are used as the conservative values of the saturation potential and repassivation potential of the steel bar obtained from one-dimensional pitting tests.

[0111] III. Electrochemical Testing of Multiple-Cycle Polarization Scanning Testing Technology

[0112] 1. Experimental Procedure of Electrochemical Testing of Multiple-Cycle Polarization Scanning Testing Technology

[0113] S1. Before the start of electrochemical testing, rinse the surface of the steel block sample with anhydrous ethanol, completely immerse it in 0.3OH0Cl SCPS, and perform pre-passivation for 3 days. The tests are all carried out at 25°C. After the pre-passivation is completed, place the exposed surface vertically upward into the above-mentioned SCPS containing different chloride ion concentrations (i.e., 0.3OH0Cl SCPS, 0.3OH0.1Cl SCPS, 0.3OH0.15Cl SCPS, 0.3OH0.2Cl SCPS, 0.3OH0.25Cl SCPS, 0.3OH0.3Cl SCPS, 0.3OH0.4Cl SCPS, and 0.3OH0.5Cl SCPS).

[0114] S2. Adopt a three-electrode system, with the steel block sample, platinum mesh, and saturated calomel electrode (SCE) as the working electrode, auxiliary electrode, and reference electrode respectively. Since SCPS is highly alkaline, place the reference electrode in a Luggin capillary for protection.

[0115] S3. After starting the electrochemical testing, wait for 300 s for the open-circuit potential (OCP) to gradually stabilize. At this time, the change in the open-circuit potential is less than 3 mV / min; subsequently, the positive potential scanning range starts from OCP - 50 mV, up to +1000 mV vs SCE or when the current density reaches 1 mA / cm 2During the reverse scan, it finally stops at OCP with a scan rate of 0.167 mV / s;

[0116] S4. Repeat step (3) four times, with a 1-minute delay between each cyclic scan, without replacing the metal sample and SCPS during this period.

[0117] 2. Deduction process of the conclusion of the electrochemical test for the multiple cyclic polarization scan test technique

[0118] After the electrochemical test of the multiple cyclic polarization scan test technique is completed, multiple cyclic polarization scan test curves of the sample under different environments can be obtained. According to the different environments, the curves can be divided into the following three types.

[0119] ① The first is the "open-loop type" CPP test curve obtained under extremely harsh environments. The specific characteristics are that in the first scan of the four cyclic scan curves, during the reverse scan, the current density continuously increases and then only slightly decreases, and does not drop to the current density during the forward scan, that is, the forward and reverse scan curves do not intersect. And in the subsequent three scans, the OCP rapidly decreases, and the passivation region of the CPP curve disappears, (as Figure 5 shown);

[0120] ② The second is the "fully negative hysteresis type" CPP test curve obtained under extremely mild environments. The specific characteristics are that during the reverse scan of all four cyclic scan curves, when the current density reaches 1 mA / cm², the curve retraces along the original path, and the phenomenon of the intersection of the forward scan curve and the negative scan curve appears. The overall type of a group of test curves showing this phenomenon is called the "fully negative hysteresis type" (as Figure 6 shown). Through such CPP curves, the repassivation potential of the last single pitting pit that completes repassivation under the corresponding environment ( E rp ) can be obtained, that is, the applied potential when the negative scan curve intersects the positive scan curve is the repassivation potential;

[0121] ③ The third is the "mixed type" CPP test curve, "open-loop type" CPP test curve or "fully negative hysteresis type" CPP test curve obtained under relatively mild environments. The specific characteristics of the "mixed type" CPP test curve are that the four cyclic scan curves irregularly show open-loop curves, negative hysteresis curves or positive hysteresis curves (as Figure 7 shown).

[0122] In a relatively mild environment, the reasons for different curve types ("mixed" curves, "open-loop" curves, or "fully negative hysteresis" curves) that may appear in different repeated tests at the same chloride ion concentration are as follows: During the CPP test in such an environment, the initiation of pitting corrosion of carbon steel has a certain probability, and it can repassivate as the potential scans downward, but the stability of the regenerated passive film varies, and pitting reactivation may occur. In other words, carbon steel has a certain pitting corrosion risk in such an environment.

[0123] Therefore, in the CPP test, at least two repeated tests are required. In a relatively mild environment, when the results of the two repeated tests are consistent, if "fully negative hysteresis" curves appear in both, it can be used as the lower limit value of the critical chloride ion concentration and the repassivation potential E rp ; when the results of the two repeated tests are inconsistent, the chloride ion concentration at this time is not considered as the lower limit value of the critical chloride ion concentration, and the repassivation potential determined by the "fully negative hysteresis" curve measured at this time is not considered. E rp .

[0124] Note: During the process of gradually increasing the chloride ion concentration, the types of multiple cyclic polarization scan test curves will gradually change from "fully negative hysteresis" to the possible existence of all three curve types and then to "open-loop".

[0125] By combining the test results of the one-dimensional pitting electrode test technology and the multiple cyclic polarization scan test technology, two determination methods can be used to determine the critical chloride ion concentration that causes steel bar corrosion in simulated concrete pore solution C th (Cl - ) range ( a ≤ C th (Cl - )≤ b ), a is the lower limit value of the critical chloride ion concentration, b is the upper limit value of the critical chloride ion concentration.

[0126] IV. Determination methods for critical chloride ion concentration

[0127] 1. Method 1 (image method): The technical route for judgment by the image method is as Figure 13 shown.

[0128] ① Observe the image type. In the one-dimensional pitting electrode test, if a "fully diffusive section" curve appears, it indicates that a single pitting pit can grow stably in a relatively harsh environment (an environment with a chloride ion concentration higher than the critical chloride ion concentration). The minimum chloride ion concentration in all environments with a "fully diffusive section" curve can be determined as the upper limit value of the critical chloride ion concentration b;

[0129] ② If the "fully diffusive segment type" curve is not presented in the one-dimensional pitting electrode test, but the "no diffusive segment type" or "partially diffusive segment type" curve is presented, replace the chloride ion concentration in the SCPS and repeat the electrochemical test of the one-dimensional pitting electrode test technology until the "fully diffusive segment type" curve is presented. The standard for replacing the chloride ion concentration in the SCPS is: gradually increase the chloride ion concentration according to the chloride ion concentration gradient of 0.05 mo / L.

[0130] ③ When the "fully diffusive segment type" curve is presented in the one-dimensional pitting electrode test, in multiple cyclic polarization scan tests, if the "fully negative hysteresis type" curve is presented, it indicates that the passive film is not broken down in a relatively mild environment (environment less than the critical chloride ion concentration), single pitting does not occur, and the maximum chloride ion concentration (except for the chloride ion concentration of 0 mol / L) in all environments presenting the "fully negative hysteresis type" curve can be determined as the lower limit value of the critical chloride ion concentration a ; (In the CPP test, if different repeated tests at the same chloride ion concentration show different curve types, this chloride ion concentration is not considered as the lower limit value of the critical chloride ion concentration)

[0131] ④ If in multiple cyclic polarization scan tests, except for the chloride ion concentration of 0 mol / L, if the "fully negative hysteresis type" curve is not presented, but the "mixed type" or "open-loop type" is presented, replace the chloride ion concentration in the SCPS and repeat the electrochemical test of the multiple cyclic polarization scan test technology until the "fully negative hysteresis type" curve is presented. The standard for replacing the chloride ion concentration in the SCPS is: gradually decrease the chloride ion concentration according to the chloride ion concentration gradient of 0.05 mo / L. If the "fully negative hysteresis type" curve is not presented even when the concentration is reduced to 0.05 mol / L, then reduce the chloride ion concentration gradient to 0.02 mol / L or 0.01 mol / L;

[0132] ⑤ The critical chloride ion concentration causing steel bar corrosion C th (Cl - ) ranges from a ≤ C th (Cl - ) ≤ b .

[0133] 2. Route 2 ( E rp method): E rp The technical route of the method judgment is as Figure 14 shown.

[0134] ① In the one-dimensional pitting electrode test, when the "fully diffusive segment type" curve is presented, the saturation potential is calculated according to the formulaE sat , based on the relationship between the saturation potential and the repassivation potential, that is E rp < E sat , the repassivation potential E rp ;

[0135] Or in multiple cyclic polarization scan tests, a "fully negative hysteresis type" curve appears, and the repassivation potential E rp is obtained. (In the CPP test, if different repeated tests at the same chloride ion concentration show different curve types, then the repassivation potential determined by the "fully negative hysteresis type" curve measured at this time is not considered E rp )

[0136] Note: If neither a "fully negative hysteresis type" curve nor a "fully diffusion section type" curve appears, it indicates that an effective repassivation potential cannot be determined in these environments E rp , then change the chloride ion concentration in the manner of the image method and repeat the experiment until a "fully negative hysteresis type" curve or a "fully diffusion section type" curve appears.

[0137] ② As the chloride ion concentration in the environment increases, when the chloride ion concentration reaches a certain critical value, E rp there will be an obvious and sudden large drop (the drop amplitude > 500 mV), as shown in Figure 8 . Sort the chloride ion concentrations in the environment from small to large, and calculate the difference between the repassivation potentials E rp in environments with different chloride ion concentrations. When the difference between the repassivation potentials E rp is first > 500 mV, the chloride ion concentrations corresponding to these two repassivation potentials E rp can be obtained. At this time, the chloride ion concentration corresponding to the higher repassivation potential E rp is the lower limit value of the critical chloride ion concentration a , and the chloride ion concentration corresponding to the lower repassivation potential E rp is the upper limit value of the critical chloride ion concentration b .

[0138] ③ If the repassivation potentials E rpIf the difference between them is not > 500 mV, replace the chloride ion concentration and repeat the electrochemical tests of the one-dimensional pitting electrode test technique and the multiple cyclic polarization scanning test technique until the repassivation potential E rp shows an obvious and sudden significant decrease (the decrease amplitude > 500 mV). The standard for replacing the chloride ion concentration in SCPS is to gradually increase the chloride ion concentration at a chloride ion concentration gradient of 0.05 mol / L.

[0139] ④ The critical chloride ion concentration causing steel bar corrosion C th (Cl - ) ranges from a ≤ C th (Cl - )≤ b .

[0140] V. Test results of this embodiment

[0141] The test results can obtain the one-dimensional pitting curve graphs and multiple cyclic polarization scanning test graphs of steel bar specimens (including steel wires and steel blocks) in 8 different chloride ion concentration environments under a hydroxide ion concentration of 0.3 mol / L. According to the test graphs, the repassivation potential ( E rp ) of the steel bar specimens in each environment can be determined, as shown in Table 3 specifically.

[0142] Table 3 Test data of steel bar specimens

[0143]

[0144] Among them, CPP is the multiple cyclic polarization scanning test technique, and 1D is the one-dimensional pitting electrode test technique. 2 repeated tests are carried out in the CPP test of this embodiment.

[0145] Among them, in the SCPS environment of 0.3OH0.1Cl, E rp is the potential at the intersection of the negative scanning curve and the positive scanning curve in the CPP curve.

[0146] Among them, in the environment from 0.3OH0.2Cl to 0.3OH0.5Cl of SCPS, E sat is the potential at the point where the current density starts to change significantly with voltage in the one-dimensional pitting curve corresponding to each chloride ion concentration of the pitting pit with the conservative minimum pitting pit depth in the hydroxide ion concentration environment ( c (OH - ) = 0.3 mol / L).

[0147] Among them, the conservative minimum pitting depth in the hydroxide ion concentration environment is obtained by performing a one-dimensional pitting electrode test in the SCPS environment of 0.3OH0.15Cl (when the hydroxide concentration is 0.3 mol / L, including the mildest environment capable of entering the diffusion stage control curve), that is, shortening the time for applying the stimulating pitting depth, and obtaining the minimum pitting depth that can make the first one-dimensional pitting curve obtained from the test enter the diffusion section control. The test results are as shown in Figure 9 and Figure 10 . When the pitting depth is 53 μm, the one-dimensional pitting curve can enter the diffusion section control; while when the pitting depth is 37 μm, the one-dimensional pitting curve cannot enter the diffusion section control, and the selected minimum pitting depth is 50 μm.

[0148] Taking the method determined in the SCPS environment of 0.3OH0.2C as an example: E sat :

[0149] According to the sample test curve in this environment, extract the key kinetic parameters of the pit solution at different pitting depths E sat , i lim , as shown in Table 4.

[0150] Table 4 Key kinetic parameters E sat , i lim

[0151]

[0152] Sample 1 and Sample 2 are two parallel experiments. According to Table 4, the parameters are fitted by the least squares method according to Equation (3) k and b a , and the corrosion kinetic equation of the saturated pit solution in this environment is obtained, as shown in Equation (6).

[0153] (6)

[0154] Through Equation (2), determine i lim = 1.463 A cm -2 at the minimum pitting depth of 50 μm, substitute it into Equation (6), and determine E sat = -4 mV in the SCPS environment of 0.3OH0.2Cl.

[0155] Route 1: Observe the type of image. As can be seen from Table 3, the SCPS environment of 0.3OH0.1Cl is the critical environment where the CPP test curve shows a "fully negative hysteresis type", and the lower limit value of the critical chloride ion concentration is determined a = 0.1 mol / L; the SCPS environment of 0.3OH0.2Cl is the critical environment where the 1D test curve shows a "fully diffused segment type", and the upper limit value of the critical chloride ion concentration is determined b = 0.2 mol / L; that is, when c (OH - ) = 0.3 mol / L, the range of the critical chloride ion concentration is 0.1 mol / L ≤ C th (Cl - ) ≤ 0.2 mol / L.

[0156] Route 2: The first significant decrease (decrease amplitude > 500 mV) in the repassivation potential ( E rp ). As can be seen from Table 3, the chloride ion concentration corresponding to the higher repassivation potential is the lower limit value of the critical chloride ion concentration a = 0.1 mol / L; the chloride ion concentration corresponding to the lower repassivation potential is the upper limit value of the critical chloride ion concentration b = 0.2 mol / L; that is, when c (OH - ) = 0.3 mol / L, 0.1 mol / L ≤ C th (Cl - ) ≤ 0.2 mol / L.

[0157] Among them, the rationality of the range of the critical chloride ion concentration obtained by the proposed method, that is, when c (OH - ) = 0.3 mol / L, 0.1 mol / L ≤ C th (Cl - ) ≤ 0.2 mol / L, will be elaborated in the following aspects.

[0158] First of all, when the Q235 steel bar is immersed in SCPS with a chloride ion concentration of 0.1 mol / L (SCPS of 0.3OH0.1Cl) for one month, and the surface of the sample is observed through an optical microscope, only the scratches of the sandpaper can be observed, and no pitting corrosion is found, as Figure 11 shown. Thus, it can be seen that taking c (Cl - ) = 0.1 mol / L as the lower limit of the critical chloride ion concentration range is reasonable.

[0159] Secondly, the determination method for pitting initiation presented by the CPP test technology has been very mature. Most studies have shown that when the CPP test image is of the "fully negative hysteresis type", the surface of the sample remains shiny after the test, and pitting does not occur spontaneously under natural conditions, and the passive film does not break down. As the severity of the environment increases, other types of CPP test images appear, indicating that pitting may or may not undergo repassivation. Finally, when the environment is so severe that the CPP test image only shows the "open-loop type", it means that pitting will definitely grow stably under natural conditions. However, since the CPP test curve showing the "open-loop type" may contain the combined information of multiple pitting pits rather than a single pitting pit, it is possible that in an environment more mild than the one only showing the "open-loop type", a single pitting pit can grow stably. Therefore, one-dimensional pitting test images are used to test the upper limit of the critical chloride concentration range. This test activates a single pitting pit and observes whether it can grow stably, simulating the growth process of a single pitting pit. If the one-dimensional pitting test image shows the "fully diffusive segment type", it means that in this environment, once the pitting pit is activated, it can grow stably. The environment with a chloride concentration lower than the critical chloride concentration is regarded as a relatively mild environment, and the environment with a chloride concentration higher than the critical chloride concentration is regarded as a relatively severe environment. Based on the LSF metal local corrosion theory, in a relatively mild environment, the initiation of pitting should be controlled by the breakdown of the passive film; in a relatively severe environment, the initiation of pitting should be controlled by the stability of pitting pit growth. The results of the one-dimensional pitting test can well describe the stability of pitting pit growth. Thus, it can be seen that c (Cl - ) = 0.2 mol / L as the upper limit of the critical chloride concentration range is also reasonable.

[0160] Looking at the E rp of a single pitting pit again, when the chloride concentration is low, the E rp obtained in the CPP test are all above 500 mV, and at this time, no pitting is found through microscopic testing. However, as the chloride concentration increases, in different repeated experiments with the same chloride concentration, the test images of the CPP test show different curve types and the "open-loop type", and it is impossible to accurately determine the E rp of a single pitting pit. Therefore, 1D testing is used to obtain the E rp of different pitting pit depths, and then it is inferred that in this environment, the maximum E rp of a single pitting pit that can be obtained through 1D testing, its E rp is less than 0 mV, which is smaller than that in a mild environment. Erp In contrast, there is a significant sudden drop, which indicates that it is very difficult for the corrosion pit to repassivate. Once pitting corrosion starts, it will grow steadily. Therefore, the method of determining the critical concentration of chloride ions by E rp the significant drop is also reasonable.

[0161] Other parts not described in detail are all prior arts. Although the above embodiments have described the present invention in detail, they are only part of the embodiments of the present invention, not all embodiments. People can also obtain other embodiments without creative efforts based on these embodiments, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for determining the critical chloride ion concentration causing steel bar corrosion in simulated concrete pore solution, characterized in that: It includes the following steps: (1) Prepare the environmental solution: Prepare simulated concrete pore solutions with chloride ion concentrations of 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol / L respectively; (2) Pretreatment: Pretreat the steel wire and the steel block respectively. The steel wire is subjected to one-dimensional pitting electrode test, and the steel block is subjected to multiple cyclic polarization scan tests; (3) One-dimensional pitting electrode test: Adopt a three-electrode system. Place the steel wire in the simulated concrete pore solutions with the above different chloride ion concentrations respectively as the working electrode, and use a platinum mesh and a saturated calomel electrode as the auxiliary electrode and the reference electrode respectively; Conduct one-dimensional pitting electrode tests to induce pitting pits with different depths, and obtain the one-dimensional pitting test curves of pitting pits with different depths on the steel wire under simulated concrete pore solutions with different chloride ion concentrations; (4) Multiple cyclic polarization scan tests: Adopt a three-electrode system. Place the steel block in the simulated concrete pore solutions with the above different chloride ion concentrations respectively as the working electrode, and use a platinum mesh and a saturated calomel electrode as the auxiliary electrode and the reference electrode respectively; Conduct multiple cyclic polarization scan tests to obtain the multiple cyclic polarization scan test curves of the steel block under simulated concrete pore solutions with different chloride ion concentrations; (5) Judgment: Judge by the image method, including the following steps: ①In the one-dimensional pitting electrode test, when a "fully diffused section type" curve appears, the minimum chloride ion concentration in all environments presenting a "fully diffused section type" curve can be determined as the upper limit value of the critical chloride ion concentration. b If a "fully diffused section type" curve does not appear, the chloride ion concentration is changed for the one-dimensional pitting electrode test until a "fully diffused section type" curve appears. ② In the multiple cyclic polarization scanning tests, except when the chloride ion concentration is 0 mol / L, when a "fully negative hysteresis type" curve appears, the maximum chloride ion concentration in all environments with a "fully negative hysteresis type" curve can be determined as the lower limit value of the critical chloride ion concentration. a ; If a "fully negative hysteresis type" curve does not appear except when the chloride ion concentration is 0 mol / L, then change the chloride ion concentration and repeat the multiple cyclic polarization scanning tests until a "fully negative hysteresis type" curve appears. ③ The range of the critical chloride ion concentration is a ≤ Critical chloride ion concentration ≤ b ; The "fully diffusion section type" means that the one-dimensional pitting curves of pitting pits with different depths can all enter the diffusion section control and conform to Fick's first law; The "fully negative hysteresis type" curve means that during the reverse scanning process of the cyclic scanning curve, when the current density reaches 1 mA / cm², the curve retraces along the original path, and the phenomenon of the intersection of the forward scanning curve and the negative scanning curve appears; The formula of the simulated concrete pore solution: A mixture of saturated Ca(OH)2, KOH solution and NaOH solution, the concentration ratio of KOH to NaOH is 4:1, and the total concentration of hydroxide ions in the simulated concrete pore solution is 0.04 - 0.4 mol / L; The source of chloride ions is sodium chloride; In the step (2), the pretreatment of the steel block includes the following specific steps: Connect one side of the steel block to the copper wire with copper tape, and encapsulate the part where the steel block is connected to the copper wire and the periphery of the steel block with epoxy resin, leaving only the only exposed surface; After the epoxy resin is completely cured, polish the exposed surface of the steel block successively with 240, 400, 600, and 800 mesh sandpapers, rinse with deionized water and absolute ethanol, dry, and encapsulate with kerosene and plastic film for standby. Before the electrochemical test, rinse the surface of the steel block with absolute ethanol, completely immerse it in the simulated concrete pore solution with a chloride ion concentration of 0 mol / L, and carry out pre-passivation for 3 days.

2. The method according to claim 1, wherein: In the judgment of step (5), through E rp the following specific steps are included for judgment by the method: s1: In the one-dimensional pitting electrode test, a "fully diffused section type" curve is presented, and the saturation potential is calculated E sat , according to the repassivation potential E rp < saturation potential E sat , the repassivation potential is obtained E rp ; Or in multiple cyclic polarization scan tests, a "fully negative hysteresis type" curve is presented, and the repassivation potential is obtained E rp ; If neither the "fully negative hysteresis type" curve nor the "fully diffusion section type" curve appears, replace the chloride ion concentration according to the method in the image method and re-conduct the experiment until the "fully negative hysteresis type" curve or the "fully diffusion section type" curve appears; S2: Sort in ascending order according to the chloride ion concentration in the environment, and calculate the repassivation potential under environments with different chloride ion concentrations E rp Calculate the difference between them. When the difference between the repassivation potentials E rp is greater than 500 mV for the first time, the chloride ion concentrations corresponding to these two repassivation potentials E rp can be obtained. At this time, the chloride ion concentration corresponding to the higher repassivation potential E rp is the lower limit value of the critical chloride ion concentration a , and the chloride ion concentration corresponding to the lower repassivation potential E rp is the upper limit value of the critical chloride ion concentration b ; If the difference between the repassivation potentials E rp is not > 500 mV, then change the chloride ion concentration and repeat the one-dimensional pitting electrode test and multiple cyclic polarization scan tests until the difference between the repassivation potentials E rp is > 500 mV; s3: The range of the critical chloride ion concentration is a ≤ critical chloride ion concentration ≤ b .

3. The method according to claim 2, wherein: In the step (5), In the step ①, the standard for replacing the chloride ion concentration is: Gradually increase the chloride ion concentration at a gradient of 0.05 mo / L. In step ②, the standard for changing the chloride ion concentration is as follows: the chloride ion concentration is gradually decreased at a chloride ion concentration gradient of 0.05 mol / L. If the "fully negative hysteresis type" curve has not appeared when the chloride ion concentration is decreased to 0.05 mol / L, the chloride ion concentration gradient is decreased to 0.02 mol / L or 0.01 mol / L. In step s2, the standard for changing the chloride ion concentration is as follows: the chloride ion concentration is gradually increased at a chloride ion concentration gradient of 0.05 mol / L.

4. The method according to claim 2, wherein: In step (2), the steel wire is pretreated, including the following specific steps: one end of the steel wire is connected to the copper wire with copper tape, and the part where the steel wire is connected to the copper wire and the periphery of the steel wire are encapsulated with epoxy resin, leaving only the other end of the steel wire as the only exposed surface; after the epoxy resin is completely cured, the exposed surface of the steel wire sample is polished successively with 240, 400, 600, 800, and 1200 mesh sandpapers, rinsed with deionized water and absolute ethanol, dried, encapsulated with kerosene and plastic film for standby, and the surface of the steel wire is rinsed with absolute ethanol before starting the test.

5. The method according to claim 2, wherein: In step (3), the specific implementation parameters for the one-dimensional pitting electrode test are as follows: a potential of 1.5 V vs SCE is applied for 200 s; further, potentials of 1.25 V vs SCE, 1 V vs SCE, 0.75 V vs SCE, and 0.5 V vs SCE are applied successively for 100 s each; then, a negative potential scan is started from 0.5 V to -0.8 V at a scan rate of 5 mV / s, and the current data is monitored and collected at a sampling rate of 1 Hz; finally, this implementation parameter step is repeated six to seven cycles.

6. The method according to claim 2, characterized in that: In the step (4), the specific implementation parameters for performing multiple cyclic polarization scan tests are as follows: wait for 300 s to stabilize the open circuit potential, the scan rate is 0.167 mV / s, the positive potential scan range starts from the open circuit potential - 50 mV, reverse scan at +1000 mV vs SCE or when the current density reaches 1 mA / cm 2 , and finally stop at the open circuit potential; repeat the steps of these implementation parameters four times, with a delay of 1 min between each repetition of the steps of the implementation parameters.

Images