Method for measuring concentration of critical chloride ions causing corrosion of steel bars in simulated concrete pore fluid

Abstract

The invention discloses a method for measuring the concentration of critical chloride ions causing corrosion of steel bars in simulated concrete pore fluid. According to the method, the one-dimensional pitting corrosion electrode technology and the multiple circulating polarization scanning test technology are combined, the corrosion starting behavior of a single pitting corrosion pit is judged based on the LSF metal local corrosion theory, and a brand-new steel bar corrosion starting judgment thought is provided. According to the method, the steel bar corrosion starting critical point can be more effectively and accurately obtained, the dispersion of a critical chloride ion concentration test result is remarkably reduced, and meanwhile, the test period is shortened, so that the critical chloride ion concentration value which is more scientific and reliable than that of a traditional test method is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of research on durability of reinforced concrete structures, and in particular to a method for determining a critical chloride ion concentration in simulated concrete pore fluid that causes steel corrosion. 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 vital impact on the safety and durability of engineering structures. This problem is particularly severe in areas with high chloride corrosion, such as marine environments. Steel corrosion caused by high-concentration chloride corrosion has become an important factor affecting the life 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 to the durability design of reinforced concrete structures and the prediction of the remaining life of structures in service.

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

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

[0005] Based on the above analysis, there is no unified standard for the determination of the critical chloride ion concentration, and the existing technology cannot accurately find the critical chloride ion concentration in theory. Therefore, there is an urgent need for a new method that can explain the mechanism of steel corrosion from the corrosion initiation behavior of a single pit and apply it to the test of critical chloride ion concentration to obtain a scientific and reliable threshold. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for determining the critical chloride ion concentration that causes steel corrosion in simulated concrete pore fluid. The present invention combines one-dimensional pitting electrode technology (One-dimensional pit test, 1D test technology) and 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 determining the initiation of steel 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 corrosion initiation, significantly reduce the discreteness of the critical chloride ion concentration test results, and shorten the test cycle, thereby obtaining a more scientific and reliable critical chloride ion concentration value than the traditional test method.

[0007] To achieve the above purpose, the technical solution designed by the present invention is as follows: The present invention provides a method for determining the critical chloride ion concentration causing steel corrosion in simulated concrete pore fluid, comprising the following steps: (1) Preparation of environmental solution: simulated concrete pore fluids with chloride ion concentrations of 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol / L were prepared respectively; (2) Pretreatment: The steel wire and steel block are pretreated separately. The steel wire is subjected to one-dimensional pitting electrode test, and the steel block is subjected to multiple cyclic polarization scanning tests. (3) One-dimensional pitting electrode test: A three-electrode system was used to place the steel wire in the simulated concrete pore solution containing the above-mentioned different chloride ion concentrations, with the steel wire as the working electrode, and the platinum mesh and saturated calomel electrode as the auxiliary electrode and reference electrode, respectively. A one-dimensional pitting electrode test was performed to induce pitting pits of different depths, and the one-dimensional pitting test curves of pitting pits of different depths of the steel wire in the simulated concrete pore solution with different chloride ion concentrations were obtained. (4) Multiple cyclic polarization scanning test: A three-electrode system was used to place the steel block in the simulated concrete pore solution containing the above-mentioned different chloride ion concentrations, with the steel block as the working electrode, and the platinum mesh and saturated calomel electrode as the auxiliary electrode and reference electrode, respectively. Multiple cyclic polarization scanning tests were performed to obtain the multiple cyclic polarization scanning test curves of the steel block in the simulated concrete pore solution with different chloride ion concentrations. (5) Judgment: Judgment is made by image method, including the following steps: ① In the one-dimensional pitting electrode test, when a "full diffusion segment" curve is presented, the minimum chloride ion concentration in all environments showing a "full diffusion segment" curve can be determined as the upper limit of the critical chloride ion concentration.b ; If the "full diffusion segment type" curve is not presented, change the chloride ion concentration to perform a one-dimensional pitting electrode test until the "full diffusion segment type" curve is presented; ② In the multiple cyclic polarization scanning tests, except for the chloride ion concentration of 0 mol / L, when the "full negative hysteresis" curve is presented, the maximum chloride ion concentration in all environments showing the "full negative hysteresis" curve can be determined as the lower limit of the critical chloride ion concentration. a ; If the “full negative hysteresis type” curve is not presented except when the chloride ion concentration is 0 mol / L, then change the chloride ion concentration and repeat the cyclic polarization scanning test several times until the “full negative hysteresis type” curve is presented; ③The range of critical chloride ion concentration is a ≤Critical chloride ion concentration≤ b .

[0008] Furthermore, in the judgment of step (5), E rp The method for judgment includes the following specific steps: s1: In the one-dimensional pitting electrode test, a "full diffusion segment" 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 scanning tests, a "full negative hysteresis type" curve is presented, and the repassivation potential is obtained E rp ; If no "full negative hysteresis type" curve or "full diffusion segment type" curve appears, change the chloride ion concentration according to the method of the image method and repeat the experiment until the "full negative hysteresis type" curve or "full diffusion segment type" curve appears; s2: Sort the chloride ion concentration in the environment from small to large, and calculate the repassivation potential under different chloride ion concentrations E rp The difference between the repassivation potential E rp When the difference between them is >500 mV for the first time, 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 of the critical chloride ion concentration a , lower repassivation potential Erp The corresponding chloride ion concentration is the upper limit of the critical chloride ion concentration b ; If the passivation potential E rp If the difference between the two values ​​is not greater than 500 mV, the chloride ion concentration is changed, and the one-dimensional pitting electrode test and multiple cyclic polarization scanning tests are repeated until the repassivation potential reaches E rp Until the difference between them is greater than 500 mV; s3: The range of critical chloride ion concentration is a ≤Critical chloride ion concentration≤ b .

[0009] Furthermore, in step (5), In the step ①, the standard for changing the chloride ion concentration is: gradually increasing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L; In the step ②, the standard for changing the chloride ion concentration is: gradually reducing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L. If the chloride ion concentration is reduced to 0.05 mol / L and does not present a "full negative hysteresis" curve, the chloride ion concentration gradient is reduced to 0.02 mol / L or 0.01 mol / L; In step s2, the standard for changing the chloride ion concentration is: gradually increasing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L.

[0010] Furthermore, the "full diffusion segment type" is a one-dimensional pitting curve of pits of different depths that can all enter the diffusion segment control and conform to Fick's first law; the "full negative hysteresis type" curve is a cyclic scanning curve that is in the reverse scanning process. When the current density reaches 1 mA / cm², the curve traces back along the original path, and the positive scanning curve and the negative scanning curve intersect.

[0011] Furthermore, the formula of the simulated concrete pore fluid is: a mixture of saturated Ca(OH)2, KOH solution and NaOH solution, the concentration ratio of KOH to NaOH is 4:1, the total concentration of hydroxide ions in the simulated concrete pore fluid is 0.04~0.4mol / L; the source of chloride ions is sodium chloride.

[0012] Furthermore, in the step (2), the steel wire is pretreated, including the following specific steps: connecting one end of the steel wire to the copper wire with a copper tape, and encapsulating the portion where the steel wire is connected to the copper wire and the surrounding 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, polishing the exposed surface of the steel wire sample with 240, 400, 600, 800 and 1200 grit sandpaper in turn, rinsing it with deionized water and anhydrous ethanol, wiping it dry, and encapsulating it with kerosene and plastic film for standby use, and rinsing the surface of the steel wire with anhydrous ethanol before starting the test.

[0013] Furthermore, in step (2), the steel block is pretreated, including the following specific steps: connecting one side of the steel block to the copper wire with a copper tape, and sealing the portion of the steel block connected to the copper wire and the surrounding of the steel block with epoxy resin, leaving only a single exposed surface; after the epoxy resin is completely cured, polishing the exposed surface of the steel block with 240, 400, 600 and 800 grit sandpaper in turn, and rinsing it with deionized water and anhydrous ethanol, wiping it dry, and sealing it with kerosene and plastic film for standby use; before the electrochemical test begins, the surface of the steel block is rinsed with anhydrous ethanol, and completely immersed in a simulated concrete pore fluid containing a chloride ion concentration of 0 mol / L for 3 days of pre-passivation.

[0014] Furthermore, in step (3), the specific implementation parameters for the one-dimensional pitting electrode test are: applying a potential of 1.5 V vs SCE for 200 s; further applying 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; then, performing a negative potential scan from 0.5 V to -0.8 V at a scan rate of 5 mV / s, and monitoring and collecting current data at an acquisition rate of 1 Hz; finally, repeating this implementation parameter step for six to seven cycles.

[0015] Furthermore, in step (4), the specific implementation parameters of the multiple cyclic polarization scanning test are: waiting for 300 s to stabilize the open circuit potential, the scanning rate is 0.167 mV / s, the potential forward scanning range starts from the open circuit potential of -50 mV, at +1000 mV vs SCE or when the current density reaches 1 mA / cm 2 The scan was reversed at the same time and finally stopped at the open circuit potential; this implementation parameter step was repeated four times with a delay of 1 min between each repetition of the implementation parameter step.

[0016] The invention also provides an application of the method in determining the critical chloride ion concentration causing steel bar corrosion.

[0017] Working principle of the present invention: The LSF theory of localized metal corrosion proposes that when the external environment is relatively harsh or the material is highly sensitive to corrosion, the breakdown of the passivation film is easier and more frequent, which makes the breakdown of the passivation film unimportant. For example, in the metastable pitting stage, the control step for the initiation of local corrosion is the stability of the pitting growth; when the external environment is relatively mild or the material is less sensitive to corrosion, the breakdown of the passivation film is very difficult. If a high potential is applied to cause the passivation film to break down, the pitting will grow very rapidly, and the control step for the initiation of local corrosion is transformed into the breakdown of the passivation film. For relatively harsh environments or materials with high corrosion sensitivity, the theory points out that the necessary condition for maintaining the active dissolution and stable growth of pits is 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 segment", it means i diss,max ≥ i lim > i diff,crit , that is, a single pit can grow stably in this environment. For a milder environment or a material with weaker corrosion sensitivity, if the multiple cyclic polarization scanning test curve shows a "full negative hysteresis type", it means that the passivation film on the metal surface will not be broken down in this environment, that is, a single pit will not start. In addition, if the repassivation potential of a single pit has a large drop (> 500mV) for the first time, it means that the pit will be difficult to repassivate, and once pitting begins, it will grow stably.

[0018] Beneficial effects of the present invention: 1. This test method combines one-dimensional pitting electrode test technology with multiple cyclic polarization scanning test technology. Based on the LSF metal local corrosion theory, it considers the steel bar corrosion initiation problem from the perspective of single pit initiation, thereby further testing the critical concentration of chloride ions and more accurately measuring the critical chloride ion concentration, effectively reducing the problem of large discreteness of test results in traditional test methods and improving the reliability and consistency of the test.

[0019] 2. The present invention can effectively make up for the limitations of a single technology by combining two testing technologies, thereby more comprehensively obtaining local corrosion parameters under different environmental conditions.

[0020] 3. The test cycle of the one-dimensional pitting electrode testing technology of the present invention is shorter than that of the current traditional testing technology, and the test results are more stable and have smaller discreteness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the full diffusion section in the one-dimensional pitting test curve; In the figure, the black dots on each curve are point B, corresponding to the saturation potential of different pit depths ( E sat ) and the limiting diffusion current density ( i lim ); Figure 2 It is a schematic diagram of a partial diffusion section in a one-dimensional pitting corrosion test curve; Figure 3 It is a schematic diagram of the non-diffusion section in the one-dimensional pitting test curve; Figure 4 It is a schematic diagram of the diffusion-controlled phase curve; In the figure, the higher black dots on each curve are point A, the lower black dots are point B, and the area between point A and point B is segment AB; segment AB is the diffusion control stage; Figure 5 It is a schematic diagram of the open loop type in the multiple cyclic polarization scanning test curve; Figure 6 It is a schematic diagram of the full negative hysteresis type in the multiple cyclic polarization scanning test curve; Figure 7 A schematic diagram of a mixed type in a multiple cyclic polarization scanning test curve; Figure 8 It is a schematic diagram of the second approach in the determination method; Fig. 9 This is a test result diagram when the pit depth is 53 μm in Example 1; Fig.10 This is a test result diagram when the pit depth is 37 μm in Example 1; Fig.11 For Q235 steel bars c (OH - )=0.3mol / L, c (Cl - ) = 0.1mol / L environment after one month of immersion under optical microscope; Fig.12 A technical roadmap for the assay method; Fig.13 Determine the technology roadmap for image methods; Fig.14 for E rp Method to judge the technology roadmap. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can understand.

[0023] Example 1 This embodiment provides a method for determining the critical chloride ion concentration in simulated concrete pore fluid that causes steel corrosion. The technical roadmap of the method for determining the critical chloride ion concentration of the present invention is as follows: Fig.12 shown.

[0024] The following steps are involved: Description of the test materials in the examples 1. The steel wire and steel block used in the test are all Q235.

[0025] 2. Formula of SCPS for simulating concrete pore fluid: a mixture of saturated Ca(OH)2, KOH solution and NaOH solution (the concentration ratio of KOH to NaOH is 4:1), c(OH - )=0.04~0.4 mol / L.

[0026] 1. Make samples 1. Preparation of samples used for one-dimensional pitting electrode testing technology: S1. The test sample is a steel wire simulating a steel bar. The diameter of the steel wire specimen is 0.3 mm. One end of the steel wire specimen is connected to the copper wire with a copper tape, and the portion where the steel wire is connected to the copper wire and the surrounding of the steel wire are encapsulated with epoxy resin, leaving only the other end of the steel wire as the only exposed surface (i.e., the cross-section of the other end of the steel wire is the only exposed surface).

[0027] S2. After the epoxy resin is completely cured, the exposed surface of the steel wire sample is polished with 240, 400, 600, 800 and 1200 grit sandpaper in turn, and rinsed with deionized water and anhydrous ethanol, wiped dry with paper towels, and sealed with kerosene and plastic film for later use. Two parallel samples are set for each test.

[0028] 2. Preparation of samples used for multiple cyclic polarization scanning test technology: S1. The sample to be tested is a steel block simulating a steel bar. The side length of the steel block is 1 cm. One side of the steel block is connected to the copper wire with a copper tape, and the part where the steel block is connected to the copper wire and the surrounding areas of the steel block (i.e., the top, left, right, front and back of the steel block) are encapsulated with epoxy resin, leaving only one side as the only exposed surface (i.e., the bottom of the steel block is the only exposed surface).

[0029] S2. After the epoxy resin is completely cured, the exposed surfaces of all steel block samples are polished with 240, 400, 600 and 800 grit sandpapers in turn, rinsed with deionized water and anhydrous ethanol, wiped dry with paper towels, and sealed with kerosene and plastic film for later use. Two parallel samples are set for each test.

[0030] 3. Preparation of SCPS: In this example, c(OH -)=0.3 mol / L, and different concentrations of chloride ions (analytical grade NaCl) were added to the solution to prepare multiple SCPS solutions with different chloride ion concentrations. The chloride ion concentrations in the solutions were 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol / L, respectively, to obtain 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.

[0031] 2. Electrochemical testing of one-dimensional pitting electrode testing technology 1. Experimental steps of electrochemical test of one-dimensional pitting electrode test technology: S1. Before the electrochemical test of the one-dimensional pitting electrode test technology, the surface of the steel wire was rinsed with anhydrous ethanol, and the exposed surface was placed vertically upward in the above-mentioned SCPS containing different chloride ion concentrations (i.e., 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). The tests were all carried out at 25°C.

[0032] S2. A three-electrode system was used, 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, the reference electrode was placed in a Luggin capillary for protection.

[0033] S3. After starting the electrochemical test, a higher potential of 1.5 V vs SCE was applied for 200 s to activate the metal surface and stimulate the occurrence of pitting pits; further, the potentials of 1.25 V vs SCE, 1 V vs SCE, 0.75 V vs SCE, and 0.5 V vs SCE were applied in sequence for 100 s each to allow the pitting pits to grow to a certain depth.

[0034] S4. Then, a negative potential scan was performed from 0.5 V to -0.8 V with a scan rate of 5 mV / s, and the current data (current density) was monitored and collected at an acquisition rate of 1 Hz.

[0035] S5. Finally, without changing the sample and the solution, steps S3 and S4 are repeated for six to seven cycles to induce pitting pits of different depths.

[0036] 2. Conclusion derivation process of one-dimensional pitting electrode test technology S1, pit depth is x , calculated by formula (1): (1) 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 dissolved iron metal cations, F is the Faraday constant; Q is the amount of charge passing through; r 1D is the radius of the one-dimensional pit.

[0037] in, K V , M alloy , n、ρ alloy , F The values ​​of are shown in Table 1.

[0038] Table 1 K V , M alloy , n、ρ alloy , F Value

[0039] S2. After the electrochemical test of the one-dimensional pitting electrode test technology is completed, the one-dimensional pitting test curves of pits with different depths under different environments can be obtained. According to different environments, the curves can be divided into the following three types: ① The first type is the one-dimensional pitting curves of different pitting depths obtained under very harsh environment (SCPS of 0.3OH0.2Cl~SCPS of 0.3OH0.5Cl in this embodiment), all of which can enter the diffusion stage control and conform to Fick's first law, which is called "full diffusion stage type" (such as Figure 1 In this case, the saturation potential of pitting pits of different depths can be determined from the one-dimensional pitting test curves of pitting pits of different depths ( E sat ) and the limiting diffusion current density ( i lim ); ② The second type is the one-dimensional pitting curve of different depths of pitting pits obtained under a relatively mild environment (SCPS of 0.3OH0.15Cl in this embodiment). Part of it can enter the diffusion section control, and part of it cannot enter the diffusion section control, which is called "partial diffusion section type" (such as Figure 2 shown); ③ The third type is the one-dimensional pitting curves of different pit depths obtained under very mild environment (SCPS of 0.03OH0Cl and SCPS of 0.3OH0.1Cl in this embodiment) in which all pits cannot enter the diffusion stage control, which is called "non-diffusion stage type" (such as Figure 3 as shown).

[0040] Note: With the increase of chloride ion concentration, the type of one-dimensional pitting test curve gradually evolves from "non-diffusion type" to "partial diffusion type" and then to "full diffusion type".

[0041] Among them, the diffusion stage control, that is, when the metal continues to dissolve, the metal cation concentration on the surface of the pit continues to increase and reaches its saturation concentration (excluding the oversaturation situation), the current at this time will not change with the change of voltage (or will change very slightly), such as Figure 4 As shown in segment AB.

[0042] Among them, Fick's first law, as shown in equation (2), states that the limiting diffusion current density decreases with the increase of pit depth.

[0043] (2) In the formula, i lim To limit the diffusion current density, n is the average oxidation state of the iron metal cation; F is the Faraday constant; D is the effective diffusion coefficient of the iron metal cation; C sat is the saturation concentration of iron metal cations on the surface of pits; x is the pit depth.

[0044] in, D , C sat The values ​​of are shown in Table 2.

[0045] Table 2 D , C sat The value of

[0046] S3, through different depths of pitting pits i lim and Esat The corrosion kinetic equation of saturated pitting solution under harsh environment is fitted, as shown in equation (3).

[0047] (3) Among them, the derivation of the corrosion kinetic equation of saturated pitting solution is as follows: The LSF metal local corrosion theory framework points out that the maximum dissolution current density of the pitting pit i diss,max It represents the charge transfer controlled dissolution rate of active metal in the etching solution. Under the condition of constant temperature, it can be known from the Tafel equation that: i diss,max is the electric potential ( E max ), as shown in formula (4).

[0048] (4) In the formula, i corr is the corrosion current density; E corr is the corrosion potential; β a is the anodic Tafel coefficient.

[0049] Taking the logarithm of both sides of equation (4) and simplifying it, we can get equation (5).

[0050] (5) In the formula, b a is the anodic Tafel slope; k For E corr , i corr and b a Related parameters.

[0051] when i diss,max = i lim hour, E max = E sat , thus we can get formula (3).

[0052] S4. By shortening the time of applying the stimulation pitting depth voltage, the minimum pitting pit depth of the minimum chloride ion concentration that enables the first curve of the one-dimensional pitting test to enter the diffusion section control is determined in the hydroxide ion concentration. Since the minimum pitting pit depth decreases with the increase of the chloride ion concentration, the minimum pitting depth obtained at a low chloride ion concentration will definitely make the one-dimensional pitting curve enter the diffusion section control at a high chloride ion concentration. Therefore, the above minimum pitting depth is used as a conservative value of the minimum pitting pit depth of the one-dimensional pitting curve entering the diffusion section control obtained by testing in a more severe environment.

[0053] According to this minimum pit depth, the following is calculated by equation (2) and equation (3): E sat As a conservative value of the saturation potential of the one-dimensional pitting pit in each harsh environment (an environment that can make the one-dimensional pitting curve present a "full diffusion segment").

[0054] The above calculated environment E sat The repassivation potential under this environment E rp Make a prediction, that is E rp < E sat .because E sat and E rp will decrease with the increase of pit depth, so the minimum pit depth is E sat and E rp Conservative values ​​of the saturation potential and repassivation potential of steel bars obtained as one-dimensional pitting tests.

[0055] 3. Electrochemical testing using multiple cycle polarization scanning test technology 1. Experimental process of electrochemical test using multiple cyclic polarization scanning test technology S1. Before the electrochemical test, the surface of the steel block sample was rinsed with anhydrous ethanol and completely immersed in 0.3OH0Cl SCPS for 3 days of pre-passivation. The tests were all carried out at 25°C. After the pre-passivation, the exposed surface was placed vertically upward in 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).

[0056] S2. A three-electrode system was used, 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, the reference electrode was placed in a Luggin capillary for protection.

[0057] S3: After starting the electrochemical test, wait for 300 s to allow the open circuit potential (OCP) to gradually stabilize. At this time, the change of the open circuit potential is less than 3 mV / min. Then, the potential forward scan range starts from OCP-50 mV and increases to +1000 mV vs SCE or when the current density reaches 1 mA / cm 2 The scan was reversed and finally stopped at OCP with a scan rate of 0.167 mV / s. S4. Repeat step (3) four times, with a delay of 1 minute between each cycle scan, and do not change the metal sample and SCPS during the period.

[0058] 2. The process of deriving the conclusions of electrochemical tests using multiple cyclic polarization scanning test technology After the electrochemical test of the multiple cycle polarization scanning test technology is completed, the multiple cycle polarization scanning test curves of the samples under different environments can be obtained. According to different environments, the curves can be divided into the following three types.

[0059] ① The first type is an "open-loop" CPP test curve obtained by testing in a very harsh environment. The specific characteristics are that in the first scan of the four-cycle scanning curve, the current density continues to increase during the reverse scan 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 drops rapidly, and the passivation zone of the CPP curve disappears (such as Figure 5 shown); ② The second type is the "full negative hysteresis" CPP test curve obtained under a very mild environment. The specific feature is that the four cycle scanning curves are all in the reverse scanning process. When the current density reaches 1 mA / cm², the curve traces back along the original path, and the positive scanning curve and the negative scanning curve intersect. The group of test curves with this phenomenon is collectively called "full negative hysteresis" (such as Figure 6 As shown in Figure 2), through this type of CPP curve, the repassivation potential of a single pit that has finally completed repassivation under the corresponding environment can be obtained ( E rp ), that is, the applied potential when the negative scanning curve intersects the positive scanning curve is the repassivation potential; ③ The third type is the "mixed" CPP test curve, "open-loop" CPP test curve or "full negative hysteresis" CPP test curve obtained under a relatively mild environment. The specific characteristics of the "mixed" CPP test curve are that the four-cycle scanning curve does not regularly appear open-loop curves, negative hysteresis curves or positive hysteresis curves (such as Figure 7 as shown).

[0060] In a milder environment, different curve types ("mixed" curve, "open loop" curve or "full negative hysteresis" curve) may appear in different repeated tests of the same chloride ion concentration for the following reasons: During the CPP test in such an environment, the initiation of pitting corrosion of carbon steel has a certain probability, and it can be repassivated as the potential is scanned downward, but the stability of the regenerated passivation film is different, and pitting reactivation may occur. In other words, carbon steel has a certain pitting risk in such an environment.

[0061] Therefore, in the CPP test, at least 2 repeated tests are required. Under a milder environment, when the results of the 2 repeated tests are consistent, if both show a "full negative hysteresis" curve, it can be used as the lower limit of the critical chloride ion concentration and the repassivation potential. E rp When the results of two repeated tests are inconsistent, the chloride ion concentration at this time is not considered as the lower limit of the critical chloride ion concentration, and the repassivation potential determined by the "full negative hysteresis type" curve measured at this time is not considered E rp .

[0062] Note: In the process of gradually increasing chloride ion concentration, the type of multiple cyclic polarization scanning test curve will gradually evolve from "full negative hysteresis type" to the possibility of all three curve types and then to "open loop type".

[0063] By combining the test results of one-dimensional pitting electrode test technology and multiple cyclic polarization scanning test technology, two determination methods can be used to determine the critical chloride ion concentration in the simulated concrete pore fluid that causes steel corrosion. C th (Cl - ) range ( a ≤ C th (Cl - )≤ b ), a is the lower limit of critical chloride ion concentration, b is the upper limit of critical chloride ion concentration.

[0064] 4. Determination of critical chloride ion concentration 1. Approach 1 (Image Method): Image method judgment technical route is as follows Fig.13 shown.

[0065] ① Observe the image type. In the one-dimensional pitting electrode test, if a "full diffusion segment" curve is presented, it means that a single pit can grow stably in a relatively harsh environment (an environment with a concentration greater than the critical chloride ion concentration). The minimum chloride ion concentration in all environments showing a "full diffusion segment" curve can be determined as the upper limit of the critical chloride ion concentration. b ; ② If the one-dimensional pitting electrode test does not show a "full diffusion segment type" curve, but shows a "non-diffusion segment type" or "partial diffusion segment type" curve, change the chloride ion concentration in the SCPS and repeat the electrochemical test of the one-dimensional pitting electrode test technology until a "full diffusion segment type" curve is shown. The standard for changing the chloride ion concentration in the SCPS is: gradually increase the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L.

[0066] ③ When a "full diffusion segment" curve is presented in the one-dimensional pitting electrode test, if a "full negative hysteresis" curve is presented in the multiple cyclic polarization scanning test, it means that the passivation film is not broken down in a relatively mild environment (less than the critical chloride ion concentration), and no single pitting occurs. The maximum chloride ion concentration in all environments showing a "full negative hysteresis" curve (except for a chloride ion concentration of 0 mol / L) can be determined as the lower limit of the critical chloride ion concentration. a ; (In the CPP test, if different curve types appear in different repeated tests of the same chloride ion concentration, this chloride ion concentration will not be considered as the lower limit of the critical chloride ion concentration) ④ If, in the multiple cyclic polarization scanning tests, except for the chloride ion concentration of 0 mol / L, if the "full negative hysteresis type" curve is not presented, but a "mixed type" or "open loop type" is presented, change the chloride ion concentration in SCPS, and repeat the electrochemical test of the multiple cyclic polarization scanning test technology until a "full negative hysteresis type" curve is presented. The standard for changing the chloride ion concentration in SCPS is: gradually reduce the chloride ion concentration according to the chloride ion concentration gradient of 0.05 mol / L. If the "full negative hysteresis type" curve is not presented when it is reduced to 0.05 mol / L, then reduce the chloride ion concentration gradient to 0.02 mol / L or 0.01 mol / L; ⑤Critical chloride ion concentration causing steel corrosion C th (Cl - ) is in the range of a ≤ C th (Cl - )≤ b .

[0067] 2. Approach 2 ( E rp Law):E rp How to judge the technical route Fig.14 shown.

[0068] ① In the one-dimensional pitting electrode test, a "full diffusion segment" curve is presented, and the saturation potential is calculated according to the formula E sat , according to the relationship between saturation potential and repassivation potential, that is E rp < E sat , the repassivation potential is derived E rp ; Or in multiple cyclic polarization scanning tests, a "full negative hysteresis type" curve is presented, and the repassivation potential is obtained E rp (In the CPP test, if different curve types appear in different repeated tests of the same chloride ion concentration, the "full negative hysteresis type" curve measured at this time is not considered to determine the repassivation potential E rp ) Note: If no "full negative hysteresis" curve or "full diffusion segment" curve appears, it means that the effective repassivation potential cannot be determined under these conditions. E rp , then change the chloride ion concentration according to the imaging method and repeat the experiment until a "full negative hysteresis type" curve or a "full diffusion segment type" curve appears.

[0069] ②As the chloride ion concentration in the environment increases, when the chloride ion concentration reaches a certain critical value, E rp There will be a significant and sudden drop (drop > 500mV), such as Figure 8 As shown. Sort the chloride ion concentration in the environment from small to large, and calculate the repassivation potential in environments with different chloride ion concentrations. E rp The difference between the repassivation potential E rp When the difference between them is >500 mV for the first time, 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 of the critical chloride ion concentration a , lower repassivation potential E rp The corresponding chloride ion concentration is the upper limit of the critical chloride ion concentration b .

[0070] ③ If the repassivation potential is different under different chloride ion concentrations E rp If the difference between the two values ​​is not greater than 500 mV, change 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 is E rp The standard for changing the chloride ion concentration in SCPS is to gradually increase the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L until an obvious and sudden large drop occurs (drop > 500 mV).

[0071] ④Critical chloride ion concentration causing steel corrosion C th (Cl - ) is in the range of a ≤ C th (Cl - )≤ b .

[0072] V. Test results of this embodiment The test results can obtain the one-dimensional pitting curves and multiple cyclic polarization scanning test graphs of steel bar specimens (including steel wire and steel block) in 8 different chloride ion concentration environments under 0.3 mol / L hydroxide ion concentration. According to the test graphs, the repassivation potential ( E rp ), as shown in Table 3.

[0073] Table 3 Test data of steel bar specimens

[0074] Wherein, CPP is a multiple cyclic polarization scanning test technology, and 1D is a one-dimensional pitting electrode test technology. In the CPP test of this embodiment, two repeated tests are performed.

[0075] Among them, in the SCPS environment of 0.3OH0.1Cl, E rp It is the potential at the intersection of the negative scanning curve and the positive scanning curve in the CPP curve.

[0076] Among them, in the environment of 0.3OH0.2Cl SCPS to 0.3OH0.5Cl SCPS, E sat is the hydroxide ion concentration in the environment ( c (OH - )=0.3 mol / L), the potential at which the current density begins to change significantly with voltage in the one-dimensional pitting curve for each chloride ion concentration.

[0077] Among them, the conservative minimum pitting depth in the hydroxide ion concentration environment is obtained by conducting a one-dimensional pitting electrode test in an SCPS environment of 0.3OH0.15Cl (the mildest environment containing a curve that can enter the diffusion stage control when the hydroxide concentration is 0.3 mol / L), that is, shortening the time of applying the stimulation pitting depth, and obtaining the minimum pitting depth that can make the first one-dimensional pitting curve obtained by the test enter the diffusion stage control. The test results are as follows Fig. 9 and Fig.10 When the pit depth is 53 μm, the one-dimensional pitting curve can enter the diffusion stage control; when the pit depth is 37 μm, the one-dimensional pitting curve cannot enter the diffusion stage control, and the selected minimum pit depth is 50 μm.

[0078] Determined under 0.3OH0.2C SCPS environment E sat As an example: According to the sample test curve under this environment, the key kinetic parameters of the pitting solution of pits with different depths are extracted. E sat , i lim , as shown in Table 4.

[0079] Table 4 Key kinetic parameters E sat , i lim

[0080] Sample 1 and sample 2 are two parallel experiments. According to Table 4, the least square method is used to fit the parameters according to formula (3): k and b a , the corrosion kinetic equation of the saturated pitting solution in this environment is obtained, as shown in equation (6).

[0081] (6) The minimum pit depth of 50 μm is determined by formula (2). i lim =1.463 A cm -2 , substituting into formula (6), determine the SCPS environment of 0.3OH0.2Cl E sat =-4 mV.

[0082] Method 1: Observe the image type. From Table 3, we can see that the SCPS environment of 0.3OH0.1Cl is the critical environment where the CPP test curve presents a "full negative hysteresis type", and the lower limit 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 presents a "full diffusion segment type", and the upper limit of the critical chloride ion concentration is determined b =0.2mol / L; that is, when c (OH - )=0.3 mol / L, the critical chloride ion concentration range is 0.1 mol / L≤ C th (Cl - )≤0.2mol / L.

[0083] Approach 2: Repassivation Potential ( E rp ) for the first time (the decrease is greater than 500mV). As can be seen from Table 3, the chloride ion concentration corresponding to the higher repassivation potential is the lower limit 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 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.

[0084] Among them, the rationality of the critical chloride ion concentration range obtained by the proposed method is that when c (OH - )=0.3 mol / L, 0.1 mol / L≤ C th (Cl - )≤0.2 mol / L, which will be elaborated from the following aspects.

[0085] First, the Q235 steel bar was immersed in SCPS with a chloride ion concentration of 0.1 mol / L (SCPS with 0.3OH0.1Cl) for one month. The surface of the sample was observed under an optical microscope. Only the scratches of the sandpaper were observed, and no pitting corrosion was found. Fig.11 As shown. c (Cl - )=0.1mol / L is reasonable as the lower limit of the critical chloride ion concentration range.

[0086] Secondly, the determination method of pitting initiation presented by CPP test technology is already very mature. Most studies have proved that when the CPP test image is "full negative hysteresis type", the sample surface remains glossy after the test, and pitting will not occur spontaneously under natural conditions, and the passivation film will not break down. As the severity of the environment increases, the CPP test image presents other types, indicating that pitting may or may not undergo repassivation. Finally, when the environment is so severe that the CPP test image only presents an "open loop type", it means that pitting will definitely grow stably under natural conditions. However, since the "open loop" CPP test curve may contain comprehensive information of multiple pitting pits, not a single pitting pit, a single pitting pit may grow stably in a milder environment than an environment that only presents an "open loop type". Therefore, a one-dimensional pitting test image is used to test the upper limit of the critical chloride ion 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 a "full diffusion segment", it means that in this environment, once the pitting pit is activated, it can grow stably. The environment below the critical chloride ion concentration is considered a relatively mild environment, and the environment above the critical chloride ion concentration is considered a relatively harsh environment. Based on the LSF metal local corrosion theory, in a relatively mild environment, the initiation of pitting corrosion should be controlled by the breakdown of the passivation film; in a relatively harsh environment, the initiation of pitting corrosion 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. It can be seen that c (Cl - )=0.2mol / L is also reasonable as the upper limit of the critical chloride ion concentration range.

[0087] Then through a single pit E rp When the chloride ion concentration is low, the CPP test results are E rp All of them are above 500mV. At this time, no pitting corrosion was found in the microscopic test. However, with the increase of chloride ion concentration, the test images of CPP test in different repeated experiments with the same chloride ion concentration showed different curve types and "open loop type", which could not accurately determine the single pitting corrosion. E rp So the 1D test was used to obtain the different pit depths. E rp , and then infer that the maximum value that can be obtained through 1D test in this environment E rp A single pitting pit E rp Less than 0mV, lower than the Erp Compared with the above, a sharp drop occurred, which indicates that the pits will be difficult to repassivate. Once pitting begins, it will grow steadily. E rp The method of determining the critical concentration of chloride ions based on a significant drop in is also reasonable.

[0088] Although the above embodiments have been described in detail, they are only a part of the embodiments of the present invention, not all of them. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all belong to the protection scope of the present invention.

Claims

1. A method for determining the critical chloride ion concentration causing steel corrosion in simulated concrete pore fluid, characterized in that: The following steps are involved: (1) Preparation of environmental solution: simulated concrete pore fluids with chloride ion concentrations of 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5 mol / L were prepared respectively; (2) Pretreatment: The steel wire and steel block are pretreated separately. The steel wire is subjected to one-dimensional pitting electrode test, and the steel block is subjected to multiple cyclic polarization scanning tests. (3) One-dimensional pitting electrode test: A three-electrode system was used to place the steel wire in the simulated concrete pore solution containing the above-mentioned different chloride ion concentrations, with the steel wire as the working electrode, and the platinum mesh and saturated calomel electrode as the auxiliary electrode and reference electrode, respectively. A one-dimensional pitting electrode test was performed to induce pitting pits of different depths, and the one-dimensional pitting test curves of pitting pits of different depths of the steel wire in the simulated concrete pore solution with different chloride ion concentrations were obtained. (4) Multiple cyclic polarization scanning test: A three-electrode system was used to place steel blocks in simulated concrete pore solutions containing different chloride ion concentrations as the working electrode, and a platinum mesh and a saturated calomel electrode were used as the auxiliary electrode and reference electrode, respectively. Perform multiple cyclic polarization scanning tests to obtain multiple cyclic polarization scanning test curves of steel blocks in simulated concrete pore fluids with different chloride ion concentrations; (5) Judgment: Judgment is made by image method, including the following steps: ① In the one-dimensional pitting electrode test, when a "full diffusion segment" curve is presented, the minimum chloride ion concentration in all environments showing a "full diffusion segment" curve can be determined as the upper limit of the critical chloride ion concentration. b ; If the "full diffusion segment type" curve is not presented, change the chloride ion concentration to perform a one-dimensional pitting electrode test until the "full diffusion segment type" curve is presented; ② In the multiple cyclic polarization scanning tests, except for the chloride ion concentration of 0 mol / L, when the "full negative hysteresis" curve is presented, the maximum chloride ion concentration in all environments showing the "full negative hysteresis" curve can be determined as the lower limit of the critical chloride ion concentration. a ; If the "full negative hysteresis type" curve is not presented except when the chloride ion concentration is 0 mol / L, then change the chloride ion concentration and repeat the cyclic polarization scanning test multiple times until the "full negative hysteresis type" curve is presented; ③The range of critical chloride ion concentration is a ≤Critical chloride ion concentration≤ b .

2. The method according to claim 1, characterized in that: In the judgment of step (5), E rp The method for judgment includes the following specific steps: s1: In the one-dimensional pitting electrode test, a "full diffusion segment" 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 scanning tests, a "full negative hysteresis" curve is presented, and the repassivation potential is obtained E rp ; If no "full negative hysteresis type" curve or "full diffusion segment type" curve appears, change the chloride ion concentration according to the method of the image method and repeat the experiment until the "full negative hysteresis type" curve or "full diffusion segment type" curve appears; s2: Sort the chloride ion concentration in the environment from small to large, and calculate the repassivation potential under different chloride ion concentrations E rp The difference between the repassivation potential E rp When the difference between them is >500 mV for the first time, 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 of the critical chloride ion concentration a , lower repassivation potential E rp The corresponding chloride ion concentration is the upper limit of the critical chloride ion concentration b ; If the passivation potential E rp If the difference between the two values ​​is not greater than 500 mV, the chloride ion concentration is changed, and the one-dimensional pitting electrode test and multiple cyclic polarization scanning tests are repeated until the repassivation potential reaches E rp Until the difference between them is greater than 500 mV; s3: The critical chloride ion concentration range is a ≤Critical chloride ion concentration≤ b .

3. The method according to claim 2, characterized in that: In the step (5), In the step ①, the standard for changing the chloride ion concentration is: gradually increasing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L; In the step ②, the standard for changing the chloride ion concentration is: gradually reducing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L. If the chloride ion concentration is reduced to 0.05 mol / L and does not present a "full negative hysteresis" curve, the chloride ion concentration gradient is reduced to 0.02 mol / L or 0.01 mol / L; In step s2, the standard for changing the chloride ion concentration is: gradually increasing the chloride ion concentration according to a chloride ion concentration gradient of 0.05 mol / L.

4. The method according to claim 2, characterized in that: The "full diffusion segment type" means that all one-dimensional pitting curves of pits with different depths can enter the diffusion segment control and conform to Fick's first law; the "full negative hysteresis type" means that the cyclic scanning curves are all in the reverse scanning process. When the current density reaches 1 mA / cm², the curve traces back along the original path, and the positive scanning curve and the negative scanning curve intersect.

5. The method according to claim 2, characterized in that: The formula of the simulated concrete pore fluid is: a mixture of saturated Ca(OH)2, KOH solution and NaOH solution, the concentration ratio of KOH to NaOH is 4:1, the total concentration of hydroxide ions in the simulated concrete pore fluid is 0.04-0.4 mol / L; the source of chloride ions is sodium chloride.

6. The method according to claim 2, characterized in that: In the step (2), the steel wire is pretreated, including the following specific steps: connecting one end of the steel wire to the copper wire with a copper tape, and encapsulating the portion where the steel wire is connected to the copper wire and the surrounding 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, polishing the exposed surface of the steel wire sample with 240, 400, 600, 800 and 1200 grit sandpaper in sequence, rinsing with deionized water and anhydrous ethanol, wiping dry, and encapsulating with kerosene and plastic film for standby use, and rinsing the surface of the steel wire with anhydrous ethanol before starting the test.

7. The method according to claim 2, characterized in that: In the step (2), the steel block is pretreated, including the following specific steps: connecting one side of the steel block to the copper wire with a copper tape, and sealing the portion of the steel block connected to the copper wire and the surrounding of the steel block with epoxy resin, leaving only a single exposed surface; after the epoxy resin is completely cured, polishing the exposed surface of the steel block with 240, 400, 600 and 800 grit sandpaper in sequence, and rinsing it with deionized water and anhydrous ethanol, wiping it dry, and sealing it with kerosene and plastic film for standby use; before the electrochemical test begins, rinsing the surface of the steel block with anhydrous ethanol, and completely immersing it in a simulated concrete pore fluid containing a chloride ion concentration of 0 mol / L for 3 days of pre-passivation.

8. The method according to claim 2, characterized in that: In the step (3), the specific implementation parameters for the one-dimensional pitting electrode test are: applying a potential of 1.5 V vs SCE for 200 s; further applying 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; then, performing a negative potential scan from 0.5 V to -0.8 V at a scan rate of 5 mV / s, and monitoring and collecting current data at an acquisition rate of 1 Hz; finally, repeating this implementation parameter step for six to seven cycles.

9. The method according to claim 2, characterized in that: In step (4), the specific implementation parameters of the multiple cyclic polarization scan test are: waiting for 300 s to stabilize the open circuit potential, the scan rate is 0.167 mV / s, the potential forward scan range starts from the open circuit potential -50 mV, at +1000 mV vs SCE or when the current density reaches 1 mA / cm 2 The scan was reversed at the same time and finally stopped at the open circuit potential; this implementation parameter step was repeated four times, with a delay of 1 min between each repetition of the implementation parameter step.

10. Use of the method according to any one of claims 1 to 9 in determining the critical chloride ion concentration causing steel bar corrosion.

Images