Method for manufacturing corrosion monitoring device and corrosion monitoring device

Abstract

To provide a method for manufacturing a corrosion monitoring device capable of highly accurately monitoring a corrosion amount of a metal piece buried in soil.SOLUTION: A method for manufacturing a corrosion monitoring device 100 including a metal piece 11 buried in soil includes: a step ST1 of acquiring in advance first and second correlations between a chloride ion concentration contained in the soil and the magnitude and number of local corrosion occurring in a sample of the same type as the metal piece buried in the soil; a step ST2 of measuring the chloride ion concentration contained in the soil in which the metal piece is buried; a step ST3 of predicting the magnitude and number of local corrosions occurring in the metal piece on the basis of the measured chloride ion concentration, the first correlations, and the second correlations; a step ST4 of determining widths, lengths, and initial thicknesses of the metal piece capable of reducing measurement errors of electric resistance values of the metal piece by performing numerical analyses using the predicted magnitude and number of local corrosions; and a step ST5 of manufacturing the metal piece.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a corrosion monitoring device capable of monitoring the amount of corrosion of metal pieces buried in the ground with high accuracy, and to a corrosion monitoring device manufactured by this manufacturing method. [Background technology]

[0002] In order to predict the lifespan and maintain metal products used in underground environments (for example, steel sheet piles and steel pipe piles, which are civil engineering and construction materials used in underground environments), or to develop products with excellent corrosion resistance, there is a need for a device that can accurately monitor the amount of corrosion of metal pieces buried in the soil.

[0003] One means capable of monitoring the amount of corrosion of a metal piece is an electrical resistance corrosion sensor as described in Patent Documents 1 to 3. An electrical resistance corrosion sensor measures the amount of corrosion (amount of corrosion thinning) of a metal piece based on an increase in electrical resistance that accompanies corrosion thinning of the metal piece that constitutes the corrosion sensor. In principle, this corrosion sensor measures the average electrical resistance of the entire metal piece, and therefore the average amount of corrosion of the entire metal piece.

[0004] However, the corrosion sensors described in Patent Documents 1 to 3 are intended to measure the amount of corrosion in an atmospheric environment, and are not intended to measure the amount of corrosion in an underground environment. When the present inventors considered using this corrosion sensor in an underground environment, they found that it may not be possible to make accurate measurements. Furthermore, when the present inventors considered using this corrosion sensor in an underground environment, they found that the form of corrosion differs depending on the concentration of chloride ions in the soil. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-3376 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-197102 [Patent Document 3] International Publication No. 2021 / 235475 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the problems of the conventional technology as described above, and an object of the present invention is to provide a corrosion monitoring device and a manufacturing method thereof that can monitor the amount of corrosion of metal pieces buried in the ground with high accuracy. [Means for solving the problem]

[0007] In order to solve the above problems, the present inventors conducted extensive research into the corrosion state of metal pieces buried in the soil. FIG. 1 shows an example of the results of the research into the corrosion state of metal pieces buried in the soil. The circled areas in FIG. 1 are locally corroded areas. Note that FIG. 1 shows the results of burying a steel pipe (material: SS400, outer diameter: approximately 100 mm) as a metal piece in highly corrosive soil and examining the corrosion state after more than one year (an image of a portion of the metal piece). As shown in Figure 1, it was found that metal pieces buried in the ground do not corrode evenly over the entire piece, but rather corrode locally. In the example shown in Figure 1, the size of the localized corrosion was about 5 mm x 5 mm, and the maximum depth was about 0.9 mm. However, it has been found that the form of localized corrosion that occurs on metal pieces, as shown in Figure 1, changes depending on the concentration of chloride ions in the soil. Specifically, as a result of extensive research by the inventors, it has been found that the size and number of localized corrosion spots are each correlated with the concentration of chloride ions in the soil.

[0008] Meanwhile, the present inventors have found through numerical analysis (described in detail below) that the cause of the deterioration in the accuracy of measuring the amount of corrosion (specifically, the accuracy of measuring the average electrical resistance value of the metal piece) when a corrosion sensor is used in an underground environment is the localized corrosion described above.The present inventors have also found that by performing numerical analysis that sets the size and number of localized corrosion pieces that may occur, it is possible to determine the width, length, and initial thickness of the metal piece that can reduce the error in measuring the amount of corrosion by the corrosion sensor (specifically, the error in measuring the average electrical resistance value of the metal piece). As mentioned above, the size and number of localized corrosion particles that may occur on a metal piece correlate with the concentration of chloride ions in the soil. Therefore, if this correlation is obtained in advance using a sample of the same type (same material) as the metal piece and the concentration of chloride ions in the soil in which the metal piece is buried is measured, the size and number of localized corrosion particles that will occur on the metal piece can be predicted from the measured chloride ion concentration and the correlation. By performing a numerical analysis using this predicted value, it is possible to determine the width, length, and initial thickness of the metal piece that can reduce the measurement error of the amount of corrosion by the corrosion sensor (measurement error of the average electrical resistance value of the metal piece).

[0009] The present invention was completed based on the above findings of the present inventors. That is, in order to solve the above-mentioned problems, the present invention provides a method for manufacturing a corrosion monitoring device comprising: a metal piece buried in the soil; an electrical resistance measurement means for measuring an electrical resistance value of the metal piece by passing a current through the metal piece; and a calculation means for calculating an amount of corrosion of the metal piece based on the electrical resistance value of the metal piece measured by the electrical resistance measurement means, the method comprising: a first correlation D=f1(C) which is a correlation between the concentration C of chloride ions contained in the soil and the magnitude D of localized corrosion that has occurred in a sample of the same type as the metal piece buried in the soil; and a second correlation N=f a concentration measurement step of measuring a chloride ion concentration C' contained in the soil in which the metal piece is buried, the corrosion monitoring device being provided with; a prediction step of predicting the magnitude D' and number N' of localized corrosion occurring in the metal piece based on the measured chloride ion concentration C', the first correlation D=f1(C) and the second correlation N=f2(C); and a method of predicting the width W, length L and initial thickness t of the metal piece, which can reduce measurement errors due to the localized corrosion in the electrical resistance value of the metal piece measured by the electrical resistance measurement means by performing a numerical analysis using the predicted magnitude D' and number N' of localized corrosion occurring in the metal piece. ini and a numerical analysis step of determining the width W, length L and initial thickness t of the metal piece. ini and a metal piece fabrication step of fabricating the metal piece according to the above.

[0010] According to the present invention, in the correlation acquisition step, a first correlation D=f1(C), which is the correlation between the concentration C of chloride ions in the soil and the magnitude D of localized corrosion that has occurred in a sample of the same type as the metal piece buried in the soil, and a second correlation N=f2(C), which is the correlation between the concentration C of chloride ions in the soil and the number N of localized corrosion that has occurred in a sample of the same type as the metal piece buried in the soil, are acquired in advance. Then, in the concentration measurement step, the concentration C' of chloride ions contained in the soil in which the metal piece equipped with the corrosion monitoring device is buried is measured, and in the prediction step, the magnitude D' and number N' of localized corrosion occurring in the metal piece can be predicted based on the measured chloride ion concentration C' and the first correlation D = f1(C) and the second correlation N = f2(C) (when the first correlation and the second correlation are expressed as functions of the chloride ion concentration C, by substituting C' for C in each function). Therefore, in the numerical analysis step, by performing (setting) numerical analysis using the predicted size D' and number N' of localized corrosion occurring in the metal piece, it is possible to reduce measurement errors due to localized corrosion in the electrical resistance value of the metal piece measured by the electrical resistance measuring means, and to calculate the width W, length L, and initial thickness t of the metal piece. ini can be determined. Finally, in the metal piece preparation step, the width W, length L and initial thickness t of the determined metal piece are ini By producing a metal piece according to the method described above, it is possible to manufacture a corrosion monitoring device that can monitor the corrosion level of the metal piece with high accuracy, regardless of the concentration of chloride ions contained in the soil in which the metal piece is buried.

[0011] In the present invention, the "size of localized corrosion" refers to the length of the long side of the circumscribing rectangle of the localized corrosion when the metal piece (or sample) is viewed from above. The "width of the metal piece" refers to the dimension in the direction perpendicular to the current flow direction and the thickness direction of the metal piece.

[0012] In the present invention, preferably, in the numerical analysis step, the predicted D' is substituted for D in the following formula (1), the predicted N' is substituted for N, and the maximum depth d of the localized corrosion that can occur is expressed as d: max R calculated by substituting ave The width W, length L and initial thickness t of the metal piece are determined so that the value of / R is equal to or greater than a predetermined threshold value. ini Determine.

number

[0013] In the above-mentioned preferred method, R in formula (1) is the electrical resistance value of the localized corrosion analysis model calculated by performing a numerical analysis on the localized corrosion analysis model (analysis model of a metal piece having localized corrosion), and R ave is the electrical resistance value of the general corrosion analysis model calculated by performing a numerical analysis on the general corrosion analysis model (an analysis model of a metal piece in which corrosion occurs evenly over the entire upper surface and the total volume of the corrosion is equal to that of the local corrosion analysis model). According to the findings of the inventors, however, the left side of Equation (1), R ave As shown on the right side, / R is the width W and length L of the metal piece, the size D and number N of localized corrosion in the localized corrosion analysis model, and the depth d of localized corrosion in the localized corrosion analysis model / initial thickness t of the metal piece. ini and function f3(L,W,N,D,d / t ini ) can be expressed as Here, the left side of equation (1) is R ave / R is an index showing the measurement error caused by local corrosion of the electrical resistance of the metal piece. ave When the value of / R is 1, the electrical resistance of the metal piece can be measured correctly even when localized corrosion has occurred, and as a result, the average corrosion amount of the metal piece can be measured correctly even when localized corrosion has occurred. ave The smaller the value of / R is below 1, the greater the measurement error in the average electrical resistance of the metal piece, and as a result, the average amount of corrosion of the metal piece cannot be measured correctly. Therefore, substitute the predicted D' for D on the right side of equation (1), substitute the predicted N' for N, and calculate the maximum depth d of localized corrosion that can occur. max R calculated by substituting ave The width W, length L and initial thickness t of the metal piece are adjusted so that the value of / R is equal to or greater than a predetermined threshold value (e.g., 0.8). ini By determining the above, it is possible to manufacture a corrosion monitoring device that can monitor the amount of corrosion of a metal piece with high accuracy. The maximum depth of localized corrosion that can occur is d max may be determined in advance by, for example, investigating the maximum depth of localized corrosion that has actually occurred in the past.

[0014] In order to solve the above problems, the present invention also provides a corrosion monitoring device manufactured by the above manufacturing method, in which the metal piece is buried in soil containing chloride ions. In the present invention, "soil containing chloride ions" means soil containing chloride ions at a concentration of 0.01 mass % or more in terms of NaCl concentration. [Effects of the Invention]

[0015] According to the present invention, it is possible to manufacture a corrosion monitoring device that can monitor the amount of corrosion of metal pieces buried in the soil with high accuracy. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing an example of the results of investigating the corrosion state of a metal piece buried in the ground. [Figure 2] 1 is a diagram showing a schematic configuration of a corrosion monitoring device according to an embodiment of the present invention. [Figure 3] 3 is a flow chart showing an outline of the steps of a method for manufacturing the metal piece 11 provided in the corrosion monitoring device 100 shown in FIG. [Figure 4] FIG. 4 is a diagram showing an example of localized corrosion observed in the correlation obtaining step ST1 shown in FIG. 3. [Figure 5]4 is a diagram showing an example of a first correlation D=f1(C) and a second correlation N=f2(C) acquired in the correlation acquisition step ST1 shown in FIG. 3. FIG. [Figure 6] 4 is a diagram schematically showing an example of an analytical model of a metal piece 11 used in the numerical analysis performed in the numerical analysis step ST4 shown in FIG. 3. FIG. [Figure 7] FIG. 11 is a diagram showing a summary of the results of a numerical analysis example. [Figure 8] FIG. 11 is a diagram showing a summary of the results of a numerical analysis example. [Figure 9] FIG. 11 is a diagram showing a summary of the results of a numerical analysis example. [Figure 10] 4 is a diagram for explaining a method for determining the width W, length L, and initial thickness tini of the metal piece 11 in the numerical analysis step ST4 shown in FIG. 3. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0018] [Configuration of corrosion monitoring equipment] First, the configuration of the corrosion monitoring device manufactured by the manufacturing method according to this embodiment will be described. Figure 2 is a diagram showing a schematic configuration of a corrosion monitoring device according to one embodiment of the present invention, where Figure 2(a) shows the overall configuration, and Figures 2(b) and 2(c) show other examples of the metal piece shown in Figure 2(a). 2(a), the monitoring device 100 according to this embodiment includes a corrosion sensor 10 buried in the ground, an electrical resistance measuring means 20 that measures the electrical resistance of the metal piece 11 by passing electricity through the metal piece 11 included in the corrosion sensor 10, and a calculation means 30 that calculates the amount of corrosion of the metal piece 11. In addition, in a preferred embodiment, the corrosion sensor 10 included in the monitoring device 100 according to this embodiment also includes a temperature measuring means 40 that measures the temperature of the metal piece 11. Below, the corrosion sensor 10, the electrical resistance measuring means 20, and the calculation means 30 that constitute the monitoring device 100 will be described in order.

[0019] <Corrosion Sensor 10> The corrosion sensor 10 comprises a metal piece 11, a substrate 12 on which the metal piece 11 is placed, and a temperature measuring means 40. The metal piece 11 is formed from the same type of metal as the metal product to be evaluated (e.g., a steel sheet pile, a steel pipe pile, etc.). In the example shown in FIG. 2(a), the shape of the metal piece 11 is rectangular in plan view, but this is not limited thereto. For example, as shown in FIG. 2(b), the metal piece 11 may be a U-shaped metal piece 11A in plan view, or as shown in FIG. 2(c), the metal piece 11 may be a serpentine-shaped metal piece 11B in plan view. The metal piece 11 has longitudinal ends 111 and 112 electrically connected to the electrical resistance measurement means 20. Wiring 211 and 221 electrically connecting the ends 111 and 112 of the metal piece 11 to the electrical resistance measurement means 20 are preferably covered with a predetermined covering member 13 to prevent disconnection due to corrosion. The same applies to the case where the metal pieces 11A and 11B are used.

[0020] The metal piece 11 has a surface that is exposed to the soil. That is, at least a portion of the metal piece 11 is exposed without being covered. In the example shown in FIG. 2, the side surfaces of the metal piece 11 are covered with resin or the like (not shown), and only the top surface is exposed to the soil. However, the present invention is not limited to this, and it is also possible to adopt a configuration in which the top surface and side surfaces of the metal piece 11 (more precisely, the parts of the side surfaces other than the parts that come into contact with the covering member 13) are exposed to the soil. The same applies when metal pieces 11A and 11B are used.

[0021] For example, a plastic substrate, which is an insulator, is used as the substrate 12. However, the present invention is not limited to this, and a substrate made of a conductor such as metal may also be used as the substrate 12. In this case, an insulator may be disposed between the substrate 12 and the metal piece 11. The same applies when the metal pieces 11A and 11B are used.

[0022] In this embodiment, a thermocouple 41 that measures the temperature of the metal piece 11 is used as the temperature measurement means 40. It is preferable to cover the thermocouple 41 with a predetermined covering member 42 to prevent the thermocouple 41 from breaking due to corrosion. In the example shown in FIG. 2(a), the tip of the thermocouple 41 is fixed to the underside of the metal piece 11 with insulating tape 43. The thermocouple 41 can be fixed in any manner, and for example, it can be fixed with a magnet or a screw. The fixing position of the thermocouple 41 (temperature measurement position) can also be arbitrary. Multiple thermocouples 41 can be used to measure temperatures at multiple positions on the metal piece 11, and the average value can be used as the temperature of the metal piece 11 used by the calculation means 30 described below. The same applies when metal pieces 11A and 11B are used.

[0023] The temperature measurement means 40 is not limited to the thermocouple 41, and any configuration can be used, such as an electrical resistance temperature sensor, as long as it can measure the temperature of the metal piece 11 buried in the soil. Also, since direct sunlight does not hit the soil, it is possible to measure the temperature of the soil and use that value as the temperature of the metal piece 11. Furthermore, the temperature measurement by the temperature measurement means 40 may be continuous or intermittent. The same applies when metal pieces 11A and 11B are used.

[0024] <Electrical resistance measuring means 20> The electrical resistance measuring means 20 includes a constant current power supply 21 , a voltmeter 22 , and an electrical resistance value calculating means 23 in order to measure the electrical resistance value of the metal piece 11 . The constant current power supply 21 is electrically connected to the ends 111 and 112 of the metal piece 11 by a wire 211 , and applies a constant current from the end 111 to the end 112 of the metal piece 11 . The voltmeter 22 is electrically connected to the ends 111 and 112 of the metal piece 11 by a wire 221 , and measures the voltage between the ends 111 and 112 of the metal piece 11 . The electrical resistance value calculation means 23 calculates the electrical resistance value of the metal piece 11 by dividing the voltage measured by the voltmeter 22 by the current supplied from the constant current power supply 21 . The measurement of the electrical resistance value of the metal piece 11 by the electrical resistance measuring means 20 may be continuous or intermittent. However, it is preferable that the timing of measuring the electrical resistance value of the metal piece 11 by the electrical resistance measuring means 20 and the timing of measuring the temperature of the metal piece 11 by the temperature measuring means 40 are synchronized. The same applies when the metal pieces 11A and 11B are used.

[0025] In addition, in order to accurately measure the electrical resistance value of metal piece 11, it is preferable to take measures to remove measurement noise caused by thermoelectric power due to the temperature difference between the connection part between end 111 of metal piece 11 and wiring 211, 221 and the connection part between end 112 of metal piece 11 and wiring 211, 221. Possible noise removal measures include, for example, (1) increasing the current flowing from constant current power supply 21 to increase the voltage measured by voltmeter 22, (2) subtracting the voltage measured by voltmeter 22 when the current flowing from constant current power supply 21 is stopped from the voltage measured by voltmeter 22 when current is flowing from constant current power supply 21, and using the resulting value as the voltage when calculating the electrical resistance value, (3) reversing the polarity of the current flowing from constant current power supply 21, and using the average of the absolute values ​​of the voltages measured when current of each polarity is flowing as the voltage when calculating the electrical resistance value, and (4) using an AC current (preferably in the low frequency range of 100 kHz or less, taking into account the skin effect) flowing through metal piece 11. Furthermore, it is preferable to use a coaxial cable or a twisted cable as the wiring 211, 221. This reduces the influence of external electromagnetic fields generated by the wiring 211, 221, and makes it possible to reduce measurement noise due to induced electromotive force.

[0026] The electrical resistance measuring means 20 is not limited to the configuration described above. For example, it is also possible to employ a configuration in which a constant voltage is applied between the end 111 and the end 112 of the metal piece 11, and the current flowing from the end 111 to the end 112 of the metal piece 11 is measured, thereby measuring the electrical resistance value of the metal piece 11.

[0027] <Calculation means 30> The calculation means 30 is composed of, for example, a computer in which a program for performing the calculations described below is stored, and calculates the amount of corrosion of the metal piece 11 based on the electrical resistance value of the metal piece 11 measured by the electrical resistance value calculation means 23. In a preferred embodiment, the calculation means 30 of this embodiment calculates the amount of corrosion of the metal piece 11 based on the electrical resistance value of the metal piece 11 measured by the electrical resistance value calculation means 23 and the temperature of the metal piece 11 measured by the temperature measurement means 40. Specifically, the calculation means 30 stores in advance a temperature-dependent function R0(T) that is the relationship between the initial electrical resistance R0 of the metal piece 11 and the temperature T. The initial electrical resistance R0 of the metal piece 11 is the electrical resistance value at a point in time before corrosion occurs in the metal piece 11, in other words, the initial thickness (t ini ) is the electrical resistance value at the point where

[0028] The temperature-dependent function R0(T) differs depending on the type of metal constituting the metal piece 11. Therefore, it is necessary to determine the temperature-dependent function R0(T) for each type of metal for which the amount of corrosion is to be measured. The temperature-dependent function R0(T) may be determined by actual measurement performed by changing the temperature each time the amount of corrosion is measured, or it may be determined by using a function obtained from a previous measurement or a function recorded in a database. Alternatively, a function theoretically determined from the chemical composition and structure of the metal may be used.

[0029] When actually measuring the temperature-dependent function R0(T) of the metal piece 11, it is preferable to measure it in a dry environment where the metal piece 11 will not corrode. In the temperature range underground, the electrical resistance of metals generally exhibits a nearly linear relationship with temperature. For this reason, for example, it is conceivable to measure the initial electrical resistance R0 of the metal piece 11 at two or more temperatures and linearly approximate the measurement results to obtain the temperature-dependent function R0(T). However, this is not a limitation, and the temperature-dependent function R0(T) may be a quadratic or higher function of the temperature T, or may be represented in the form of a table that records the correspondence between the initial electrical resistance R0 and the temperature T.

[0030] The calculation means 30 calculates the temperature-dependent function R0(T) described above and the electrical resistance R of the metal piece 11 measured by the electrical resistance calculation means 23. t and the temperature T of the metal piece 11 measured by the temperature measuring means 40. t The corrosion amount Δt of the metal piece 11 is calculated using the above. Specifically, the calculation means 30 calculates the measured temperature T based on the temperature-dependent function R0(T). t The initial electrical resistance value R0(T t Next, the calculation means 30 calculates the calculated initial electrical resistance value R0(T t ) and the measured electrical resistance R t R0(T t ) / R t Next, the calculation means 30 calculates the calculated ratio R0(T t ) / R t Based on this, the remaining thickness t' of the metal piece 11 at the time of measurement is calculated by the following formula (A). t'=t ini ×R0(T t ) / R t (A) In the above formula (A), t ini is the initial thickness of the metal piece 11. Finally, the calculation means 30 calculates the corrosion amount Δt of the metal piece 11 at the time of measurement using the following formula (B). Δt=t ini -t' (B)

[0031] In this embodiment, the calculation means 30 calculates the temperature T of the metal piece 11 measured by the temperature measurement means 40. t However, in an underground environment, there is no direct sunlight and there is little temperature change compared to an atmospheric environment, so it is not necessary to consider the temperature dependency of the electrical resistance value. When the temperature dependency of the electrical resistance value is not taken into consideration, R0(T t ) can be the initial electrical resistance value R0 (fixed value) measured at a representative temperature in the soil. Furthermore, based on the same concept as in Patent Documents 1 to 3, it is also possible to provide a reference part (a reference part of the same shape as the metal piece 11) that is isolated from the soil environment so that corrosion does not occur, separate from the metal piece 11 for measuring the amount of corrosion, and to adopt a configuration in which the amount of corrosion of the metal piece 11 is measured based on the electrical resistance value of the metal piece 11 and the electrical resistance value of the reference part. In this case, the temperature measuring means 40 is not necessary, and the initial thickness of the reference part is set to t ref_ini (= initial thickness t of metal piece 11 ini ), the initial electrical resistance of the metal piece 11 is R ini , the electrical resistance value of the metal piece 11 at the time of measurement is R t , the initial electrical resistance of the reference section is R ref_ini , the electrical resistance value of the reference part at the time of measurement is R ref_t Then, the corrosion amount Δt of the metal piece 11 at the time of measurement can be calculated by the following formula (C). Δt=t ref_ini ×(R ref_ini / R ini -R ref_t / R t ) ···(C)

[0032] [Manufacturing method for corrosion monitoring device] A method for manufacturing the corrosion monitoring device 100 having the above configuration will be described below, focusing mainly on a method for manufacturing the metal piece 11 provided in the corrosion monitoring device 100. FIG. 3 is a flow diagram showing an outline of the steps of a method for manufacturing the metal piece 11 provided in the corrosion monitoring device 100. As shown in FIG. 3, the manufacturing method according to this embodiment includes a correlation obtaining step ST1, a concentration measuring step ST2, a prediction step ST3, a numerical analysis step ST4, and a metal piece producing step ST5. Each of steps ST1 to ST5 will be described below in order.

[0033] <Correlation acquisition step ST1> In correlation acquisition step ST1, a first correlation D=f1(C) is acquired in advance, which is the correlation between the concentration C of chloride ions contained in the soil and the magnitude D of localized corrosion that has occurred in a sample of the same type (same material) as metal piece 11 buried in the soil. Also, in correlation acquisition step ST1, a second correlation N=f2(C) is acquired in advance, which is the correlation between the concentration C of chloride ions contained in the soil and the number N of localized corrosion that has occurred in the sample buried in the soil. Specifically, for example, soil containing different concentrations of NaCl aqueous solutions is placed in multiple containers, and multiple samples are buried in the soil in each container. After a certain period of time, the magnitude D and number N of localized corrosion that has occurred in each sample are measured, thereby making it possible to obtain the first correlation D = f1(C) and the second correlation N = f2(C).

[0034] Figure 4 shows an example of localized corrosion observed in correlation acquisition step ST1. Figure 4(a) shows an example of an image of a sample, and Figure 4(b) shows the definition of the magnitude of localized corrosion D. The area surrounded by a dashed line in Figure 4(a) is the localized corrosion LC that occurred on the sample. The number N of localized corrosion LC can be measured, for example, by a person visually inspecting the sample directly or by visually inspecting an image of the sample (in the example shown in FIG. 4, N=7). Furthermore, the size D of the localized corrosion LC can be determined, for example, by a person drawing a circumscribing rectangle of the localized corrosion LC on an image of the sample, as shown in FIG. 4(b), and the length of the long side of this circumscribing rectangle can be treated as the size D of the localized corrosion LC.

[0035] FIG. 5 shows examples of the first correlation D = f1(C) and the second correlation N = f2(C) acquired in correlation acquisition step ST1. Figure 5 shows correlations acquired from corrosion conditions after two months of burying four SM490A steel specimens (25 mm wide x 50 mm long) in soil at 40°C in each container. Figure 5(a) shows the first correlation D = f1(C), and Figure 5(b) shows the second correlation N = f2(C). The vertical axis of Figure 5(a) represents the average value of the magnitude D of multiple localized corrosion LCs that occurred in each specimen, and the vertical axis of Figure 5(b) represents the average number N of localized corrosion LCs that occurred in each specimen. The horizontal axes of Figures 5(a) and 5(b) represent the chloride ion concentration C converted to NaCl concentration. The chloride ion concentration C can be calculated by proportionally allocating the NaCl concentration according to the ratio of the atomic weight of Na to the atomic weight of Cl. In the example shown in FIG. 5(a), the magnitude D of localized corrosion LC that occurred in each sample buried in soil with each chloride ion concentration C (data plotted as "◯" in FIG. 5) is used to perform an approximate calculation using the least squares method or the like to calculate an approximate curve (shown by a dashed line) that is the first correlation D = f1(C). That is, the first correlation D = f1(C) is expressed as a function, but the present invention is not limited to this, and the first correlation can also be expressed in table format. The same applies to the second correlation N = f2(C).

[0036] <Concentration measurement step ST2> In the concentration measurement step ST2, the concentration C' of chloride ions contained in the soil in which the metal piece 11 provided in the corrosion monitoring device 100 is buried is measured. Specifically, for example, a portion of the soil in which the metal piece 11 is buried is taken as a sample, the weight of this sample is measured, and the sample is then dried. The weight of the dried sample is then measured. The value obtained by subtracting the weight after drying from the weight before drying is considered to be the weight of water contained in the sample before drying. Next, the dried sample is added to, for example, a predetermined amount of ion-exchanged water, and the solution is filtered. Then, the chloride ions contained in the filtered solution are quantified, for example, by ion chromatography. Finally, the quantified chloride ions are divided by the weight of water contained in the sample to measure the concentration of chloride ions contained in the sample before drying. In concentration measurement step ST2, the concentration of chloride ions contained in the measured sample can be treated as the concentration C' of chloride ions contained in the soil in which the metal piece 11 is buried.

[0037] <Prediction step ST3> In the prediction step ST3, the size D' and number N' of localized corrosion LC occurring in the metal piece 11 are predicted based on the measured chloride ion concentration C' and the first correlation D = f1(C) and the second correlation N = f2(C) (when the first correlation and the second correlation are expressed as functions of the chloride ion concentration C, by substituting C' for C in each function).

[0038] <Numerical analysis step ST4> In the numerical analysis step ST4, a numerical analysis is performed using the predicted size D' and number N' of localized corrosion LC occurring in the metal piece 11, thereby reducing the measurement error due to localized corrosion LC in the electrical resistance value of the metal piece 11 measured by the electrical resistance measuring means 20. ini Determine. The content of the numerical analysis performed in the numerical analysis step ST4 will be described below.

[0039] 6A and 6B are diagrams schematically illustrating an example of an analytical model of a metal piece 11 used in the numerical analysis performed in the numerical analysis step ST4. FIG. 6A is an analytical model (localized corrosion analytical model) of a metal piece 11 having a rectangular shape in plan view with localized corrosion, and FIG. 6B is an analytical model (general corrosion analytical model) of a metal piece 11 having a rectangular shape in plan view with corrosion having the same total volume as that of FIG. 6A evenly occurring over the entire upper surface. As shown in FIG. 6A, when five pieces of localized corrosion LC with length D × width D × depth d have occurred, the total volume of corrosion is 5 × D 2 On the other hand, as shown in Figure 6(b), if corrosion (amount of corrosion thinning) is Δt and occurs evenly over the entire upper surface, the total volume of corrosion is the length of the metal piece L × width of the metal piece W × amount of corrosion Δt. In the numerical analysis, these total volumes are set equal (i.e., 5 × D 2 Numerical analysis is performed using each analytical model set (xd = L x W x Δt), and the extent of error between the electrical resistance value calculated for the local corrosion analytical model of FIG. 6(a) and the electrical resistance value calculated for the general corrosion analytical model of FIG. 6(b) is evaluated. If there is no error in the electrical resistance value calculated for the local corrosion analytical model of FIG. 6(a), then, in accordance with the principles of the electrical resistance-type corrosion sensor 10, that value should be the same as the electrical resistance value calculated for the general corrosion analytical model of FIG. 6(b). Specific numerical analysis examples 1 to 5 will be described below in order. Note that FIGS. 7 to 9, which will be described later, are diagrams summarizing the results of the numerical analysis examples. Also, FIG. 10, which will be described later, shows the width W, length L, and initial thickness t of the metal piece 11 in numerical analysis step ST4. ini FIG. 10 is a diagram for explaining a method for determining the

[0040] (Numerical analysis example 1) In the numerical analysis example 1, the local corrosion analysis model shown in FIG. 6(a) was used. The length L of the metal piece 11 was 100 mm, the size D×D of the local corrosion LC was 3 mm×3 mm, and the number N of the local corrosion LC was fixed at 5. Under these conditions, the width W of the metal piece 11 was changed to 5 mm, 10 mm, 20 mm, and 30 mm, and the initial thickness t iniThe electrical resistance value R of the localized corrosion analysis model under each condition was calculated by numerical analysis under the conditions where the thickness of the localized corrosion LC was changed to 0.1 mm, 0.5 mm, 1.0 mm, 3.0 mm, and 6.0 mm, and the depth d of the localized corrosion LC was changed to various values ​​until the localized corrosion LC penetrated the metal piece 11. In calculating the electrical resistance value R, 14.2 μΩ cm was used as the volume resistivity ρ of the metal piece 11. In addition, for the general corrosion analysis model shown in Fig. 6(b), the corrosion volume Δt was set according to the depth d of the localized corrosion LC in each localized corrosion analysis model shown in Fig. 6(a) (i.e., the corrosion volume Δt was set so that the total volume of corrosion was equal to that of the localized corrosion analysis model shown in Fig. 6(a)). ave was calculated by numerical analysis. The localized corrosion LC of the localized corrosion analysis model shown in Fig. 6(a) was set to a condition that it did not overlap in the width direction of the metal piece 11. Furthermore, the electrical resistance value R of the localized corrosion analysis model shown in Fig. 6(a) was calculated by dividing the metal piece 11 in the longitudinal direction into areas with localized corrosion LC and areas without localized corrosion LC, calculating the electrical resistance value for each divided region, and assuming that these divided regions were connected in series.

[0041] 7(a) is a diagram summarizing the results of Numerical Analysis Example 1. The horizontal axis of FIG. 7(a) is the ratio of the depth d of the localized corrosion LC of the localized corrosion analysis model shown in FIG. 6(a) to the initial thickness t ini Therefore, when the value on the horizontal axis is 1, it means that the localized corrosion LC penetrates the metal piece 11. The vertical axis of FIG. 7(a) is the electrical resistance value R of the general corrosion analysis model shown in FIG. 6(b). ave This is the value obtained by dividing the value by the electrical resistance value R of the corrosion analysis model shown in Figure 6(a). Therefore, when the value on the vertical axis is 1, the electrical resistance value R can be measured correctly even when localized corrosion LC occurs as shown in Figure 6(a) (the electrical resistance value R of the general corrosion analysis model shown in Figure 6(b)). ave As a result, even if localized corrosion LC occurs, the average corrosion amount of the entire metal piece 11 can be measured correctly. As shown in Figure 7(a), under any condition, d / t iniThe larger the value of R ave If the width W of the metal piece 11 increases, the value of d / t ini R corresponding to ave It can be seen that the value of / R increases, and the measurement error of the corrosion amount can be reduced. ini Regardless of the value of (t ini (d / t is any value between 0.1 and 6.0 mm) ini and R ave The relationship between W and R is along the same curve (for example, when W=5 mm, t ini Regardless of the value of d / t between 0.1 and 6.0 mm, ini and R ave (The relationship between the initial thickness t of the metal piece 11 and the thickness t of the metal piece 11 is along the same curve plotted with "◇"). ini It can be seen that the larger the value of d (the smaller the value on the horizontal axis in FIG. 7(a) for the same depth d of localized corrosion LC) is, the easier it is to maintain the effect of reducing measurement errors.

[0042] [Numerical analysis example 2] In numerical analysis example 2, the electrical resistance value R of the localized corrosion analysis model shown in Figure 6(a) was calculated by numerical analysis under the same conditions as in numerical analysis example 1, except that the size D × D of the localized corrosion LC was fixed to 5 mm × 5 mm. In addition, as the general corrosion analysis model shown in Fig. 6(b), the electrical resistance value R of each general corrosion analysis model was calculated by setting the corrosion amount Δt according to the depth d of localized corrosion LC of each localized corrosion analysis model shown in Fig. 6(a). ave was calculated by numerical analysis.

[0043] Fig. 7(b) is a diagram summarizing the results of Numerical Analysis Example 2. The vertical and horizontal axes in Fig. 7(b) are the same as those in Numerical Analysis Example 1 shown in Fig. 7(a). As shown in Figure 7(b), under any condition, d / t ini The larger the value of R ave The relationship in which the value of / R becomes smaller is the same as in the case of numerical analysis example 1 shown in Figure 7(a), butini R corresponding to ave The value of / R is different from that in the first numerical analysis example.

[0044] [Numerical analysis example 3] In Numerical Analysis Example 3, the electrical resistance value R of the localized corrosion analysis model shown in Figure 6(a) was calculated by numerical analysis under the same conditions as in Numerical Analysis Example 1, except that the size D × D of the localized corrosion LC was fixed to 8 mm × 8 mm. In addition, as the general corrosion analysis model shown in Fig. 6(b), the electrical resistance value R of each general corrosion analysis model was calculated by setting the corrosion amount Δt according to the depth d of localized corrosion LC of each localized corrosion analysis model shown in Fig. 6(a). ave was calculated by numerical analysis.

[0045] Fig. 8(a) is a diagram summarizing the results of Numerical Analysis Example 3. The vertical and horizontal axes in Fig. 8(a) are the same as those in Numerical Analysis Example 1 shown in Fig. 7(a). As shown in Figure 8(a), under any condition, d / t ini The larger the value of R ave The relationship in which the value of / R becomes smaller is the same as in the case of numerical analysis example 1 shown in Figure 7(a), but ini R corresponding to ave The value of / R is different from that in the first numerical analysis example.

[0046] From the numerical analysis examples 1 to 3 explained above, R ave The value of / R is determined by the width W of the metal piece 11, the size D of the local corrosion LC, and d / t ini It can be seen that it changes depending on the value of .

[0047] [Numerical analysis example 4] In Numerical Analysis Example 4, the electrical resistance value R of the localized corrosion analysis model shown in Figure 6(a) was calculated by numerical analysis under the same conditions as in Numerical Analysis Example 2, except that the number N of localized corrosion LCs was fixed to 2. In addition, as the general corrosion analysis model shown in Fig. 6(b), the electrical resistance value R of each general corrosion analysis model was calculated by setting the corrosion amount Δt according to the depth d of localized corrosion LC of each localized corrosion analysis model shown in Fig. 6(a). ave was calculated by numerical analysis.

[0048] Fig. 8(b) is a diagram summarizing the results of Numerical Analysis Example 4. The vertical and horizontal axes in Fig. 8(b) are the same as those in Numerical Analysis Example 2 shown in Fig. 7(b). As shown in Figure 8(b), under any condition, d / t ini The larger the value of R ave The relationship in which the value of / R becomes smaller is the same as in the case of numerical analysis example 2 shown in Figure 7(b), but ini R corresponding to ave The value of / R is different from that in Numerical Analysis Example 2.

[0049] [Numerical analysis example 5] In Numerical Analysis Example 5, the electrical resistance value R of the localized corrosion analysis model shown in Figure 6(a) was calculated by numerical analysis under the same conditions as in Numerical Analysis Example 2, except that the number N of localized corrosion LCs was fixed to 10. In addition, as the general corrosion analysis model shown in Fig. 6(b), the electrical resistance value R of each general corrosion analysis model was calculated by setting the corrosion amount Δt according to the depth d of localized corrosion LC of each localized corrosion analysis model shown in Fig. 6(a). ave was calculated by numerical analysis.

[0050] Fig. 9 is a diagram summarizing the results of Numerical Analysis Example 5. The vertical and horizontal axes in Fig. 9 are the same as those in Numerical Analysis Example 2 shown in Fig. 7(b). As shown in Figure 9, under any condition, d / t ini The larger the value of R ave The relationship in which the value of / R becomes smaller is the same as in the case of numerical analysis example 2 shown in Figure 7(b), but ini R corresponding to ave The value of / R is different from that in Numerical Analysis Example 2.

[0051] From the numerical analysis examples 2, 4, and 5 explained above, R ave The value of / R is determined by the width W of the metal piece 11, the number N of localized corrosion LC, and d / t ini It can be seen that it changes depending on the value of .

[0052] In all of the numerical analysis examples 1 to 5, the length L of the metal piece 11 was fixed at 100 mm. ave The value of / R also changes depending on the length L of the metal piece 11. Therefore, in the numerical analysis, R ave / R is the width W and length L of the metal piece 11, the size D and number N of localized corrosion in the localized corrosion analysis model, and the depth d of localized corrosion in the localized corrosion analysis model / initial thickness t of the metal piece 11. ini and function f3(L,W,N,D,d / t ini ) can be expressed as Specifically, according to the findings of the present inventors, R ave / R can be expressed by the following formula (1).

number

[0053] Therefore, as shown in FIG. 10, in the numerical analysis step ST4 of this embodiment, D′ predicted in the prediction step ST3 is substituted for D on the right side of the above formula (1), N′ predicted in the prediction step ST3 is substituted for N, and the maximum depth d of the localized corrosion LC that may occur is substituted for d. max (i.e., the size D and number N of the localized corrosion LC are fixed to constants D' and N', respectively), and the unknown variables on the right side of equation (1) are replaced by the width W, length L, and initial thickness t of the metal piece 11. ini and the depth of localized corrosion LC is d. In other words, R ave / R, W, L, t ini The maximum depth d of localized corrosion LC that can occur is expressed as a function of d on the right side of equation (1). max R calculated by substituting ave The width W, length L and initial thickness t of the metal piece 11 are determined so that the value of / R is equal to or greater than a predetermined threshold value Th. iniDetermine the maximum depth d of localized corrosion LC that can occur. max is, for example, the maximum depth d of localized corrosion LC that actually occurred in the past. max should be investigated and determined in advance. For example, if we substitute L1 for L on the right side of equation (1) and W1 for W, t ini to ini1 When substituted, d / t ini (=d / t ini1 ) and R ave When the relationship between the temperature and R is expressed by the dashed curve in Figure 10, d = d max When ave The value of / R is less than the threshold value Th (the data is plotted as "●" in Figure 10). Therefore, it is determined that the measurement error caused by the local corrosion LC of the electrical resistance value R of the metal piece 11 cannot be reduced. On the other hand, if we substitute L2 for L on the right side of equation (1) and W2 for W, t ini to ini2 When substituted, d / t ini (=d / t ini2 ) and R ave When the relationship between the temperature and R is expressed by the solid curve in Figure 10, d = d max When ave The value of / R is equal to or greater than the threshold value Th (the data is plotted as "◯" in FIG. 10). Therefore, it is possible to reduce the measurement error of the electrical resistance value R of the metal piece 11 due to the local corrosion LC, and the width W, length L, and initial thickness t of the metal piece 11 can be reduced. ini are W2, L2 and t ini2 can be determined.

[0054] <Metal piece production step ST5> In the metal piece preparation step ST5, the width W, length L and initial thickness t of the determined metal piece 11 are ini (In the example shown in FIG. 10, W2, L2 and t ini2 ) is prepared. That is, the width W, length L and initial thickness t ini A metal piece 11 having the following structure is prepared.

[0055] The procedure described above allows the manufacture of the metal piece 11 included in the corrosion monitoring device 100. The corrosion monitoring device 100 can be manufactured by placing this metal piece 11 on the upper surface of a substrate 12 to manufacture a corrosion sensor 10, attaching an electrical resistance measuring means 20 to this corrosion sensor 10, and attaching a calculating means 30 to the electrical resistance measuring means 20.

[0056] According to the manufacturing method of this embodiment, in the correlation acquisition step ST1, a first correlation D=f1(C), which is the correlation between the concentration C of chloride ions in the soil and the magnitude D of localized corrosion that has occurred in a sample of the same type as the metal piece 11 buried in the soil, and a second correlation N=f2(C), which is the correlation between the concentration C of chloride ions in the soil and the number N of localized corrosion that has occurred in a sample of the same type as the metal piece 11 buried in the soil, are acquired in advance. Then, in the concentration measurement step ST2, the concentration C' of chloride ions contained in the soil in which the metal piece 11 equipped in the corrosion monitoring device 100 is buried is measured, and in the prediction step ST3, the size D' and number N' of localized corrosion LC occurring in the metal piece 11 can be predicted based on the measured chloride ion concentration C' and the first correlation D = f1(C) and the second correlation N = f2(C) (when the first correlation and the second correlation are expressed as functions of the chloride ion concentration C, by substituting C' for C in each function). Therefore, in the numerical analysis step ST4, by performing (setting) numerical analysis using the predicted size D' and number N' of localized corrosion LC occurring in the metal piece 11, the width W, length L, and initial thickness t of the metal piece 11 can be reduced, which can reduce the measurement error caused by the localized corrosion LC of the electrical resistance value R of the metal piece 11 measured by the electrical resistance measurement means 20. ini can be determined. Finally, in the metal piece preparation step ST5, the width W, length L and initial thickness t of the determined metal piece 11 are ini By producing a metal piece 11 according to the above, it is possible to manufacture a corrosion monitoring device 100 that can monitor the amount of corrosion of the metal piece 11 with high accuracy, regardless of the concentration of chloride ions contained in the soil in which the metal piece 11 is buried. [Explanation of symbols]

[0057] 10. Corrosion sensor 11, 11A, 11B...Metal piece 20. Electrical resistance measurement means 30...Arithmetic means 100 Corrosion monitoring device ST1: Correlation acquisition step ST2: Concentration measurement step ST3···Prediction step ST4: Numerical analysis step ST5: Metal piece preparation step

Claims

1. A metal piece buried in the ground; an electrical resistance measuring means for measuring the electrical resistance of the metal piece by passing a current through the metal piece; a calculation means for calculating the corrosion level of the metal piece based on the electrical resistance value of the metal piece measured by the electrical resistance measuring means; A method for manufacturing a corrosion monitoring device, comprising: A first correlation D=f, which is a correlation between the concentration C of chloride ions contained in the soil and the magnitude D of localized corrosion that occurred in a sample of the same type as the metal piece buried in the soil. １ (C), and a second correlation N=f, which is a correlation between the concentration C of chloride ions contained in the soil and the number N of localized corrosion that occurred in the sample buried in the soil. ２ a correlation acquisition step of acquiring (C) in advance; a concentration measurement step of measuring a concentration C' of chloride ions contained in the soil in which the metal piece is buried, the concentration C' being provided by the corrosion monitoring device; The measured chloride ion concentration C′ and the first correlation D=f １ (C) and the second correlation N=f ２ (C) a prediction step of predicting the size D' and number N' of localized corrosion occurring on the metal piece based on By performing a numerical analysis using the predicted size D' and number N' of the localized corrosion occurring in the metal piece, a measurement error due to the localized corrosion of the electrical resistance value of the metal piece measured by the electrical resistance measuring means can be reduced. ｉｎｉ a numerical analysis step for determining The determined width W, length L and initial thickness t of the metal piece ｉｎｉ and a metal piece manufacturing step of manufacturing the metal piece according to the above. A method for manufacturing a corrosion monitoring device.

2. In the numerical analysis step, the predicted D' is substituted for D in the following formula (1), the predicted N' is substituted for N, and the maximum depth d of the localized corrosion that can occur is substituted for d. ｍａｘ R calculated by substituting ａｖｅ The width W, length L and initial thickness t of the metal piece are determined so that the value of / R is equal to or greater than a predetermined threshold value. ｉｎｉ Determine A method for manufacturing the corrosion monitoring device according to claim 1. [Equation 3] In the above formula (1), d represents the depth of localized corrosion set in the analysis model of a metal piece having localized corrosion (localized corrosion analysis model). R represents the electrical resistance value of the localized corrosion analysis model calculated by performing a numerical analysis on the localized corrosion analysis model. R ａｖｅ means the electrical resistance value of the general corrosion analysis model calculated by performing a numerical analysis on an analysis model of a metal piece in which corrosion occurs evenly over the entire upper surface and the total volume of the corrosion is equal to that of the local corrosion analysis model (general corrosion analysis model).

3. The metal piece is manufactured by the manufacturing method according to claim 1 or 2, and is buried in soil containing chloride ions. Corrosion monitoring equipment.

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