Corrosion estimation device and method

Abstract

This corrosion estimation device comprises a sensor (101), a first processing circuit (102), a second processing circuit (103), and a third processing circuit (104). The sensor (101) measures the water content of soil of a target site. The first processing circuit (102) estimates, on the basis of a water content measurement value measured by the sensor (101), an attenuation function representing a change over time in the proportion of area wetted with water on the surface of a metal structure embedded at the target site. The second processing circuit (103) estimates, on the basis of the estimated attenuation function, a speed increase function for increasing the corrosion speed of the metal structure derived from the thickness of a water film formed on the surface of the metal structure. The third processing circuit (104) estimates a change over time function of the corrosion speed given by the product of the attenuation function and the speed increase function.

Description

[Technical field]

[0001] The present invention relates to a corrosion estimation device and method for estimating corrosion of a structure buried in the ground. [Background technology]

[0002] The infrastructure facilities that support our lives are numerous and of great variety. In addition, infrastructure facilities are exposed to a variety of environments, including not only urban areas but also mountainous areas, coastal areas, hot spring areas, cold regions, and even underwater and underground, and the forms and speeds of deterioration vary. For this reason, it is necessary to grasp the deterioration state of the facilities through visual inspection and other means and to carry out appropriate maintenance. However, for metal underground facilities such as steel pipe columns, support anchors, and steel piping, visual inspection of the parts hidden underground is not possible, making it difficult to carry out maintenance according to the deterioration state.

[0003] Metal underground facilities, being in contact with soil, corrode and deteriorate at different rates depending on the external environment (Non-Patent Document 1, Non-Patent Document 2, Non-Patent Document 3). The corrosion rate of metals in soil can be calculated by the following methods. For example, the corrosion rate can be calculated by taking out metal that has been buried for a certain period of time, and dividing the amount of corrosion measured directly by the period of burial. However, this method requires a long period of time to calculate the corrosion rate, and requires burying and excavating samples, removing rust from the samples that have been taken out, and the work efficiency is extremely low. In addition, this method only provides information on the changes in condition before and after burial, and it is not possible to monitor the changes in the corrosion rate over time.

[0004] On the other hand, there are also methods to evaluate the corrosion rate using electrochemical techniques such as the AC impedance method. In the AC impedance method, the charge transfer resistance Rct in the soil is measured by applying a small AC current to electrodes. Since the inverse of the charge transfer resistance Rct is proportional to the corrosion rate, the corrosion rate can be evaluated by measuring the charge transfer resistance Rct. In addition, the change in the corrosion rate over time can also be measured by monitoring the charge transfer resistance Rct. However, the measuring equipment used in the AC impedance method is generally expensive and is not suitable for, for example, multi-point observation or long-term continuous measurement. In addition, when trying to measure a location deep underground, the distance between the electrodes and the measuring equipment becomes long, and there is a problem that the measurement becomes difficult due to the influence of external noise, etc. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Morio Kadoi et al., "Study on the Soil Corrosion of Metallic Materials (Part 1) - Fundamental Experiments on Soil", Corrosion Prevention Technology, Vol. 16, No. 6, pp. 238-246, 1967. [Non-Patent Document 2] Yoshikazu Miyata and Shuji Asakura, "Soil corrosion measurement mainly based on electrochemical techniques (part 2) - Estimation of corrosion rate by combining multiple information and proposals for future measurements -", ​​Materials and Environment, Vol. 46, No. 10, pp. 610-619, 1997. [Non-Patent Document 3] Tomomi Tsunoda and Tetsuro Akiba, "Perspectives for Evaluating Soil Corrosion," Corrosion Prevention Technology, Vol. 36, No. 3, pp. 168-177, 1987. Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, the conventional methods have had the problem that it is difficult to measure the corrosion rate of metals buried underground simply and inexpensively.

[0007] The present invention has been made to solve the above problems, and has an object to make it possible to measure the corrosion rate of metals buried underground simply and inexpensively. [Means for solving the problem]

[0008] The corrosion estimation method of the present invention includes a measurement step of measuring the moisture content of soil at a target site, a first processing step of estimating a decay function representing the change over time in the proportion of the area of ​​the surface of a metal structure buried in the target site that is wet with water based on the moisture content measurement value measured in the measurement step, a second processing step of estimating from the decay function a rate increase function that increases the corrosion rate of the metal structure, which is derived from the thickness of the water film formed on the surface of the metal structure, and a third processing step of estimating a function of the change over time of the corrosion rate given by the product of the decay function and the rate increase function.

[0009] In addition, the corrosion estimation device of the present invention includes a sensor that measures the moisture content of soil in a target area, a first processing circuit that estimates a decay function that represents the change over time in the proportion of the area of ​​the surface of a metal structure buried in the target area that is wet with water based on the moisture content measurement value measured by the sensor, a second processing circuit that estimates a rate increase function that increases the corrosion rate of the metal structure from the decay function, which is derived from the thickness of the water film formed on the surface of the metal structure, and a third processing circuit that estimates a time-dependent function of the corrosion rate given by the product of the decay function and the rate increase function. Effect of the Invention

[0010] As described above, according to the present invention, a decay function is estimated based on the measured moisture content value, a rate increase function derived from the thickness of the water film is estimated from the decay function, and a time-dependent corrosion rate function given by the product of the decay function and the rate increase function is estimated, thereby making it possible to measure the corrosion rate of metals buried underground simply and inexpensively. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a configuration diagram showing the configuration of a corrosion estimation device according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a flowchart illustrating a corrosion estimation method according to an embodiment of the present invention. [Diagram 3] FIG. 3 is a configuration diagram showing a hardware configuration of the corrosion estimation device according to the embodiment of the present invention. [Figure 4]FIG. 4 is an explanatory diagram showing an image of estimation using the corrosion estimation device according to the embodiment of the present invention. [Diagram 5] FIG. 5 is a characteristic diagram showing the change over time in the corrosion rate of metals in soil measured by the AC impedance method. [Figure 6A] FIG. 6A is a cross-sectional view that typically shows the change over time of water on the surface of a metal structure 201 buried in soil. [Figure 6B] FIG. 6B is a cross-sectional view that typically shows the change over time of water on the surface of metal structure 201 buried in soil. [Figure 7] FIG. 7 is a characteristic diagram showing the change in the wet area ratio on the surface of a metal structure buried in soil. [Figure 8A] FIG. 8A is a cross-sectional view that illustrates the state of a water film 204 on the surface of a metal structure 201. As shown in FIG. [Figure 8B] FIG. 8B is a cross-sectional view that illustrates a state in which the water on the surface of the metal structure 201 has changed into a thinner water film 204a as the water drains away. [Figure 9] FIG. 9 is a characteristic diagram showing the change over time in the corrosion rate caused by a decrease in the thickness of the water film on the surface of a metal structure buried in soil. [Figure 10] FIG. 10 is a characteristic diagram showing an example of a model of the change over time in the corrosion rate of metals in soil. [Figure 11] FIG. 11 is a characteristic diagram showing a state in which a corrosion rate model is fitted to the measured values ​​shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A corrosion estimation device according to an embodiment of the present invention will be described below with reference to Fig. 1. This corrosion estimation device includes a sensor 101, a first processing circuit 102, a second processing circuit 103, a third processing circuit 104, a fourth processing circuit 105, a storage unit 106, and a display unit 107.

[0013] The sensor 101 measures the moisture content of soil in a target area. The sensor 101 obtains the moisture content of the soil, for example, by measuring the dielectric properties of the soil. The sensor 101 also measures the moisture content of the soil by utilizing the matric potential, which indicates the soil's moisture retention capacity. The measurement results can be stored in the memory unit 106.

[0014] The first processing circuit 102 estimates an attenuation function that represents a change over time in the ratio of the surface area of ​​the metal structure buried in the target ground that is wet with water, based on the moisture content measured by the sensor 101. The estimated attenuation function can be stored in the memory unit 106.

[0015] The second processing circuit 103 estimates a rate increase function that increases the corrosion rate of the metal structure, which is derived from the thickness of the water film formed on the surface of the metal structure, from the estimated attenuation function. The second processing circuit 103 estimates the rate increase function from the attenuation function stored in the storage unit 106. The estimated rate increase function can be stored in the storage unit 106.

[0016] The third processing circuit 104 estimates a function of change in corrosion rate over time given by the product of the attenuation function and the rate increase function. The third processing circuit 104 estimates the function of change in corrosion rate over time from the attenuation function and the rate increase function stored in the memory unit 106.

[0017] The fourth processing circuit 105 uses a time-varying function to determine the corrosion rate of the metal structure from the measured moisture content of the soil of the target site. The fourth processing circuit 105 also displays the determined corrosion rate on the display unit 107.

[0018] Next, the corrosion estimation method according to the present invention will be described with reference to FIG.

[0019] First, in step S101, the moisture content of the soil of the target land is measured by the sensor 101 (measurement step). The measurement information of the soil moisture content of the target land can be obtained by any principle or means as long as information on the amount of moisture or the ratio of moisture contained in the soil can be obtained. For example, a method of deriving the moisture content by measuring the dielectric properties of the soil, or a case where the moisture content is measured using the matric potential that indicates the soil moisture retention capacity can be given. For example, a soil moisture sensor is used to measure the change in the soil moisture content over time.

[0020] Next, in step S102, based on the moisture content measured in step S101 (measurement step), the first processing circuit 102 estimates an attenuation function that represents the change over time in the proportion of the area of ​​the surface of the metal structure buried in the target ground that is wet with water (first processing step).

[0021] Next, in step S103, the second processing circuit 103 estimates a rate increase function that increases the corrosion rate of the metal structure, which is derived from the thickness of the water film formed on the surface of the metal structure, from the attenuation function (second processing step).

[0022] Next, in step S104, the third processing circuit 104 estimates a function of change over time of the corrosion rate given by the product of the attenuation function and the rate increase function (third processing step).

[0023] Next, in step S105, the fourth processing circuit 105 uses the obtained time-varying function to obtain the corrosion rate of the metal structure from the measured moisture content of the soil of the target site (fourth processing step). The obtained corrosion rate is displayed on the display unit 107.

[0024] The first processing circuit 102, the second processing circuit 103, the third processing circuit 104, the fourth processing circuit 105, and the storage unit 106 of the corrosion estimation device according to the embodiment described above may be a computer device including a CPU (Central Processing Unit) 131, a main storage device 132, an external storage device 133, a network connection device 134, and the like, as shown in FIG. 3, and the above-mentioned functions (corrosion estimation method) may be realized by the CPU 131 operating (executing the program) according to a program deployed in the main storage device 132. The above-mentioned program is a program for causing a computer to execute the corrosion estimation method shown in the embodiment described above. The network connection device 134 is connected to a network 135. Also, each function may be distributed to a plurality of computer devices.

[0025] Here, the moisture content of the soil of the target site can be obtained by installing a sensor 101 in soil 142 in which the equipment, which is a structure made of metal to be estimated, is buried, as shown in Fig. 4. It is preferable to install the sensor 101 in an environment as close as possible to the installation environment of the equipment, and installing the sensor 101 as close as possible to the equipment increases the accuracy.

[0026] Furthermore, the corrosion estimation apparatus 100 can be a computer device as described above, and can be realized, for example, by an electronic device such as a general personal computer or a tablet. For example, the sensor 101 can be provided with a transmission function 144 for transmitting measured information, and configured to be able to communicate with the corrosion estimation apparatus 100 via a communication network 145. It is also possible for one corrosion estimation apparatus 100 to correspond to a plurality of sensors 101. The display unit 107 can be realized by a personal computer monitor, a wireless device, or the like.

[0027] A more detailed explanation is given below. Corrosion in soil progresses through chemical reactions between the surface of a metal structure and the soil in contact with it. Figure 5 shows an example of the change over time in the corrosion rate of metal in soil (1 / Rct, Rct: charge transfer resistance) measured using the AC impedance method. Time = 0 is when it is raining, and this is a typical example showing the change in corrosion rate as drainage progresses over time after the rain stops.

[0028] As drainage progresses, there is no significant change in the corrosion rate for a while, but as time passes, the corrosion rate increases rapidly and then begins to decrease. Figure 5 shows the change in corrosion rate due to a single rainfall, but it is known that in actual soil, the behavior shown in Figure 5 is repeated every time there is rainfall.

[0029] As mentioned above, a characteristic of soil environments is that the amount of moisture in the soil changes over time due to rainfall, and the corrosion rate changes depending on the state of the water on the surface of the buried metal structure and the dissolved oxygen in the water. First, consider the water on the surface of the metal structure. When it rains, the soil is filled with water, but when the rain stops and the water drains away, the amount of moisture in the soil (soil moisture content) decreases. Therefore, the amount of water present on the surface of the buried metal also changes over time.

[0030] 6A and 6B are schematic diagrams showing the change over time of water on the surface of a metal structure 201 buried in soil. There are a plurality of soil particles 202 on the surface of the metal structure 201. Since metal corrosion occurs in the parts in contact with water, the rate of corrosion on the surface of the metal structure 201 is equal to the rate of the area wetted with water. The state in which the soil is filled with water during rainfall is defined as a wet area ratio of 1. In this state, as shown in FIG. 6A, the spaces between adjacent soil particles 202 are filled with soil water 203. After this, as the rain stops and the water drains, the soil water 203 filling the spaces between adjacent soil particles 202 decreases, and as shown in FIG. 6B, a water film 203a is formed around each soil particle 202, and a space without water is formed between adjacent soil particles 202, and the wet area ratio on the surface of the metal structure 201 decreases.

[0031] In general soil, when drainage begins, water initially drains into the ground due to gravity. After this, water trapped by the soil's capillary phenomenon remains for a long time and dries out due to evaporation, etc., so the change in the wetted area ratio shows a decay behavior over time as shown in Figure 7.

[0032] On the other hand, the effect of dissolved oxygen in water on the corrosion rate changes depending on the thickness of the water film on the surface of the metal structure. The thickness of the water film on the surface of the metal structure decreases as the water drains. Figures 8A and 8B show the state in which the thickness of the water film 204 on the surface of the metal structure 201 changes to a thinner water film 204a as the water drains.

[0033] Here, as shown in FIG. 8A, when the thickness of the water film 204 existing on the surface of the metal structure 201 is equal to or greater than the steady diffusion layer thickness D, the corrosion rate is constant regardless of the water film thickness, and therefore the effect of the water film thickness on the corrosion rate remains almost constant from the time of rainfall until drainage progresses to a certain extent.

[0034] In contrast, when the water drains to a certain extent and the thickness W of the water film 204a on the surface of the metal structure 201 becomes thinner than the steady diffusion layer thickness D as shown in FIG. 8B, the corrosion rate increases as the water film thickness W becomes thinner.

[0035] The thickness of the water film is related to the diffusion limit current density of dissolved oxygen, and is theoretically proportional to the oxygen partial pressure of the gas in contact with the thin water film, and inversely proportional to the water film thickness. However, since there is a limit to the amount of dissolved oxygen that can react on the surface of the metal structure, the corrosion rate does not increase infinitely. Therefore, the change over time in the corrosion rate caused by the change in water film thickness (decrease in water film thickness) will show the behavior shown in Figure 9.

[0036] As described above, there are effects on the corrosion rate due to the wetted area and the thickness of the water film, and the actual corrosion rate changes taking into account both of these effects simultaneously.

[0037] In the present invention, as described below, the effect of the wetted area on the corrosion rate is expressed as a decay function, and the effect of the water film thickness on the corrosion rate is expressed as an increasing function, and the function expressed as the product of these is used as a model for expressing the corrosion rate of metal in soil.

[0038] FIG. 10 is an example of a model of the change over time in the corrosion rate of metals in soil. In FIG. 10, (a) shows the change in wetted area, (b) shows the change in corrosion rate caused (or resulting from) the thickness of the water film, and (c) shows the change in corrosion rate of a metal structure buried in soil. Also, FIG. 11 is an example of fitting the above-mentioned corrosion rate model (solid line) to the measured values ​​(white circles) shown in FIG. 5. In this way, it has been found that the corrosion rate model well explains the tendency of the corrosion rate of soil actually measured by the AC impedance method.

[0039] It is known that the effects of the wetted area and the water film thickness are both determined by the state of the solid phase, i.e., the soil particles, present on the surface of the metal structure. For example, when the soil particles are large and uniform, drainage is fast and capillary action is difficult, so the wetted area decreases quickly and the thin water film does not remain for a long time.

[0040] On the other hand, in the case of soil with fine particle size, such as clay, drainage is slow and the decrease in wetted area is extremely gradual, but as the water drains away, the small amount of water retained by capillary action tends to form a thin water film on the surface of the metal structure.

[0041] In this way, the effects of the wetted area and the water film thickness are not independent of each other, but are determined by the state of the soil particles and are deeply related to each other. Therefore, if we know the decay function that expresses the change in the wetted area over time, we can also define the increase function that expresses the change in the water film thickness over time, so in order to know the change in the corrosion rate over time, we only need to know the change in the wetted area over time.

[0042] Therefore, if information on the change in soil moisture content over time after one rain event can be obtained, a model for the change in corrosion rate over time can be derived based on this, and then this model can be used to determine (estimate) the corrosion rate from only the soil moisture content information.

[0043] Next, we will explain the estimation of the decay function that expresses the change over time in the proportion of wetted area based on the measured moisture content. This estimation can be performed by directly using the change over time in soil moisture content to derive the best fitting decay function. There are no particular restrictions on the function to be fitted, but for example, the following decay function Y1 can be used. In the following formula, t is time, and a, b, and t1 are parameters.

[0044]

number

[0045] However, the soil moisture content and the wet area ratio on the surface of the metal structure are not strictly the same, so it is advisable to derive the relationship between the soil moisture content and the wet area ratio on the surface of the metal structure in advance, for example, by the following method, so that the change over time in the wet area ratio on the surface of the metal structure can be estimated from the measurement results of the soil moisture content.

[0046] First, a soil moisture sensor and electrodes for the AC impedance method are buried in the soil, and changes in the moisture content and corrosion rate due to the soil becoming wet or dry are measured.

[0047] Next, the change in corrosion rate is fitted with a time-dependent corrosion rate function given by the product of the decay function and the rate increase function, and the relationship can be derived by comparing the decay function with the change in moisture content.

[0048] Based on this previously obtained relationship, the change over time in the wet area ratio on the surface of the metal structure is derived from the measurement results of the soil moisture content. The function used for fitting the derived change over time in the wet area ratio on the surface of the metal structure is not particularly limited, but the aforementioned attenuation function Y1 can be used.

[0049] Next, the estimation of the rate increase function using the attenuation function will be explained. From the attenuation function obtained (estimated) as described above, the rate increase function derived from the water film thickness on the surface of the metal structure is estimated. As described above, the influence of the wetted area and the influence of the water film thickness are not independent of each other, but are determined by the state of the soil particles and are deeply related to each other. Therefore, if the attenuation function expressing the change in the wetted area over time is known, the increase function expressing the change in the water film thickness over time can also be defined, so that in order to know the change in the corrosion rate over time, it is necessary only to know the change in the wetted area over time.

[0050] That is, by deriving the relationship between the attenuation function expressing the change in wetted area over time and the increase function expressing the change in water film thickness over time in advance through experiments, etc., the velocity increase function derived from the water film thickness on the surface of the metal structure can be estimated from the estimated attenuation function. Note that there are no particular restrictions on the velocity increase function, but for example, the following function Y2 is one of the functions that can well describe the phenomenon. Note that K, α, β, γ, and t0 are parameters.

[0051]

number

[0052] Next, we will explain how to estimate the function of change in corrosion rate over time. We estimate the function of change in corrosion rate over time, which is given by the product of the decay function and the rate increase function. When Y1 is used as the decay function and Y2 is used as the rate increase function, the estimated change in corrosion rate over time is expressed by the following function Y.

[0053]

number

[0054] By going through the steps described above, it is possible to estimate information about changes in corrosion rate over time from information about changes in soil moisture content over time. This means that even when you want to know the corrosion rate, you can estimate it simply by measuring the soil moisture content, without using conventional expensive AC impedance devices.

[0055] As described above, according to the present invention, a decay function is estimated based on the measured moisture content value, a rate increase function derived from the thickness of the water film is estimated from the decay function, and a time-varying corrosion rate function given by the product of the decay function and the rate increase function is estimated, thereby making it possible to measure the corrosion rate of metals buried underground simply and inexpensively.

[0056] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0057] 101: sensor, 102: first processing circuit, 103: second processing circuit, 104: third processing circuit, 105: fourth processing circuit, 106: memory unit, 107: display unit.

Claims

1. A measuring step of measuring the moisture content of soil of the target site; A first processing step of estimating an attenuation function representing a change over time in the ratio of an area of ​​a surface of the metal structure buried in the target land that is wet with water based on the moisture content measured in the measuring step; A second processing step of estimating, from the attenuation function, a rate increase function that increases the corrosion rate of the metal structure, the rate increase function being derived from the thickness of the water film formed on the surface of the metal structure; a third processing step of estimating a function of change in corrosion rate over time given by the product of said decay function and said rate growth function; The corrosion estimation method includes:

2. The corrosion estimation method according to claim 1, A corrosion estimation method, further comprising a fourth processing step of determining a corrosion rate of the metal structure from the measured moisture content of the soil of the target site using the time-dependent change function.

3. A sensor for measuring the moisture content of soil in a target area; a first processing circuit that estimates an attenuation function that represents a change over time in the ratio of an area of ​​a surface of a metal structure buried in the target ground that is wet with water based on the moisture content measured by the sensor; A second processing circuit that estimates a rate increase function that increases a corrosion rate of the metal structure, the rate increase function being derived from a thickness of a water film formed on the surface of the metal structure, from the attenuation function; a third processing circuit for estimating a corrosion rate function over time given by the product of the decay function and the rate growth function; A corrosion estimation device comprising:

4. The corrosion estimation device according to claim 3, A corrosion estimation device further comprising a fourth processing circuit that uses the time-dependent change function to determine a corrosion rate of the metal structure from the measured moisture content of the soil at the target site.

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