JPH0423748B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for detecting corrosion of buried pipes such as gas pipes and water pipes, underground steel tanks, or reinforcing bars in concrete. [Prior Art] Conventionally, the degree of corrosion of buried pipes has been inspected by digging up the ground, but the labor and cost required for inspection are high, so there is a desire to develop a simple detection method. Regarding this point, there is a method of estimating the degree of corrosion of buried pipes by burying metal pieces of the same material close to the buried pipes and electrochemically measuring the degree of corrosion of these metal pieces (Japanese Patent Laid-Open No. 56
There is a method (Japanese Patent Application Laid-open No. 10695/1983) in which a conductive wire is attached to the outer periphery of the buried pipe in advance and changes in the electrical resistance of the wire are detected. In addition, since there is a large difference in the corrosion rate of buried pipes depending on the nature of the soil in the buried part, there are methods to estimate the corrosion rate of buried pipes by electrochemically measuring the corrosivity of the soil (for example, JP-A-58 −208654 Publication, JP-A-58
-37398 Publication). [Problems to be solved by the invention] However, in the above cases, detection means must be installed at the time of piping installation, and there is a drawback that existing ones cannot be used. This is to determine whether the environment is conducive to corrosion, and not to detect the degree of corrosion. The present invention was developed in view of the above points, and provides a method for easily detecting the state of corrosion of buried pipes, etc., without excavating the ground. [Means for Solving the Problems] In order to solve the above-mentioned drawbacks, the present invention supplies an alternating electric signal between the measurement electrode provided above the ground surface of a buried pipe, etc. and the pipe concerned, and The method is characterized by measuring the AC impedance at each frequency, determining the boundary frequency that separates the characteristics of the impedance response at each frequency, and analyzing the relationship between the AC impedance and frequency on the high frequency side to detect the corrosion layer thickness. be. [Example] Corrosion of buried pipes is caused by electrochemical reactions with the soil, and the electrochemical reactions are determined by the presence and properties of the corroded layer, and the speed and magnitude of this reaction are This can be explained by replacing it with electrical impedance. As a result of many experiments, the inventor determined that the alternating current impedance at each frequency was determined by supplying an alternating electric signal with the frequency sequentially changed between the measurement electrode installed on the ground surface above the buried pipe and the pipe concerned. If you measure this, analyze it, and display it in a diagram such as the absolute value of impedance and frequency, the phase difference of impedance and frequency, the real component of impedance and frequency, and complex number representation, you will see two responses: a high-frequency response and a low-frequency response. As the corrosion layer thickens, the number of high-frequency sections that can be approximated as a straight line in the absolute value of impedance vs. frequency, the phase difference of impedance vs. frequency, and the real component of impedance vs. frequency increases, and the complex number representation increases. The figure shows that the impedance of the high-frequency side element increases. The present invention utilizes this phenomenon to determine the thickness of the corroded layer. Embodiments of the present invention will be described below with reference to FIGS. 1 to 9. In FIG. 1, 20 is a building, and 10 and 11 are branch pipes led from the main pipe, such as gas or water pipes. First, the measuring electrode 12 connected to the potentiostat 14 is installed on the ground above the buried pipe 10, and the piping section 11 and the potentiostat 14 are connected. The potentiostat 14 is connected to a frequency response analyzer 16, and the frequency response analyzer 16 is connected to an arithmetic display device 18. The impedance measured by this measurement device is an average value near the pipe 10 closest to the measurement electrode 12. After measurement, the side electrode 12 is sequentially moved to detect the corrosion layer in a certain section of the pipe 10. Measure the presence or absence and thickness of the corroded layer. FIG. 2 is a graph showing the relationship between the absolute value of impedance |Z| and frequency (f) by measuring impedance in a certain frequency band. In this diagram, there are sections where the absolute value of impedance |Z| approximates the frequency with a straight line and sections where it is a curved line, and the frequency at this boundary (referred to as the boundary frequency fcorr) is calculated and determined. Then, the average thickness of the corroded layer is calculated and compared with the average thickness value of the corroded layer corresponding to the boundary frequency fcorr, which is stored in the calculation display device 18 and determined in advance through experiments, and the average thickness of the corroded layer is determined and displayed. The average thickness values of the corroded layer with respect to the boundary frequency fcorr are shown in FIG. In actual detection work, the impedance is measured by the frequency response analyzer 16 while decreasing the frequency at regular intervals from a high frequency (for example, 100 kHz), and the absolute value of the measured impedance |Z|
is compared with the absolute value |Z| for the immediately preceding frequency on the calculation display device 18, and when the difference is larger than a predetermined value, it is determined that the section approximating the straight line, which is the high frequency side element, has ended, and the current The measurement may be terminated by setting the frequency as the boundary frequency fcorr. Next, as another example, a case will be described in which the average thickness of the corroded layer is determined from the relationship between the phase and frequency of impedance. FIG. 3 shows the relationship between impedance phase difference (θ) and frequency (f) after measuring impedance in a certain frequency band. In this diagram, there are sections where the phase difference approximates the frequency as a straight line and sections that are curved, and the frequency at this boundary (boundary frequency fcorrθ) is calculated and found. Then, the average thickness of the corroded layer is calculated and compared with the average thickness value of the corroded layer corresponding to the boundary frequency fcorrθ, which is stored in the calculation display device 18 and determined in advance through experiments, and the average thickness of the corroded layer is determined and displayed. The average thickness of the corroded layer with respect to the boundary frequency fcorrθ is as shown in FIG. In actual detection work, similarly to the above case, the phase difference is compared with the phase difference for the previous frequency on the calculation display device 18, and when the difference is larger than a predetermined value, the section approximating to a straight line ends. Then, determine the boundary frequency fcorrθ. As yet another example, a case will be described in which the average thickness of the corroded layer is determined from the relationship between the real component of impedance and frequency. FIG. 4 shows the relationship between the real component (R) of the impedance and the frequency (f) by measuring impedance in a certain frequency band. In this diagram, there are sections where the real component (R) approximates the frequency with a straight line and sections where it is a curved line, and the frequency at this boundary (boundary frequency fcorrR) is calculated and determined. Then, the average thickness of the corroded layer is calculated and compared with the average thickness value of the corroded layer corresponding to the boundary frequency fcorrR, which is stored in the calculation display device 18 and determined in advance through experiments, and the average thickness of the corroded layer is determined and displayed. The average thickness of the corroded layer with respect to the boundary frequency fcorrR is as shown in FIG. The procedure for determining the boundary frequency fcorrR in actual detection work is the same as in the above case. As yet another example, a case will be described in which the corrosion layer thickness is determined by the impedance difference A of the elements on the high frequency side. FIG. 5 shows impedance measured in a certain frequency band expressed as a complex number. The impedance difference A in this diagram is compared with the average thickness value of the corroded layer with respect to the impedance difference A, which is stored in the calculation display device 18 and determined in advance through experiments, and the average thickness of the corroded layer is calculated and displayed. The average thickness value of the corroded layer with respect to the impedance difference A is as shown in FIG. In actual detection work, the frequency response analyzer 16 uses three points in the low frequency region (for example, 0.1Hz,
0.05 Hz, 0.01 Hz) and high frequency (for example, 10 kHz), and the data is transmitted to the arithmetic display device 18 after being separated into real and imaginary components. The arithmetic display device first approximates the impedance values at three points on the low frequency side to a circular arc on a complex number display diagram, and calculates the difference between the impedance value at the intersection with the real axis on the high frequency side and the real component of the impedance measurement value at the high frequency ( Find A). Table 1 shows the actual measurement results and the results of cutting and measuring the cross section of the buried pipe for the four methods described above.

〔Effect of the invention〕

As explained above, the present invention supplies an alternating electric signal between the measuring electrode provided above the ground surface of a buried pipe and the pipe, measures the alternating current impedance at each frequency, and responds to the impedance at each frequency. The thickness of the corroded layer can be detected by finding the boundary frequency that separates the characteristics of the pipe and analyzing the relationship between AC impedance on the high frequency side and frequency. Easy to measure. Furthermore, the present invention is capable of detecting corrosion not only in buried pipes but also in underground storage tanks, reinforcing bars in concrete, and the like.

[Brief explanation of drawings]

FIG. 1 is an overall view showing an embodiment of the method for detecting the thickness of a corroded layer of a buried steel pipe, etc. according to the present invention, and FIGS.
The diagrams shown are measurement diagrams of absolute value of impedance and frequency (|Z|-f), phase difference and frequency (θ-f), real value of impedance and frequency (R-f), and impedance difference (A), respectively. Figure 6 is a diagram for evaluating the average thickness of the corrosion layer from the boundary frequency fcorr determined from the relationship between the absolute value of impedance and frequency, and Figure 7 is a graph showing the boundary frequency fcorrθ and the corrosion layer similarly determined from the relationship between phase difference and frequency. Figure 8 is an evaluation diagram of the average thickness of the corrosion layer and the boundary frequency fcorrR obtained from the relationship between the real component (R) of impedance and frequency. Figure 9 is an evaluation diagram of the average thickness of the corrosion layer.
and an evaluation diagram of the average thickness of the corroded layer. 10... Piping (buried), 11... Piping, 12...
... Measuring electrode, 14 ... Potentiostat, 1
6... Frequency response analyzer, 20... Building.

Claims (1)

[Scope of Claims] 1. An alternating electric signal is supplied between a measurement electrode provided above the ground surface of a buried pipe, etc. and the piping, AC impedance at each frequency is measured, and the response of the impedance at each frequency is determined. A method for detecting corrosion in buried pipes, etc., characterized in that the thickness of the corrosion layer is detected by determining the frequency of the boundary that separates the characteristics and analyzing the relationship between AC impedance and frequency on the high frequency side. 2. The method of detecting the corrosion thickness of buried pipes, etc. as set forth in claim 1, wherein the frequency of the boundary that distinguishes the characteristics of the impedance response is determined from the relationship between the absolute value of the measured AC impedance and the frequency. Corrosion detection method. 3. The method of detecting the corrosion thickness of buried pipes, etc. according to claim 1, wherein the frequency of the boundary that distinguishes the characteristics of the impedance response is determined from the relationship between the phase difference and the frequency of the measured AC impedance, and the corrosion thickness is detected. Corrosion detection method. 4. The buried pipe, etc. according to claim 1, characterized in that the corrosion thickness is detected by determining the frequency of the boundary that separates the characteristics of the impedance response from the relationship between the real number component of the measured AC impedance and the frequency. Corrosion detection method. 5 Find the difference (A value) between the real component at the intersection of the response on the low frequency side and the real axis on the high frequency side of the measured impedance and the real component obtained by extrapolating the frequency to infinity to detect the corrosion thickness. A method for detecting corrosion of buried pipes, etc., as set forth in claim 1.