JP3105666B2 - Diagnosis method for corrosion of buried metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion diagnosis method for detecting a corrosion state of a metal material buried in soil or concrete, for example, a gas pipe or the like.

[0002]

2. Description of the Related Art Many pipelines for transporting gas, oil, and the like are buried underground, and thus are susceptible to soil corrosion and electric corrosion (corrosion caused by direct current leaking from a railway track into the ground). Therefore, it is necessary to inspect the state of corrosion before a corrosive hole is formed in the transport pipe due to soil corrosion or electric corrosion, and to take measures such as replacing the pipe in a place with severe corrosion. In addition, pipes and reinforcing bars in concrete, which are said to be less likely to corrode, also undergo corrosion due to neutralization, salt damage, and the like, which has resulted in the formation of corrosion holes and a decrease in strength.

[0003] In order to inspect the state of corrosion of buried metal from the surface of soil or concrete,
An AC impedance measurement method using an AC current, which is one of the electrochemical measurement methods, is known. This AC impedance measuring method will be described with reference to FIGS. 9 and 10 based on an example of a steel pipe buried in soil. The steel pipe 14 is a straight pipe,
It is buried at a certain depth in the soil 16. Measurement electrode 4
Consists of a counter electrode 6 and a reference electrode 8, is connected to a constant potential power supply 10, and is further connected to a frequency characteristic analyzer 12. Further, the constant potential power supply 10 and the steel pipe 14 are connected by an electric wire 18. FIG. 10 shows an overall equivalent circuit. Rp1 is the polarization resistance existing between the soil 16 and the surface of the steel pipe 14, and it is known that when this value is high, the possibility of corrosion is small. Cdl1 is a charge double layer capacitance existing in parallel with the polarization resistance Rp1. Rs1 is the soil resistance in the depth direction and is connected to Rp1 and Cdl1, and Rs2 is the soil resistance in the pipe axis direction and exists continuously. Rp2 is the polarization resistance existing between the counter electrode 6 and the surface of the soil 16, and Cdl2 is the charge double layer capacity. The measurement electrode 4 is installed on the surface of the soil 16 immediately above the steel pipe 14, and an alternating current flows between the counter electrode 6 and the steel pipe 14 by the constant potential power supply 10. Then, the voltage between the steel pipe 14 and the reference electrode 8 and the current flowing from the counter electrode 6 are measured, and the result is subjected to AC impedance analysis.

[0004] The AC impedance analysis is briefly described as follows. That is, alternating currents of various frequencies flow between the counter electrode 6 and the steel pipe 14. When the current has a low frequency, the capacitor has an infinite resistance.
The current flows through p1 and Rs1, and in the case of a high frequency, the resistance of the capacitor disappears, so that the current flows through Rs1 and Cdl1. In each case, the impedance between the steel pipe and the reference electrode is determined as Rp1 + Rs1 and Rs1 from the voltage between the steel pipe and the reference electrode and the current flowing through the counter electrode, and Rp1 is calculated from the difference between the two. This value is used to diagnose the progress of corrosion.

[0005]

However, in the conventional measuring method, since the current spreads widely other than immediately below the measuring electrode 4, to what extent the applied alternating current is dispersed in the soil and flows to the steel pipe 14. Therefore, it was impossible to quantitatively know the progress of corrosion of the steel pipe 14 from the obtained value. For example, if there is a corroded part in a part of the steel pipe and the resistance value at that part is low,
When the measurement electrode is moved along the surface of the soil and measurement is performed at a plurality of locations, an average value including the resistance of other portions is obtained, not only at the corroded and uncorroded portions.
Therefore, it is possible to perform a relative evaluation of the progress of corrosion, that is, to detect the places where corrosion is progressing the most, but it is not possible to perform a quantitative evaluation, that is, to judge how fast corrosion is progressing. There was a problem. In other words, since the range of influence of the alternating current is unknown, the area of the steel pipe involved in the measurement cannot be clarified, and the impedance per unit area required for performing a corrosion diagnosis must be determined from the measured impedance. Was not possible.

[0006]

SUMMARY OF THE INVENTION In the present invention, the object of the present invention is to solve the above-mentioned problems, to specify a measurement range, and to detect a resistance value per unit area of a metal material. The measurement electrode is placed on both sides if the metal material is linear, and if the metal material is flat, the measurement electrodes are placed around the measurement location together with the measurement location. A current flows from each counter electrode so that the values are equal, and the amount of current flowing to the counter electrode of the main measurement electrode placed at the measured position and the voltage between the metal material and the reference electrode at this time are analyzed by AC impedance analysis. Then, the state of corrosion was diagnosed by detecting the resistance of the metal material immediately below the main measurement electrode.

[0007]

[Function] Since the voltage value between the reference electrode of each measurement electrode arranged around the main measurement electrode and the metal material is equalized, the comparison between the reference electrode of the main measurement electrode and the reference electrode of the other measurement electrodes is performed. The voltage between them becomes zero, so that the current from the main measurement electrode flows only up to almost the middle of the distance from the adjacent measurement electrode. Therefore, since the reaching range of the current of the main measurement electrode can be specified, and the area of the metal material involved in the measurement can be limited, the unit obtained from the value obtained by analyzing the current and the voltage between the reference electrode and the metal material by AC impedance analysis is used. The impedance per area, that is, the corrosion state can be diagnosed.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG.
Is a cross section of soil, and FIG. 2 shows an equivalent circuit of FIG. The steel pipe 14 is buried in a straight line at a certain depth in the soil 16 as a burying medium. On the surface of the soil 16, three measuring electrodes 4 are disposed immediately above the steel pipe 14, and from the left, a first measuring electrode 4 a, a second measuring electrode 4 b, and a third
The measurement electrode 4c is used, and the central second measurement electrode 4b is used as a main measurement electrode. Each measurement electrode 4 includes a counter electrode 6 and a reference electrode 8, and is connected to a constant potential power supply 10. The constant potential power supply 10 is further connected to a frequency characteristic analyzer 12.
The electric wire 18 is also directly connected to the steel pipe 14 and connected to each of the constant potential power supplies 10.

The equivalent circuit of FIG. 2 will be described. Rp1 is the polarization resistance generated between the soil 16 and the steel pipe 14, and this value has a correlation with the corrosion state. When the resistance value is low, it indicates that the state is a corrosion state. Cdl1 is a charge double layer capacity which is also generated between the soil 16 and the steel pipe 14, and exists in parallel with the polarization resistance Rp1. Rs1 and Rs2
Is the soil resistance, Rs1 is the soil resistance in the depth direction, and Rs2 is the soil resistance in the pipe axis direction. Rp2 and Cdl2 occur between the counter electrode 6 and the surface of the soil 16, and Rp2 is a polarization resistance,
Cdl2 is a charge double layer capacitance.

In the measurement, alternating currents of various frequencies are applied between the constant potential power supply 10 and the counter electrode 6 and the steel pipe 14 so that the voltages between the reference electrodes 8 and the steel pipe 14 are equal to each other. The impedance value of the current value of the second measurement electrode 4b and the voltage value between the reference electrode 8 and the steel pipe 14 are analyzed by Rc1.
Find the value of In the AC impedance analysis, as described in the conventional example, currents of various frequencies are passed, the absolute value and the phase difference of the impedance at each frequency are obtained, and Rs1 and Rp1 + Rs1 obtained from these by analysis such as Cole-Cole plot. Rp1 is determined from the value of.

In this manner, the current applied to the counter electrode 6 of the second measuring electrode 4b is reduced by the first and third measuring electrodes 4 on both sides.
a, 4c, which is interrupted by the current applied to the counter electrode 6, and directly below the second measurement electrode 4b, theoretically, the second measurement electrode 4b and the first measurement electrode 4a, and the second measurement electrode 4b and the third measurement electrode 4c. It flows only to almost the middle position of each, and the length l of FIG.
That is, all of the current flows within the range of the distance between the main measurement electrode 4b centered on the main measurement electrode 4b and the other measurement electrodes 4a and the like. Thus, since the diameter of the steel pipe 14 is known, the surface area of the steel pipe 14 involved in the measurement is clarified, and the impedance per unit area of the steel pipe 14 is obtained from the obtained impedance. There is a correlation between the impedance per unit area and the corrosion state of the steel pipe 14, and the corrosion state of the steel pipe 14 can be quantitatively and reliably diagnosed by this value.

Experimental Example Hereinafter, a test performed using a simulated pipe buried in concrete will be described. 3 and 4 show an outline of the simulation pipe. The simulation pipe 22 has a length of 9 cm and an outer shape of 2.8 c.
It has a total length of 209 cm, in which 21 m black tubes 26 are fixed with a 1 cm insulator 28 interposed therebetween. This is length 200
cm, a width of 60 cm and a height of 40 cm were placed at the center in the width direction so that the cover was 10 cm. (See FIG. 3). FIG. 4 shows a cross section. As shown in FIG. 4, lead wires 30 are attached to both ends of each of the black tubes 26 so that they can be electrically connected to an adjacent tube from the outside. Further, 95-11 corresponding to the central portion of the simulation pipe 22
Three holes 5 mm in diameter reaching the pipe surface are provided in three places within a range of 5 cm so that corrosion occurs only at those places.
While the t% aqueous sodium chloride solution was added dropwise, the wet state was always maintained.

The AC impedance was measured by the following method. (1) When the lead wire 30 of each black tube 26 is not connected to another black tube 26, that is, when each black tube 26 is in an insulated state, the measurement electrode 4 is connected to each black tube 26 with respect to the black tube. 26, and is measured using one constant potential power supply 10. This is a comparative example. (2) Connect the lead wire 30 of each black tube 26 to the adjacent black tube 26
Is measured by the measuring method of the present technology when the black tube 26 is connected to the lead wire 30, that is, when all the black tubes 26 are in a conductive state. This is referred to as the present measurement method. (3) Connect the lead wire 30 of each black tube 26 to the adjacent black tube 26
When the black tubes 26 are in a conductive state, that is, when all the black tubes 26 are in a conductive state, the measurement is performed by a conventional measuring method.
Hereinafter, the conventional method will be used.

In the above measurement method, the measurement was carried out while sequentially moving the measurement points in the tube axis direction. These results are shown in FIGS. In the figure, ○ indicates the value obtained by the present measurement method, □ indicates the value obtained by the comparative example, and △ indicates the value obtained by the conventional method. 5 and 6 show the case where the frequency is 1 kHz. FIG. 5 is a graph showing the absolute value of the impedance, and FIG. 6 shows the phase difference θ at this time.
7 and 8 show the case where the frequency is set to 10 mHz.
FIG. 7 is a graph showing the absolute value of the impedance,
FIG. 8 is a graph showing the phase difference θ at this time.

Regarding the absolute value of the impedance from each graph, at any frequency, the measured impedance value by the conventional method is considerably lower than the measured impedance value of the comparative example, and the distribution is gentler. The measured impedance value of the measurement method is in good agreement with the measured impedance value of the comparative example. As for the phase difference, 1
At kHz, although the measured impedance value of the present measurement method varies, the impedance measured value of the comparative example almost matches the allowable range within the allowable range. At 10 mHz, the measured impedance value of the conventional method is considerably more negative than the measured impedance value of the comparative example. It can be seen that the error between the impedance measurement value according to the present measurement method and the impedance measurement value of the comparative example is reduced in contrast to the values on the side.

In the above embodiment, a straight steel pipe 14 has been described as an example. However, the present invention is not limited to this, and can also be applied to a case where reinforcing bars are laid in a grid in concrete. In this case, the measurement electrode is arranged around the main measurement electrode. Specifically, if you know the location of the rebar,
The main measurement electrode is arranged immediately above the intersection of the reinforcing bars, and the other measuring electrodes are respectively arranged on the four reinforcing bars. Then, as described above, a current is applied and AC impedance analysis is performed.
Find the impedance just below the main measurement electrode. Also in this case, the current from the main measurement electrode does not spread widely due to the current from the surrounding measurement electrodes, and the reach can be specified, so that accurate impedance per unit area is obtained, and a quantitative corrosion state is obtained. be able to. Further, the embedding medium is not limited to soil or concrete, but may be other materials.

[0017]

According to the corrosion diagnostic method of the present invention, a measuring electrode is arranged around a main measuring electrode arranged at a position to be inspected,
An alternating current is passed from the counter electrode of each measurement electrode to the metal material so that the voltage between the reference electrode of all the measurement electrodes and the metal material becomes equal, and the current value of the counter electrode of the measurement electrode placed at the inspection location at that time Since the impedance value is obtained by analyzing the AC impedance from the voltage between the reference electrode and the metal material, the range of the current flowing from the counter electrode of the main measurement electrode arranged at the position to be inspected can be specified, and the inspection is performed. Since the surface area of the metal material can be obtained, the impedance per unit area of the metal material, which could not be obtained in the past, can be obtained, and the corrosion state of the metal material embedded inside the embedding medium such as concrete or soil can be quantitatively determined from the outside The diagnosis can be made quickly and reliably.

[Brief description of the drawings]

FIG. 1 is a sectional view of soil illustrating a corrosion diagnostic method according to the present invention.

FIG. 2 is an equivalent circuit diagram of the soil section shown in FIG.

FIG. 3 is a perspective view of concrete showing an experimental example.

FIG. 4 is a sectional view of the concrete shown in FIG. 3;

FIG. 5 is a graph showing the results of an experiment.

FIG. 6 is a graph showing the results of an experiment.

FIG. 7 is a graph showing the results of an experiment.

FIG. 8 is a graph showing the results of an experiment.

FIG. 9 is a cross-sectional view showing a conventional measurement method.

FIG. 10 is an equivalent circuit diagram of a conventional method.

[Explanation of symbols]

 4 Measurement electrode 6 Counter electrode 8 Reference electrode 10 Constant potential power supply 12 Frequency characteristic analyzer 14 Steel pipe 16 Soil

Continuation of front page (56) References JP-A-63-163266 (JP, A) JP-A-1-167650 (JP, A) JP-A-4-95867 (JP, A) JP-A-59-190647 (JP) , A) Tokiko Sho 61-29457 (JP, B2) Shibata, Kajiyama "Development of AC Impedance Distribution Measurement Technique Using Multi-Point Potential Control Method" Proceedings of the 39th Symposium on Corrosion Protection, October 10, 1992 , P. 53-56 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 17/00-17/04 G01N 27/02 G01N 27/26 F17D 5/00-5/08

Claims (3)

(57) [Claims] 1. A measuring electrode comprising a counter electrode and a reference electrode is provided on a surface of an embedding medium in which a metal material is buried therein by a plurality of appropriate amounts along the inspected location and the embedding direction of the metal material around the inspected location. Arranged at an interval, an alternating current is passed from the counter electrode of the measurement electrode to the metal material so that the voltage between the reference electrode of each measurement electrode and the metal material is equal,
The resistance value between the metal material and the embedding medium is obtained by performing an AC impedance analysis of the current value of the measurement electrode disposed at the inspection location and the voltage applied between the reference electrode and the metal material. A method for diagnosing corrosion of a buried metal, comprising detecting a corrosion state of the metal. 2. The burying device according to claim 1, wherein the metal material is formed of a linear member, and the measurement electrodes are arranged at three places along the metal material, and diagnosis is performed with the measurement electrodes arranged at the center thereof. Diagnosis method for metal corrosion. 3. The buried metal corrosion according to claim 1, wherein the metal material has a lattice shape, a measurement electrode is arranged at a position to be measured, and the measurement electrodes are arranged at least on three sides around the measurement electrode. Diagnostic method.