JP5450468B2 - Pitting depth calculation method, pitting depth calculation device, and pitting depth calculation system - Google Patents

Description

  The present invention relates to a pitting corrosion depth calculation method, a pitting corrosion depth calculation device, and a pitting corrosion depth calculation system for metal materials, and in particular, when underground disposal of radioactive waste generated from a nuclear power generation facility or the like is performed. The present invention relates to a pitting depth calculation method, a pitting depth calculation device, and a pitting depth calculation system for calculating the depth of pitting corrosion of a waste disposal container to be used.

  Conventionally, in a chemical plant or the like, corrosion monitoring is performed by various methods for metal devices (reactors, storage tanks, etc.) where corrosion is a problem. For example, as a general method of corrosion monitoring, the mass loss measurement method, the polarization resistance measurement method (DC polarization resistance method, AC polarization resistance method, impedance method), and the electrical method widely known as corrosion measurement methods for metal materials, Resistance measurement methods exist.

  An electrochemical noise method is known as a method for detecting the occurrence of corrosion of a metal material. For example, as an example of means using this electrochemical noise method, Patent Document 1 discloses a corrosion measuring apparatus that can detect corrosion occurring in pipes and structures of a chemical plant or the like.

Here, the measurement object is not only pipes and structures of chemical plants and the like as envisaged by the invention described in Patent Document 1, but also waste used for underground disposal of radioactive waste A disposal container is also conceivable.
This radioactive waste disposal container is made of carbon steel, etc., which is difficult to form a passive film so as not to cause pitting corrosion that causes perforation in consideration of prevention of the outflow of radioactive waste to the surrounding environment. In addition to being used as a material, it is buried underground with sufficient consideration so that the environment in which it is buried is not alkaline (since carbon steel may form a passive film with strong alkali).

  However, even when careful consideration is given to materials and the environment, pitting corrosion may occur because the waste disposal container for radioactive waste is buried in the basement for a very long time. Sex is not zero. And the pitting corrosion that occurs in such a controlled environment is usually so small that it can be ignored without the need to consider it, but if you are targeting a waste disposal container for radioactive waste, Considering the impact of radioactive waste spills on the environment, no matter how small the pitting can be overlooked.

  Therefore, when a waste disposal container for radioactive waste is targeted, it is necessary to detect any minute change and evaluate the change. In particular, when evaluating anomalies, it is very important to examine the depth of pitting corrosion in order to estimate the progress of drilling.

Here, in order to investigate the depth of pitting corrosion, it is troublesome to dig up the waste disposal container for radioactive waste once buried in the ground.
Therefore, at present, an acoustic emission method is known as a method for estimating the penetrating life of a damaged portion. For example, as an example of means using this acoustic emission method, Patent Document 2 discloses a corrosion damage evaluation system that estimates the penetrating life of a damaged portion of a tank buried in the ground.

Special table 2009-544008 JP 2006-250823 A

However, since the techniques according to Patent Document 1 and Patent Document 2 assume pipes and structures such as chemical plants or tanks buried in the ground as measurement targets, minute pitting corrosion is accurately detected. I can't. In addition, since the technique according to Patent Document 2 uses the acoustic emission method, it is an effective means when damage occurs in a short time such as a crack. It is not accurate enough to detect food.
Therefore, with the technologies according to Patent Document 1 and Patent Document 2, it is natural that the depth of pitting corrosion of the radioactive waste disposal container cannot be calculated.

  The present invention has been made in view of the above problems, and its problem is the depth of minute pitting corrosion that occurs in a waste disposal container for radioactive waste that cannot be measured and calculated by conventional methods. Another object is to provide a pitting depth calculation method, a pitting depth calculation device, and a pitting depth calculation system for calculating the depth.

  In order to solve the above problems, the pitting depth calculation method according to the present invention includes a volume calculation step for obtaining a pitting volume from a current signal obtained by measuring a metal material over time by an electrochemical noise method, By calculating the pitting area from the impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by the AC impedance method, and dividing the pitting volume by the pitting area, And a depth calculation step for calculating the depth.

As described above, the pitting depth calculation method according to the present invention obtains the pitting volume from the current signal obtained by measuring the metal material with time by the electrochemical noise method. The volume of the pitting corrosion can be obtained not only when the damage occurs but also when the initial small pitting corrosion occurs. Therefore, when the waste disposal container for radioactive waste is targeted, the volume of minute pitting corrosion generated in the waste disposal container can be obtained.
And the pitting corrosion depth calculation method according to the present invention is to obtain the pitting corrosion area from the impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by the alternating current impedance method. Can be obtained accurately.

  And since the pitting corrosion depth calculation method according to the present invention obtains the pitting corrosion depth based on the above-described pitting corrosion volume and area, it is so small that it cannot be measured and calculated by the conventional method. The depth of pitting corrosion can be obtained.

In the pitting depth calculation method according to the present invention, the current signal measurement interval is preferably 0.1 to 5 seconds.
As described above, the pitting depth calculation method according to the present invention can set the data amount to be acquired to an appropriate amount by defining the measurement interval of the current signal to a predetermined time. Therefore, it is possible to avoid a situation that occurs due to an excessive amount of data to be acquired, such as unnecessary electrical noise generated from a device such as a corrosion measurement device or a pitting depth calculation device affecting the measured data. In addition, it is possible to avoid a situation that occurs due to too little data to be acquired, such as detection of occurrence of local corrosion such as pitting corrosion and calculation of the pitting volume becomes difficult.

In the pitting depth calculation method according to the present invention, the metal material is preferably buried in the ground. Furthermore, it is preferable that the metal material constitutes a waste disposal container for radioactive waste.
Thus, the pitting corrosion depth calculation method according to the present invention can be suitably applied to a waste disposal container for radioactive waste buried in the ground.

  Further, the pitting corrosion depth calculation apparatus according to the present invention includes a volume calculation means for obtaining a pitting volume from a current signal obtained by measuring a metal material over time by an electrochemical noise method, and pitting corrosion by an AC impedance method. An area calculating means for obtaining a pitting area from an impedance spectrum obtained by measuring the metal material before and after the occurrence, and a depth for calculating the depth of pitting by dividing the pitting volume by the pitting area. And calculating means.

  The pitting depth calculation system according to the present invention measures the current signal of a metal material over time by an electrochemical noise method and measures the impedance spectrum of the metal material before and after the occurrence of pitting corrosion by an AC impedance method. From a corrosion measuring device, a volume calculating means for obtaining a pitting volume from a current signal of the metal material obtained by the corrosion measuring device, and an impedance spectrum of the metal material before and after the occurrence of pitting corrosion obtained by the corrosion measuring device A pitting depth calculating device comprising: an area calculating means for obtaining a pitting area; and a depth calculating means for calculating a pitting depth by dividing the pitting volume by the pitting area. It is characterized by providing.

  As described above, the pitting depth calculation device and the pitting depth calculation system according to the present invention obtain the pitting volume from the current signal obtained by measuring the metal material over time by the electrochemical noise method, and Since the pitting corrosion area is obtained from the impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by the impedance method, the volume and area of minute pitting corrosion can be accurately obtained.

  According to the pitting depth calculation method, the pitting depth calculation device, and the pitting depth calculation system according to the present invention, a hole is generated from a current signal obtained by measuring a metal material over time by an electrochemical noise method. The pitting corrosion area is obtained from the impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by the alternating current impedance method, and the volume and area of the micro pitting corrosion are accurately obtained. Can do. And since the depth of a pitting corrosion is calculated | required based on the volume and area of an above-described pitting corrosion, the depth of a micro pitting corrosion which cannot be measured and calculated with the conventional method can be calculated | required.

1 is a schematic diagram of an entire corrosion measurement device, pitting depth calculation device, and measurement environment to which a pitting depth calculation method according to the present invention is applied. It is a schematic diagram of the measurement environment to which the pitting depth calculation method, the pitting depth calculation device, and the pitting depth calculation system according to the present invention are applied. It is a block diagram of the pitting depth calculation apparatus concerning the present invention. It is a graph (range of 900-1200 second) which shows the electric current value measured with the corrosion measuring apparatus. It is a graph (range of 0-2000 second) which shows the electric current value measured with the corrosion measuring apparatus. (A) is a vector locus diagram displayed with the real part of impedance on the horizontal axis and the imaginary part on the vertical axis, and (b) is the absolute value and phase difference of impedance on the vertical axis and frequency on the horizontal axis. FIG. It is a circuit diagram of a general equivalent circuit.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out a pitting depth calculation method, a pitting depth calculation apparatus, and a pitting depth calculation system according to the present invention will be described in detail with reference to the drawings as appropriate.

[Pitting corrosion depth calculation system]
First, an embodiment of a pitting depth calculation system according to the present invention will be described with reference to FIG. This pitting depth calculation system S includes a corrosion measurement device 8 and a pitting depth calculation device 9, and measures measurement data such as current signals and potential signals of metal materials obtained from the corrosion measurement device 8. This is a system for calculating the depth of pitting corrosion by performing signal processing with the corrosion depth calculating device 9.

(Pitting corrosion depth calculation device)
As shown in FIG. 3, the pitting depth calculation device 9 is implemented by a personal computer or the like provided with at least a storage unit and a processing unit. In detail, the input unit 901, the first storage unit 902, and the determination Means 903, volume calculation means 904, depth calculation means 905, time notification means 906, signal output means 907, second storage means 908, area calculation means 909, and output means 910 are provided.

Note that the first storage unit 902 and the second storage unit 908, which are storage units, include, for example, a storage device such as a memory or a hard disk. The determination unit 903, the volume calculation unit 904, the depth calculation unit 905, the time notification unit 906, and the area calculation unit 909, which are processing units, are configured by a CPU (Central Processing Unit), for example. The input unit 901, the signal output unit 907, and the output unit 910 are configured by a predetermined input or output interface.
Further, the pitting depth calculation device 9 may be configured (or configured to be connected) to a display device such as a CRT (Cathode Ray Tube) or an LCD (Liquid Crystal Display) as display means.

(Configuration of each means of the pitting depth calculation device)
The input unit 901 receives measurement data such as a current signal and a potential signal of a metal material from the corrosion measuring device 8. The measurement data input to the input unit 901 is output to the first storage unit 902 and the second storage unit 908.

The first storage unit 902 is a unit that stores the current signal input from the input unit 901.
The determination unit 903 is a unit that determines whether or not the current signal accumulated in the first storage unit 902 exceeds a predetermined value (a value that the pitting corrosion has occurred). Then, the determination unit 903 has a current from when it exceeds a predetermined value until it reaches a steady value (a current value measured when the metal material is measured in a steady state in which corrosion such as pitting corrosion does not occur). The signal is output to the volume calculation means 904.
The discriminating means 903 can set a predetermined value from the outside. Further, the determination unit 903 may be configured so that a steady value can be set from the outside, or the steady value may be calculated by the determination unit 903 itself.

  The volume calculation unit 904 is a unit that calculates the pitting corrosion volume based on the current signal input from the determination unit 903. Then, the volume calculation means 904 outputs the calculated pitting corrosion volume to the depth calculation means 905.

The time notification unit 906 is a unit that notifies the signal output unit 907 and the second storage unit 908 that each process is performed when a predetermined time comes. The time notification means 906 can set a predetermined time from the outside.
The signal output unit 907 is a unit that outputs a sine wave potential (or current) signal having an arbitrary frequency to the metal material 1 when a notification is input from the time notification unit 906.

  The second storage unit 908 is a unit that stores the sine wave current (or potential) signal input from the input unit 901. When the notification is input from the time notification unit 906, the second storage unit 908 outputs a sine wave current (or potential) signal for a predetermined period to the area calculation unit 909 from when the notification is input.

  The area calculation unit 909 is a unit that calculates the area of pitting corrosion based on the sine wave current (or potential) signal output from the second storage unit 908. Then, the area calculation unit 909 outputs the calculated pitting corrosion area to the depth calculation unit 905.

  The depth calculation unit 905 is a unit that calculates the pitting corrosion depth by dividing the pitting corrosion volume input from the volume calculation unit 904 by the pitting corrosion area input from the area calculation unit 909. . Then, the depth calculation unit 905 outputs the calculated pitting corrosion depth to the output unit 910.

The output unit 910 is a unit that outputs the depth of pitting corrosion input from the depth calculation unit 905 to a display unit or the like connected to the outside (or installed inside).
Here, the display means may be configured to display not only the calculated pitting corrosion depth but also a measured current signal, a vector locus diagram, a Bode diagram, and the like.

  The configuration of the pitting corrosion depth calculation device 9 is not limited to the above configuration. For example, when calculating the volume of pitting corrosion based on all current signals input from the input unit 901, the first storage unit 902, the determination There is no need to provide the means 903, and a configuration in which a current signal is directly output from the input means 901 to the volume calculation means 904 may be employed. When the determination unit 903 is not provided, the volume calculation unit 904 can set the steady value used when calculating the pitting corrosion volume from the outside, or the steady value can be set by the volume calculation unit 904 itself. It can be calculated.

  Further, the time notification unit 906 is not provided, and after a few minutes have elapsed since the determination unit 903 determined that the current signal has exceeded a predetermined value, each process is performed from the determination unit 903 to the signal output unit 907 and the second storage unit 908. It may be configured to notify that it is performed.

  In addition, a notification is input from the time notification unit 906 (or determination unit 903) to the input unit 901 instead of the second storage unit 908, and a sine wave current (or potential) signal for a predetermined period from when the notification is input. The configuration may be such that the input means 901 directly outputs to the area calculation means 909. In this case, it is not necessary to provide the second storage unit 908.

  Further, a frequency response analyzer (not shown) may be provided as a separate body from the pitting depth calculator 9. In this case, the frequency response analyzing apparatus includes a time notifying unit 906, a signal output unit 907, a second storage unit 908, and an area calculating unit 909 of the pitting corrosion depth calculating unit 9 described above, and further an input. Means and output means. Specifically, after the sine wave current (or potential) signal is input from the corrosion measuring device 8 to the input means, and the process of calculating the pitting area is performed in the frequency response analyzer, the calculated pitting area is calculated. Is output to the depth calculation means 905 of the pitting corrosion depth calculation device 9 by the output means. In this case, it is not necessary to provide the time notification means 906, the signal output means 907, the second storage means 908, and the area calculation means 909 in the pitting corrosion depth calculation device 9.

  As mentioned above, although the structure of each means of the pitting depth calculation apparatus 9 was demonstrated, the detail of the process process (volume calculation process, area calculation process, depth calculation process) performed by each means of the pitting depth calculation apparatus 9 Will be described later.

(Corrosion measuring device)
The corrosion measuring device 8 is a device for monitoring corrosion of a metal material, in particular, a waste disposal container for radioactive waste buried in the ground. An existing measuring device used when applying the electrochemical noise method is used. It can be used as it is. Therefore, according to the pitting corrosion depth calculation method according to the present invention, it is not necessary to use a new measuring device.
And the corrosion measuring apparatus 8 is not specifically limited, For example, what is necessary is just to use the potentiostat galvanostat provided with the electrometer 6 with the zero resistance ammeter (zero resistance ammeter) 7 and high input resistance.

In addition, as a measurement environment by the corrosion measuring device 8, a current value between the measurement object 1 (waste disposal container) and the counter electrode 2 and a potential value between the measurement object 1 and the reference electrode 3 are obtained by the potentiostat / galvanostat. May be measured at a predetermined measurement interval (time) and output to the pitting depth calculation device 9 (or the pitting depth calculation device 9 and the frequency response analysis device). In addition, when assuming corrosion monitoring of a radioactive waste disposal container, the measurement object 1, the counter electrode 2 and the reference electrode 3 are covered with bentonite 4, and these are immersed in a test solution 5 assuming groundwater or the like. That's fine.
Furthermore, when actually monitoring the corrosion of the radioactive waste disposal container, as shown in FIG. 2A, the whole or part of the overpack 11 having a box shape is set as the measurement object 1 and is underground. The counter electrode 2 and the reference electrode 3 may be installed in the vicinity of the overpack 11 while being embedded. Moreover, as shown in FIG.2 (b), you may install the counter electrode 2 which exhibits a cylinder shape or a box shape so that the overpack 11 and the reference electrode 3 which were embed | buried under the ground may be enclosed.
2 is connected to the electrometer 6 and the ammeter 7, the counter electrode 2 is connected to the ammeter 7, and the reference electrode 3 is connected to the electrometer 6.

  The counter electrode 2 and the reference electrode 3 may be made of the same metal as the object 1 to be measured, an inert metal such as graphite or platinum, or the like. Further, the shapes of the counter electrode 2 and the reference electrode 3 are not particularly limited, but it is preferable that the area of the surface facing the measurement object is large. Moreover, it is preferable to install the counter electrode 2 and the reference electrode 3 so that the distance with a measuring object may become near. The number of counter electrodes 2 and reference electrodes 3 may be two or more.

[Pitting depth calculation method]
Next, an embodiment of a pitting corrosion depth calculation method according to the present invention will be described with reference to FIGS. 4, 5, and 6.
The pitting depth calculation method according to this embodiment is a pitting depth calculation method using an electrochemical noise method and an alternating current impedance method, and includes a volume calculation step, an area calculation step, a depth calculation step, Consists of.

Here, the electrochemical noise method is a method of measuring a current signal generated at the time of corrosion from a measurement object and detecting a corrosion state (pitting corrosion volume or the like) based on the signal. Note that the measurement of the current signal or the like is performed under a condition that does not have an electrical influence from the outside (natural state), and this current signal is a current flowing between the measurement object 1 and the counter electrode 2 (see FIG. 1). ).
The AC impedance method is a method of detecting a corrosion state (such as a pitting corrosion area) by applying an AC signal to a measurement object and analyzing the response signal after the application.

(Volume calculation process)
The volume calculation step is a step of obtaining the pitting corrosion volume from a current signal obtained by measuring a metal material over time by an electrochemical noise method.

The measurement of the current signal by the electrochemical noise method is continuously performed with time even during the measurement by the AC impedance method (between the measurements).
The measurement intervals of the current signal are preferably equal intervals and preferably not less than 0.1 seconds and not more than 5 seconds. If the measurement interval is less than 0.1 seconds, the electrical noise caused by the corrosion measuring device 8, the pitting depth calculating device 9 and the like becomes excessive, and this electrical noise is likely to affect the measurement data. Because it becomes. Also, if the measurement interval exceeds 5 seconds, it is difficult to detect the occurrence of pitting corrosion and calculate the volume.

Normally, when pitting corrosion occurs in a metal material, a large value protruding in the current signal is measured, and thereafter gradually attenuates to the original steady value (in the vicinity of 1000 to 1100 seconds in FIGS. 4 and 5).
In the measurement graph, the area formed between the straight line indicating the steady value (for example, the straight line indicating 5.01 × 10 ⁻⁸ in FIG. 5) and the waveform of the measured current signal is necessary for elution of the metal material when pitting corrosion occurs. It corresponds to the amount of electricity. Note that the area may be calculated by integration.

Here, when iron is used as the metal material, Fe is eluted as a divalent ion, the atomic weight of Fe is 55.845 g / mol, and the density of Fe is 7.87 g / cm ^3. By taking this point into consideration, the volume of pitting corrosion can be calculated. Specifically, the volume is (calculated quantity of electricity [C]) × (1/96485 [mol / C]) × (1/2) × 55.845 [g / mol] × (1/7. 87 [cm ³ / g]).
The steady value is a current value measured when the metal material is measured in a steady state in which corrosion such as pitting corrosion has not occurred in the metal material, for example, excluding 5 minutes before and after the occurrence of pitting corrosion. This is the median current value.

Here, regarding the area formed between the straight line indicating the steady value in the measurement graph and the waveform of the measured current signal, the electric quantity may be calculated from the entire area. Also, when the current signal exceeds a predetermined value, it is determined that pitting corrosion has occurred, and only in that case, the area of the period from when it exceeds the predetermined value until it returns to the steady value (a straight line indicating the steady value) And the amount of electricity calculated from the area between the measured current signal waveform and the measured current signal waveform. By setting it as such a structure, the calculation amount at the time of calculating a pitting corrosion volume can be reduced.
In addition, a predetermined value is not specifically limited, For example, when measuring the measuring object whose area is 3.14 cm < ² >, it is 5.0 * 10 < ^-5 > A. Further, the predetermined value may be a value that becomes 3σ when the standard deviation of the current value in the steady state is σ, and may be determined statistically.

  As described above, the volume calculation process may be performed by the volume calculation unit 904 of the pitting depth calculation device 9 or by a person.

(Area calculation process)
The area calculating step is a step of obtaining a pitting area from an impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting by the AC impedance method.

  Measurements using the AC impedance method are performed at regular intervals on a metal material. For example, the measurement is repeated at intervals such as every day, every week, or every month. For this measurement, it is only necessary to measure the metal material before and after the occurrence of pitting corrosion. Therefore, the measurement interval may be determined by examining the frequency and probability of pitting corrosion and the amount of corrosion on the entire surface.

  Note that the measurement by the AC impedance method is a technique in which a small amount of potential (or current) signal is applied to a target metal material, and therefore a load is applied to the metal material (measurement target) each time measurement is performed. Therefore, it is preferable to make the measurement interval as long as possible within a range where about one pitting corrosion can be expected during the measurement interval.

  In addition, the measurement by the alternating current impedance method is performed after a few minutes from the time when it is detected that pitting corrosion has occurred in the volume calculation process (for example, after 10 minutes from the time when the current value exceeds a predetermined value). Any configuration may be used. By adopting such a configuration, for example, when pitting corrosion has not occurred for two months, measurement by the alternating current impedance method is not performed during the period, so the load due to measurement on the metal material is not increased. It can be reduced as much as possible. In addition, even if the measurement environment changes so that pitting corrosion frequently occurs every day, the impedance spectrum is measured every time it is detected that pitting corrosion has occurred. Therefore, it can be detected without missing the occurrence or progress of pitting corrosion.

However, since the measurement of waste disposal containers for radioactive waste takes a long time, not only local corrosion such as pitting corrosion, but also the phenomenon that the entire waste disposal container (metal material) corrodes uniformly Is assumed. Therefore, it is necessary to set the measurement interval of the measurement by the AC impedance method so that the amount of uniform corrosion on the entire surface (total corrosion amount) can be ignored. This is because the pitting depth calculation method is a method for obtaining the pitting area by comparing two consecutive measurement results obtained by the AC impedance method, so that elements other than pitting corrosion are measured between the two consecutive measurements. This is because it is important not to change.
Therefore, when the impedance spectrum is measured every time the occurrence of pitting corrosion is detected without setting the measurement interval to be equal, when a predetermined period has elapsed (for example, 6 months have elapsed) without detecting the occurrence of pitting corrosion, It is preferable that the impedance spectrum be measured.

Next, the principle of calculating the area of pitting corrosion by the AC impedance method will be described.
The measurement by the AC impedance method is an unsteady measurement as described above, and the response signal in the relaxation process until it returns to the steady state after shifting from the steady state by an external signal (potential or current signal) applied to the metal material. By analyzing, it is possible to obtain information such as elementary processes and reaction intermediates.

  Since the input external signal uses a minute alternating current signal, the damage to the metal material (measurement target) due to the measurement is generally at a level that can be ignored. Then, spectrum analysis can be performed by changing the frequency of the AC signal. In the measurement in the high frequency range, spectrum analysis can be divided into charge transfer, which is a fast process, and diffusion, which is a slow process in the low frequency range.

When a potential signal having a sine wave is input as an external signal, a sine wave current signal whose phase is shifted by an angle θ is generated as a response signal. This current signal can be displayed as an exponential function from Euler's formula, and the (complex) impedance displayed in a complex number is characterized by the absolute value of the impedance and the phase shift θ.
With respect to the impedance calculated for each frequency as described above, by taking the real part (Z ′) of the impedance on the horizontal axis and the imaginary part (Z ″) on the vertical axis, as shown in FIG. A vector trajectory diagram (or Cole-Cole plot) can be obtained. Further, by taking the absolute value of impedance (| Z |) and phase difference (θ) on the vertical axis and the frequency at that time on the horizontal axis, a Bode diagram as shown in FIG. 6B can be obtained.

Here, a circuit representing a corrosion reaction occurring in a metal material is a general equivalent circuit as shown in FIG. 7 (a circuit in which an element in which a charge transfer resistance and a capacitor are connected in parallel to a solution resistance is connected in series. ), The impedance Z of the circuit showing this corrosion reaction is Z = R, where solution resistance R _sol , charge transfer resistance R _ct , electric double layer capacitance C, each frequency ω, and imaginary unit j _sol + R _ct / (1 + jωR _ct C). The characteristic values of the respective elements are obtained by arbitrarily substituting the characteristic values of the respective elements so as to be well suited to the measurement results drawn on the vector locus diagram and the Bode diagram.

  In addition, there are already many known literatures regarding such an analysis technique of electrochemical impedance. For example, Shiro Haruyama, Toru Minoru and Shoji Anan, “Anticorrosion Technology” Vol. 27, (1978), no. 9, p. There are detailed descriptions in 449-456 and the like.

  For this equivalent circuit, it is desirable to consider a presumed corrosion reaction and examine it in a preliminary experiment simulating the literature and the environment. The element of the equivalent circuit used at this time is based on its electromagnetic definition. For example, in the case of a capacitor, the capacitance C is εS / d (ε: dielectric constant of electric double layer, S: electrode area of electric double layer, d : Electric double layer thickness), and resistance R is expressed by ρd / S (ρ: resistivity of charge transfer resistance, S: electrode area of charge transfer resistance, d: thickness of charge transfer resistance).

  If pitting corrosion has occurred in the measurement object during the two consecutive AC impedance measurements, only the area (S) of the measurement object changes, and the other dielectric constant (ε) and resistivity (ρ) The thickness (d) does not change. Therefore, the pitting area on the electrode surface can be obtained by comparing the characteristic values of each element of the equivalent circuit obtained from the measurement result (impedance spectrum) by the two consecutive AC impedance methods.

  In other words, if the area of the entire measurement target is S and the pitting corrosion has an area that is x times S, the area other than pitting corrosion is (1-x) S. Therefore, the capacitance of the capacitor after the occurrence of pitting corrosion is (1-x) times and the resistance is 1 / (1-x) times that before the occurrence of pitting. Then, by calculating the characteristic value of each element of the equivalent circuit that best fits using the relationship between the capacitance and resistance of this capacitor, and calculating the x value by solving simultaneous equations for the resistance and the capacitor, the pitting area is calculated. can do.

Here, a distributed constant type equivalent circuit often used in the model of the porous electrode as an assumed equivalent circuit (for example, Masayuki Itagaki, Akihiro Taya, Daishi Ishidani, Kunihiro Watanabe, “ZAIRYO-TO-KANKYO Materials and Environment” Vol.50, No.1 (2001) p.24-29) may be used for fitting. In this case, the inventors have confirmed that if the calculation is performed assuming that one of the pores has become pitting corrosion, the same result is obtained with an average error within 5%.
As described above, the process of the area calculation process may be performed by the pitting corrosion depth calculation device 9 or the area calculation means 909 of the frequency response analysis device, or may be performed by a person.

(Depth calculation process)
The depth calculation step is a step of calculating the pitting depth by dividing the pitting volume by the pitting area. By dividing the pitting corrosion volume calculated in the volume calculation step by the pitting corrosion area calculated in the area calculation step, the pitting corrosion depth can be obtained.

  In general, if the pitting area is known, it is assumed that the pitting is formed to have an ideal hemispherical shape for a metal material, and the size of the pitting is often calculated. However, the inventors have observed examples where pitting corrosion is not an ideal hemisphere. In addition, there are many examples in which the shape of the cross section of the pitting corrosion appearing on the metal material surface is not a circle but an ellipse or a polygon. Therefore, according to the pitting corrosion depth calculation method of the present invention that considers the shape of pitting corrosion to be an elliptical column or a polygonal column, the depth of pitting corrosion can be calculated without any problem.

  As described above, the depth calculation process may be performed by the depth calculation unit 905 of the pitting corrosion depth calculation device 9 or may be performed by a person.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and the design can be appropriately changed without departing from the gist of the present invention described in the claims.
For example, in the area calculation process, the current signal greatly fluctuates while the metal material is measured by the AC impedance method. Therefore, it is good also as a structure which does not calculate the volume of pitting corrosion based on an electric current signal during the said period.

Next, with respect to the pitting corrosion depth calculation method, it was confirmed that the effect of the invention was achieved by an example satisfying the requirements of the present invention.
[Measurement environment]
First, the measurement environment of the embodiment will be described with reference to FIG.
Carbon steel was used as a measurement object (test electrode) 1 instead of a radioactive waste disposal container. Further, platinum was used for the counter electrode 2 and an Ag / AgCl reference electrode was used for the reference electrode 3. In addition, since the test electrode 1 was a disk shape having a diameter of 2 cm and was coated on one surface so as not to cause corrosion, the area of the measurement target was 3.14 cm ² .

Assuming that groundwater or the like has infiltrated into the actual disposal environment, the test electrode 1, the counter electrode 2 and the reference electrode 3 are pH 9, NaHCO ₃ —Na ₂ CO ₃ : 10,000 ppm, NaCl: 19,000 ppm, 30 ° C., The sample was immersed in the test solution 5 adjusted to be open to the atmosphere.

A personal computer (pitting corrosion depth calculation device) that uses a 1287 type potentiostat and galvanostat manufactured by Solartron, equipped with an ammeter and an electrometer, and measures the current signal using the electrochemical noise method. Recorded. The current signal at this time is shown in FIGS.
The current signal measurement interval is 1 count / 1 second.

  The measurement by the alternating current impedance method was continuously performed once every two days. The measurement was to connect and control a Solartron 1255B type frequency response analyzer to the 1287 type potentiostat and galvanostat to obtain an impedance spectrum. The measurement frequency range was 10 kHz to 1 Hz, and the amplitude was measured within 10 mV.

[Measuring method]
An increase in the current value at which it was possible to determine that pitting occurred on the 13th day from the start of measurement, and the pitting area was determined using the impedance spectra on the 12th and 14th days. Then, the median value (5.01 × 10 ⁻⁸ A) of the current value (excluding 5 minutes before and after the occurrence of pitting corrosion) measured by electrochemical noise for 2 hours before and after the occurrence of pitting corrosion was taken as the steady value (FIG. 5), The area formed by the current signal at the time and the steady value was obtained by integration to calculate the pitting volume.

  In order to verify the accuracy of the results of the pitting corrosion depth calculation method of the present invention, after the measurement by the AC impedance method on the 14th day is completed, a sample is taken out, rinsed with pure water and dried, and pitting corrosion is made by Keyence Corporation laser. The pitting corrosion shape and depth were calculated using a microscope VK-9500 / 9510.

[Calculation results]
In the measurement graph, the area formed by the waveform of the current signal in the section where pitting corrosion occurred and the straight line indicated by the steady-state value was calculated by integration to obtain the amount of electricity required for pitting corrosion. The section in which pitting corrosion can be determined occurred for 121 seconds, and the amount of electricity was 4.91 × 10 ⁻² C (Coulomb). Assuming that the Faraday constant is 96485 C / mol and all Fe is eluted with 2 valences, Fe eluted by the occurrence of pitting corrosion is 2.55 × 10 ⁻⁷ mol. Here, since the atomic weight of Fe is 55.845 (g / mol), it is understood that 1.42 × 10 ⁻⁵ g of Fe was eluted. Since the Fe density is 7.87 g / cm ³ , pitting corrosion with a volume of 1.80 × 10 ⁻⁶ cm ³ occurs.

On the other hand, as an equivalent circuit of AC impedance, fitting was performed assuming a circuit (FIG. 7) in which an element in which a charge transfer resistance and a capacitor are connected in parallel to a solution resistance was connected in series. When the impedance spectra (FIG. 6) measured on the 12th day and the 14th day were fitted, the pitting area was 6.7 × 10 ⁻⁵ times the total test electrode area (3.14 cm ² ), that is, the pitting area. Was best fitted with 2.1 × 10 ⁻⁴ cm ² .

From these, the pitting depth was determined to be 1.80 × 10 ⁻⁶ cm ³ ÷ 2.1 × 10 ⁻⁴ cm ² = 86 μm.
At this time, when pitting corrosion was actually observed with a laser microscope, the shape was an elliptical column and the maximum pitting depth was 82.6 μm.

  Compared with the actual measurement value not limited to the above-described example, the pitting corrosion depth calculated according to the present invention has an error within 5%, and it was confirmed that the calculated value was sufficiently reliable.

1 Measurement object (metal material)
2 Counter electrode 3 Reference electrode 4 Bentonite 5 Test solution 6 Electrometer 7 Ammeter (Non-resistance ammeter)
8 Corrosion measuring device (potentiostat / galvanostat)
9 Pitting depth calculation device 11 Overpack 901 Input means 902 First storage means 903 Discrimination means 904 Volume calculation means 905 Depth calculation means 906 Time notification means 907 Signal output means 908 Second storage means 909 Area calculation means 910 Output means S Pitting depth calculation system

Claims (6)

A volume calculating step for obtaining a pitting corrosion volume from a current signal obtained by measuring a metal material over time by an electrochemical noise method;
An area calculation step for obtaining a pitting area from an impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by an AC impedance method;
And a depth calculation step of calculating a depth of pitting by dividing the pitting volume by the pitting area.   The pitting corrosion depth calculation method according to claim 1, wherein the measurement interval of the current signal is 0.1 to 5 seconds.   The pitting corrosion depth calculation method according to claim 1, wherein the metal material is buried in the ground.   The pitting corrosion depth calculation method according to any one of claims 1 to 3, wherein the metal material constitutes a waste disposal container for radioactive waste. A volume calculating means for obtaining a pitting volume from a current signal obtained by measuring a metal material over time by an electrochemical noise method;
An area calculating means for determining a pitting corrosion area from an impedance spectrum obtained by measuring the metal material before and after the occurrence of pitting corrosion by an AC impedance method;
A depth calculating means for calculating the depth of the pitting by dividing the pitting volume by the pitting area;
A pitting corrosion depth calculating device comprising: Corrosion measuring device that measures the current signal of the metal material over time by the electrochemical noise method, and measures the impedance spectrum of the metal material before and after the occurrence of pitting corrosion by the AC impedance method,
Volume calculation means for obtaining a pitting corrosion volume from the current signal of the metal material obtained by the corrosion measuring device, and a pitting corrosion area from the impedance spectrum of the metal material before and after the occurrence of pitting corrosion obtained by the corrosion measuring device. A pitting corrosion depth calculating device comprising: an area calculating means; and a depth calculating means for calculating a pitting corrosion depth by dividing the pitting corrosion volume by the pitting corrosion area;
A pitting corrosion depth calculation system comprising:

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