JP6403618B2 - AC corrosion risk measurement and evaluation method for buried metal bodies - Google Patents

Description

  The present invention relates to an AC corrosion risk measurement evaluation method for buried metal bodies.

An evaluation method based on a measured value of coupon current density is generally known as a method for measuring and evaluating the cathodic protection status and corrosion risk of a buried metal body such as a cathodic protection buried metal pipeline. This uses a metal piece of the same material as the buried metal body called a coupon that simulates the coating defect part of the buried metal body, and electrically connects this to the buried metal body that has been coated, in the vicinity of the buried metal body The coupon DC current density I _dc and coupon AC current density I _ac are obtained by measuring the current (coupon current) flowing through the wire connecting the coupon and the buried metal body, and the average value of the measurement time (I _dc ^ave , I _ac ^ave ) are ^compared with the cathodic protection standard using the coupon current density as an index.

At that time, in the evaluation of the AC corrosion risk based on the coupon AC current density I _ac, it is determined whether or not the measured value of the coupon current corresponds to a sine wave of a commercial frequency (for example, 50 Hz). ) And (ii) are determined to correspond to a commercial frequency sine wave (see Patent Document 1 below). (i) The time difference between the maximum value and the minimum value of the measurement values measured within the unit measurement time (20 ms) is ½ · unit measurement time (10 ms). (ii) In the measurement value measured within the unit measurement time (20 ms), (maximum value) − (average value) = (average value) − (minimum value).

JP 2010-190658 A

  The polarity of the coupon current is such that a current (cathode current) flowing from the electrolyte in the direction of the coupon is positive, and a current (anode current) flowing from the coupon to the electrolyte is negative. When evaluating the AC corrosion risk based on the coupon current density, it is necessary to determine whether the measured value waveform of the coupon current (time-series change in measured value) corresponds to a sine wave of commercial frequency, but more importantly As a discriminating factor, the polarity of the measured value waveform should be reversed (the maximum value is a positive value and the minimum value is a negative value). This is because when the AC corrosion occurrence mechanism is considered, the surface roughness increases due to repetition of the anode current and cathode current on the local surface of the buried metal body, and the surface of the metal (iron) is exposed. This is because the accumulation of anode reaction that turns metal into metal ions (iron turns into iron ions) is considered to be AC corrosion.

On the other hand, in the conventional evaluation of the AC corrosion risk by the coupon current density, the maximum value I _ac ^max of the coupon AC current density I _ac is obtained for each fixed measurement category (for example, 10 s), and the measurement time of the maximum value (for example, for example, (1 hour) An average value was obtained, and the average value was compared with a reference value (30 A / m ^{2 in the} case of ISO 15589-1) to evaluate the presence or absence of AC corrosion risk.

  However, even if the average value described above is within the reference value, such as the influence of the operation of the AC electric railway vehicle, such as the phenomenon that the fluctuation of the coupon AC current density is affected, There is a concern that a large amount of anode current that affects AC corrosion is included in the fluctuation of coupon current density in a short time, and risk management cannot be said to be sufficient only by comparing the average value with the reference value. In particular, the metal that causes AC corrosion becomes metal ions (iron becomes iron ions). The anode reaction is an irreversible reaction, so quantitatively grasping the accumulation of the anode reaction makes the AC corrosion risk more precise. It becomes important in evaluating.

  The present invention has been proposed on the basis of such new knowledge, and an object thereof is to more accurately evaluate the AC corrosion risk by quantitatively grasping the accumulation of the anode reaction.

In order to achieve such an object, an AC corrosion risk measurement and evaluation method according to the present invention has the following configuration.
The coupon current of a coupon installed near the buried metal body and electrically connected to the buried metal body is measured at a set sampling interval at a unit measurement time corresponding to one cycle of the commercial frequency, and within the unit measurement time. The ratio of the time when the measured value of the coupon current is on the anode side to the unit measurement time is obtained as an anode current ratio, and the AC corrosion risk of the buried metal body is evaluated by the anode current ratio. AC corrosion risk measurement evaluation method.

  The AC corrosion risk measurement and evaluation method of the present invention having such characteristics introduces a new evaluation index called the anode current ratio, thereby quantitatively grasping the accumulation of anode reaction and more accurately measuring the AC corrosion risk. Can be evaluated.

It is explanatory drawing which showed an example of the measured value (measured value waveform) of the coupon current measured within unit measurement time. It is explanatory drawing which showed specifically the alternating current corrosion risk measurement evaluation method which concerns on embodiment of this invention. It is explanatory drawing which showed the system configuration | structure for implementing the alternating current corrosion risk measurement evaluation method which concerns on embodiment of this invention. It is explanatory drawing which showed the flow of the alternating current corrosion risk measurement evaluation method which concerns on embodiment of this invention. It is explanatory drawing (graph showing the combination of the maximum value of an anode current ratio and a coupon alternating current density) which showed the alternating current corrosion risk measurement evaluation method which concerns on embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the measurement and evaluation of the AC corrosion risk, a unit measurement time Ts corresponding to one cycle of the commercial frequency is set, and a coupon current is measured at a set sampling interval within the unit measurement time Ts. As is well known, the coupon current can be measured by installing a coupon near the buried metal body to be measured and evaluated and electrically connecting the coupon and the buried metal body.

  The unit measurement time Ts is 20 ms when the commercial frequency is 50 Hz, 16.7 ms when the commercial frequency is 60 Hz, and 60 ms when the commercial frequency is 16-2 / 3 Hz. The sampling interval to be set is preferably about 0.1 ms in order to grasp the influence of a high-speed phenomenon such as operation of an AC electric railway vehicle. The polarity of the coupon current to be measured is a positive current (cathode current) flowing from the electrolyte to the coupon and a negative current (anode current) flowing from the coupon to the electrolyte.

FIG. 1 shows an example of a coupon current measurement value (measurement value waveform) I (t) measured within the unit measurement time Ts described above. In this example, the maximum measurement value I (t _max ) is obtained at the appearance time t _max within the unit measurement time Ts, and the minimum measurement value I (t _min ) is obtained at the appearance time t _min . Whether or not such a measured value I (t) affects the AC corrosion risk is determined by whether or not the following conditions i) ii) or i) to iii) are satisfied by an AND condition.

i) about half of the maximum measured value I (t _max) of the appearance time t _max and the minimum measured value I (t _min) appearance time t time difference _min Delta] t is the unit measurement time Ts. For example, when the unit measurement time Ts is 20 ms, Δt is about 10 ms.

ii) The polarities of the maximum measured value I (t _max ) and the minimum measured value I (t _min ) are reversed. That is, I (t _max )> 0 and I (t _min ) <0.

iii) The strain rate ε is obtained by the following equation (1), and the strain rate ε is equal to or less than a set threshold value. This distortion rate ε is a value that quantitatively indicates how much the waveform of the measured value I (t) is distorted from the sine wave of the commercial frequency. The lower the distortion factor ε, the closer to a commercial frequency sine wave. I _ave in the following formula (1) is the average value (T is the number of samples of the unit measurement time) within the unit measurement time Ts of the measurement value I (t), and I _p is the maximum measurement value I (t _max ) And the midpoint value of the minimum measured value I (t _min ).

  Then, when the above conditions i) ii) or i) to iii) are satisfied by the AND condition, the measured value I (t) is on the anode side (I (t) <0 or I (t) ≦ Find the time Tan that is 0). The time Tan is calculated from the number of samples of the measured value I (t) that becomes negative (or negative or 0) and the sampling interval by allocating I (t) from t = 0 to t = Ts to positive, negative, and 0 can do. Then, the anode current ratio α is obtained by the ratio of the time Tan to the unit measurement time Ts, α = Tan / Ts.

  This anode current ratio α quantitatively represents the accumulation of the anode reaction that is the basis of AC corrosion. By grasping how much the anode current ratio α is expressed within the measurement time, It is possible to quantitatively determine whether or not to perform preferential preventive maintenance of AC corrosion risk.

Hereinafter, a specific measurement evaluation method based on the anode current ratio α will be described. Here, the maximum coupon alternating current density I _ac from the measured value I of the coupon current measured in the unit measurement time Ts (t) calculated a coupon alternating current density I _ac, the measurement division every unit measurement time Ts is more consecutive The value I _ac ^max is extracted, and the anode current ratio α is obtained from the measured value I (t) of the coupon current constituting the maximum value I _ac ^max .

  This will be described more specifically with reference to FIG. In FIG. 2, the unit measurement time Ts corresponding to one cycle of the commercial frequency is set to 20 ms, the sampling interval within the unit measurement time Ts is set to 0.1 ms (200 samples), and the unit measurement time Ts is measured for n1 consecutive measurements. A division is set, and a measurement time Tm in which n measurement divisions are continued is set.

  As a setting example, if the measurement time Tm is 1 hour, a 10-second measurement division is set by dividing the measurement time Tm into 360 equal parts, and 400 (8 sec) continuous unit measurement times Ts (20 msec) in the 10-second measurement division Can be set.

The coupon alternating current density I _ac obtained every unit measurement time Ts is obtained by the following equations (2) and (3). The coupon DC current density I _dc is obtained from the equation (2), and the coupon AC current density I _ac is obtained from the equation (3) using the coupon DC current density I _dc .

  In the following formulas (2) and (3), A is the coupon surface area, I (t) is the measured value of the coupon current at time t, and T is the measured value of the coupon current I (t) measured within the unit measurement time Ts. The number of samples.

Then, the maximum value I _ac ^max for each section of the coupon alternating current density I _ac is extracted for each set measurement section. In the example shown in the figure, n1 coupon alternating current densities I _ac are obtained in the measurement section, and the m1 value among them is extracted as the maximum value I _ac ^max for each section. Here, as described above, the anode current ratio α is obtained from the measured value waveform of the measured value I (t) constituting the maximum value I _ac ^max for each section. This anode current ratio α is an evaluation value representing the cathodic protection situation at the same time as the maximum value I _ac ^{max for} each section. Then, a combination (α, I _ac ^max ) of the anode current ratio α and the maximum value I _ac ^max for each section is obtained for each measurement section.

  FIG. 3 is an explanatory diagram showing a system configuration for executing the AC corrosion risk measurement evaluation method according to the embodiment of the present invention. In the following description, an embedded metal pipeline coated with a plastic coating as an embedded metal body (hereinafter simply referred to as a pipeline) will be described as an example. However, embodiments of the present invention are not particularly limited to this. Absent. Further, the buried metal body here is premised on cathodic protection such as an external power source system or a galvanic anode system.

  The pipeline 1 to be evaluated for AC corrosion risk includes a plastic coating 1C and is embedded in an electrolyte (soil) S. A coupon 2 simulating a defective portion of the plastic coating 1C is embedded in the vicinity of the pipeline 1 in the electrolyte S. The coupon 2 is a metal piece made of the same material as the pipeline 1 and has a known surface area A. The pipeline 1 and the coupon 2 are electrically connected by an electric wire L. FIG. 3 shows a situation where a stray current Is including an anticorrosion current Ic and an alternating current component flows in the peripheral electrolyte of the pipeline 1 and the coupon 2.

  Such a measurement evaluation system in the pipeline 1 can be configured by the measurement device 10 and the external arithmetic processing device 20. Here, for convenience, for the pipeline 1 and the coupon 2, the measurement device 10 that is always connected during the measurement time and the external processing device 20 that acquires information from the measurement device 10 after the measurement time and performs arithmetic processing are separately configured. However, these can also be integrated.

  The measuring device 10 is provided with a shunt 11 on the electric wire L, and measures the coupon current detected by the shunt 11. The measured coupon current is calculated by the arithmetic processing means 14 via the low-pass filter 12 and the AD converter 13. Is input. The arithmetic processing means 14 includes a data storage means 15 for storing the sampled coupon current and the result of arithmetic processing of the sampled coupon current, a coupon current measuring means 14A, a coupon DC current density calculating means 14B, a coupon AC current density calculating means 14C, Functions such as maximum coupon AC current density extraction means 14D are provided.

  The coupon current measuring unit 14A has a function of sampling and measuring the coupon current at a set sampling interval in a unit measurement time corresponding to one cycle of the commercial frequency. When the commercial frequency is 50 Hz, the unit measurement time is 20 ms. When the sampling interval is set to 0.1 ms, 200 coupon current measurement values are sampled per unit measurement time. The coupon current measuring means 14A sequentially outputs the waveform of the measured value I (t), which is time series data of the 200 coupon current measured values (data relating the time t and the measured value sampled at that time). The waveform of the measured value I (t) output by the coupon current measuring unit 14A is stored in the data storage unit 15 every unit measurement time.

The coupon DC current density calculating unit 14B has a function of calculating the coupon DC current density I _dc by the above equation (2) from the waveform of the coupon current measurement value I (t) measured by the coupon current measuring unit 14A. One coupon DC current density I _dc is output per unit measurement time.

The coupon AC current density calculating means 14C is calculated by using the above equation from the coupon current measurement value I (t) measured by the coupon current measuring means 14A and the coupon DC current density I _dc calculated by the coupon DC current density calculating means 14B. (3) has a function of calculating the coupon alternating current density I _ac . One coupon alternating current density I _ac is output per unit measurement time and stored in the data storage means 15.

The maximum coupon AC current density extraction unit 14D extracts the maximum value I _ac ^max of the coupon AC current density I _ac for each measurement section. For example, if the measurement section is 10 s, the unit measurement time of 20 ms is continued 400 times in 8 s in the measurement section. During that time, the coupon AC current density calculation means 14C outputs 400 coupon AC current density I _ac . To do. Maximum coupon alternating current density extracting unit 14D is sequentially compares the coupon alternating current density I _ac output from the coupon alternating current density calculating unit 14C, eventually a maximum value I _ac from the coupon alternating current density I _ac 400 Extract ^max . The extracted value of the coupon alternating current density for each section is stored in the data storage means 15 with the maximum value I _ac ^max and the waveform of the measured value I (t) constituting the value I _ac ^max .

  The external arithmetic processing unit 20 includes arithmetic processing means 21 having functions such as polarity determination means 21A, appearance time difference calculation means 21B, strain rate calculation means 21C, anode current ratio calculation means 21D, and AC corrosion risk evaluation means 22. Yes.

The polarity discriminating means 21A takes in the measured value I (t) of the coupon current constituting the maximum value I _ac ^max stored in the data storage means 15 of the measuring device 10, and the maximum measured value I in the measured value I (t). It is determined whether the polarities of (t _max ) and the minimum measured value I (t _min ) are different. Here, it is determined whether or not I (t _max ) × I (t _min ) is a negative value. When I (t _max ) × I (t _min ) is a negative value, I (t _max ) and I (t _min ) have different polarities, which means that the maximum measured value I (t _max ) Indicates a positive value and the minimum measured value I (t _min ) is a negative value.

The appearance time difference calculation means 21B takes in the measurement value I (t) constituting the maximum value I _ac ^max stored in the data storage means 15 of the measurement apparatus 10, and the appearance time t _{max of the} maximum measurement value I (t _max ). And a time difference Δt between the appearance times t _min of the minimum measured value I (t _min ). Further, the strain rate calculating means 21C has a function of taking a measured value waveform constituting the maximum value I _ac ^max stored in the data storage means 15 of the measuring apparatus 10 and calculating the strain rate ε by the above equation (3). doing.

The anode current ratio calculating means 21D takes in the measured value I (t) constituting the maximum value I _ac ^max stored in the data storage means 15 of the measuring device 10, and I (t) from t = 0 to t = Ts. Is divided into positive, negative, and 0, and the time Tan is obtained from the number of samples of the measurement value I (t) that is negative (or negative or 0) and the sampling interval, and the ratio of the time Tan to the unit measurement time Ts, α = Tan The anode current ratio α is calculated from / Ts.

  The AC corrosion risk evaluation means 22 has a function of evaluating the AC corrosion risk based on the outputs of the polarity determination means 21A, the appearance time difference calculation means 21B, the strain rate calculation means 21C, and the anode current ratio calculation means 21D.

  FIG. 4 shows an example of an evaluation flow of the AC corrosion risk evaluation means 22 in this case. In step S1, the initial value of the number n of measurement categories is set to 1. Step S2 specifies the measurement category (n) to be processed for the measurement categories divided into a plurality of categories, and processing is performed sequentially from the measurement category (1).

In step S < _b > 3, the maximum value I _ac ^max (n) for each category of the coupon alternating current density measured in each measurement category (n) in the measurement device 10 is captured. Step S4 is a determination process for the output of the polarity determining means 21A. When I (t _max ) × I (t _min ) is a negative value (“YES”), that is, the maximum measured value I (t _max ). Is a positive value and the minimum measured value I (t _min ) is a negative value, the process proceeds to step S5, and if I (t _max ) × I (t _min ) is not a negative value (“NO”) The process proceeds to step S7.

Step S5 is a determination process of the outputs of the appearance time difference calculating means 21B, the maximum measured value I (t _max) appearance time t _max and the minimum measured value I (t _min) the time difference Δt is the unit of appearance time t _min of the It is determined whether or not the measurement time Ts is about ½. If the time difference Δt is about ½ of the unit measurement time Ts (“YES”), the process proceeds to step S6, and the time difference Δt is the unit measurement time. If it is not ½ of Ts (“NO”), the process proceeds to step S7.

  Step S6 is a determination process for the output of the strain rate calculation means 21C. When the strain rate ε is smaller than the threshold Et (“YES”), the process proceeds to Step S8, and when the strain rate ε is greater than or equal to the threshold Et (“NO”). ") Proceeds to step S7. Here, the threshold Et can be set to 0.01, for example.

When the waveform of the measured value I (t) of the coupon current measured in each measurement section is not identified as a commercial frequency sine wave through steps S4, S5, and S6, the process proceeds to step S7. In S7, it is determined that there is no AC corrosion risk in the maximum value I _ac ^max in the coupon AC current density obtained in the measurement category at that time, and the process proceeds to step S10.

If the measured value I (t) of the coupon current constituting the maximum value I _ac ^max extracted in each measurement section is identified as a commercial frequency sine wave through steps S4, S5 and S6, the process goes to step S8. Thus, the anode current ratio calculation means 21D calculates the anode current ratio α. Here, any of the steps S4, S5, and S6 can be omitted. For example, only in steps S4 and S5, the coupon current constituting the maximum value I _ac ^max extracted in each measurement section can be reduced. The measured value I (t) can be identified as a sine wave having a commercial frequency.

When the anode current ratio calculation means 21D calculates the anode current ratio α in step S8, the combination (α, I _ac ^max ) of the anode current ratio α and the maximum value I _ac ^{max for} each section is stored in the data storage means 15. Remember.

In step S10, it is determined whether or not the current measurement time n is final. If it is not final ("NO"), the process proceeds to the next time (n + 1) (step S11), and the measurement category is determined. On the other hand, the process of step S2-step S9 is performed. If the current n is final (“YES”), (α, I _ac ^max ) for each measurement category stored in the data storage means 15 is output (step S12).

FIG. 5 shows the output result. Here, the horizontal axis represents the anode current ratio α, and the vertical axis represents the maximum value I _ac ^max of the coupon alternating current density extracted for each category, and the output result of (α, I _ac ^max ) is shown. . As is apparent from the figure, when the anode current ratio α increases, the maximum value I _ac ^{max for} each coupon AC current density section increases, and this is an appropriate index for evaluating the AC corrosion risk. It shows that it is a thing.

As shown in FIG. 5, when the anode current ratio α is 0.3 or more, the maximum value I _ac ^{max for} each section of the coupon AC current density increases sharply, and the anode current ratio of 0.3 or more If it is possible to grasp the proportion of occurrences of α in the entire measurement time, it is possible to quantitatively grasp the necessity of preferential preventive maintenance against AC corrosion risk from the measurement results. become.

In step S13 of the flow shown in FIG. 4, based on the above-described knowledge, a threshold αt is set (for example, αt = 0.3), and the maximum coupon AC current density that satisfies α> αt (or α ≧ αt) is satisfied. The number of values I _ac ^max evaluates whether preferential preventive maintenance against AC corrosion risk is necessary.

As described above, according to the embodiment of the present invention, the average value within the measuring time of the maximum value I _ac ^max of the coupon alternating current density obtained within the measuring time Tm is the reference value (30A if ISO 15589-1 is used). / M ² ), even if it is lower, the anode current ratio α can be obtained from the measured value I (t) of the coupon current that constitutes each maximum value I _ac ^max to enable precise evaluation of AC corrosion risk Become. At that time, it can be said that the anode current ratio α is an index that clearly represents the accumulation of an irreversible anode reaction that causes AC corrosion, and therefore whether or not priority preventive maintenance against AC corrosion risk is necessary. Can be grasped quantitatively.

10: Measuring device, 11: Shunt
12: low-pass filter, 13: AD converter, 14: arithmetic processing means,
14A: Coupon current measuring means, 14B: Coupon DC current density calculating means,
14C: Coupon AC current density calculation means,
14D: Maximum coupon AC current density extraction means, 15: Data storage means,
20: External processing unit, 21: Processing unit,
21A: Polarity discrimination means, 21B: Appearance time difference calculation means,
21C: strain rate calculation means, 21D: anode current ratio calculation means,
22: AC corrosion risk evaluation means, L: Electric wire

Claims (5)

The coupon current of a coupon installed near the buried metal body and electrically connected to the buried metal body is measured at a set sampling interval in a unit measurement time corresponding to one cycle of the commercial frequency,
The ratio of the time when the measured value of the coupon current within the unit measurement time is the anode side to the unit measurement time is determined as the anode current ratio,
An AC corrosion risk measurement evaluation method for a buried metal body, wherein the AC corrosion risk of the buried metal body is evaluated based on the anode current ratio. Measure the coupon current of a coupon installed near the buried metal body and electrically connected to this buried metal body,
The measured value of the coupon current measured at each set sampling interval in a unit measurement time corresponding to one cycle of the commercial frequency is
i) The time difference between the maximum value appearance time and the minimum value appearance time of the measured value of the coupon current within the unit measurement time is ½ of the unit measurement time.
ii) The polarities of the maximum value and the minimum value of the measured value of the coupon current within the unit measurement time are reversed,
When satisfying both
The ratio of the time when the measured value of the coupon current within the unit measurement time is the anode side to the unit measurement time is determined as the anode current ratio,
An AC corrosion risk measurement evaluation method for a buried metal body, wherein the AC corrosion risk of the buried metal body is evaluated based on the anode current ratio. 3. The AC corrosion risk measurement and evaluation method according to claim 2, wherein the anode current ratio is obtained when the following condition iii) is satisfied in addition to the above i) ii).
iii) When the strain rate ε determined by the following formula from the measured value of the coupon current within the unit measurement time is equal to or less than a threshold value,
ε = | I _ave − | I _p || / | I _p |
Where I _{ave is} an average value of the measurement values in the unit measurement time, I _{p is} an intermediate value between the maximum value and the minimum value of the measurement values in the unit measurement time. The coupon alternating current density I _ac is determined from the measured value of the coupon current measured during the unit measurement time,
Extracting the maximum value I _ac ^max of the coupon AC current density I _ac for each measurement section in which a plurality of unit measurement times are continuous,
The AC corrosion risk measurement evaluation method according to any one of claims 1 to 3, wherein the anode current ratio is obtained from a measured value of the coupon current constituting the maximum value I _ac ^max . Within a measurement time in which a plurality of measurement categories are continuous,
Extracting a plurality of the maximum values I _ac ^max ;
Comparing the anode current ratio determined for each maximum value I _ac ^max with a set threshold value;
5. The preferential preventive maintenance for AC corrosion is determined based on the number of the maximum value I _ac ^max having the anode current ratio that is greater than or equal to or greater than the threshold. AC corrosion risk measurement evaluation method.