JP2018141215A - Measurement and evaluation method for DC stray current corrosion risk - Google Patents

Abstract

The present invention provides a method for measuring and evaluating the risk of DC stray current corrosion in buried metal objects, capable of accurately evaluating the risk of DC stray current corrosion in buried metal objects without overestimating or overestimating the risk. [Solution] A method for measuring and evaluating the risk of DC stray current corrosion of buried metal objects by measuring a probe current, in which a probe DC current density Id.c. is calculated for each sub-unit time from instantaneous values of the probe current measured multiple times during a sub-unit time, and a minimum value Id.c.min and an average value Id.c.ave of the probe DC current density Id.c. are calculated for each unit time consisting of multiple consecutive sub-unit times, and if the minimum value Id.c.min and the average value Id.c.ave are both negative values in the unit time, the evaluation value for each unit time is the minimum value Id.c.min x sub-unit time + average value Id.c.ave x (unit time - sub-unit time), and if the minimum value Id.c.min is negative and the average value Id.c.ave is positive in the unit time, the evaluation value for each unit time is the minimum value Id.c.min x sub-unit time. [Selected Figure] Figure 2

Description

  The present invention relates to a method for measuring and evaluating a direct current stray current corrosion risk of a buried metal body such as a pipeline that is managed for cathodic protection.

  Metal bodies buried in the ground that are electrolytes are exposed to the risk of corrosion due to stray currents flowing in the ground. There are various causes of stray current, but DC stray current, which is leakage current from DC electric facilities such as DC electric railways, has a large current value and fluctuation range and shows high-speed fluctuation, so the corrosion risk is managed. Detailed measurement evaluation is required above. When the direct current stray current flows into the buried metal body, it flows out of the buried metal body again into the ground at a position higher than the potential of the electrolyte, and a high corrosion risk occurs at the outflow site.

  In a buried metal body that has a plastic coating with high insulation performance on its surface, such as a buried metal pipeline, a probe (also called a coupon) that simulates a coating defect is placed around the buried metal body. It is known that the corrosion risk of the buried metal body is evaluated by electrically connecting the buried metal body and measuring the current (probe current) flowing through the connecting wire (see Non-Patent Document 1 below). .

In addition, as a specific measurement and evaluation method for corrosion risk using the probe current, the instantaneous value I (t) of the probe current is measured every 0.1 ms, and one cycle of the commercial AC frequency (for example, 50 Hz or 60 Hz) is measured. As one sub unit time (20 ms for 50 Hz), one probe DC current density I _dc and one probe AC current density I _ac are obtained from the instantaneous value I (t) of the probe current measured in this sub unit time. The average value (I _dc ^ave , I _ac ^ave ) and maximum value (I _dc ^max , I _ac ^max ) of the probe DC current density I _dc and the probe AC current density I _ac are obtained every predetermined unit time (for example, 10 s). ), Minimum values (I _dc ^min , I _ac ^min ), average values (I _dc ^ave , I _ac ^ave ) and maximum values (I _dc ^max , I _ac ^max ) obtained within the measurement time (for example, 24 h) ), the minimum value (I _dc ^min, I _ac ^min), the probe DC current density I _dc and the probe alternating current density I _ac indicative Caso Be compared to a de corrosion criteria it has been proposed (see Patent Document 1).

JP 2006-145492 A

`` Protection against corrosion by stray current from direct current systems '' BRITISH STANDARD EN 50162: 2004 P25-26

  The DC stray current changes in a short time according to the situation that changes every moment, such as the operating state of the DC electric railway and the moisture state of the surrounding soil (electrolyte) due to rain, etc., and regularity (periodicity) to the change There is no feature. For this reason, in order to accurately grasp the corrosion risk due to DC stray current, whether the probe current I (t) measured every 0.1 ms is the anode current (current flowing out from the probe toward the electrolyte) or the cathode It is conceivable to always check whether the current (current flowing from the electrolyte into the probe) and to obtain the time integral of the instantaneous value I (t) of the probe current that has become the anode current.

  However, in order to grasp the influence of the DC stray current that changes in a short time, the measurement interval of the probe current needs to be shortened to at least about 0.1 ms. Since it is necessary to store or process the amount of data, the hardware burden becomes too great if measurement evaluation in the field is assumed.

On the other hand, when the corrosion risk is evaluated with the average value I _dc ^ave of the probe DC current density I _dc obtained for each unit measurement time as in the conventional technique described above, the peak value of the anode current is overlooked, Corrosion risk may be underestimated. In addition, when trying to determine the generation status of the anode current from the time change of the minimum value I _dc ^min of the probe DC current density I _dc obtained per unit time, one minimum value I _dc ^min becomes a negative value (anode current). In such a case, the negative value is evaluated as being continued within the unit time, and there is a problem that the corrosion risk is excessively evaluated.

  This invention makes it a subject to cope with such a situation. That is, the present invention is to provide a measurement and evaluation method for DC stray current corrosion risk that can accurately evaluate the DC stray current corrosion risk in the buried metal body without being excessive or too small.

  In order to solve such a problem, the present invention has the following configuration.

A method for measuring and evaluating the DC stray current corrosion risk of a buried metal body by measuring a probe current, wherein the probe DC current is measured at each sub unit time from the instantaneous values of the probe currents measured during the sub unit time. The density I _dc is obtained, and for each unit time in which a plurality of the sub unit times are connected, the minimum value I _dc ^min and the average value I _dc ^ave of the probe DC current density I _dc are obtained, and the minimum value I in the unit time is obtained. _{When both dc} ^min and the average value I _dc ^ave are negative values, the minimum value I _dc ^min × the sub unit time + the average value I _dc ^ave × (the unit time−the sub unit time) When the minimum value I _dc ^min is a negative value and the average value I _dc ^ave is a positive value in the unit time, the minimum value I _dc ^min × the sub unit time is set as the evaluation value for each unit time. The evaluation value per unit time is used. Measurement method for evaluating the DC stray current corrosion risk to.

  The present invention having such a feature makes it possible to accurately evaluate the risk of corrosion of a buried metal body due to a DC stray current that changes from moment to moment, without being too large or too small. Can increase the sex.

It is explanatory drawing which showed the system configuration | structure for measuring a probe electric current and a tube ground potential. It is explanatory drawing which showed the corrosion risk measuring method of direct current stray current.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, a buried metal pipeline will be described as an example of a buried metal body to be measured and evaluated. However, the measurement and evaluation object of the present invention is not particularly limited to a pipeline.

  As shown in FIG. 1, a buried metal pipeline (hereinafter referred to as a pipeline) 1 to be measured is covered with a coating 2 having a high insulating property, and is buried in soil S as an electrolyte. It is assumed that the pipeline 1 is cathodic-proofed, and a corrosion-proof current flowing out from a buried electrode such as an external power supply device (not shown) is supplied around the pipeline 1.

A probe (or coupon) 3 is installed in the soil S in the vicinity of the pipeline 1. The probe 3 simulates a defective portion of the coating 2 in the pipeline 1 and has a predetermined surface area (for example, 10 cm ² ) made of the same material as that of the pipeline 1. The probe 3 is electrically connected to the pipeline 1 and a reference electrode (for example, saturated copper sulfate electrode) 4. Between the probe 3 and the pipeline 1, a shunt 5 and a switch (usually for measuring the probe current) ON) 6 are connected in series. Further, a voltmeter 7 for measuring the probe potential is connected between the probe 3 and the verification electrode 4.

  The probe current is a current flowing through the shunt 5, and a current flowing in the direction from the probe 3 to the soil S (anode current) is negative, and a current flowing in the direction from the soil S to the probe 3 (cathode current) is positive. The probe current can quantitatively evaluate the corrosion risk caused by the anode current in the defective portion when the coating 2 of the pipeline 1 has the defective portion.

The probe potential is measured by measuring the potential between the probe 3 and the soil S as an electrolyte. When the coating 2 of the pipeline 1 has a defective portion, the potential of the electrolyte against the defective portion is less than the anticorrosion potential due to cathodic protection. Can be quantitatively evaluated. Regarding the probe potential, the potential when the switch 6 is in the on state is referred to as the probe on potential E _on, and the potential measured with the switch 6 turned off is referred to as the probe instant off potential E _off .

The probe instant off potential E _off is measured with the switch 6 turned off in a state where the pipeline 1 is cathodic protected. As shown in FIG. 1, when the verification electrode 4 is installed on the ground surface, the voltage between the probe 3 embedded in the ground and the verification electrode 4 includes an IR drop due to a current flowing through the electrolyte (soil S). The probe instant-off potential E _off is a true probe excluding IR drop by temporarily cutting off the electrical connection between the probe 3 and the pipeline 1 and measuring the voltage between the probe 3 and the reference electrode 4. The potential to the electrolyte can be obtained, which is an important evaluation value for grasping the cathode polarization state of the pipeline 1.

  The probe current and probe potential in the pipeline 1 are measured as shown in FIG.

  The instantaneous value I (t) of the probe current measured by the shunt 5 and the instantaneous value E (t) of the potential of the probe 3 measured by the voltmeter 7 are sampled every 0.1 milliseconds (ms). Such a sampling interval is effective in accurately grasping high-speed fluctuations in the DC stray current.

Instantaneous values I (t) and E (t) sampled every 0.1 ms are calculated by the following formulas (1) to (4) from a plurality of values for each set sub unit time (Sub1, Sub2, Sub3,...). By obtaining evaluation values (probe DC current density I _dc , probe AC current density I _ac , probe on potential E _on , probe instant off potential E _off ), the data capacity to be stored is reduced. In the following formula, A is the surface area of the probe 3, E _on (t) is an instantaneous value E (t) measured when the switch 6 is on, and E _off (t) is the switch 6 Is the instantaneous value E (t) measured in the off state.

Sub unit time (Sub1, Sub2, Sub3, ...) is one cycle of commercial AC frequency (for example, when commercial AC frequency is 50Hz, it is 20ms, and when commercial AC frequency is 60Hz, it is about 16.67ms. ) Is preferable. This is because the influence of the AC induction on the DC stray current is offset and the probe AC current density I _ac is obtained to separately evaluate the AC corrosion countermeasure. In the above formulas (1) to (4), the sub unit time is set to 20 ms and 200 instantaneous values (I (1) to I (200), E _ON (1) sampled every 0.1 ms. to E (200), seeking the value from _{E OFF (1) ~E (200} )).

  When one continuous measurement time (for example, a measurement time of 24 hours) is set, the on time measured with the switch 6 turned on and the off time measured with the switch 6 turned off are combined. Set the unit measurement time (Unit1, Unit2, ..., Unit43200). In the example shown in the figure, the unit measurement time is set to 2 s, of which 1.7 s is the on time and the remaining 0.3 s is the off time.

In the ON time, the average value I _dc ^ave , the maximum value I _dc ^max , the minimum value I _dc ^min , and the probe AC current density I _ac of the probe DC current density I _dc for each unit time in which a plurality of sub unit times are connected. I _ac ^ave , maximum value I _ac ^max , minimum value I _ac ^min , average value E _on ^ave , maximum value E _on ^max , and minimum value E _on ^min of probe-on potential E _on are obtained. In the illustrated example, a unit time of 1.6 s obtained by connecting 80 sub unit times is set, and an average value, a maximum value, and a minimum value are obtained from 80 I _dc , I _ac , and E _on , respectively. In the off time, one sub-unit time is set, and the instant-off potential E _off is obtained.

When evaluating the corrosion risk of the DC stray current, it is assumed that the potential of the pipeline 1 to be measured and evaluated is less than the anticorrosion potential. In order to confirm this premise, an off time is provided for each unit measurement time, and the probe instant off potential E _off is obtained there. Although the probe DC current density I _dc measured when the switch 6 is on and the probe instant-off potential E _off measured when the switch 6 is off cannot be measured simultaneously, the unit measurement time is as short as 2 s. By setting, for the situation that occurs at almost the same time in the situation that changes frequently, the evaluation of the potential to the electrolyte correctly without the IR drop due to the probe instant off potential E _off and the probe DC current density It becomes possible to evaluate the DC stray current corrosion risk by I _dc .

In each unit measurement time (Unit1, Unit2, ..., Unit43200), the instantaneous values I (t), E (t) (for each sub-unit time (Sub1, Sub2, Sub3, ...) for a unit time of 1.6s ( E _on (t) and E _off (t)) data are stored. The average value (I _dc ^ave , I _ac ^ave , E _on ^ave ), maximum value (I _dc ^max , I _ac ^max , E _on ^max ), minimum value (I _dc ^min , After _obtaining I _ac ^min and E _on ^min ), these values are stored, the instantaneous value E _on (t) of the sub unit time indicating the maximum value E _on ^max of the probe on potential, and the probe DC current The instantaneous value I (t) of the sub unit time indicating the minimum density value I _dc ^min is stored, and the other instantaneous value data is deleted.

  When the measurement time (24h) ends, the DC stray current corrosion risk of the pipeline 1 is evaluated as follows.

[1] When the minimum value I _dc ^min and the average value I _dc ^ave of the probe DC current density are both negative in the unit time (1.6 s) set to one unit measurement time (2 s), The DC stray current corrosion risk is evaluated using the minimum value I _dc ^min × sub unit time + average value I _dc ^ave × (unit time−sub unit time) as an evaluation value for each unit measurement time (unit time). That is, when the sub unit time is 20 ms, the DC stray current corrosion risk is evaluated on the assumption that the minimum value I _dc ^min continues for 20 ms and the average value I _dc ^ave continues for 1980 ms.

[2] When the minimum value I _dc ^min of the probe DC current density is negative and the average value I _dc ^ave is positive in the unit time (1.6 s) set for one unit measurement time (2 s) Using the minimum value I _dc ^min × sub unit time as an evaluation value for each unit measurement time (unit time), the DC stray current corrosion risk is evaluated. That is, when the sub unit time is 20 ms, the DC stray current corrosion risk is evaluated assuming that the minimum value I _dc ^min continues for 20 ms.

According to such an evaluation method, it is possible to accurately grasp the state of the anode current flowing from the probe 3 to the soil S as the electrolyte every short unit measurement time (for example, 2 s). At this time, since the anode current status is grasped by the positive / negative of the minimum value I _dc ^min and average value I _dc ^ave of the probe DC current density, the situation is precisely grasped without storing excessive instantaneous value data. be able to. Further, it is possible to avoid excessive risk evaluation as compared with the case where one unit measurement time is evaluated by the minimum value I _dc ^min of the probe current density.

In addition, the anticorrosion management of the pipeline 1 with cathodic protection is defined in ISO 15589-1: 2015 that the anode shift induced by the DC stray current is allowed if the cathodic protection standard range is maintained. It has been. The cathodic protection standard range is maintained by confirming that the probe instant-off potential E _off has passed the cathodic protection standard. In the measurement method described above, the probe instant-off potential E _off is frequently obtained every short unit measurement time (2 s), and the pipeline 1 is cathodic-proofed against frequent changes due to DC stray current. Whether or not it is within the reference range can be properly grasped.

1: buried metal pipeline (pipeline), 2: coating,
3: probe (coupon), 4: reference electrode, 5: shunt
6: Switch, 7: Voltmeter, S: Soil (electrolyte)

This is a method for measuring and evaluating the DC stray current corrosion risk of the buried metal body by measuring the probe current flowing between the buried metal body and the probe . Assuming that the current is positive , the probe DC current density I _dc is obtained for each sub unit time from the instantaneous value of the probe current measured a plurality of times during the sub unit time, and for each unit time in which a plurality of the sub unit times are connected, When the minimum value I _dc ^min and the average value I _dc ^ave of the probe DC current density I _dc are obtained and the minimum value I _dc ^min and the average value I _dc ^ave are both negative values in the unit time, The minimum value I _dc ^min × the sub unit time + the average value I _dc ^ave × (the unit time−the sub unit time) is an evaluation value for each unit time, and the minimum value I _dc ^min in the unit time. Is In the case where the average value I _dc ^ave is a positive value with a negative value, the minimum value I _dc ^min × the sub unit time is used as an evaluation value for each unit time. Evaluation method.

Claims (4)

A method for measuring and evaluating the DC stray current corrosion risk of buried metal bodies by measuring the probe current,
_{Obtain the} probe DC current density I _dc for each sub unit time from the instantaneous value of the probe current measured multiple times during the sub unit time,
For each unit time in which a plurality of the sub unit times are connected, the minimum value I _dc ^min and the average value I _dc ^ave of the probe DC current density I _dc are obtained,
When the minimum value I _dc ^min and the average value I _dc ^ave are both negative values in the unit time, the minimum value I _dc ^min × the sub unit time + the average value I _dc ^ave × (the above Unit time-the sub unit time) as an evaluation value for each unit time,
In the unit time, when the minimum value I _dc ^min is a negative value and the average value I _dc ^ave is a positive value, the minimum value I _dc ^min × the sub unit time is set as an evaluation value for each unit time. A method for measuring and evaluating DC stray current corrosion risk.   The method for measuring and evaluating DC stray current corrosion risk according to claim 1, wherein the sub-unit time is one period of a commercial AC frequency, and an instantaneous value of the probe current is measured every 0.1 milliseconds. . After adding the off time for _{obtaining the} probe instant off potential E _off after the unit time, to be one unit measurement time,
The method for measuring and evaluating a DC stray current corrosion risk according to claim 1 or 2, wherein a plurality of the unit measurement times are connected to form a measurement time.   5. The DC stray current corrosion risk measurement and evaluation method according to claim 4, wherein the unit time is 1.6 seconds, the unit measurement time is 2 seconds, and the measurement time is 24 hours.

Images