JP2012193414A - Cathodic protection method and cathodic protection system of buried metal pipeline - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce the alternating current corrosion risk of a plastic coated steel pipe P2 when a buried metal pipe line where the plastic coated steel pipe P2 and the cast iron pipe P1 are connected via an insulating joint 1, is influenced by alternating current induction; and to appropriately supply a generated cathodic protection current to both of the plastic coated steel pipe P2 and the cast iron pipe P1, thereby bringing both pipelines into a good cathodic protection state.SOLUTION: A cathodic protection system includes: a galvanic anode 2 connected to the plastic coated steel pipe P2; an alternating current induction attenuator 3 which is provided between the plastic coated steel pipe P2 and a galvanic anode 2, and connects the plastic coated steel pipe P2 and the galvanic anode 2 via at least a non-polar capacitive element 30; and a bond current regulator 4 including a bond rectifying element 40 which is connected between the plastic coated steel pipe P2 and the cast iron pipe P1 while interposing the insulating joint 1 therebetween and takes the flow from the cast iron pipe P1 to the plastic coated steel pipe P2 as a forward direction.

Description

  The present invention relates to a cathodic protection method and a cathodic protection system for a buried metal pipeline, and more specifically, in a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint, The present invention relates to a cathodic protection method and a cathodic protection system that are performed in a situation where a steel pipe is subjected to AC induction.

  Since cast iron pipes do not require welding at joints, piping work is easy, and they have been used as pipelines for water, gas, etc. in Japan, Europe and the United States for many years. Because this cast iron pipe is a low-strength material, the ground environment of the buried part has changed, for example, the buried part has become under the intersection of a highway with heavy wheel load or under a direct current electric railroad rail with increased rail transport capacity. In some cases, there is a problem in strength in the pipeline embedded therein. In response to such changes in the ground environment at the buried site, a new high-strength steel pipe having ductility characteristics is being replaced in place of the existing cast iron pipe. A steel pipe will be connected.

Thus, when connecting a new steel pipe to an existing cast iron pipe, the existing cast iron pipe is a so-called bare pipe without coating, and the steel pipe is coated with plastic. In this case, assuming that both pipelines are electrically connected, the existing cast iron pipe has an iron oxide product formed on its surface, so that the pipe-to-ground potential is shifted to the plus side (pipe). Since the ground potential is about -0.5V _CSE (saturated copper sulfate electrode CSE reference potential)), the tube ground potential (about -0.8V _CSE if the plastic coating is defective) When a defect occurs in the plastic coating of the steel pipe, the existing cast iron pipe becomes the cathode and the steel pipe becomes the anode, and a combination of the large cathode and the small anode causes the corrosion to progress in the coating defect of the steel pipe.

  In order to prevent such corrosion, it is required to insert an insulating joint between the cast iron pipe and the steel pipe, and to further provide cathodic protection to the steel pipe in order to ensure the corrosion prevention of the steel pipe. . Cathodic protection methods include an external power supply method and a galvanic anode method. Generally, when both ends of a steel pipe having a short distance are insulated joints, a galvanic anode method is adopted.

  On the other hand, when connecting an embedded pipeline with an insulation joint, make sure that surges (high voltage) due to lightning damage and power accidents do not occur on the insulation joint so that the insulation joint does not burn, cause sparks, or cause an electric shock. It is necessary to do so. In contrast, as shown in FIG. 1, silicon diodes D and D are connected in reverse parallel to pipelines J2 and J3 connected through an insulating joint J1 (the following non-patent document). 1).

The Electric Society of Electrical Corrosion Prevention Research Committee, “New Edition of Electric Corrosion / Soil Corrosion Handbook” The Institute of Electrical Engineers of Japan, May 1977, p. 263-264

  An embedded metal pipeline, in which a plastic-coated steel pipe and an uncoated cast iron pipe (hereinafter simply referred to as a cast iron pipe) are connected via an insulation joint, is parallel to a high-voltage AC power transmission line or an AC electric railway transport route. When subjected to induction, there is an AC corrosion risk for plastic coated steel pipes. When a plastic-coated steel pipe and cast iron pipe connected via an insulating joint are connected in reverse parallel with a silicon diode as shown in the prior art, an AC voltage exceeding the forward voltage Vf (operating voltage) of the silicon diode is applied to both pipelines. If an electric current flows and there is a defect in the coating of the plastic-coated steel pipe, the risk of AC corrosion increases as the area of the defective part decreases.

  In response to this, AC induction of the plastic-coated steel pipe is reduced by connecting a galvanic anode (Mg anode) as a ground electrode to the plastic-coated steel pipe. However, in the situation where various stray currents exist, stray current flows into the plastic-coated steel pipe from the connected galvanic anode and flows into the cast iron pipe side via the diode, and the cast iron pipe has a low ground resistance. This may cause a corrosion risk on the cast iron pipe side. To avoid this, it is conceivable to insert a backflow preventer into the wire connecting the plastic-coated steel tube and the galvanic anode. According to this, a one-way current from the plastic-coated steel tube to the galvanic anode is proposed. Therefore, the AC current that escapes to the ground is half-wave rectified and the function as a ground electrode is reduced by half, and there is a problem that the effect of reducing AC induction by connecting the galvanic anode cannot be obtained sufficiently.

  On the other hand, in order to eliminate the risk of corrosion on the cast iron pipe side, it is conceivable that the cathodic protection of the cast iron pipe side is considered separately, and an galvanic anode is also connected to the cast iron pipe. Most of the cathodic protection current that flows out of the galvanic anode connected to the coated steel pipe and the galvanic anode connected to the cast iron pipe will flow into the cast iron pipe with a low ground resistance, so that the plastic coated steel pipe is properly cathodic protected. Problems that can not be.

  This invention makes it an example of a subject to cope with such a problem. In other words, when the buried metal pipeline in which the plastic-coated steel pipe and the uncoated cast iron pipe are connected via an insulating joint is affected by AC induction, the AC corrosion risk of the plastic-coated steel pipe can be effectively reduced. That both the pipeline and the cast iron pipe are properly supplied with the generated cathodic protection current, so that both pipelines are in good cathodic protection, and part of the cathodic protection current is cast iron. When the steel pipe side is subjected to cathodic protection on the pipe side with the remaining part, the anticorrosion state of both pipes is individually grasped and the conditions of the cathodic protection system are properly adjusted so that the anticorrosion state of both pipes is appropriate. Setting, etc. is an object of the present invention.

  In order to achieve such an object, the cathodic protection method and cathodic protection system for buried metal pipelines according to the present invention include at least the following configurations.

  A cathodic protection method in a situation where a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint is affected by alternating current induction, wherein the plastic-coated steel pipe is coated with the plastic-coated steel pipe. A galvanic anode for supplying a cathodic protection current is connected to both the steel pipe and the cast iron pipe, the plastic-coated steel pipe and the galvanic anode are connected via at least a non-polar capacitive element, and the plastic-coated steel pipe and the cast iron A buried current pipeline including a rectifying element including a rectifying element having a forward direction from the cast iron pipe to the plastic-coated steel pipe with the insulating joint interposed therebetween is connected between the pipe and the pipe. Cathodic protection method.

  A cathodic protection system in a situation where a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint is affected by alternating current induction, and is connected to the plastic-coated steel pipe An AC induction reducer that is provided between a current-carrying anode, the plastic-coated steel pipe, and the current-carrying anode, and connects the plastic-coated steel pipe and the current-carrying anode through at least a nonpolar capacitive element; and the insulation A bond current regulator including a bond rectifying element that is connected between the plastic-coated steel pipe and the cast iron pipe with a joint interposed therebetween and has a forward direction from the cast iron pipe toward the plastic-coated steel pipe; Cathodic protection system for buried metal pipelines.

  The present invention has such a feature, so that when a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint is affected by alternating current induction, the plastic-coated steel pipe is used. The AC corrosion risk can be effectively reduced. Further, according to the present invention, by appropriately supplying the generated cathodic protection current to both the connected plastic-coated steel pipe and the cast iron pipe, both pipelines can be brought into a good cathodic protection state. In addition, the present invention allows a part of the cathodic protection current to flow to the cast iron pipe side, and when the steel pipe side is cathodic protected by the remaining part, the anticorrosion state of both pipes is grasped individually, and the anticorrosion state of both pipes Thus, the conditions of the cathodic protection system can be set appropriately so as to be appropriate.

It is explanatory drawing of a prior art. It is explanatory drawing explaining the cathodic protection method and cathodic protection system of a buried metal pipeline concerning one embodiment of the present invention. It is explanatory drawing which showed the specific circuit structure of the alternating current induction reducer and bond current regulator in embodiment of this invention (FIG. 2 (a) has shown the alternating current induction reducer, FIG.2 (b) is FIG. Bond current regulator is shown). It is explanatory drawing which showed the voltage-current characteristic of the bond current regulator in embodiment of this invention. A graph showing the cathodic protection criteria as an index coupons current density, and the horizontal axis the vertical axis coupon flowing direct current density I _DC indicates the coupon alternating current density I _AC. It is explanatory drawing explaining the measuring method of the instant-off electric potential in a cast iron pipe.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 is an explanatory diagram illustrating a cathodic protection method and cathodic protection system for a buried metal pipeline according to an embodiment of the present invention. The object of corrosion prevention here is a buried metal pipeline in which a steel pipe P2 is connected to a part of a cast iron pipe P1. The cast iron pipe P1 is a so-called bare pipe with no coating, and the steel pipe P2 is coated with a plastic coating such as a polyethylene coating. The cast iron pipe P1 and the steel pipe P2 are connected via the insulating joint 1. The buried metal pipeline is affected by AC induction, for example, in parallel with the high-voltage AC power transmission line _LAC and the AC electric railway transportation route.

  Under such circumstances, the cathodic protection system installed for the cast iron pipe P1 and the steel pipe P2 includes an galvanic anode (for example, Mg anode) 2 connected to the steel pipe P2, and a steel pipe P2 and the galvanic anode 2. An AC induction reducer 3 provided therebetween and a bond current regulator 4 connected between the cast iron pipe P1 and the steel pipe P2 with the insulating joint 1 interposed therebetween are provided. The AC induction reducer 3 is connected to the electroplating anode 2 by an electric wire w1 and is connected to the steel pipe P2 by an electric wire w2. The bond current regulator 4 is connected to the steel pipe P2 by the electric wire w3 and is connected to the cast iron pipe P1 by the electric wire w4.

Moreover, in order to grasp | ascertain the cathodic protection condition of the cast iron pipe P1, the reference electrode (for example, saturated copper sulfate electrode) 5 is connected to the cast iron pipe P1 via the electric wire w5, and the cast iron pipe P1 is connected to this electric wire w5. A voltmeter 6 for measuring the tube-to-ground potential is installed. On the other hand, in order to grasp the cathodic protection situation of the steel pipe P2, a coupon 7 installed in the vicinity of the steel pipe P2 is connected to the steel pipe P2 via the electric wire w6, and the coupon current density of the steel pipe P2 is connected to the electric wire w6. A DC / AC ammeter 8 for measuring (coupon inflow DC current density I _DC and coupon AC current density I _AC ) is installed.

Here, the galvanic anode 2 not only functions as a ground electrode for reducing the _AC voltage VAC applied to the steel pipe P2 by a high-voltage AC power transmission line _LAC or an AC electric railway transport path, but also uses the steel pipe P2 as a ground current. The flowing alternating current is released to the ground, and this is supplied to the cast iron pipe P1 and the steel pipe P2 as the cathodic protection current Ic.

  FIG. 3 is an explanatory diagram showing a specific circuit configuration of the AC induction reducer 3 and the bond current regulator 4 in the embodiment of the present invention (FIG. 3A shows the AC induction reducer 3). FIG. 3B shows the bond current regulator 4). The AC induction reducer 3 shown in FIG. 3A has a non-polar capacitive element (capacitor) 30 inserted between at least the steel pipe P2 and the galvanic anode 2. Since the steel pipe P2 and the galvanic anode 2 are connected via the nonpolar capacitive element 30, an AC ground current can be effectively released to the ground, and a DC stray current can be transferred from the galvanic anode 2 to the steel pipe P2. Can be prevented from flowing toward

  The AC induction reducer 3 includes a backflow preventing rectifying element 31 (diode) connected in parallel with the nonpolar capacitive element 30 and configured to forward flow from the steel pipe P2 toward the galvanic anode 2, A surge protection element 32 connected in parallel with the nonpolar capacitance element 30 and the backflow prevention rectification element 31 is included. The surge protection element 32 here includes a coil element 32a, an arrester 32b, a varistor 32c, and the like. If necessary, the connection line of the AC induction reducer 3 can be provided with a switch element 33 and a fuse for electrically disconnecting the connection between the steel pipe P2 and the galvanic anode 2.

  The bond current regulator 4 shown in FIG. 3 (b) is connected at least between the cast iron pipe P1 and the steel pipe P2 with the insulating joint 1 interposed therebetween, and the flow from the cast iron pipe P1 to the steel pipe P2 is defined as the forward direction. And a surge protection element 41 connected in parallel with the bond rectifier element 40. The surge protection element 41 here includes a coil element 41a, an arrester 41b, a varistor 41c, and the like. If necessary, the connection line of the bond current regulator 4 can be provided with a switch element 42 and a fuse for electrically disconnecting the connection between the cast iron pipe P1 and the steel pipe P2.

  FIG. 4 is an explanatory diagram showing voltage-current characteristics of the bond current regulator 4 in the embodiment of the present invention. The voltage-current characteristics of the bond current regulator 4 are basically specified by the characteristics of the bond rectifier element 40. The forward voltage (Vf) is applied to the voltage V applied in the forward direction (arrow direction). If it exceeds the voltage V, the current I flows, and with respect to the voltage V having a polarity opposite to the forward direction (arrow direction), the current I is not flowed up to a certain level of voltage. And the bond current regulator 4 is provided with the coil element 41a, so that the current rise is suppressed against the voltage rise at a voltage higher than the forward voltage (Vf) of the bond rectifier element 40. That is, if the characteristic indicated by the broken line in the figure is the voltage-current characteristic of the bond rectifying element 40, the characteristic is adjusted as indicated by the solid line by the function of the coil element 41a. According to this, even if the forward voltage between the cast iron pipe P1 and the steel pipe P2 increases, the magnitude of the current flowing in the forward direction can be limited to a certain level.

  The function of the cathodic protection system and the cathodic protection method in the above-described buried metal pipeline will be described below.

  First, the AC induction reducer 3 installed between the steel pipe P2 and the galvanic anode 2 firstly applies a ground current to the AC voltage applied to the steel pipe P2 to effectively produce a ground effect (AC The second effect is to prevent the direct current stray current from flowing into the steel pipe P2 from the galvanic anode 2. In addition, the galvanic anode 2 has a function of first reducing the AC induction of the steel pipe P2 as an earth electrode, and secondly, supplying the cathodic protection current Ic to the cast iron pipe P1 and the steel pipe P2 by the output ground current. It has the function to do.

  The bond current regulator 4 is configured so that the alternating stray current flows into the galvanic anode 2 and the direction of the steel pipe P2 to the cast iron pipe P1 when the ground potential of the steel pipe P2 becomes more positive than the ground potential of the cast iron pipe P1. By preventing the current flowing through the cast iron pipe P1, it has a function of avoiding the risk of corrosion of the cast iron pipe P1. Furthermore, due to the voltage-current characteristics of the bond current regulator 4 described above, the cathodic protection current Ic output from the galvanic anode 2 is prevented from flowing excessively into the cast iron pipe P1, thereby supplying the steel pipe P2 side. The cathodic protection current that is generated is kept from becoming extremely small.

According to such a cathodic protection system, a buried metal pipeline in which a steel pipe P2 is connected to a part of a cast iron pipe P1 via an insulating joint 1 is affected by AC induction by a high-voltage AC transmission line _LAC or the like. Below, the risk of AC corrosion of the steel pipe P2 can be eliminated, and the cast iron pipe P1 and the steel pipe P2 can be in a good cathodic protection state.

Here, the cathodic protection status of the steel pipe P2 is managed so as to pass the cathodic protection standard using the coupon current density (coupon inflow DC current density I _DC and coupon AC current density I _AC ) as an index. FIG. 5 is a diagram showing the cathodic protection standard using the coupon current density as an index, and the horizontal axis shows the coupon inflow DC current density I _DC and the vertical axis shows the coupon AC current density I _AC . The inside of the bold line frame of the diagram is the range of the appropriate cathodic protection situation, so that the coupon inflow DC current density I _DC and the coupon AC current density I _AC obtained from the measured values of the DC / AC ammeter 8 fall within this frame. To manage the cathodic protection situation.

  On the other hand, the cast iron pipe P1 applies the 100 mV cathode polarization standard, and manages the cathodic protection state so that the pipe ground potential of the cast iron pipe P1 shifts from the natural potential to at least 100 mV minus side. Here, the tube-to-ground potential is measured by the voltmeter 6, and it is preferable to use an instant-off potential measured by turning off the connection line of the bond current regulator 4.

  In the cathodic protection method according to the embodiment of the present invention, the instant-off potential of the cast iron pipe P1 is measured, and whether or not the difference between the natural potential of the cast iron pipe P1 and the measured instant-off potential is equal to or greater than a reference value (100 mV). The cathode anticorrosion state of the cast iron pipe P1 is grasped. The difference between the natural potential of the cast iron pipe P1 and the measured instant-off potential is the amount of cathode polarization of the cast iron pipe P1 due to the cathodic protection current output from the galvanic anode 2 flowing into the cast iron pipe P1. If the amount of cathode polarization is not less than the reference value (100 mV), it can be understood that the cathodic protection state of the cast iron pipe P1 is appropriate. The difference between the natural potential of the cast iron pipe P1 and the measured instant-off potential being 100 mV or more indicates that the corrosion rate of the cast iron pipe P1 is an order of magnitude smaller than the natural corrosion rate.

  FIG. 6 is an explanatory view illustrating a method for measuring the instant-off potential in the cast iron pipe P1. In the figure, the vertical axis shows the pipe ground potential of the cast iron pipe P1, and the horizontal axis shows the elapsed time, and shows the change with time of the pipe ground potential of the cast iron pipe P1. In the figure, the potential of A is the potential (natural potential) when the cast iron pipe P1 is not cathodic-protected, and the potential of B is the above-described cathodic anti-corrosion system and the bond current regulator 4 is connected at time T1. The potential immediately after the on state, the potential of C, is a potential in a state where the cathode polarization has sufficiently progressed after the bond current regulator 4 is turned on, and the potential of D is the potential state of C by the switch element 42 at time T2. The potentials immediately after the connection of the bond current regulator 4 is turned off are shown.

  The potential difference from the potential of A to the potential of B or the potential difference from the potential of C to the potential of D shown in the figure is caused by measuring the tube-to-ground potential of the buried pipe with the reference electrode 5 installed on the ground. The potential difference is the product of the cathodic protection current Ic and the soil resistance R. The instant-off potential can be defined as a potential (D potential) obtained by subtracting an IR drop from a tube-to-ground potential (C potential) in a state where the cathode polarization is sufficiently advanced. The potential difference from the potential B to the potential C in the figure is the cathode polarization amount of the cast iron pipe P1 due to the cathodic protection current Ic. In a state where the cathode polarization has advanced to C, the bond current regulator 4 is turned off, the instant-off potential indicated by the potential of D is measured, and the known potential of A (natural potential) and the measured potential of D ( The cathode polarization amount of the cast iron pipe P1 is obtained from the difference from the instant-off potential.

  The setting on the system for the cast iron pipe P1 and the steel pipe P2 to be in an appropriate cathodic protection state is performed by setting the ground current by the AC induction reducer 3 and setting the voltage-current characteristic in the bond current regulator 4. The voltage-current characteristic of the bond current regulator 4 is set so that the instant-off potential of the cast iron pipe P1 is sufficient to shift from the natural potential to at least 100 mV minus side, and the current against the voltage rise to satisfy this condition. Rise is suppressed. As a result, the cathodic protection current Ic output from the galvanic anode 2 can be prevented from excessively flowing into the cast iron pipe P1, and the steel pipe P2 side can be appropriately cathodic protected.

  As described above, according to the cathodic protection method and cathodic protection system for the buried metal pipeline according to the embodiment of the present invention, the steel pipe P2 coated with plastic and the cast iron pipe P1 without coating are connected via the insulating joint 1. When the connected buried metal pipeline is affected by AC induction, the AC corrosion risk of the steel pipe P2 can be effectively reduced. Further, by properly supplying the generated cathodic protection current Ic to both the cast iron pipe P1 and the steel pipe P2, both pipelines can be brought into a good cathodic protection state. In addition, when a part of the cathodic protection current Ic flows to the cast iron pipe P1 side and the remaining part of the steel pipe P2 side is cathodic protected, the anticorrosion state of both pipes is individually grasped, and the anticorrosion state of both pipes is determined. Cathodic protection system conditions can be set to be appropriate.

P1: Cast iron pipe, P2 steel pipe,
1: insulation joint, 2: galvanic anode (Mg anode),
3: AC induction reducer, 30: Nonpolar capacitance element (capacitor),
31: Backflow prevention rectifier, 32: Surge protective element,
32a: coil element, 32b: arrester, 32c: varistor,
33: Switch element,
4: Bond current regulator, 40: Bond rectifier, 41: Surge protective element,
41a: coil element, 41b: arrester, 41c: varistor,
42: switch element,
5: Reference electrode (saturated copper sulfate electrode), 6: Voltmeter,
7: Coupon, 8: DC / AC ammeter,
w1-w6: Electric wire, Ic: Cathodic protection current, L _AC : High-voltage AC transmission line,
V _AC : AC voltage

Claims (11)

A cathodic protection method under a situation where a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint is affected by AC induction,
A galvanic anode that supplies a cathodic protection current to both the plastic-coated steel pipe and the cast iron pipe is connected to the plastic-coated steel pipe, and the plastic-coated steel pipe and the galvanic anode are connected via at least a nonpolar capacitive element. ,
A bond current regulator including a rectifying element having a forward flow from the cast iron pipe to the plastic-coated steel pipe is connected between the plastic-coated steel pipe and the cast iron pipe with the insulating joint interposed therebetween. Cathodic protection method for buried metal pipelines.   2. The buried metal according to claim 1, wherein a voltage-current characteristic of the bond current regulator is such that an increase in current is suppressed with respect to an increase in voltage at a voltage higher than a forward voltage of the rectifying element. Pipeline cathodic protection method. Cathodic protection of the plastic-coated steel pipe and the cast iron pipe as a grounding current flowing out from the galvanic anode by AC induction as a cathodic protection current,
While the coupon inflow DC current density and the coupon AC current density of the plastic-coated steel pipe have passed the cathodic protection control standard with the coupon current density as an index,
3. The cathodic protection method for buried metal pipelines according to claim 1 or 2, wherein a pipe ground potential of the cast iron pipe is shifted from a natural potential to at least 100 mV minus side.   4. The method of cathodic protection of buried metal pipeline according to claim 3, wherein the pipe ground potential of the cast iron pipe is an instant-off potential measured by turning off the bond current regulator. A cathodic protection system in a situation where a buried metal pipeline in which a plastic-coated steel pipe and an uncoated cast iron pipe are connected via an insulating joint is affected by AC induction,
A galvanic anode connected to the plastic-coated steel pipe;
An AC induction reducer provided between the plastic-coated steel pipe and the galvanic anode, and connecting the plastic-coated steel pipe and the galvanic anode through at least a non-polar capacitive element;
A bond current regulator including a bond rectifying element that is connected between the plastic-coated steel pipe and the cast iron pipe with the insulating joint interposed therebetween, and that forward flows from the cast iron pipe to the plastic-coated steel pipe. Cathodic protection system for buried metal pipelines.   The said alternating current induction reducer is connected in parallel with the said nonpolar capacitive element, The backflow prevention rectifier element which makes the flow toward the said electroactive anode from the said plastic covering steel pipe a forward direction is included. Cathodic protection system for buried metal pipelines described in 1.   7. The cathodic protection system for an embedded metal pipeline according to claim 6, wherein the AC induction reducer includes a surge protection element connected in parallel with the nonpolar capacitance element and the backflow prevention rectifier element.   8. The cathodic protection system for an embedded metal pipeline according to claim 7, wherein the bond current regulator includes a coil element connected in parallel with the bond rectifier element.   9. The embedded structure according to claim 8, wherein the voltage-current characteristic of the bond current regulator is such that an increase in current is suppressed with respect to an increase in voltage at a voltage higher than a forward voltage of the bond rectifier element. Cathodic protection system for metal pipelines.   The voltage-current characteristic of the bond current regulator is set such that the tube-to-ground potential of the cast iron pipe is sufficient to shift from a natural potential to at least 100 mV minus side. Cathodic protection system for buried metal pipelines.   11. The cathodic protection system for a buried metal pipeline according to claim 10, wherein the pipe ground potential of the cast iron pipe is an instant-off potential measured by turning off the bond current regulator.

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