Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547, doi: 10.20964/2021.05.

59

International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org

Study of Impressed Current Cathodic Protection (ICCP) on the

Steel Pipeline under DC Stray Current Interference

Yong Guo1,2, Jifeng Ding1,2, Xiangyang Li3,* and Jiarun Li4,*

1
Central Iron and Steel Research Institute, Beijing 100081, China;
2
Qingdao NCS Testing and Protection Technology Co., Ltd., Qingdao 266071, China;
3
Beijing Advanced Innovation Center for Materials Genome Engineering, China Iron & Steel
Research Institute Group, Beijing 10081, China;
4
School of Environment and Safety Engineering, Qingdao University of Science and Technology,
Qingdao 266042, China;
*
E-mail: lixy@cisri.com.cn and lijiarun@qust.edu.cn

Received: 6 January 2021 / Accepted: 16 March 2021 / Published: 31 March 2021

In this work, a detection method of stray direct current on buried pipelines and a determination method
for anodic/cathodic regions on pipelines is proposed. To compensate for the current interference, a
current requirement test is adopted to provide a reference for an additional impressed current cathodic
protection system. When the buried pipeline is severely disturbed, the implemented impressed current
cathodic protection system, which is based on the results of the requirement test, negatively shifts the
instant-off potential of the pipeline by at least 350 mV during the measurement period. This shift meets
the cathodic protection criteria, suggesting that the impressed current cathodic protection system can
effectively suppress stray subway current-induced corrosion.

Keywords: Impressed current cathodic protection; Current requirement test; DC stray current;
Drainage; Instant-off potential

1. INTRODUCTION

In the southeastern coastal area of China, the well−developed high−tension electricity network
and electrified mass transit strikingly flourish in the local economy; however, the rail transit traction
systems and high-voltage transmission lines generate stray DC that inevitably causes the corrosion of
adjacent metallic pipelines [1]. These underground pipelines include gas pipelines, oil pipelines, water
pipelines, and heating pipelines, all of which form a complex network downtown. Some pipelines lay
parallelly for a dozen miles or cross metro tracks; thus, some of the DC current of the metro circuit may
flow through the pipelines and result in severe metallic pipeline corrosion accidents [2]. The metro mass
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 2

transit is powered by DC current, and the track serves as one part of the circuit when running; therefore,
stray DC will be generated due to the electric connection between the track and ground [3-6].
The corrosion issue caused by stray current was noticed within ten years of the first DC-powered
rail line in Virginia of the United States in 1888. Since then, the control of stray current has been a
critical issue [7]. In China, the water pipelines in the tunnel of the first-stage project of the Beijing metro
leaked in the 1970s due to corrosion perforation, which was attributed to stray DC [8]. In another case,
the DN300 gas pipelines under the Century Avenue of Shanghai leaked 10 times before 2008, which
was also ascribed to the stray DC generated by its accompanied metro No. 2 line [9]. Stray DC can
promote serious corrosion of its adjacent metallic structures because the location where DC current flows
out always acts as an anode area associated with a considerable corrosion rate. Therefore, the prevention
of corrosion induced by stray DC remains incomplete and needs to be solved for many underground
systems.

Figure 1. Implementation process of the analysis of an impressed current cathodic protection (ICCP)
system.

Only passive defense techniques are currently available for alleviating the corrosion of metallic
pipelines adjacent to metro mass transit, including grounding drainage, direct drainage, polarity
drainage, and impressed drainage, which have been summarized in the file of BS EN 50162:2004 [10].
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 3

Nonetheless, these methods have some defects in practice. For instance, direct drainage can promote
potential danger for metro operation because it demands an electric connection between the track and
the disturbed pipeline to guide the stray DC returning to the electric loop of the metro. Ground and
polarity drainages are widely adopted by using sacrificial or belt anodes, but the effective protection
distances of both methods are generally less than 150 m [11]. In the case of a high-intensity stray current,
the drainage effect is quite limited and may be less than 50 m. In view of this, ICCP is an effective way
to overcome the limitation of the abovementioned techniques. It has been reported that the effective
distance can reach 14 km via the impressed current method [12].
In Dongguan city of Guangdong Province in China, one of the gas pipelines is located across the
metro No. 2 line. The stray DC derived from the power supply system of the metro may affect the
pipe−to−ground potential, promoting stray current corrosion of the pipeline. In this work, the
interference of stray DC on a gas pipeline was evaluated, and an ICCP system was proposed in view of
the results of the current requirement test and installed underground. Systematic and specific
considerations in the design and protection results are reported, aiming to provide a practical reference
for corrosion protection engineers to resolve stray DC issues. The implementation process is shown in
Figure 1.

2. EXPERIMENTAL

2.1. Measurement of stray current interference

The cathodic protection (CP) effect evaluation aims to test the potential (versus a reference
electrode) of a preburied steel coupon electrically connected to cathodic protected pipelines. The steel
coupon has an identical grade to that of the pipelines. Moreover, a reference electrode is guided to the
vicinity of the steel coupon via a plastic pipe throughout the ground and underground [13-15].
Nonetheless, when a CP system operates on a specific pipe, a high voltage drop (IR drop) accounts for
a considerable portion of the measured structure-to-ground potential. In this case, a polarization probe
(PP) is adopted to evaluate the CP effect by collecting the instant-off potentials of the pipe in this work
[16]. PP is a combination of a reference electrode and steel coupon (see Figure 2). The reference
electrode and steel coupon are both mounted in PP, in which two identical steel coupons are used to
study the electric properties of the coating defects on pipelines as well as the self-corrosion state. The
shortened distance between the reference electrode and steel pipe compared to the traditional CP effect
evaluation can mitigate the IR drop in the signal circuit [17]. The specific structure of the PP and the
connection diagram of the potential collector are presented in Figure 2.
In Figure 2, the stray current testing system in this work is composed of a PP (NCS PP2000,
Qingdao NCS Testing Protection Technology Co., Ltd. China), a potential collector (UDL2, Mobiltex
Data Co., Ltd. Canada), and the pipeline with stray current interference, respectively. The UDL2
potential collector can automatically record the on and instant-off potentials of structure-to-ground by
connecting its blue terminal to the pipeline, red terminal to the Cu/CuSO4 reference electrode, and black
terminal to the steel coupon with an exposed surface of 6.5 cm2. The grade of the steel coupon in this
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 4

work is Q345, which is in harmony with that of the pipe. The specific testing procedures are listed as
below:
(i) Burying the PP in the vicinity of the test station, backfilling the soil, and watering and tamping
the soil.
(ii) Connecting the UDL2 with the pipe, the reference electrode and the steel coupon in PP. The
UDL2 was placed in the test post on ground.
(iii) Collecting the potentials of the pipe in the presence of a pre-existing CP system using a ten-
second ON and one-second OFF cycle.

Figure 2. Schematic of the stray current testing system.

2.2. Cathodic protection criteria with stray current interference

In BS EN12954:2019 [18], under soil and water conditions, the protection potential should be
more negative than −0.85 V (IR free versus the saturated Cu/CuSO4 reference electrode, CSE). Under
aerated conditions, T < 40 C and 100 Ω·m < ρ < 1000 Ω·m in solid water, the protection potential should
be more negative than −0.75 V (IR free, versus CSE).
In AS2832.1-2015[19]: The protection criteria for structures subject to traction current effects
varies with the structure polarization time. Structures with sound coating characteristics, or those that
have otherwise been proven to be polarized and depolarized rapidly in response to stray current, shall
comply with the following criteria:
i) The potential should not be 5% more positive than the protection criterion of the test period.
ii) The potential should not be more positive than the protection criterion plus 50 mV, i.e., −800
mV for ferrous structures with more than 2% of the test period.
iii) The potential should not be more positive than the protection criterion plus 100 mV, i.e., −750
mV for ferrous structures with more than 1% of the test period.
iv) The potential should not be more positive than the protection criterion plus 850 mV, i.e., 0
mV for ferrous structures with more than 0.2% of the test period.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 5

2.3. Current requirement test

The current requirement test is generally adopted for pipelines with indescribable stray current.
The complexity of the buried pipe system associated with the absence of previous construction data of
the pipeline makes the current requirement test necessary because an appropriate ICCP design cannot be
achieved in this regard. The current requirement test refers to using a temporary ICCP system at the
construction site to provide cathodic current to the present pipeline and targeting to obtain the magnitude
of impressed current, which can cathodically polarize the pipeline to meet the potential principle of the
cathodic protection standard. The tentative results can supply a reference for further CP design.
The temporary ICCP system associated with the potential testing system constitutes the current
requirement test. The temporary ICCP system comprises an anode bed (DN40  1000 mm, 30 pcs), a
cathodic cable, a current rectifier (NCSRC01, Qingdao NCS Testing Protection Technology Co., Ltd.
China), a slide rheostat (SF041, Shanghai Hanbiao Electronic Technology Co., Ltd. China), and some
jumper wires connecting different pipelines to be protected. The potential testing system includes a test
station, a PP, a digital multimeter (FLUKE289C, Fluke Corporation, USA), and a UDL2 potential
collector. The connection diagram of the current requirement test is depicted in Figure 3, and the
procedure of the current requirement test is listed as follows.
(i) Thirty pcs of steel tubes (DN40  1000 mm) were knocked into the wet zone approximately
50 m away from the pipeline that was affected by stray current. These steel tubes, serving as anode beds,
were electrically connected to each other in parallel. The anode bed was electrically connected to the
slide rheostat and further connected to the positive terminal in the current rectifier. The ground resistance
of the anode bed in this work was approximately 2.2 Ω.
(ii) The negative terminal of the current rectifier was connected to the pipelines via the amphenol
connector in the test post, powering the rectifier and then cathodically polarizing the pipelines.
(iii) The on and instant-off potentials of the structure were collected by the UDL2 instrument
with a three-second ON and one-second OFF cycle after 30 min of polarization.
(iv) The output current of the rectifier was adjusted as soon as the instant-off potential of the
pipeline revealed by the UDL2 instrument was beyond the required potential scope.
(v) The adjusted current was kept running for more than 24 h to ensure the desirable instant-off
potential of the pipe.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 6

Figure 3. Connection diagram of the current requirement test.

The current magnitude of the current requirement test initially adopted 5 A. The output current
was increased to 8 A, 12 A and 15 A step by step if the potentials varied insignificantly. The current was
maintained at 15 A for 4 h because the potentials of the pipe to ground met the criteria in Section 2.2.

2.4. On and instant-off potentials

For buried steel pipelines disturbed by stray subway current, the stray current flows into the
pipelines thorough the defects of anticorrosive coatings, where the pipeline potential deviates negatively
and the pipeline is cathodically protected; thus, these defects are regarded as cathodic regions. The stray
current in the pipeline subsequently flows out from the defects of the pipeline close to the subway, where
pipeline is severely corroded, and these defects are regarded as anodic regions [20,21]. The anodic
regions are the most dangerous part in the pipeline because they are the very sites where corrosion
initiates on the pipeline. Nonetheless, the anodic regions are alterable with the states of subway operation
[22-23].
When the pipeline is cathodically protected regardless of the stray current, the relationship
between the on and instant-off potentials can be expressed as:
Uon = Uoff + I0 R (1)
where Uon is the on potential, Uoff is the instant-off potential, I0 is the output current of the
rectifier, and R is the loop resistance; thus, I0R signifies the IR voltage drop of the measurement loop.
In the presence of stray current interference,
Uon-stray = Uoff-stray + (I0 + Istray)R (2)
where Uon-stray and Uoff-stray are the on potential and instant-off potentials of the pipeline in the
presence of stray current interference, respectively. IstrayR is the IR voltage drop derived from the stray
current interference.
The instant-off potential of the cathodically polarized pipeline is related to the IR voltage drop
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 7

of the measurement loop. The instant-off potential is formulated as,

Uoff-stray = (I0 + Istray)Rp (3)
where Rp is the polarization resistance, which can be deemed a constant in a short time.
Consequently, the variation in the instant-off potential (Uoff-stray) is determined by the flow direction of
Istray. In the case of no interference, the instant-off potential of the pipeline equals Uoff. As soon as the
metro operates, the resultant stray current at the anodic region flows in the opposite direction compared
with that at the cathodic region, resulting in the instant-off potential of the anodic region shifting
positively. In addition, the magnitude of the potential deviation relates to the interference degree of the
stray current.

3. RESULTS AND DISCUSSION

3.1. Analysis of stray current interference on pipelines

A sketch map of the gas pipeline associated with subway line No. 2 in Dongguan city of
Guangdong Province is presented in Figure 4. The DN300 gas pipeline with a wall thickness of 11 mm
is 15.13 km in length. The pipeline is protected against soil corrosion by a 3PE coating associated with
sacrificial magnesium anodes. Eleven test posts were distributed along the pipeline.

Figure 4. Comparative distribution of gas pipeline and subway lines No. 2, CS-1 to CS-11 signify the
numbers of the test posts along the pipeline.

Figure 5a presents the minimum and maximum potentials obtained at each test post of the
pipeline in the absence and presence of stray DC interference within 24 h. It is obvious that the stray DC
strikingly affects the on and instant-off potentials of the pipeline. In Figure 5a, the fluctuation scope of
the on potentials of the pipeline in the absence of DC interference (0:00-6:00 AM) is much smaller than
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 8

that in the presence of DC interference (6:00 AM-12:00 PM), e.g., the potentials of pipeline revealed by
CS-7 signify the most serious interference caused by stray DC, of which the minimum and maximum on
potentials are −9.239 and 5.084 V, respectively; in contrast, the minimum and maximum instant-off
potentials are −1.101 and −0.088 V, respectively. The highest potential fluctuation of CS-7 can be
attributed to its location, which is the closest to the metro line among these test posts. The potential
fluctuation decreases with an increasing distance of the test posts away from the subway line, indicating
that the stray DC undoubtedly influences the potentials of the pipeline. It has been reported that buried
gas pipelines located in the neighboring area of rail transit in Shanghai have pipe-to-soil potentials that
fluctuate dramatically. The positive shift in the average potential is approximately 40 mV, and the
instantaneous maximum positive shift of the potential reaches 200 mV [24], the results were consistent
with that in this work. Nonetheless, the potential fluctuation of CS-4 in the absence and presence of stray
DC are the lowest among these test posts, which can be ascribed to the presence of an adjacent value
station, by which the stray DC can be drained by its grounding system.
In Figure 5b, the fluctuation scope of the instant-off potentials is much smaller than that of the
on potentials in Figure 5a for each test station. The decayed fluctuation range can be due to the IR drop
in the current loop that has been eliminated by the method mentioned in Section 2.1 using the UDL2 and
PP.

Figure 5. Extremes of the on (a) and instant-off (b) potentials of the pipeline monitored within 24 h of
the absence and presence of stray DC interference. CS-1 to CS-11 signify the numbers of the test
posts along the pipeline.

3.2. Effect evaluation of the subsistent CP

The earth resistivities obtained at each test post are listed in Table 1. The earth resistivities were
acquired using the four-probe method, and the distance between the adjacent probes was two meters.
The difference in earth resistivities can be ascribed to the variety of soil characteristics because the gas
pipeline crosses through farmland, hills, and green belts, as depicted in Figure 4.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 9

Table 1. Earth resistivities along with the gas pipeline route

Test post CS−1 CS−2 CS−3 CS−4 CS−5 CS−6 CS−7 CS−8 CS−9 CS−10 CS−11

Earth resistivity
26.4 14.4 31.4 85.4 40.2 301.4 389.4 101.7 116.2 74.1 138.2
(Ω m)
Note: CS-1 to CS-11 signify the numbers of the test posts along the pipeline.

According to the criteria of BS EN 12954 [18], the cathodic protection potential of pipelines
subjected to the stray current effect should not be more positive than −0.85 V (vs. CSE) in the soil with
an earth resistivity less than 100 Ω·m, whereas the value is −0.75 V (vs. CSE) in the case of an earth
resistivity greater than 100 Ω·m. Two dashed lines signifying −0.85 V and −0.75 V are portrayed in
Figure 5b, in which the instant-off potentials revealed by each test post indicate that the pipeline seems
to be effectively protected cathodically in the absence of stray DC (0:00-6:00 AM). However, Figure 5b
cannot reveal the actual state of the cathodic polarization of the pipeline because it merely illustrates the
extrema of the instant-off potentials. It is worth noting that the fluctuations of the on and instant-off
potentials in the presence of stray DC (6:00 AM-12:00 PM) validate that most of the pipelines perform
as polarity alternating regions due to interference. The alternating area signifies the area where stray DC
flows into or alternately flows out; for instance, most of the areas of the pipeline suffer corrosion when
the stray current flows out from them. Consequently, the CP system of the pipeline is evidently affected
by the DC stray current, and the pipeline is corroded for long running times.
Figure 6 presents the ratios of the instant-off potentials that are more positive than the CP criteria
concerning the abovementioned AS2832.1−2015 obtained in the presence of DC interference (6:00 AM-
12:00 PM).

Figure 6. Ratios of the instant-off potentials that fail to meet the cathodic protection criteria obtained at
each test post. CS-1 to CS-11 signify the numbers of the test posts along the pipeline.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 10

In Figure 6, the ratios of the instant-off potentials failing to meet the criteria of the CS-6 and CS-
7 test posts reach 56.46% and 60.47%, respectively. The subway intersects the pipeline at the CS-7 test
station, while the CS-6 test station is located closest to the intersection, both of which validate that most
of the time, some portions of the pipeline located in these areas lack cathodic protection compared with
others and suffer the most serious corrosion. Within the monitoring period, most of the instant-off
potentials obtained from these test posts cannot meet the cathodic protection criteria, indicating that the
previous cathodic protection system cannot remedy the effect of stray current. In this case, it can be
inferred that the pipeline corrodes for a long time. In the laboratory, under a DC current density of 10
A/m2, the DC on-potentials of X52 pipe steel with a self-corrosive potential of -0.800 V (vs. CSE) at the
anode and cathode can reach approximately −0.400 V and −1.500 V (vs. CSE), respectively;
furthermore, in the soil solution and anodic areas, the DC current results in accelerated corrosion of the
steel, which presents an essential threat to the integrity of the pipeline [25].

3.3. Determination of the anodic and cathodic regions

When the metro is running, the average value of the instant-off potential (Uoff-avg) of the pipeline
can be determined as:
 U n
i 1
U 
off  avg
off

n (4)
where n is the total number of Uoff within a certain monitoring period.
The Uoff-avg derived from each test post is adopted to evaluate the effectiveness of the subsistent
CP system, namely, when Uoff-stray is more positive than the potential criteria introduced in Section 2.2,
the corresponding portion of the pipeline lacks cathodic protection.
The Uoff-avg values of these test posts along the pipeline are listed in Table 2, all of which meet
the cathodic protection principle mentioned in Section 2.2 concerning the earth resistivities in Table 1.
This result shows that the previous CP system can effectively protect the pipeline from corrosion in the
absence of interference. Nonetheless, Uoff-avg cannot be adopted to determine the effectiveness of the CP
system in the presence of stray current interference because the stray current will flow in and out in the
very region of the pipeline with the variation in the operating state of the metro. In view of this, we
introduce a new parameter Uoff-stray-avg, which is the average value of Uoff-stray that is more positive than
Uoff-avg for the potentials derived from a specific test post (Eq. 5). When Uoff-stray is more negative than
Uoff-avg, it is believed that the current would flow into the specific region of the pipeline, leading to an
enhanced CP. Nonetheless, as soon as Uoff-stray > Uoff-avg in the presence of stray current interference, the
corresponding region of the pipeline must be in the state of current discharge; thus, Uoff-stray-avg signifies
the average potential state of a specific region of the pipeline. Consequently, the Uoff-stray-avg can signify
the degree of anodic polarization aroused by stray current to some extent. Combined with the potential
criteria introduced in Section 2.2, we can propose a reasonable method to determine the anodic/cathodic
regions on pipelines in the presence of DC interference, viz. comparing the value of Uoff-stray-avg with the
potential criteria introduced in Section 2.2, namely, when Uoff-stray-avg > −0.85 V (ρ < 100 Ω·m, CSE), the
anodic region is determined, and vice versa.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 11

 n i 1U off  stray

U off  stray  avg  , (U off  stray  U off  avg )
n (5)

Table 2. Evaluation of the anodic and cathodic areas of pipelines disturbed by stray subway current.

Potential criteria (vs. CSE)

Test post
Uoff−avg Uoff−stray−avg concerning the earth Determination
number
resistivities in Table 1

CS−1 −1.091 V −0.964 V −0.850 V Cathodic region

CS−2 −1.068 V −1.033 V −0.850 V Cathodic region
CS−3 −1.031 V −0.893 V −0.850 V Cathodic region
CS−4 −1.182 V −1.087 V −0.850 V Cathodic region
CS−5 −1.135 V −0.904 V −0.850 V Cathodic region
CS−6 −0.794 V −0.638 V −0.750 V Anodic region
CS−7 −0.784 V −0.611 V −0.750 V Anodic region
CS−8 −0.976 V −0.777 V −0.750 V Cathodic region
CS−9 −1.155 V −0.896 V −0.750 V Cathodic region
CS−10 −1.152 V −0.911 V −0.850 V Cathodic region
CS−11 −0.809 V −0.627 V −0.750 V Anodic region

Based on the abovementioned discussion, the calculated and evaluated results are shown in Table
2, from which CS-6, CS-7 and CS-11 are determined to be the anodic regions in the presence of stray
current interference. The CS-6 and CS-7 test stations are located closer to the intersection of the pipeline
and the metro line than others, and the comparatively low loop resistances there result in the current
flowing into the coating defects elsewhere on the pipeline being more apt to flow out at the CS-6 and
CS-7 sites, namely, the current discharges there and preferentially corrodes the pipeline.
It is worth noting that CS-11, located at the end of the pipeline, which is comparatively far from
the intersection of the pipeline and metro line, is determined to be the anodic region. This result is
because CS-11 is located near the river, as depicted in Figure 4, resulting in the comparatively low earth
resistance in the deep earth layer (Table 1). Combining the high potential fluctuation of CS-11 in Figure
5a, it can be inferred that the anti-corrosion coating of the pipeline within this region has a large number
of defects. In addition, an insulation joint is installed at the end of the pipeline of the CS-11 region, both
of which promote the stray current being more apt to flow out from the coating defects and leading to
the anodic characteristic of this region. Referring to the polarity of CS-1, which is also located far from
the intersection but exhibits cathodic characteristics due to the superior coating performance on the
pipeline of the CS-1 region (Figure 5a), the comparatively low difference between the maximum (2.468
V vs. CSE) and minimum potentials (−3.871 V vs. CSE) with interference indicates the desirable coating
integrity of the CS-1 region (see Figure 4 and Table S1).
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 12

3.4. Current requirement determination

Impressed current cathodic protection commonly used in long-distance pipeline systems can
effectively mitigate stray current effects and provide cathodic protection [24]. However, as discussed
above, because of cathodic protection system failure, the subsistent CP system cannot effectively protect
the whole pipeline against corrosion due to the presence of anodic regions, e.g., CS-6, CS-7 and CS-11.
The criteria for selecting the position for the current requirement test should meet the following:
(i) be close to the anodic region of the pipeline;
(ii) be near the pipeline region with the most serious interference, such as the intersections of the
subway and the pipeline; and
(iii) be nearby the valve chest of the pipeline, which facilitates the current requirement test and
subsequent installation of the ICCP system in the valve chest.
In this case, the region of the test post (CS-11) is the most suitable site for the current requirement test
because it meets the above three principles.
Figure 7a shows the extrema and average instant-off potentials of the pipeline with a 15 A
temporary cathodic current. Compared with Figure 5b, the maximums of the instant-off potentials
deviate negatively in Figure 7a, suggesting that the impressed current weakens the influence of positive
potential deviation caused by the stray current. In Figure 7b, the quantities of the instant-off potentials
failing to meet the criteria are thoroughly within the limitations that the CP criteria suggests.

Figure 7. Extremes and averages of the instant-off potentials of the pipeline under 15 A cathodic current
with DC interference (a). (b) Ratios of the instant-off potentials that fail to meet the cathodic
protection criteria obtained at each test post with 15 A cathodic current. CS-1 to CS-11 signify
the numbers of the test posts along the pipeline.

3.5. Performance of the ICCP and effect analysis

3.5.1. Calculation the ICCP system

Based on the results of the current requirement test, an additional ICCP system was installed near
the valve chest near the No. 11 test post. The ICCP system was composed of a deep anode bed, a rectifier,
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 13

a junction box, confluence, and connecting cables. The anode body was 40 m long and was installed in
an auxiliary anode bed with a depth of 60 m. There were 10 mixed metallic oxide anodes connected in
series in the anode body, whose leaving space was filled with coke. A sketch map of the anode body
associated with the connection diagram is shown in Figure 8.

Figure 8. Schematic of the anode body [26].

The computational formula of the ground resistance of a deep auxiliary anode bed can be
expressed as
 2L
R ln( )
2L D (6)
where R is the ground resistance of the anode body (Ω), ρ is the soil resistivity (75.6 Ω·m, which is the
average earth resistivity adjacent to the anode body), L is the length of the anode body (40 m), and D is
the diameter of the auxiliary anode length (0.25 m). In this case, the ground resistance of the anode body
can be determined (R≈1.74 Ω). Concerning the degradation of the coating on the pipeline with an
increasing operating time, the current (15 A) derived from the current requirement test will not meet the
increasing current demand. Consequently, the rated current of the rectifier is estimated to be 40 A, and
the corresponding rated voltage is 60 V.

3.5.2. Effect analysis on the impressed current cathodic protection

As soon as the deep anode bed was installed, the performance of the ICCP system was evaluated
using the UDL2 instrument, which was connected in the form shown in Figure 2. Three different
magnitudes of cathodic current (11 A, 13 A, and 15 A) were impressed on the pipeline for 24 h. Figure
9a depicts the extremes and averages of the instant-off potentials of the pipeline under 15 A cathodic
current within 24 h.
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 14

Figure 9. Extremes and averages of the instant-off potentials of the pipeline under 15 A cathodic current
within 24 h (a). (b) Ratios of the instant-off potentials that fail to meet the cathodic protection
criteria obtained at each test post with 15 A of cathodic current. CS-1 to CS-11 signify the
numbers of the test posts along the pipeline.

Compared with the instant-off potentials without the external supplemental ICCP in Figure 5b,
the instant-off potentials of the pipeline show striking negative shifts. Comparatively more positive
instant-off potentials are also revealed by the CS-6 and CS-7 test posts, which are more positive than
−0.75 V; thus, the protective effect is obvious compared with that in Figure 5b. Figure 9b presents the
ratios of the instant-off potentials that fail to meet the CP potential criteria; 0.1%, 1.5% and 0.6% of the
CS-7, CS-10 and CS-6 test posts, respectively, are observed, all of which are lower than the ratio of 5%
as depicted in Section 2.2. This result suggests that the impressed current (15 A) successfully prevents
the pipeline from corrosion promoted by stray DC.
Stray DC subway current corrosion is one form of electrochemical corrosion because the driving
force of electron transfer is the potential difference between different regions of pipelines in different
states [27]. The electrochemical corrosion process of the stray current comprises cathodic and anodic
processes simultaneously. The cathodic reaction occurs in the region where current flows into the
pipeline associated with the oxygen depolarization process when presented in neutral and alkaline
environmental media but is associated with the hydrogen evolution reaction in acidic environments
[28,31].
Under neutral or alkaline conditions, the cathodic reaction is
1
2
O  H O  2e  2OH
2


2 (7)
Under acidic conditions, the reaction is
2 H  2e  H

2 (8)
Moreover, the anodic reaction exists in the region where the stray current flows out of the
pipeline. The anodic reaction is
Fe  Fe2+  2e (9)
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 15

The CP technique can suppress stray DC corrosion and relieve the interference influence of stray
current [32] by the compensation of impressed cathodic current. The charge consumption by the
interfacial capacitance effect, as well as the extra reactions of other species rather than iron under
dynamic DC current corrosion, account a significant part in depressing the metal dissolution when the
current flows out of the pipeline [33]. Regarding the previous cathodic region on the pipeline, the
impressed cathodic current will further polarize this region with a more negative potential. It is worth
mentioning that the cathodic current should not be too high to prevent hydrogen-induced cracking [34,
35].
As shown in Figure 9, the ICCP system doubtlessly relieves stray current corrosion, although it
cannot fundamentally solve the interference problem of stray current. Table 3 presents the extremums
of the instant-off potentials in the absence and presence of external ICCP systems derived from Figures
5b and 9a, in which the deviations are also listed for fluctuation comparison. As soon as the ICCP system
is in operation, the minimums of the instant-off potentials negatively shift slightly, whereas the
maximums notably shift. This result validates that the cathodic current will preferentially flow to the
electropositive regions of the pipeline and that the flow direction will vary with the variation in the
anodic/cathodic region of the pipeline with stray DC interference. In addition, the impressed current
drives the cathodic reduction of dissolved oxygen to produce hydroxyl ions, elevating the solution pH.
The surface of buried pipelines will cause local alkalization of the solution through the production of
OH− by cathodic reactions [25]. Therefore, a calcium-magnesium deposing layer would be formed in an
alkaline environment, resulting in a mitigation of DC stray current corrosion [36].

Table 3. Extremums of the instant-off potential in the absence and presence of external ICCP derived
from Figures 5b and 9a, respectively.

Type of test CS-1 CS-2 CS-3 CS-4 CS-5 CS-6 CS-7 CS-8 CS-9 CS-10 CS-11

Maximums of the
instant-off potentials −0.539 −0.474 −0.409 −0.837 −0.501 −0.154 −0.088 −0.355 −0.252 −0.211 −0.258
without ICCP, V
Maximums of the
instant-off potentials −0.944 −0.852 −0.913 −0.885 −0.852 −0.736 −0.702 −0.8457 −0.811 −0.757 −0.796
with ICCP, V

Deviations, V −0.405 −0.378 −0.505 −0.048 −0.351 −0.582 −0.614 −0.490 −0.559 −0.546 −0.538

Minimums of the
instant-off potentials −1.154 −1.220 −1.175 −1.179 −1.174 −1.060 −1.101 −1.186 −1.204 −1.200 −1.156
with ICCP, V
Minimums of the
instant-off potentials −1.224 −1.252 −1.207 −1.222 −1.287 −1.141 −1.194 −1.230 −1.285 −1.325 −1.273
with ICCP, V

Deviations, V −0.070 −0.032 −0.032 −0.043 −0.114 −0.081 −0.093 −0.046 −0.081 −0.125 −0.117
Int. J. Electrochem. Sci., 16 (2021) Article ID: 210547 16

4. CONCLUSIONS

In summary, the DC subway stray current causes DC interference of buried pipelines. The
maximum fluctuation section of the structure-to-electrolyte voltage in the intersection of the pipeline
and subway reaches up to 14 V, and the proportion that the off potential covers above 60% does not
meet the cathodic protection rule. As a test method to simulate that the ICCP system suppresses stray
current corrosion, the current requirement test has validated the functional effects of the impressed
current cathodic protection system, ensuring the reliability of the implementation effect from stray
current protection.
By adding the cathodic protection system and applying cathodic current to the pipeline, namely,
completely deviating the pipeline potential negatively to the corrosion-free region and generating a
settled calcium and magnesium layer on the pipeline surfaces by cathodic polarization, the
cathodic/anodic polarization arising from stray current is relieved, the potential fluctuation is reduced,
and the current distribution becomes more even [36]. Therefore, the ICCP system of buried pipelines
can effectively solve the corrosion problem caused by stray current. Moreover, before impressed current
protection is conducted, the current requirement test results can be used to provide a basis for the proper
design of ICCPs for metro systems in the future.
The influence of the metro line on the corrosion of the gas pipeline in Dongguan City was
evaluated in this work, in view of which the polarity of each test post was determined. After that, an
additional ICCP system was installed, and some tentative conclusions are provided below:
(1) The stray DC stemming from the metro evidently affects the corrosion of the gas pipeline,
especially at the intersection site.
(2) The pre-existing ICCP system cannot protect the pipeline from corrosion in the presence of
DC stray current.
(3) Another ICCP system was added to compensate for the stray current after the current
requirement test, and the potentials of the pipeline shift negatively and meet the criteria of CP.
(4) A new method for the determination of anodic/cathodic regions is proposed.

ACKNOWLEDGEMENTS
We acknowledge the financial support from the National Natural Science Foundation of China (Code:
51771057) and Qingdao NCS Testing Protection Technology Co., Ltd. China (NCS Control Code:
NJS18019).

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