Corrosion of Buried Pipelines by Stray Current in Electrified Railways: Mechanism, Influencing Factors, and Protection

Abstract

Metal pipes are often interfered with by currents of irregular frequency and wavelength. This is called stray current interference. The corrosion of metal pipes caused by stray current interference is one of the many factors contributing to their deterioration. Pipeline corrosion and leakage caused by stray currents can significantly impact the safety, environment, and economy of the surrounding areas. Over the past few years, stray current interference has become one of the research hotspots in the petrochemical and natural gas transportation industries. This review article investigates accident cases caused by stray currents and compares the corrosion mechanisms of DC stray currents and AC stray currents, analyzes the influence of pipeline material, environment, voltage and current, stress, and other factors on corrosion, and puts forward corrosion protection measures, such as cathodic protection, coating technology, internal corrosion control, and means of detecting stray current corrosion. Finally, it provides an outlook on future research directions on corrosion protection against stray currents.

1. Introduction

With the rapid development of China’s urbanization, the construction of high-speed railways and urban subways has gradually improved, leading to an increase in the total mileage. However, this has brought about the problem of stray current corrosion. When a high-speed train moves, not all the traction current can return from the rail to the traction substation [1], and the track and the earth cannot be completely insulated. During this period, a portion of the stray current flows into the ground through the rails; this current that leaks through is called a stray current.

Pipeline corrosion problems resulting from stray currents were noted after the opening of the first DC transit rail in Richmond, VA, USA, as early as 1888 [2]. Construction of China’s Shanghai Subway started in January 1990, but attention was not paid to stray current corrosion in the early stages, which caused old pipelines with no protection to be seriously corroded by the stray current, and gas pipelines running alongside the subway suffered many leaks [3]. To date, protective measures to mitigate the impact of stray currents in urban railways primarily include source control methods, drainage methods, and other auxiliary measures. Stray current can cause serious electrochemical corrosion of the buried oil and gas pipelines and destroy the protective coatings and cathodic protection system of the pipeline itself. Pitting corrosion, perforation, and other phenomena can occur, leading to pipeline leakage and resulting in explosions, fires, and other serious accidents. This not only affects the safety and efficiency of oil and gas transportation but also poses significant risks and harm to public safety, as well as to the lives and property of people in surrounding areas, causing substantial economic losses to the nation. Therefore, preventing and mitigating stray current corrosion and addressing related issues are of great significance in modern electrified railway systems (Figure 1).

Stray currents can be divided into two categories according to the different interference sources: DC and AC stray currents. DC stray current is generated by nearby DC transmission equipment and welding devices, etc., while AC stray current mainly originates from high-voltage AC transmission lines and AC electrified railway equipment [4,5]. Compared with the serious impact of DC stray current on metal equipment, AC stray current causes less corrosion of pipes but leads to a reduction in the service life of pipes; therefore, equal attention should be paid to the ramifications of DC and AC stray current on pipeline corrosion. The corrosion caused by DC stray current is often associated with DC running subways and light rail [6]. Anode interference resulting from the DC usually extends for a maximum period of about 250 s. AC corrosion mainly occurs near electrified railways. These two types of currents can cause various degrees of corrosion to underground oil and gas pipes. This paper introduces the mechanism of stray current corrosion in electrified railway systems compared to previous studies that focused on the effects of stray current corrosion in isolated systems. This paper also provides a more comprehensive comparison of the causes and interference patterns of DC and AC stray currents on the corrosion of buried pipelines, analyzes the various factors influencing stray current corrosion, and finally discusses the protection measures and methods of detecting stray current corrosion to reduce its influence on buried pipelines, and provides an outlook on future research directions. The overall research approach of the article is shown in Figure 2.

2. Stray Current Corrosion Mechanism and Interference Characteristics

2.1. Corrosion Mechanism and Interference Characteristics of DC Stray Current

2.1.1. Corrosion Mechanism of DC Stray Current in Urban Rail Transit

DC stray currents are more common in long-distance high-voltage lines or urban rail transit. Because the grounding network into the ground current is especially large in the vicinity of the substation, stray currents will cause serious corrosion of the grounding electrode and its nearby buried metal [7]. The corrosion of stray current can be explained by an electrochemical mechanism [8]. The formation of electrochemical corrosion requires metal materials with anode and cathode, electrolyte, and electron flow. When the rail transit passes through, DC stray current flows into the soil. When this happens, the inflow point area of the pipeline current generates a positive potential gradient and becomes the cathode, which is negatively charged. This process generates hydroxide ions, which do not have a great impact on the pipeline; however, as long as the potential in the cathode region is excessively low, metallic pipeline materials are more prone to losing electrons and reacting with oxidizing agents, causing hydrogen evolution corrosion to occur. As the hydrogen is released, it causes the corrosion protection layer to peel off. The outflow point of the stray current in the pipeline generates a negative potential gradient, which becomes an anode and is positively charged. The pipeline metals undergo electron loss oxidation reaction and become ionized near the pipeline. In moist soil environments, the water and dissolved electrolytes become the electrolytes in the corrosion reaction, leading to electrochemical corrosion on pipelines. This is also the reason why DC stray currents accelerate pipeline corrosion. The corrosion protection layer of the pipeline is destroyed and peeled off, leading to pitting corrosion and crevice corrosion. The specific chemical reaction equation for the corrosion process is as follows:

Hydrogen evolution in anaerobic environments:

Anode reaction: Fe → Fe²⁺ + 2e

Cathode reaction in anaerobic acidic environment: 2H⁺ + 2e → H₂↑

Cathode reaction in anaerobic alkaline environment: 2H₂O + 4e → 2OH⁻ + H₂↑

Oxygen reduction in aerobic environments:

Anodic reaction: Fe → Fe²⁺ + 2e

Cathodic reaction in aerobic acidic environments: O₂ + 4H⁺ + 4e → 2H₂O

Cathodic reaction in aerobic alkaline environments: O₂ + 2H₂O + 4e → 4OH⁻

The above chemical reactions represent the general mechanism of DC stray current corrosion. Based on this, some researchers have conducted studies on the DC stray current corrosion of pipeline steel using various methods. Taking the electrified railway as an example, Tan et al. [7] investigated the corrosion of Q235 steel induced by DC stray current and concluded that DC stray current could accelerate metal corrosion through the electrochemical impedance method and weight loss measurement method. Liu X [9] researched the corrosion characteristics of Q235 steel under stray current interference in a red soil environment and disclosed a linear correlation between the potential offset and corrosion rate through the weight loss technique and current density method. In the actual subway rail transportation process, stray currents will not only influence the electrical equipment of the subway facilities but also lead to varying degrees of damage to the ballast, structural steel, and pipelines of the tunnel.

2.1.2. Interference Characteristics of DC Stray Current

In terms of DC stray current interference characteristics, it is noted that interference is severe during the operation of the interference source system and relatively minimal during non-operational periods [10]. Stray current interference can be categorized into the following types based on its origin: DC stray current interference in high-voltage direct current (HVDC) transmission systems, DC stray current interference in rail transit DC traction systems, and DC stray current interference in other systems such as DC electrolysis systems and DC welding systems.

Based on the stray current interference characteristics of the rail transit DC traction system, the traction current is transmitted through the track, and these currents can leak into the surrounding metal structures. Simultaneously, the asymmetric distribution of the traction current and the poor grounding of the power supply system may generate stray currents. The grounding system may cause the current to flow into the path that should not be used, thus causing interference [11,12].

DC rail transit traction system is mainly a dynamic interference. The existence of stray currents may cause electromagnetic interference, leading to serious interference with the communication of the rail system. Influencing factors that affect the magnitude and distribution of stray currents include the material and layout of the track, the environment in which it is located, the asymmetric current distribution, the humidity and temperature of the soil, the design of the traction system, the grounding conditions, etc. [13,14]. Many scholars have investigated the interference laws of dynamic DC stray currents generated by rail transit DC traction systems using different research methods, such as modeling, experimental testing, and field data analysis.

Liu W [15] proposed a dynamic diffusion model to evaluate the diffusion of stray current and its impact. Through simulation and measurement of the potential gradient, it was found that the dynamic fluctuations of the stray current will increase the potential gradient, thereby generating greater electromagnetic interference, which is particularly serious in low soil resistivity environments. Li Zhong et al. [16] analyzed the impact of asymmetric stray current distribution through 3D modeling. They found that the asymmetry of current distribution will aggravate the anode corrosion of the pipeline and greatly increase the stray current density in the defective area. Zhang Y et al. [17] simulated the stray current corrosion of buried pipelines through numerical simulation using COMSOL Multiphysics software. The model took into account the influence of soil resistivity, voltage, and pipeline layout on stray current corrosion. It was found that high voltage, low soil resistivity, and asymmetric current distribution aggravated pipeline corrosion. Du Y et al. [18] studied stray current corrosion through simulation experiments combined with field tests, focusing on analyzing the impact of dynamic DC interference parameters on the corrosion rate. The study found that the dynamic period of the current and asymmetric current fluctuations significantly aggravated corrosion and proposed two methods: the positive potential offset time ratio and the time ratio of symmetrical or asymmetrical current density to evaluate corrosion under time-lapse and anode interference. Risk assessment methods: Cai Z et al. [19] established a multiphysics finite element model (FEM) to simulate the stray current dynamic process during the traction process and studied the corrosion behavior of pipelines caused by stray currents in the subway DC traction system. Cai et al. considered the dynamic changes of the traction current and the potential changes of the pipeline and used a finite element model to simulate the stray current distribution in the subway acceleration project. The N20 carbon steel corrosion experiment was also used to verify the simulation results. Research has found that in the subway traction system, a sudden increase in traction current when the subway accelerates will aggravate the corrosion of the pipelines (especially the anode area). The corrosion rate in the current-intensive area will increase significantly, and the degree of corrosion will be similar to that of the track, related to the distance between them. Zhu C et al. [20] proposed a track potential control method and verified it through simulation. By optimizing the train operation schedule, the current fluctuations and track potential were reduced, which greatly improved the stability of the track traction system, reducing stray current corrosion. Gu J et al. [21] proposed a negative resistance converter (NRC-TPS). Through circuit analysis and dynamic modeling, they verified that it could improve the potential fluctuation problem, improve the stability of the traction system, and reduce stray currents causing corrosion. In summary, the source of stray current in the DC traction system is mainly current leakage and asymmetric current distribution. The current flows in the track and grounding system, resulting in potential difference and electromagnetic interference. Controlling the potential difference and potential fluctuation can reduce the interference effectively. Measures include optimizing the grounding system, using insulating materials, improving track design, and adopting control strategies.

2.2. Corrosion Mechanism and Interference Characteristics of AC Stray Current

The AC stray current corrosion mechanism is also electrochemical in nature; however, scholars have not yet reached a consensus on the AC stray current corrosion mechanism. The more supported theories now include the rectification model, alkalization mechanism, AC depolarization theory, autocatalytic mechanism, oscillation model, AC corrosion mechanism model, passivation damage, and pitting AC corrosion theory. The basic reaction of AC corrosion is the same as the DC stray current corrosion reaction above.

2.2.1. Rectification Model

The rectification model states that the occurrence of AC corrosion is due to the current released by the metal during anodic polarization being greater than the current during cathodic reduction. During the positive half-cycle of the alternating current, metal ions are released from the metal surface. In the negative half-cycle, some of these metal ions should be reduced and deposited back onto the metal surface, but this is not fully realized. The accumulated loss of metal ions ultimately leads to the occurrence of corrosion [22]. The overall current produced by polarization is different from the total current produced by cathodic polarization, resulting in a net current. However, the rectification model assumes that only oxygen reduction occurs in the cathode process and is based on the absence of cathodic protection. This makes it have significant limitations.

2.2.2. Alkalization Mechanism and Autocatalytic Mechanism

Nielsen [23] proposed an alkalization mechanism for the study of the AC corrosion mechanism under cathodic protection. The alkalization theory proposes that when AC flows over metal surfaces, the electrolytic reaction generates hydroxide ions, which increases the pH value of the surrounding area and forms an alkaline environment. At the same time, the high-frequency and high-amplitude AC potential cycle and potential fluctuation accelerate the reaction, further promoting the increase in the alkalization rate [24,25,26,27]. At the same time, he proposed an autocatalytic mechanism on this basis, pointing out that there are three necessary conditions for the occurrence of AC stray current corrosion in pipelines under cathodic protection: AC-induced voltage, small anti-corrosion layer damage, and excessively negative cathodic polarization potentials.

2.2.3. AC Depolarization Theory

The theory of AC depolarization proposes that AC current changes the electrochemical kinetics of corrosion through potential fluctuations. The polarization of the cathode is faster than the polarization of the anode, triggering a depolarization effect and causing a negative shift in potential. The corrosion current density becomes larger. When the current frequency is higher, and the intensity is greater, the depolarization effect is more significant, and the corrosion rate is higher [28,29].

The above studies show that potential fluctuations are going to accelerate the corrosion of pipelines, and when the current intensity increases and the current frequency increases, the depolarization effect will be more obvious, and the corrosion process will be further accelerated.

2.2.4. Oscillation Model

Panossian [30] proposed an oscillation model, pointing out that the AC voltage oscillates continuously between the outside of the pipeline and the soil environment in contact with it, causing the passive film to be damaged and corrosion to occur. The oscillation model is proposed based on the alkalinization theory combined with the electrochemical process. The model proposes that in acidic, neutral, and alkaline environments, high current interference will cause the pipeline potential to oscillate between the corrosion-free zone and the active dissolution zone, as well as the passivation zone and the corrosion-free zone. The reason for AC corrosion at this time is that the cyclic oscillation of the AC voltage destroys the passivation film on the pipeline surface and forms a porous, non-protective oxide film. This non-protective oxide film continues to thicken as the reaction proceeds, and the pipeline is not corroded during this period [31]. Yang Y et al. [32] executed an experiment on the corrosion behavior of pipelines in alkaline environments and found that the AC current affects the polarization potential and the pH value of the environment, resulting in periodic destruction of the passivation layer of the pipeline steel. This periodic destruction is a typical manifestation of the oscillation model.

2.2.5. Passivation Damage and Pitting AC Corrosion Theory

Buchler et al. [33] proposed the AC corrosion theory of passivation destruction and pitting corrosion. They believed that when cathodic protection and AC interference work together, the passivation film generated on the metal surface during the positive half cycle of AC interference is eliminated during the negative half AC cycle interference. It is reduced to Fe²⁺ and accumulates to form a porous rust layer. In the next half cycle, a new passivation layer will be formed under rust layers, and then it will continue to be reduced to Fe²⁺ in the negative half cycle. When the cathode protection current is large, the Fe³⁺ passivation film generated in the anode half cycle will be continuously reduced, leading to continuous metal corrosion. The potential fluctuation of the alternating current causes periodic damage to the passivation film. When potential fluctuation exceeds a certain threshold, its corrosion rate increases significantly. The frequency and intensity of alternating current are important factors that influence the passivation film and corrosion rate. Lu C [34] studied the corrosion behavior of X65 steel under AC current density without cathodic protection. The study found that AC interference promoted uniform corrosion and pitting corrosion, and the anode dissolved in a weak passivation environment. When the AC current density increased, the corrosion rate and pitting depth also increased significantly.

The above mechanisms were later tested by some scholars. Although they have not been unified, through summarizing the above mechanisms and models, it can be seen that most of these theories divide AC corrosion into two cycles: AC positive half cycle and AC negative half cycle. The corrosion in the two cycles affects each other and continues during AC current disturbances, resulting in continuous metal corrosion.

3. Stray Current Corrosion Morphology, Measurement Method, and Influencing Factors

3.1. Corrosion Morphology of DC Stray Current

DC stray current is more harmful to the corrosion strength of metals than AC stray current. When there is no stray current influencing metal gas pipes, there is only natural corrosion, most of which is galvanic cell type. Additionally, the corrosion current is only a few tens of milliamps, and the driving potential difference is only a few hundred millivolts; however, the principal of stray current interference with corrosion is the electrolytic cell. That means potential can reach several volts and the current can be as high as 100 amperes; thus, the harm of stray current corrosion is far greater than that of natural corrosion.

The appearance of corrosion caused by DC interference is a continuous bubble-shaped round pit, which is caused by an electrolytic corrosion process caused by a large instantaneous direct current. In addition, the corrosion is mostly pitting, the wound surface is generally smooth, and the edge is neat. Its product is mainly carbon black fine powder.

3.2. Corrosion Morphology of AC Stray Current

In contrast to DC stray current corrosion, AC stray current leads to a different corrosion morphology. Kou J found that when the AC stray current density is no more than 100 A/m², the steel corrosion is dominated by uniform corrosion; however, as the AC stray current density increases, the steel specimen surface starts to become uneven, and the corrosion morphology changes. The phenomenon of uniform corrosion turning into pitting corrosion occurs, and at this time, the corrosion becomes more localized. When the AC stray current density reaches 150 A/m², there is obvious localized corrosion pitting [35]. Ding Q et al. analyzed the corrosion morphology and products of steel by SEM and EDS. They found that as the interference current increases, the corrosion degree becomes more intense, and the corrosion products are mainly iron oxides [36].

In terms of morphology, Nielsen found that the corrosion products of AC corrosion are mainly magnetite (Fe₃O₄) mixed with soil to form soil crust. Moreover, the products reduce the soil resistivity and enlarge the damaged coating surfaces, accelerating the corrosion rate; thus, the corrosion process is essentially autocatalytic. In addition, Ragault found that at low current density, the presence or absence of AC interference has little influence on the SCC of pipeline steel. When the current density is high, corrosion pits will be generated near the fracture surface, which is also the local corrosion (pitting) that accelerates the generation of microcracks, as mentioned above.

Kim observed that low-carbon steel corrosion without an AC power supply and negative protection is much better than that without negative protection; however, as the AC current density increases, the pH of the solution becomes higher. The higher cathodic protection potential is better than the lower cathodic protection potential [37,38,39].

3.3. Methods for Measuring DC and AC Stray Currents

In terms of the criteria for determining the presence of stray current, the ground potential of the pipeline is usually used. According to SY0007-1999 [40], for pipelines located near DC electric railways, cathodic protection systems, and other DC interference sources, DC interference is confirmed when the pipe ground potential at any point on the pipeline is shifted forward by 20 mV from the natural potential, or when the potential gradient of soil near the pipeline is greater than 0.5 mV/m.

Therefore, as the interference potential decreases, it is more difficult to reduce the interference, and the monitoring cost increases; therefore, considering the protection technology level and engineering economic strength of our country, the limit value of stray current protection measures in our country is different from the European Union standard EN50152 [41]. When the pipe/ground potential at any point on the pipeline is shifted forward by 100 mV from the natural potential or the soil potential gradient around the pipeline is greater than 2.5 V/m, direct current drainage protection measures should be taken for the pipeline. Cheng et al. proposed that it is feasible to use the positive offset of 20 mV pipe ground potential as the basis of the existence of DC interference [42]. When it is confirmed that there is DC interference on buried metal gas pipelines, long-term monitoring should be carried out, and remedial measures should be taken according to the specific conditions.

For buried pipelines with cathodic protection applied to the outer layer, the current monitoring method is frequently employed to measure stray current levels. The ratio i_c of monitoring current to reference current and the ratio T_c of monitoring current display time to total interference time are commonly used in engineering as the basis for judging the degree of interference of stray current by the current method.

In addition to measuring stray current with a potential recorder and current recorder, the SCM (stray current mapper) method is still used at present. This method is a set of detection methods jointly developed by the Reddy Company of England and the Canadian Pipeline Bureau [43]. SCM is a tool used to measure and locate stray currents around metal structures or equipment, precisely detecting current distribution through several steps. First, SCM captures the current on metal surfaces or in soil by placing current probes at the desired measurement points. Then, by measuring the voltage difference between different points, the current’s intensity and direction are inferred. In some cases, the device introduces a known current source to excite the system, helping differentiate stray currents from background currents. The collected data are analyzed to reveal the intensity, distribution, and potential sources of stray currents, enabling technicians to pinpoint the source and affected areas and ultimately assess the corrosion risk caused by stray currents. The stray current mapper has also been used to detect the pipe ground potential and DC-induced current. If there is a stray current on the pipeline, the magnetic field formed by the stray current will be captured by the SCM magnetometer. Deng J utilized the SCM method to verify the high accuracy of the method by using multiple SCM stray current detection sensing panels in conjunction with each other and used the RD-PCM pipeline current mapping equipment to monitor the pipeline corrosion protection layer, which made the data monitored by the SCM stray current detection sensing panels more accurate [44]. The stray current status can be analyzed by measuring the magnetic field of the stray current, and the magnitude and direction of the stray current can be identified based on the characteristic that the current and the magnetic field have a correlation of direction and magnitude; however, due to the limitation of the sampling frequency, this method cannot measure AC stray current.

Initially, most stray current detection devices are supposed to be set directly above the pipeline and perpendicular to the pipeline axis so as to obtain the best detection effect. However, in field operations, the location of the pipeline is often difficult to determine, so ascertaining the general orientation of the pipeline becomes the primary task of the SCM detection method. In order to measure the track current and stray current leakage point, Xu [45] proposed a magnetostrictive current sensor. FEM simulation is performed to improve the sensitivity and accuracy of the sensor. The adjustable coil design is used to reduce the problem of reduced linear range caused by increased sensor sensitivity. Studies have shown that when the sensor measures a current from 0 to 500 A, the sensitivity of the sensor is 0.391 µ/A, and when the current is 500 to 1000 A, the sensitivity of the sensor is 0.418 µ/A. High sensitivity can be achieved while measuring a large range, which can be used to monitor stray current leakage.

Another way of detecting AC stray current corrosion is by measuring the corrosion rate. This method uses a resistance probe; the pipeline is connected to one probe, and a probe that is not connected to the pipeline is set as a reference, and the corrosion rate is determined using the following formula:

$$C_t=C_i{\displaystyle \frac{R_{\mathrm{ci}}{R}_{\mathrm{rt}}}{R_{\mathrm{ri}}{R}_{\mathrm{ct}}}}$$

(1)

$$V_{\mathrm{corr}}={\displaystyle \frac{d_{ct}}{d_t}}$$

(2)

where C_t is the thickness before probe corrosion, R_ci is the original resistance of the probe, and R_ri is the reference resistor. R_rt is the reference probe’s resistance at time t. R_ct is the corrosion probe’s resistance at time t. Resistance probe technology provides continuous corrosion data and can provide real-time monitoring. Additionally, its principle is simple, it is easy to operate, and has strong applicability.

SeonYeob [46] used a thin film resistance probe to measure the corrosion rate. The probe loses weight after AC corrosion, and the steel plate resistance changes. The experimental data are recorded by a monitor. Afterward, these data are delivered to specialized analysis software, such as the M-Report processing software (https://www.metricorr.com/pipeline/ (accessed on 10 January 2007)) of Metricorr Company.

Compared to the measurement of DC stray currents, the standards for measuring AC stray currents are not as uniform. AC voltage is used as the interference criterion in SY/T0087.1-2006 [47] in China. On the other hand, GB50698-2011 [48] also uses AC current density as the AC interference criterion; the reason is that the SY/T0087.1-2006 standard is largely based on data set by foreign standards and has not been modified according to national conditions. In addition, many investigations indicate that when the AC voltage is relatively large, the AC density is not necessarily large, and the pipeline corrosion is not necessarily serious; therefore, there is still some controversy over using the AC voltage as a criterion for judging the AC stray current.

Internationally, German standards DIN50925 [49], CEN/TS15280 [50], ISO 15589-1-2003 [51], and EN1295 [52], etc., have usually been used as the standards for AC stray current. The evaluation standard of stray current serves as a reference. The criteria for judging stray currents are mainly AC current density and DC current. The ratio of current density and AC current to DC current are evaluated; thus, these criteria are relevant. The core is the parameter of alternating current density [53,54,55,56]. AC current density is selected as the main evaluation basis for the magnitude of AC stray current interference, and AC interference voltage is used as the basis for auxiliary evaluation. At present, AC current density is commonly used at home and abroad to detect AC stray currents. The specific method is to prepare one or several corrosion detection pieces with a surface area of 1 cm², bury them in the ground at the same depth as the buried pipeline, and connect them to the pipeline that has both cathodic protection and AC interference influence. Then, use an AC ammeter to record AC current and calculate AC current density J_AC according to the formula. Finally, make a comparison with domestic current density evaluation standards to draw a conclusion.

$$J_{AC}={\displaystyle \frac{8V}{\rho d}}$$

(3)

where J_AC is the current density, V is the AC interference voltage, $\rho$ is soil resistivity, and d is the area diameter of the test piece. According to M. Ormellese, the corrosion rate fluctuates significantly when the AC current density is below 30 A/m², but when it is greater than 100 A/m², the corrosion rate is usually 2–5 times higher; however, this is all based on the cathodic protection current of the same size. The corrosion rate varies differently with different cathodic protection currents. M. Ormellese [57] and his team conducted tests by first turning off AC interference, then turning off cathodic protection, and then turning off cathodic protection and then turning off AC interference. They found that the protection current of the cathode protection sample increased with the increase in the current density of AC stray current. When the AC stray current interference fades away gradually, the magnitude of the negative protection current also decreases.

According to Thabane H [58], the influence of AC on cathodic protection potential in the CP potential of −1150 mV was found to be firmer, with a small deviation from the set value.

Whether the corrosion is caused by AC or DC interference, the relationship between AC stray current corrosion and DC stray current corrosion is essentially the effect of current on the electrode surface after the current passes through, thus enabling the metal to exchange electrons with the electrolyte, leading to metal corrosion. According to Faraday’s law of electrolysis,

$$M=KQ=KIt$$

(4)

M is the mass of the precipitated metal, K is the proportional constant, Q is the quantity of the passing electricity, I is the current intensity, and T is power on time. A current of 1 A flows to the soil solution through the steel pipe surface, which can dissolve about 9.15 kg per year. The corrosion caused by stray currents is much more serious than that caused by other reasons; therefore, when AC and DC exist simultaneously, they can cause interference. According to Yan X [59], AC enhances the corrosion of steel pipes. When the current density is increased from 0 to 800 A/m², the corrosion rate decreases. In addition, as the current increases, the corrosion potential increases up to 400 A/m²; after this, the corrosion potential becomes negative. This is because the steel produces corrosion products at a sufficiently high AC current density. At the same time, it creates a well-knit film on the surface of the steel [60]. The response of DC potential to applied alternating current is instantaneous and irreversible. However, when the AC current density is less than 400 A/m², the AC causes the corrosion potential to shift negatively, and the DC potential is restored and maintained at a more negative level that is larger than the original corrosion potential when the AC is removed. Therefore, the influence of alternating current on steel is irreversible. High AC current density causes a hydrogen evolution reaction on the metal surface and enhances the corrosion of steel. In addition, although AC electric energy causes steel corrosion, only AC electric energy enhances steel corrosion, and lower AC current can cause steel dissolution (acting as Faraday current). Most of the alternating current is involved in the charging and discharging processes in the double charge layer (acting as non-Faraday current) or participates in the redox reaction of water. Therefore, understanding the relationship between AC and DC and the corrosion mechanism is the basis for constructing the stray current corrosion prediction model. Only by performing the preparatory work well can the corrosion prediction model be established in the future, and the set value of cathodic protection can be adjusted according to different current densities and current frequencies. Comparing the methods for measuring DC stray current and AC stray current shows that DC stray current and AC stray current share similarities in their measurement methods and basic principles, but AC stray current exhibits periodic variations compared to DC stray current. Although the corrosion hazard of AC stray current is not as severe as that of DC stray current, its corrosion distribution is more complex and exhibits uneven corrosion patterns. When measuring, factors such as the current frequency and waveform of the AC stray current must be considered, making its corrosion protection strategy more complex.

3.4. Factors Affecting Stray Current Corrosion

3.4.1. Environmental Factors

Regarding environmental factors, the key factors affecting stray current corrosion include soil resistivity, salt concentration in the soil, oxygen concentration in the soil, soil porosity, and buried depth of the pipeline. Among them, temperature, humidity, and soil pH value affect soil resistivity, and thus affect stray current corrosion. Ferry R [61] conducted an experimental study of the impacts of soil temperature, humidity, and pH value on soil resistivity. The study found that when soil humidity increases, soil resistivity decreases. Temperature had little influence on soil resistivity. The effect of pH value on resistance depended on the chemical properties of the soil. Bai F et al. [62] found that when soil resistivity is low, the stray current generated by the electrified railway is more likely to be transmitted to pipelines via the soil, and the stronger conductivity enhances the distribution of stray current. Zhang X [63] used numerical simulation methods to study the impact of subway stray currents on the corrosion of buried pipelines. The study found that the degree of corrosion of the buried pipeline will be immensely reduced when the soil resistivity exceeds 103 Ω per meter. When the resistivity of the pipeline’s anti-corrosion layer exceeds 106 Ω per meter, its ability to prevent stray current corrosion will be greatly enhanced. This shows that high soil resistivity and high anti-corrosion coating resistivity have a positive inhibitory effect on stray current corrosion. Yang Y et al. [32] used the boundary element method to research the effect of soil resistivity on stray current corrosion and analyzed the relationship between soil resistivity and current density. High resistivity makes the value of the current passing through the soil smaller and the current density lower. Li X [64] used numerical simulation methods to analyze the influence of various environmental factors, focusing on the influence of soil’s physical and chemical properties. The study showed that soil resistivity, soil porosity, and soil oxygen concentration affect the corrosion conditions. Lower soil resistivity and higher porosity make it easier for stray currents to penetrate through the soil to the surface of the buried pipeline. At the same time, when the oxygen concentration in the soil increases, it further promotes point chemical reactions and aggravates corrosion. Li Z [16] studied the influence of factors such as soil conductivity and the angle between pipes and cables on stray current corrosion under high-voltage DC systems. The study found that highly conductive soil improved the distribution of stray currents across the surface of buried pipelines. Moreover, the angle between the high-voltage DC cable and the buried pipeline affects the distribution of current and corrosion intensity. When the angle between the two is larger, the corrosion effect of stray current is more serious. The rational design of buried pipelines and cables and the importance of layout were also studied. Dzakyprasetyo R et al. [65] used CIPS and DCVG methods to analyze the effectiveness of pipeline corrosion protection and found that soil pH impacts corrosion significantly. Acidic soils increase the corrosion tendency of metals, while the corrosion rate of the pipe is extremely reduced in neutral and alkaline soil environments. Chung N et al. [66] used Response Surface Methodology and Box–Behnken Design methods to analyze the impact of soil pH, chloride, and sulfate factors on stray current corrosion. The degree of corrosion under different conditions was simulated through electrochemical experiments, and the corrosion current density under each condition was observed. The research found that in a lower pH environment, high sulfate and chloride concentrations aggravate corrosion, and when both exist at the same time, the impact of chloride on corrosion is more significant. These results show that the properties of the soil itself, temperature, humidity, pH value, and other factors greatly affect the corrosion of buried pipelines by stray currents. Generally speaking, soil with elevated temperatures, high moisture, and low pH will not only accelerate the transmission of stray currents to the surfaces of buried pipelines, but also increase the tendency of metal ions on the surface of the pipeline to dissolve, further aggravating corrosion.

3.4.2. Electrical Factors

In terms of stray current corrosion in DC systems, research has found that factors that affect corrosion include the average track voltage in the system, the magnitude of the traction current, the applied voltage, the insulation of the track, the design of the grounding system, the distance from the interference source, and the interference cycle. Among them, the size of the traction current, the insulation of the track, and the design of the grounding system have an important impact on the magnitude of the stray current. Many scholars have studied this. Ogunsola A and others [67] found through field tests that the main factors affecting DC stray current corrosion include the size of the traction current, the insulation of the track, and the resistivity of the soil. Cai Z et al. [68] used multiphysics models and electrochemical methods combined with finite element simulation to study the impact of buried pipeline corrosion under different traction current intensities in DC traction systems. The study found that higher traction current intensity will immensely increase the corrosion rate. Corrosion mainly appears in the anode area in pipelines, especially near the track; thus, corrosion is more serious here. Peng Brenna [69] used a combination of numerical simulations and field tests to study the impact of average track voltage on stray current corrosion of buried pipelines in a DC-electrified railway system. They found that the larger the average track voltage affected by stray current, the greater the intensity of current interference. Meng F and others [70] proposed an approximate reference electrode method combined with simulation to study the influence of dynamic DC stray current interference on buried pipelines under different applied voltage conditions in the soil. They found that applied voltage and burial depth exert a stronger influence on corrosion. Qin Y [71] et al. studied the interference of X56 pipeline steel by stray currents and found that the higher the applied voltage, the closer the stray current is to the pipeline, and the greater the impact of corrosion on the buried pipeline.

The factors influencing AC traction systems are quite different from those influencing DC systems. They include AC current frequency, current direction, waveform, and potential fluctuation. Compared with the constant current flow in DC systems, AC systems have frequency characteristics. Guo Y [72] studied the corrosion behavior of X60 pipeline steel under the influence of AC currents of different frequencies (10–200 Hz). The study found that when the AC current frequency increases, the corrosion potential of X60 steel shifts positively, the anode current density decreases, and the rate of corrosion by the stray current is greatly reduced. Li X et al. [64] conducted an experimental study on the corrosion behavior of X65 steel under different AC current frequencies and densities. By adjusting different current densities and frequencies, they found that when the current density increases, the corrosion potential of X65 steel shifts negatively, making the corrosion more serious. A higher current frequency will shift the corrosion potential positively, shortening corrosion time and reducing corrosion rate. Regarding the influence of current direction, Moran et al. [73] believed that the asymmetry of the direction of the AC current is the key factor leading to the increase in corrosion rate. The influence of the change in stray current direction on cathodic protection was analyzed by modeling. The change in the current direction reduces the protection performance of the cathodic protection system and increases the risk of corrosion. Guo Y et al. [74] found, through stress corrosion testing and simulation of the protection system of the cathodic system, that the corrosion of X80 steel is rapidly aggravated when the current direction is repeatedly switched, which has a huge impact on the cathodic protection system. Li M et al. [75] found, through electrochemical testing, that the continuous change in the AC current direction will cause the local potential to shift negatively, which will intensify the anodic dissolution of the defects of the buried pipeline coating. In addition to the above-mentioned current frequency and current direction causing more serious corrosion to the buried pipeline, the waveform also has a corresponding effect on the corrosion rate. Potential fluctuations cause the local potential to shift negatively and accelerate the dissolution of the anode. Chin [28] studied the corrosion effect of AC current with different waveforms at the same frequency. The study found that when the current frequency is the same, the corrosion rate of the triangular wave is greater than that of the sine wave and the square wave. It is believed that the reason for this phenomenon is related to its peak voltage. The potential difference in the AC system fluctuates with the periodic change in the current. Unlike the DC system, where the potential difference between the soil and the pipeline is constant, the potential fluctuation seriously affects the intensity and nature of corrosion. Chen L [76] analyzed the effect of dynamic AC currents on the corrosion rate of pipeline steel by the electrochemical method. AC potential fluctuation caused great interference with the cathodic protection system. Under high current density, the corrosion continued to intensify with the increase in interference time. Based on this, a dynamic corrosion risk assessment method based on the cathodic protection level was proposed. Feng J [77] analyzed the stray current interference generated by the traction current of urban rail through field measurement and studied the effect of potential fluctuation between soil and pipeline on corrosion. The study found that when the potential fluctuates with the train operation cycle, the corrosion rate increases significantly. Du Y [18] used the dynamic potential measurement method to study the influence of potential fluctuation on the corrosion rate of pipelines and found that when the potential fluctuation period is short, the corrosion rate increases significantly. In addition to the above factors, like the DC system, the traction current, the applied voltage, and the insulation of the track also affect the AC system.

3.4.3. Other Factors

(1) Stress factors

Among the many factors that cause stray current interference of pipelines, stress is an important factor that cannot be ignored. The stress concentration area of the pipeline is a fragile area that is very susceptible to stray current interference. Common stress concentration areas include the welds, joints, and pipeline turns. Such areas are more susceptible to the influence of electrochemical reactions. Over time, cumulative effects occur, and the corrosion rate continues to increase. Corrosion in high-stress areas gradually becomes serious, and corrosion-induced cracking occurs and continues to expand.

Lin S and others [78] studied the cumulative corrosion effect of stray currents in the subway system with the finite element model combined with the influence of mechanical stress. The study found that the distribution of stray currents on the track system is uneven. During the long-term operation of the system, stray current interference continues to accumulate in areas where stress is concentrated, causing damage to local areas and significantly increasing the corrosion rate [79].

(2) The quality of the anti-corrosion layer and the effectiveness of the cathodic protection system

Anti-corrosion coating and cathodic protection systems are both important measures for pipeline anti-corrosion. The high quality and integrity of the coating and the effectiveness of the cathodic protection system are important factors in slowing down corrosion. In terms of the quality and integrity of the anti-corrosion coating, Wang X et al. [80] simulated the corrosion behavior of X80 steel in 3.5% sodium chloride solution through electrochemical experiments. Through SEM analysis, they found that the protective layer of the pipe showed debonding behavior under the influence of stray currents, and the corrosion in the debonding area intensified with the increase in current density. Wang X [24] studied the corrosion effect of AC stray current on X70 pipeline steel at the polyethylene coating defect by electrochemical method. The study found that when the coating defect area is small, the depth of the corrosion pit is deeper, and the corrosion current density is higher. The stray current corrosion in the area with larger coating defects is more severe. This study shows that the size of the coating defect will also have a corresponding effect on corrosion. Some scholars have introduced detection methods, such as CIPS and DCVG, for pipeline anti-corrosion coating damage and evaluated pipeline corrosion in different areas based on field data. Studies have found that coating damage will intensify anode dissolution and cause local potential negative shifts in this area, accelerating the corrosion process. Additionally, coating damage will also influence the effectiveness of the cathodic protection system. The coating at the damaged location will prevent the current from being evenly distributed on the surface of the pipeline, making the buried pipeline more susceptible to stray current corrosion [81]. In terms of the design of the cathodic protection system, Yang C [82] evaluated the design of the cathodic protection system by simulating the interference of high-voltage DC electrodes on buried pipelines and found that the layout of the insulating flange greatly affected the effectiveness of the cathodic system protection. An unreasonable layout made it easier for stray current to return to the ground, destroying the original current path and forming new corrosion points. Reasonable distribution can prevent the spread of stray current and reduce its flow into the protected area. Emadoddin M and others [83] reviewed the impact of AC stray current on the cathodic protection system and believed that the potential fluctuation of AC current makes its interference area larger, while the potential control range of the cathodic protection system cannot cope with the impact of its fluctuation. It is proposed that the cathodic protection system should adopt a higher potential control range and add decoupling devices to adjust the frequency of AC current to reduce its interference. Moreover, Emadoddin M believes that there are multiple sources of stray current, and the superimposed stray current will make it difficult for a single cathodic protection method to cope with it. The impact of each stray current source should be fully evaluated, and a multi-layer protection strategy should be specified. Interactive monitoring, dynamic monitoring, and timely adjustment to the cathodic protection system should be undertaken. Guo Y et al. [74] believe that the design of the cathodic protection system needs to consider the mechanical stress factors of the pipeline. The stress concentration area and the damaged area of the anti-corrosion layer are very weak, and conventional cathodic protection cannot achieve the ideal effect. It is proposed that the design of the cathodic protection system should be combined with the stress monitoring system to provide additional current protection for the stress concentration area and improve the corrosion resistance of the pipeline. Through the research of the above scholars, it is shown that damage to the anti-corrosion layer and the effect of the cathodic protection system will greatly affect the degree of corrosion of the pipeline by stray current. The cathodic protection system should be designed accordingly and more effectively according to the specific situation while ensuring the integrity and high quality of the anti-corrosion layer.

(3) Pipe material factors

In order to ensure the safety and stability of long-distance pipelines, more and more high-grade steels are being developed and applied. At present, the steels commonly used for oil and gas pipelines include X60, X65, X70, and X80. More advanced X100 and X120 steels are also being gradually developed and used. SMI announced the research results of X100 high-strength pipeline steel in 1988; by September 2002, TransCanada had successfully used X100 high-strength pipeline steel in the Saratoga test section of the WESTPATH project and achieved corresponding research results. In 1993, the Exxon Corporation of the United States began to study X120 pipeline steel. In 1996, Exxon Corporation cooperated with other companies to jointly promote the research and development of X120 pipeline steel. Canada included X100 steel in the CSAZ245.1-2002 standard [84] in 2002 and built a 2 km long X100 high-strength steel pipeline with a diameter of 914 mm in 2004. However, X100 and X120 pipeline steels have not been widely used due to manufacturing and cost reasons, and they are still in the research stage. Many scholars at home and abroad have studied the performance of different grades of steel, such as X60, X65, X70, and X80. These studies have shown that the overall corrosion resistance of the pipeline under the influence of stray current increases with the improvement of steel grade. At the same time, the higher mechanical strength of pipeline steel can better reduce the corrosion interference caused by complex interference environments. However, compared with low-strength steel, high-strength steel is more brittle and is susceptible to hydrogen embrittlement after stress concentration occurs in certain weak parts, making pipeline steel more prone to fatigue cracks. The corrosion resistance of different steels varies in complex current environments, and reasonable material selection and protection measures can reduce corrosion [85].

The environment, electricity, and some other factors, such as stress factors, have a corresponding impact on stray current interference. Further protection measures need to be taken against these influencing factors to reduce the interference of stray currents on pipelines in traction systems.

Table 1. Factors affecting stray current corrosion.

Factor Categories	Specific Influencing Factors	Impact Descriptions	References
Environmental factors	Soil resistivity	Lower resistivity soil allows stray current to pass through more easily, leading to increased corrosion. High-resistivity soil effectively inhibits stray current corrosion.	[62]
	Soil salt concentration	Higher salt concentration increases conductivity, promoting the spread of stray current, which leads to more severe corrosion.	[64]
	Soil oxygen concentration	Higher oxygen concentration in the soil promotes localized chemical reactions, accelerating corrosion.	[16]
	Soil porosity	Higher porosity allows stray currents to penetrate the pipeline surface more easily, exacerbating corrosion.	[64]
	Buried depth	The burial depth of the pipeline can reduce the impact of stray currents, but soil properties still influence corrosion.	[63]
	Temperature, humidity, pH value	High temperature, high humidity, and low pH soil will accelerate the transmission of stray current to the buried pipeline surface and make metal ions more easily dissolved, aggravating corrosion.	[61,64]
Electrical factors	Traction current size	Higher traction current increases the corrosion rate, especially near the track.	[67]
	Track insulation	Poor insulation of the track causes stray currents to leak into the surrounding environment, increasing the risk of corrosion.	[68]
	Grounding system design	The design of the grounding system affects the distribution and intensity of stray currents. Poor design may lead to more concentrated stray currents impacting pipelines.	[64]
	Applied voltage	Higher applied voltage results in stronger stray currents, impacting the corrosion rate of the buried pipeline.	[71]
	AC current frequency	Higher AC current frequency shifts the corrosion potential positively, reducing the corrosion rate. Low frequency increases corrosion rate.	[72]
	AC current direction	Changes in the direction of the AC current lead to asymmetry, increasing the corrosion rate and reducing the effectiveness of cathodic protection.	[73]
	AC current waveform	Triangular waveform leads to a higher corrosion rate compared to sine and square waves due to higher peak voltage.	[28]
	Potential fluctuations	Potential fluctuations cause local negative shifts, promoting anode dissolution and increasing corrosion rate.	[76]
Other factors	Stress factors	Stress concentration areas (such as welds, joints, and pipeline bends) are more susceptible to stray current interference, leading to increased corrosion and possibly stress corrosion cracking.	[24,78]
	Quality of anti-corrosion coating and effectiveness of cathodic protection system	High-quality coatings and effective cathodic protection systems significantly reduce corrosion. Damaged coatings exacerbate corrosion. A well-designed cathodic protection system reduces stray current corrosion.	[74,80]
	Pipe material factors	Higher-strength steels (e.g., X80) offer better resistance to stray current corrosion but may be more susceptible to hydrogen embrittlement and fatigue cracking. Proper material selection and protection measures can reduce corrosion.	[24,74]

summarizes the factors influencing the stray current corrosion mentioned above.

4. Corrosion Protection Measures

In addition to understanding the source of stray currents, many scholars have focused on studying methods to prevent corrosion caused by these currents. There are several common anticorrosive measures, as shown in Figure 3.

4.1. Anticorrosive Coating

Reducing anti-corrosion by coating is one of the main anti-corrosion measures. From ancient animal and plant oils and asphalt to various metals and metal compounds, resin materials, and various other anti-corrosion materials, corrosion-resistant coatings can provide a physical barrier to protect the pipeline itself. Additionally, some coatings contain inhibitors that are released when corrosive media penetrate, helping to reduce corrosion of the pipeline material. Furthermore, certain coatings protect the pipeline material through cathodic protection via sacrificial anodes. Corresponding coating methods are also emerging, including spraying, flow coating, electroplating, and electrophoresis. The coating can also be melted on the protected surface by explosive methods. As shown in

Table 2. Types of common anticorrosive coatings.

Type of Coating	Classification	Characteristics	Advantages	Disadvantages	Applications
Asphalt-based coating	Petroleum asphalt or enamel	Rust removal requirements are not high; Poor resistance to mechanical damage; Resistance to soil surplus stress differences;	Good electrical insulation, water resistance, and chemical resistance.	Low cohesive force, easily eroded by soil bacteria and penetrated by plant roots and relatively short service life.	Non-cohesive soil with low humidity
	Coal tar pitch or enamel		Its performance is better than that of petroleum asphalt, its adhesiveness and mechanical strength with steel pipelines are improved, and it will not be damaged by microorganisms	There are still weaknesses, such as high-temperature softening and low-temperature hardness and brittleness.	It is suitable for swampy, underwater, seabed, saline–alkali soils and other environments, but not for gravel and cohesive soil sections.
Epoxy powder coating	(fusion-bonded epoxy, FBE)	Epoxy resin can generate a strong chemical bond with steel pipe and hardly generate volatile matter during installation, thus causing no pollution.	Strong adhesion, firm anticorrosive coating, corrosion resistance, solvent resistance, and soil stress resistance.	FEB has a large water absorption rate, weak damp–heat resistance, and limited impact damage resistance.	It is suitable for most soil environments but not for anti-corrosion coating of metal pipelines conveying mediums with too high temperatures.
	Double-layer powder structure anticorrosive coating		DPS has better impact resistance than FEB	Cost is higher than for FEB	It is often used as an anticorrosive coating for steel elbows.
Polyolefin-based anticorrosive material	Polyethylene powder	Polyolefin-based anti-corrosion materials include mainly polyethylene and polypropylene. The former has stable performance and a strong ability to isolate corrosive medium, but it is nonpolar and needs a mixture to be bonded to steel pipelines.	A multi-purpose spraying method is used to spray molten polyethylene powder onto heated steel pipes. The equipment is simple and can be sprayed simultaneously.	Polyethylene is poorly bonded to steel and has limited water vapor permeability resistance.	It is mainly used for anti-corrosion spraying of small-caliber steel pipelines in urban public buildings and factories.
	(2-Layer Polyethylene)	Bottom layer binder, top layer polyethylene	It has excellent mechanical properties, low water vapor permeability, good anti-corrosion performance, resistance to soil stress, and low cost of anti-corrosion materials and coating.	Bonding ability with steel is not strong, especially at high temperatures. The coordination with the cathode protection is poor, and under the action of the protection current, the viscous viscosity may be lost, and the current may be shielded.	Commonly used in low-temperature environments, small-diameter steel pipes
	3PE (3-Layer Polyethylene)	Low-layer epoxy powder, intermediate adhesive layer, outer polyolefin layer	Combining the advantages of FBE and polyolefin, it has excellent anti-corrosion performance and good mechanical-damage resistance.	It is difficult to construct, repair damages on site, and prevent corrosion of pipe fittings.
	polypropylene	The characteristics are similar to polyethylene, but the use temperature is higher.	Similar to polyethylene	Similar to polyethylene	It has not been applied on a large scale, but it has been exported, for example, the three-layer polypropylene anticorrosive coating exported to Sudan.

, there are three types of anticorrosive coatings: organic anticorrosive coatings, inorganic anticorrosive coatings, and metal anticorrosive coatings.

4.1.1. Organic Anti-Corrosion Coating

Organic corrosion-resistant coatings are the most widely used type of corrosion-resistant coatings. Organic coatings typically possess excellent electrochemical, chemical, and mechanical properties, allowing them to effectively perform their physical barrier function. In environments where corrosive media penetration is more severe, coatings containing corrosion inhibitors can be used to provide additional protection. Some organic coatings can also achieve electrochemical protection through sacrificial anodes by incorporating metal anode materials.

(1) Good electrochemical properties

Theoretically, in a complex corrosive environment, it is inevitable that the corrosive medium permeates the organic anticorrosive coating to contact the protected metal, so the coating is often used in conjunction with cathodic protection. However, in the cathodic protection operation, ions combine to generate alkali, which affects the anticorrosive coating and accelerates the peeling of anticorrosive coating [86]. Some coatings are too insulating, thus isolating the protection current. Therefore, according to Shujun Liu [87], it is better to carry out a cathodic stripping test before the organic coating is used as a pipeline anticorrosive coating for the project; the radial stripping distance of the anticorrosive coating around the defect should not exceed 12.5 mm after the test.

(2) Good chemical properties

In modern engineering, metallic pipelines are often used as permanent underground facilities, with long design service life and large investments in manpower and material resources for maintenance and repair. Moreover, pipelines are mostly located in sparsely populated areas (except urban underground pipelines) [88] and sometimes pass through acid and alkali, swamps, and industrial pollution areas. Therefore, the coating must be able to resist acid, alkali, and some mixed solutions.

Researchers such as Dmitri Fix [89] studied the chemical properties of organic anticorrosive coatings using simulation methods and then used SEM to observe coating performance, for example, coating cracking, dissolution, peeling, etc. They also carried out adhesion detection.

(3) Good mechanical properties

Usually, the operating environment of buried pipelines is complicated. Landslide disasters along the pipeline can threaten the safety of construction personnel during the pipeline construction and can also cause deformation and damage to the pipeline during the operation period, including serious pipeline rupture and oil and gas leakage, leading to explosions. The huge landslide disaster in Ecuador in 1983 caused damage to pipelines within 4 km of the explosion. In 1995 and 1996, three landslides occurred in the Northwest pipeline in the United States, resulting in pipeline fracture accidents. In 2002, the South American Nor Andino pipeline was cut off for 90 days due to a landslide disaster [90].

In addition, similar disasters occur frequently in southwest China. Take the Guizhou section of the China-Myanmar oil and gas pipeline as an example. The terrain along its route is highly undulating, the geomorphology is complex, and a variety of geologic hazards have developed under such conditions [91]. For example, active land structures and concentrated rainfall easily lead to landslides, mudslides, and other disasters [92].

Therefore, as the first barrier to protect the pipeline, the organic coating of the pipeline must have a certain hardness, adhesion, impact resistance, or bending resistance in addition to corrosion resistance. Laboratories often carry out different testing experiments on anticorrosive coatings of different materials; for example, Tekkanat B tested the environmental cracking resistance of the 3LPE organic anticorrosive coating, which is prone to cracking; Yang Z [93] used a new method to obtain the concentration conductivity and porosity of shale by fitting the experimental results. The radial permeability of shale was deduced by using the relationship between them. Moreover, Peng Y studied the landslide early warning model under the influence of multiple factors. The United States has made considerable progress in shaping the mechanical damage on the surface of pipelines. However, a lot of work needs to be done to improve the accuracy of these technologies in inhibiting the adverse effects of microstructures on pipelines, especially in terms of density variation characteristics and plastic deformation, etc.

In the past, petroleum asphalt or coal tar were commonly used as the main anti-corrosion coating materials for pipelines, while 3PE (three-layer polyethylene) or polypropylene, single-layer or double-layer epoxy powder, heat shrinkable tape/shrink tube, etc., are the modern anti-corrosion materials that are commonly used. However, due to the restrictions of the construction performance, equipment, and environment of the anticorrosive coating, the fields of application of these anticorrosive materials also have their own specialization. This paper takes common organic anticorrosive coatings in engineering as an example and compares them according to their performance, advantages, and disadvantages, as well as their areas of specialization [94,95].

In addition to the anti-corrosion materials mentioned above, there are also anti-corrosion products made from these coatings; for example, polyethylene, coal tar, petroleum asphalt, and paraffin base can be made into anti-corrosion adhesive tapes. They use cold winding (polyethylene anti-corrosion tape) or hot winding (coal tar, petroleum asphalt anti-corrosion tape) to form an anti-corrosion layer on steel pipelines. In the 1960s, Daqing Oil Field, Changchun Institute of Applied Chemistry, and Shanghai No. 3 Factory of Pen Parts collaborated in the production of anti-corrosion tape, which was appraised by the Ministry of Petroleum and Chemical Industry in 1977. As early as the last century, anti-corrosion tape was considered one of the most cost-effective anti-corrosion measures for pipelines in the United States [96], and it has developed rapidly since then. During the recent overhaul of northeast oil pipelines, anti-corrosion tape was one of the most popular anti-corrosion construction technologies.

4.1.2. Inorganic Nonmetal Anticorrosive Coating

At present, pipeline corrosion is mainly prevented using organic coatings. However, due to aging and deterioration, heat resistance, and cold resistance of organic coatings, the service life of pipelines coated with these coatings is limited; therefore, inorganic corrosion prevention technologies have emerged. Compared with organic corrosion prevention coatings, the variety of inorganic, nonmetallic corrosion-resistant coatings is limited. Currently, there are mainly inorganic corrosion-prevention materials used in engineering applications, such as cement mortar corrosion-resistant coatings and concrete corrosion prevention coatings, ceramic corrosion-resistant coatings, enamel corrosion prevention coatings, and glass corrosion prevention coatings.

(1) Cement mortar and concrete anticorrosive coating

In urban pipe networks, cement lining with a thickness of 5 mm is a common anti-corrosion method. Moreover, in underwater pipeline facilities such as rivers and river bottoms, concrete with a thickness of 5–25 mm is commonly used as the counterweight layer and outer mechanical protection layer of the pipeline. However, there are also restrictions on the use of this anticorrosive coating. Generally speaking, cement is alkali-resistant but not acid-resistant because the basic components of cement, tricalcium aluminate, and C4AF, easily react with acids [97]. The barbed wire mesh or steel skeleton in the concrete anticorrosive coating makes the concrete anticorrosive coating have gaps, so they easily corrode when encountering media such as water and NaC. The volume of the steel bar becomes larger after corrosion, and the stress makes the concrete protective layer crack [98]. Therefore, it is mostly used in pipes with hard water or non-aqueous medium.

(2) Ceramic coating

Ceramic materials are composed of compounds of metal elements, such as alumina, magnesia, zirconia, etc. Ceramic materials may include bricks, stone, coarse ceramics, porcelain, concrete, refractory materials, glass, etc. Ceramics are resistant to high temperature and wear and can be made into finished products resistant to various strong corrosive media. Combined with various modern coating technologies, it is possible to use ceramics as anticorrosive coatings [99]. Sun J Y invented a ceramic coating method, where sanding is carried out before the coating is applied to the alumina ceramic body so that the coating can reach maximum adhesion and strength during the coating process; finally, polishing is carried out, thus avoiding delamination between the coating and the ceramic body as much as possible. The modern preparation method can also be applied to protect ceramic coatings on metal pipelines.

Ceramic anticorrosive coating is a kind of high corrosion-resistant anticorrosive coating with high-temperature resistance and very good wear resistance. However, the ceramic anticorrosive coating has poor impact resistance and is not suitable for prefabricated outer anticorrosive coating of steel pipes in buried pipeline engineering in the general sense. Therefore, ceramic coatings are generally only used for internal corrosion protection of process pipelines and process equipment in chemical industries or special environments and are rarely used for external corrosion protection.

(3) Enamel coating

Enamel is a kind of glass-like porcelain layer closely combined with metal, which is obtained by coating enamel on the surface of a metal and then “enameling”. The traditional enamel process is technically perfect, and the coating is generally used as an internal and external anticorrosive coating for pipelines. However, the “enameling” process requires high temperatures of up to 900 degrees Celsius; at this temperature, some steel products will undergo changes in their metal structures. Therefore, enameling is generally only used as an anticorrosive coating on low-carbon steel and other high-temperature-resistant steel products. In addition, there is evidence that enamel coatings can also be used as adhesives for mortar coatings and steel bars in some places. P. G. Allision’s team identified the mechanism through which reactive enamel coating (RPEC) increases the bond strength between cement mortar and steel bars [100]. Dan Song [101] found that enamel coatings can effectively improve the service life of the grey cast iron substrate in a complex frictional environment.

(4) Glass anticorrosive coating

Although the cost of the glass anti-corrosion coating is lower than that of the enamel coating, the glass anti-corrosion coating has not been adopted on a large scale because the production needs a heating process, the cooling speed of the glass is faster, the adhesion between the glass glaze melt and the substrate is not strong, and the glass anti-corrosion coating is brittle and easily falls off due to external force.

4.1.3. New Coatings

(1) polyurethane (PU) coating

PU coating is composed of a polyol compound and an isocyanate solution. Its viscosity is regulated by low molecular weight resin. PU coating has excellent performance; its hardness is as high as HS 80~86, it has good wear resistance, strong cathodic stripping resistance, and can conduct cathodic protection even after long-term failure.

Branch M’s research [94] shows that PU has good wear resistance and corrosion resistance, with especially excellent microbial corrosion resistance. It is widely used on the inner and outer surfaces of pipelines. It does not contain volatile solvents, is non-toxic and harmless, and has simple construction and good anticorrosive coating quality. It is mainly suitable for repairing damage, joints, and anticorrosive coatings and has wide application prospects.

(2) Nano-modified anticorrosive coating

As we all know, the surface materials involved in corrosion protection are determined by the microstructure. However, the addition of nanomaterials to such structures can improve the properties of materials. For example, nano-silicon dioxide has the characteristics of serious surface coordination deficiency, large specific surface area, and surface oxygen deficiency. Therefore, the strength, ductility, and toughness of the coating can be greatly improved by adding nanomaterials into the coating. However, the factors limiting the development of nano-modified coatings come from the nanomaterials themselves, mainly the dispersion of the nanoparticles. How to avoid the large-scale agglomeration of nanoparticles during the drying process and the content of nanoparticles added to the coating are the main difficulties limiting the application of nano-modified materials.

Ammar S [102] found that the anti-corrosion behavior and hydrophobicity of the nano-composite coating formed by embedding ZnO nanoparticles in the polymer are significantly improved compared with the anti-corrosion effect of the original coating. Kongparakul S [103] developed a kind of self-repairing anticorrosive coating, which added nano-silica and self-healing microcapsules (POT/ETA/DEA) to the epoxy coating, significantly improving the corrosion resistance of the coating and reducing the surface corrosion rate of cold rolled steel. The schematic of corrosion protection is shown in Figure 4. The research also proves that the modified nano-coating can effectively increase the length of the diffusion path and reduce the permeability of the O₂ nano-composite film.

4.2. Cathodic Protection

4.2.1. Sacrificial Anode Method of Cathodic Protection

Cathodic protection [104,105,106] is one of the most widely used methods for pipeline anticorrosion protection. In the field of petroleum and petrochemical industry, cathodic protection is widely used and has obvious anti-corrosion effects. It is often used in combination with anti-corrosion coatings to achieve the best anti-corrosion effect. Cathodic protection is generally divided into three categories, as shown in

Table 3. Types of cathodic protection.

Protection Type	Classification	Material	Advantages	Disadvantages	Applicable Place
Traditional sacrificial anode–cathode protection type	sacrificial anode	Mg and magnesium alloy, Zn and Zinc alloy, AI and AI alloy, etc.	The self-corrosion is small, and the long-term discharge process rarely polarizes; large discharge per unit weight, uniform output current, good mechanical properties, low price, wide source	High requirements for anticorrosive coating after metal consumption to replace regularly	Not suitable for high-resistance environments
	Shallow anode bed	Scrap steel, magnetic iron oxide, high silicon cast iron, platinum-plated anode	Convenient construction, easy maintenance and replacement, low cost	High grounding resistance. It has great influence on the surrounding buildings, and it is difficult to evenly distribute the protection current of complex structures	Shallow surface soils with low resistivity and where the protected object is relatively simple
	Deep-well anode bed		The anode grounding resistance is small, the current distribution is even, and the interference with other buildings is small	Construction is more complex, maintenance and replacement difficulties, high cost	Areas with low resistivity or high soil resistivity underground; Landmark metal complex with regional cathodic protection
New type of sacrificial anode protection	Flexible anode based on conductive polymer	Mostly doped conductive polymer materials, such as graphite, are added to polyethylene medium	Close to the anode; can protect objects with complex shapes; high current utilization; low current loss; when used for pipeline protection, the potential distribution is relatively uniform. The cost is low	Current reliability and life are lower than MMO under high currents	It is not suitable for cases with abnormally large discharge densities
	MMO Flexible anode	Noble metal oxide	Inherits the advantages of high discharge density and long service life of MMO anode	The cost is higher than the flexible anode of conductive polymer	It can be used in almost any situation

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4.2.2. Impressed Current Method of Cathodic Protection

The forced current method involves directly applying a cathode current from an external direct current to the protected metal to polarize the cathode to achieve cathode protection. As shown in Figure 5, It consists of an auxiliary anode, reference electrode, DC power supply, and related connecting cables. The function of the auxiliary anode is to send the protective current to the electrolyte to flow to the protective body. In addition to external coating, cathodic protection is also a common measure for effectively preventing stray current interference in long-distance pipelines, but it is often used in combination with coatings; for example, impressed current cathodic protection is often used as the main method for regional cathodic protection and sacrificial anode protection is used as the auxiliary method for combined cathodic protection. C. Christodoulo [107] found that after the existing impressed current protection disappeared, the protected steel still showed no signs of corrosion for a certain period of time. This indicates that ICCP has the characteristic of continuous corrosion prevention. Law D W [108] found that the corrosion evaluation of the ICCP system in seawater was not similar to that of other areas due to the influence of the tide and splash zone of seawater. The ICCP system has a prominent protection effect on pipelines, but it is not without defects. For instance, impressed current cathodic protection is also effective. According to TW Cain, U Angst’s study [109,110], the negative potential in the range of −850 mV to −1.12 V has the best effect on the prevention and control of stray currents. When the protection voltage is lower, the cathodic protection system often has a poor effect. However, when the voltage is too high, hydrogen evolution reactions occur easily, resulting in changes in the element composition and grain size of the pipe following hydrogen embrittlement and hydrogen damage (SCC). Residual stress concentration at corrosion pits also occurs more easily, leading to significant reductions in mechanical properties, thus degrading the mechanical properties of the metal [111]. Additionally, SCC has been identified as one of the major failures of pipeline steel in humid environments, resulting in cracking of high-pressure gas transmission pipelines and serious economic losses and disasters. Zvirko O [112] found that in the decommissioned pipeline, the degraded X60 steel showed higher resistance to SCC in comparison with the degraded X52 steel. Zhu [113] found that AC and CI^- have synergistic effects on metal corrosion. Taking X80 pipeline steel as the research object, he found that the fracture had obvious brittle fracture characteristics, and the preliminary analysis was the embodiment of the combined action of anodic dissolution and SCC. This is because after H⁺ enters the surface of pipeline steel to obtain electrons, the generated gas often brings nonmetallic substances back to the surface of the pipeline, thus causing hydrogen-induced cracking of the pipeline or peeling of the coating. In addition, the ICCP system is affected by various complicated factors, so there may be inaccurate adjustments in some cases. For example, in areas of southwest China [114] with large soil displacement and frequent surface activities, buried pipelines are vulnerable to stress corrosion caused by soil displacement [115]. Pipelines in silty soil under salt–temperature coupling environments [116] are vulnerable to stronger corrosion than those in the interior. Pipelines in some humid areas are also vulnerable to microbial corrosion; however, with the development of pipeline intelligent equipment, intelligent ICCP equipment is emerging. CA Sibiya [6] aimed to develop an ICCP system that integrates real-time monitoring and intelligent adjustment of the value of cathodic protection current potential. Xinhua Wang [24] used COMSOL Multiphysics to study the influence of AC current on coated X70 steel and the delamination of 3-layer polyethylene coating with defects. They found that the pit current density of small defects was greater than that of large defects. Zhang Y [17] used COMSOL Multiphysics to simulate the stray current corrosion dynamics. They also calculated the distribution and intensity change in electrolyte potential in the cathodic protection system by solving Laplace’s three-dimensional equation, which shows that the increased applied voltage causes the pipeline potential to move further forward, resulting in accelerated corrosion by stray currents and the corrosion effect with sacrificial anode cathodic protection is much smaller than that without cathodic protection when the two pipelines are in the crossing state. Zhang Z [117] simulated and analyzed the corrosion model with COMSOL Multiphysics. The simulation mainly includes the effects of DC stray current on electrode potential, pH value, iron dissolution, and corrosion rate. His team found that the higher the DC stray current was, the higher the electrode potential, and the electrode potential moved forward. In addition, the pH value of the soil around the grounding electrode was also affected by direct current, and additional direct current injection led to an increase in hydrogen ion concentration. Finally, DC stray current can accelerate the dissolution rate of anode iron and change the rate of electrode thickness. The higher the stray current density, the faster the anode dissolution rate and the faster the grounding electrode corrosion rate. The research results are of great guiding significance for the establishment of corrosion prediction models.

4.2.3. Drainage Protection of Cathodic Protection

In an environment with stray currents, applying cathodic protection to protected structures by eliminating stray currents is called drainage protection. There are four common drainage methods.

(1) Direct drainage

When the polarity of stray current interference potential is stable, the protection body and interference source can be directly connected by cable to eliminate stray current. This method is simple and feasible, but it also has some disadvantages. Take stray currents caused by urban rail transit as an example; if the pipe ground potential is lower than the rail and ground potential, the current on the rail will flow into the pipe, causing corrosion.

(2) Polar drainage

When the polarity of the stray current interference potential alternates between positive and negative, the stray current can be discharged back to the interference source through a series of diodes. Due to the unidirectional conduction performance of the diodes, only the stray current is allowed to be discharged in the positive direction, while the negative direction is reserved for cathodic protection. This method is widely used at present.

(3) Forced drainage

The above two methods can only protect the protective body when discharging, whereas the protective body is in a natural corrosion state when it is not discharging; thus, the advantages of forced drainage are obvious. Forced drainage means drainage through a rectifier. A discharge current is used for protection under intense stay current, while this system is used to supply protection current to place the protection body into a cathode protection state. A constant potential meter is usually used for forced discharge, and it is better to leave a small amount of protection current output when there is discharge protection.

(4) Grounding drainage H₂O

Grounding drainage is conducted to change the connection object of the drain cable so that one end of the drain cable changes from the running rail to the auxiliary anode. This method is convenient to use, but the auxiliary anode needs to be replaced regularly.

To sum up, as an important part of cathodic protection, discharge protection is often arranged according to the specific conditions of the site instead of using a discharge method. In actual situations, cathodic protection schemes in which auxiliary anode, ICCP, and grounding discharge are arranged at the same time are often used.

4.3. Internal Anti-Corrosion Technique

4.3.1. Corrosion Inhibitor

Usually, corrosion inhibitors are used in the interior of pipelines to slow down the corrosion of the pipelines. The corrosion inhibitor forms a single or multi-layer dense protective film on the surface of the substrate. The inhibitors have the advantages of high corrosion inhibition efficiency, long-lasting corrosion inhibition, and no environmental pollution [118]; thus, they can mitigate the pipeline’s corrosion.

4.3.2. Inner Coating Technology

Anticorrosive coatings are used not only for the exterior of steel pipelines but also for the interior of pipelines. Anticorrosive inner coatings include epoxy type, modified epoxy type, epoxy phenolic type, polyurethane type, and nylon type. They are difficult to apply due to the limitation of the inside of the pipeline and the method of preparation of the inner coating [119]. H Fujiyama [120] used a special method called “plasma sputtering” to spray the coating. The inner coating system was applied to reactive sputter deposition on metallic tubes with inner diameters of 30 mm and lengths of 480 mm.

4.3.3. Composite Pipe Technology

With the popularity of trenchless repair technology, composite pipe-lining technology for pipeline corrosion repair has attracted more people’s attention. For instance, Fiberglass lining has creep resistance and good performance at high temperatures. Bimetal composite pipes not only improve the corrosion resistance of pipelines but also retain the mechanical strength of steel pipes. The ceramic lining has good wear resistance, high strength, and high-temperature resistance. Moreover, a new composite pipe technology named multilayer lining anticorrosive technology is gaining popularity. The outer layer is a fabric-reinforced flexible plastic pipe, which is usually made of wear-resistant, temperature-resistant, puncture-resistant, and hydrolysis-resistant materials, such as wear-resistant PE/PU/PUX, while the middle layer is made of fiber composite materials, usually made of fiber materials such as polyester or aramid. Different materials are selected for the inner layer according to different application scenarios, but the performance satisfied is nothing more than hydrolysis resistance, swelling resistance, chemical resistance, temperature resistance, and wear resistance. Most of them are made of PE/PU/PUX, PVDF, or other materials. For example, oil-resistant TPU was selected for the inner layer of a fuel transmission pipeline, and polyester fiber was selected for the intermediate fiber layer.

Although the above three technologies can reduce the corrosion of pipelines to a certain extent, a corrosion prediction and protection system with feedback is the most accurate and reflects the future of the development of anti-corrosion technology. Establishing a high-precision corrosion prediction and protection model has been a hot topic in the research of corrosion protection in recent years. The specific application of corrosion technology needs to take into account the actual working conditions, including the external environment (e.g., soil zone, temperature, water quality), construction conditions, and factors such as cost, on-site installation, and supply of engineering materials. In recent years, researchers have carried out predictive research on the application of coatings, the cathodic protection effect, and the use of corrosion inhibitors in different situations. The main purpose of such research is to optimize the performance of anti-corrosion technology, save costs, and avoid wasting resources. In addition, it will pave the way for the establishment of a real-time model of a corrosion monitoring system in the future.

5. Common Methods for Studying Corrosion-Resistant Coatings

5.1. Weight-Loss Method

The weightlessness method is a relatively simple and convenient corrosion test method. By comparing the quality of corrosion samples before and after the corrosion test, the corrosion rate can be calculated by analyzing the corrosion time. Since this method does not need to consider the accumulation of corrosion products on metal specimens, only poor quality can be considered; hence, it is widely applicable to various corrosion tests.

Domestic scholars often use GB/T 16545-2015 [121] as the measurement benchmark, and corrosion products are usually removed by chemical, electrolytic, and mechanical methods. Wang R [122] studied the degree of adhesion of a resin coating with different phytic acid dipotassium and Q235 steel, using the weightless process to remove corrosion products. Shi X [123] studied the effect of Cu in pipeline steel on microbial corrosion resistance and the mechanical properties of pipeline steel. They also used this method to remove the corrosion production.

5.2. Advanced Observation Method (SEM, EDS, and AFM)

Scanning electron microscopy (SEM), EDS, and atomic force microscopy (AFM) are methods that are commonly used to analyze the corrosion morphology of metals. Among them, SEM can be used not only to observe corrosion products but also to observe the anti-corrosion effect of corrosion inhibitors. M. Yadav et al. [124] used EDS to reveal the corrosion of low-carbon steel in different corrosion solutions. The authors used polished low-carbon steel, low-carbon steel with 15% HCI, and a 15% HCI solution of (2-(2-(2-Phenyl)-3-(Isonicotinado)-4-oxotiazolin-5-yl) acetic acid (PITA) and (N-(2-Phenyl-4-oxotiazolin-3-yl)-Isonicotinade (POTI) as the corrosion testing environment. The results showed that the Fe contents of low-carbon steel measured by the EDS were 85.26%, 83.12%, 69.24%, and 71.23%, respectively. They proved that the polished low-carbon steel was completely exposed to EDS, while the two corrosion inhibitors formed protective films on the low-carbon steel; thus, the Fe content was relatively lower in the two samples that were protected. In order to confirm these findings, M. Yadav used AFM to observe the roughness morphology. The study found that the surface roughness of low-carbon steel in POTI and PITA corrosion inhibitor solutions was 160 nm and 150 nm, respectively, while the average roughness of 15% HCI solution without the corrosion inhibitor was 650 nm; thus, the study proved that both corrosion inhibitors can protect the surface of low-carbon steel, and PITA has a better effect than POTI.

5.3. Electrochemical Impedance EIS

In recent years, as demand for improved coating performance has grown in many countries, EIS has been chosen to research the process of destruction of organic coatings.

As shown in Figure 6, the reference electrode, indicator electrode, and working electrode make up the loop current. There are different test curves and analog circuits for different immersion periods. The most important observations of the EIS method are the Nyquist plot, Bode plot, phase Angle, and impedance modulus.

For instance, Mahdavian [125] used EIS to conduct corrosion research on low-carbon steel, and the anticorrosive coating was prepared with a mixture of zinc phosphate, zinc chromate (SNCZ), and epoxy resin. According to the Nyquist plots, it can be concluded that the radius of the arc represents the high-frequency band of the interface reaction impedance. On the other hand, in the middle and late stages of the reaction, there is a line with a slope of 1 behind the arc, indicating that the reaction has been controlled by diffusion. According to the Bode diagram, it can be clearly seen that there are two “peaks”, and the time constant is two (shows a “peak” when the reaction is a capacitive–reactance response). The time constant shows that the electrode reaction process can be determined by the number of variables, and the degree of influence of state variables can be determined by the numerical value, which is very useful for analyzing the mechanism of the electrode reaction. In addition, the polarization curve method and electrochemical impedance method belong to the electrochemical measurement method, so their principles are similar, and a reference electrode, counter electrode, and working electrode are also required; these will not be described here. Thus, during actual research, researchers do not use only one method to test coating performance, but often use the above methods to comprehensively judge the coating performance. For example, A Ehsani [126] used EIS, electrochemical noise analysis, and density functional theory to evaluate thyme plant extract as an environmentally friendly corrosion inhibitor for stainless steel 304 in an acidic solution. Kuiren L measured the corrosion resistance of silicon coating on ZNALMGRESI alloy by the copper-accelerated salt spray test and corrosion weight loss method. First, XRD was used for physical property analysis and structure analysis, and then SEM was used for observation. Finally, a polarization curve and EIS diagram were used to study the electrochemical corrosion process of the coated alloy.

6. Conclusions

There are many reasons for stray currents, most of which are related to the functioning of the electrified tracks, UHV transmission lines, and cathodic protection systems themselves. Metal components in infrastructure near high-speed railway stations and UHV transmission lines are particularly vulnerable to corrosion from stray currents. Based on this, this paper compared the corrosion mechanisms and interference characteristics of DC and AC stray currents. DC stray current corrosion, on the basis of electrochemical corrosion, further lowers the potential of the pipeline’s cathodic region, making the pipeline metal more susceptible to losing electrons, which accelerates the corrosion rate of the pipeline. DC stray current interference is a dynamic disturbance where asymmetric current distribution and poor grounding of the power supply system increase the stray current density, further accelerating corrosion. As for the corrosion mechanism of AC stray current, scholars have not reached a consensus; however, the theories supported include the rectification model, alkalization mechanism, and AC depolarization theory. However, it remains fundamentally an electrochemical corrosion process, with the positive and negative half-cycles of the alternating current continuously influencing each other, resulting in ongoing corrosion of the pipeline metal.

In terms of corrosion morphology, the corrosion damage caused by DC stray currents is generally more severe than that caused by AC stray currents. The corrosion appearance due to DC interference is a continuous bubble-shaped round pit, mostly pitting. For AC stray current corrosion, as the current density increases, the corrosion morphology gradually transitions from uniform corrosion to localized pitting corrosion. In terms of measurement methods, DC stray current can be measured using a potentiometer, current recorder, and SCM (stray current mapper). Among these, the SCM method offers higher measurement accuracy. In comparison to DC stray current measurements, the standards for measuring AC stray current are not unified. Typically, AC current density is used as the primary indicator of current interference, while AC voltage serves as an auxiliary evaluation criterion. AC current meters can be used to measure the AC current and its density. In terms of influencing factors, they can primarily be divided into environmental and electrical factors. In addition, factors such as stress, the effectiveness of protective measures, and pipeline materials also play a role.

In response to the corrosion damage caused by stray currents on underground pipelines, this paper proposes three corrosion prevention measures: the use of corrosion-resistant coatings, cathodic protection, and internal corrosion protection technologies for pipelines. Among these, corrosion-resistant coatings serve as a physical barrier. Additionally, some coatings contain corrosion inhibitors, which are released when corrosive media penetrate, providing further corrosion protection. Furthermore, some coatings achieve protection for pipeline materials through sacrificial anode cathodic protection. Cathodic protection, as one of the most widely used pipeline corrosion prevention methods, is typically combined with corrosion-resistant coatings for enhanced protection. The sacrificial anode–cathode protection, impressed current method of cathodic protection, and drainage protection of cathodic protection are three of the most common methods. The former is suitable for smaller structures where external power sources are not easily available, while the latter two methods are applicable in environments with more complex corrosion and denser stray currents. In terms of internal corrosion protection technologies, both corrosion inhibitors and internal coating technologies can effectively suppress pipeline corrosion. In recent years, composite pipe technology has received increasing research and application attention. Composite pipe technology integrates the excellent corrosion resistance of the inner lining and the superior mechanical properties of the outer layer, making it one of the key technologies for future pipeline corrosion prevention.

To better suppress the damage caused by stray currents to underground pipelines, we believe that future research should focus on the following three aspects:

Suppressing the generation of stray currents. Building on the improvement of the grounding system, more current isolation devices can be used to prevent stray currents from spreading along underground pipelines. At the same time, optimizing the design of the power system to reduce uneven current distribution can help concentrate the current along the intended current paths, minimizing current leakage.

Developing materials with better corrosion resistance for pipeline construction and creating more advanced composite pipes. Composite pipes have excellent overall performance, integrating the advantages of both the internal and external materials of the pipeline. In the future, with increasing requirements for pipeline corrosion resistance, the performance of the internal material will be further enhanced. The use of new corrosion-resistant alloys or coating technologies can significantly improve the service life of pipelines in harsh environments, showing even more outstanding performance in environments with severe corrosion, such as marine and underground settings.

Establishing a real-time corrosion monitoring and protection system. The real-time monitoring and protection system can continuously collect data through sensors to obtain information such as the corrosion status and potential of underground pipelines. Once the data are transmitted, the system can use algorithms to predict the corrosion rate and trends of the pipeline, providing early warnings that can significantly reduce risks such as pipeline leakage. In terms of future development, with the improvement of sensor performance, monitoring systems with longer distances, larger coverage, and higher precision can be established. By combining artificial intelligence and big data analysis technologies and utilizing machine learning and deep learning methods, more accurate monitoring and predictions can be achieved.