## 2. Stray Current Corrosion Mechanism and Interference Characteristics
### Corrosion Mechanism and Interference Characteristics of DC Stray Current
#### 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. [PERSON] [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. [PERSON] 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. [PERSON] 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. [PERSON] 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: [PERSON] 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. [PERSON] 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.