## 3 Stray Current Corrosion Morphology, Measurement Method, and Influencing Factors
### 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. [PERSON] [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. [PERSON] 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. [PERSON] [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 \(\Omega\) per meter. When the resistivity of the pipeline's anti-corrosion layer exceeds 106 \(\Omega\) 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. [PERSON] 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. [PERSON] [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. [PERSON] [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. [PERSON] 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. [PERSON] 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.