## 2. Stray Current Corrosion Mechanism and Interference Characteristics  
### 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 \(\rightarrow\) Fe\({}^{2+}\) + 2e  
Cathode reaction in anaerobic acidic environment: 2H\({}^{+}\) + 2e \(\rightarrow\) H\({}_{2}\)\(\uparrow\)  
Cathode reaction in anaerobic alkaline environment: 2H\({}_{2}\)O + 4e \(\rightarrow\) 2 OH\({}^{-}\) + H\({}_{2}\)\(\uparrow\)  
Oxygen reduction in aerobic environments:  
Anodic reaction: Fe \(\rightarrow\) Fe\({}^{2+}\) + 2e  
Cathode reaction in aerobic acidic environments: O\({}_{2}\) + 4H\({}^{+}\) + 4e \(\rightarrow\) 2H\({}_{2}\)O  
Cathode reaction in aerobic alkaline environments: O\({}_{2}\) + 2H\({}_{2}\)O + 4e \(\rightarrow\) 4 OH\({}^{-}\)  
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  
Figure 2. Overall flowchart of the article’s research content.  
corrosion of pipeline steel using various methods. Taking the electrified railway as an example, [PERSON] 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. [PERSON] [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.