## 3 Stray Current Corrosion Morphology, Measurement Method, and Influencing Factors
### Methods for Measuring DC and AC Stray Currents
The specific method is to prepare one or several corrosion detection pieces with a surface area of 1 cm\({}^{2}\), 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. \[J_{AC}=\frac{8V}{\rho d} \tag{3}\]  
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 [PERSON], the corrosion rate fluctuates significantly when the AC current density is below 30 A/m\({}^{2}\), but when it is greater than 100 A/m\({}^{2}\), 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. [PERSON] [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 [PERSON] [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 [PERSON]'s law of electrolysis,  
\[M=KQ=KIt \tag{4}\]  
\(M\) 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 [PERSON] [59], AC enhances the corrosion of steel pipes. When the current density is increased from 0 to 800 A/m\({}^{2}\), the corrosion rate decreases. In addition, as the current increases, the corrosion potential increases up to 400 A/m\({}^{2}\); 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/m\({}^{2}\), 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.