## 3 Stray Current Corrosion Morphology, Measurement Method, and Influencing Factors
### 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 pipelineslocated 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. [PERSON] 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\({}_{\text{c}}\) of monitoring current to reference current and the ratio T\({}_{\text{c}}\) 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. [PERSON] 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.