## 3 Stray Current Corrosion Morphology, Measurement Method, and Influencing Factors
### Methods for Measuring DC and AC Stray Currents
Initially, most stray current detection devices are supposed to be set directly above the pipeline and perpendicular to the pipeline axis so as to obtain the best detection effect. However, in field operations, the location of the pipeline is often difficult to determine, so ascertaining the general orientation of the pipeline becomes the primary task of the SCM detection method. In order to measure the track current and stray current leakage point, [PERSON] [45] proposed a magnetostrictive current sensor. FEM simulation is performed to improve the sensitivity and accuracy of the sensor. The adjustable coil design is used to reduce the problem of reduced linear range caused by increased sensor sensitivity. Studies have shown that when the sensor measures a current from 0 to 500 A, the sensitivity of the sensor is 0.391 u/A, and when the current is 500 to 1000 A, the sensitivity of the sensor is 0.418 u/A. High sensitivity can be achieved while measuring a large range, which can be used to monitor stray current leakage. Another way of detecting AC stray current corrosion is by measuring the corrosion rate. This method uses a resistance probe; the pipeline is connected to one probe, and a probe that is not connected to the pipeline is set as a reference, and the corrosion rate is determined using the following formula:  
\[C_{t}=C_{i}\frac{R_{\text{ci}}R_{\text{rt}}}{R_{\text{ri}}R_{\text{ct}}} \tag{1}\]  
\[V_{\text{corr}}=\frac{d_{ct}}{d_{t}} \tag{2}\]  
where Ct is the thickness before probe corrosion, Rci is the original resistance of the probe, and Rri is the reference resistor. Rrt is the reference probe's resistance at time t. Rct is the corrosion probe's resistance at time t. Resistance probe technology provides continuous corrosion data and can provide real-time monitoring. Additionally, its principle is simple, it is easy to operate, and has strong applicability. [PERSON] [46] used a thin film resistance probe to measure the corrosion rate. The probe loses weight after AC corrosion, and the steel plate resistance changes. The experimental data are recorded by a monitor. Afterward, these data are delivered to specialized analysis software, such as the M-Report processing software ([[https://www.metricorr.com/pipeline/](https://www.metricorr.com/pipeline/)]([https://www.metricorr.com/pipeline/](https://www.metricorr.com/pipeline/)) (accessed on 10 January 2007)) of Metricorr Company. Compared to the measurement of DC stray currents, the standards for measuring AC stray currents are not as uniform. AC voltage is used as the interference criterion in SY/T0087.1-2006 [47] in China. On the other hand, GB50698-2011 [48] also uses AC current density as the AC interference criterion; the reason is that the SY/T0087.1-2006 standard is largely based on data set by foreign standards and has not been modified according to national conditions. In addition, many investigations indicate that when the AC voltage is relatively large, the AC density is not necessarily large, and the pipeline corrosion is not necessarily serious; therefore, there is still some controversy over using the AC voltage as a criterion for judging the AC stray current. Internationally, German standards DIN50925 [49], CEN/TS15280 [50], ISO 15589-1-2003 [51], and EN1295 [52], etc., have usually been used as the standards for AC stray current. The evaluation standard of stray current serves as a reference. The criteria for judging stray currents are mainly AC current density and DC current. The ratio of current density and AC current to DC current are evaluated; thus, these criteria are relevant. The core is the parameter of alternating current density [53; 54; 55; 56]. AC current density is selected as the main evaluation basis for the magnitude of AC stray current interference, and AC interference voltage is used as the basis for auxiliary evaluation. At present, AC current density is commonly used at home and abroad to detect AC stray currents.