Buried pipelines near DC-electrified railways and metro systems experience dynamic stray-current (dynamic DC) interference whose anodic and cathodic half-waves are typically asymmetric. Qin et al. (2020) showed that negatively asymmetric dynamic DC interference induces localized (pitting) corrosion of X70 pipeline steel in a simulated soil environment, and proposed — on indirect electrochemical evidence — that pits initiate by breakdown of a passive film that forms in the high-pH environment created by the dominant cathodic current. The initiation mechanism has not been directly visualized or quantitatively modeled, the true interfacial pH and its spatial gradients are unknown because near-surface extraction measurements carry systematic errors, and transfer of the laboratory findings to heterogeneous real soils is unresolved. Follow-up work through 2026 has clarified a conditional pitting mechanism in bulk simulated solutions, measured and modeled interfacial pH under static cathodic protection, and produced rate-kinetic models of dynamic DC corrosion, but the coupled initiation problem — waveform to interfacial chemistry to film breakdown to pit nucleation, in porous media and real soils — remains open.

Background

Stray current corrosion is the accelerated electrochemical corrosion suffered by buried metal structures when an external direct current (DC), leaked from systems such as DC-electrified railways or metro traction systems, enters and leaves the structure through the surrounding soil; the current leaves the metal at anodic spots, where iron dissolves. Because train traffic varies with time, the interference is dynamic: the pipe-to-soil potential fluctuates with train operating periods (tens to hundreds of seconds), and the positive and negative half-waves generally have unequal amplitudes and durations — the interference is asymmetric. When the negative (cathodic) half-wave dominates, the current entering the steel surface drives oxygen reduction and hydrogen evolution, generating hydroxide ions that raise the local pH; iron can then passivate, i.e. become covered by a thin, protective oxide passive film that suppresses dissolution. A leading hypothesis for stray-current pitting is that the subsequent anodic half-waves break down this high-pH passive film at weak points, nucleating stable pits, but this hypothesis had been supported only by indirect electrochemical evidence (cyclic voltammetry, a potential-sweep measurement of the electrode's current response; Mott–Schottky capacitance analysis of the passive film's semiconductor properties; and real-time potential monitoring) rather than direct observation. Pipelines are normally protected against soil corrosion by cathodic protection (CP) — imposing an external cathodic current on the structure to suppress metal dissolution — and stray DC superimposes on this protection. X70 is a high-strength low-alloy steel widely used for oil and gas pipelines. A further difficulty is that the pH actually prevailing at the steel/electrolyte interface — the quantity that controls passivity — is hard to measure: in the reference study, the in-situ near-surface pH was obtained by withdrawing solution through an extraction tube ending roughly 1 mm from the steel surface, so the measured value is diluted and spatially smeared relative to the true interfacial pH and its gradients. Finally, laboratory media (quartz sand fully saturated with one simulated soil solution) idealize real soil, whose varying water content, resistivity, composition, and oxygen transport control the local electrochemistry. Before the source study, Qian and Cheng (2020) had shown that dynamic DC interference accelerates corrosion of X52 pipeline steel in bicarbonate solution, and the source group had documented dynamic-DC corrosion rates of X70 (Qin et al. 2018–2023); adjacent literature on AC (alternating current) corrosion shows that film-based corrosion mechanisms under fluctuating potentials (oscillation, passivation-damage, and alkalization models) remain actively contested, which frames why direct evidence rather than inference alone is demanded here. Quantitative spatio-temporal modeling of when and where initiation occurs as a function of the interference waveform parameters, and transfer of a mechanism obtained in simplified saturated-sand/simulated-solution media to heterogeneous real soils with varying water content, resistivity, composition, and oxygen transport, remain open.

Problem Statement

Establish the mechanism by which localized (pitting) corrosion initiates on X70 pipeline steel under negatively asymmetric dynamic DC (stray-current) interference in a simulated soil environment, as posed by Qin et al. (2020): determine, with direct evidence going beyond the indirect electrochemical inference available in that study (real-time DC potential monitoring, in-situ near-surface pH measurements, cyclic voltammetry, and Mott–Schottky analysis), whether and how the breakdown of the passive film formed on the steel surface exposed to the high-pH conditions generated by the negatively asymmetric DC current induces pit initiation. An answer that leaves the causal chain between the asymmetric waveform, the interfacial chemistry, the film state (passive film versus corrosion-product film), and pit nucleation undetermined does not resolve the question.

References

  1. Qin, Huimin; Du, Yanxia; Lu, Minxu; Sun, Xuepeng; Zhang, Yi. 'Experimental study on the corrosion behavior of X70 steel under asymmetric dynamic DC interference.' Materials and Corrosion 71(11) (2020) 1856–1871. DOI 10.1002/maco.202011725. https://doi.org/10.1002/maco.202011725
  2. Qian, Shan; Cheng, Y. Frank. 'Corrosion of pipelines under dynamic direct current interference.' Construction and Building Materials 261 (2020) 120550. DOI 10.1016/j.conbuildmat.2020.120550. https://doi.org/10.1016/j.conbuildmat.2020.120550
  3. Qin, Huimin; Zhang, Tao; Du, Yanxia; Zhang, Lei; Zhang, Yuxing; Liu, Hui. 'Experimental study of dynamic DC stray current corrosion of X70 steel.' Materials and Corrosion 74 (2023) 33–52. DOI 10.1002/maco.202213415. https://doi.org/10.1002/maco.202213415
  4. Yu, Xin; Yuan, An; Cui, Gan; Liu, Jianguo; Xing, Xiao; Li, Zili. 'Pitting corrosion behavior of X70 steel in weakly alkaline solution under dynamic DC interference.' Construction and Building Materials 384 (2023) 131406. DOI 10.1016/j.conbuildmat.2023.131406. https://doi.org/10.1016/j.conbuildmat.2023.131406
  5. Yu, Xin; Liu, Jianguo; Jiang, Hailei; Cui, Gan; Luo, Xiaoming. 'The formation of pitting under dynamic DC interference: Corrosion product film defects or passive film induction.' Corrosion Science 231 (2024) 111961. DOI 10.1016/j.corsci.2024.111961. https://doi.org/10.1016/j.corsci.2024.111961
  6. Yu, Xin; Liu, Jianguo; Mao, Jialin; Cui, Gan; Luo, Xiaoming. 'A computational study on the kinetic process of metal corrosion under dynamic DC interference.' Construction and Building Materials 443 (2024) 137746. DOI 10.1016/j.conbuildmat.2024.137746. https://doi.org/10.1016/j.conbuildmat.2024.137746
  7. Li, Xiang; Wang, Jun; Qian, Wenbin; Xu, Haohao; Du, Yanxia; Duan, Rujiao; Liu, Jianguo; Cui, Gan. 'Corrosion Kinetics and Prediction Modeling of X70 Steel Under Dynamic DC Interference.' Materials and Corrosion 76(5) (2025) 650–660. DOI 10.1002/maco.202414679. https://doi.org/10.1002/maco.202414679
  8. Yang, Chao; Dong, Xingxin; Yang, Wendi; Cui, Gan; Liu, Jianguo. 'Influence and Mechanism of Potential Fluctuation Parameters on X70 Steel Corrosion under Dynamic DC Interference.' Journal of Materials Engineering and Performance (2026). DOI 10.1007/s11665-026-14542-2. https://doi.org/10.1007/s11665-026-14542-2
  9. Du, Yanxia; Wierzbinski, Emil; Waldeck, David H. 'Research on the difference of characteristics at Steel/electrolyte interface under cathodic protection and in High-pH alkaline solution.' Journal of Electroanalytical Chemistry 925 (2022) 116878. DOI 10.1016/j.jelechem.2022.116878. https://doi.org/10.1016/j.jelechem.2022.116878
  10. Martinelli-Orlando, Federico; Mundra, Shishir; Angst, Ueli M. 'Cathodic protection mechanism of iron and steel in porous media.' Communications Materials 5 (2024) 15. DOI 10.1038/s43246-024-00454-y. https://doi.org/10.1038/s43246-024-00454-y
  11. Attarchi, Mehdi; Ormellese, Marco; Brenna, Andrea. 'Evaluation of pH variation in cathodic protection conditions by FEM simulation.' Materials and Corrosion 75(3) (2024) 322–334. DOI 10.1002/maco.202213684. https://doi.org/10.1002/maco.202213684
  12. Attarchi, Mehdi; Ormellese, Marco; Brenna, Andrea. 'pH Measurement During Cathodic Protection and DC Interference.' CORROSION 2021 Conference and Exposition, paper no. 16766 (AMPP, 2021), pp. 1–9. DOI 10.5006/c2021-16766. https://doi.org/10.5006/c2021-16766
  13. Ormellese, Marco; Beretta, Silvia; Brugnetti, Fabio; Brenna, Andrea. 'Effects of non-stationary stray current on carbon steel buried pipelines under cathodic protection.' Construction and Building Materials 281 (2021) 122645. DOI 10.1016/j.conbuildmat.2021.122645. https://doi.org/10.1016/j.conbuildmat.2021.122645
  14. Ding, Jifeng; Liu, Xiaolong; Ma, Guangjiao; Guo, Yanming; Li, Xiangyang. 'Corrosion Behavior of Pipelines Influenced by Dynamic Stray Currents Derived from Underground Rail Infrastructure.' ACS Omega 11(5) (2026) 7475–7485. DOI 10.1021/acsomega.5c08604. https://doi.org/10.1021/acsomega.5c08604
  15. Wang, Chengtao; Qin, Guojin. 'Corrosion of underground infrastructures under metro-induced stray current: A review.' Corrosion Communications 14 (2024) 23–38. DOI 10.1016/j.corcom.2023.08.005. https://doi.org/10.1016/j.corcom.2023.08.005
  16. Liang, Haiming; Wu, Yuxi; Han, Bin; Lin, Nan; Wang, Junqiang; Zhang, Zheng; Guo, Yanbao. 'Corrosion of Buried Pipelines by Stray Current in Electrified Railways: Mechanism, Influencing Factors, and Protection.' Applied Sciences 15(1) (2025) 264. DOI 10.3390/app15010264. https://doi.org/10.3390/app15010264
  17. Zhang, Xinyan; Guo, Guangwei. 'Simulation of Influencing Factors of Buried Pipeline Interfered by Subway Stray Current.' Highlights in Science, Engineering and Technology 35 (2023) 140–148. DOI 10.54097/hset.v35i.7046. https://doi.org/10.54097/hset.v35i.7046
  18. Liang, Yi; Du, Yanxia; Tang, Dezhi; Chen, Le; Zhang, Lei; Qiao, Lijie. 'Research on AC corrosion behavior and corrosion product film evolution of X70 steel under the combined action of AC interference and CP.' Corrosion Science 197 (2022) 110085. DOI 10.1016/j.corsci.2022.110085. https://doi.org/10.1016/j.corsci.2022.110085