Alternating Current Interference as a Plausible Dominant Factor Affecting Corrosion Risk in a Mixed Steel/Polyethylene Urban Gas Distribution Pipeline: A Field Case Study

Mixed steel/polyethylene gas distribution pipelines are increasingly used in congested urban environments where conventional layouts are restricted by existing underground utilities, safety constraints, and site-specific construction conditions. In such systems, buried steel transition sections may become particularly vulnerable to electrical perturbation and corrosion, especially when installed near electrified transport infrastructure. This paper presents a field case study on a recently installed mixed steel/polyethylene gas distribution pipeline located on Lunca Street, Petroșani, Romania, approximately parallel to an electrified railway. Electrical and electrochemical investigations were carried out eight months after installation and included 24 h monitoring of pipe-to-soil potential versus Cu/CuSO4, 24 h monitoring of alternating current pipe-to-soil voltage, mixed alternating current and direct current signal visualization, and coating insulation resistance measurements. The results showed that alternating current pipe-to-soil voltage was present at all monitored points, with weighted mean values ranging from 0.41 to 1.23 Vrms, while pipe-to-soil potential values ranged from −0.120 to −0.238 V versus Cu/CuSO4. Although the measured average coating insulation resistance remained relatively high, the combined electrical and electrochemical data indicate that alternating current interference associated with the nearby electrified railway is the most plausible dominant contributing source of the recorded electrical perturbation. Within the analyzed site perimeter, no other nearby electrical infrastructures with comparable interference potential were identified. The highest alternating-current exposure and the least favorable electrochemical values were recorded on the longer metallic segment, showing that metallic length and local configuration strongly influenced the severity of the effect. A mitigation strategy based on polarized electrical decoupling and dedicated grounding is proposed as a practical means of improving electrical safety and reducing corrosion risk in the exposed and buried steel sections.