AC Interference on Cathodically Protected Pipelines
AC interference occurs when nearby AC power systems induce or transfer alternating voltage and current onto a metallic structure or corrosion-control system.
Three Coupling Mechanisms
| Mechanism | Typical Situation | Why It Matters |
|---|---|---|
| Capacitive coupling | Aboveground pipe near energized conductors, especially during construction. | Can create shock hazard before the pipe is grounded or buried. |
| Inductive coupling | Pipeline parallel to AC transmission lines. | Can induce steady-state AC voltage along the pipeline. |
| Conductive coupling | Fault current or ground potential rise near towers or substations. | Can create high transient voltages and coating stress. |
Safety Comes First
AC voltage on a pipeline is not just a corrosion topic. It can be a personnel-safety hazard at test stations, valves, insulating joints, casings, rectifiers, and other appurtenances. Testing and mitigation should consider touch voltage, step voltage, fault conditions, and whether decoupling devices are needed to preserve CP while safely passing AC.
AC Corrosion Concern
AC corrosion risk depends on more than AC voltage alone. Important factors include AC current density at coating defects, DC CP level, soil resistivity, defect size, coating condition, grounding, and time. A pipeline can meet a DC CP criterion and still require AC evaluation if AC current density is high at small defects.