Cathodic protection is a well-established method for controlling corrosion on buried metallic infrastructure. When a CP system is correctly designed and operating within accepted potential criteria, an asset should be protected. However, there is a category of corrosion risk that conventional CP criteria do not fully address: AC-induced corrosion caused by electromagnetic interference from nearby electrical infrastructure.
AC interference is a growing concern for operators of buried pipelines and other metallic assets located within corridors shared with high-voltage power lines, HVAC cables, and electrified rail traction systems.
What Is AC Interference and How Does It Differ from DC Stray Current?
AC interference occurs when alternating current from an adjacent electrical source induces voltage and current onto a buried pipeline or metallic structure. Inductive coupling is the most common mechanism: as alternating current flows through an overhead power line or buried cable, it generates a fluctuating magnetic field that induces a corresponding voltage along any parallel or crossing metallic conductor in its vicinity.
DC stray current interference, typically from impressed current CP systems or electrified rail networks, is directional and causes corrosion at points where the current leaves the structure and enters the surrounding soil. AC interference operates differently. The induced current alternates at 50 Hz, meaning the conventional understanding of anodic and cathodic sites does not apply in the same way. Despite this, AC-induced corrosion is a well-documented and significant risk.
A further distinction is that DC stray current tends to affect larger sections of a structure, whereas AC-induced corrosion is typically highly localised; concentrated at coating defects or areas of disbonded coating where the pipeline is directly exposed to soil.
Why Well-Protected Assets Can Still Suffer AC-Induced Corrosion
A pipeline can meet the standard CP criterion of a pipe-to-soil potential more negative than -850 mV (Cu/CuSO₄) and still suffer significant corrosion damage due to AC interference. The reason is that AC-induced corrosion is not governed by the same electrochemical mechanisms that CP criteria are designed to address. At coating defects, the rapid alternating of anodic and cathodic half-reactions can disrupt the protective alkaline environment maintained by CP polarisation and drive localised metal dissolution that conventional potential measurements do not capture.
EN ISO 18086, the standard specifically addressing AC-induced corrosion of cathodically protected pipelines, defines risk in terms of AC current density at the metal surface rather than pipe-to-soil potential alone. The standard identifies an AC current density below 30 A/m² as low risk, with values above 100 A/m² representing a significant corrosion threat. Significant metal loss has been documented in fewer than five years where these thresholds are exceeded. Operators cannot rely solely on CP potential readings to confirm asset integrity in corridors with active AC interference sources. In addition to corrosion concerns, there are also important safety risks associated with AC interference, including steady-state voltage induction as well as fault current conditions that can give rise to hazardous touch and step voltages.
Sources of AC Interference
The main sources affecting buried infrastructure are:
High-voltage AC overhead transmission lines: The most common source. Long parallel runs between a pipeline and an overhead line create conditions for sustained inductive coupling, influenced by line voltage, current loading, phase configuration, soil resistivity and corridor geometry.
Buried HVAC and HVDC export cables: The growth of offshore wind and solar infrastructure has introduced new interference sources, including buried AC export cables. HVDC cables can be associated with DC stray current effects and require separate assessment.
Electrified rail traction systems: AC traction systems operating at 25 kV generate significant electromagnetic fields that can induce interference on buried assets in adjacent corridors.
Substations and switching equipment: Areas around substations may introduce both inductive and conductive interference, particularly in low-resistivity soils.
Survey Methods: Quantifying AC Interference
Accurately characterising AC interference requires a structured site investigation. Corrpro Europe’s engineering team uses a combination of field survey techniques and data logging to build a detailed picture of interference conditions along the affected corridor.
AC voltage and current density measurements provide the primary metric for assessing corrosion risk under EN ISO 18086. Because interference levels vary with power line loading, traction system operation, and seasonal ground conditions, high-resolution data logging is used to capture both steady-state and fluctuating conditions over time. Electromagnetic field mapping identifies the proximity and operational load of power infrastructure, supporting the interference modelling process.
DCVG and close interval potential surveys (CIPS) are used alongside AC interference assessment to establish coating condition and identify defects where AC current density will be highest. Corrpro Europe also uses industry-standard computer modelling to simulate interference levels under both normal and fault conditions, enabling the pipeline’s exposure to be assessed systematically and mitigation measures to be designed for worst-case scenarios.
Mitigation Strategies and Compliance with EN ISO 18086
Once interference levels are quantified, a mitigation strategy is designed using computer modelling to reduce AC current density to below the thresholds defined in EN ISO 18086. Corrpro Europe designs and implements site-specific solutions using a combination of the following:
Zinc ribbon grounding: Installed parallel to the pipeline and connected at regular intervals, zinc ribbon provides a low-resistance path to earth for AC-induced current, reducing AC voltage on the pipeline while also contributing galvanic protection at coating defects.
Ground rods and gradient control mats: Used at locations where AC interference may present a safety hazard. The risk to personnel depends on touch and step voltages, soil resistivity, and exposure conditions rather than a fixed voltage threshold; however, elevated AC voltages can lead to hazardous body currents. Gradient control mats are installed to reduce touch and step potentials at above-ground pipeline features such as valve stations and test points, thereby improving personnel safety.
Solid-state decouplers: Decoupling devices allow AC to discharge safely to a grounding system while maintaining DC isolation of the pipeline. Corrpro Europe supplies and installs Dairyland solid-state decouplers, which present low impedance to AC while blocking DC current flow, preserving CP system integrity.
Each system is engineered to the specific interference profile identified during the assessment phase. Full details of Corrpro Europe’s AC mitigation capabilities and materials are available in the AC Interference and Mitigation datasheet.
Ongoing Monitoring to Verify Performance
Installing a mitigation system is not the end of the process. EN ISO 18086 requires that mitigation performance is verified through ongoing monitoring. Corrpro Europe provides post-mitigation monitoring including periodic AC voltage measurements, long-term data logger deployment, and regular review of AC and DC parameters. Changes in power infrastructure loading or deterioration of grounding materials can affect system performance over time; regular monitoring ensures any changes are identified and addressed promptly.
Corrpro Europe’s AC Interference Capability
Corrpro Europe offers a complete end-to-end service for AC interference assessment and mitigation. Our AMPP and ICorr-certified engineers carry out site investigations, interference studies, and computer modelling to quantify risk and develop compliant mitigation strategies. We supply high-quality mitigation materials and provide on-site supervision and commissioning support throughout the UK, Europe, Africa and Asia supporting operators across oil and gas, utilities, renewables, and transport infrastructure.
For a closer look at our AC mitigation materials and design approach, download the AC Interference and Mitigation datasheet. To discuss an assessment or mitigation project, contact the Corrpro Europe engineering team.
