Why Cathodic Protection Fails: Lessons from the Field
Cathodic Protection (CP) systems are essential for extending the life of critical infrastructure such as pipelines, storage tanks, and marine structures. However, field experience and industry case studies reveal that CP systems can and do fail, often for preventable reasons. Understanding these failure modes is crucial for asset owners, engineers, and inspectors seeking to maximize asset integrity and safety.
Common Causes of Cathodic Protection Failure
Poor System Design
Inadequate current output, improper anode selection, or failure to account for structure geometry and soil resistivity can leave parts of an asset underprotected.
Uneven current distribution is a frequent issue, with some areas overprotected (risking hydrogen embrittlement or coating disbondment) and others underprotected (leading to localized corrosion).
Improper Installation
Shallow or poorly placed anode beds, substandard connections, or incorrect cable routing can cause shorts, insufficient protection, or rapid anode consumption.
Flexible anode systems, for example, may fail due to cable breakage or electronic shorting between the anode and the protected structure.
Lack of Monitoring and Maintenance
Without routine potential surveys and system checks, issues such as broken connections, depleted anodes, or interference can go undetected until significant damage occurs.
Inaccurate or infrequent monitoring can lead to underprotection, especially if potential readings are not taken at close intervals or with proper equipment.
Environmental and Operational Changes
Changes in soil resistivity (e.g., due to seasonal moisture variation), introduction of new electrical infrastructure (such as railways or power lines), or construction activities can alter the effectiveness of an existing CP system.
High-resistivity soils, in particular, require special design considerations to ensure adequate current reaches all protected surfaces.
Coating Breakdown and Shielding
CP is most effective when used in conjunction with high-quality coatings. If coatings fail or disbond (often due to excessive polarization or poor application), the exposed metal increases the current demand on the CP system, potentially overwhelming it.
Certain coatings, especially solid film-backed types, can shield the underlying metal from CP current, rendering the system ineffective in those areas.
Stray Currents and Electrical Interference
Stray DC currents from nearby railways, power lines, or other infrastructure can interfere with CP operation, causing corrosion at unintended locations.
These currents may enter and leave the pipeline at different points, leading to unpredictable corrosion patterns unless properly mitigated.
Case Example: Stray Current Interference from an LRT System.
Diagnostic Field Testing
Conducting stray current diagnostics at a local transformer rectifier unit (TRU) on a buried pipeline beneath an active light rail line. Testing confirmed interference from the railway’s DC traction system, which had not been accounted for in the original CP design.
With direct bonding no longer permitted, a localized CP retrofit using galvanic anodes and isolation components was installed to mitigate interference and restore protection levels.
Lessons Learned and Technical Recommendations
1. Comprehensive CP System Design
Incorporate detailed site assessments that consider all present and future sources of AC and DC interference, including mass transit systems, high-voltage power lines, and electrified infrastructure.
Utilize advanced modeling tools (e.g., boundary element or finite element analysis) to simulate current distribution, optimize anode configuration, and evaluate interference scenarios during the design phase.
Account for variable soil resistivity profiles, depth of cover, and complex geometries to ensure robust current delivery.
2. High-Quality Installation Practices
Adhere to recognized standards such as NACE SP0169 and ISO 15589-1 for CP system installation.
Verify all electrical connections using appropriate testing (e.g., continuity checks, resistance measurements) and document baseline performance metrics prior to system commissioning.
Conduct pre-energization verification of anode bed performance to ensure effective current output and distribution.
3. Routine Monitoring and Maintenance
Implement regular pipe-to-soil potential surveys in accordance with established intervals, and deploy close interval potential surveys (CIPS) in suspected interference zones to precisely locate underprotected areas.
Integrate remote monitoring and data logging systems for critical or remote assets to enable continuous performance tracking and rapid identification of anomalies.
Establish a maintenance program that includes timely repair of damaged CP components and periodic re-evaluation of system effectiveness.
4. Coating Integrity and Compatibility
Specify high-performance coatings suitable for the service environment and compatible with CP systems. Avoid coatings known to cause cathodic shielding (e.g., certain polyolefin-based materials).
Enforce rigorous surface preparation and coating inspection protocols per NACE/SSPC standards to ensure long-term adhesion and effectiveness.
Address coating holidays and defects immediately to minimize current demand and reduce the risk of localized corrosion.
5. Stray Current Detection and Mitigation
Identify all potential sources of stray DC during the design stage through interference testing and stakeholder coordination with utility and transit operators.
Where bonding is not permitted, explore alternative mitigation strategies such as:
Installation of sacrificial or impressed current anodes at interference-prone locations.
Use of dielectric isolation joints and polarization cells to control current paths.
Deployment of monitoring test stations equipped for stray current measurement and trend analysis.
How Zenith Integrity Can Help
At Zenith Integrity, we support asset owners and operators with practical, field-tested solutions for cathodic protection system performance and reliability. Our team has experience working with infrastructure adjacent to electrified transit corridors, where stray current interference poses unique challenges to pipeline and structural integrity.
We provide end-to-end support for CP systems, from initial assessments and system design through to installation oversight and performance monitoring. Our approach emphasizes early identification of risks such as coating degradation, shielding effects, and external electrical interference—including DC traction systems—and the implementation of appropriate mitigation measures tailored to site conditions.
Through collaboration with clients and stakeholders, including transit agencies and utilities, we’ve developed and implemented effective mitigation strategies such as isolation solutions, localized CP enhancements, and long-term monitoring programs.
Our goal is to help extend asset life and reduce risk through technically sound, cost-effective corrosion control strategies grounded in real-world application.