DC Stray Current Interference
DC stray current interference occurs when direct current travels through unintended paths and enters or leaves a buried or submerged metallic structure.
Pickup and Discharge
Where DC current enters a structure from the electrolyte, that area is generally cathodic during the pickup condition. Where current leaves the structure and enters the electrolyte, corrosion risk increases because the discharge area behaves anodically.
For field interpretation, the discharge location is usually the critical corrosion concern. The difficult part is that dynamic stray current may move the pickup and discharge locations over time.
Common Sources
- DC transit systems and rail return-current systems.
- Nearby impressed current CP systems.
- HVDC transmission systems.
- DC welding operations.
- Interference bonds, grounds, or isolation changes that alter current paths.
Detection Methods
Useful methods include time-based structure-to-electrolyte monitoring, line-current measurements, current-flow direction checks through bonds, exposure and mutual surveys, side-drain measurements, and correlation with the suspected source.
A single static reading is weak evidence for dynamic DC interference. The better evidence is correlation: the affected structure changes when the source changes.
Mitigation Concepts
- Reduce or relocate the interfering current source where practical.
- Use resistance bonds, drainage bonds, or controlled bonds when technically justified.
- Install galvanic anodes or impressed current distribution at discharge areas.
- Improve coating condition to reduce current pickup and discharge.
- Use electrical isolation carefully; isolation can help or worsen current-distribution problems depending on the circuit.