What Is Cathodic Protection?
Cathodic protection (CP) is an electrochemical technique that prevents corrosion of buried or submerged steel by making the steel structure the cathode of an electrochemical cell. By shifting the steel’s potential to a more negative value (typically -850 mV or more negative vs. Cu/CuSO4 reference electrode), the anodic dissolution reaction that causes corrosion is suppressed. CP works alongside protective coatings as the primary corrosion prevention system for pipelines, offshore platforms, storage tanks, and marine structures.
There are two CP methods: sacrificial anode (galvanic) and impressed current (ICCP). Selection depends on the structure size, coating quality, soil/water resistivity, and design life.
| Parameter | Sacrificial Anode (SACP) | Impressed Current (ICCP) |
|---|---|---|
| Driving force | Galvanic potential difference | External DC power supply (rectifier) |
| Anode materials | Zinc, magnesium, aluminum alloys | High-silicon cast iron, MMO/Ti, graphite |
| Voltage output | 0.2-1.0 V (natural) | 1-50 V (adjustable) |
| Current output | Low (mA range per anode) | High (up to hundreds of amps) |
| Best for | Well-coated pipelines, short structures, offshore | Long pipelines, poorly coated structures, large tanks |
| Soil resistivity | Low-medium (< 5000 ohm-cm for Zn, < 10000 for Mg) | Any resistivity |
| Maintenance | Minimal (anode replacement every 10-30 years) | Regular (rectifier monitoring, anode bed inspection) |
| Power required | None | Continuous AC power supply |
| Design standard | NACE SP0169, ISO 15589-1/2, DNV-RP-B401 | NACE SP0169, ISO 15589-1/2 |
Sacrificial Anode Systems (SACP)
SACP uses anodes made of metals more active (electronegative) than steel—zinc, magnesium, or aluminum alloys. When connected to the pipeline, these anodes corrode preferentially, supplying protective current to the steel.
Key applications:
- Subsea pipelines: Aluminum alloy bracelet anodes (Al-Zn-In) welded to the pipe at regular intervals
- Well-coated onshore pipelines: Magnesium anodes in backfill packages, installed every 300-1000 m
- Offshore structures: Aluminum or zinc anodes welded to jacket legs and nodes
- Underground storage tanks: Zinc or magnesium anode beds
SACP is self-regulating: current output increases at coating defects (holidays) where bare steel is exposed, and decreases where the coating is intact.
Impressed Current Systems (ICCP)
ICCP uses an external DC power source (transformer-rectifier) to force current from inert or semi-inert anodes through the soil or water to the pipeline. The rectifier converts AC power to DC and allows operators to adjust the output voltage and current.
Key applications:
- Long-distance transmission pipelines (hundreds of kilometers)
- Pipelines with deteriorated or poor-quality coating
- High-resistivity soil environments where sacrificial anodes cannot deliver sufficient current
- Large tank farms and terminal facilities
ICCP ground beds can be shallow (horizontal trenches with distributed anodes) or deep (vertical boreholes with centralized anodes at 15-100 m depth). Deep anode beds are preferred in congested areas to minimize stray current interference with adjacent structures.
CP Monitoring
Pipeline operators must verify that the CP system maintains adequate protection by measuring pipe-to-soil potentials using Cu/CuSO4 reference electrodes at test stations installed along the pipeline route. The standard protection criterion is -850 mV (instant-off) per NACE SP0169. Surveys are conducted annually or continuously using remote monitoring systems.
CP is a required part of the overall pipe coating and protection strategy for buried and submerged pipelines.
Leave a Comment
Have a question or feedback? Send us a message.