Cathodic Protection for Buried Pipelines
Auto-filled from coating type. Override if known.
Typical: 500-5,000 (low), 5,000-10,000 (moderate), >10,000 (high)
Typical: 1.0-2.0 coated, 1.5-3.0 bare pipe
Mg HP: -1.75V, Mg Std: -1.55V, Zn: -1.10V (vs Cu/CuSO4)
Typical 0.85 for packaged anodes (NACE SP0572)
Understand cathodic protection theory, galvanic series, NACE standards, and soil resistivity effects
Anode sizing for buried pipelines requires calculating total current demand based on pipeline surface area, coating efficiency and degradation, and required current density. The number of anodes is determined by the greater of: (1) current output capacity per anode divided into total current demand, and (2) total amp-hours needed over the design life divided by each anode's capacity. Anode resistance is calculated using the Dwight formula for soil resistivity.
Magnesium anodes have a higher driving voltage (-1.55 to -1.75V vs Cu/CuSO4) and higher capacity (1,100 Ah/lb), making them suitable for higher-resistivity soils (1,000-10,000 ohm-cm). Zinc anodes have lower driving voltage (-1.10V) and capacity (370 Ah/lb) but are more efficient in low-resistivity soils (<1,000 ohm-cm) and marine environments.
NACE SP0169 specifies a minimum protection potential of -850 mV vs Cu/CuSO4 reference electrode (instant-off potential). Alternative criteria include 100 mV of cathodic polarization from the native corrosion potential. The pipeline-to-soil potential must be maintained more negative than -850 mV at all points along the pipeline.
Coating degradation increases the bare (unprotected) area over time, requiring more cathodic protection current. FBE coatings typically start at 98% efficiency and degrade 0.5-1% per year. The average bare area over the design life is used for anode sizing to ensure adequate protection throughout the system's service life.