Galvanic CP Design: Principles and Procedures Across Structures
Core principle
A galvanic (sacrificial anode) system protects a structure by coupling it to a more active metal. The anode's open-circuit potential must be sufficiently negative relative to the structure's protection criterion, and the driving voltage must overcome polarization plus circuit resistance. Design always starts from the protection criterion (typically −0.85 V vs Cu/CuSO4 for steel in soil or water, or 100 mV of cathodic polarization), then works backward to current demand, anode mass, and anode count.
Design sequence
Estimate wetted/coated surface area and current density (i). Current demand I = i × A. Then apply coating breakdown and safety factors. Anode resistance R (Dwight or McCoy formulas for vertical/horizontal anodes) sets output via Ohm's law: I_anode = (E_anode − E_protection)/R_total. Anode count = total current demand divided by allowable output per anode, then check mass for design life using Faraday's law and anode efficiency.
Structure-specific considerations
Buried pipelines: distributed bracelet or can anodes, attenuation over coated pipe, interference with foreign structures. Offshore jackets: anode arrays sized for splash-zone and submerged areas, allowance for storm damage. Tank bottoms and internal surfaces: anode spacing for current throw and shielding. Reinforced concrete: high resistivity demands dense anode spacing.
Common traps
Confusing open-circuit vs closed-circuit anode potential; forgetting coating breakdown increases current demand over life; using anode efficiency near 100%; ignoring anode passivation; and mixing design life of anode mass with structural design life. Watch unit consistency (mA vs A, m² vs ft²).
Rule of thumb
For a coated buried steel structure, size anode mass for end-of-life current demand (coating degraded), not initial demand. Always verify both current output capability AND mass for life; the governing constraint is the larger of the two.