Galvanic (Sacrificial) Anode Systems
How galvanic systems work
A galvanic anode protects because its open-circuit potential is more negative than the structure's. Magnesium (roughly −1.5 to −1.7 V CSE), zinc (about −1.1 V CSE), and aluminum alloys (used mainly in seawater and brines) are the three families. The driving voltage is small - typically 0.3 to 0.9 V - so anode-to-earth resistance controls output far more than in impressed current systems.
Selection and design
Soil resistivity drives the choice. Magnesium is used in moderate to low resistivity soils (commonly below a few thousand ohm-cm); zinc is used in low-resistivity soils, freshwater, and marine environments and is preferred where hydrogen evolution or overprotection must be avoided; aluminum is for seawater and saline muds. Magnesium anodes are normally installed in a gypsum-bentonite backfill, which lowers resistance and keeps the anode active. Design steps: estimate current requirement, calculate anode resistance (Dwight vertical formula, R = (ρ/2πL)[ln(8L/d) − 1]), compute output from I = ΔE/R, then size the anode for life from its current capacity in ampere-hours and the expected current.
Traps
Using magnesium on well-coated, high-resistivity lines and overprotecting them; forgetting the backfill; treating the resistance formula as exact (it is an approximation); ignoring that anode output falls as the structure polarizes and the driving voltage shrinks; confuse anode consumption rate with theoretical capacity.
Worked example
A 7.7 kg magnesium anode with a practical capacity of about 500 A·h/lb (≈1100 A·h/kg) delivers 50 mA. Ampere-hours available ≈ 8,500. Life = 8,500 / 0.050 = 170,000 hours ≈ 19 years, before any efficiency correction.