Sacrificial Anode Service Life
How long a boat's zinc, aluminum, or magnesium anode lasts, by Faraday's law.
Example
You enter
- Anode material zinc
- Anode net mass (lb) 5
- Protective current draw (A) 0.15
- Utilization factor (0-1, ~0.85) 0.85
You get
- Life (yr) 1.145
- Consumption rate 4.37 lb/yr
Checked against published worked examples from: Appalachian Underground Corrosion Short Course (Donlan / George).
Details, formula, and sources
Scopelife = anode mass x electrochemical capacity x utilization / (protective current x 8760 h). A 5 lb zinc (354 A-h/lb) at 0.85 utilization drawing 0.15 A lasts about 1.1 years -- the annual haul-out replacement. Aluminum (Al-Zn-In, 907 A-h/lb design) of equal mass lasts far longer per amp and works in brackish water where zinc passivates, which is why it has largely replaced zinc. The protective current depends on wetted area, coating, and water; measure it with a reference electrode. Replace at about half consumed. A planning estimate; ABYC E-2 and a corrosion survey govern.
Formula or tablelife_hours = anode_mass_lb x capacity_Ah_per_lb x utilization / current_A; life_years = life_hours / 8760. Capacity: zinc 354, aluminum (Al-Zn-In) 907, magnesium 500 A-h/lb.
Source and editionSacrificial-anode life by Faraday's law of electrolysis (ABYC E-2 cathodic protection; DNV-RP-B401 anode capacities), by name; a corrosion survey and reference-cell reading govern.
Free accessFaraday's law is public; the electrochemical capacities are published anode-material properties. The protective current comes from a reference-electrode measurement of the boat.
What governsEstimate. AHJ and licensed professional govern.
API field namesanode_material, anode_mass_lb, current_draw_a, utilization_factor, life_years, consumption_lb_per_year
- Electrochemical capacity zinc ~354, aluminum (Al-Zn-In) ~907, magnesium ~500 A-h/lb (zinc and aluminum are the DNV-RP-B401 seawater design values; magnesium is the common design figure)DNV-RP-B401 / ABYC E-2
- Utilization factor ~0.85 for a slender standoff anode (the anode is ineffective before fully consumed)DNV-RP-B401