Galvanic Corrosion: How Cruisers Catch It Early
Galvanic corrosion eats boats slowly — bronze through-hulls dissolving, prop shafts pitting, swim ladders disappearing — until something fails dramatically. The trigger is dissimilar metals in seawater with a current path. Marinas with bad wiring are the most common cause. Detection is the difference between a boat repair and a boat tragedy.
The warning signs
- Zincs disappearing fast — replaced every few months instead of annually
- Pink/white deposits on bronze fittings (dezincification)
- Aluminum hardware (Wichard fittings, anodised) showing pitting
- Stainless rigging showing pitting at terminals
- Underwater metals (rudder shaft, prop, struts) showing rapid loss
Catch it before it costs
- Galvanic meter: Reads boat-to-water potential. Healthy bronze reads around -550 to -700 mV vs Ag/AgCl reference. More negative = anodic = corroding. Less negative = cathodic = OK.
- Visual zinc inspection: Every haul, photograph zinc condition. Track wear rate.
- Diver report: Have the diver photograph through-hulls and report rapid changes.
- Marina hopping: Symptoms after staying in a particular marina suggest marina electrical issue.
Prevention
- Galvanic isolator: Critical when on shore power. Blocks low-voltage galvanic current while passing fault current safely. USD 200–400 installed.
- Proper bonding: All underwater metals tied to a common bonding system with zinc anodes.
- Adequate anodes: Shaft, rudder, hull anodes sized properly. Replace before fully consumed.
- Watch your neighbours: A boat next to you with leaking electronics will eat your zincs too.
- Anchor more, marina less: Anchoring eliminates shore power as a corrosion source.
If you find active corrosion
Disconnect shore power immediately. Inspect for ground faults (megger test if possible). Check the boat's bonding system. Replace consumed anodes with appropriate-size fresh ones. If marina is the source, move berths or consider a different facility.
Frequently asked questions
How fast does it act?
Weeks to months in bad cases. Zincs that should last a year can be gone in a month.
Most common cause?
Marina shore power leakage to ground through neighboring boats.
Galvanic isolator necessary?
Strongly recommended for any boat that uses shore power.
Anodes: shaft, rudder, hull?
All needed. Shaft zinc most critical; rudder and hull anodes provide additional protection.
Cost of damage if missed?
Through-hull replacement, shaft replacement, hardware replacement — can run thousands.