Technical maintenance

Electrolysis on board a catamaran

Electrolysis (or to be more precise, galvanic corrosion) is something that attacks boats via their various metal fittings.

March 3, 2023

Electrolysis (or to be more precise, galvanic corrosion) is something that attacks boats via their various metal fittings. It is an electro-chemical phenomenon that results in rapid oxidation of the hull, which can eventually lead to water ingress.

Electrolysis is the reaction caused by two different metals coming into contact with each other in a corrosive atmosphere, in this case sea water. It is essential to regularly check for this phenomenon on your boat. A visual and physical check should be carried out every 6 months of each through-hull or skin fitting.


Consequences of electrolysis:

Electrolysis exacerbates the aging of a boat

  • Blisters in the paint below the waterline.
  • Metal erosion and pitting on the submerged areas.
  • Whitish deposits resembling calcium that settles around the zinc anodes or any blisters.
  • Blistering along weld lines.
  • Depleted zincs or anodes
  • Zincs that still look like new (an anode may look new but can have been completely consumed)

Note: These problems are usually more developed toward the back of the hulls where electrical activity is often higher. To check an anode, weigh it and compare with information in the supplier’s manual.

Example of anodes and zinc protection blocks


Examples of electrolysis:

Rudder bearing corroded by electrolysis


Hull being eaten away by electrolysis

Depleted anodes

Causes of the problem:

  • Deterioration of zinc/aluminum anodes.
  • Stray electrical current from shoreside installations.
  • Stray electrical currents from onboard equipment such as the windlass, radio equipment, starter motors and other power-hungry electrical devices.
  • Incorrectly wired or poorly grounded onboard equipment.
  • Incorrect cable sizing for grounding or wiring of onboard equipment.
  • Zincs or anodes that have been painted over.


How to deal with the problem:

  • Once the physical causes of the problem have been found and eliminated, the process of repairing these paint systems is relatively simple.
  • A repair can be made to small areas.
  • If the affected areas are very large, it may be necessary to haulout and repaint the entire catamaran.



  • Do not paint metallic parts under where the anodes are fitted
  • Depending on the waters on which you sail, different types of anodes are used:

– Salt water: zinc anode.

– Fresh water: magnesium anode.

– Brackish water: aluminum anode.


If you have any questions, please contact your Lagoon dealer or complete this contact form.