What is galvanic corrosion
Galvanic corrosion is an electrochemical process that occurs when two different metals are in electrical contact and submerged in a conductive electrolyte, such as seawater. The less noble metal (anode) corrodes to protect the more noble metal (cathode).
On a boat, this can destroy propellers, shafts, stern drives, seacocks, lead keels, and any submerged metallic component. It is one of the most common and costly causes of damage to vessels, and many owners do not detect it until the damage is severe.
How it occurs on a boat
The galvanic cell
For galvanic corrosion to occur, three elements are needed:
- Two different metals (for example, a bronze propeller and a stainless steel shaft)
- Electrical contact between them (through the boat's structure or wiring)
- An electrolyte (seawater, which is an excellent conductor due to its salt content)
Seawater completes the electrical circuit and galvanic currents flow from the less noble metal to the more noble one, dissolving the former.
The galvanic series
Metals are ranked on a scale of electrochemical potential. The greater the potential difference between two metals, the faster the less noble one will corrode:
Less noble (anodic) -- will corrode: - Zinc - Aluminum - Carbon steel - Cast iron
More noble (cathodic) -- protected: - Stainless steel (316) - Bronze - Copper - Titanium
For example, if a bronze seacock is connected to an aluminum hull, the aluminum will corrode rapidly to protect the bronze.
Types of corrosion on boats
Direct galvanic corrosion
Occurs between the metals on the boat itself: bronze propeller + stainless steel shaft + steel stern drive. It is the most common type and is prevented with sacrificial anodes.
Stray current corrosion
Stray currents come from other boats in the same marina or from faulty shore electrical installations. A neighboring boat with an electrical leak can send currents through the water that accelerate corrosion on your vessel.
This type is especially dangerous because it can be very rapid: an aluminum boat can suffer serious damage in weeks if strong stray currents are present.
Differential aeration corrosion
Occurs when one area of the same metal receives more oxygen than another. For example, under a barnacle encrustation, the metal has less access to oxygen and corrodes faster. This is an additional reason to keep the hull clean.
Sacrificial anodes: your first line of defense
How they work
Sacrificial anodes are pieces of less noble metal installed in the submerged areas of the boat. Being the most anodic metal in the system, they corrode instead of the propeller, shaft, or hull. It is a controlled and economical sacrifice.
Anode materials
Zinc: - The most traditional and affordable - Suitable for saltwater - Does not work well in freshwater (becomes passive and stops protecting)
Aluminum: - Works in saltwater, freshwater, and brackish water - Greater protection capacity per kilogram than zinc - Increasingly popular as a universal option
Magnesium: - For freshwater only - Corrodes too quickly in saltwater - Greater protection potential, ideal for lakes and rivers
Where to install anodes
- On the propeller shaft: collar or disc of zinc/aluminum
- On the stern drive (outboards and stern drives): manufacturer-specific anode
- On the hull: anodes bolted below the waterline, evenly spaced
- On the rudder: one on the blade and one on the stock
- In the engine heat exchanger: internal cylindrical anode (pencil anode)
When to replace them
The general rule is to replace anodes when they have lost 50% of their original mass. Inspect anodes:
- Every 6 months if the boat is always in the water
- At every haulout
- Immediately if you notice they have been consumed faster than expected (may indicate stray currents)
If the anodes are not being consumed at all, that is also a problem: it means they are not making electrical contact with the boat's structure and therefore not providing protection.
Galvanic isolation
What is a galvanic isolator
When you connect your boat to shore power, the ground wire creates an electrical connection between your boat and every other boat connected to the same dock. This allows galvanic currents to flow between vessels.
A galvanic isolator is installed on the shore power ground wire and blocks low-voltage galvanic currents (below 1.2V) while maintaining the safety ground protection.
Isolation transformer
For complete protection, an isolation transformer fully separates the boat's electrical system from the marina's ground network. It is the safest solution but also the most expensive and bulky. Recommended for aluminum boats and high-value vessels.
Warning signs of galvanic corrosion
Detecting corrosion early can save you very costly repairs:
- Pitting or blistering on the propeller or stern drive
- Pink discoloration on bronze parts (dezincification)
- Anodes consumed very quickly (in less than 3 months)
- White deposits on aluminum parts
- Cracks or weakening in seacocks or valves
- Abnormal readings on a potential meter (silver-silver chloride multimeter)
Prevention: best practices
1. Use compatible metals
When replacing parts, choose metals that are close on the galvanic series. Avoid combining aluminum with bronze or copper directly.
2. Isolate different metals
If you must use different metals in contact, place an insulating gasket of nylon, PTFE, or rubber between them to interrupt the electrical contact.
3. Keep anodes in good condition
Replace anodes before they are completely consumed. A boat without active anodes is unprotected.
4. Inspect the electrical system
An electrical leak on your boat can dramatically accelerate corrosion. Make sure that: - There are no bare wires touching the hull - Connections are clean and well insulated - The negative return circuit goes to the bus bar, not to the hull
5. Install a galvanic isolator
If your boat spends time connected to shore power, a galvanic isolator is a small investment (100-250 EUR) that prevents serious problems.
6. Measure hull potential
A technician with a reference electrode (silver-silver chloride or zinc) can measure the electrical potential of your hull and determine whether the anodes are working correctly. The target value for steel is between -800 and -1,050 mV, and for aluminum between -900 and -1,100 mV.
Repairing corrosion damage
When corrosion has already caused damage, the options depend on the severity:
- Surface pitting on the propeller: a propeller shop can grind and repaint it
- Dezincification of bronze: the part loses strength and must be replaced
- Damage to an aluminum hull: requires specialized welding by a certified professional
- Damaged seacocks: immediate replacement (risk of flooding)
Repair is always more expensive than prevention. Keeping your anodes current and inspecting the electrical system is the best investment for the health of your vessel.
Cost of cathodic protection
| Item | Approximate cost | |---|---| | Set of zinc anodes (35' sailboat) | 80-150 EUR | | Propeller shaft anode | 15-40 EUR | | Galvanic isolator | 100-250 EUR | | Professional potential measurement | 80-150 EUR | | Isolation transformer | 800-2,000 EUR |
Compared to the cost of a new propeller (500-3,000 EUR) or an aluminum hull repair (several thousand EUR), cathodic protection is a tiny fraction of the annual maintenance budget.
