Galvanic Corrosion: How to Protect Your Boat

Galvanic corrosion: what it is and why it affects (almost) every boat

Seawater is a natural electrolyte — it conducts electricity extremely well. When two different metals are submerged in it and connected to each other (even indirectly, through the hull or the boat's electrical system), an unintentional galvanic cell is created. The less noble metal corrodes as it gives up electrons to the more noble one. This is not a matter of luck or material quality: it is chemistry, and it happens on every boat.

You already know the visible results: the propeller that thins out, the corroded rudder shaft, anodes consumed too quickly, bronze fittings that leach zinc until they become porous. What is less visible is often more worrying: internal corrosion at hull fittings, attack on metal hulls, silent deterioration of the drive shaft.

Understanding how galvanic corrosion works — and how to protect against it — is one of the fundamental skills of boat maintenance. You don't need an engineer: you need to know what to look for and when to act.

How it works: the galvanic series

Every metal has its own electrochemical potential. When two metals with different potentials come into contact in the presence of an electrolyte (salt water, harbour water, even condensation moisture), a galvanic current flows between them. The metal with the lower potential — the less noble one — oxidises and is consumed. The one with the higher potential — the more noble — is protected at the expense of the first.

The galvanic series in seawater, simplified, runs from most active (corrodes first) to most noble (protected):

• Zinc — the most active, the basis of sacrificial anodes
• Aluminium — active, used for anodes on aluminium boats
• Magnesium — used for protection in fresh water
• Mild steel / cast iron
• Stainless steel 304 — intermediate position, vulnerable in oxygen-depleted zones
• Lead
• Stainless steel 316 — more noble than 304, more resistant in marine environments
• Naval bronze / brass
• Copper
• Nickel
• Titanium
• Platinum / gold — the most noble

The further apart two metals are in the series, the more aggressive the galvanic couple they form. A zinc anode in contact with a bronze bushing: an acceptable, purpose-designed pairing. A mild steel bolt fastening a stainless steel component: problematic over time. An absolutely avoid combination is aluminium in direct contact with copper or brass in a marine environment.

Galvanic corrosion vs. electrolytic corrosion: don't confuse them

These are two different phenomena, often mixed up. The distinction matters because it changes both the diagnosis and the solution.

Galvanic corrosion — caused by the potential difference between two dissimilar metals in contact within an electrolyte. It is a passive phenomenon: it requires no external current, it generates itself. Its intensity depends on the potential difference between the metals, the relative surface area (a small anode on a large structure is consumed very quickly) and the conductivity of the water.

Electrolytic corrosion (or stray current corrosion) — caused by electrical current flowing through the water from external sources: leaks from the boat's electrical system, shore power currents, interference from neighbouring boats connected to the same marina electrical network. It is often far more aggressive than pure galvanic corrosion and can destroy a propeller in a matter of weeks if left unchecked.

How to tell them apart in practice: galvanic corrosion is slow and progressive, specifically targets the less noble metals in the system, and is managed with anodes and isolation. Electrolytic corrosion is rapid and irregular, can attack metals that would normally be protected, and requires first finding and fixing the source of the electrical leak.

Sacrificial anodes: the primary protection system

The principle is straightforward: an even more active metal — zinc, aluminium or magnesium — is deliberately introduced into the system, placed in electrical contact with the metals to be protected. The sacrificial anode becomes the negative pole of the cell and corrodes in place of the propeller, shafts, hull and hull fittings. Once it has lost more than 50% of its original mass, it must be replaced: the remaining surface is no longer sufficient to guarantee protection.

The choice of anode material depends on the environment in which the boat operates:

• Zinc — the classic material, optimal for salt water. It has the right electronegative potential to effectively protect bronze, stainless steel and brass in marine environments. Do not use in fresh water: the oxide film that forms in the absence of chlorides drastically reduces its effectiveness.
• Aluminium — the best choice for mixed use (brackish water, lagoons, estuaries) and increasingly preferred in open sea as well. It lasts longer than zinc for the same mass, contains no cadmium (lower environmental impact) and maintains effectiveness across a wider range of salinity.
• Magnesium — for fresh water, where zinc and aluminium lose effectiveness. Very electronegative potential: do not use at sea — it would be consumed too quickly and could cause over-protection with hydrogen as a by-product.

A common mistake is fitting anodes of the wrong material on aluminium hulls. Aluminium hulls require anodes made of pure aluminium or a specific alloy: using zinc on an aluminium hull can create a problematic galvanic couple instead of providing protection.

Where to place anodes: the critical zones

Cathodic protection only works if the anodes are in electrical contact with the metals to be protected and physically close to them. Mounting one anode on the hull and expecting it to protect the propeller three metres away is not enough: effectiveness decreases with distance, especially in fresh or brackish water with low conductivity.

The zones requiring dedicated protection:

• Propeller and hub — anode on the shaft, anode on the hub (if specified by the manufacturer), collar anode at the sterntube exit. The propeller is always exposed and rotates: electrical contact must be maintained even with moving parts.
• Rudder — shaft and blade in contact with salt water, often stainless steel. Anode applied directly to the blade or shaft.
• Hull fittings and seacocks — bronze fittings passing through the hull are a high-risk position. On a fibreglass hull they are not connected to the system, so they must be protected individually or via a bonding wire linked to the general protection system.
• Outboard motor — the lower unit always has its own specific anode, often integrated into the motor body. Replace every season or more frequently in aggressive waters.
• Steel or aluminium hulls — require a structured protection plan with anodes distributed across the entire underwater hull, calculated according to the submerged surface area.

The marina problem: stray currents and galvanic isolators

When the boat is moored with the shore power cable connected, the boat's electrical system is in continuity with the marina's electrical network. If the marina has leakage — or if a neighbouring boat has an electrical fault — current can flow through the water and attack your submerged metals even with the boat sitting still and switched off.

This phenomenon is particularly insidious in leisure marinas where dozens of boats are connected to the same network: a single boat with a significant leak is enough to put the propellers and shafts of all its neighbours at risk.

The solution is the galvanic isolator: a device installed on the earth conductor of the shore power cable that blocks low-frequency galvanic currents while still allowing the safety current to pass in the event of a fault. It is a small, relatively inexpensive component that is installed once and works silently. For boats that spend a lot of time in marina on shore power, it is an investment that pays for itself quickly in saved anodes and components.

For those wanting even more comprehensive protection — typically on steel or aluminium boats, or on high-value vessels operating in particularly aggressive waters — there is Impressed Current Cathodic Protection (ICCP): an active system that generates a controlled current to maintain a constant cathodic potential across the hull. It requires professional installation and a control unit, but eliminates the need for conventional sacrificial anodes.

How to recognise an ongoing galvanic problem

Galvanic corrosion rarely announces itself with obvious signs until the damage is already significant. Knowing what to look for — and when — is the difference between a timely intervention and a costly replacement.

• Anodes consumed too quickly: if an anode lasts less than a season, there is a problem. Either the anodes are undersized, or there is an electrical leak draining them abnormally. Before buying larger anodes, find the cause.
• Anodes that don't consume: sounds positive, but often isn't. A passivated anode loses electrical contact through oxidation of the mounting surface and is protecting nothing. Metal-to-metal contact must be verified every time the boat is hauled out.
• Propeller with irregular edges or localised erosion: galvanic corrosion produces rough, porous surfaces with uniform material loss; cavitation produces localised "orange peel" erosion on the pressure face of the blades. They must be distinguished because they require different remedies.
• Bronze fittings with a pinkish colour: this is dezincification — the bronze loses zinc from the alloy, leaving behind porous, brittle pure copper. Dezincified fittings must be replaced immediately: they cannot withstand pressure.
• Accelerated corrosion after yard work: any work involving the electrical system or submerged metal components can upset the system's balance. After any major haul-out, carry out a systematic anode inspection.

Preventive maintenance: the minimum schedule

• At every haul-out: visual inspection of all anodes. Replace everything that has lost more than 50% of its original mass. Clean mounting surfaces with abrasive paper before fitting new anodes — no paint under the anode, it must make metal-to-metal contact.
• During the season (every 2–3 months in aggressive waters): check accessible anodes. An anode consuming very rapidly mid-season signals a problem that cannot be put off until the next haul-out.
• Before a long mooring on shore power: verify that the galvanic isolator is present and working.
• After any electrical work: check that no new leaks have been created. A multimeter is sufficient for a basic earth leakage check.
• Every 2–3 years: professional inspection of the boat's electrical system with a specific stray current test — especially if you spend a lot of time in marina and notice abnormal anode consumption.

Conclusion: galvanic protection is not optional

Galvanic corrosion is one of the few forms of damage that acts constantly, even with the boat sitting in the marina. It does not depend on how much you sail, how new the boat is or how good the antifouling paint is: it depends on the metals you have on board and the environment you put them in.

The good news is that protection is neither complicated nor expensive: anodes of the right material, correctly fitted, checked regularly, supplemented by a galvanic isolator if you are often on shore power. A simple maintenance programme that protects components — propeller, rudder, hull fittings — whose restoration would cost many times more.