Galvanic Corrosion and Your Boat's Electrical System

What Is Galvanic Corrosion

Galvanic corrosion happens when two dissimilar metals are electrically connected in the presence of an electrolyte, such as seawater. One metal acts as the anode and gradually gives up electrons to the other, corroding in the process. Every boat with underwater metal, a propeller shaft, a strut, seacocks, has this reaction happening to some degree.

Stray AC and DC current makes the problem far worse. A boat sitting quietly at the dock can lose metal at a fraction of a millimeter per year from galvanic action alone, but a stray current fault can accelerate that same loss to a matter of weeks, especially in a saltwater marina where the water itself is a much better conductor than fresh water.

This threat is not limited to your own boat. Most stray current problems originate from a boat's own onboard DC wiring, but faulty wiring on a neighboring vessel, or a fault in the marina's shore power pedestals and underground wiring, can also inject stray current into the water and attack your underwater metal even if your own system is wired correctly.

How Wiring Errors Increase Galvanic Corrosion

Without a galvanic isolator or an isolation transformer, this problem gets significantly worse. The green safety ground wire that connects your boat to shore power also ties your underwater metal, through the boat's bonding system, to every other boat sharing that same shore power ground. In effect, the green wire turns your propeller shaft, seacocks, and other bonded components into one plate of a giant battery, with a neighboring boat's underwater metal or the dock's grounding grid as the other plate.

That reaction can rapidly degrade the structural integrity of expensive components and, if it reaches the hull fittings, the hull itself. Zincs cannot fight this kind of sustained current on their own. They are sized to absorb the ordinary galvanic action between dissimilar metals, not to indefinitely soak up a current path introduced by a wiring fault somewhere else in the marina.

Zincs and Anodes

Plenty of anodes still sold and installed as "zincs" today are not actually zinc. We keep the name out of habit, since zinc was the standard material for decades, but a lot of what goes on a boat now is an aluminum alloy, which works across salt, brackish, and fresh water. True zinc anodes are still zinc alloys, but they are only suited to saltwater, and magnesium anodes exist too, reserved for freshwater use since they corrode too quickly in salt.

The anode is sacrificial by design. It willingly gives up its electrons and, in the process, degrades into metal oxides and other compounds over time. The important part is what does not degrade: your propeller shaft, fittings, and propeller are spared because the anode is offering itself up as the more easily corroded metal instead.

Galvanic Isolator

A galvanic isolator is a largely passive device that uses a set of diodes to filter the green safety ground wire, blocking low-voltage stray DC current from reaching the underwater metal that is in contact with seawater.

The advantages are that it is passive, inexpensive, and can last decades when properly installed to the ABYC A-28 standard, which governs how these devices must fail. The drawback is that it is prone to a slow, silent failure that is easy to miss, which is why it is often overlooked as the cause of corrosion damage until an inspection catches it. A galvanic isolator also does not completely block stray current, it significantly reduces it, so regular testing is part of keeping it effective. It is also current-limited and cannot stop stray AC or stabilize voltage the way an isolation transformer can, which is why larger or more demanding installations sometimes call for a transformer instead.

Isolation Transformer

An isolation transformer is an active component, and the name is a bit of a misnomer since it typically does not change voltage. Most marine isolation transformers are wound 1:1, though step-up and step-down versions exist for boats that need to convert between different shore power standards. A 1:1 unit fully isolates your boat's electrical system from the shore power grid, which removes the metallic ground path back to shore that stray current relies on.

The advantages are near-total isolation and virtually no stray current through that ground path. The drawback is inefficiency: converting power generates some heat, which is both a fire consideration and simply makes the boat warmer and less comfortable on hot days. Watch for unusual heat or breakers tripping randomly with no other changes aboard, since these can be early indicators of a developing fault, though a transformer can also fail without obvious warning. Because the isolation transformer becomes a new AC power source aboard, its secondary winding requires its own neutral-to-ground bond. The ELCI itself stays on the shore side at the main shore power breaker, and ABYC actually waives that requirement when the transformer is mounted within 10 feet of the shore power inlet. Isolation transformers generally handle much higher loads than a galvanic isolator, which makes them the right choice where a traditional galvanic isolator is not sufficient.

Testing Your Galvanic Isolator

Testing a galvanic isolator is a yearly maintenance task that takes only a few minutes, using the diode test function on a multimeter rather than resistance or ohms mode. Disconnect both green wires from the isolator and connect your meter leads to the studs, then test across the device in both directions, since it is possible for one side's diodes to fail while the other side still reads fine.

A healthy isolator settles at somewhere between 0.7 and 1.0 volts in each direction, after briefly climbing as the meter's test current stabilizes. A reading of 0 volts or an OL (open circuit) reading means the isolator has failed. A reading that settles outside the 0.7 to 1.0 volt range, for example around 0.45 volts, indicates a partial failure. Either way, this test only confirms the diodes are functioning; it does not replicate real-world stray current blocking, and an older isolator that fails will typically fail open, silently losing your safety ground. If you are due for a replacement, we recommend a newer fail-safe isolator that fails closed instead, preserving the ground connection. For a more detailed walkthrough of this test, see Marine How To's guide to testing a galvanic isolator.

Testing Your Isolation Transformer

Testing an isolation transformer is a bit more involved. Start by identifying the primary side, where incoming shore power connects, and the secondary side, where power leaves to feed the boat. That gives you four terminals to work with.

There should be measurable impedance across the two primary terminals, confirming the field windings are not shorted, and the resistance across the two secondary terminals should be similar. The specific values vary by manufacturer, so check your unit's documentation for the expected range. Next, measure resistance between the primary and secondary sides with a standard multimeter. You should see OL, or overload, indicating resistance too high for the meter to read, which rules out a dead short between the windings. A multimeter only applies a few volts, though, so it cannot reveal insulation that has degraded but not yet failed outright. A proper insulation test, using a megohmmeter at several hundred volts or more, is the more thorough way to confirm the transformer is isolating cleanly and is worth having done periodically by a professional.

A visual inspection is also worthwhile. Look for worn components, signs of thermal damage, and loose lugs. If it is safe to do so, check for an acrid odor, which can indicate burned or degraded insulation.

Summary

Galvanic corrosion is a fact of life for any boat with underwater metal, but stray AC and DC current, whether from your own wiring or a neighboring boat and the marina's infrastructure, can turn a slow, manageable process into rapid, expensive damage. A galvanic isolator or an isolation transformer, paired with properly sized zincs and a yearly test of whichever device you rely on, is what keeps that risk in check.