Yacht galvanic corrosion, isolators and hull testing

Galvanic corrosion can consume sacrificial anodes and damage immersed metals, including shafts and propellers. The risk is not limited to aluminium or steel hulls. Any yacht with metallic parts in the water can be affected.

This guide explains how the galvanic circuit forms, why shore power matters, and the different roles of galvanic isolators and isolation transformers. It also sets out the visual checks crew can make and when hull-potential, bonding-continuity or shore-earth diagnostics need an engineer.

How the galvanic circuit forms

Galvanic corrosion occurs when dissimilar metals are electrically connected while exposed to a conductive fluid. Within that pairing, the more active alloy corrodes preferentially.

The rate is not fixed. It depends on the nature of the materials, the relative exposed areas, fluid conductivity and temperature. A change of marina, a temperature change or an alteration to the yacht's underwater-metal configuration can therefore change anode consumption.

Hull construction does not remove the risk. A GRP yacht can still have a shaft, propeller and other immersed metallic components that form part of the corrosion circuit. The condition of those parts and their sacrificial anodes should therefore be considered separately from the hull material.

This also means that anode condition should be treated as a trend, rather than a single observation. Photographs and written records from routine dives and haul-outs make it easier to identify a change after moving berth, connecting shore power or altering immersed metalwork.

Why the shore-earth path matters

Connecting shore-side ground to the yacht's metallic parts can create a galvanic path. The cited Victron Energy transformer manual warns that this path can rapidly consume sacrificial anodes and attack the shaft, propeller and other immersed metals.

The correct response is not to disconnect the safety earth. The same manual warns that, without the shore earth, GFCI protection and the clearing of a short circuit to vessel metal will not work. Corrosion concerns require diagnosis of the electrical path while retaining protective earthing.

A shore-power connection and its earth arrangement must therefore be assessed as part of the complete vessel system. If corrosion appears after shore-power connection, or anodes begin disappearing rapidly, the investigation should include the isolator or transformer arrangement, shore earth and bonding continuity.

The yacht's status also matters. For the transformer arrangement covered by the cited manual, vessel PE must be connected directly to shore PE when the yacht is out of the water for winter storage or maintenance. The responsible electrician should confirm that arrangement before shore connection.

Galvanic isolator or isolation transformer

A galvanic isolator and an isolation transformer address the shore connection differently. They should not be treated as interchangeable components or modified by crew.

The cited Victron galvanic isolator uses anti-parallel diodes. It blocks low-voltage DC current entering through the shore-power earth wire, helping to prevent electrolytic corrosion of underwater metal. Its diodes conduct above about 1.4 Vdc and allow current at a higher AC fault voltage so that the residual-current device can break the circuit.

Installation position is fundamental to that function. The isolator must be wired into the green safety-earth conductor ahead of vessel grounding connections, with no onboard earth path bypassing it. This is not a crew modification.

The unit must also be mounted in a ventilated, heat-resistant location. It can become hot while conducting fault ground current, so heat damage, an obstructed mounting space or an unexpectedly hot enclosure requires professional attention.

An isolation transformer works differently. It removes shore-to-vessel AC electrical continuity and, when correctly arranged, can provide galvanic isolation together with an onboard ground-fault path. Correct earthing remains part of the installation, including the direct PE connection specified by the cited manual while the vessel is ashore.

Reading anode and underwater-metal condition

Paint blistering, white powder and pitting on underwater aluminium are reported signs of galvanic corrosion. If any of these are found, inspect the sacrificial anodes immediately and record their condition.

Useful observations include:

• Anode condition: Photograph each accessible anode during routine dives or haul-out so that later consumption can be compared with the earlier record.
• Paint blistering: Record its position and the underwater metal on which it appears. This gives the engineer a clear location for follow-up inspection.
• White powder: Photograph the deposit and surrounding metal without treating it as a cosmetic defect.
• Pitting: Record the affected component and arrange an assessment of the metal and corrosion circuit.
• Timing: Note whether the condition appeared after shore-power connection, a marina change or a change to the underwater-metal configuration.

Shaft and propeller damage may require both electrical diagnosis and inspection of the shaft and propulsion system. Paint blistering or exposed underwater metal may also require coordination with gelcoat, paint and antifouling work, but coating work should not replace investigation of the corrosion path.

Hull-potential and bonding-continuity checks belong in the professional diagnostic process. The fact sheet does not support using visual appearance alone as a substitute for those tests, particularly when anodes are being consumed rapidly or damage begins after connecting shore power.

What crew can check safely

Crew checks should remain visual and documentary. Do not open electrical equipment, alter protective-earth wiring or bypass the galvanic isolator.

• Log underwater condition: Record anode condition during routine dives and haul-outs. Photograph paint blistering, white powder and pitting on underwater metal.
• Inspect the isolator externally: Check the accessible enclosure, mounting and ventilation clearance. Look for visible heat damage, but do not open or rewire the unit.
• Check the ashore procedure: If the yacht is out of the water and uses the transformer arrangement described above, ask the responsible electrician to confirm the required PE arrangement before shore connection.
• Keep the safety earth connected: Do not remove shore earth in an attempt to slow anode consumption. The cited manual warns that this prevents GFCI protection and short-circuit clearing to vessel metal from working correctly.
• Record relevant changes: Note a marina change, temperature change or alteration to the underwater-metal configuration alongside the anode log, because these variables can affect corrosion rate.

If the isolator enclosure is hot, damaged or appears to have been bypassed, stop at the visual check. The unit can become hot while carrying fault ground current and needs assessment by an engineer.

When to call an engineer

Arrange professional diagnostics when anodes are rapidly consumed, corrosion appears after shore-power connection, or underwater metal develops blistering, white powder or pitting. These conditions call for inspection of the affected metal and the electrical paths associated with shore power.

An engineer is also required when:

• The galvanic isolator is hot or damaged: Its condition, installation and ventilation need checking without crew opening the enclosure.
• A bypass is suspected: Any earth path around the isolator can make it ineffective.
• Testing or replacement is required: The isolator is installed in the safety-earth conductor, so this is not a crew task.
• Hull-potential testing is needed: This forms part of professional corrosion diagnostics rather than a visual crew check.
• Bonding continuity needs checking: The bonding and shore-earth paths must be assessed as part of the vessel's electrical arrangement.
• The transformer installation is in question: Its isolation, onboard ground-fault path and PE arrangement require electrical verification.

Do not respond to any of these conditions by removing shore earth. Keep protective earthing intact and have the corrosion circuit investigated.

Blueward Marine provides marine electrical and electronics services in Mallorca, including galvanic-isolator, isolation-transformer, bonding and shore-earth diagnostics. Submit fault details and photographs through the online service booking, or call the 24/7 emergency line on +34 971 251 374.