Galvanic Corrosion: Causes and Countermeasures

Two different metals, an electrolyte, and a conductive connection create a galvanic cell. It supplies current as long as the less noble metal dissolves. In water, all three conditions are permanently present – and a fourth is added when connected to shore power.

How the Cell is Formed

Metals have different electrochemical potentials. If two of them are conductively connected in an electrolyte, a current flows, and the less noble metal releases material. This is precisely the setup that a boat naturally has in water: bronze on the propeller, stainless steel on the shaft, aluminum on the drive, plus salt or brackish water.

The further apart two metals are in this series, the stronger the effect. The surface ratios further amplify it: a small, less noble surface next to a large, noble one will be eroded particularly quickly.

The classic countermeasure is the sacrificial anode: a deliberately less noble metal that dissolves instead of the parts to be protected. It is a wear part and must match the water—zinc for saltwater, aluminum as a more universal choice, magnesium for freshwater.

Shore Power Increases the Size of the Cell

Up to this point, this is a process happening on your own boat. As soon as shore power is plugged in, the scale changes: the protective earth conductor connects the metal parts on board via the jetty to those of all other connected boats — and to the shore revetment, often made of steel.

This creates a common cell across the entire harbor. The consequence is well-known but rarely correctly attributed: anodes disappear remarkably quickly, or the boat's own drive corrodes, even though there had been no problems for years. This is particularly noticeable on boats with plastic hulls, as the drive unit is the only large metal part in the water.

WarningDisconnecting the protective earth conductor to solve the problem is life-threatening and impermissible. It is the device that triggers shutdown in case of a fault. The solution lies in blocking DC currents and allowing fault currents to pass.

The Galvanic Isolator

A galvanic isolator is integrated into the protective earth conductor of the shore power connection. It consists of anti-parallel and series-connected diodes that only conduct above a certain voltage. Galvanic voltages are typically one to two volts – below this, the isolator blocks and the corrosion current does not flow.

A fault current, on the other hand, occurs at mains voltage. It immediately overcomes the blocking threshold, flows off via the protective earth conductor, and triggers the protective device. The safety function thus remains fully intact – that is the real trick.

When selecting, the shore power protection is important: a device for 16 amperes and one for 32 amperes cover the usual cases. What is important is the load capacity in case of a fault - the isolator must withstand the short-circuit current until the fuse has tripped. It is installed near the shore power connection, in a dry, accessible area.

The Isolation Transformer

The more thorough solution completely separates the onboard system from the shore power grid. An isolation transformer transfers energy magnetically; there is no longer a conductive connection between the dock and the boat. This interrupts the corrosion path — and at the same time alleviates the problem of reversed polarity or poorly grounded dock outlets, because the onboard electrical system gets its own defined potential.

The price for this is weight, space requirements, cost, and operating losses. For boats that are frequently connected to shore power in changing locations, it is nevertheless the most stable solution.

Both methods maintain the protective function. Neither replaces anodes — the galvanic processes on your own boat continue regardless of the shore power connection.

Stray Current Corrosion — the Other Case

Stray current corrosion is to be distinguished from galvanic corrosion: here, current is not driven by a natural potential, but by a faulty system. A DC consumer with poor return current seeks a path through water and fittings — and in doing so, removes material, orders of magnitude faster than any galvanic cell.

It can be recognized by unusually rapid anode consumption, by localized erosion at a specific point, and in systems with chain ground or ground return via the structure. It is detected using fault current monitoring that compares forward and return current and reports the difference.

NoteIf anodes disappear remarkably quickly, the following sequence is recommended: first rule out stray current, then consider the shore power connection, and only then think about larger anodes. More anode mass does not address the root cause.

How to Identify the Case

The easiest test for the fourth case: don't use shore power for a few weeks and compare the anode condition.

Typical Errors

• Protective earth conductor disconnected. Life-threatening and impermissible — that's what isolators and isolation transformers are for.
• More anodes than solution. Treats the symptom, not the cause.
• Incorrect anode material. Zinc in freshwater passivates and no longer works.
• Anodes painted or insulated. Without conductive contact to the protected part, they do not work.
• Stray current not excluded. It causes erosion faster than any galvanic element.
• Isolator chosen too small. It must withstand the fault current until the fuse trips.