Corrosion Mechanisms Galvanic corrosion

Occurs when two dissimilar metals in contact with an electrolyte (e.g., seawater) create a galvanic cell, causing one metal to corrode.

Stray current corrosion

Occurs when an electrical current flows through a conductor (e.g., water) from a foreign source (e.g., a faulty shore power connection) and causes corrosion on metal structures in contact with the conductor.

Isolation Transformer

An isolation transformer is a device that isolates a vessel's electrical system from shore power by creating a physical and electrical barrier. It blocks stray currents from entering the vessel and also prevents AC or DC currents from the vessel from escaping.

Expert Opinion

Both opinions you received contain elements of truth. DC can cause corrosion

DC current creates a galvanic cell when it flows between dissimilar metals, leading to corrosion.

AC can stray

Stray AC currents can occur if there is a fault in the shore power connection or if the vessel's electrical system is not properly isolated.

Zincs can act as diodes

In some cases, zincs on the vessel can act as diodes, allowing AC currents to flow in one direction (from the vessel to the water) but blocking them in the other direction. This creates a DC component to the AC stray current, which can then cause corrosion.

Recommendation

Based on this information, it is highly recommended to install an isolation transformer on the vessel to mitigate the risks of stray current corrosion. This is especially important given the presence of an old vessel nearby with potentially faulty wiring.

Additional Considerations Ensure the isolation transformer is properly sized and installed by a qualified electrician. Regularly monitor the electrical system for stray currents and take corrective action if necessary. Consider using additional corrosion protection measures, such as sacrificial anodes and cathodic protection, to further reduce the risk of corrosion. Flag for review