Marine Electrolysis & Galvanic Corrosion

Any vessel moored in a harbor is subject to corrosion of its metal surfaces that are exposed to the marine environment. Two different processes: galvanic corrosion and electrolysis can cause damage to vessels. Galvanic corrosion is caused when two dissimilar metals come into contact with each other while immersed in an electrolyte like salt water. Electrolysis is caused by the existence of potential current between two different objects. These objects can be aboard a single vessel or involve multiple vessels. This document isn’t meant to discuss the entire subject of corrosion, but is meant to offer some ideas on how to reduce its effects on your boat.

Galvanic Corrosion – is similar to the operation of a battery. Two dissimilar metals in contact with an electrolyte create a natural electrical current. Lead/zinc-acid battery in your boat is an excellent example. Prevention of this problem is based on good planning and knowledge of what’s being used on your vessel. These are some potential causes of galvanic corrosion:

• Using dissimilar metals on the vessel that come into contact with the salt water. This could include using steel bolts or copper based bottom paint on an aluminum hull.
• Mixing different types of bottom paint on the same vessel.
• Mooring vessels with incompatible hull construction next to each other. An example would be a steel vessel placed immediately next to aluminum vessel.

Electrolysis – is the result of potential current existing between two different objects. It is normally caused by stray electrical current. Electrolysis is much more destructive than galvanic corrosion. To prevent electrolysis, the vessel must be wired properly and outside sources of stray current must be identified. These are a few possible sources of electrolysis:

• Using a common automotive battery charger. Most automotive chargers have their primary and secondary transformer windings electrically connected which allows stray current to enter the vessel’s grounding system. Use a marine rated charger, which will have an isolation transformer to prevent current passing across the charger’s windings.
• A welder using DC current can induce a current into the water if any of the welding cables are frayed or bare. A better solution is to place the welder onboard the vessel being repaired and connect welder ground directly to project, or as close as possible.
• Having two bilge pumps with their connections reversed, i.e., the frame of one pump is negatively grounded and the frame of the second pump is connected to the positive side of the electrical system.
• The ground and neutral of the vessel’s electrical system are not isolated or the shore power cord is improperly connected. Power should be taken only from hot and neutral terminals. Drawing power from hot and ground terminals will induce stray current into the harbor, and poses a hazard to anyone in the water near the vessel.

Prevention - Kodiak harbor has policies in place that regulate the use of shore power cords to help reduce stray current and fire hazards:

• Electrical systems and equipment used within Kodiak municipal harbors must comply with NFPA 70, National Electrical Code and Chapter 3 of NFPA 303.
• Cords need to be designed for marine use and kept out of the water at all times.
• The typical household extension cord is not acceptable.
• Cords with current carrying capacity of less than thirty (30) amps may not be used. Flexible cords will be used only in continuous lengths without splicing or taps.
• Cords, attachment plugs and connector bodies will not be smaller than required for the rated current of the attached cord or connected equipment.
• Infrared heating lamps may only be used with porcelain type sockets.
• Any heater capable of causing a fire if overturned must be equipped with a safety switch that will disconnect electric current to the heater if overturned.

Source: Northern Harbors & Small Ports: Operations and Maintenance, with permission from author, Allan Sorum, Valdez Harbormaster.