Galvanic Corrosion

Galvanic corrosion is caused by dissimilar metals in electrical contact with each other while immersed in the same electrolyte (like seawater). This situation forms an electrical battery, and a current flows between metals as long as there is a circuit path. Corrosion associated with this situation typically takes a long time (months to years) since the currents are very small. Vessels are normally protected with some form of system designed to prevent this type of corrosion. Examples are sacrificial  anodes and impressed current systems (like the Mercathode system). There are also examples of this type of corrosion which are not prevented by corrosion protection systems. An example of this might be the installation of a stainless steel fastener in an aluminum mast without proper isolation (the aluminum corrodes in this situation if the two parts become wetted with a conductive fluid, like salt spray, for example.

Underwater galvanic corrosion can be controlled by the proper installation and maintenance of a cathodic protection system. Any system should be periodically checked (just like you check your oil and water in the engine) to ensure it's doing its job.  All those green wires running throughout the boat (connected to hull fittings, rudders, etc.) are part of this system and also require maintenance. Galvanic corrosion can also be limited by ensuring that the direct contact of dissimilar metals in a moisture-laden environment is minimized.

Galvanic attacked is significantly accelerated when a vessel plugs into shore power unless the boat is isolated using an approved device. The green ground wire in the shore cord effectively connects all the underwater metal of all boats on the shore power system. The effects of plugging in are routinely worse behind private residences and very small marinas since there is more metal from dock structures and ground stakes in the water compared to the amount of anode materials on boats. Typically, if you are not properly isolated, a small DC current will flow off your boat. This current acts to quickly consume your anodes (zincs are one form of anode) after which your underwater metals begin to slowly corrode.

Isolation strategy involves installation of an approved Galvanic Isolator (one with a failsafe rating is a must ensure the grounding connection is not lost in the event of an electrical transient which damages the isolator). However, beware: NEVER, NEVER disconnect the green ground wire in the shore cord. This will place you and your guests at substantial risk of electric shock injury or death.

We have the expertise and experience to analyze your vessel and determine the state of protection, and to recommend proven strategies to minimize the likelihood of corrosion damage. See our various services offered here.

The pictures below are examples of galvanic corrosion where, in most cases, the cathodic protection system was generally ignored or the boat was not properly isolated from the dock grounding system..

Pitted stainless shaft from lack of galvanic protection. This probably took multiple months to years to happen. The repair bill for both shafts was in the multiple thousands.

Stainless trim tabs corroded after zincs were consumed.

Outboard engine lower unit damage occurred after the zinc anode on the bracket was depleted. This boat was attached to shore power without galvanic isolation. 

This is the forward end of an outboard’s lower unit. The zinc anodes were consumed by dock structures behind the owner’s residence. The lower unit case is eaten through and all the trim cylinders began to leak oil into the water.

Here’s a zinc anode that was painted. It doesn’t function in this condition, and the boat suffered lower unit corrosion damage as a result.

Here a stainless fastener was not isolated from the aluminum casing and the aluminum was corroding under the paint (which was probably not properly applied).

Imagine the shock of the owner of this boat when it was hauled. This is a perfect example of galvanic corrosion taking place over a relatively long time.