Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals form a natural battery when placed in contact with each other while immersed in an electrolyte. Seawater is a perfect electrolyte for metals. Metals vary in their reaction to being part of this natural battery. Metals that are more active are anodes or less noble. In a galvanic cell a less noble metal, like zinc, will dissolve sooner a more noble metal like brass. Prevention includes proper choices for metals used in vessel construction, use of protective paints and sacrificial anodes. Some suggestions to reduce corrosion on your boat can include:
- Avoid using bottom paint that is incompatible with the vessel hull material. A good example is copper-based bottom paint on an aluminum hulled boat
- Steel boats should not be moored directly next to aluminum vessels
- Try to use fasteners on your boat made of similar metals throughout the vessel. Use all brass or all stainless steel for example
Electrolysis
Electrolysis is also the corrosion of dissimilar metals, but a different process causes it. A difference in the direct potential current that exists between two dissimilar metals is the reason for the problem. This is a stray current or circuit that is not following an anticipated path. Improper wiring, poor choices in building materials, or defective electrical appliances can cause it. Introduced currents will flow by the path of least resistance. Stray current can also cause Electric Shock Drowning.
Electrolysis is much more destructive to boats than galvanic corrosion. You can reduce electrolysis with these iteas:
- Avoid the use of common automotive battery chargers on your boat. These chargers use an autotransformer in which the primary and secondary windings are electrically connected. Marine battery chargers use an isolation transformer. An isolation transformer has separate windings and does not allow current to pass into the vessel from the utility connection
- Vessels using a positive ground electrical system should avoid mooring next to one with a negative ground system
- Check the connections to any two bilge pumps that are electrically connected. Ensure that each pump shares the same positive and ground connection. If one pump is grounded to the frame and the second is not, a stray current will be introduced
- Electric welders can introduce current into salt water due to frayed or damaged connectors. If welding is required on a vessel, it is better if the welder is mounted on the boat being welded
- Check your vessel for reversed polarity
- Check to see if any severe corrosion is present near telephone connections. Telephone service voltage can introduce 48 volts of current into the water
- Install sacrificial zincs on your boat and properly maintain them
- Keep a layer of quality bottom paint applied to all hull surfaces in contact with the water
- Tilting your outboard motor out of the water may completely eliminate corrosion
One ideal solution for vessel owners is to purchase and install an isolation transformer for their vessel electrical system. Vessels moored along each other act like the plates within a battery. One boat can introduce current into the water and damage may not occur until it reaches a boat six spaces down. Using an isolation transfer effectively takes you out of the battery and protects the boat from introduced stray current.
Marinas should have a policy in place to address the proper use of shore power cords. Shore power cords need to be designed for use in the marine environment and UL approved. Don't use a common household extension cord for your boat! Marine power plugs and receptacles are built to standards set by the National Fire Protection Association. Using the wrong plug in the wrong receptacle or use of a worn fixture can cause fire and corrosion problems.
Corrosion can be caused by multiple factors. Items addressed in this article will give you a good start on preventing damage to your vessel. Talk to your harbormaster and boat mechanic about additional safeguards that may work for you.
Alan Sorum - Alan Sorum is the Maritime Operations Project Manager for the Prince William Sound Regional Citizens' Advisory Council (PWSRCAC). Alan is ...
