How To Prevent Outdrive Corrosion

By Charles Fort







Most anodes are passive, meaning they simply sacrifice themselves as needed, but others, called impressed-current anodes, are active, relying on 12-volt battery power and a controller to force a small amount of DC current through a permanent anode. Impressed current anodes are commonly used on aluminum outdrives in addition to sacrificial anodes, when the sacrificial anodes by themselves would not provide sufficient protection. Impressed current systems, like passive anodes, need to be inspected regularly. They commonly fail due to poor or corroded electrical connections. Many don't have indicators to show they're working properly, so most manufacturers recommend testing yearly by a qualified shop.

Shore Power

You might not think that a boat's power connection could have anything to do with a corroding outdrive, but it can. Aside from missing anodes, it's probably the most common cause of severe corrosion. Boats plugged into shore power share the green ground wire with every other plugged-in boat in the marina. The ground wire is critical to a safe boat because it carries off dangerous current from a fault in the shore power system. But because the shore power ground wire is also connected to the DC ground system (which is critical to preventing shocks to swimmers in the area), it extends to a boat's bonded underwater metals — including the outdrive.

Photo of corroded overdrive

The outdrive is then connected to the neighbor's grounded underwater metals (even one several boats away), which may be stainless steel or bronze. Because the two different metals are in the water and connected through the ground wire, it's the same as if they were in direct contact. If a boat's underwater metal isn't protected by an anode, it will use a neighboring boat's anode for protection. When that's gone, it will use other less-noble metals, likely an outdrive, to protect itself. The solution is to install a galvanic isolator, which will prevent low-level DC current from flowing through the green wire — effectively breaking the connection to neighboring boats — but still allowing shore power current to flow to ground in an emergency.

Stray Current

Another way an outdrive can be destroyed by corrosion is through a voltage leak in a DC system. This problem is less common and typically caused by a fault in a boat's 12-volt DC system. For example, if a bilge pump sitting in saltwater were to have a break in the DC wire's insulation, current could destroy metal fittings in the bilge.

Avoiding stray current corrosion means making sure that there are no wires in the bilge (all wires should be supported above the level of expected bilge water), except the wire to the bilge pump. The latter should be inspected to make sure it's in good shape. Aside from keeping wires out of the bilge, the most effective way to prevent stray-current corrosion is to have all of your underwater metal bonded to the boat's ground. That way, any leaks will be brought back to your battery before they have a chance to destroy fittings. Most new boats are bonded from the factory, but older boats can be retrofitted. It should be noted that a galvanic isolator won't normally prevent stray current corrosion damage because the damaging voltage exceeds what the isolator can stop. 

Charles Fort writes for Seaworthy, the BoatUS Marine Insurance publication dedicated to helping members to avoid injury and boat damage due to accidents and storms. For an insurance quote, please call 800-283-2883 or go to

— Published: December 2012

AC Corrosion





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