Metal parts underwater can be subjected to two basic styles of
electrolysis: galvanic corrosion and stray current corrosion. Both
can damage the drive, propeller, underwater parts, boat and motor
if not correctly monitored (testing at 2 week intervals) and avoided.
Galvanic corrosion is an electrochemical reaction between two or
more metals. Drive systems consist of several di erent metals. Some
are more active than others.
Galvanic corrosion of the more chemically active metals can occur
whenever two or more dissimilar metals that are “grounded”
(connected by actually touching each other, or through a wire or
metal part) are immersed in a conductive solution (any material
that can conduct electricity). Anything but pure water is conductive.
Saltwater, fresh water with a high mineral content and polluted
freshwater are highly conductive. Conductivity increases with
temperature. at is why Florida boats experience more corrosion
than boats in Maine.
Speci cally look at a typical marine drive unit with a stainless steel
propeller. e aluminum is the more chemically active metal (called
the anode) and the stainless steel propeller is the less chemically active
metal (called the cathode).
CORROSION TABLE
Gold
Stainless Steel
Bronze
Copper
Brass
Steel
Aluminum
Zinc
Magnesium
Least Active
Most Active
GALVANIC CORROSION
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