Preventing galvanic corrosion is a vital consideration
when installing an onboard electric system. Galvanic
corrosion is the corroding of metal under the influence
of an electric current. As you can see in the table,
every type of metal has a difference in potential with
respect to other metals. If components made of two
different metals are dipped into a liquid (electrolyte)
and short-circuited, a (low) current will flow. This
will result in corrosion of the metal with the lowest
potential, eventually dissolving it completely.
There are three situations that can cause two different kinds
of metal to be submerged in electrolyte on a vessel. And it is
important to remember that while saltwater is an excellent
conductor, brackish water and freshwater can also conduct
Although the first situation is not directly related to
the onboard power circuit as such, it is a major cause
of corrosion, especially pitting. A propeller made
of, for example, manganese bronze is connected to the hull
via the propeller shaft, the engine and the negative pole of the
battery. On a steel boat, this will result in a difference in potential
between the hull and the propeller. The bottom of the boat is
normally protected by paint and, therefore, insulated in theory.
However, any scratch in the paint will result in two different
metals being dipped in electrolyte and short-circuited, and an
electrical current will immediately start flowing.
To solve this problem, you will need to fit a sacrificial anode
made of a metal with a lower potential than the hull, such as zinc
or aluminium. The difference in potential between the anode and
the propeller ensures that the anode is corroded, not the hull.
The second situation does concern the onboard
power system. The negative pole of the battery is
usually connected to the hull, via the engine for
instance. If the boat is used as a conductor, perhaps because
the negative pole of the lighting system is not wired directly to
the battery but connected through the hull, a small difference
in potential can arise between these two connections. This
can also cause corrosion and the risk is especially high with
aluminium boats if the hull is used as a conductor. In this case,
all equipment, including engines, generators, alternators and
navigation equipment, needs to be unearthed and the negative
pole of the battery has to be connected with the hull at a single
central point only.
The third relevant situation involves the shore power
earth connection. In power installations, the neutral
and protective earth are connected to each other
at the power station and connected to groundwater via a thick
steel rod. This means that all protective earth connections in a
harbour are linked to each other. Steel sheet pile walls and quays
are also connected to the protective earth via groundwater.
When an aluminium boat is moored next to one made of steel,
for instance, the two different metals (steel and aluminium)
are dipped in electrolyte (water) and a small difference in
potential arises between them. If both hulls are connected to the
protective earth, a short-circuit will arise and lead to corrosion.