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Preventing corrosion

onboard boats

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

electricity.

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.

TECHNICAL BACKGROUND

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