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Electrochemical corrosion
Galvanic corrosion
A boat that lies in the water constitutes a galvanic ele-
ment since different metals (or metal alloys) such as
steel and bronze are in electrical contact with each
other while in the same electrolyte, namely salt water.
This produces galvanic corrosion.
In popular terms, this can be compared with the simi-
lar electrochemical process that goes on in a battery.
It is called a galvanic element. In a non-battery con-
text the result of this process is called galvanic corro-
sion.
There needs to be certain conditions for galvanic cur-
rents to occur: The metals need to be in an electro-
lyte, containing oxygen and galvanically connected to
each other. A corrosion current will then flow from the
metal with the lower potential (the anode) to the metal
with the higher potential (the cathode). Two chemical
reactions are needed for this - oxidation and reduction.
Oxidation (release of electrons) takes place at the an-
ode and reduction (absorption of electrons) takes
place at the cathode.
General
NOTE! For more detailed information about electro-
chemical corrosion see manual
Marine Electrical
Systems Part 1
.
Electrochemical corrosion
Electrochemical corrosion of metals can cause very
serious and expensive damage to propellers, propeller
shafts, rudders, keels and other equipment fitted to
the boat.
The types of corrosion that are particularly significant
to boats are:
• Galvanic corrosion
• Crevice and deposit corrosion
• Stray current corrosion
Galvanic and stray current corrosion are of major im-
portance when doing the installation work. It may be
caused by mistakes in the electrical installation or
choice of wrong materials in submerged components.
The damaging effects of electrochemical corrosion
start as soon as the boat is launched. Since corrosion
takes place “out of sight” below the surface of the wa-
ter, damage can be severe and is often not detected
until the boat is brought up on land.
Consequently, propellers and other components
should be protected against corrosion by the use of
sacrificial anode made of zinc or magnesium. This
protection may not be sufficient if electrical equipment
is connected incorrectly or incorrect material is used
in external boat equipment.
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