Electrochemical Corrosion; General - Volvo D5 series Installation Manual

General

NOTE! Please refer to the Marine Electrical Sys-
tems Part 1 manual, chapter Electrochemical cor-
rossion for more detailed information.

Electrochemical corrosion

Electrochemical corrosion of metals can cause very
serious and expensive damage to propellers, propel-
ler shafts, rudders, keels and other equipment fitted
to the boat.
The types of corrosion that are particularly significant
for boats are:
•
Galvanic corrosion
•
Crevice and deposit corrosion
• Stray current corrosion
Galvanic and stray current corrosion are of major
importance 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 corro-
sion takes place "out of sight" below the surface of
the water, damage can be severe and is often not
detected until the boat is brought up on land.
Consequently, drives and propellers should be pro-
tected against corrosion by the use of sacrificial
anodes made of zinc, aluminum, magnesium or iron.
This protection may not be sufficient if electrical
equipment is connected incorrectly or incorrect mate-
rial is used in external boat equipment.
Electrochemical corrosion
Galvanic corrosion
A boat that lies in the water constitutes a galvanic
element, 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,
called a galvanic element. In a non-battery context
the result of this process is called galvanic corrosion.
There must be be certain conditions for galvanic
currents to occur: The metals need to be in an elec-
trolyte 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 reac-
tions are needed for this; oxidation and reduction.
Oxidation (release of electrons) takes place at the
anode and reduction (absorption of electrons) takes
place at the cathode.
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