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2.2. Types of aluminium corrosion in commercial vehicles . . . . . . . . . . . . . . . . . . . . 147
2.3. Further references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
CHAPTER XI
CORROSION RESISTANCE
145
EUROPEAN ALUMINIUM ASSOCIATION
1. Definition of corrosion
Corrosion is a electrochemical
interaction between a metal and
its environment which results in
changes in the properties of the
metal and which may often lead
to impairment of the function of
the metal, the environment, or
the technical system of which
these form a part (definition as
per EN ISO 8044).
Corrosion can occur locally (“pitting”),
or it can extend across a
wide area to produce general
deterioration.
2. Corrosion of aluminium
2.1. The natural oxide layer
A clean aluminium surface is very
reactive and will react spontaneously
with air or water to form
aluminium oxide. This oxide
builds a natural protective layer
on each aluminium surface with
a thickness of around 1 – 10 nm.
The oxide layer is chemically very
stable, has a good adhesion to
the metal surface, repairs itself
and protects the aluminium from
further corrosion. (Figure XI.1)
The oxide layer can be destroyed
in strong acidic or alkaline environments
or where aggressive
ions are present. Aggressive ions
can destroy the layer locally and
lead to local corrosion attack
(“pitting”). A typical case for this
reaction is the contact between
aluminium and chloride ions,
which are present in seawater or
road salts.
Some alloying elements might
increase the corrosion resistance
of the oxide layer, while others
can weaken it.
Vehicle manufacturers or fleet
operators should contact the aluminium
supplier in any case of
critical working conditions like
elevated temperatures or aggressive
loads.
Aluminium substrate
Natural oxide
layer (Al2O3)
FIGURE XI.1
146
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER XI CORROSION 146 | 147
Although highly resistant to corrosion
through its natural oxide
film, the following types of corrosion
can occur in commercial
vehicle construction or operation:
• Galvanic corrosion
• Crevice corrosion
• Pitting corrosion
• Filiform corrosion
2.2.1. Galvanic corrosion
Galvanic or bimetallic corrosion
can occur when two different
metals (or electroconductive
non-metallic materials) come
into direct or indirect contact
with each other in the presence
of an electrolyte. The reason for
this type of corrosion is the difference
in the electrochemical
potential of the two metals.
Aluminium is a very electronegative
metal and therefore special
attention has to be paid when
aluminium is used in combination
with other metals under the
presence of an electrolyte (such
as water). In an electrochemical
reaction, the aluminium is working
as an anode and is dissolving,
while the other metal retains its
integrity.
In this case, the aluminium ions
react with the oxygen of the
water to alumina (Al2O3), which
builds a white layer on the aluminium
surface.
2.2. Types of aluminium corrosion in commercial vehicles
147
EUROPEAN ALUMINIUM ASSOCIATION
There are 3 main prerequisites for
galvanic corrosion:
• 2 different metals with different
electrochemical potential
• presence of an electrolyte
• direct or indirect contact
between the 2 metals
The electrolyte enables the flow
of electrons between the 2 metals.
This can happen if the metals
are wetted by the electrolyte
(e.g. water containing salt) or
emerged in the electrolyte. In
commercial vehicles, this type of
corrosion can occur where steel
and aluminium parts are bolted,
riveted or screwed together and
where rainwater or road splash
water can come in contact with
the metal parts. (Figure XI.2)
To avoid direct contact between
the 2 metals and to prevent
entrapment of water, it is necessary
to work with insulating
material (such as neoprene or
other elastomers) between the
metals and to use sealing compounds
to close constructive
gaps. (Figure XI.3)
PRINCIPLE OF A GALVANIC CELL BUILT WITH ALUMINIUM
AS ANODE
FIGURE XI.2
Electron Flow
Electrolyte
Cathode
(e.g. copper)
Anode
(Aluminium)
2Al 2Al3+ + 6e- 6H+ + 6e- 3H2
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