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Aluminium
Gasket ( PVC,
elastomer)
Other metal
(Steel…)
Bolt
Sleeve and
insulating
washers
PREVENTION OF GALVANIC CORROSION
FIGURE XI.3
e-
148
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER XI CORROSION 148 | 149
2.2.2. Crevice corrosion
Crevice corrosion occurs in small
constructive recesses. In a crevice
there will be the possibility for
accumulation of moisture
because of capillary forces and
deposits of aggressive media.
Therefore, especially in the road
water splash zone, constructive
gaps should always be closed as
far as possible, as the penetrating
water might contain aggressive
ions (e.g. from road salts).
The corrosion rate of crevice corrosion
is normally very low due to
the corrosion product – alumina
– being very stable and building a
sealing of the crevice. (Figure
XI.4)
2.2.3. Pitting corrosion
Pitting corrosion is the most common
corrosion form seen on aluminium,
characterised by the
development of small local pits in
the surface. The diameter and the
depth of the pits varies and
depends on different parameters
related to the aluminium itself
(type of alloy, rate of cold working,
heat treatments) or to its environment
(presence of aggressive ions).
Pitting corrosion occurs on sites
where the natural oxide film is
damaged or imperfect due to
diverse reasons like manufactur-
Sealing of access
to crevice
Possible
crevice
Sealing of the
crevice itself
Tight fastening
of bolts and rivets
necessary
CREVICES AND HOW TO PREVENT THEM
FIGURE XI.4
ing related circumstances (areas
which have been ground, weld
discontinuities etc.). The pits are
formed with a rapid increase in
depth after initiation followed by
a slower growth. This is due to
the corrosion product - alumina –
that is not soluble in water and
therefore adheres to the surface
of the metal inside the pits. The
alumina then obstructs the direct
contact between the aluminium
surface and the corrosive medium
and by this slows down the
corrosion speed. (Figure XI.5)
This slowdown in the rate of pitting
corrosion explains the fact
that aluminium equipment can be
used for decades in certain environments
(country air, sea air, sea
water) without any protection.
In other words, pitting corrosion
is quite normal and does not
impact the durability of vehicles.
PITTING CORROSION
FIGURE XI.5
Alumina
Alumina
CICICICorrosion
Attack
149
EUROPEAN ALUMINIUM ASSOCIATION
2.2.4. Constructive measures
to prevent corrosion
Some general rules shall be
applied to prevent corrosion (in
most cases to prevent any kind of
water trap or areas where condensation
can occur):
• Constructive gaps should be
avoided or, if not possible, should
be sealed. (Figure XI.6)
• Water traps should be avoided.
Assemblies should be constructed
with the open side
downwards. (Figure XI.7)
• Weld discontinuities shall, also
with regard to other issues like
stress, fatigue etc., be strictly
avoided. (Figure XI.8)
• Materials having different electrochemical
potential have to be
separated from each other by
coatings or insulating materials.
2.2.5. Filiform corrosion
Filiform corrosion (also known as
under-film corrosion) occurs
under paint or enamel layers. It
depends mostly on environmental
conditions and the quality of
the surface treatment prior to
painting. The corrosion filaments
Unsuitable solutions
Better solutions
Run of
Run of
FIGURE XI.7
Watertight seal
FIGURE XI.6
Weld discontinuities
FIGURE XI.8
have a worm-like appearance
and are readily visible. Filiform
corrosion does not attack the
metal surface, but affects the
surface appearance.
The mode of corrosion is quite
similar to pitting with the front of
the attack being supported by
moisture which penetrates the
surface layer and leads to oxygen
concentrated areas and by thus
acting as an anode. Filiform corrosion
is mainly an aesthetic effect,
but could lead in certain construction
parts to delaminating of the
surface layer system.
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