aluminium in commercial vehicles - European Aluminium Association

EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER XI 151
2.2.6. 5000 series alloys
and elevated temperatures
When held for long periods at
elevated temperatures (between
65°C and 200°C), aluminiummagnesium
alloys containing
more than 3% of magnesium
undergo metallurgical changes
that can lead to intergranular
corrosion if the two conditions
below are both satisfied:
• Precipitation of a continuous
bead of Al Mg intermetallic
8 5
compounds occurs along the
grain boundaries (sensitization).
These Al Mg precipitations are
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anodic to the bulk material.
• Presence of an aggressive
medium, e.g. a saline solution on
the bare surface of the material.
This phenomenon has been studied
many times with a view to gauging
the influence of the following
parameters for sensitization:
• The magnesium content and the
production process largely determine
the kinetics of sensitization of
5000 series material. Proper routes
to minimize susceptibility are well
established at suppliers.
Filiform corrosion coming from default in paint
• Manufacturing processes like
forming and thermal joining
(welding) might reduce resistance
of final product to sensitization.
• The thermal load (i.e. temperature
multiplied by time of exposure)
is more important than the
temperature alone. For example,
if 65°C is often given as a limit in
catalogues or manuals, it takes
two years to sensitize a 5086
alloy at that temperature, while
at 100°C, several months are
necessary. The fastest sensitizations
are generally observed
between 130°C and 150°C.
But even if a material is sensitized,
corrosion will only happen
in aggressive environments, i.e.
when a corrosive electrolyte gets
in contact with the metal surface.
Experience has confirmed this.
There are road tankers for heavy
fuel oil, which have seen 20 years
and more of service, running 8 to
10 hours a day, which is at least
50,000 hours of cumulative
operation at 65-70°C.
As a general guideline, the use of
alloys with a maximum of 3% of
magnesium is strongly recommended
where exposure for long
periods to temperatures in excess
of about 75°C occurs. When the
use of 5000 alloys with higher
Mg content is desired, consultation
with the material producer is
necessary and their applicability
must be evaluated in detail, taking
into account the thermal
exposure of the part during its
total lifetime.
2.2.7. Other forms of corrosion
Other forms of corrosion do exist,
but the alloys and tempers most
currently used in commercial
vehicles are not prone to these
types of corrosion.
2.3. Further references
• Corrosion of aluminium,
C. Vargel, ed. Elsevier
• www.corrosion-aluminium.com