Aluminium Design and Construction John Dwight

times that of the actual pits) produces a false impression as to the true
condition of the metal. Tensile tests on 6063 alloy glazing bar material,
after long service at polluted sites, have shown the metal to be only
very slightly weakened by the pitting and still well able to meet the
specified strength. The only possible worry is the fall-off in ductility
that can occur with thin material.
The colour of weathered aluminium varies from a soft blue-grey in
rural environments to dark grey or black in industrial ones, In terms of
appearance, it is a help if the surface is washed from time to time. When
a component has its upper and lower surfaces both exposed to a polluted
atmosphere, the lower surface appears to be more heavily attacked, because
the corrosion products are not washed away by the rain in the way they
are on the upper surface. Regular washing is especially important for an
anodized surface, if it is to retain its good looks. Anodizing confers no
permanent benefit in appearance unless properly maintained.
Apart from pitting attack, it is sometimes possible for exposed
aluminium surfaces to suffer from other more abnormal forms of corrosion,
such as: (a) exfoliation, namely a delamination or peeling away of layers
of metal parallel to the surface, in much the same way as the rusting
of steel; and (b) intercrystalline attack or ‘stress-corrosion’.
Exfoliation is liable to occur with the strong aircraft-type alloys (2xxx,
7xxx), when used unprotected in some adverse environments. Stresscorrosion
is a potential hazard with the stronger 5xxx materials (Mg >
3.5%), when they have to operate for a long time at an elevated
temperature (say, over 70°C). It is also a consideration in the HAZ at
welds made on the weaker type of 7xxx material, such as 7020.
Another hazard is the poultice effect, which arises when an aluminium
surface is denied oxygen, thus causing it to go on corroding because
the oxide layer cannot reform. An example would be when aluminium
sheet lies in close contact with certain insulation boards and condensation
is present, causing chemicals to leach out of the board and attack the
aluminium.
4.7.2 When to protect against corrosion
In most aluminium installations, no protection against surface corrosion
is necessary, except for the sake of appearance. When called for, protection
can be in the form of painting (Section 3.7.4) or anodizing (Section
3.7.3). Table 4.9 gives recommendations as to when protection is needed
in outdoor situations, depending on the durability rating (A, B, C or D)
of the alloy and the severity of the environment [9]. The data on ratings
A, B and C are taken from BS.8118.
In welded construction, it is possible for the weld filler material, and
hence the weld deposits, to be less durable than the parent metal. This
must be taken into account when deciding whether or not to paint.
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