Aluminium Design and Construction John Dwight

Table 4.8 Nominal composition of weld filler alloys
Note. * The use of these fillers may lead to corrosion problems under prolonged exposure in a hot
environment
Atmospheric corrosion of aluminium is by a process of pitting. Often
referred to as ‘weathering’, this is radically different from the rusting
of steel. Air-borne pollutants, such as sulphuric acid and sodium chloride,
cause local pits to form, the depth of which may be used as a measure
of the severity of the attack. In the first year, the pit depth increases
relatively quickly, but with the passage of time the corrosion products
effectively stifle the process. Eventually, after five years or so, further
corrosion almost stops. This pattern of behaviour is illustrated in Figure
4.12, which shows the likely variation of average pit depth with time for
3103 sheet in an outdoor industrial location [32].
The severity of pitting depends on the alloy and the environment. For
a very durable alloy, such as 3103, the worst pits may have a depth after
five years of roughly 0.04 mm in a rural location, 0.08 mm in an inland
industrial one and something like 0.16 mm at an industrial site by the sea.
The surface of weathered aluminium always looks much worse than
it really is, and the immense volume of the corrosion products (some 200
Figure 4.12 Pitting corrosion. Typical growth of average pit depth with time for exposed
3103 sheet in an inland industrial location (Alcan.)
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