Page 366 - Corrosion Engineering Principles and Practice

334 C h a p t e r 9 A t m o s p h e r i c C o r r o s i o n 335

In the absence of moisture, most contaminants would have little or
no corrosive effect.
Rain is not always corrosive. It may even have a beneficial effect
by washing away atmospheric pollutants that have settled on exposed
surfaces. This effect is particularly noticeable in marine environments.
On the other hand, if the rain collects in pockets or crevices, it may
accelerate corrosion by supplying continued wetness in such areas as
shown in Fig. 9.6.
Dew and condensation are undesirable from a corrosion stand-
point if they are not accompanied by frequent rain washing to dilute
or eliminate surface contamination. A film of dew, saturated with
sea salt or acid sulfates (acid rain), and acid chlorides coming from
an industrial atmosphere provide an aggressive electrolyte for the
promotion of corrosion.
Temperature plays an important role in atmospheric corrosion in
two ways. First, there is a normal increase in corrosion activity which
can theoretically double for each 10° increase in temperature. Secondly,
a little-recognized effect is the temperature lag of metallic objects, due
to their heat capacity, behind changes in the ambient temperature. The
period of wetness is often much longer than the time the ambient air is
at or below the dew point and varies with the section thickness of the
metal structure, air currents, RH, and direct radiation from the sun.

FIGURE 9.6 Galvanized bolting assembly after 10 years of exposure to
a deicing salt environment.