Page 194 - Corrosion Engineering Principles and Practice

168 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 169

FIGURE 6.24 Sand trapped behind trim can wear away the protective paint
coating and expose the steel to the environment and subsequent severe
corrosion damage.

wet, but during the drying period, adjacent wet and dry areas
develop. A differential aeration cell develops near the edges of the
wet zones leading to corrosion.
Areas on automobiles where poultice has occurred, for example,
include the hem flange within doors, wheel wells, and inside frames.
Sand trapped behind trim can wear away the protective paint coating
and expose the steel to the environment and subsequent severe
corrosion damage (Fig. 6.24). These areas remain wet almost
continuously with a highly corrosive liquid due to the moisture
entrapment effect of the poultice. The aggravation caused by deicing
salts can be quite serious in these areas due to wet dry cycling and
accumulation that can reach saturation. A detailed analysis of
poultices collected on 50 cars driven in four major North American
cities has revealed the presence of large quantities of ions such as
sodium, calcium, sulfate, and chloride (Table 6.1) These ions were
thought to be most probably coming from road deicing and dust
control agents [13].

Corrosion under Insulation. Corrosion under insulation (CUI), for
example, is a particularly severe form of localized corrosion that has
been plaguing chemical process industries ever since the energy crisis
of the 1970s forced plant designers to include much more insulation
in their designs [14].