Page 361 - Corrosion Engineering Principles and Practice

330 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 331

FIGURE 9.1 Rusting galvanized roof with regular bands of rust.

9.2 Types of Corrosive Atmospheres

While atmospheres have been traditionally classified into four basic
types, most environments are in fact mixed and present no clear
demarcation. Furthermore, the type of atmosphere may vary with the
wind pattern, particularly where corrosive pollutants are present, or
with local conditions (Fig. 9.2) [2].

9.2.1 Industrial
An industrial atmosphere is characterized by pollution composed
mainly of sulfur compounds such as sulfur dioxide (SO ), a precursor to
2
acid rain, and nitrogen oxides (NO ), the backbone of smog in modern
x
cities. Sulfur dioxide from burning coal or other fossil fuels is picked up
by moisture on dust particles as sulfurous acid. This is oxidized by some
catalytic process on the dust particles to sulfuric acid, which settles in
microscopic droplets and fall as acid rain on exposed surfaces. The
result is that contaminants in an industrial atmosphere, plus dew or fog,
produce a highly corrosive, wet, acid film on exposed surfaces.
In addition to the normal industrial atmosphere in or near
chemical plants, other corrosive pollutants may be present. These are
usually various forms of chloride which may be much more corrosive
than the acid sulfates. The reactivity of acid chlorides with most
metals is more pronounced than the reactivity of other pollutants
such as phosphates and nitrates.