Types of Corrosive Atmospheres

While atmospheres have been traditionally classified into four basic types, most environments are in fact mixed and present no dear 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]. 

An industrial atmosphere is characterized by pollution composed mainly of sulfur compoimds such as sulfur dioxide (SO ), a precursor to acid rain, and nitrogen oxides (NO ), the backbone of smog in modern dties. Sulfur dioxide from burning coal or other fossil fuels is picked up by moisture on dust particles as sulfurous add. 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 maybe much more corrosive than the add sulfates. The reactivity of add chlorides with most metals is more pronounced than the reactivity of other pollutants such as phosphates and nitrates. 

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