330 Fundamentals of Corrosion
seawater composition all have a bearing on the corrosion rates. However, we
are primarily concerned with conditions in the temperate category. But keep
in mind that in the southern sections of the temperate zone, ambient temper-
atures may be relatively high, which can have an effect on corrosion rates.
A rural atmosphere is generally that of an inland farm with little or no
heavy manufacturing operations. In such areas, the problems of atmospheric
corrosion will be somewhat diminished. The same will apply in very dry
climatic conditions.
Urban atmospheres, even when free from industrial pollution, are subject
to pollution from road trafc and the burning of fossil fuels. The former pro-
duces oxides of nitrogen, which may be oxidized to nitric acid. The latter has
the potential of generating sulfur dioxide, which is converted to sulfuric and
sulfurous acid in the presence of moisture.
In an industrial atmosphere, all types of contamination by sulfur in the
form of sulfur dioxide or hydrogen sulde are important. The burning of fos-
sil fuels generates a large amount of sulfur dioxide, which is converted to sul-
furic and sulfurous acids in the presence of moisture. Combustion of these
fossil fuels and hazardous waste products should produce only carbon diox-
ide, water vapor, and inert gas combustion products. This is seldom the case.
Depending on the impurities contained in the fossil fuel, the chemical com-
position of the hazardous waste materials incinerated, and the combustion
conditions encountered, a multitude of other compounds may be formed.
In addition to the most common contaminants previously mentioned, pol-
lutants such as hydrogen chloride, chlorine, hydrogen uoride, and hydro-
gen bromide are produced as combustion products from the burning of
chemical waste. When organophosphorous compounds are incinerated, cor-
rosive phosphorous compounds are produced. Chlorides are also a product
of municipal waste incinerators.
Energy production leads to the formation of oxides of nitrogen, which may be
oxidized to nitric acid. This reaction has a very low rate; therefore, in the vicin-
ity of the emission source, the concentration of nitric acid and nitrites is very
low. The greatest effect will occur at some distance from the emission source.
The antipollution regulations that have been enacted do not prevent the
escape into the atmosphere of quantities of these materials sufcient to pre-
vent corrosion problems. The corrosivity of an industrial atmosphere dimin-
ishes with increasing distance from the area.
Marine environments are subject to chloride attack resulting from the
deposition of droplets of crystals formed by evaporation of spray that has
been carried by the wind from the sea. The quantity of chloride depositions
from a marine environment is directly proportional to the distance from
the shore — the closer to the shore, the greater the deposition and corrosive
effect. The atmospheric test station at Kure Beach, North Carolina, shows
that steel exposed 80 ft from the ocean corrodes 10 to 15 times faster than
steel exposed 800 ft from the ocean. The distance from the shore at which
marine atmospheres cease depends on the prevailing winds, the distance