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The atmospheric environment varies drastically with regards to corrosivity
depending on the geographical location.
Electrolyte.
In atmospheric corrosion, the electrolyte is moisture from precipitation,
fog or dew, sea spray, or other sources. The three factors that have the most
influence on the corrosivity of the atmosphere at a given site are:
(1) the amount of time that exposed surfaces remain wet at the site,
(2) the amount of chloride from the sea that reaches the surfaces
(3) the amount of industrial pollutants (mainly acids) that reach the
surfaces.
In all atmospheric environments there is an excess of oxygen, thus the
corrosion of most metals in atmospheric environments is not limited by the
amount of oxygen present and can proceed rapidly when the electrolyte is
present. The corrosivity of the atmosphere varies drastically.
The corrosion rate of steel, for example, can vary by a factor of 100. In
general, the least corrosive atmospheres are found in dry inland (desert) sites
and the most corrosive sites are industrial or industrialmarine sites.
Moist tropical locations are very corrosive due both to the time of wetness
and the high temperatures experienced. However, local conditions and features of
design have an influence on corrosive attack that often exceeds the differences
experienced due to geographical conditions.
Thus, generalizations regarding specific site corrosivity based on the
corrosion of a single metal at a single location at a given site can be
misleading.
It is not prudent to ignore the possibility of corrosion at a dry inland
site nor to consider corrosion inevitable at a marine industrial site. The rates
of attack can vary at different sites, but the mechanisms of attack, features
that cause accelerated attack and corrective measures for
corrosion
prevention and
corrosion
protection that can reduce the attack are similar.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
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