Atmospheric Corrosion 93
a metal as a result of washing away the pollutants deposited during the dry
spell.
Whether the rain will increase or decrease the corrosive action depends
on the ratio of deposition between the wet and dry contaminants. When the
dry period deposition of pollutants is greater than the wet period deposition
of surface compounds, the washing effect of the rain will dominate and the
corrosive action will decrease.
In areas where the air is less heavily polluted, the corrosive action of the
rain will assume much greater importance because it will increase the cor-
rosion rate.
Another factor affecting the relative corrosion rate resulting from rain is
the orientation of the metal surface. In areas of heavy industrial pollution,
skyward-facing metal surfaces benet from rain. In those areas where dry
deposition is considerably greater than wet deposition of sulfur pollutants,
the washing effect of rain predominates, and the corrosion rate decreases. In
areas having less pollution, the situation is reversed and the corrosive action
of the rain predominates.
4.3.2.3 Fog
In areas having a high degree of air pollution, high acidity and high concen-
trations of sulfate and nitrate can be found in fog droplets. The pH of fog water
has been found to be in the range of 2.2 to 4 in highly contaminated areas.
4.3.3 Deposit of Pollutants
Atmospheric pollutants can be deposited into the aqueous layer by either
wet or dry deposition. For wet deposition to take place, it is necessary for the
rain, fog, dew, or snow to be present; whereas for dry deposition, precipita-
tion of any kind is not involved. Dry deposition is considered predominant
indoors or in highly polluted areas close to emission sources. It is difcult to
determine the relative importance of wet deposition because of the inciden-
tal nature of the precipitation.
Dry deposition is controlled by two factors: (1) aerodynamic processes and
(2) surface processes. Aerodynamic processes relate to the actual depletion
of the gaseous constituent in the atmosphere (e.g., SO
2
), in the atmospheric
region adjacent to the aqueous phase, and the ability of the system to add
new SO
2
into this region. Whether or not new SO
2
can be introduced into
the region depends on the actual wind speed, type of wind ow, and shape
of the sample. The ability of the surface layer to accommodate the gaseous
constituent (SO
2
) is a phenomenon of the aqueous layer, which is a surface
process. Greater amounts of gaseous constituents (SO
2
) can be accommo-
dated with increases in the thickness of the aqueous layer, resulting from
increased relative humidity, the pH of the solution, and the alkalinity of the
solid surface.