Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 401

Atmospheric zone

Splash zone high
tide

Mean high tide

Tidal zone

Mean low tide

Continuous
submerged
zone

Mud line

Mud zone

Relative metal thickness loss

FIGURE 6.6 Corrosion profile of steel piling after 5 years of exposure ( Humble, 1949 ).

6.3 CORROSION STRESSES DUE TO THE ATMOSPHERE,
WATER AND SOIL
Atmospheric corrosion is a process that takes place in a film of moisture on the
metal surface. The moisture film may be so thin that it is invisible to the naked
eye. The corrosion rate is increased by an increase in relative humidity; by the
occurrence of condensation, when the surface temperature is at or below the
dew point; and by an increase in the amount of pollution in the atmosphere,
because the corrosive pollutants can react with the steel and may form deposits
on the surface. Experience has shown that significant corrosion is likely to take
place if the relative humidity is above 80% and the temperature above 0 C.

It is worth mentioning that, when pollutants or hygroscopic salts are present,
corrosion occurs at much lower humidity levels. The atmospheric humidity and
air temperature in a particular region will depend on the climate conditions in
that part of the world.
The location of the constituent element of a structure also influences corro-
sion. If the steel structures topside are exposed to the open air, climatic

°