Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 402

parameters such as rain and sunshine and pollutants in the form of gases or
aerosols affect corrosion.
Special care should be taken when considering structures that are partly
immersed in water or partly buried in soil, such as the offshore structure
jacket. Corrosion under such conditions is often restricted to a small part of
the structure, where the corrosion rate can be high. Exposure tests for estimat-
ing the corrosivity of water or soil environments are not recommended. The
type of water

has a significant influence on the cor-
rosion of steel. Corrosivity is also influenced by the oxygen content of the
water, the type and quantity of dissolved substances and the water tempera-
ture. For offshore structure jackets, animal or vegetable growth can accelerate
corrosion.
Three different zones of immersion in water can be defined. First, the under-
water zone is the area that is permanently exposed to water. Second, the inter-
mediate zone is a fluctuating level zone, where the water level changes due to
natural or artificial effects, thus giving rise to increased corrosion due to the
combined impact of water and the atmosphere. Third, the splash zone is the
area subject to wave and spray action, which can give rise to exceptionally
high corrosion stresses, especially with seawater.
Corrosion in soil is dependent on the amount and the nature of mineral content
of the soil and on the presenting organic matter, the water content and the oxygen
content. The aeration of the soil is the main factor that influences its corrosivity.
The oxygen content will vary and corrosion cells may be formed. Different
types of soil and differences in soil parameters are not considered classification cri-
teria in ISO 12944.
It is worth mentioning that, for the selection of a protective paint system, the
special stresses to which a structure is subjected and the special situations in
which a structure is located should be taken into consideration. Both the design
and the use of the structure may lead to corrosion stresses not taken into con-
sideration in the classification system.

— fresh, brackish or salt —

6.3.1 Classification of Environments
For the purposes of ISO 12944, atmospheric environments are classified into six
atmospheric corrosivity categories. To determine corrosivity categories, the
exposure of standard specimens is strongly recommended. Table 6.7 defines
the corrosivity categories in terms of the mass or thickness loss of standard spe-
cimens made of low-carbon steel and/or zinc after the first year of exposure. For
details of standard specimens and the treatment of the specimens prior to and
after exposure, see ISO 9226. Extrapolation of thickness losses to one year
from shorter exposure times, as well as back-extrapolation from longer times,
will not give reliable results and are therefore not permitted. The thickness
losses obtained for steel and zinc specimens may sometimes fall into different
categories. In such cases, the higher corrosivity category should be used. If it is