over these zones, particularly in the low-water and splash zones (see BS 6349 (8.1)). The
development of ‘macro-cells’in the steel at the junction of the tidal and low water zones can
limit the corrosion rate to that in the immersed zone. As in the case of canals, abrasion from
fenders and floating debris also causes increased corrosion in the low-water zone.
The presence of marine growth also has a considerable influence on protective measures.
There is no growth within the atmospheric and splash zones, but in the intertidal and
continuously immersed zones heavy growths of barnacles and weeds can develop, which
damage the paint treatment and prevent its renewal. However, the growth can shield the steel
from exposure to oxygen and in this way reduce the rate of corrosion, counter-balanced by
the removal of the growth by abrasion and wash from ships, particularly those with bow
thrust propellers. Evidence for bacterial corrosion is limited and it is generally concluded
that, although bacterial activity occurs to some extent on most marine structures, it does not
cause a significant amount of corrosion damage.
In discussing protective measures Morley and Bruce noted that it is improbable that the life
of paint coatings from application to first maintenance will exceed 12 to 15 years, although
recent improvements in glass flake epoxy and polyester coatings can extend the maintenance
period to over 25 years. However, the cost of painting should be balanced against the alter-
native of increasing steel thickness, or the use of high tensile steel at mild steel stresses (e.g.
grade S355GP steel at S270GP steel stresses, see Section 2.2.6). This provides an additional
corrosion loss of 30% without loss of load-bearing capacity at an additional steel cost of
about 7%. It should also be noted that steel thicknesses may be determined by the stresses
caused during driving (see Section 2.2.6) giving a reserve available for the lower stresses
under service conditions. Also, maximum stresses for working conditions in marine
structures may be at or near the soil line where corrosion losses are at the minimum rate.
Corus(2.4)recommend the following protective measures for marine structures:
Atmospheric zone and splash zone Organic coatings or high-quality concrete encase-
ment, well compacted with appropriate cover,
extending 1m below mean high water level. Care
is needed to ensure that the splash zone, and pos-
sibly the tidal zone, is fully encased, otherwise
increased electro-chemical corrosion can occur at
the steel-concrete junction. Coatings should have
a 400 m dry film thickness to give estimated
20-year life.
Intertidal zone Bare steel to nominal or increased thickness to allow
for corrosion loss (because of uncertainty in driving
depths, it may be necessary to extend the coating
from the splash zone into the intertidal zone).
Continuously immersed zone Bare steel or cathodic protection.
Underground zone No protection necessary.Piles forming the main supporting structures in important jetties or in offshore platforms
exposed to a marine environment require elaborate and relatively expensive treatment to
ensure a long life. The steel in the atmospheric zoneis protected by paint and the first essential
is to obtain thorough cleaning of the metal. This is achieved by the application of sand or
grit blasting to obtain a white metal or near white metal condition. Coating systems using
The durability of piled foundations 495