zinc silicate epoxy primer and aliphatic polyurethane or epoxy topcoats at a total dry film
thickness of 240 m would be suitable for 10- to 15-year maintenance periods.
The most severe conditions of corrosion are experienced in the splash zonewhere
Hedborg(10.18)quotes corrosion rates of 0.13 to 0.25 mm per year in the Canal Zone of the
USA and the Hawaiian Islands, and a rate of 0.88 mm per year which has been observed on
a platform at Cook Inlet, Alaska. Paint coatings used in the past comprised a zinc silicate
primer followed by three or more coats of epoxy coal-tar paint to obtain an overall dry
film thickness of 400 m. However, such coatings had a life of only a few years, after
which wastage on the exposed steel continued at the rates quoted above and are no longer
recommended. Alternative high-build, shop-applied, organic coatings, such as glass flake
epoxy(2.5), are more durable. These have been subjected to bioreactor tests and have shown
good protection against localized corrosion due to bacteria at the low water level. The
problem of pinholes and abrasion allowing potential bacterial attack remains. Cathodic
protection is ineffective in the splash zone and the thickness of metal should be such as to
ensure that wastage due to corrosion will not curtail the design life of the structure. This is
achieved over the length in the splash zone, either by increasing the thickness of the steel,
or by providing cover plates of steel to the same specification as the piles, bracings, or jacket
members or of a corrosion-resistant material such as Monel metal. Above the splash zone
continual maintenance by periodic cleaning and painting is needed on exposed steelwork.
Below the splash zonethe bare or painted steel is protected by cathodic means or a
compatible combination of coating and cathodic measures. Cathodic protection utilizes the
characteristic electrochemical potential possessed by all metals (see BS 7361-1:1991
Cathodic Protection–except for offshore applications). The metals which are higher in the
electromotive series act as anodes to the metals lower in the series which form the cathodes.
Thus if a steel structure is connected electrically to a zinc anode the current escapes to the
soil or water through the anode, and the structure thus forms the cathode, so preventing the
escape of metallic ions from the structure. The two methods of cathodic protection used in
marine structures are the sacrificial anode system and the power-supplied (or impressed-
current) system. In the former, large masses of metal such as magnesium, aluminium, or
zinc, which are higher in the electromotive series than steel, are used as the anodes. In the
power-supplied system the anodes are non-wasting and consist of graphite, lead-silver, or
other noble metals. They supply direct current from a generator or transformer rectifier to
the structure acting as the cathode. The anodes are not welded directly to the piles as they
would become detached when driving through guides or jacket members. Instead they are
electrically bonded to the jacket or to bracing members. Appropriate coating can reduce the
direct current requirements.
The sacrificial anode system is generally preferred for marine structures since it does not
require the use of cables which are liable to be damaged by vessels or objects dropped or
lowered into the water from the structures. However, Hedborg(10.18)points out that divers are
required to replace sacrificial anodes which are designed to have, for example, a 10-year
life. In depths of water of up to 60 m the anodes can be replaced by divers at a reasonable
cost, but in deep-water platform structures, the diving costs increase steeply with increasing
depths. Sacrificial systems can be designed to be replaced without using divers, and the life
of the anodes can be extended by reducing the area of steel requiring protection, i.e.
by painting the steel. While a long life cannot be achieved by painting, marine growth will
replace the paint and hence maintain the protection. The choice between sacrificial anode
systems, with or without a coated structure, and power-supplied systems is a matter of
496 The durability of piled foundations