Page 566 - Corrosion Engineering Principles and Practice

528 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 529

FIGURE 13.5 Sacrificial zinc anodes on the interior wall of a drydock water
handling pipe. (Courtesy of Defence R&D Canada-Atlantic)

conductive waters. Industry data have provided estimates for the
1998 sales of various hardware components totaling $146 million. The
largest share of the cathodic protection market is taken up by sacrificial
anodes at $60 million, of which magnesium has the greatest market
share. Major markets for sacrificial anodes are the water heater and
the underground storage tank markets [7].
The oil and gas industries have probably been responsible for the
greatest seawater applications of sacrificial anodes in seawater
applications. New technologies had to be developed to support the
exploitation of deep sea resources which had expanded at a great
pace since the mid 1970s. Corrosion protection of the expensive and
intricate structures had to be based on CP systems, for which the
available scientific data were sparse. Designs were often based more
on inspired guesswork than on the application of science, particularly
for impressed current systems. Designers preferred to use copious
quantities of inexpensive zinc anodes in the belief that overprotection
was safer than the risk of underprotection [8].

13.2.2 Impressed Current Cathodic Protection
ICCP of ships is always used in conjunction with protective coatings.
The coatings are intended as the primary protection, and the ICCP is
a backup in those areas where coating defects may be present. In the
period immediately subsequent to the application of the coating,
there is very little demand on the impressed current system. As the
coating deteriorates during the operational life of the ship, the CP
current demand increases. Eventually, the current demand placed
may exceed the capabilities of the design, with high anode currents
causing even more damage to the coating, particularly in areas