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sacrificial anodes. The anodes must be buried or submerged near the structure
to be protected and electrically connected to it with a low resistance bond. As the
anodes are consumed, they must be periodically monitored and replaced when
over 80 percent of the metal is consumed, or when they will be consumed before
the next scheduled inspection. The level of protection provided can be
determined by measuring the potential of the structure being protected by
comparison with a standard reference electrode.
3-18.104.22.168 Impressed Current. Impressed current cathodic protection systems
use an external source of electrical alternating current, and a rectifier, to provide
the protective direct current to be impressed across the system. This system also
requires anodes buried or submerged in the vicinity of the structure being
protected, but these anodes can last much longer than galvanic anodes, since
they only conduct the protective current into the water or soil and are not the
source of the current. Impressed current cathodic protection systems also require
periodic inspection and maintenance to ensure effectiveness in controlling
NONFERROUS METALS AND ALLOYS. A variety of materials are
available that, if used properly, are more resistant to corrosion by seawater and
marine atmosphere than steel. These materials are used for specialized
applications and are not used as much as steel due to higher costs. The common
nonferrous metals are aluminum, copper, nickel, titanium, and alloys of each.
Aluminum. Many alloys of aluminum are available for applications
requiring high corrosion resistance to the marine atmosphere as well as good
strength-to-weight ratios. The common uses of aluminum at the waterfront
include: Brows and platforms, decking and catwalks, and light poles and bases.
Aluminum should not be used as a substitute for steel solely for its
corrosion resistance quality. Aluminum and its alloys are subject to pitting and
crevice corrosion in marine environments, especially in submerged conditions. If
pitting can be tolerated and crevices eliminated, aluminum alloys may be used
successfully where low weight and other unique properties are required.
ASTM specifications define compositions and mechanical properties of
aluminum alloys. Alloys 5083, 5086, 5052 and 6061 are the most popular alloys
for structures exposed to the marine atmosphere.
Copper. Copper and copper alloys are suitable for waterfront use
because of their uniform, low corrosion rate. Copper is used for electrical
conductors, pipe, sheathing, and many hidden uses on supporting equipment at
the waterfront. The copper alloys usually selected for marine corrosion resistance
are: copper, cupro-nickel 90-10, cupro-nickel 70-30, arsenical admiralty brass,
and most true (zincless) bronzes. These alloys form films of corrosion products
that provide protection even in flowing water.