Magnesium Anodes.-- Magnesium anodes have
Hull protection is provided at all times until the
a half-cell potential of about negative 1.5 volts. They
anode is completely consumed.
are not used in seawater applications because of rapid
DISADVANTAGES OF SACRIFICIAL AN-
loss of the anode material and overprotection due to
ODES.-- The following is a list of the disadvantages
the high driving voltage. They are used in fresh or
of sacrificial anodes:
brackish water areas where the resistivity of the
electrolyte is relatively high and a higher driving
The anode current is uncontrollable.
voltage is required to produce the proper amount of
Water turbulence around the hull increases the
Iron Anoes.-- Iron anodes are installed to
Frequent replacement is necessary when stray
increase the presence of iron ions in the water. This
dc is present (especially when welding ma-
strengthens the formation of the oxide film produced
chines are used).
on copper alloy surfaces,
Fuel consumption is increased.
Steel Waster Pieces.-- Steel waster pieces are
sleeves of mild steel installed at nonferrous metal
Replacement is usually necessary before sched-
junctions to protect sea valves and sea chests.
uled overhaul (every 3 years).
USES OF SACRIFICIAL ANODES.--
Impressed Current Cathodic Protection
Sacrificial anodes are used in small boats, mothballed
ships, and submarines. They may be installed in piping
systems, bilge pumps, valves, ballast tanks, fuel tanks,
The impressed current cathodic protection (ICCP)
sewage collection holding tanks (CHTs), sonar
system (fig. 11-9) uses an external source of electrical
domes, voids, and stem tubes.
power provided by a regulated dc power supply to
ADVANTAGES OF SACRIFICIAL AN-
provide the current necessary to polarize the hull. The
ODES.-- The following is a list of the advantages of
protective current is distributed by specially designed
inert anodes of platinum-coated tantalum. The
No external electrical power is required.
principal advantage of an ICCP system is its automatic
control feature, which continuously monitors and
They are relatively foolproof and little mainte-
varies the current required for corrosion protection. If
nance is required.
the system is secured, no corrosion protection is
They are easy to install.
Figure 11-9.--Basic impressed current cathodic protection system.