Figure 11-9.--Basic impressed current cathodic protection system.

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

polarizing current.

noise level.

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

System

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

sacrificial anodes:

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.

provided.

Figure 11-9.--Basic impressed current cathodic protection system.

11-9