major differences in environment the ship
rudder stock and the hull. To permit full rotation of the
rudder stock from port to starboard, a large loop is
encounter--seawater and brackish or fresh water.
required in the ground strap.
HULL POTENTIAL SETTING OF SHIPS IN
DIELECTRIC SHIELD.-- The dielectric shield
SEAWATER.--The impressed current system is
designed to operate automatically and requires a
prevents shorting of the anode current to the hull and
minimum amount of maintenance. The operator
aids in wider current distribution. The dielectric shield
normally sets the hull potential at -0.85 volt. When the
is applied as a thick coating around each anode. It
voltage between the hull and the reference electrode is
consists of a high-solids epoxy with a high-dielectric
more positive than the voltage set by the operator, the
strength.
output of the controller increases. This causes an
Operation
increase in the anode current output from the power
supply until the voltage between the hull and the
The requirements for operating the ICCP system on
reference electrode approaches the set voltage. A
ships is provided in the manufacturer's technical
voltage between the hull and the reference electrode that
manual. The system should be operated at all times,
is negative to the set voltage causes a decrease in
with the following exceptions:
controller output, thereby decreasing the anode current
1. During diving operations
output.
2. During equipment repair
The optimum range of polarization or
3. During planned maintenance
hull-to-reference electrode potential for a ship with an
ordinary steel hull is from a -0.80 to a -0.90 volt to the
4. During drydocking
silver/silver chloride reference electrode. Increased
The system must be reactivated within 2 hours after
anode current will result in hull potentials more negative
the activity is completed. Never energize the system if
than the optimum amount. Increasing the negative
the ship is out of the water (drydocked).
potential does not provide more protection. If exceeded,
this will result in hydrogen generation at the hull
Before the reference electrode is connected to the
surface.
controller, you should check the voltage between the
reference electrode and the steel hull. The voltage
HULL POTENTIAL SETTING OF SHIPS
should be approximately 0.6 volts dc. The hull will be
ENTERING BRACKISH OR FRESH WATER.-- As
negative (), and the reference electrode will be positive
a ship enters a port or bay that is riverfed, the resistivity
(+). If the voltage is zero, the reference electrode has
of the water will change as the salinity changes.
an open lead or the lead or electrode is shorted to the
Operation of the ICCP system will be affected by the
hull. When the voltage is 0.6 volts or higher, the ship is
changing water resistivity. The operator will notice the
receiving cathodic protection from an external source,
ICCP system operating at higher voltage outputs and
which could be zinc anodes or an electrical leakage.
lower current outputs. The lower current output is
caused by the higher impedance of the water. A higher
You need to inspect the controller and power supply
voltage output is required to drive the same current in
wiring to ensure the unit is properly grounded. Before
the higher-resistivity electrolyte. The operator will
connecting the anode leads to the power supply, check
record this condition on the ICCP log. Do not take
for possible shorts. The voltage developed between a
action to correct this condition by equipment calibration
disconnected platinum anode and the steel hull will
while the ship is in brackish water.
range from 1.0 to 2.0 volts dc and can be read on a
Cathodic Protection Log
positive (+) and the polarity of the hull is negative (-).
If this voltage is zero, you could have an open lead wire
Normal operating procedures require maintaining a
Cathodic Protection Log, NAVSEA Form 9633/1 (fig.
or a shorted anode. When the voltage reads between 2.0
11-9A and 11-9B) on ICCP system operation. The
to 5.0 volts, it indicates that the anode lead is immersed
readings are recorded on these logs daily and submitted
in seawater.
to NAVSEA monthly. Logs submitted to NAVSEA are
Hull Potential Settings
analyzed to identify those systems that are not operating
correctly. After analysis of the logs is complete, a
As the ship's water environment changes, so must
response is sent to the ship or TYCOM indicating the
the level of protection from the cathodic protection
operational status of the equipment as interpreted from
system. The following paragraphs address the two
11-11
