 System Setup & Operation
Configuration
Most vessels are fitted with independent forward and aft systems
for redundancy and uniform distribution of current. Forward systems
are usually composed of recessed hull-mounted anodes and reference
electrodes (cells) for physical protection. The aft system usually
includes surface-mounted anodes and electrodes for maximum output.
Anode materials must be inert to the environment and proven effective,
consequently noble metal combinations are used. Zinc reference
electrodes are manufactured with mil-spec type zinc.
Maintenance
The automatically-controlled system is virtually maintenance-free.
All that is required is the monthly upload of system readings from the
IP Logger to Wilson Walton for analysis, and periodic cleaning
of shaft slipring brushes to maintain adequate ground. Should the
system fail, it is equipped with overprotection circuitry which
initiates automatic shutdown to manual mode. Drydock inspection of
hull components is mandatory, at which time electrical equipment is
inspected, calibrated and commissioned.
An ICCP system is the best type of system for the prevention of
corrosion. This permanent system will ensure against hull failures for
the life of the vessel.
The Aquamatic III system employs the latest in computer technology
to insure the finest in reliability, accuracy and serviceability. The
backbone of the system is the computer-controlled switch-mode power
supply. This unit has been refined over the years based on customer
feedback and the vast experience of Wilson Walton International
engineers. In summary, Aquamatic III operates quietly and
conveniently, requiring the minimum of attention and maintenance.
Understanding the Display
Aquamatic III has a distinct advantage over galvanic types of
cathodic protection -- control over the amount of current supplied --
which allows the degree of protection to be easily regulated.
Performance can be monitored from its integrated vacuum fluorescent
(VF) display which is always visible on the front panel of the
administrator power unit or RAD. The following readouts are provided:
- CELL [#] - Displays hull potential in millivolts at the specified reference cell.
- SHAFT [#] - Displays propeller shaft potential in millivolts. Range of operation is 0mV to 250mV.
- OUTPUT VOLTS - Displays the output voltage in VDC. This relates
to the total output voltage applied to the hull mounted anodes with respect to the hull.
- ANODE [#] AMPS - Displays the output current for the specified
anode in amperes DC. This relates to the individual amount of
current each anode supplies to the hull.
The ability of Aquamatic III to report the output of each
anode facilitates analysis and troubleshooting. Less
sophisticated systems provide only an aggregate anode reading which
can inadvertently disguise developing or critical anode failures.
Properly functioning anodes may increase output to compensate for an
anode that has experienced a fault. Therefore, the combined
anode reading may appear normal even while a portion of the hull is
under-protected. With Aquamatic III, an individual anode fault can be
quickly identified and corrected.
At a glance, the display informs the crewmember of the system's
status as well as the general condition of the immersed hull. A
correct interpretation, however, must include an analysis of the
present display readings collectively, and ideally, the present
readings should be compared to prior readings in order to complete the
picture. The Aquamatic III IP Logger
automates this process for accuracy and convenience.
The reference cell potentials indicate the electrical potential of
the outer hull. One terminal of the display's input is connected
directly to the hull; the other terminal is connected to a zinc
electrode fixed to but insulated from the outer hull. When this
zinc/steel assembly is immersed in an electrolyte (such as seawater),
a galvanic potential is created. The magnitude of this potential,
measured by the computer, varies according to several parameters:
1) Amount of exposed steel area
a) Vessel draft
b) Condition of external coating
2) Conductive properties of the electrolyte (seawater)
3) Miscellaneous factors
a) Composition of the steel (on a molecular level)
b) Vessel speed
c) Temperature
d) Availability of oxygen
When a bare piece of unprotected steel is immersed in a strong
electrolyte, it will normally have a potential (with respect to zinc)
of between 300 and 750 mV, depending on the composition of the actual
materials involved. When the steel is provided with a coating,
however, the coating (ideally) insulates the steel from the
electrolyte. In theory, the potential is reduced to zero and no
corrosion occurs, because the steel is insulated from corrosive
elements. If a coating existed which could maintain this insulating
effect indefinitely, cathodic protection would become unnecessary. No
coating is capable of this performance, however, and thus cathodic
protection of immersed steel is essential.
By supplying to a steel structure an electrical current with a
higher electro-negativity than that of unprotected steel, the
corrosion process is retarded. Utilizing anodes fabricated of inert
metals such as silver, an electrical current is forced or
"impressed" through the electrolyte, forming an electrical field on
the surface of the immersed steel. As more current is impressed, the
steel becomes more cathodic. By controlling the amount of current
supplied in such a way as to exactly match the variable amount of
current required to maintain an arbitrary hull-potential, any desired
degree of cathodic protection may be obtained simply by choosing the
corresponding potential value.
Extensive research indicates that the most desirable potential
level is about 200mV (with respect to zinc.) This value is comfortably
within the "protected" range yet not so greatly negative as to cause
coating damage from excessive electrolytic action. So, under ideal
conditions, the display will always read +200mV for all cell inputs.
From a practical standpoint, however, some tolerance above or below
that desired measurement should be expected.
Next: Remote Administration Display (RAD)
Previous: Understanding Impressed Current Cathodic Protection
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