Reference Electrodes
A high purity zinc reference electrode is utilized in all Aquamatic
ICCP systems. There are many types of reference electrodes in use by
various ICCP equipment manufacturers. It has been determined that high
purity mil-spec zinc is most suitable for marine applications. The two
main benefits are its strength and safe failure modes.
Cell failures come under the following four categories:
- Painted cell
- Broken electrical connection
- Broken cell
- Consumed cell
Any of these failures would cause the system to return to minimum
output. Similar failure conditions on systems which utilize
silver/silver chloride cells would cause the rectifier/power unit to
go to maximum output, possibly causing paint damage. With Aquamatic
III, the computer can identify each of the above failure modes and
automatically identify and isolate the defective cell, thus preventing
such potential catastrophes.
The zinc reference cell is positive (+) with respect to steel.
Therefore, the steel hull is negative (-) relative to the cell.
Unprotected Hull-to-Reference is +300 mV to +600 mV. The "desired"
protected Hull-to-Reference is +200 mV (usual "desired" setting for
automatic control). "Maximum" protected Hull-to-Reference is +50 mV.
If, for example, the desired voltage is set at +200 mV on the
computer and the actual voltage is +600 mV, the switch mode power
supply will automatically adjust to maximum output. As the hull
becomes polarized (less positive) by the cathodic protection current,
the actual Hull-to-Reference voltage shifts from normal toward 0
mV. When it reaches the preset desired voltage, (typically 200 mV)
the output will diminish to the level required to maintain that
voltage. Full protection is achieved whenever the actual voltage is
equal to or slightly less than the desired voltage.
The following table offers general guidelines for
the interpretation of zinc reference electrode potentials. These
readings are valid at sea only. They should not be used in port.
Unprotected Steel = +300 mV to +600 mV
Protected Steel = + 50 mV to +300 mV
Optimum Protection Range = +125 mV to +250 mV
Overprotected Steel = 0 mV to -999 mV
Anodes
All anodes must have a dielectric shield under and around them.
This shield consists of an epoxy layer at least 1/4" (6mm) thick which
is tapered to 1/8" (3mm) at its periphery. The shield is essential
for proper operating efficiency. Without it, high
concentrations of current would be wasted in the steel areas adjacent
to the anode. By utilizing a shield, the current is "thrown" great
distances.
All anodes use a double gland cofferdam assembly on the inside of
the hull for cable entry to insure watertight integrity.
Previous: Switch-Mode Power Supply
Return to Aquamatic Main Page
|
