Impressed Current Cathodic Protection
The basic cause of corrosion is the instability of metals in their refined forms. All metals tend to revert back to their natural states, especially when exposed to an environment such as saltwater. Thus iron, as the dominant element in steel, tends to oxidize or corrode to form rust.
The corrosion of metals in seawater is electrochemical in nature. This means that a flow of electricity (electrons) occurs between different areas on the same metal surface, through a solution capable of conducting an electric current (electrolyte). This electrochemical action causes deterioration or wastage of the metal.
Electrochemical reactions are precise and measurable in volts, making it possible to calculate the amount of metal which will be lost to corrosion. For this same reason, it is possible to exercise control.
The purpose of cathodic protection is to prevent corrosion on the exterior underwater areas of steel-hulled vessels. An Impressed Current Cathodic Protection (ICCP) system, such as Aquamatic III, is a sophisticated method of corrosion control that supplements the coating, which is generally considered the primary means of hull protection.
An ICCP system is known for its versatility, effectiveness, controllability and long-term service. The system includes seven major components:
• Reference electrodes
• Controller
• Power supply
• Anodes
• Tailshaft & propeller grounding assembly
• Rudder stock grounding assembly
• Datalogger
These units are electrically connected together to form a closed loop with the ship hull for maintaining a selected constant level of protection. The combination results in a sophisticated electrical system which performs the following functions:
1) The reference electrode measures a voltage difference between itself and the hull, which is directly related to the amount of protection received by the hull.
2) The controller compares the voltage difference produced by the reference electrode with a preset internal voltage. The output is then automatically adjusted to maintain the electrode voltage equal to the preset voltage.
3) The power supply, in response to the signal from the controller, regulates the amount of shipboard alternating current fed into the rectifier, and converts the regulated AC to direct current which is impressed on the anode.
4) The anode is mounted on the ship's hull in an insulated reinforced resin holder. The DC from the power supply is fed through the anode to the hull, thus completing the electrolytic circuit.
5) The tailshaft & propeller grounding assembly feeds protective current to the intermediate tailshaft and propeller, in addition to safeguarding against premature main engine bearing and journal failure.
6) The rudder stock grounding assembly allows protection to be provided to the rudder.
7) The datalogger is an IP Logger intelligent system which twice-daily records and uploads ICCP system readings to the ship's computer for e-mail transmission to WWI and/or owner's office.
The demand for current is governed by the wetted surface area of the ship and propeller, the condition of the hull coating, temperature, pH and conductivity of the seawater, and the speed and draft of the ship.
Basically, as the current is increased, the hull corrosion rate decreases substantially until the protective polarization level is reached. Increasing the current beyond this point offers no additional benefit.
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.