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.

Next: System Setup and Operation

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