Impressed current cathodic protection (ICCP) on ships consists of multiple anodes (lead or platinum-coated titanium) installed at selected locations below the waterline in the hull, and control equipment that automatically adjusts the anode current to the required value.
After transformation and rectification, DC power is supplied to the anodes from the ship's 440 V 60 Hz three-phase AC power distribution system. The control equipment includes a reference electrode, amplifier components and one or more transformer rectifier units.
Anode current control is usually regulated by an electronic thyristor controller and this figure outlines a typical scheme.
The control equipment automatically monitors the amount of anode current required, which varies with ship speed, water temperature, salinity, paint job conditions, etc.
Typical anodic current densities range from 10 mA/m 2 to 40 mA/m 2 for protection of painted surfaces and 100 to 150 mA/m 2 fbr bare steel surfaces.
The total impressed current for a hull in good condition may be as low as 20 A. The maximum controller output may be as high as approximately 600 A at 8 V. Cathodic protection does not seem to prevent the growth of soft tissue on the hull, so a top coat of antifouling (toxic) paint is still necessary.
Typical reference and main anode profiles are shown above. Monitoring facilities in cathodic protection control cabinets can provide the following measurements:
Reference electrode voltage (hull potential)
amplifier output voltage
Total anode current
Individual anode current
Measurement results and ship operating conditions should be recorded regularly,
Such as location, draft, water temperature, etc.
Changes in underwater hull area, speed, water temperature/salinity, and paint condition can all cause changes in anode current. However, in a properly tuned system, the hull potential should remain constant.
Although reference electrodes and monitoring facilities provide reasonable daily checks, they only take measurements in the vicinity of the installed electrodes.
Voltage readings can be taken between portable silver or zinc test electrodes and the hull while the vessel is alone or anchored at sea. The portable electrode is placed 2-3 meters below the surface of the water and as close to the hull as possible at designated locations around the vessel.