A system of Impressed Current Cathodic Protection (ICCP) is installed on each vessel and this is a method of corrosion protection which automatically controls the electrochemical corrosion of the ship’s hull below the waterline. This includes the external hull surface and the internal surfaces of tanks containing seawater, e.g. ballast tanks.
Cathodic protection, which is frequently used in conjunction with coatings, can be achieved through impressed current, galvanic anode techniques, or a combination of the two.
Cathodic protection is analogous to a simple battery cell with two plates immersed in electrolyte. If the two battery electrodes are connected electrically, one of the battery plates in the electrolyte will deteriorate as a result of the flow of electrical current, because when two metals are submerged in seawater, which serves as the electrolyte, one of the metals will act as the anode and deteriorate. Which metal in any given pair serves as the anode is determined by their relative positions in the electrochemical series; however, steel can serve as an anode for copper, brass, or bronze. The strength of the electric current generated in the corrosion cell, and thus the rate of deterioration, is dependent on the metals involved and the electrolyte strength.
If a third electrode is added to the cell and the current is forced to flow, the third electrode becomes the cathode and the old anode becomes the new cathode.
This is the operation of an impressed current cathodic protection system. When ICCP is installed on a vessel, the hull steel is maintained at a more negative electrical potential than the surrounding sea water. For this reason, loading and discharging terminals typically adhere to ISGOTT Recommendation 20.6, Earthing, Bonding and Cathodic Protection, which states, in reference to IMO recommendations for the safe transport, handling, and storage of hazardous substances in port areas, that ship shore bonding cables should be discouraged. High currents that can occur in earthing cables and metallic connections are avoided. These are the result of potential differences between the ship and terminal structure, specifically the residual potential difference that can exist for up to twenty-four hours after the shipboard ICCP has been turned off.
On any connections, these terminals typically employ insulating flanges to electrically isolate the ship and terminal structure. During preparations for berthing at terminals where such insulation is not used, or where earth connections are required by local regulation, or when bunker barges approach, the ICCP must be turned off at least 24 hours in advance.
When a vessel enters a river estuary, the fresh or brackish water’s increased resistance may limit the spread of current from the anodes. Normally, the voltage output increases to compensate for this and would be accompanied by very low current levels and the reference electrode potentials may indicate under protection. However, in this system, the output is adjusted automatically and the system returns the hull to the optimum protection level when the vessel returns to sea water.
As a system working principle, protection is achieved by passing a low-voltage DC current between the metal of the hull and anodes that are insulated from the hull but in contact with seawater.
The electrical potential of the hull is maintained to be more cathodic than that of the anodes. This condition minimizes corrosion. The flow of impressed current, which varies based on the ship’s speed, the salinity and temperature of the water, and the condition of the hull’s paint, requires careful management. If the hull’s potential is made too negative relative to the anode, the paint film can be damaged electrolytically or by the evolution of hydrogen gas between the hull steel and paint. Usually the vessel’s system automatically regulates the incoming electrical current to ensure maximum safety. Titanium anodes located port and starboard on the ship provide current and prevent the anodes from corroding, and the surfaces are integrated into the vessel hull to reduce drag. Port and starboard zinc reference electrodes are utilized to compare the potential of the hull to that of unprotected steel and zinc electrodes. A sufficient amount of current is impressed through the anodes to lower this to between 150 and 250mV.
Upon activation, the unit’s transformer rectifier converts the ship’s 440V AC supply to a low-voltage, precisely controlled DC current, with the positive terminal connected to the anodes and the negative terminal connected to the ship’s hull.
In normal use, the system is completely automatic. In normal mode, the display will display the following information: Anode current and voltage; Reference cell No.1 mV reading; Reference cell No.2 mV reading (average value).
The system should be monitored frequently, with daily readings taken.
In general, depending on the vessel size, the system consists of a monitoring panel and one or two rectifier units. The rectifier units are wired to port and starboard reference electrodes and port and starboard anodes.
The monitoring unit is also equipped with facilities to raise an external alarm to give warning of any system abnormalities, via the main alarm system.
Note that the monitoring unit has a switch that allows the forward or aft system rectifier readings to be displayed. In the EDIT mode, alarm and operation set points can be modified. This, however, requires the input of a password. The unit can also be operated in the MANUAL mode, which allows the manual reference level to be adjusted and the correct operation of the system to be monitored as the outputs adapt to the new settings.
With regard to rudder and propeller earthing, in order to avoid electrolytic corrosion of the propeller shaft, a slip ring is clamped to the shaft and is earthed to the hull via brushes.
A second set of brushes, insulated from earth, monitors the shaft mV potential and this signal is fed to a millivolt meter.
To ensure efficient bonding, the slip ring should be cleaned on a regular basis. The shaft potential value should ideally remain below 75mV.
The rudder stock is also earthed for protection via a flexible earth cable between the deck head and rudder stock to minimize any electrolytic potential across the bearings and bushes.
The areas of the hull shielded from the hull face, such as thruster tunnels, rudder and sea water intakes, ballast tanks receive only limited protection from the ICCP system. These areas are therefore fitted with separate sacrificial anodes.
In general, the following routine checks apply to all vessels:
- Record the output current and all voltages on a daily basis.
- Check the reference electrode voltage on a daily basis.
- Check and clean the propeller shaft slip ring and brushes every week (when possible).
- Inspect the rudder stock earth strap every month.
- Return completed log sheets to the manufacturer for scrutiny every month.
- Inspect and clean the control unit cooling fans and grills every three months.
- The anodes and reference cells must be externally inspected every dry dock period. The anodes are fitted with an insulating shield cover to prevent excessive local over-protection and the condition of this shield must be closely inspected at this time.
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