Corrosion damage to the steel or aluminium hull of any seagoing vessel is expensive to repair and not always easy to detect. Although a bolted or welded anode will provide cathodic protection, the degree of protection it provides can only be practically checked when the vessel is out of the water and so the extent of any problem only becomes apparent during dry-docking.
Simple galvanic anode systems rely on the difference in natural potentials between themselves and the cathode -- hull, propulsion and steering systems etc -- created by their relative positions in the electro-chemical table. This difference in potentials causes a current to flow, but the current cannot be varied. A technology that is increasingly being used for larger craft is that of impressed current cathodic protection (ICCP).
ICCP systems can be monitored and have made corrosion protection much more controllable. Such systems use inert (non-galvanic) anodes with an external source of DC power to impress a current from anode to cathode (hull). Reference electrodes, installed adjacent to the anodes, are electrically connected into the system to provide a remote control and monitoring facility than can continually present information, both visual and in printed form, locally and on the bridge if required.
ICCP systems offer long term control of corrosion and much reduced maintenance and inspection times for larger vessels, hence their increasing popularity. Such systems operate evenly over the hull to control any corrosion as the coating deteriorates between dry docking schedules. As well as hull protection, other dedicated systems have been developed to protect and monitor valuable components and steelwork over the lifetime of the vessel. Bow thrusters, water jets and sea chests are among typical applications.
Where water jets are installed in place of traditional propellers and rudders, ICCP is the best solution to protect the tunnels and components that are subject to high velocity water flow and greater than usual corrosion. An additional vulnerable area of water jet propulsion systems which can be protected by ICCP is the void space between the jet impeller housing and the ship’s hull, particularly where the latter is of aluminium.
Although aluminium produces a natural passivating oxide layer which resists seawater corrosion, practical experience has shown that additional protection is needed to prevent galvanic corrosion on aluminium hulled catamarans with water jet propulsion systems. This is due to the presence of stainless steel and copper alloys in the waterjets.
A particular void space anode assembly comprises a mixed metal oxide coated titanium wire anode mounted into a header cable for strength and enclosed in a perforated plastic housing. This housing also incorporates a reference electrode for monitoring purposes. The anode is wound around the external surface of the stainless steel water jet impeller cone housing and electrical connections are taken through a glanded void space inspection hatch.
Installation and commissioning of ICCP systems on aluminium vessels should always be in the hands of specialist contractors to ensure correct set-up as applied voltages need to be controlled very precisely. Too low a voltage can create an over protection condition, where transpassive corrosion actually causes the natural aluminium oxide surface to break down, thus reducing corrosion resistance. An over protection condition is not peculiar to ICCP. Such a situation can exist with sacrificial anode systems but the condition only becomes evident during dry-docking. A correctly set and closely controlled ICCP system will avoid over protection.
In vessels with ICCP, it is vital to incorporate a shaft earthing system to eliminate the risk of spark erosion where steel shafts pass through bearings. Shaft earthing systems are designed around modular components thus enabling earthing and monitoring for all shaft diameters.
The power supply for ICCP is traditionally via a DC/DC converter powered from a ship’s 440 volt system. Because not all vessels run their power all the time, an ICCP system has been developed that operates from the battery circuit used for engine starting, thus ensuring that cathodic protection is continuous, even when a ship is in port.
The Stena Line HSS fast ferries operating between the UK, Holland and Ireland are aluminium hulled catamarans with water jet propulsion systems and travel at up to 40 knots. Impressed current cathodic protection systems continue to provide successful monitored corrosion protection for these vessels, the original systems being supplied by BAC Corrosion Control when the vessels were constructed. The company now undertakes regular checks both during normal operation and during dry-docking as part of its on-going service to Stena Line. The cathodic protection systems have now been in place for nearly four years with no corrosion problems being experienced in the critical areas, thus indicating the effectiveness of ICCP systems for these high speed vessel applications.
Austal Ships, Australia’s leading fast ferry shipbuilders are keen advocates of ICCP systems for the aluminium hulled fast ferries it builds. BAC Award systems were supplied for water jet protection on three Austal aluminium catamarans in 1998 and 1999, and three vehicle/passenger aluminium hulled catamarans currently under construction for Minoan Flying Dolphins in Greece will also have water jet protection systems. Additionally, these vessels will have shaft earthing.
The Bornholms Trafikken gas turbine powered aluminium catamaran, also built by Austal, has BAC systems installed in the water jet tunnel, void space and bow thruster tunnel. The sea chests and hull are also being monitored and drive shafts have shaft earthing and monitoring systems to prevent erosion.
There are those that will say that a properly applied paint coating on a hull should be all that is required to prevent a vessel suffering corrosion. In an ideal world that is fairly true, but what happens to the coating when it suffers damage through impact below the water line? Without corrosion protection, the base metal will be attacked. With ICCP, controlled and monitored protection is provided.