As iron is present everywhere in ships from small screws in machinery to body of the ships either in pure form or mixed form which are prone to environmental corrosion which increases with temperature, oxygen content, water velocity, conductivity, contaminants. The seawater containing salt (Na+ ions) forms a perfect electrolyte with ship’s hull (made up of mild steel) to form a Galvanic cell. The ferrous hydroxide in the presence of excess oxygen in water is oxidized to form ferric oxide and water.
2Fe (OH)2 + O2 ———-> Fe2O3 .2H2O (rust)
Conventionally by coupling the ship’s structure with more active metal such as zinc or magnesium and by virtue of more active metal working as anode subjected to rusting the parental metal is protected as in case of ship the hull is parent metal and zinc, magnesium or alloys serves as sacrificial anode, because of some dereliction(majorly maintenance costs) the alternate Impressed Current Catholic Protection (ICCP) came in practice which makes the Hull to remain always cathode in addition keep potential difference minimum and introduces a current opposite to the natural corrosion current thereby protecting anode and avoiding corrosion.
Mechanism of ICCP
The mechanism of ICCP introduces an insoluble Impressed current anode which is added to system and we pass a DC current opposite to natural corrosion current between the anode and cathode which is equal or slightly greater than natural corrosion so that anode is now protected and does not corrode. Major components of ICCP are-
- DC 24V output power supply unit and control panel (Quantum ICCP panel)
- Impressed current anode
- Zinc Reference Cell
- Remote monitoring panel
- Rudder bonding cable
- Propeller shaft grounding assembly with shaft hull
As anything in this world is not perfect similarly this ICCP system also have some limitations such as heavy current can also flow into the main bearings and thrust bearings and causes pitting marks leading to Main Engine Damage.
When a ship moves from Sea Water to Fresh Water, its electrical conductivity reduces and its resistivity increases. Because of reduced conductivity, the reference cell will not be able to detect a potential difference. At the destination the ICCP if switched on will protect pier also increases the current drastically which may lead to overload. The maintenance .i.e. inspection and overhauling of ICCP are usually performed at dry dock only. Corrosion costs money for instance, U.S.A. economy costs almost $300 billion per year and it is estimated that 1/3rd of it can be controlled by corresponding techniques like this. Also India has been losing Rs.1.52 lakh crore per year due to corrosion in various sectors.
Control over on/off system
The systems are fabricated for automated current output, nevertheless the output voltage varies. The required characteristics of ICCP systems is continuous automatic monitoring of electrical potential at the contact point between the sea water and hull, and accordingly the output of anode deviates as per measurements. So, the mechanism is much more efficient and reliable than systems with sacrificial anodes where the level of protection is unknown and uncontrolled.
Why ICCP turned off prior to port or Ship-to-Ship transfer or berth alongside terminal ?
ICCP is designed to operate under dependence on the salinity of the water. This is because in salty water the impedance is less which allow easy flow of current to the anodes and helps in efficient protection of the hull. At times of berthing and Ship-to-Ship transfer if ICCP remains on it would start protecting berth or hull of other ship too, which decreases efficiency. Hence in ports or in fresh water the ICCP is turned off. Consequently the ICCP is one of the most underrated machinery which must be modified to reduce the demerits which are minute and yes with the proctored installation and functioning the ships life as well as its efficiency can be enhanced.