Preface
The manufacturing materials of seagoing ships and their seawater piping systems are basically steel, and seawater has strong corrosiveness to steel. Without effective protection measures for hull plates and seawater pipelines, these materials will undergo chemical and/or electrochemical reactions during use, leading to corrosion, which will greatly shorten the service life of the ship. Currently, common protection methods include applying anti-corrosion coatings below the ship’s waterline, painting anti-rust paint on the exterior of seawater pipelines, coating the inner walls with anti-corrosion thin film coatings, or using anti-corrosion liners.
However, when the ship berths or departs from the dock, the hull inevitably scrapes against the dock, fenders, tugs, etc., causing damage to the hull’s anti-rust paint or anti-corrosion coating; when removing attached marine organisms from the hull, the hull’s anti-rust paint or anti-corrosion protective layer may be damaged while the marine organisms are peeled off; the hull and seawater pipelines are subject to thermal stress, torsional stress, aging of the protective layer, and tiny pinholes in the coating; during maintenance operations on seawater pipelines (especially when electric welding is used), the anti-corrosion protective layers inside and outside the pipelines are easily damaged; in addition, the ship’s rudder blades, propellers, and propeller shafts inevitably come into contact with seawater… These damages, defects, and destructions will eventually lead to direct contact between the exposed metal parts of the ship and seawater, resulting in local chemical and/or electrochemical corrosion.
To prevent corrosion of damaged hulls, seawater pipelines with damaged internal and external protective layers, as well as rudder blades, shafting, propellers, and other parts in contact with seawater, and to extend the service life of the ship, ship anti-corrosion protection devices such as the Impressed Current Cathodic Protection (ICCP) system, the Shaft Earthing Device, and the Marine Growth Preventing System (MGPS) have emerged.
Electrochemical Reactions
When two metals or metals containing impurities are placed in an electrolyte (seawater is an electrolyte), the metal with higher activity (zinc Zn in the figure below) tends to lose electrons, undergo oxidation, and become a consumed anode; the metal with lower activity (iron Fe in the figure below) gains electrons, undergoes reduction, and becomes a protected cathode.
According to the different ways of providing cathodic current, ship cathodic protection is divided into sacrificial anode method and impressed current method.
The sacrificial anode method connects a more active metal (such as aluminum, zinc, etc.) to the protected metal structure (such as steel). Through the continuous dissolution and consumption of the more active metal, it provides protective current to the protected metal structure, thereby protecting it.
For ship sacrificial anode protection, a certain number of zinc block alloys are generally installed as sacrificial anodes along the outer side of the hull along the bilge keel streamline, seawater valve boxes, double bottoms, internal areas of double-hull compartments, and rudder propellers. If aluminum alloy materials are used, the effect is better, but they are prohibited in areas such as engine rooms and cargo oil tanks (due to the possibility of sparks caused by excessive potential difference). The design service life of sacrificial anodes is generally 2-3 years, and they must be fully inspected and replaced during each dry docking.
The impressed current method reverses the electrochemical corrosion process. It changes the potential of the surrounding environment through an external power supply, keeping the potential of the part to be protected lower than that of the surrounding environment, thus making it the cathode in the entire environment. In this way, the protected part will not corrode due to the loss of electrons.
(Image: Protected Part – Seawater – Auxiliary Anode)
Impressed Current Cathodic Protection (ICCP) System
The ICCP system utilizes the principle of electrochemical corrosion to convert external alternating current into low-voltage direct current, and applies a certain amount of direct current to the hull through auxiliary anodes. When the circuit is connected, a rectifier or potentiostat is used to maintain the hull at a low potential. According to the potential change of the hull in seawater, electrons are actively released to protect the hull, rudder propellers, etc., from corrosion: the anode connected to the external direct current power supply directly applies current to the protected hull, continuously providing electrons to enrich the hull surface with electrons. By controlling the hull potential or current intensity, the hull undergoes cathodic polarization (turning the entire hull into a cathode). The current forms a closed loop from the auxiliary anode through seawater to the hull, effectively inhibiting the corrosion and dissolution process of the metal on the hull surface. Thus, it reduces or even completely inhibits the electrochemical corrosion of the submerged part of the hull, which can replace the zinc or aluminum blocks of the sacrificial anode protection device to protect the hull.
In addition, the current from the cathodic protection device also enters the rudder. To prevent corrosion caused by the current returning to the rectifier through the rudder bearing, the rudder blade is connected to the hull with a fixed cable, which is the rudder blade earthing arrangement.
The main components of the ICCP system include the ICCP control box, an intelligently controlled direct current power supply, auxiliary anodes that transmit the direct current output by the direct current power supply to the hull, reference electrodes for comparison, relevant connecting cables, and insulation shielding materials. Of course, seawater is indispensable to form a complete working loop.
The ICCP control box is generally installed in the engine room, containing a step-down transformer, rectifier, inductor, fuse, switch, cooling fan, and LCD display. Its main function is to monitor the hull potential through the reference electrode and output a certain amount of compensation direct current to the auxiliary anode. Daily inspections of the ICCP’s voltage and current should be carried out and recorded, and all parameters should comply with the requirements of the manual.
Currently, two types of direct current power supplies are widely used: rectifiers and potentiostats. Their function is to provide a stable current of a certain magnitude according to changes in the submerged area of the hull, water quality, etc. When the submerged area of the hull and water quality do not change much, a manually controlled rectifier is usually adopted; when the submerged area of the hull and water quality change frequently, an automatically controlled potentiostat is generally used to control the potential of the submerged part of the hull within the optimal protection potential range.
The auxiliary anode is composed of an inert metal plate (such as platinized titanium, mixed metal oxide, etc.) embedded in an insulating base, installed in a watertight isolation cabin, with the positive terminal connected to the ICCP control box. Its function is to transmit the compensation current output by the direct current power supply to the hull through seawater, maintaining the hull potential at a normal level, which can effectively protect the large-area and complex-structured submerged part of the hull.
The reference electrode is used to measure the hull potential. Based on the measurement results, it provides control signals to the rectifier or automatically controlled potentiostat, and adjusts the output current through comparison and reference to keep the hull in a good protective state. The potential difference between the reference electrode and the hull is affected by many factors such as seawater salinity, ship speed, temperature, and ocean currents. If the reference electrode is damaged or short-circuited, an error message will appear on the ICCP control panel.
Connecting cables: The current flows out from the positive pole of the DC power supply transformer rectifier, passes through the anode cable to the auxiliary anode, then reaches the damaged hull through seawater, and flows back to the negative pole of the power supply from the hull through the cathode cable, forming a working loop. Similarly, the current flows out from the positive pole of the DC power supply transformer rectifier, passes through the reference electrode via the measuring cable, and flows back to the negative pole of the power supply through the cathode cable.
Insulation shielding is installed around the auxiliary anode to achieve complete electrical insulation and isolation of the hull, inhibit the generation of corrosion current, and thus prevent hull corrosion. That is, the insulation shielding ensures that the output current from the auxiliary anode will not be short-circuited near the auxiliary anode and can reach farther parts of the hull.
If the ICCP system fails or is shut down for a long time, it will lead to severe corrosion of the hull plates (especially the welds between plates), which will greatly shorten the service life of the ship.
To ensure that the hull is always in a well-protected state, the power supply should be kept on, and the system should operate in automatic mode, usually without manual adjustment. However, when the automatic mode fails (e.g., the control board needs to be replaced) or when entering fresh water areas, the system should be switched to manual mode or shut down. Shutting down the system for a few hours has little impact, but if the system is shut down for a long time, it will take some time for the system to reach its optimal protection state once restarted.
Regular preventive maintenance will extend the service life of the equipment, shorten the system shutdown time, and reduce damage to system components; during the stay in port, open the isolation cabin covers of the auxiliary anodes and reference electrodes (ensure there is no water pressure inside) to check their water tightness with the hull, preventing seawater leakage or condensation accumulation; if conditions permit, visually inspect the auxiliary anodes, reference electrodes, insulation shielding, and their surrounding conditions from the outside.
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