Ships are often exposed to very harsh marine environments. Due to the operating environment, ship hulls are often susceptible to environmental corrosion. The corrosiveness of seawater, which relates to the overall corrosion of steel, increases with increasing temperature. Oxygen content, water flow velocity, the content of corrosive impurities, eroding particles, and electrical conductivity. Seawater containing salt forms a perfect electrolyte ICCP in the ship. With a ship hull made entirely of iron (mild steel), forming a galvanic cell.
What Is A Galvanic Cell? Why Does A Ship Hull Corrode?
When two different metals come into contact with each other in a corrosive medium (electrolyte). The more active metal in the series of galvanic voltages acts as an anode and corrodes. This means that in the galvanic of a series of an metals. The more active metal acts as an anode and an corrodes. While the less active metal acts as an cathode and remains protected.
When these two metals are placed in seawater, they come into direct electrical contact. A current flows through the electrolyte from the more active metal (anode) to the less to active metal (cathode). This current is called the corrosion current and is nothing but a transfer process of metal ions. And electrons from the anode, which dissolves the metal ions and flows into the solution. This simple cell in which the corrosion process occurs is called a galvanic cell.
So, How Does Corrosion Occur On Ships?
Ships are made of steel, which is mainly composed of iron. Iron is an electrochemically positive element and tends to donate electrons and convert them into free ions.
- The anodic reaction that occurs is 2Fe → 2Fe++ + 4e-
- Seawater is composed of oxygen and hydrogen. Which produces electrochemically negative to hydroxyl ions is H2 O + O2 + 4 e- → 4 (OH)-, which receive electrons. Donated by an iron can.
- The iron ions combine with hydroxide ions in seawater to form iron hydroxide. 2 Fe++ +2 (OH)- → 2 Fe (OH)2. This is called the oxidation of iron.
- This iron hydroxide is oxidized in the presence of excess oxygen in the water to form iron oxide and water. This is the so-called “rust”. 2 Fe (OH)2 + O2 → Fe2 O3 + 2 H 2 O (rust)
Inhomogeneity of mild steel in the hull and factors such as unevenness, thickness, thickness and quality of paint. Variability In welds in ship construction, due to different metals and oxygen levels in seawater. Areas of the hull act as cathode and anode, forming a galvanic cell.
It turns out that thanks to a series of galvanic voltages. Instead of less active metals (cathode), more active metals (anodes) can also corrode. Combining a ship’s structure with a more active metal such as zinc or magnesium creates a galvanic cell. In which the active metal acts as the anode. Providing a flow of electrons to the structure, and then becomes the cathode. The cathode is protected, and the anode slowly dissolves, which is called a sacrificial anode.
The Need for ICCP in ship
From galvanic cells and sacrificial anodes, we learned that a potential difference between two electrodes is required for corrosion currents to occur. These corrosion currents cause the anode to dissolve in the electrolyte. But if there was no potential difference inside the hull, the corrosion currents would be minimal, and no corrosion would occur. This is what ICCP in ship does. Minimizing the potential difference and introducing a current that is opposite to the natural corrosion current ensures that the body always remains the cathode, thereby protecting the anode and preventing corrosion.
External Current Cathodic Protection
The External Current Cathodic Protection system is a technologically advanced, long-term solution to corrosion problems and is considered a better alternative to sacrificial anode systems. In ICCP in ship , the metal to be protected is connected to an insoluble anode and a current is passed through it using a DC power source that is opposite to the corrosion current, thereby converting the corroding metal from an anode to a cathode and protecting it from corrosion.
This insoluble anode can be made of platinum, platinized titanium, or another inert element. A similar galvanic cell with an anode (more active metal) and a cathode (less active metal) with some modifications. Here, an insoluble external current anode is added to the system. Under normal circumstances, without the insoluble anode, the corrosion current would build up and corrode the anode. However, in this case, a direct current flows between the anode and the cathode, which is opposite to the natural corrosion current.
The Same Principle Applies to Ships
Here, a contact point on the hull is taken and connected to a reference electrode. This reference electrode is a completely insoluble metal. The reference electrode measures at the natural corrosion current, which is an nothing but the potential difference between at the hull and this reference cell. This corrosion current in the hull must be measured so that an equal or slightly stronger (reverse) direct current can be supplied to the external anode.
The Workings Of ICCP in Ship and Its Components
1. Power Supply And Control Panel With 24V DC Output
The DC power supply may include a rectifier unit that converts 440V AC to DC. Larger ships usually have two power supplies, one at the bow and one at the stern. Power supply units are also known as Quantum ICCP in ship panels and contain a network of thyristors and PCBs that monitor the voltage and current parameters from the reference cells and send signals accordingly to the external power anodes. These power units are in a master-slave configuration between the front and rear units, and their performance can be monitored by the ECR monitor panel. The control panel is equipped with alarms for abnormal readings.
2. External Current Anodes
External current anodes are usually made of a strong, insoluble material such as titanium. They can be in disc or strip shape. Two or four are placed symmetrically at the fore and aft of the ship.
The anodes are welded to the double-plated hull surface and are completely flush with the outer hull plate. This anode material serves only as an electron source for the hull and is not consumed in providing this protective current.
3. Zinc Reference Cell
The potential is monitored by reference electrode assemblies mounted port and starboard between the anodes where at the lowest possible potential is likely to occur to detect the lowest corrosion current between the hull and seawater. This measurement is reported to the control panel, which automatically adjusts the anode power of the impressed current.
These are connected via cable lugs and seals and screwed to the hull surface via a cutoff body. These zinc reference cells are very stable in nature and provide a stable reference for measuring hull and sea potentials as well as weak currents in the system. These electrodes are fully interchangeable.
4. Remote Monitoring Panel
The engine control room has a remote monitoring panel for daily monitoring and recording of ICCP in ship parameters. When a vessel enters port, the ICCP in ship power supply must be switched off. Otherwise there is a risk that the currents of the ship and shore ICCP in ship systems will interact and cause damage to the hull paint. Excessive current flow on the hull surface can cause the paint to peel off.
5. Rudder Earthing Cable
To protect the rudder from corrosion caused by the ICCP in ship, a flexible rubber earthing cable is used, one end of which is fixed to the top of the rudder stock and at the other end to the hull structure, either through a lug or an eye plate. This creates a special electrical connection.
6. Propeller Shaft Grounding
Assembly with the Shaft Hull MV Voltmeter Even on vessels with ICCP in ships, the propeller shaft bearings are susceptible to corrosion by spark erosion. This is because a rotating propeller shaft is electrically isolated from the aircraft body by a lubricating oil film in the bearings and the use of non-metallic bearings on the tail shaft. This insulation can create an electrical potential between the shaft and the hull, which can cause strong currents to flow in the bearings. It can also cause strong currents to flow in the main and thrust bearings, causing pitting corrosion and damaging the main motors. This problem is solved by grounding the propeller shaft to the aircraft body using a slip ring and contact brush assembly.
Passage of Freshwater
When a ship moves from seawater to fresh water, the electrical conductivity decreases, and the resistivity increases. Due to the decrease in conductivity, the reference cell cannot detect the potential difference. At this time, it is necessary to limit the output protection current of the external current anode.
When sailing in fresh water, the automatic control of the ICCP in ship ‘s power supply increases the rectified voltage to the maximum value, limiting the external current and avoiding over-protection and protecting the ship When a vessel’s ICCP in ship is switched on, the terminal tries to protect the dock and increases the current in the system. This can cause an overload. Therefore, the ICCP in ship power in the port is always switched off.
