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Corossion - The Deadly Enemy
In the previous article we saw as to how the process of corrosion works and also the different chemical and electrochemical reactions that are responsible for it. In this article we will learn about the different methods to fight corrosion on ship.
There are many methods to prevent corrosion on ship. For all the methods the first thing we need to do is to stop the electro chemical reactions. The three important ways to stop corrosion are:
- By painting the metal surface that is continuously exposed to water or moisture. (see image below for crew painting ship side)
- By reversing the current using sacrificial anode
- By reversing the current using ICCP system (impressed current)
The process of painting is simple. In this article we would see as to how corrosion can be prevented by using sacrificial anode and ICCP system. If you want to see what corossion can do to a ship just see the picture and below and hope you dont find yourself on such a ship while on a vacation or a cruise tour
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The sacrificial anode method uses less noble metals such as zinc or aluminium. These metals are welded using steel strips on different parts of the ships surfaces which are continuously exposed to water. As the metals are less noble they attract the current of the steel plated on the ship's surface before it enters through some copper part into the skin of the ship. Thus till the time the metals are less noble they keep on dissolving into the water, preventing corrosion.
To prevent the process of corrosion from taking place the ship's surface is painted a thick layer. The paint is to prevents the current from entering the ship's skin. If the paint has come off, the sacrificial anode will disappear faster. The main advantage of sacrificial anode is that it is a low investment system but its life span is short and unpredictable. Also the dissolving of the metal is irregular and uneven. It is sometimes seen that if the anodes are newly installed there occurs a counter effect on the surface paint.
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Impressed Current Cathodic Protection(ICCP system)
An ICCP system uses positive current to prevent corrosion. A large amount of positive current is passed in the water surrounding the ship which prevents any kind of reactions from taking place. This is done by using a rectifier which is connected to the surface steel with the negative terminal and the positive terminal is connected to the anodes that are fixed inside the ship's surface. The anodes are made of inert or non reactive substances which cannot dissolve in the water. The anodes are generally formed of oxides. This is because oxides cannot oxidate further. Now the anodic reaction wont have any metal to consume, the formation of oxygen bubbles would take place but those oxygen bubbles are not harmful to the surface. The impressed current is in the Range of 10A - 600 A. The impressed current depends on many factors such as size, speed etc The voltage can reach as high as 20-30V.
Though the installation charges are high, it is a long lasting and reliable system. The maintenance required is too less and the system can be controlled easily. Also the self monitoring system enables less human interference and thus make it more user friendly. It controls the current rate with the change in the temperature and salinity of water.
An error in the ICCP system can cause severe damage to the part of the ship below the water line, damaging both the surface and the paint. It is generally used for ships which are long and have high speed.
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Ship knowledge - A Modern Encyclopedia by K. Van Dokkum