Corrosion is one of the highest enemies of the ship and its equipment. It is also the toughest enemy to fight against for the people working on the ship. Iron is one substance which is used in great quantity on the ship. From the main body of the ship to the smallest equipment used in operations, iron makes its presence felt in almost every type of equipment used on board the ship.
A ship is continuously in contact with water and moisture laden winds which makes it highly vulnerable to corrosion. The other body of the ship (mainly hull) is continuously in contact with water, making it extremely vulnerable to corrosion. It is for this reason sacrificial anodes are used to protect the parent material. In this article we will have a look at the working of sacrificial anodes on ship.
It is to note that the sacrificial anodes which are protecting the parent material should lie higher in the electromotive series or galvanic series of metals.
How Sacrificial Anodes Function
Sacrificial anodes works on the standard similar to electrolysis, according to which if an anode and a metallic strip are dipped in electrolytic solution, anode electron will dissolve and deposit over the metallic strip and make it cathode.
In the case of a ship, sea water acts as an electrolyte and transfers the electrons from the anode by oxidizing it over the steel plate and making a protecting layer. If the metal is more active it will be easily oxidized and will protect the metallic compound by making it act as cathode. The anode will corrode first sacrificing itself for the other compound and it is thus called sacrificial anode.
Electromotive series or galvanic series metals
- Zinc (Zn)
It can be seen from the table that for protecting iron any material above in the series are useful. These metals are preferred because they are easy and cheap to change the anodes rather than complete a large sheet of metal.
These anodes are used in various applications such as:
- Protecting the ship’s hull.
- Protecting the ballast tanks corrosion.
- Protecting the heat exchangers.
- Sea chests
The most common metals used for sacrificial anode is zinc.
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Frequency for Changing of Anodes
The frequency for changing of anodes depends on the application where the anodes have been used.
In case the anodes are fixed to the ship’s hull, then they are to be examined during dry dock which takes place after 2 to 3 years. If the anodes are found completely corroded then anodes of bigger size should be fitted, for fully corroded means that the material used was of poor quality or a large amount of material is required to protect the hull. Normally, sacrificial anodes are changed at every dry dock.
If sacrificial anodes are used for heat exchangers and it is found during inspection that the anode left is only 10% then also it has to be changed.
How to assess the Anodes
During the inspection of heat exchangers or sea chest, if the condition of anode is same as it was fixed then it indicates that the sacrificial anodes are ineffective.
The main cause for this is that the electrical continuity between the parent materials is not made. Because of this the parent metal starts getting corroded instead of the anodes. It is therefore vital to check the electrical continuity during installation.