In this type of system, the ship’s hull is forced to become a cathode with respect to electrodes that are positive with respect to the ship’s hull. The electrodes used for this purpose are Zinc, Magnesium and Aluminium, However, in all the three, Zinc is very popular. These electrodes provide galvanic current and in the process get eaten away. They provide protection to the ship hull by sacrificing itself and thus are called as Sacrificial anodes. “Sacrificial anodes” have also been developed from composite materials using aluminium alloys.
A Sacrificial anode is made up of from alloys of metal that have lesser electro-chemical potential in comparison to the metal which is to be protected. A sacrificial anode normally consists cast-m straps or lead wires so that it can be connected to the under water hull which needs protection against corrosion. This cast-m strap or lead wire can be connected with the hull using welding method or any other physical attachment. The cast-m strap does not need any further precautions post welding. However, if lead wire is being used, then proper care is required to be taken. The resistance of the lead wire must be low for proper functioning. It is therefore, the lead wire must be properly insulated to avert the condition of increase in its resistance or any damages from corrosion.
The Relative Merits/Demerits of Sacrificial Anodes:
- Initial cost is low
- Provides protection even when power is not available
- No maintenance is required
- Simple in installation
- No training required for operation
- The anodes have limited life requiring periodic replacements
- Current demand increases under varying cruising conditions and also under enhanced sea water corrosivity due to pollution.
- Necessity of a large number of anodes results in increase of frictional drag and weight
- Anodes contribute to increase noise level
- Current output cannot be regulated
- The periodic replacement costs heavily as it needs dry docking