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Zinc Vs Aluminium as Alloy for Sacrificial Anodes

General
Traditionally many companies prefer to use zinc instead of aluminium. It is advised to use aluminium for the following reasons:

1. Price of anode material
   Due to the fact that the capacity of aluminium (2700 A hr/Kg) is almost 3.5 times higher than zinc (780 A hr/Kg) the total amount of required anode material (Kg) is less. This results in a cheaper Cathodic Protection System to that given by zinc with an equal leval of protection.
   
   
2. Price of installation
   As the total weight of anode material for a system using aluminium is less compared to a system using zinc anodes the installation costs for a system using aluminium anodes will also be less.
   
   
3. Multi Effective
   In brackish water aluminium anodes work more effectively than zinc, due to the higher driving voltage of aluminium.
   
   
4. Environmental Pollution
   Compared to aluminium alloys, zinc alloys contain more elements which are detrimental for the environment.

The use of Sacrifical Anodes in Tanks

General
Traditionally many vessels are fitted with sacrificial anodes for external protection (hull). For achieving the same protection of the internal construction of the vessel (mainly water based tanks) sacrificial anodes can also be used. However due to the fact that tanks are inherently complex structures as well as being confined spaces the use of anodes has to be well engineered and the following rules adhered to.

1. Effect of ballast tank anodes
   The ballast tank anodeswill be without effect when the ballast tanks are empty. It will take some time to obtain full effect (polarisation) of submerged steel surfaces after filling with seawater. In the under deck area on top of tanks the anodes will not be effective unless the tanks are completely filled.
   
   
2. The use of zinc / aluminium or magnesium anodes
   Anode alloy materials based on zinc or aluminium are acceptable, magnesium alloy is not allowed. Ballast tanks adjacent to tanks for liquid cargo with a flash point <60° are to be considered as dangerous areas. Aluminium alloy anodes are to be located in such a way the kinetic energy which is developed in case of loosening and falling down will be <275 J. This means that in these areas the maximum height (in mtr) of an aluminium anode above tank bottom, deck or stringer may not exceed the value obtained by "28 / W", where "W" is the gross weight of the anode. Futhermore classification society may require structures to be installed to protect the aluminium anodes from being hit by falling objects. Please consult your local surveyor / classification society branch office for full details.
   
   
3. Welding or bolting type anodes
   For a good operation of the anodes it is vital to have a good electrical continuity between the ship structure and the anodes. This can be obtained either by welding (preferably) or bolting the anodes to the ship structure. In case bolting type anodes are requested the optional 'M' clamps can be used.