Choosing an anode is not just a matter of selecting zinc or aluminium but is also dependent upon the design of the anode.
When a general anode is required for ballast tanks, the installation of a ZT or an AT design is considered.
These anodes are welded to the tank and have the advantage, except for the smallest size, of having brackets which are the same distance apart.
When an individual anode needs replacing, the spent anode top con be broken off, leaving the brackets in place. New ZT or AT anodes can then be easily bolted through the holes in these brackets thus saving welding and avoiding the risk of damage to the coating system. This is not the only choice of anode. Frequently the bottom deck area of the tank is always immersed in the few cetimetres of ballast water. This can lead to pitting and repair to the coating is very difficult.
The problem can be solved by bolting an Pitguard Anodes. These anodes can be treated as `seastock`. If pitting is discovered when the crew undertake their routine inspection they can bolt on Pitguard anodes. The problem can be avoided in the first place by intalling Pitquard anodes at newbuilding or during drydocking.