Aluminium anodes are generally more efficient and economical than zinc anodes.
The driving voltage of aluminium anodes is higher than of zinc, resulting in better protective current distribution of aluminium anodes.The capacity of aluminium (2,650 Ah/kg) is about 3.5 times higher than zinc (780 Ah/kg). Hence, the total required anode weight by use of aluminium is far less then for zinc anodes.Using aluminium anodes involves considerably lower material cost compared to zinc anodes.
Generall aluminium anodes are technically and economically better than zinc anodes.However, for ballast tanks adjacent to tanks for liquid cargo with flash point <60 °C there are some limitations of using aluminium anodes:
• According to DnV Rules for Ships such tanks are considered dangerous areas.
• Aluminium alloyed anodes are to be so located that a kinetic energy of ≤275 J is developed in case of their loosening or falling down.
• That means that an aluminimum anode weighing for instance 10 kgs must be located lower than 2.8 m from the tank bottom.
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