Al or Zn anodes in ballast and cargo tanks?
Aluminium anodes are generally preferred over zinc anodes, as a result of its technical and economical advantages. There are, however, concerns using aluminium alloyed anodes in certain tanks due to the risk of sparking if the anodes fall off.
Restrictions for aluminium anodes in certain tanks
If an aluminium anode strikes a rusty surface, a thermite spark may be generated. Therefore its use is restricted in tanks where there may be explosive atmospheres and there is a risk of the anode falling. The restrictions apply under the following circumstances:
- Tanks for liquid cargo with flash point <60 °C.
- Ballast tanks adjacent to tanks for liquid cargo with flash point <60 °C.
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Design consequence
In accordance with DNV-CG-0288 Corrosion protection for ships, the following parameters must be considered for sacrificial anodes in tanks:
- Anode alloy materials based on aluminium or zinc are acceptable. Magnesium based alloys are not acceptable.
- Aluminium anodes must be located so that a kinetic energy less than 275 J is developed (in case the anodes become loose and fall down).
- Based on elevation vs. weight, a 10 kg aluminium anode must be located lower than 2.8 m from the tank bottom.
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