A. CURRENT REQUIREMENTS

The wetted surface of the hull times protective current density.

B. SYSTEM DESIGN LIFE

The total weight of anode material required is:

As a result the number of anodes selected must at least satisfy both the total current and total weight requirements as follows:

ANODE LOCATION

The calculated anodes must be installed in equal distances around the hull, about 6 meters apart. The 15% - 20% of the total calculated QTY must be installed to the stern and rudder area of the vessel. The anodes should not be mounted in the zone close to the propeller's edges.

INTERNAL CATHODIC PROTECTION

The cathodic protection of the ship's tanks is effected with sacrificial anode system. Of course, this type of protection can be applied only to water ballast or cargo/ballast tanks. Zinc aluminum or magnesium anodes may be used but for the installation precautions must be taken for avoiding hydrogen evolution (in the case of magnesium) and sparking (in the case of aluminum). The final selection of the alloy to be used will also depend on the ballast water (seawater), the compatibility of the alloy with any cargo, which might be carried in the tank, the size of the protected tank and the type of the vessel.

CURRENT DENSITY REQUIRED FOR PROTECTION

In case, the tank surface is being painted, the anodes will be installed to suppress corrosion on any area with paint defects. The current density of 5mAmp/m2 is normally adequate. In case, of non-coated surface the following current densities are recommended:

FORE AND AFT PEAK TANK 108mAp/m²
TOP SIDE TANKS 120mAp/m²
DOUBLE BOTTOM TANKS 86mAp/m²
CARGO/CLEAN BALLAST TANK 86mAp/m²
LOWER WING TANKS 90mAp/m²

The system life should be at least four (4) years. The above-recommended current densities are applicable to ships for which the normal ballast trip is longer than approx. 5 days. In case, the trip is shorter than 5 days, then the above densities should be increased by 20%.
 

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