The number of anodes for the protection of the hull is calculated based on the following criteria:
A. CURRENT REQUIREMENTS
The wetted surface of the hull times protective current density.
B. SYSTEM DESIGN FILE
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:
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%.
Methods for Hull and Tank Anodes installations
US MIL SPEC
|Al||0.10 - 0.50|
|Cd||0.025 - 0.07|
|POTENTIAL Ag / AgCl (mV)||-1050|
(Amp. Hours / kg)
|Si||0.05 - 0.20|
|Zn||3.50 - 5.00|
|Ti||0.01 - 0.05|
|In||0.02 - 0.05|
|Mn||0.15 - 0.50|
|POTENTIAL Ag / AgCl (mV)||-1100|
(Amp. Hours / kg)