Zinc Anodes in Water

Zinc anodes are consumed in seawater at a rate of about 12 kg/A year. On bare steel, the average current density is about 0.10 A/m, so to protect 100 m of bare steel requires about 120 kg/year of zinc anodes. Heinrich (1980) refers to a theoretical capacity of 820 A-h/kg and an efficiency of 95% in seawater, giving a practical capacity of 780 A-h/kg (i.e., 0.089 A-year/kg, and close to the figure quoted earlier. However, in the North Sea, higher currents (up to 0.13 A/m ) may be needed than in the Gulf of Mexico (Eliassen and Steensland, 1977). 

Fully or partially immersed structures, including piers, harbors, and oil rigs, make use of steel pilings and shuttering that in general cannot be 

In the case of pipes in abyssal seawater or deep mud, a good painting scheme is applied in the factory using a chemically resistant paint, and zinc anodes are placed at intervals along the pipe before construction. These may be in the form of bracelets or attached by welding to the pipe. 

Cathodic protection can be successful only under immersed conditions and, therefore, only the parts of a ship below the waterline (and holds filled with seawater ballast) can be protected by this method. Ideally, the ship is painted in dry dock to the required standard, and zinc anodes are fitted below the waterline to protect the steel and paint. Since, in general, a zinc anode in seawater will protect all the steel within a radius of about 3 m, anodes are spaced at distances of about 6 m. Because of the turbulence around the stem and because bronze propellers often are present, corrosion rates are higher in this area and more anodes are placed there. 

Modem anodes are streamlined to reduce drag, and the amount of zinc used is sufficient to protect the ship s hull for at least the period between dry dockings, which is determined for other reasons. 

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