Internal Cathodic Protection of Tanks and Containers

Magnesium anodes must not be used in tanks. Aluminum anodes may be installed in all tanks according to the agreements of the International Association of Classification Societies, which are included in the individual regulations [5,6], but in tanks (b) in the event of the anode falling off, the kinetic energy must not exceed 275 J, i.e., a 10 kg anode must not be fixed more than 2.8 m above the bottom of the tank. There are no restrictions on the use of zinc anodes. The restrictions on the use of aluminum anodes are due to the possible danger of sparks if the anode falls off. 

Anodes are designed to last 4 years so that ballast times for (a) are 40% of the service time and for (b) 25%, which corresponds to 146 and 91 days per year, respectively. The shortest filling time should amount to 5 days per ballast journey. Protection current densities are given in Table 17-3. 

The surfaces to be protected should be the total surface, including inserts, spars and pipes. The upper 1.5 m of the side walls and the covers should be provided with a coating of recognized quality [10] to protect against corrosion. 

The horizontal surfaces should be coated because there is residual water in the ballast and there are water-oil mixtures in the crude oil tanks when ships travel empty and these can cause severe corrosion attack. In the lower part of the tank, up to about 1.5 m from the base, a combination of coating and cathodic protection with special anodes is chosen. Basically the anodes could take over the exclusive protection in this area, but with empty ballast tanks containing residual water or empty crude oil tanks with aggressive oil-water mixtures containing sulfur compounds, they do not prevent corrosion. 

Suitable anode shapes have been developed for different types and constructions of tanks. For example, there are long flat anodes which when installed on the 

---