The method of surface preparation which can be used for ballast tanks before drydocking. It bases on a breakdown of the rust using very strong electrical current. The current is produced by galvanic reaction between steel and large amount of magnesium anodes fitted for this purpose in the ballast tank, which is completely filled with seawater (electrolyte). This process requires one to two weeks and is accompanied by heavy hydrogen gas emission, so appropriate ventilation must be fitted. After descaling, an efficient washing of the ballast tank must be carried out, for the removal of rust and calcareous deposits formed on the tank surfaces. See also BLASTOMATIC system.