17 - Cathodic Protection of Ships

Abstract

Cathodic protection of ships extends from the external protection of the underwater area including all attachments and openings, to the internal protection of various tanks, pipework, and bilges. Ships are exposed to water of very different compositions. The salt content and conductivity are particularly important because they have a profound influence on the action of corrosion cells. In addition, on ships problems of dissimilar metals must be considered. A distinction is made between complete and partial protection of the underwater area, depending on the extent of the protected region. In partial protection, only the stern is protected; it is particularly endangered because of the high flow rate and aeration as well as the formation of cells on attachments such as the propeller and rudder. Partial protection can also be extended to the bow, which also experiences high rates of flow. The complete protection of the ship with galvanic anodes or impressed current is becoming increasingly important because defects in the coating due to mechanical damage are more frequent at the bow and amid ships. The extent to which attachments such as propellers and rudders can be covered by the protection or be given individual cathodic protection depends on the design of the ship and the method of protection. The design of cathodic protection must involve the calculation of the Protection Current Requirement, Protection by Galvanic Anodes, and Protection with Impressed Current.