Paints alone cannot accord complete protection to underwater hull. Areas above keel blocks are not always possible to protect by painting. Stern areas suffer erosion due to high turbulence caused by propeller. In these areas, chances of galvanic corrosion are more due to presence of bronze propeller which is directly coupled to mild steel hull by ‘A’ brackets. In addition to these factors, paint films possess permeability to water vapour and aggressive ionic species, which adversely affect their a/c properties with ageing. In view of the above facts, Hull protections methods are used to increase the life of hull.
Cathodic protection in conjunction with a/c paints offers protection against corrosion of ships hull more efficiently and optimizes the current requirements. Direct current for cathodic protection of ships hull can be applied either by use of sacrificial galvanic anodes or by use of impressed current from ships’ supply using inert anodes as carriers of current (impressed current cathodic protection). Accordingly, the most important and commonly used hull protection methods are :
It is well known that electrical shorting of two dissimilar metals in a conductive electrolytic solution produces a galvanic current. The baser one between the two gets consumed and the nobler metal is protected. This principle is used in devising sacrificial anodes for the purpose of cathodic protection. Metals like magnesium, zinc and aluminium are baser in comparison to carbon steels and therefore can produce galvanic current when coupled with steel in seawater resulting their sacrificial dissolution and protecting mild steel.
A Sacrificial anode is manufactured from alloys of metal that have lesser electro-chemical potential in comparison to the metal which is to be protected. It normally comes with cast-m straps/lead wires for ease in connection with the under water hull. To know about Sacrificial anode in details, check our post “Sacrificial anodes“.
This technique makes use of inert anodes of larger current carrying capacity for impressing direct current from ship’s supply on the hull. For further details of ICCP, visit Impressed Current Cathodic Protection