The prevention of corrosion on ships deals with the provision of protective coating for the ship structure steel and its continued maintenance. Also a means of preventing electrochemical wastage is required, which is known as Cathodic protection.
There are two types of cathodic protection method, Sacrificial anode system and Impressed current system.
Sacrificial Anodic Protection
Sacrificial anodes are metal or alloy attached to the hull, which have a more anodic potential than steel when immersed in seawater. These anode supply the cathodic protection current, but will be consumed in doing so and therefore required replacement for the protection to be maintained.
Sacrificial anode may be fitted within the hull, and are often fitted in ballast tank. Magnesium anodes are not used in the cargo tank of oil tankers owing to the spark hazard. Should any part of the anode for and strike the tank structure when gaseous condition exist an explosion could result.
Zinc anode may be safely employed.
Impressed Current Cathodic Protection
The impress current system consist of a source of direct current, anodes, apparatus for measuring and controlling the current and a high quality inert protective coating around the area of the hull nearest to the anode.
Continuous control of the impress current required for adequate protection varies with the immersed area, the ship speed, the salinity of the water and condition of the hull paint work.
This control is usually obtained by the use of reference anode positioned some distance from the operating anode. If too great a current well to flow it could destroy paint coatings on the ship hull. Around the anode a protective coating is epoxy resin is applied directly to hull for a radius of one meter or more, since highly alkaline condition arise near the anode. A propeller bonding arrangement must be fitted with impress current system to ensure propeller protection.
Prevention of Marine Growth and Fouling of Ship Hull
Anti fouling paints are used for this purpose. They consist of a vehicle with pigments, which give body and colour together with materials poisonous to marine vegetable and animal growth.
Mercury and copper are the best-known poisons used in anti fouling paint. To prolong the useful life of the paint the poisonous compound dissolve slowly in seawater. Once the release rate falls below a level necessary to prevent settlement of marine organism the anti fouling composition is no longer effective.
A recently developed anti fouling paint is non-toxic in operation it is based on a metallised acrylic polymer and psychically influences the film which form on immersed hull. Adherence by marine organism is made almost impossible by the paint altering the critical surface of the film.
International Convention on the Control of Harmful Anti-fouling Systems on Ships prohibits the use of harmful organotins in anti-fouling paints used on ships and establishes a mechanism to prevent the potential future use of other harmful substances in anti-fouling systems.
How Zinc Blocks Protect Ship Hull from Corrosion
Many ship corrosion problems are associated with the coupling of metallic parts of differential, which consequently form corrosion cells under service condition. The corrosive rate of metal and alloy and seawater has been extensively investigated and as a result galvanic series of metal and alloys in seawater have been obtained.
Zinc block attached to the hull, in way of bronze propeller and other immersed fitting, have a more anodic potential than steel when immerse in seawater. These anodes supply the cathodic protection current, but will be consumed and doing so and therefore required replacement for the protection to be maintained.
Oral Guide by – MIN ZAR TAR