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Marine Cathodic Protection
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Marine Cathodic Protection

Due to the environment in which they operate, ships are among the structures most exposed to environmental corrosion. The sea water is a very corrosive environment because the salt present in it makes it a very good conductor of electricity.

Almost every part of the ship is subjected to corrosion, with varying intensity. Parts of the vessel underwater or exposed to water (e.g. ballast tanks and pipes) are more affected by corrosion. Some of the parts highly exposed to corrosion by sea water are

·        Ship’s external hull – exposed to water

·        Rudder

·        Propeller shaft

·        Bilge Keel

·        Bow Thruster

·        Cargo Tanks

·        Ballast Tanks

·        Other tanks

·        Pipes carrying ballast/cargo

 

There are various methods of protecting the ship hull and other areas from corrosion.

There are three major methods of corrosion protection

1.     Anti-corrosion paints – metallic/organic

2.     Cathodic Protection – ICCP (Impressed Current Cathodic Protection)

3.     Cathodic Protection – Sacrificial Anodes

 

Sacrificial anodes

 

Anodes can be classified based on their shape, size, material, mounting method and method of securing to the surface to be protected.

 

Anode Shape

Following are some widely used shapes for anodes

·       Flat or block shaped

·       Cylindrical or semi-cylindrical

·       Tear-drop anodes

·       Bracelet anodes

·       Disc shaped

·       Tubular anodes

 

Anodes can be of different shapes based on their applicability. The selection of the shape of anode depends on several factors. Some of these factors are:

·       shape of the surface to be protected,

·       availability of space,

·       accessibility,

·       ease of installation

·       special considerations, e.g., effect on resistance for small boats

 

For example, flat anodes are used mostly for flat, large surfaces like the ship’s hull. Tear-drop anodes are used in high speed boats where streamlining of water is important as flat anodes will increase the boat’s resistance. Bracelet anodes are used for pipelines and propeller shaft, while tubular anodes are used for cables. There are no fixed rules here though, and the choice depends on the availability, cost and flexibility in design. For example, cylindrical anodes can also be used to protect pipelines, and it is not necessary to use bracelet anodes if they are costlier.

 

Anode Mounting Method

Based on mounting technique, there are two major types of anodes which are used in ships:

1.     Flush mounted anodes – in this type of anode, is in direct contact with the surface to be protected. The insert is generally a flat bar which can be welded or bolted to the surface

 

Slender stand-off anodes – In these types of anodes, the anode material is not in direct contact with the surface to be protected, and there is a gap. The insert is generally a tubular one which can be welded or bolted to the surface.

 

Method of Securing the anode to the surface to be protected

There are three basic methods of securing the anode to the surface which is to be protected. They are

1.     Welding

2.     Bolting

3.     Using studs/brackets

Welding ensures the closest electrical contact between the anode and surface to be protected, thus ensuring good conductivity between anode and the surface through the insert material. However, due to issues of accessibility, some locations (e.g., stringers, girders etc.) may not be conducive to welding, and bolting or bracket installations may be preferred. Additionally, if the anodes have to be replaced relatively frequently, then bolted ones are relatively easier to replace compared to welded ones. Anodes can also be bolted to small studs or brackets which in turn are welded to the hull.

Induced current cathodic protection system (ICCP)

In induced current cathodic protection system, a flow of electric current is generated from an external source, protecting the parts of the hull which are in contact with sea water, thereby causing a cathode reaction which, in turn, protects the steel from corrosion. The hull is usually connected via amplifiers to the negative half of the power source, while the positive pole is connected to the anode and due to possible damages, built into the bottom shell. Anodes are made from precious metals such as platinum, conductive polyplastics, etc. The area around the set of hull anodes should be protected by special coatings. Currents and voltages of the cathode protection vary depending on the size of the ship, the damaged surface of the ship, speed, sea water salinity, and the number and position of the anodes.

Suzhou SYNCOR Cathodic Protection Co., Ltd. working more than 30 years in Cathodic protection doing design, supply materials, installation and commissioning’ trying to deliver some of most complex project in the world, on budget and on time for our clients.

 

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