Immersed structures

Coal tar enamel This is derived from the coking of coal and is further distilled to produce coal tar pitches. It is used for hot application on-site. It will crack and craze if exposed to sunlight but has been employed successfully for over 50 years for the protection of underground or immersed structures. The main use is now for the exteriors of buried or immersed pipelines. Different types of enamel are available to give various degrees of heat resistance. It is now generally used for pipelines below 155 mm diameter. 

This group includes platinised-titanium, platinised-niobium, lead alloys and lead-platinum anodes, which are used for immersed structures, e.g. jetties, sheet piling and power stations. 

The method of applying cathodic protection to immersed structures will depend on several factors including ... 

Conductive Polymers Anodes currently available consist of a conductive-polymer graphite material coated on to a multistrand copper conductor. The polymer provides an active surface but shields the conductor from chemical attack. A non-conductive outer braid may be used to give abrasion resistance and avoid direct contact with the cathode. The finished anode has the appearance of an electric cable and is claimed to have applications for buried/immersed structures and for internal protection of tanks, etc. Anode current densities are typically given as 14-30mAm ... 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

Submarine ieiecomthunications cables operate at less than 1 A but sub-. marine power cables may operate at several hundreds Of amperes and will require larger spacing between the earth, electrodes and nearby burled or immersed structures. IfpOssible, testsShould be made with trial earth elfec. trOdes at both ends of. the submarine cable and on Off tests made before bringmg the power supply bii the system into permanent Use., ... 

Stray currents are produced in the electrolyte during the operation of cathodic-protection systems and part of the protection current may traverse nearby immersed structures which are not being cathodically protected. The resultant corrosion produced on the unprotected structure is referred to as corrosion interaction or corrosion interference. 

Attenuation the decrease in potential and current density along a long buried or immersed structure from the drainage point. 

Corrosion Interaction (or interaction) increase (or decrease) in the rate of corrosion of a buried or immersed structure caused by interception of part of the cathodic protection current applied to another buried or immersed structure. 

Drainage (electric drainage) (a) flow of positive current through the soil or the electrolyte solution from the cathodically protected structure to the groundbed of the impressed current system, or (b) protection of an immersed structure from electrochemical corrosion by making an electrical connection between the structure and the negative return circuit (rail, feeder, busbar) of a d.c. electric traction system. 

Primary Structure a buried or immersed structure cathodically protected by a system that rpay constitute a source of corrosion interaction with another (secondary) structure. 

Fully or partially immersed structures, including piers, harbors, and oil rigs, make use of steel pilings and shuttering that in general cannot be... 

Volumetric controls. Volumetric inspection methods are indispensable to be able to diagnose a problem on an immersed structure. The points to be considered are ... 

When a very long, unprotected or poorly protected embedded structure such as a pipeline crosses different types of soil, the dissolution potentials of the metal with respect to the soil are not consistently the same. This leads to the circulation of currents, which results in localised corrosion at the exit zones into the soil. This is also observed with immersed or semi-immersed structures such as ship hulls. For this reason, the return current should not flow through the hull, as one would be inclined to do on a small craft with battery-powered electric equipment. One conductor for each polarity is required if the system is distributing direct current, and one conductor per phase (plus one for the neutral, if required) for alternating current. 

Provide for electrical continuity of all metallic components, in immersed structures, that are... 

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