Polymer Cable Anodes

Polymer cable anodes are made of a conducting, stabilized and modified plastic in which graphite is incorporated as the conducting material. A copper cable core serves as the means of current lead. The anode formed by the cable is flexible, mechanically resistant and chemically stable. The cable anodes have an external diameter of 12.7 mm. The cross-section of the internal copper cable is 11.4 mm and its resistance per unit length R is consequently 2 mQ m l The maximum current delivery per meter of cable is about 20 mA for a service life of 10 years. This corresponds to a current density of about 0.7 A m. Using petroleum coke as a backfill material allows a higher current density of up to a factor of four. 

Without coke backfill, the anode reactions proceed according to Eqs. (7-1) and (7-2) with the subsequent reactions (7-3) and (7-4) exclusively at the cable anode. As a result, the graphite is consumed in the course of time and the cable anode resistance becomes high at these points. The process is dependent on the local current density and therefore on the soil resistivity. The life of the cable anode is determined, not by its mechanical stability, but by its electrical effectiveness. 

Impressed current anodes must be insulated from the surface that is being protected. Also, the current connections must be well insulated to prevent the free ends of the cable from being attacked and destroyed. 

The demands on insulating materials in soil and fresh water are relatively low. Anodically evolved oxygen makes the use of aging-resistant insulating materials necessary. These consist of special types of rubber (neoprene) and stabilized plastics of polyethylene, and polyvinylchloride, as well as cast resins such as acrylate, epoxy, polyester resin and many others. 

A single or multicored plastic-coated cable of the type NYY or NYY-O is used as the connecting cable between a protected object and an anode in soils and fresh water, and particularly in seawater, medium heavy or heavy rubber-sheathed connections of an NSHou or NSSHou type are used. Heavy welded connections of type NSLFSou are used for severe mechanical loading. In addition to these, for ships, marine cable of type MGCG or watertight cables must be considered. 

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Fig. 13-6 Potential variation of a galvanized steel easing pipe ehannel eathodi-cally protected with a flexible polymer cable anode.

Figure 19-1 shows the experimental setup with the position of the steel test pieces and the anodes. The anodes were oxide-coated titanium wires and polymer cable anodes (see Sections 7.2.3 and 7.2.4). The mixed-metal experimental details are given in Table 19-1. The experiments were carried out galvanostatically with reference electrodes equipped to measure the potential once a day. Thus, contamination of the concrete by the electrolytes of the reference electrodes was excluded. The potentials of the protected steel test pieces are shown in Table 19-1. The potentials of the anodes were between U(2u-cuso4 = -1-15 and -1.35 V. 

The anode systems used today consist of a fine-meshed, oxide-covered titanium network [55,56] (see Section 7.2.3), polymer cable anodes of high flexibility... 

Anodic processes may cause premature failure of oxidisable anode materials, however. A CP system based on a carbon-filled polymer cable anode functioned properly until 6 to 8 y of service. Later, it became increasingly difficult to achieve the criterion of 100 mV depolarisation. Detailed examinations after 15 y showed that the carbon had dissolved from the outer layers of the cable and the polymer had become brittle. This caused high-resistance build-up in the circuit and decreasing current density [40]. In another case using the same anode, however, the material itself was found to be in good condition after 12 y. This was probably related to lower operation current densities. In this case, the system required maintenance in that the power sources, the coimections and the reference electrodes had failed and needed to be replaced [41]. 

Cable anodes of conducting polymers have an advantage when there are site problems with the installation of other anodes. They are extensively used for the cathodic protection of reinforcing steel in concrete (see Section 19.5.4). 

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 ... 

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