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. 

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). 

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. 

Anode 1 = plastic cable anode anode 2 = mixed metal oxide-titanium. 

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]. 

Polymeric anodes produced from 1975, also called cable and composite anodes, ensure a beneficial linear distribution of current and potential on the protected structure in cathodic protection systems. They have found applications for the cathodic protection of underground pipelines, tanks, and reinforced concrete structures in the vicinity of factories and municipal areas. Cable anodes, similar to graphite and ferrosilicon anodes, are placed in a conducting backfill enabling the transfer of some electrochemical reactions to the carbon backfill/environment phase interface. The applied anodic current density usually ranges from 0.5 to 1.5 A m . Conduction of current in polymeric anodes can take place in two ways ... 

LIDA type cable anodes were applied in the form of chains (cylindrical anodes connected with sections of cable) and highly efficient Polish rectifiers. The control system was made up of ... 

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