Backfill anode

Coke breeze anode backfill (Fluid or granular) 

Anode beds are sometimes installed deep below the surface (deeper than 30 m) (Fig. 13.15), a design that is particularly useful for installations where electrical interference problems are severe or where the resistivity of the soil near the surface is high. The deep installation causes the current flowlines to become parallel and thus provide a more uniform distribution of the protective current. 

Suitable coke or graphite backfill materials are available from suppliers. Coke should be either coal coke or calcined (heat-treated) petroleum coke. Non-calcined petroleum coke can have high electrical resistivity and be unsuitable for groundbed backfill use. 

Fiqure 13.18 Conventional groundbed using petroleum coke backfill and 5 cm cast iron anodes. (Courtesy of Dean Rookes, West Coast Corrosion Prevention Ltd.) 

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The effect of the backfill is to lower the circuit resistance and thus reduce potential loss due to the environment. The additive resistances of the anode/backfill and backfill/soil are lower than the single anode/soil resistance. Backfills attract soil moisture and reduce the resistivity in the area immediately round the anode. Dry backfill expands on wetting, and the package expands to fill the hole in the soil and eliminate voids. 

Walaszkowski et al. (1995b) applied the EIS technique in the investigation of anodic systems and backfills this technique can provide information on the mechanism of anode-backfill interactions. By this method the ohmic resistance of the backfill layer can be determined, as well as the resistance of anodic processes occurring on the anode-backfill-environment phase interface. The authors proposed an electric equivalent circuit of polarized anodic systems (Fig. 8-16). 

The possibility of gas blocking caused by oxidation of carbon to CO2, and the formation of oxygen gas as a result of the oxidation of water are disadvantages of carbon backfills. That is why the ventilation of anodic beds is used. More information on anodic backfills is given by Shreir etal. (1994) and Walaszkowski (1991). 

Figure 8-16. An electric equivalent circuit of an anode-backfill system and its impedance spectrum. Notation Rc electrolyte resistance, backfill resistance, anodic process resistance, and CPE and CPE. the constant phase elements.

A piece of zinc may also be used as a reference electrode by immersing it in an electrolyte that contains some quantity of chloride or sulfate ions or it can be used in the ground if surrounded by a suitable backfill. The anode backfill for zinc is usually bentonite, sodium sulfate, g5q3sum, and sodium chloride. Zinc electrodes made from high purity material usually provide a stable potential that is constant t within a5 mV bracket. The electrode may slowly polarize and may have marine growth on it in seawater but this is generally prevented if the zinc is allowed to act as an anode at a very low current density. 

The following relationship is widely used to determine the resistances of the anode backfill ... 

The backfill is either poured into the borehole, or the anodes are enclosed in sacks of permeable material filled with backfill. Such anodes are sunk into the borehole and backfilled with water and fine soil. Anodes installed in this way deliver their maximum current after only a few days. 

Galvanic anodes must not be backfilled with coke as with impressed current anodes. A strong corrosion cell would arise from the potential difference between the anode and the coke, which would lead to rapid destruction of the anode. In addition, the driving voltage would immediately collapse and finally the protected object would be seriously damaged by corrosion through the formation of a cell between it and the coke. 

Fig. 7-1 Material consumption from impressed current anodes. graphite anode without coke backfill, O graphite anode with coke backfill, FeSi anode without coke backfill, A FeSi anode with coke backfill.

Table 7-2 Composition and properties of solid impressed current anodes (without coke backfill)...

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. 

Where there is available ground and the specific resistivity of soil in the upper layers is low, the anodes are laid horizontally [3]. A trench 0.3 to 0.5 m wide and 1.5 to 1.8 m deep is dug with, for example, an excavator or trench digger (see Fig. 9-2). A layer of coke 0.2-m thick is laid on the bottom of the trench. The impressed current anodes are placed on this and covered with a 0.2-m layer of coke. Finally the trench is filled with the excavated soil. No. IV coke with a particle size of 5 to 15 mm and specific gravity of 0.6 t m" is backfilled at a rate of 50 kg per meter of trench. The anodes are connected in parallel and every three to four anode cables are connected to the anode header cable by a mechanical cable crimp encapsulated in an epoxy splice kit to give an economical service life at high current output. 

For installations with continuous coke backfill, the anodes can be installed at double the spacing of the anode bed extension. The lower the ratio p /p (i e., the higher the specific soil resistivity), the further apart the anodes can be placed. 

Fig. 9-4 Effective anode lengthening by coke backfill with Eq. (9-3) with Pc = 1 G m and C = 0.31.

In total, three high-silicon iron anodes of 3 kg each were installed at points a, aj and as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and J = 0.2 m so that the length of the coke backfill was about 1 m. Each anode was connected by a separate cable to the anode bus bar of the transformer-rectifier to allow the current of individual anodes to be monitored. Three cathode cables 2x4 mm were installed for the return path of the protection current and attached on the tank end to the connecting clamps of the dome support. 

The grounding resistance of the three anodes with the stated dimensions of the coke backfill, a soil resistivity of 75 D m and an interference factor, F = 1.2, was calculated from Eq. (24-35) as about 14 Q. After the anode installation was in operation, measurements of the grounding resistance gave a value of about 12 Q. 

Deep anodes are mainly used for injecting such high protection currents (see Section 9.1). The advice given in Section 9.1 on resistances and potential distribution relates to anodes in homogeneous soils. Large deviations are to be expected in soil used as backfill and in the neighborhood of structures [2]. This is generally the case with local cathodic ( hot-spot ) protection. 

Additional individual anodes must be installed at points on the protected object where a sufficiently negative pipe/soil potential cannot be achieved. Since usually only the voltage cone is of interest, the place of installation does not depend on the specific soil resistivity. Coke backfill is not necessary, and the place of installation is determined by the local circumstances. Individual horizontal anodes are conveniently installed parallel to the pipeline at the depth of the pipe axis. The voltage, length and distance of the anodes from the protected object are chosen according to Section 9.1 so that criterion No. 6 or No. 7 in Table 3-3 is fulfilled. 

Installation of an anode bed with 4 FeSi anodes in coke backfill, cable and connecting sleeves, including earthworks DM 6000... 

The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

When used underground they are usually placed in a backfill, consisting of gypsum, sodium sulphate and clay, which may be added loose, shipped in a bag around the anode, or obtained in cast form. 

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