Graphite backfill

On subterranean pipeline, look for graphitic corrosion on the very bottom of the line where it rests on the backfill. When graphitic corrosion occurs under these conditions, the affected region may be a narrow zone running along the pipe bottom over some distance (Case History 17.1). 

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.

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. 

Graphite anodes when used in soils are invariably placed in a carbonaceous backfill. This helps to compensate for the lower electrical resistivity of graphite when compared with silicon iron. In such an environment, no build-up of a film of high resistance between the anode and backfill occurs, unlike silicon-iron anodes where the resistance can increase with... 

A land-based arrangement for CP in soil is shown in Figure 10.18. A backfill of a carbonaceous material such as coke breeze or graphite particles is used around the anode. The backfill is electron-conducting, which makes the effective size of the anode larger. This in turn reduces the resistance between the anode and the structure, and it reduces the consumption of anode material. If the resistivity of the soil is very high, buried pipelines may be protected by means of a continuous anode along the pipeline at a suitable distance from it. 

Indeed, when buried in soils containing SO Jakobs and Hewes report graphite consumption rates of I-56kg A" y at 21 6Am which is considerably higher than the theoretical maximum consumption rate. These factors must be considered with regard to the operating environment and the chemical treatment of backfill. 

Graphite anodes are made by mixing calcined petroleum coke particles and coal tar pitch binder. The desired shape is heated at 2800°C in order to convert amorphous carbon to crystalline carbon (graphite), which is immune to chloride solutions [10-11]. These anodes are normally impregnated with linseed oil or synthetic resins for reducing porosity and spalling, hi addition, the anodes are buried in the soil and backfilled with coke breeze for a uniform distribution... 

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 ... 

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. 

For another method of cathodic protection, the source of electrons is an impressed current from an external dc power source, as represented in Figure 17.23h for an underground tank. The negative terminal of the power source is connected to the structure to be protected. The other terminal is joined to an inert anode (often graphite), which, in this case, is buried in the soil high-conductivity backfill material provides good electrical contact between the anode and the surrounding soil. A current path exists between... 

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