Deep anodes local cathodic protection

Deep anodes are installed where the resistivity is high in the upper layers of soil and decreases with increasing depth. This type of installation is recommended for densely populated areas and for local cathodic protection (see Chapter 12) on account of the small space needed and the smaller voltage cone, which avoids interference with foreign structures. 

Fig. 12-2 Local cathodic protection in a power station. deep anodes O horizontal anodes Potential readings Ccu-cuso4 volts (A) free corrosion potential before commissioning the cathodic protection (B) 4 months after switching on...

Cathodic protection with impressed current anodes is used predominantly with cables or steel casing in which the cable is inserted, outside built-up areas where it is possible to build large anode installations without damaging interference with other lines. In densely populated areas, protection with impressed current anodes is often only possible with deep anodes, with surface anodes or locally at individual problem points (local cathodic protection, see Chapter 12). 

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. 

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

Note, however, that there are conditions under which inhibitors can give rise to detrimental local corrosion, that is, pitting corrosion. This is the case when the amount of inhibitor is insufficient. Under these conditions, only part of the surface can be covered, thus giving rise to a local element. Corrosive attack is particularly extensive at the uncovered anode areas because of increased corrosion current density and deep cavities penetrating into the material. Similarly, if the inhibitor is too readily reduced at the cathodic areas of the metal surface, increased corrosion can result because compact protective films are not formed. Since there are no universally applicable inhibitors, they must be carefully selected and examined for each specific case. In doing so, inhibition of metal dissolution is not the only point to be considered—there is also hydrogen absorption. 

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