Equipment Using Galvanic Anodes

3 Examples of the Design of Protective Installations 11.3.3.1 Equipment Using Galvanic Anodes 

Magnesium anodes were chosen as the source of the protection current in this old example, because on one hand a sufficient current, including current reserve, could be achieved due to the relatively low soil resistivity, and on the other hand, use of an impressed current protection system would have required much greater expenditure. 

Since to achieve the necessary protection current, including a current reserve (in total about 15 mA), the total resistance of the protection current circuit at a voltage of 0.6 V between steel and magnesium anodes [Eq. (11-1)] must not exceed 40 2, two 5-kg magnesium block anodes were provided whose grounding resistance for each anode amounted to about 20 2. The grounding resistance of the two anodes connected in parallel, taking into account an interference effect factor, F = 1.1 with = 2 in Eq. (24-35), comes to about 11 2. 

Basically the maximum anode currents given by Eq. (6-14) indicate that the grounding resistance of the cathode is considerably smaller than that of the anode. Since this is not necessarily the case, particularly with storage tanks with good coating, the current output of the anodes is considerably lower [see Eqs. (6-13) and (11-1)]. 

At the relatively low protection current density of 200 juA vtr and with the anode positioned on one side, it is to be expected that with this storage tank sufficient reduction in potential would be achieved on the other side of the tank from the anode. The off potential was measured using a measurement point at a depth of about 2 m as t/cu-cuso = -0.88 V at the tank. At the other side of the tank as well as above it, off potentials of-0.90 to -0.94 V were found. These potentials were measured with a protection current of 10 mA (anode 1 6 mA, anode 2 4 mA) with an additional resistance of 8 2 in the protection current circuit (see Fig. 11-2). With a direct connection between the tank and the group of magnesium anodes, the initial current was about 16 mA, which after 1 h of polarization decreased to about 14 mA. The reserve current, based on a long-term current of 10 mA, amounted to ca. 40% in the operation of the cathodic protection installation. 

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