Magnitude of Current Required

The potential decay, E, along an infinite pipeline measured from the point of attachment to the dc source having potential Ea is expressed as an exponential relation with respect to distance, x, along the pipeline in accord with 

Both E and Ea represent differences between polarized potential with current flowing and corrosion potential in absence of current, Rl is the resistance of pipe of radius r per unit length, k is a constant, and z is the resistance of pipe coating per unit area (for derivation, see Appendix, Section 29.4). This equation is derived by assuming that polarization of the cathodically protected surface is a linear function of current density. Note that E becomes zero at x = oo. 

Considering a finite pipeline for which all is half the distance to the next point of bonding, and potential E at all = Eb, 

Cathodic protection is optimum within a specific potential range (see Section 13.7), so that the length of pipeline protected by one anode increases as the metallic pipe resistance, Rl, decreases, and coating resistance, z, increases. 

The current density required for complete protection depends on the metal and on the environment. It can be seen from Fig. 5.15 in Section 5.11 that the applied current density must always exceed the current density equivalent to the measured corrosion rate in the same environment. Hence, the greater the corrosion rate, the higher must be the impressed current density for protection. 

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