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Published: Kem. Ind. 51 (7-8) (2002) 343–348
Paper reference number: KUI-48/2001
Paper type: Professional paper

Electrochemical Protection of Metallic Structures Cathodic Protection of an Underground Tank Bottom

A. Rešetić and S. Martinez


Modeling of the cathodic protection system consisting of an underground tank bottom protected by cylindrical anodes placed below it, has been elaborated. A mathematical expression is given for calculation of the maximum cathode potential difference ΔEmax that can be compared to the allowed potential difference ΔEZ set as an electrochemical criterion for efficient protection. According to this criterion, steel underground structures should be polarized to the electrode potential between -0.85 V and -1.2 V with respect to the saturated Cu/CuSO4 reference electrode. By solving the Laplace equation under the assumption of the uniform current density of oxygen reduction on the cathode and constant potential of the anode, the expression for the potential distribution was obtained. 3D and contour plots of the potential distribution are shown in case of the tank bottom protected by three anodes. The knowledge of potential distribution enables computation of the maximum potential difference occurring at the cathode for the specified values of model quantities. These parameters include protection current density jz, electrolyte conductivity k, depth of the underground anodes d and their distance c. jz and k may be determined experimentally from polarization and conductometric measurements, respectively. In order to satisfy the boundary condition on the cathode, jz should correspond to the limiting current density of oxygen reduction process. Hence, jz and k being predetermined for a specific cathodic protection system leave d and c as parameters of the model, that may be varied in order to satisfy the condition of efficient protection, ΔEmax ≤ ΔEZ. Simplified expressions for maximum potential difference on the cathode were derived in case of d/c <<1 and d/c >> 1. The expressions given in this work may be used for modeling of the cathodic protection systems, having electrode arrangements, that are well approximated by the investigated two-dimensional geometry. The model may also be used as an educational tool for determination of relevant cathodic protection quantities under various conditions.

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cathodic protection, potential distribution, current distribution, Laplace's equation, modelling cathodic protection