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33 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 and surface condition lead to the differ- ences between polarization curves (Figure 1). The corrosion potentials and corrosion current densities shown in Figure 1 are dif- ferent for coated and bare structures. Boundary Element Analysis System Cathodic Protection Modeling The model for Figure 2 was set up according to boundary element analysis system recommendations, with the simula- tion box 20 times the size of the model. Since the elements of the box are small, four elements are adequate. The polarization curve is used as the boundary condition of the cathode; how- ever, it is a nonlinear boundary condition. Therefore, the polarization data must be manipulated using a piecewise linear inter- polation approach. 8 Experimental Verification of the Mathematical Model To verify the accuracy of the mathemat- ical model, laboratory experiments were conducted. The experiment box, 8 by 1.8 by 0.8 m, was made of wood and covered with insulating board and a polyvinyl chloride (PVC) board at the bottom and sides. A steel pipe (Q235), with an outside diameter FIGURE 1 Polarization curves of coated and bare steel. FIGURE 2 Indoor experimental schematic diagram. FIGURE 3 Comparison of experimental data with numerical results. o f 2 0 m m , w a l l thickness of 3 mm, length of 6,000 mm, and coated with a three-layer polyeth- ylene (3PE) anticor- rosion coating, was buried in the box at a depth of 0.5 m . T h e c y l i n d r i c a l au xi li ar y sur fa c e- treated anode had a diameter of 0.03 m, length of 0.1 m, and output current of 1 mA; and was placed 0.3 m from the pipe, with no fillers, at a depth of 0.5 m. The cylindrical ground- ing had a diameter of 0.05 m and length of 0.1 m, and was placed 0.1 m from the pipe at a depth of 0.2 m. The test points were set up on the pipe at 0.5-m intervals, excluding the two ends. Three cables were connected to the ener- gized points of the protected pipe, the aux- iliary anode, and the reference electrode (Figure 2). The CP potential data were acquired over time, using the test block power interruption method. Comparison Between the Simulation and Experimental Results The experimental results are shown in Figure 3, and simulation results are listed in Table 1. The experimental data and simulation results are in good agreement, with negli- gible errors of 1.6 to 3.0%. Therefore, the mathematical model is representative of the grounding effect on CP.

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