Study on the Boundary Conditions of Coated Steel in Numerical Simulation of Cathodic Protection Systems

Boundary element method (BEM) algorithm has been used in cathodic protection (CP) described by the Laplace equation with boundary conditions given by a polarization curve for nearly three decades. Polarization curve of bare steel can be adopted directly as a boundary condition, and the calculated results are polarized potentials or called Off potentials which can evaluate the protection level. However, for steel covered with coating, an IR drop is involved in the polarization curves, and the calculated results will be the On potentials. In order to eliminate the IR drop, current interrupter technique and electrochemical impedance spectroscopy (EIS) were applied in this paper to obtain the resistances of Fusion Bonded Epoxy (FBE) coatings under different conditions, i.e., good FBE coating with no holiday, holidays with various sizes, degraded coatings, without CP and with different CP levels. Results showed that the resistances changed with different coating conditions and different CP levels. IR free polarization curve of coated steel was gained after eliminating IR drop. A simulation experiment was conducted and the calculated results were compared with the calculated data. Results showed that BEM combined with IR free polarization curve could give a good polarized potential profile of coated metallic structures in cathodic protection systems.