Abstract
Numerical calculation 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 ‘Of’ 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, linear polarization, current interrupter technique and electrochemical impedance spectroscopy (EIS) has been utilized. Compared with the first two methods, EIS has the best verification and applied in this paper to obtain the resistances of two types of Fusion Bonded Epoxy (FBE) coatings without CP and with different CP levels. Results show that the resistances barely changed with different CP levels. IR free polarization curve of coated steel gained after eliminating IR drop has been expressed by involving electrochemical reactions. The polarization performance of coated steel will be well understood by such described polarization curves, and as boundary conditions, good on-off potential profiles of coated metallic structures in CP systems will be gained.
