On-line analysis is very promising in the field of corrosion monitoring in industrial conditions. In some circumstances, when Tafel coefficients are not known, satisfactory assessment of corrosion intensity is possible. This technique is related to electrochemical impedance spectroscopy (EIS), in that an alternating potential perturbation is applied to one sensor element in a three element probe, with a resultant current response. Not only the primary frequency but higher order harmonic oscillations are analyzed in this technique. Theory has been formulated, whereby all kinetic parameters, e.g. Tafel slopes, can be calculated explicitly. No other technique offers this facility. At present the technique remains largely unproven and rooted in the laboratory domain.
Investigations by the frequency intermodulation method can be carried out in wide range of frequency. Moreover the necessity of additional impedance measurement aimed at selection of appropriate perturbation signal is eliminated. Due to this technique intermodulation harmonic components cleared from the capacitive component can be analyzed. Intermodulative amplitude spectrum supplies broad range of data concerning investigated system.
Analysis of the harmonic and/or intermodulation response gives the possibility to obtain the polarization resistance and both Tafel parameters (and, hence, the corrosion rate) within one measurement. This unique feature of the harmonic and intermodulation techniques will be further developed to create a corrosion monitoring tool for systems under cathodic protection.