Harmonic synthesis: A novel electrochemical method for corrosion rate monitoring

This paper presents a method of instantaneous corrosion rate determination based on harmonic current measurements of an electrode perturbed by a sinusoidal voltage signal of low frequency and of relatively low amplitude of the order of 10-30 mV, ensuring a nondestructive character of investigations. The theoretical principles of this method, called harmonic synthesis (HS), have been described. It is based on discrete synthesis of a quasi-stationary polarization curve from harmonic currents in a potential range determined by the amplitude of the perturbation signal. The polarization curve obtained in this way can be analyzed by appropriate software tools in order to determine the required kinetic parameters of the corrosion process, namely the corrosion current and Tafel coefficients. Contrary to the known harmonic analysis (HA) method, the HS method allows corrosion rate monitoring of systems with mixed control. On the basis of computer simulations for corrosion systems with activation, activation-diffusion, and activation-ohmic polarization, it has been found that limiting the synthesis to the third harmonic component is quite sufficient for practical purposes. The error of the current summation did not generally exceed 1% under these conditions. The method of choosing the perturbation signal frequency, allowing the elimination of the nonfaradaic current connected with the presence of the electric double layer, has been presented. The usefulness of the HS method has been illustrated by examples of mild steel and copper corroding in artificial seawater. The mean corrosion rate of the metals, determined by the HS method with ohmic drop correction, differed only by ca. 2-8% from that found by the gravimetric method, while the classic HA method gave results underrated by ca. 18-24%. (C) 2003 The Electrochemical Society.