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 ͑HS͒, 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 ͑HA͒ method, the HS method allows corrosion rate monitoring of systems with mixed control. On the basis of computer simulations for corrosion systems with activation, activationdiffusion, 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%.

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A critical review has been presented of papers on application of AC electrochemical methods for determination of metal corrosion rates in cathodic polarisation conditions. Application of the classical linear electrochemical impedance spectroscopy (EIS) with the use of low-amplitude perturbation signals has been discussed. Further AC techniques have been characterised using signals of higher amplitude, hence in the non-linear range of the electrode polarisation characteristic. Faradaic rectification (FR) and harmonic analysis (HA) with other derived methods such as harmonic impedance spectroscopy (HIS) and harmonic synthesis (HS) for monitoring of the effectiveness of cathodic protection have been presented.

A critical review has been presented of papers dealing with application of DC electrochemical methods for corrosion current determination of cathodically polarised metals. Methods of evaluation of cathodic protection (CP) effectiveness have been described, based on polarisation curve and polarisation resistance measurements. Mathematical methods and software have been characterised for computer analysis of polarisation curves used to determine electrochemical parameters describing the corrosion rate.

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