When monitoring corrosion in laboratory and plant situations, changes in the chemical environment, temperature, pressure, flow rate, and so on may severely influence the corrosion processes, in terms of both mechanism and rate. Under these conditions, it becomes difficult to apply techniques such as linear polarization resistance with any degree of confidence or reliability. The basic Linear Polarization Resistance (LPR) technique may be applied in a number of ways, but the accuracy of the corrosion rate measurement is based upon some estimated value for the Stern Geary “constant”, which is system dependant and can change dramatically as the corrosion processes evolve. The fundamental assumptions regarding the LPR response are based on a linear resistive model of the corrosion processes, whereas in practice the corroding interface behaves in a non-linear fashion and the charge transfer (Faradaic) processes behave very much like diodes. These non-linear characteristics can be studied using a number of techniques to measure the “distortion” effects and produce fundamental information regarding the corrosion rate and the anodic and cathodic Tafel coefficients, thus providing some theoretical advantages over the basic LPR technique. There are principally two related techniques currently being used, Harmonic Distortion Analysis (HDA) and Inter-Modulation Distortion (IMD) also called Electrochemical Frequency Modulation (EFM) both of which will be discussed in detail.

This paper describes the application of electrochemical non-linear analysis techniques to corrosion monitoring in laboratory and field situations. The measurement technology, application, and interpretation are discussed, with emphasis on some of the advantages and disadvantages of the use of these techniques.

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