Comparison Between Electrochemical Noise and Electrochemical Frequency Modulation Measurements during Pitting Corrosion

The electrochemical noise (EN) and electrochemical frequency modulation (EFM) techniques have been compared for their use to detect pitting corrosion. To do this, experiments on different corroding systems showing passivation and pitting corrosion were carried out. These corroding systems were: (1) Aluminum in borate buffer solution with and without chlorides (pitting corrosion + passivation behavior), (2) AISI 304SS in 0.3 wt.% FeCl3 solution and 6 wt.% FeCl3 solution at room temperature (passivation behavior + pitting and/or crevice corrosion), and (3) AISI 304SS in 6 wt.% FeCl3 at elevated temperature of 57 °C (pitting corrosion). Both EN and EFM were measured on-line meanwhile changing the corrosive environment by adding chlorides or by increasing the temperature. A potential perturbation composed of two sine waves is applied with the help of EFM to get current response at various frequencies. As the corrosion process is nonlinear in nature, the ac-response contains components at harmonic and intermodulation frequencies. Analysis of current components at different frequencies yields the information about the corrosion behavior under investigation. EFM may be considered to detect pitting initiation and its further development due to the results obtained by measuring the so-called "causality factors", which are the ratio of the current components in the ac-response.