Electrochemical Noise and Impedance Analysis of Iron in Chloride Media

Abstract

Potential and current noise data have been collected for pure iron foils exposed to 0.5N sodium chloride (NaCl) that was aerated, deaerated, or aerated with sodium nitrite (NaNO2) added as inhibitor. Potential and current noise data were obtained at the beginning of each hour over a 24-h period either sequentially or simultaneously. Statistical and spectral analyses were performed resulting in noise resistance values. For the noise resistance Rn = σV/σI, where σV and σI are the standard deviation values of potential and current, respectively, similar values as for the polarization resistance Rp determined with electrochemical impedance spectroscopy (EIS) at the end of the tests were obtained. The spectral noise resistance Rosn obtained from spectral analysis of simultaneously collected current and potential data agreed satisfactorily with Rp and Rn for aerated and inhibited solutions. Electrochemical noise and impedance data have also been obtained for two polymer coatings on cold-rolled steel during immersion in 0.5N NaCl for five months. The two types of measurements clearly distinguished between the relatively poor performance of the alkyd system and the excellent performance of the epoxy polyamide system. Similar trends with exposure time were observed for the pore resistance Rpo and Rp, on the one hand, and Rn and Rosn on the other hand. However, the numerical values of the parameters obtained with EIS were different from those obtained with electrochemical noise analysis (ENA).