2003 Volume 52 Issue 9 Pages 488-495
An electrochemical noise analysis to evaluate corrosion rate of mild steel was studied theoretically and experimentally. It was revealed that a signal source involved in an external measurement system and that in corroding electrode were equivalent to calculate impedance of the electrode. Sudden and large fluctuations in potential and/or current were not suitable to estimate the corrosion rate by noise impedance, however, these should be applicable to monitor occurrence of crack propagation of SCC or initiation of pits. Based on theoretical analysis, both the correlation coefficient of potential and current data in time domain and the coherence of those of power spectrum densities in frequency domain are considered as a fundamental index to estimate the corrosion rate from the impedance spectrum. High correlation coefficient in time domain was required, but not enough to judge the reliability of impedance from noise data. The impedance calculated from noise data, that was measured for a mild steel in sodium chloride solutions, coincided well with that measured by AC method, when both the correlation coefficient and the coherence showed high values. Limits of estimation of corrosion rate from the noise impedance and the noise resistance defined by the ratio of standard deviations of potential and current fluctuations are discussed.