Power Spectral Density Analysis of the Corrosion Potential Fluctuation of Stainless Steel 316L in Early Stages of Exposure to Caribbean Sea Water

Stainless Steel 316 L samples were exposed for one month to Caribbean sea water. The fluctuations of the free corrosion potential values (E_corr at o.c.p.), considered as electrochemical noise (EN), were correlated with the stainless steel pitting corrosion in sea water. E_corr fluctuation analysis was carried out using the power spectral density (PSD) vs frequency. Figure 1 presents the initially changes of E_corr with time vs frequency in bilogarithmic scale. For the first three days, the PSD slopes present a negative β exponent (β > 1) at lower frequencies [0.002 – 0.05 Hz], indicating a release of spontaneous energy from the steel surface (corrosion). However, at major frequencies β tends to values close to zero (0.12 – 0.07). At later stages the PSD slopes show a unique value close to zero (0.02 – 0.06), in all range of frequencies [0.002 – 1 Hz]. The exponent β is associated with persistent processes (forces) in data series (2¹⁶) over time. The calculated Hurst coefficient based on β values suggested that initially the localized corrosion process occurs with the contribution of fractional Brownian motion (fBm) and fractional Gaussian noise (fGn). After the first three days, the E_corr fluctuations were only dominated by fGn (H≈0.5), which corresponds to the special case when the corrosion occurs at the border between anti-persistent (H<0.5) and persistent (H>0.5) processes. The E_corr fluctuations were correlated with the data surfaces provided by SEM-EDS and XRD.