Power Spectral Density Analysis of the Corrosion Potential Fluctuation of Aluminium in Early Stages of Exposure to Caribbean Sea Water

摘要

Aluminium samples were exposed for six months to Carribean sea water. The fluctuations of the freecorrosion potential values (Ecorr at o.c.p.), considered as electrochemical noise, were correlated withthe aluminium pitting corrosion in sea water, influenced by different crystalline phases formed on themetal surface: Al(OH)3 as a corrosion product in pits, and likewise crystals of MgCaCO3 and SiO2proceeding from the sea water. At three months the film of bio-fouling had grown on aluminiumsurface, formed by two diatoms (microalgae). Ecorr fluctuation analysis was carried out using the powerspectral density (PSD) vs frequency. The decrease of the PSD slopes (negative exponent) indicated arelease of spontaneous energy during from the aluminium surface, with the progress of pit formation insea water. The calculated Hurst exponent values suggested that pits formed randomly on the surface insea water could be as a consequence of highly persistent processes, dominated by fractional Gaussiannoise (fGn) at almost all periods of testing in sea water, with the contribution of fractional Brownianmotion (fBm) during some periods of time.