The object of this study is to investigate the electrochemical behaviour and surface changes on AA6061-T6 alloy, exposed to Caribbean seawater (Cozumel Channel) under laminar flow, contrasting these with stationary flow. In our previous studies, EN technique was carried out to characterise the first stages of corrosion in stationary seawater of copper, aluminium and aluminium alloys, as well as the initial stages of AZ31B Mg alloy in simulated body fluid. The β exponent is a parameter correlated with the strength of persistence in a process. The fGn is a stationary process, and the fBm is nonstationary. PSD plots display a slope, β exponent, which enables the differentiation between series with fractional Gaussian noise ( fGn, β from −1 to 1) and fractional Brownian motion (fBm, β from 1 to 3). ĮN measurements can be analysed transforming the data in the frequency domain by fast Fourier transform (FFT) to obtain power spectral density (PSD). EN measurements can be performed under corrosion potential or any constant potential/current, depending on the research objective, to analyse the corrosion mechanism and obtain the corrosion rate. The main sources of EN observed in corrosion systems are attributed to microscopic and macroscopic events. This technique can provide information concerning the nature of the corrosion process and the rate thereof. Electrochemical noise is presented by random fluctuations of corrosion potential or current, typically with frequencies below 10 Hz and low amplitude. Fluctuations may be interpreted as electrochemical noise (EN), which is useful for the purposes of corrosion mechanism characterisation. The monitoring of the open circuit potential (φ corr, free corrosion potential) or corrosion current is one of these and is easy to handle. The characterisation of the corrosion process requires electrochemical nondestructive techniques, and the preferred methods are those that do not apply external polarisation. The principal parameters that affect the corrosion behaviour of metals immersed in this electrolyte are oxygen content, dissolved mineral salts, pH, temperature, specific contaminants and flow velocity. Seawater is a complex electrolyte of different ions, with a high salinity (3.5%, density 1.023 g/cm 3 at 25☌), which causes damage to metals in a short time. Moreover, the heterogeneity of the surface could result in favourable nucleation sites. The localised corrosion becomes greater in the presence of aggressive ions, such as chlorides, and corrosion pits initiate in oxide film sites, weakened by chloride attack. The electrochemically active anodic particles are rich in Mg, Si and Al, with Mg preferential dissolution, leaving a cavity in the oxide layer, while the cathodic particles are rich in Fe, Si and Cr, acting as preferential sites for oxygen reduction. IMPs present electrochemical behaviour different from the alloy matrix, and they may be classified in two types: cathodic and anodic. The addition of alloying elements to aluminium increases its mechanical properties however, the precipitated intermetallic particles (IMPs) have a harmful effect on the corrosion resistance of the Al alloys. The alloy presents good corrosion resistance in many environments having neutral pH, because of the formation of protective amorphous aluminium oxide film on its surface of approximately 2–3 nm thickness and is insoluble in water. The changes in the morphology and elemental composition of the formed layers revealed that the localised attacks occurred in the vicinity of intermetallic particles rich in Fe and Cu, which act as cathodes.Īluminium alloy (AA) 6061-T6 is popular as a nonferrous material for structures in seawater and is characterised by various properties, such as strength-to-weight ratio, extrudability-particularly for the manufacture of profiles with complex geometry, low thermal expansion coefficient, good wear resistance and corrosion resistance. The value of the exponent β in PSD graphs suggested that the localised corrosion on AA6061-T6 surface occurs as a persistent stationary process, which dynamic is controlled by oxygen diffusion. ![]() Their power spectral density (PSD) plots were obtained in order to gain information concerning the dynamic of the spontaneous release of energy during the corrosion process. ![]() The fluctuations of current were transformed in the frequency domain. The calculated corrosion current, based on Rn, was one order higher in laminar flow. Open circuit potential monitoring and electrochemical current fluctuations, considered as electrochemical noise (EN), were employed as two nondestructive methods. The electrochemical behaviour and surface changes on AA6061-T6 alloy exposed to Caribbean seawater from the Cozumel Channel for 30 days under laminar flow (0.1 ms−1) were studied, these contrasting then with stationary conditions.
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