Electrochemical Noise Analysis of Aluminum Alloy 6061-T6 in Laminar Flow of Seawater

Abstract

Aluminum alloy 6061-T6 is widely used in many fields of engineering, especially in marine applications1. In the presence of chlorides localized corrosion occurs2. The goal of this study is characterize the alloy surface and its corrosion resistance in laminar flow conditions (0.1 ms-1), during the exposure in Caribbean seawater of Cozumel channel. As electrochemical techniques, open circuit potential (OCP) monitoring and electrochemical noise (EN) have been employed. OCP was correlated with changes on the alloy surface. The fluctuations of current, considered as EN3, were acquired up to 30 days and were transformed in the frequency domain. Their power spectral density (PSD) plots were obtained in order to gain information concerning dynamic of spontaneous release of energy. At the end of the experiment, the value of β exponent in PSD graph (Fig. 1a) suggests that AA6061-T6 corrosion process occurs as a persistent non-stationary process, which dynamics is controlled by fractional Brownian motion (fBm). The EN data provided the pitting index (PI = 0.95) and Rp value (1.44 kΩ cm2). The PI value indicates that Al6061-T6 presents pitting corrosion. Corrosion current (Icorr = 18.08 μA cm-2) was calculated based on Rp and Stern-Geary coefficient (B = 0.026 V) values. Removing the corrosion products, SEM-EDX analysis showed undissolved intermetallic particles on the alloys surface, rich in Fe and in the presence of Si, as well as the presence of pit cavities (Fig. 1b). Figure 1. AA6061-T6 after 30 days of exposure in Caribbean seawater of Cozumel channel with a laminar flow (0.1 ms-1): a) PSD plot of current noise signal and β exponent value and b) SEM-EDS of the surface after remove of the corrosion layer.