Corrosion Studies On Pure Aluminum In Aqueous Neutral Sodium Chloride Solutions

This thesis consists of two parts. In part one, the corrosion of pure aluminum in 3.0wt% neutral sodium chloride solutions was studied by electrochemical impedance spectroscopy (EIS) techniques, combining the examination of scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The results show that during the initial immersion segment, the polarization resistance (Rp) obtained at low frequency (10mHz) and the C_dl increase simultaneously, which coincides with the growth of oxide film on pure aluminum. While the discrepancy between the time when R_p and free potential arrived at their maximum respectively indicates that, during the growth process of oxide film, chloride ions have commenced their attacking to the pure aluminum substrate. And two times constant were confirmed, the high frequency one originates from the protective layer on the corroding substrate including the oxide film and the corrosion products, while the low frequency one comes from thediffusion process or the corrosion reaction and so on. Meanwhile, It is found that satisified relationship between the EIS features and the corroding morphologies has been observed, which has been interpreted according to the characteristics of corrosion process such as auto acceleration of pitting corrosion and the protection of local anodic reaction to the area around them.In part two, electrochemical noise (EN) measurements in conjunction with the scanning electron microscopy (SEM) technique have been used to study the corrosion behavior of pure aluminum in neutral 3.0wt% sodium chloride solution. The FFT analysis shows that none of the spectral power density plot's (PSD)three parameters which including high frequency linear (k), the white level (W) and the cut-off frequency (f_c) can properly describe the propensity and intensity of corrosion process respectively. Two new parameters (Se and Sg) were derived by using dimensional analysis method, and the value of S_E can characterize the severity of pitting corroding material. While the value of the other parameter (SG₎ was confirmed into the range of 5-20 and it's meaning remains for further investigationIn the other hand, the Wavelet analysis indicated that the energy distribution plot (EDP) can be used as fingerprints of EN signal and can be used to characterize the corrosion process. For metastable pitting, the relative energy mainly defined at the D crystals with small scales, while for pits propagation and pits growth, the relative energy mainly defined at the D crystals with middle and large scales respectively. Meanwhile, Good relationship between the EN features and the corroding morphologieshas been observed during the pitting corrosion process of pure aluminum in neutral 3.0wt% sodium chloride solution.