AFM and EIS investigation of the influence of pH on the corrosion film stability of Al4Cu2Mg8Si7 intermetallic particle in aqueous solutions

Abstract

The stability of corrosion films on the Q-phase (Al₅Cu₂Mg₈Si₆) under different pH environments ranging from pH 1 to pH 13 in 0.1 M NaCl solution has been investigated using Atomic Force Microscopy (AFM) and Electrochemical impedance spectroscopy (EIS). The magnitude of R_ct and R_f of Q-phase was noted to vary with pH. At pH 4 and 6, the values of R_ct and R_f were higher by about 3 orders of magnitude than the values at pH 1 and 13. This indicates better protection of the metal towards corrosion under neutral conditions. The polarization resistance (R_ct + R_f) obtained from the fitting parameter is observed to follow the order pH 6 > pH 4 > pH 10 >> pH2 > pH12 > pH13 > pH 1. The EIS response showed a capacitive behavior. From surface topography and cross section topography characterization there were significant differences in the surface characteristics of Q-phase under different pH values. The microscopic topographic analysis suggests copious accumulation of corrosion products in near neutral environments and the lowest surface roughness was observed in the alkaline environment. Quantitative surface roughness characterization shows that a maximum surface roughness was at pH 2 which decreases with increased basicity of the environment. Results indicate that the corrosion film stability decreases with increased deviation from the neutral pH environment.