Role of S^2- ions on the microstructure change and the pitting behaviour of aluminum in saline solution

The electrochemical behaviour and the passive film microstructure of aluminum during its exposure to 3.5 wt% NaCl solution in the absence and presence of S^2- ions are investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy measurements, XRD, XRF, SEM and AFM. Electrochemical measurements show that the presence of S^2- ions enhances the uniform corrosion of aluminum in NaCl solution, but delay its susceptibility to the pitting corrosion. In addition, EIS analysis illustrate that the formation of more compact and protective passive layer in the presence of S^2- ions compared to its rough surface in the absence of S^2- ions as evidenced by the lower value of constant phase element (CPE) and higher value of phase shift (N). Cracks, non- homogenous and open large pits with high degree of roughness are clearly observed on the aluminum surface in the absence of S^2- ions, compared to oriented grooves, elongated ridges with the accumulation of the corrosion products inside the pits in the presence of S^2- ions. The inhibitory effect of S^2- ions for the pitting corrosion of aluminum is interpreted on the basis of the change in its microstructure of the passive film in the absence and presence of S^2-ions.