Electrochemical impedance characteristics of pure Al and Al–Sn alloys in NaOH solution

Electrochemical impedance analysis of pure Al and Al–Sn alloys in alkaline solution is reported. The impedance spectrum of pure Al is found to consist of two intersecting capacitive semicircles with a loop at intermediate frequencies. The capacitive semicircles occurring at the high and low frequencies are due to the charge transfer reaction at the metal/surface oxide film interface and the dielectric properties of surface oxide film, respectively. The inductive loop at medium frequencies results from the surface charge build-up at the metal/oxide film interface caused by the rate difference of introduction into and transport through the oxide film of Al 3+ ions. Al–Sn alloys show discrete impedance spectra according to their electrode potential. In the potential range where Al is in the active state and Sn is in immunity, the impedance spectra of Al–Sn alloys are composed of only a capacitive semicircle and then converted into a reverse semicircle indicating a negative polarization resistance with the electrode potential. In addition, Al–Sn alloys have the same impedance spectra as that observed for pure Al at the potentials in which Sn is dissolved.