Impedance response of aluminum alloys with varying Mg content in Al-Mg systems during exposure to chloride corrosion environment

This research discusses the corrosion behavior of as-cast Al alloys with different Mg content by potentiostatic electrochemical impedance spectroscopy (PEIS). The complex plane spectra of all samples feature a high-frequency loop, followed by semi-infinite diffusion impedance characteristics at low frequencies, with the corrosion-induced formation of a defined porous structure of a layer making finite diffusion through the pores dominant upon prolonged exposure. The most compact layer causes the most pronounced and well-resolved finite diffusion features in the impedance spectra of the sample with the highest Mg content, while the sample with the lowest Mg content has a highly porous layer unable to slow down the corrosion rate at the layer/sample surface interface. The highest layer capacitance and diffusion admittance are found in the sample with the highest Mg content, with a more adherent protective film expected to form. However, the growth rate of the layer was not adequate for the remarkable closing of the pits, indicating the weakness of this sample towards pit activity. The results show that increasing Mg content improves corrosion resistance and clearly separates bulky corrosion from localized pitting corrosion, but it also increases the thickness of a more compact, poorly adhesive layer.