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

Abstract

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 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.