Revisiting the Electrochemical Impedance Spectroscopy of Magnesium with Online Inductively Coupled Plasma Atomic Emission Spectroscopy

The electrochemical impedance of reactive metals such as magnesium is often complicated by an obvious inductive loop with decreasing frequency of the AC polarising signal. The characterisation and ensuing explanation of this phenomenon has been lacking in the literature to date, being either ignored or speculated. Herein, we couple electrochemical impedance spectroscopy (EIS) with online atomic emission spectroelectrochemistry (AESEC) to simultaneously measure Mg-ion concentration and electrochemical impedance spectra during Mg corrosion, in real time. It is revealed that Mg dissolution occurs via Mg2+, and that corrosion is activated, as measured by AC frequencies less than approximately 1 Hz approaching DC conditions. The result of this is a higher rate of Mg2+ dissolution, as the voltage excitation becomes slow enough to enable all Mg2+-enabling processes to adjust in real time. The manifestation of this in EIS data is an inductive loop. The rationalisation of such EIS behaviour, as it relates to Mg, is revealed for the first time by using concurrent AESEC.