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.
A novel coupling between atomic emission spectroelectrochemistry (AESEC) and electrochemical impedance spectrometry (EIS) is demonstrated. In this way, it is possible to distinguish situations in which the oxidation of the metal leads directly to the formation of dissolved ions or passes through a slightly soluble film intermediate. It was found that Zn dissolution 0.1 M NH4Cl occurs without any significant solid intermediate as evidenced by the excellent correlation between the AC components of the electrical current and the zinc dissolution rate. Analysis of the dissolution rate and total current transients as a function of potential directly yields the anodic and cathodic Tafel slopes for this system. In contrast, for Zn/0.5 M NaCl, electrochemical oxidation leads directly to the formation of an intermediate corrosion product film with subsequent dissolution.
A novel spectroelectrochemical method based on the thermal-lens effect from electrolyte solution using regular Joule heat generation by current focussing in a small-sized channel is under development. Numerical calculations using finite-element modelling were used for a thorough estimation of experimental conditions (cell and electrode shape and size). The major interferring effects were estimated. A change in the analyte concentration near electrodes due to electrolysis is significant and is overcome by large-radius ring elctrodes and AC current. The calculations were confirmed by experiments. An advanced cell design with good reproducibility and the sensitivity of measurements down to 10–6 M is proposed.