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

Abstract

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 Mg(2+) , 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 Mg(2+) dissolution, as the voltage excitation becomes slow enough to enable all Mg(2+) -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.

Application of Side-View Imaging and Real-Time Hydrogen Measurement to the Investigation of Magnesium Corrosion

“…However, a number of recent experimental evidences strongly suggest that univalent magnesium is not responsible for the negative difference effect, and the process of hydrogen evolution is entirely cathodic in nature. Discussion of the arguments supporting this statement is not reported here for brevity, but can be found in the literature 2,[4][5][6][15][16][17] .…”

Role of Segregated Iron at Grain Boundaries on Mg Corrosion

“…12,15 Even a very low content of impurities (usually < 150 ppm in "pure" Mg) 41 can lead to formation of such surface film and the morphology of this film depends on electrolyte composition and Cl − concentration. 45,46 The effects of different alloy precipitates and alloy microstructure were widely studied in the case of Mg alloys [47][48][49][50][51][52][53][54][55] but not pure Mg.…”

“…This phenomenon called "the negative difference effect" (NDE) 9-16 and more recently "the anodic hydrogen evolution" process 17,18 has been also observed for other metals than Mg such as Al, Li. [19][20][21][22] Despite the abundant literature on Mg corrosion and many models of Mg corrosion (such as: the formation of Mg(I) as an intermediate product, 23,24 the formation and breakdown of the partially protective film, 11,[25][26][27] the formation of hydride intermediates, [28][29][30][31][32] formation of dark filiform patterns 13,16 and increase the hydrogen production in aqueous chloride solutions.12,15 Even a very low content of impurities (usually < 150 ppm in "pure" Mg) 41 can lead to formation of such surface film and the morphology of this film depends on electrolyte composition and Cl − concentration. 45,46 The effects of different alloy precipitates and alloy microstructure were widely studied in the case of Mg alloys [47][48][49][50][51][52][53][54][55] but not pure Mg.…”

“…36,37 It has to be noted that there is no direct evidence of the formation of Mg + species as indicated by several authors. 16,[38][39][40][41] The other mechanisms such as breakdown of corrosion films can be influenced by the surface reactivity of Mg substrate, which depends on the presence of alloying elements or noble metal impurities, leading to increased catalytic activity of Mg. The noble metal impurities, such as Fe, Cu or Ni, 34 which have been considered as the active sites of the cathodic reaction, 38,[42][43][44] lead to formation of dark filiform patterns 13,16 and increase the hydrogen production in aqueous chloride solutions.…”

“…Data acquisition and post processing analyses were performed using Ion-Spec software (version 4.1). 24 Mg + ions characteristic of metallic and/or oxidized magnesium, 40 MgO + and 41 MgOH + characteristic of magnesium oxide and hydroxides, respectively, and 13 Al + , 25 Mn + and 26 Fe + characteristic of metallic (and/or oxidized) impurities (Al, Mn and Fe, respectively), were chosen for ion depth profiling and imaging.…”