On the corrosion mechanism of Mg investigated by electrochemical impedance spectroscopy

Abstract

This work reported a detailed analysis of the electrochemical impedance spectra obtained for the Mg electrode during immersion in a sodium sulfate solution. A model was proposed which took into account the presence of: (i) a thin oxide film (MgO) which progressively covered the Mg electrode surface, (ii) film-free areas where the Mg dissolution occurs in two consecutive steps, (iii) a thick layer of corrosion products (Mg(OH) 2), (iv) an adsorbed intermediate ðMg þ ads Þ which is responsible for the chemical reaction allowing the negative difference effect to be explained. From the impedance data analysis, various parameters were extracted such as the thin oxide film thickness, the resistivity at the metal/oxide film interface and at the oxide film/electrolyte interface, the active surface area as a function of the exposure time to the electrolyte, the thickness of the thick Mg(OH) 2 layer and the kinetic constants of the electrochemical reaction.

Corrosion of Mg alloys EV31A, WE43B, and ZE41A in chloride‐ and sulfate‐containing solutions saturated with magnesium hydroxide

“…The quality of the fit is attributed to the fact that there were slow changes in the corrosion rate in the present work as is evident from Figures . The fit was not as good as those by Gomes et al and those of Scully and co‐workers; however, those works used solutions that were much less corrosive, so the corrosion behavior was much more stable, and there was much less change in corrosion behavior during the measurement of EIS spectra. The corrosion rates evaluated from the EIS data, P i,EIS,1 and P i,EIS,2 , were consistently smaller than the corrosion rates evaluated from the evolved hydrogen P H .…”

“…Gomes et al found that steady‐state corrosion for pure Mg in 0.1 M Na 2 SO 4 , for times between 2 and 58 hr, was consistent with a reaction sequence involving the Mg + ion as an intermediate, occurring mainly in the breaks in the MgO surface film, 5–7 nm thick. These breaks in the MgO layer represented a fraction of ≈0.003 of the surface.…”

“…The Mg corrosion mechanism was reviewed by Atrens and co‐workers, who found that the experimental evidence supported the mechanism involving the Mg + ion as an intermediate, as found by Gomes et al Recently, Fajardo and Frankel showed that the experimental evidence for Mg corroding in 0.1 M NaCl directly contradicted the catalytic activity model for Mg corrosion, showing that this model could not explain the negative difference effect in which the hydrogen evolution rate increases with anodic polarization.…”

“…The corrosion mechanism may be elucidated by electrochemical studies, including polarization curves and electrochemical impedance spectroscopy (EIS) . The EIS study of Gomes et al found steady‐state corrosion for pure Mg in 0.1 M Na 2 SO 4 for times between 2 and 58 hr. Their EIS data were consistent with a reaction sequence involving the unipositive Mg ion (Mg + ) as an intermediate, under the conditions shown in Figure , in which the anodic hydrogen is exclusively produced by the chemical splitting of water by the Mg + ion.…”

From frequency dispersion to ohmic impedance: A new insight on the high-frequency impedance analysis of electrochemical systems

“…It should be noted that it is this last contribution that is analyzed when using the complex capacitance representation to determine the thickness of the oxide films formed on the surface of the electrodes. [17][18]33 The parameter determined from the fitting procedure is 0.77, and will be further discussed in the following section.…”