Interpretation of Inductive Loop in Electrochemical Impedance of Magnesium Dissolving in Sodium Sulfate Solution

Magnesium is known as it has small density and high strength. Therefore magnesium and its alloys are widely used in the industrial fields such as the automobile and electronic devices, which are helpful for weight saving. However, it is also known that magnesium has large corrosion rate, which limits the application of magnesium. Furthermore, hydrogen evolution rate on dissolving magnesium is increased when the anodic polarization is increased, so called “Negative Difference effect (NDE)” [1].

Some mechanisms have been proposed in previous researches.  Petty et al. [2] calculated the valence number of magnesium ions formed during anodic dissolution and proposed that the theory of generation of monovalent ions of magnesium. Supporting this theory, Baril et al. [3] suggested the following dissolution mechanism of magnesium by measuring the polarization curves and impedance spectra combined with SECM.

Mg → Mg⁺_ads + e^-                                                                      (1)

Mg⁺_ads ⇄ Mg²⁺+ e^-                                                                    (2)

 A chemical reaction was expressed by the following reaction because of the NDE.

Mg⁺_ads + H₂O → Mg²⁺ + OH^- + 1/2H₂                                       (3)

They suggested that the monovalent ions of magnesium were formed on the magnesium surface as the adsorbed intermediates during anodic polarization [3]. On the other hand, Williams et al. [4] investigated the local current on the magnesium surface by in-situ scanning vibrating electrode technique when magnesium is polarized to noble potential. They found that the anodic dissolution of magnesium is shown to be highly localized in nature and accompanied by significant cathodic activity. The reactions for both anodic dissolution of magnesium and hydrogen evolution occurred on the magnesium surface are expressed by the following reactions.

Mg → Mg²⁺ + 2e^-                                                                      (4)

2H₂O + 2e^- → H₂ + 2OH^-                                                          (5)

They implied that the hydrogen evolution on the magnesium surface is accelerated by the applied noble potential [4]. Furthermore, it has been reported that the localized corrosion was observed on the magnesium surface during the anodic dissolution of the magnesium, and enhanced by the applied noble potential [4, 5]. However, the anodic dissolution mechanism of magnesium is not well understood.

In the present study, the dissolution and hydrogen evolution mechanisms of the pure magnesium in 1 M Na₂SO₄ were investigated by 3D electrochemical impedance measurements. The inductive loops were observed in the impedance spectra measured at various electrode potentials. Focusing on the inductive loops, the relation between the impedance spectra and mechanisms of magnesium dissolution and hydrogen evolution on the magnesium surface were discussed.

References

[1] G. Song, A. Atrens, D. Stjohn, J. Nairn, Y. Li, Corros. Sci, 39 (1997) 855-875.

[2] R.L. Petty, A.W. Davidson, J. Kleinberg, J. Am. Chem. Soc., 76 (1954) 363-366.

[3] G. Baril, G. Galicia, C. Deslouis, N. Pebere, B. Tribollet, V. Vivier, J. Electrochem. Soc, 154 (2007) C108-C113.

[4] G. Williams, N. Birbilis, H.N. McMurray, Electrochem. Commun, 36 (2013) 1-5.

[5] M. Taheri, J.R. Kish, N. Birbilis, M. Danaie, E.A. McNally, J.R. McDermid, Electrochim. Acta, 116 (2014) 396-403.