Enrichment efficiency of noble alloying elements on magnesium and effect on hydrogen evolution
Introduction
Magnesium (Mg) alloys, due to their high specific strength, remain a promising alternative to aluminium (Al) alloys for light-weighting in automotive and aerospace applications [1]. The corrosion of Mg and its alloys limits their broader usage and extension to new applications [2], therefore, in order to develop corrosion-resistant Mg alloys, it is necessary to study the attendant corrosion mechanism(s) operational in aqueous solutions. Magnesium ( dissolves to form Mg2+ ions (the anodic reaction) as shown in Eq. (1), whilst hydrogen evolution reaction (HER) ( is the dominant accompanying cathodic reaction, Eq. (2), in aqueous environments.
During the anodic dissolution of Mg and its alloys, in many electrolytes, the phenomenon of increased hydrogen evolution (cathodic) kinetics with applied anodic potential or current density occurs with respect to that at the open circuit potential (OCP). This phenomenon of hydrogen evolution is popularly known as the negative difference effect (NDE) [3]. A variety of models including the metal spalling [4,5], magnesium hydride formation [6,7], Mg univalent ion formation [8,9], impurity enrichment [10,11] and film-breakdown [[12], [13], [14]] have been proposed to explain NDE. Detailed discussions on these models and their shortcomings have been presented in some of the recent reviews on Mg corrosion [9,15]. However, the understanding of this phenomenon at molecular length scales remains elusive.
It has long been revealed that so-called ‘cathodic impurities’ such as Fe, Cu, Ni and Co, if present beyond an allowable limit, cause accelerated corrosion of Mg [10]. In fact, such observations form the basis of commercial Mg alloys being produced with compositions that avoid critical impurity tolerance limits. The incongruent dissolution of Mg during anodic polarisation is expected to cause the enrichment of even trace level cathodic impurities and/or alloying elements (despite being present in very small quantities) since the OCP of the Mg alloys are usually well below the reversible potentials of such impurity or most alloying elements. Recent work on Mg and its alloys using the scanning vibrating electrode technique (SVET) shows the formation and growth of cathodic sites during anodic dissolution [[16], [17], [18]], revealing definitively that it is anodically induced ‘cathodic activation’ of the surface which occurs. In the light of these findings, focused interest in the possibility of enriched cathodic impurities or alloying elements acting as sites for hydrogen evolution (cathodic) reaction has been generated. Taheri et al. revealed one of the first qualitative experimental validations for the enrichment of impurities on Mg at the nanoscopic trace level by showing the existence of Fe impurity particles on(in) the corrosion film formed on Mg after dissolution in dilute NaCl using focused ion beam-transmission electron microscopy (FIB-TEM) [19].
In other work, Lysne et al. [20] and Höche et al. [21] demonstrated that increasing the concentration of Fe in Mg (which is a major impurity element) increases the attendant hydrogen evolution during anodic polarisation and the anodically induced ‘cathodic activation’. However, in both the studies, Fe enrichment was not experimentally observed and quantified for a direct correlation. The quantification of cathodic impurities or alloying elements in pure Mg and the extent of their enrichment upon anodic dissolution of Mg is critical to establishing the role of enrichment on the observed ‘cathodic activation’ and NDE, and the FIB-TEM technique employed by Taheri et al. [19] is – whilst definitive - too localised for this purpose. Recent studies using Rutherford backscattering spectroscopy (RBS) [22] and particle-induced X-ray spectroscopy (PIXE) [23] show that impurity elements (namely Fe) that are present in Mg can be quantified with high sensitivity (in the parts per million range). These studies further revealed that the Fe enrichment efficiency is less than 1% on Mg.
The enrichment of other intentional alloying elements also needs to be quantified in a similar manner to shed light on the role of the enrichment of cathodic elements on the NDE and the ‘cathodic activation’ behaviour that is observed in Mg and its alloys. The local rate of hydrogen evolution reaction on these enriched cathodic elements (in Mg alloys) would depend on the local exchange current density for hydrogen evolution ( at or around these alloying elements [24]. The enhancement in global HER on an alloy containing such impurities would seem to depend of the site coverage of the enriched species and the specific . To date, there exists no systematic work in the open literature, in which the hydrogen evolution on Mg alloys has been scrutinised from the perspective of enrichment of alloying elements with distinctly different exchange current densities of hydrogen evolution (. Accordingly, the present work is intended to investigate (i) the enrichment of two different alloying elements on Mg alloys; (ii) correlating the observed enrichment of the noble alloying elements with different and the overall or global hydrogen evolution kinetics on these Mg alloys.
Indium (In) and gold (Au) with of -0.342 and +1.5 V, respectively, are both cathodic to Mg ( = – 2.38 V) but widely differ in such that, In and Au have as 10−10 A cm-2 and 10-5 A cm-2, respectively [25]. Hence, In and Au were chosen as alloying elements to facilitate an investigation on the effect of enrichment of noble alloying elements with different on the overall hydrogen evolution behaviour on the Mg alloy surface during anodic dissolution. Indium has an extensive solid solubility in Mg (≈ 35 wt. % at room temperature) [26] while gold has only a very limited solid solubility (with a maximum solid solubility of 0.8 wt. % at ≈570 °C and ≈ 0 wt. % at room temperature) in Mg [27]. Therefore an alloying addition of 0.1 wt. % (greater than total impurity content in Mg) of indium and gold to Mg was chosen for this work and hence, Mg-0.1 wt. % In and Mg-0.1 wt. % Au alloys were cast, solutionised and quenched for this investigation. Various electrochemical methods were employed to study the kinetics of these alloys, while the enrichment of the alloying elements was detected and quantified using PIXE.
Section snippets
Materials
Mg-0.1 wt. % In and Mg-0.1 wt. % Au alloys were cast for this investigation. Melt casting was carried out by heating the alloy mixture to 750 °C in an induction melting furnace (Leybold–Heraeus ISO1/III induction furnace) in a graphite-coated steel crucible, where it was held for 30 min, during which it was repeatedly stirred to ensure proper mixing. The melt was cast in the induction furnace atmosphere where it cooled to room temperature. The chemical composition of pure Mg and Mg-0.1 wt. % In
Results
The backscattered SEM images of the as-cast and homogenised Mg-In and Mg-Au alloys are shown in Fig. 1.
In the Mg-In alloy, heat treatment has led to complete dissolution of the dendritic structure and the formation of homogenous equiaxed grains in contrast with the incomplete dissolution of the Mg3Au phase in the Mg-Au alloy, noted by the etch pits where Mg3Au particles existed prior to etching. Such a variation arises out of the fact that In has an extensive solubility in Mg (close to 35 wt. %
Discussion
The alloying of Mg with In and Au led to increased hydrogen evolution (cathodic) kinetics with Au being most effective. Despite the Mg-In and Mg-Au alloy having different microstructures and different enrichment pattern (segregation of an enriched element in the case of Mg-Au alloy) the overall enrichment efficiencies of Au and In due to prior alloy dissolution were low, with Au showing slightly higher enrichment efficiency. However, irrespective of the alloy composition the cathodic activation
Conclusions
The influence of the noble alloying element enrichment on the cathodic activation of Mg-0.1 wt. % In and Mg-0.1 wt. % Au was systematically evaluated by quantification of enrichment using particle-induced X-ray spectroscopy and electrochemical techniques. The following conclusions can be drawn from the results:
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Anodically induced alloying element enrichment was observed, and quantified. However, in response to anodic dissolution the enrichment and enrichment efficiency of indium on the
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Acknowledgements
PG would like to thank R. Liu and X. Xia for casting and heat treatment of the alloys. NB is supported by Woodside Energy. TWC and JRS are supported by the Office of Undersecretary for Defense.
References (38)
Potentiostatic study of the magnesium electrode in aqueous solution
J. Electroanal. Chem.
(1970)Anodic behavior of Mg in Hc03-/Co23- buffer solutions, quasi-steady measurements
Electrochim. Acta
(1992)- et al.
Corrosion mechanism and hydrogen evolution on Mg
Curr. Opin. Solid State Mater. Sci.
(2015) - et al.
The source of hydrogen evolved from a magnesium anode
Electrochem. commun.
(2013) - et al.
The localised corrosion of Mg alloy AZ31 in chloride containing electrolyte studied by a scanning vibrating electrode technique
Electrochim. Acta
(2013) - et al.
Towards a physical description for the origin of enhanced catalytic activity of corroding magnesium surfaces
Electrochim. Acta
(2014) - et al.
Evolution of hydrogen at dissolving magnesium surfaces
Corros. Sci.
(2013) - et al.
Evidence for enhanced catalytic activity of magnesium arising from anodic dissolution
Electrochim. Acta
(2014) - et al.
A labview based FPGA data acquisition with integrated stage and beam transport control
Nucl. Instrum. Methods Phys. Res. B
(2013) - et al.
A study on magnesium corrosion by real-time imaging and electrochemical methods: relationship between local processes and hydrogen evolution
Electrochim. Acta
(2016)
Modeling reaction pathways for hydrogen evolution and water dissociation on magnesium
Electrochim. Acta
Magnesium: industrial and Research Developments over the last 15 years
Corrosion
Fundamentals and advances in magnesium alloy corrosion
Prog. Mater. Sci.
On the development of hydrogen from the anode
Philos. Mag. Ser.
Corrosion of anodically and cathodically polarized magnesium in aqueous media
J. Electrochem. Soc.
Disintegration of magnesium while dissolving anodically in neutral and acidic solutions
J. Electrochem. Soc.
The anodic oxidation of magnesium metal: evidence for the existence of unipositive magnesium1,2
J. Am. Chem. Soc.
Corrosion mechanisms of magnesium alloys
Adv. Mater. Eng.
Some corrosion characteristics of high purity magnesium alloys
J. Electrochem. Soc.
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