Investigating the Effect of Ferrous Ions on the Anomalous Hydrogen Evolution on Magnesium in Acidic Ferrous Chloride Solution

Abstract

Electrochemical testing, hydrogen collection and on-line inductively coupled plasma mass spectroscopy are used to assess the dissolution of high purity magnesium and the attendant hydrogen evolved in solutions containing ferrous ions. It was revealed that ferrous ions in solution had a minimal effect on the hydrogen evolved upon anodically polarized magnesium. The results show that deposition of ferrous ions could not account for enhanced hydrogen evolution during anodic polarization of magnesium. Further analysis of the results obtained under the testing conditions in this study show that the anomalous hydrogen evolution exhibited by dissolving Mg was associated with the regions of anodic dissolution. Magnesium (Mg) corrosion in aqueous electrolytes of any pH is accompanied by hydrogen evolution (HE) as the primary cathodic reaction. This is due to the low open circuit potential (OCP) exhibited by Mg, commonly around −1.5 V SCE , which is below the reversible potential (E rev,H) for the HE reaction (HER). 1 The electrochemical theory (as exemplified by the Butler-Volmer equation) predicts that, when a metallic electrode is polarized to higher potentials, the rate of the cathodic reaction will decrease exponentially. 2 However, this is not the case for Mg where the rates associated with the HER increase on the Mg surface when the applied potential is increased above its E corr. This phenomenon, first observed by Beetz in the 19 th century, 3 is still the subject of investigation and lacks a generally accepted molecular or atomic level mechanism in the literature. Even though it has traditionally been termed the negative difference effect (NDE) it will be referred to here as anomalous HE. This phenomenon is most studied for Mg but has been observed on other active metals such as Al. 4-6 The anomalous HE on anodically polarized Mg has been rationalized on the basis of an enhanced HER rate in response to anodic dissolution-whereby the HER remains a cathodic reaction. 1 In this context, the increase in cathodic reaction rates on previously anodically dissolved metals (i.e. cathodic activation) has also been shown to be less unique a phenomenon than previously considered. 7 A number of different theories have been proposed to explain anomalous HE on anodically polarized Mg. The unipositive Mg (Mg +) theory, which was first invoked by Turrentine in 1907, 8 persisted for decades after Petty et al. reported indirect evidence of the existence of monovalent (n = 1) Mg +9 and was further developed by Song and Atrens. 10-12 This theory was discredited after it was shown that the observations of Petty et al. did not support evidence for the existence of Mg +13 and that numerous independent studies have revealed that n (the number of electrons transferred during anodic dissolution of Mg) equals 2. 14-16 The effect of noble metal impurities has attracted attention as one rationale for the anomalous HE observed during anodic polarization of Mg. The importance of impurities on Mg corrosion under open circuit conditions has been known since the work by Boyer 17 and later by Hanawalt and McNulty. 18 However, the idea that the accumulation of noble metal impurities due to incongruent dissolution of Mg may be responsible for the HE observed during anodic polarization of Mg is more recent. the regions enriched with noble metals to be the primary source for anomalous HE due to higher catalytic properties toward the HER than Mg and the overpotential for HE shown by these metals at the low potentials reached during Mg dissolution. 1 Brady et al. 23,24 indicated that alloying elements such as Al and Zn can concentrate in the inner MgO corrosion product layer formed on Mg alloys. The enrichment of iron impurities on Mg specimens (unalloyed with the exception of deliberate Fe impurities) was modelled by Lysne et al., 25 who reported poor enrichment efficiencies that could not account for the large amount of H 2 exhibited at potentials above the OCP. Trace analysis using Rutherford backscattering spectroscopy (RBS) 16 and particle induced X-ray emission (PIXE) 21 confirmed that, even though noble metal enrichment indeed occurs as a consequence of Mg dissolution , the enrichment process has a low efficiency. Finally, it has been shown that, even though impurities affect the kinetics of the HER, the influence is small relative to the total HE rates measured during anodic polarization. 6,26,27 These studies used ultra-high purity Mg with a nominal purity of 99.9999% (concentration of Fe was about 0.1 ppm), whilst still exhibiting strong anomalous HE during anodic polarization. Recently, Höche et al. 28 proposed an impurity-based mechanism in which Fe impurities present in the Mg electrode leave the surface as a consequence of non-faradaic release, and then re-plate in their metallic form after 'self-corrosion' in solution at their OCP. In this model, analogous to that exhibited by Cu-containing particles in Al alloys, 29 Fe electrodeposition would form thin regions of metallic iron on the electrode surface that may act as reactive cathodic sites for the HER following the Heyrovsky pathway (H 2 O + H ads + e − → H 2 + OH − , where H ads is a reduced hydrogen atom adsorbed on the electrode surface). This concept was further developed by Lamaka et al., 30 who evaluated the impact of Fe sequestration on Mg corrosion by exposing commercially pure Mg (220 ppm Fe) to different strong Fe-complexing agents. Hydrogen collection measurements under open circuit conditions showed decreased rates of HE in NaCl solutions containing species such as cyanide, salicylate, oxalate, methylsalicy-late and thiocyanate that can complex ferrous ions and thus prevent iron replating. However, cyanide, which provided the highest corrosion inhibition, poisons the HER by strongly adsorbing on surfaces and suppressing hydrogen recombination. 31 Additionally, cyanide has also been reported to increase hydrogen permeation into α-Fe 32 by promoting hydrogen entry into the metal and decreasing the number of reduced hydrogen atoms available for chemical recombination via the Heyrovsky pathway. It is possible that cyanide and the other com-plexing agents studied may cause the same effect on Mg. Therefore,) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.130.126.222 Downloaded on 2018-10-22 to IP