There is considerable current interest in identifying alloying elements for magnesium which produce an intrinsically corrosion resistant surface, thus precluding the need for additional protection by surface treatments and coatings. Recent work has demonstrated the promise of certain group IV and V elements as binary micro-alloying additions, which produce significant reduction in the corrosion rate of Mg 1,2
. In the case of alloyed arsenic (As), a well-known cathodic poison, the reduced rate of corrosion is principally attributed to a profound suppression of cathodic activity on transition-metal impurity containing phases by inhibiting recombination of atomic hydrogen 3
. Recent work has also demonstrated a similar phenomenon for Ge alloyed Mg 2
, although the underlying reasons are less obvious than for the case of As. To clarify the role of Ge in significantly reducing the corrosion rate of Mg, an in situ scanning vibrating electrode technique (SVET) is herein employed to study localised corrosion behaviour of Mg-Ge alloys at open circuit conditions and during anodic polarisation when immersed in chloride containing aqueous solution. Upon immersion in the absence of external polarisation in aqueous solution containing concentrations of 0.5 M NaCl and below, SVET-derived area-averaged anodic current densities are shown to be extremely low, with no visible evidence of breakdown in the form of the dark filiform-like tracks reported over 24 h (and up to 90 h for a concentration of 0.1 M). However, when using concentrations of 1 M NaCl (aq) and greater, filiform-like localised corrosion features become established after initiation periods of ca.2 h or longer. In contrast to pure Mg, no cathodic activation of filament clusters are observed and residual tracks are shown to develop a white colour instead of the typical surface blackening observed on pure Mg. The addition of Ge is shown to strongly depress free corrosion potential (Ecorr
) compared to pure Mg and it is proposed that it acts to maintain Ecorr
below the apparent breakdown potential indefinitely for chloride ion concentrations of 0.5 M and lower. Under galvanostatic anodic polarisation, hydrogen evolution rates are significantly lower in comparison to pure Mg and no localised cathodic activity is detected for the duration of the experiment. In this respect the behaviour of Ge-alloyed Mg closely mirrors that of the Mg-As system, but with the benefit of intrinsic corrosion resistance bestowed by a non-toxic alloying addition.
1. N. Birbilis , G. Williams, K. Gusieva, A. Samaniego, M.A. Gibson and H.N. McMurray, Electrochem. Commun., 34 (2013), 295.
2. R. L. Liu, M. F. Hurley, A. Kvryan, G. Williams, J. R. Scully and N. Birbilis, Nature Scientific Reports, 6: (2016) 2874.
3. G. Williams, H. Ap Llwyd Dafydd, H.N. McMurray and N. Birbilis, Electrochim. Acta, 219 (2016) 401.