The Ecorr of the six Ca-BMG were found to be centred about
1.4 VSCE, which is nobler than the known Ca65Mg15Zn20 BMG
and almost comparable with Mg-rich BMG. Existing electrochemical
data on Ca65Mg15Zn20 by Morrison et al. [25], obtained Ecorr of
1.54 V, while Zberg et al. [16] obtained 1.28 V for Mg66Zn29Ca5.
Owing to the varying experimental parameters, a comparison between
the actual Ecorr values is difficult.
Mass loss data (Fig. 5a) revealed that, depending on composition,
mass loss rate varied from 2.2 to 25.1 g cm2 day1
. This is
a relatively wide window, which suggests that the rate of dissolution
is heavily dependent on BMG composition. Nevertheless, there
is good correlation between the corrosion rate obtained by mass
loss and PDP tests (Fig. 7). Both tests were considered essential,
since one yields a ground truth of the physical damage, whereas
the other reveals the electrochemical characteristics that indicate
the mechanistic basis for the corrosion.
The Ca-BMG herein were designed to contain high Zn content in
order to increase their corrosion resistance, and this has been largely
successful. Compared with Ca65Mg15Zn20, which disintegrated
within 3 h after exposure to simulated body conditions [22], it was
possible to maintain a Ca-rich BMG content, with significantly improved
corrosion resistance, surviving for up to 16 days immersed
in MEM at 37 C in a humidified CO2 atmosphere. It is also pertinent
to note that the corrosion behaviour of Ca and Mg varies significantly,
with Ca being much more reactive than Mg [34].
Correlation between the Zn content and mass loss rate was observed,
whereby reduced corrosion rate is accompanied by a higher
Zn ratio (Fig. 5), However, among the alloys studied, for a given Zn
concentration, the Ca content varied by as much as 5 at.%, giving
some scatter of the data, which was useful in highlighting the impact
of the Ca:Mg ratio. CaBMG1 (Ca57.5Mg15Zn27.5) and CaBMG2
(Ca55Mg17.5Zn27.5), both with 27.5 at.% Zn, revealed a notable difference
in mass loss rate (Fig. 5).
To understand better the mechanistic aspects associated with
the rapid dissolution rates of the alloys, PDP testing provided an
interesting insight. While Ca-BMG reveal corrosion potentials in
the range 1.4 VSCE, the currents realized in the vicinity of Ecorr
are high. Such high dissolution rates for Ca-rich alloys have also
been observed in crystalline Mg–Ca alloys by Kirkland et al. [34].
Compared with Mg, Ca is capable of undergoing very high rates
of anodic dissolution, resulting in rapid corrosion. Conversely, Zn
additions also raise the ability to sustain cathodic reactions, since
Zn is a more potent cathode than Mg. As such, the elevated Ca
and Zn contents in Ca-BMG result in high rates of dissolution.
A close inspection of the polarization curves reveals that the
cathodic reaction kinetics are comparable for the various CaBMG.
However, it was revealed that, while the Ca-BMG exhibit significantly
higher rates of corrosion compared with pure crystalline
Mg, they all evolve hydrogen to a similar extent. This is believed to
Fig. 10