Page 110 - IJB-8-3

Rare Earth Magnesium Alloy
3.3. Electrochemical behavior contains only one wave crest and one wave trough, which
was consistent with the Nyquist plots.
The OCP test results of Mg, Mg-Sc, and mMg-Sc in In general, high-frequency capacitive loop was
SBF are shown in Figure 5A. It was noted that the OCP attributed to the electric double layer at the interface
of three samples gradually reached a saturation value of Mg substrate and electrolyte [49] . The capacitive
within 20 min, and the OCP of Sc-containing Mg alloys loop observed at the middle-frequency region was
was slightly nobler than that of Mg. In addition, the related to the presence of protective film [50] . The low-
positive shift rate (ΔE/Δt) of the OCP curve for Mg was frequency inductance loop was ascribed to the chemical
relatively higher than that of Sc-containing Mg alloys, reaction of Mg with H O in the region of corrosion
+
suggesting a faster corrosion layer formation on the product film rupture and the desorption of corrosion
2
sample surface. The polarization curves, as displayed products [51] . Moreover, the EIS spectra of the samples
in Figure 5B, showed that Mg-Sc alloy exhibited high could be interpreted using the equivalent circuit, as
cathodic current density. It was believed that the large shown in Figure 6A. In the equivalent circuit, R stood
amount of second phase led to the presence of more for solution resistance. The R and R represented
s
f
ct
hydrogen evolution sites at the cathode [47] . In the anodic the charge transfer resistance and the film resistance.
branches, the corrosion current density (i ) increased The CPE was the associated constant phase element
corr
f
slowly with corrosion potential (E ), and the i of of product film, and CPE represents the double
corr
corr
Mg grew at the highest rate before breakdown potential layer capacitance [52] . The L and R were the inductive
dl
L
(E ). However, the i of Mg was gradually overtaken elements, which represented inductance and inductance
bd
corr
by that of Mg-Sc alloy after E . It might be due to the resistance.
bd
rupture of the local protective film and the re-exposure The parameters were fitted by ZSimpWin software,
of the micro-galvanic couples to the corrosive medium, and the results are summarized in Table 3. However,
which resulted in an accelerated dissolution rate of the previous researchers reported that the CPE parameter
Mg substrate. The detailed corrosion parameters of the did not completely describe the precise value of the
polarization curves are listed in Table 2. It was shown capacitance, since the CPE behavior involves dielectric
that the E of Sc-containing Mg alloys was more system dispersion or conductive system dispersion, which
corr
positive than that of Mg. Meanwhile, mMg-Sc also had a much narrower range of expected values for the
showed a smaller i as compared to Mg and Mg-Sc. dielectric constant . Therefore, the capacitance could be
[53]
corr
In a nutshell, the mMg-Sc alloy with higher E and replaced by C in the following equation :
[54]
corr
lower i had better corrosion resistance. In addition, f
corr
mMg-Sc alloy also exhibited a more positive E . It was n  RR f  1 ( − nn)/
s
bd
indicated that mMg-Sc had a better pitting corrosion C = Q 1/  R + R  (3)
f
resistance.  s f 
The Nyquist plots of samples are depicted in Where, C was the effective capacitance of
f
Figure 5C. Obviously, the impedance diagram of Mg corrosion film in μF/cm and n was the CPE index. The
2
was composed of one high-frequency capacitive loop calculation results of C for the samples are shown in
f
and an inductive arc at low frequency. As a comparison, Figure 6B. It was shown that Mg-Sc had the smallest
the Sc-containing Mg alloys also included a capacitive C , which was only 31.69 μF/cm . Wang et al. [55]
2
f
loop in the medium-frequency region, which represented reported that the variation of C and film thickness
f
that the protective film was built up on the surface . was in inverse proportional relationship. The smaller
[48]
Correspondingly, the Bode plots are presented in C value indicated the thicker corrosion film formed on
Figure 5D. The |Z| versus frequency plot of the mMg-Sc f
alloy exhibited a relatively high value. For phase angle Table 3. The detailed parameters of equivalent circuit elements.
versus frequency, two wave crests (represent capacitance
loop) and one wave trough (represent inductance loop) Samples Mg Mg-Sc mMg-Sc
2
were included in Sc-containing Mg alloys. The Mg R (Ω/cm ) 8.89 8.24 8.92
s
R (Ω/cm ) 252.4 238.3 360.9
2
ct
Table 2. Fitted Tafel parameters from the polarization curves Q (μF/cm s ) 19.66 20.18 22.34
2
n-1
depicted in Figure 2B. n f 0.9032 0.9186 0.9049
1
2
Samples E corr i corr E P R (Ω/cm ) 139.8 423.6 620.3
f
bd
i
(V) (A/cm ) (V) (mm/y) Q (μF/cm s ) 1893 2048 2135
2
2
n-1
ct
Mg −1.62 4.16×10 −5 / 0.961 n 2 0.8663 0.8729 0.9161
Mg-Sc −1.58 7.94×10 −5 −1.44 1.834 R (Ω/cm ) 154.1 478.3 539.5
2
L
mMg-Sc −1.52 1.31×10 − 5 −1.20 0.303 L (H/cm ) 2808 4216 5281
2
102 International Journal of Bioprinting (2022)–Volume 8, Issue 3