Biomedical Engineering Reference
In-Depth Information
Mg and the Mg-4.0Zn-0.2Ca alloy degraded quickly, during the early stage of immersion in
SBF, accompanied by the rapid formation of an insoluble protective corrosion layer, which
retarded degradation. The degradation process of Mg-4.0Zn-0.2Ca alloy could be roughly
summarized as follows: just after immersion in SBF solution, magnesium alloy react with
fluids on the surface and get dissolved in the surrounding fluids. With the increasing time of
immersion, more Mg 2+ , Zn 2+ and Ca 2+ ions were dissolved into the solution, the local pH
near the surface of the Mg could be >10[30]. As a result, a magnesium-containing calcium
phosphate would precipitate from the SBF solution and deposited on the surface of the
magnesium samples, per the following equation:
Anodic reaction:

2
Mg
Mg
2
e
Cathodic reaction:
OH
2
HO
 
2
e
H
2
2
2
2
Mg
2
OH
Mg OH
(
)
2
3-
2+
2
PO
+Ca
Mg
Mg Ca
(
PO
)
4
xy
4
Moreover, when Mg2+, Zn2+ and Ca2+ ions were dissolved into the solution, phosphate-
containing Mg/Ca insoluble protective layer was formed and tightly attached to the matrix.
Previous studies [31] have shown that this corrosion layer promotes the osteo-inductivity
and osteo-conductivity, predicting good biocompatibility of magnesium and retarded
degradation. Therefore, it is proposed that the Mg 2+ , Zn 2+ and Ca 2+ released during
degradation are safe. Hence, we come to the conclusion that the degradation of the Mg-
4.0Zn-0.2Ca alloy was harmless and has good biocompatibility.
E(V)
Current(mA/cm 2 )
V(mm/year)
As-cast
-1.60
2.67
2.05
Extruded
-1.57
2.43
1.98
Table 5. Corrosion potential, corrosion current and corrosion rate of Mg-4.0Zn-0.2Ca alloys
3.3.3 Corrosion morphology and products
The samples after electrochemical measurements were observed by SEM. The typical
Surface morphology of Mg-Zn-Ca alloys after electrochemical measurements was shown in
Fig.11. Corrosion attack on a large area was observed. At the same time, the filiform
corrosion and pitting corrosion were found on the Mg-Zn-Ca alloys sample's surface after
electrochemical measurements. The former mainly distributed on the grain boundary, and
the latter mostly occurred in second phase location.
XRD patterns of the corrosion products on the surface of Mg-Zn-Ca alloys immersed in
Hank's solution were presented in Fig.12. The XRD results suggest that magnesium
hydroxide [Mg (OH) 2 ], other phosphates and hydroxyapatite (HA) were precipitated on the
Mg-Zn-Ca alloys surface.
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