Biomedical Engineering Reference
In-Depth Information
Hank's solutions and rapidly generated bubbles. And these reactions of Mg and H
2
O in
Hank's solution generated a large amount of OH
-
and leaded to the pH values of the
solutions be obviously increased. Comparing the pH values of Mg-4.0Zn-xCa alloys, it can
be found that the pH variations of pure Mg and Mg-4.0Zn-0.2 Ca alloy are much lower than
those of Mg-4.0Zn-1.5Ca and Mg-4.0Zn-2.0Ca alloys. The pH values of Mg-4.0Zn-1.5Ca and
Mg-4.0Zn-2.0Ca alloys are remarkably increased to 8.2 from 7.4 after 12hrs immersion tests,
which is even equal to those of pure Mg and Mg-4.0Zn-0.2Ca alloy after 96hrs immersion
tests. At the end of the immersion tests,the pH values are increased to 8.22 and 8.32 for pure
Mg and Mg-4.0 Zn-0.2 Ca alloy, respectively. In particular, the pH value is elevated to 11
from 7.4 for Mg-4.0Zn-2.0Ca alloy. This phenomenon can be explained as follows. The
standard potential of coarse second phases of Mg-4.0Zn-1.5Ca and Mg-4.0Zn-2.0Ca alloys is
higher than that of the pure Mg. Therefore, the selective attack occurred between α-Mg and
the second phase, and the reaction in Hank's solutions is acute. Thus, the pH values of Mg-
4.0Zn-1.5Ca and Mg-4.0Zn-2.0Ca alloys are rapidly increased. However, the uniform
microstructure and lower reaction rates of pure Mg, Mg-4.0Zn-0.2Ca and Mg-4.0Zn-0.5Ca
alloys cause a slow increase of pH values. After 48hrs immersion, bubbles are quite
decreased which corresponds a slow reaction rate and leads a slow increase of the pH
values for the samples. In addition, the increasing corrosion films including HA and other
phosphates formed by the reaction during the immersion test can further reduce the
reaction rates or degradation of the alloys [29].
Fig. 10. pH variation of Hank's solution versus the immersion testing time for as-cast Mg-
4.0Zn-xCa alloys.
According to the above-mentioned search, we found that Mg-4.0Zn-0.2Ca alloy have an
excellent corrosion resistance among the Mg-Zn and the Mg-Zn-Ca alloys. Thus, the
immersion test was only performance on the Mg-4.0Zn-0.2Ca alloy. The degradation rates of
the alloy after 30-day immersion were listed in Table 5. The degradation rates of Mg-4.0Zn-
0.2Ca alloy degraded were slower than pure Mg, which was adherent to the electrochemical
results.
