Page 73 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

IMPROVEMENT OF CORROSION RESISTANCE OF COMMERCIAL PURE MAGNESIUM AFTER ITS MODIFICATION BY SINGLE AND TWO-STEP ANODIZATION
After 6 days, the degradation gradually decreased due to the passivation i.e. formation of a protective layer. Coated
samples also corroded faster during the first hours of immersion with maximum values of around 0.09, 0.08 and
0.03 mm·year-1 for NAF, HMT and MAN respectively. Similar to bare c.p Mg, the NAF and HMT coated samples then
slowed the degradation rate until reaching a steady state level of 0.032 and 0.016 mm·year-1, respectively. In the case
of MAN, after an initial increase, it maintained a similar corrosion rate of around 0.028 mm·year -1 during all the test.
Degradation profiles of two-step coatings were similar to single anodized coatings with a maximum corrosion rate
of 0.09mm year-1 for NAF-HMT and 0.03 mm year-1 for NAF-MAN. These samples reached the steady state degrada-
tion levels at about 0.006 mm year-1 for NAF-HMT and 0.003mm year -1 for NAF-MAN which was approximately ten
fold lower than single anodized coatings. Even both samples with two-step anodizing presented the best corrosion
resistance in the end of the immersion test, it is important to notice that the sample NAF-MAN showed also an at-
tenuated degradation in the beginning, making this system to have the better corrosion behavior of all. Using the
information of the hydrogen evolution test presented in Fig.·7, it is possible to calculate the average corrosion rate 4 for each sample, the results are as follows: 0.067, 0.033,0.018,0.025,0.018 and 0.008 mm year-1 for bare c.p Mg, NAF,
HMT, MAN, NAF-HMT and NAF-MAN, respectively.
Fig.7 Corrosion rate (mm/year) vs time (days) for modified samples of Mg exposed to 0.9% m/v NaCl under hydrogen evolution assay
Finally, surfaces and cross-sections of the samples were analyzed by SEM after 1 month of immersion (Fig. 8). In samples anodized with NAF, HMT, MAN and NAF-MAN the structure and morphology of the coatings were conserved whereas samples anodized with NAF-HMT showed loss of the porous structure. The cracks observed in the SEM im- ages are normally obtained for PEO coated Mg samples which were exposed to a corrosive media and consequently a corrosion products layer was formed underneath the PEO coating. It seems that such layer structure (Mg substrate/ corrosion product/PEO coating), suffers cracking under the high vacuum environment of the SEM chamber. In order to quantify the corrosion attack, the area of the corrosion products layer (Fig. 8) was calculated as an indicative
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