Page 69 - 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
dropped to 60 mA·cm-2 after 14s for NAF MAN. Finally, in NAF HMT the current decrease to 13 mA·cm-2and form NAF MAN to 5 mA·cm-2.
Fig.1 Current density-time and voltage –time responses during PEO process on c.p Mg with different electrolyte additives by operating the system under galvanostatic and potentiostatic mode.
Appearance of the sparks at different stages of the process is shown in Fig.2. At initial stages, gas evolution occurred on the sample surface (10s), and then the formation of micro-discharges was observed in all conditions. Intensity and size of the micro-discharges were small and weak with NAF which suggests that this coating was more compact. In contrast, larger sparks were observed on coatings obtained with HMT and MAN thus a more porous morphology is expected here. In general, micro-discharges difficult to see on the surface of samples both using the galvanostatic and potentiostatic process. In the potentiostatic process, the micro-discharges density decreased due to the sharp incline of the current density when the potential reached a steady state level.
Fig.2. Micro-discharge appearance during different stages of PEO.
Images of surfaces and cross-sections were taken for each condition and are showing in Fig. 3. The NAF-based sur- faces consisted of non-uniform small pores approximately 0.4 μm in diameter and occasional large around 1.2 μm. The cross-section allows to see a coating with a low surface porosity and non-visible interconnected pores with the surface. The thickness of the layer was about 3.2 ±0.4 μm with a barrier layer of around of 0.3μm. In this sample, the polishing lines of the base material can be still observed.
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