Page 56 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 3
The ratio between thicknesses of coatings and the charge density passed through the system was calculated as an approach to the electrical efficiency of the systems. Results are summarized in Table.4.
Table.4 Relationship between film thickness measured and the charge density passed during forma- tion of the anodic films.
The ratios presented in Table 4 indicate how much charge was employed to increase the coating thickness in one mi- crometer. Therefore, the lower the ratio, the more efficient the anodization process. The reduction in the process ef- ficiency could be due to gas evolution, high dissolution rate of the coating in the electrolyte solution or both. Samples processed under galvanostatic mode presented the higher ratios in comparison with the potentiostatic, particularly for anodization in NAF and HMT; galvanostatic anodization in MAN appears to be the more efficient of this mode with ratio values about half of the other two processes. Potentiostatic coatings in HMT seems to be the more efficient process of all, in this treatment the lower amounts of charge were consumed despite generating coatings of similar thickness to the rest of potentiostatic tests. On the other hand, the efficiency of the potentiostatic processes in NAF and MAN decreased as the anodizing voltage was raised whereas for HMT it remained nearly constant. Therefore, considering all the different conditions employed in this study, the more efficient way to form PEO coatings on c.p Mg was using as additive HMT and anodizing under potentiostatic control.
Discussion
Magnesium anodization should be carried out in alkaline solutions to avoid dissolution of the material [20–22]. NaOH and KOH has been extensively used due to they generate environment friendly solutions, with low toxic elements thinking in biomedical applications. These solutions generally are supplemented with silicates, aluminates or phos- phates in order to improve the corrosion resistance and to generate some species according to the required appli- cation [16,18,22]. Additionally, other additives can be added to increase the solution conductivity, to optimize the coating formation and/or to promote the incorporation of the foreign atoms into the growing oxide. Previous studies has reported the use of other additives such as borate, sulfate, sodium citrate, ammonium, phosphate, among others [13,20,23–25]. For instance, according with studies by X. Guo et al [9], the addition of benzotriazole to a silicate base solution, allowed the formation of an absorption layer on the surface of the material, generating a smooth, compact and uniform anodized film. The present work aimed to study the effect of three additives in the formation and mor- phology of the anodic films. NAF is a compound often used to improve the resistance of the material [26–28], and other organics additives such as glycerol, ethylene glycol, among others [29,30] are also frequently used; however, HMT and MAN have not been studied before as additives for anodization of magnesium alloys.
On the other hand, in the process of anodization of c.p Mg, factors such as applied current, treatment time, con- ductivity and composition of the electrolyte, played a crucial role in the determination of the characteristics of the coatings. During the process of PEO, both formation and dissolution of the coating, occur simultaneously. This phe- nomena is highly dependent of the reactions that appear in the interface electrolyte-material, which at the same time depend on the composition of the electrolyte solution. In this context, the additives used for the present study, affected the conductivity of the base solution which was of 23.20 mS·cm-1 but after the addition of NAF, it increased
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