Page 58 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

CHAPTER 3
On the other hand, the addition of organic compound to a silicate base solution during the Mg anodizing process, it is related with the mechanism of the film formation but not changing its composition by the formation of additional phases. Contrary, it was reported that organic compounds only may cause changes in the intensity of the peaks of the existing phases [36].
For instance, HMT, which is a nontoxic and water soluble amine, it has been used as an additive or buffering com- pound during the anodizing process of Mg [34]. HMT is used as arc suppression in anodization of Mg-alloys which is directly proportional to the concentration of the compound in the solution [37]. HMT is also used to remove cracks by the reduction of the thermal stress during solidification of the molten oxide. This effect have a direct impact in the improvement of the corrosion resistance of the coating [37]. It was also showing that the presence of HMT improves the breakdown voltage of the coating. Gozuacik et al, observed an increment in the pore size after the addition of HMT in a silicate solution in the anodization process of AZ91 Mg-alloy, accompanied of a decrease in the thickness of the anodic films with respect of the solution without HMT [36].
On the other hand, Although MAN it has not been studied before in Mg anodizing similar compounds like glycerol has been reported previously. MAN and glycerol are nontoxic sugar alcohols which can be used to adjust the conductivity of the solution and avoid high discharges in the process. It has been reported that the unit area adsorptive capacity at the anode-electolyte solution interface can increase in presence of compounds such as glycerin [38][39][40]. Op- posite to that reported for HMT, reduction in thickness of the coating are related with increment in the concentration of the compound in solution [38]. According with Herbert [41][42] the addition of low volatility organic alcohols as glycerol or mannitol, in a silicate base solution, allows higher voltages to be attained for the same current density which increases the hardness of the obtained coating and improve its uniformity
Mixture of silicate solution with either HMT or MAN acts as a hybrid organic/inorganic system where electron trans- fer induce a specific growth of the anodic film which highly influence the final performance of the coating related with tits wear and corrosion resistance [43].
Regarding with the anodizing process, once the NAF solution was compared with HMT and MAN, it was observed that after the breakdown voltage, oxygen gas and products from the molten oxide were expelled out for the dis- charged channels reaching the surface of the coating. As a consequence of this, interconnected pores were formed which were only present in the outer layer. An increment on the pore size was associated with the increment in the current/voltage applied. In addition, for the samples operated at potentiostaic mode except NAF-P2, it was possible to observe most uniform and free of defects films, compared with the treatments under galvanostatic mode. Examination of the cross-sections of the formed films revealed differences in the structure and dimensions of the layers according with variations in voltage/current applied. It was found that the thickness of the inner barrier layer was closely related to the additive employed, it was around 360nm for NAF, 560nm for HMT and 750nm for MAN; these values were in the range of other studies reported previously were PEO was used in Mg alloys by using electro- lytic solutions containing potassium fluoride and silicate [44][32]. In general, the ratios (C.cm-2/μm) were lower for coatings obtained under potentiostaic mode than in galvanostatic showing higher efficiencies in the process for the first one. Samples treated with MAN solution presented the thicker anodic films in both modes whilst with NAF and HMT similar values were obtained.
Regarding with the dimensions and structures of the coatings, it is expected that both compact and thicker anod- izing films will present more favorable corrosion protection than porous coatings. Structures with pores and defects can lead the penetration of corrosive agents through the coating reaching the substrate. Corrosion products can be formed in the interface substrate-coating affecting the adhesion of the anodic film and producing the liberation of particles from it [45].
4. Conclusions
- Composition of electrolyte solution plays an important role in the characteristics of the obtained coatings. Addi- tion of fluoride in the base silicate solution produce and increment in its conductivity, as consequence of this, the
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