Page 57 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

NOVEL COATINGS FOR COMMERCIAL PURE MAGNESIUM OBTAINED BY PLASMA ELECTROLYTIC OXIDATION THROUGH THE ADDITION OF ORGANIC ADDITIVES
to 35.90 mS·cm-1. Similar results were observed by other authors as reported by Yerokhin et al [14]. According with this, if the conductivity is high, the electrical requirements of operation (current and voltage) are lower [17]. Con- ductivity of the solution has a direct influence on the ion transport in the electrolytic solution. At high conductivity the differential ion mobility is improved and the breakdown potential is lower. Opposite effect was observed when HMT and MAN were added, where the conductivity slightly decreased to 18.98 mS·cm-1 and 21.40 mS·cm-1, respec- tively. The effect on the pH of the electrolyte can be also very influential in the anodization process as it determines the stability of the species formed during the surface treatment. On that respect, none of the additives induced an important change on the electrolyte; only HMT induced a small increment in the pH value of around 0.8 pH units. The increase in conductivity caused by addition of NAF resulted, as expected, in lower anodizing voltages and a lot less surface porosity, compared to the other electrolytes. In fact, when the anodizing voltage was maintained at 140 V, the anodic film obtained appears to be nearly free of pores and in general, NAF samples appear to have pores only towards the surface of the anodic film. In addition, to the best of our knowledge, there is no other study showing similar structures of superposed layers as those obtained in the present study in NAF. Despite the higher conductivity, the film thicknesses in the NAF samples are of the same order than for the other samples. The lower conductivity of HMT and MAN electrolytes, much certainly is responsible for the larger sparks formed in those cases, which lead to the formation of larger pores. For the organic additives, although there was much difference on the surface morphology of the films, it is clear that the anodizing process in some way depends on the kind of additive. It is clearly observed when looking at the film thickness and electric charge values obtained for each case (Table 4).
During the growth process, microdischarges visualized as sparks occurred randomly on the surface. The number, size and duration of the sparks change through the different stages of the anodization and are highly influenced by the composition of the electrolyte. As product of this reaction, gas evolution, composed of oxygen and aqueous vapor is observed around the anode. Sparks occurrence also have an effect in the increase of the local temperature at certain points on the surface. Anodization in NAF resulted in formation of some particulate material in the elec- trolyte which was higher for samples treated under galvanostatic mode than potentiostatic. This event was also observed by Němcová et al. [31] who related this with the intensity of the sparks and the constant melting and ejection of the material from the coating influenced by the addition of fluoride which affect the melting point and the viscosity of the coating. However, according to the much higher voltages involved in the anodizing process in HMT and MAN, which are consistent with the morphology of the film surfaces, with a lot bigger pores and showing signs of melting of the anodic film material, more energetic sparks are expected to take place during anodizing in these two electrolytes than in NAF.
It is clear that, besides pH, conductivity and the electrical parameters (voltage/current), the results of the anodizing process depends on the chemical composition of the electrolyte employed. Fluoride containing electrolytes have been used for anodizing of Mg alloys [26-28]. Similar results were founded by Hwang et al, who study the effect of potassium fluoride in the anodizing of a Mg alloy and obtained similar results[32].
Our results were consistent with other studies, in which the addition of fluoride in a silicate base solution leads to an increase in the overall conductivity of the solution which implies a reduction in the work and final voltage [33] [14]. As a consequence of this, the size of the sparks also decreased producing pores with smaller size but higher density on the outer layer of the coating [33]. Other studies reported the use of NaF in the electrolytic solution with the purpose to improve the final appearance of the samples regarding to the texture, opacity and color [34]. Although the presence of fluoride changes the structure of the obtained coatings, being these more compact than those obtained in the absence of this compound, the dimensions of the thicknesses are not significantly affected [34]. By the addition of fluoride, an insoluble magnesium fluoride (MgF2) film is growing on the surface of the ma- terial inducing its passivation and avoiding the excessive dissolution of the Mg during the anodization process [33] [35]. This film entails an improvement in the harness, wear and corrosion resistance of the coating [33].
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