Page 47 - 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
1. Introduction
Magnesium (Mg) is a lightweight metal widely used in industrial applications due its properties such as high ductil- ity and machinability, relatively low elastic modulus, low density, good electromagnetic shielding, good damping capacity and high strength [1–4]. This material also is consider as a good option in the biomedical field where tem- poral implants are required, which is the case of fixation screws for bones i.e [5,6]. Despite this, Mg is highly reac- tive and its wear and corrosion resistance are low [7,8]. Currently, this problem has been handled through basically two strategies, the first is by the incorporation of other elements in the Mg-matrix (Mg-alloy) in order to generate new phases that can improve the performance of the material at mechanical and corrosion resistance level. The second option is by modification of the material either by changing the physicochemical properties of the material or by producing of a protective coating [9,10]. In this last context, plasma electrolytic oxidation (PEO), also known as microarc oxidation (MAO), is a favorable option due it is a simple, low cost and reproducible technique. In PEO the material is oxidized in a controlled way allowing to obtain surfaces with specific morphology, thickness and composition by the modification of the operation parameters of the system such as voltage, current density, time and electrolytic solution [11–13]. In PEO the system can be operated under different modes, galvanostatic, in which the current density is fixed and remains constant during all the process and the changes in the voltage during the process are registered; or under potentiostatic mode, where the voltage remains constant and the changes in the current density are recorded [11,14]. Coatings obtained under both processes differ in terms of dimensions and mor- phology. Another important parameter which plays a major role in the formation and composition of the films is the electrolytic solution composition. Anodization of Mg should be in alkaline solutions where the material is more stable, according with this, KOH and NAOH are compounds widely used as base solution in order to reach adequate pH values. Furthermore, the addition of compounds base on silicate, aluminate or phosphate to the base solution is commonly reported [13,15–18]. Additionally and with the purpose to modify either the composition or structure of the coating, other additives can be added to the electrolyte. The aim of this work is to compare the coatings obtained when three additives were added separately to the electrolytic solution and operated in different mode (galvano and potentiostatic mode). The chosen additives were one inorganic, sodium fluoride (NaF-NAF), which was previously studied and reported for other authors and two organics, hexamethylenetetramine (C6H12N4-HMT) and mannitol (C6H14O6-MAN), non-reported before for anodization of Mg.
2. Materials and methods
2.1 Sample processing
Square samples of 99.9% commercially pure magnesium (c.p Mg) of dimensions of 1 cm x 1 cm and approximately 1 mm thickness were mechanically polished with silicon carbide paper up to 1000 grade. All specimens were cleaned in distilled water and then degreased with acetone in an ultrasonic bath for 30 minutes.
2.2 Modification of Mg samples by plasma electrolytic oxidation
Samples were processed in an electrochemical cell where the c.p Mg was set as anode and a stainless steel beaker was used as cathode. The electrolyte was an alkaline solution based on KOH and sodium silicate with addition of other compounds such as sodium fluoride (NAF), hexamethylenetetramine (HMT) and mannitol (MAN) Table.1. A DC power supply (Kepco BHK 500-0.4 MG) was used for sample anodization. For the potentiostaic mode, the system was operated under the maximal values of voltage (500V) and current (0.4A) and according with the curve described, breakdown voltage, critical voltage and maximal reached voltage were chosen for evaluation in each one of the electrolytes.
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