Page 65 - 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
1. Introduction
Magnesium (Mg) is a material with a favorable ductility, easy processing and light weight. These characteristics render Mg widely used in aerospace and automotive industries, in the manufacture of electronic devices and also in the bio- medical field. However, its high chemical reactivity is a major obstacle to use this material as it is easily corroded. For instance, in biomedical applications, the biocompatibility of an Mg-based implant is compromised by the accumula- tion of hydrogen gas and rapid changes in pH in the environment. The use of Mg alloys or of surface coatings which decrease the degradation rate, is a manner to solve reactivity-related problems. Spontaneous oxidation in ambient air produces a protective (oxide) surface layer on Mg as well as other metals, such as Ti and Al. However, this oxide layer is not thick enough to provide long term protection to the metal against accelerated degradation. Therefore various techniques such as plasma electrolytic oxidation (PEO) also known as micro-arc oxidation (MAO), can be used to grown a protective oxide layer from the metal in a controlled way [1–3]. This technique is a modification of the conventional anodizing where upon establishing a maximally possible voltage, a dielectric breakdown is produced as a consequence of a localized dissolution of the new formed film due to punctual micro-discharges accompanied with gas evolution. Modification of parameters such as voltage, current density, electrolyte solution and time, determine the morphology, thickness, composition, and physiochemical properties of the coating. Together, these characteristics directly impact the corrosion resistance of the material. In general, anodization requires a base electrolytic solution such as silicates, phosphates, or aluminates, [4–16]. However this solution can contain some additives which in order to create new phases in the coating by addition of new elements or to improve the corrosion resistance of the material [17],
With the purpose to improve the corrosion resistance of the Mg, different formulation of electrolytic solution were studied. Initially and according with suggested in literature, the base solution consisted of sodium metasilicate and potassium hydroxide was chosen [18]. This solution may contain other additives to improve the corrosion properties of the material. Previous studies has been reported the use of additives such as borates, sulfate, glycerol, sodium citrate, ammonium, phosphate, ethylene glycol, among others [6,11,12,16,25]. In the present work c.p Mg were modified by PEO by using initially three different formulations in which different additives were studied under specific operative mode of the system according with previous studies carried out for the authors. The used compounds were added with the purpose to improve the corrosion resistance of the material. One of the compounds used was sodium fluoride (NAF) which has been used and reported in previous studies [17]; it was compared with the addition of two new additives; hexamethylenetetramine(HMT) and mannitol (MAN). These compounds were chosen because of their organic nature and their ability to change the electrical conductivity of the solution without alter the basic MgO/Mg(OH)2 composition of the final coating. Further, considering the future possible application of the material as a biomedical implant the organic compounds such HMT and MAN are considering as promising due to there are non-toxic elements for the body. Summarizing, the aim of this study was to evaluate the corrosion resistance: of coatings obtained by PEO in presence of two unusual additives in the electrolyte solution HMT and MAN and compare them with NAF. Additionally, double- coating is proposed as a new alternatively trying to combine the properties of a protective and compact layer (obtained with NAF) with the porous morphology (obtained with HMT and MAN).
2. Materials and methods
2.1 Sample processing
Commercial pure magnesium (99.9%) samples with dimensions of 1 cm x 1 cm and 1 mm thickness were polished up to 1000 grade with silicon carbide paper. After that, samples were sonicated in acetone for 30 min in an ultrasonic bath.
4
63