Electrodeposition of Tantalum Coatings on Nitinol Stents

Abstract

The Ni-Ti shape memory alloys are currently a topic of notable interest in medicine. They provide a unique opportunity to make novel surgical implants and instruments for vascular and orthopedic surgery. However, a high nickel content in these alloys could cause the problem of biocompatibility because of nickel toxic effect. In turn, a tantalum is successfully used in medicine as a wire or sheet. It does not irritate the living tissue and does not harm the functioning of the organism, but it has a high specific weight. Therefore, using of tantalum coatings on various materials is more appropriate than utilization of a bulk metal. Thus, it is necessary to improve a corrosion resistance of nickel-titanium alloys (nitinol) that can be achieved with applying of protective porousless tantalum coating.For obtaining of tantalum coatings the NaCl-KCl-NaF(10 wt%)-K2TaF7(10 wt%) melt was used at a temperature 1023 K. Direct (DC), unsteady state (USC) and pulsed current (PC) were supplied for electrodeposition. A cathodic current density was changed from 5 to 200 mA/cm2 at the galvanostatic mode. The same cathodic current densities and the time of electrolysis (tel = 0.5-5.0 s) with pauses (tp = 1-5 s) were used at unsteady state and pulsed electrolysis. It was found that a pulsed electrolysis allow to produce coatings with lower roughness than galvanostatic electrolysis.Electroreduction of tantalum complexes in chloride-fluoride melt at a nitinol electrode was investigated. A voltammetric curve registered at a nitinol electrode except of the electroreduction peak of tantalum fluoride complexes to tantalum has several peaks corresponding to the formation of intermetallic compounds of nickel and tantalum.In the case of formation intermetallic compounds between a substrate and coating is an important aspect of the coating adhesion to substrate. The adhesion was measured by cross-sections method using a tester of adhesion Elcometer 107. The measurements showed that coatings obtained by DC and USC could be classified as adhesion to international standards ISO (1) and ASTM (4B). At the same time, coatings deposited on nitinol by PC have a maximum class of adhesion ISO (0) and ASTM (5B). These results were obtained for intermetallic compounds with a thickness around 1-2 μm.Additional experiments were carried out for obtaining of thick layers of intermetallics and study their composition by SEM and EDX.The porosity of tantalum coatings was determined by Erhard’s technique based on measuring a current dissolution at a certain potential, in which dissolves a substrate material and coatings remain passive.Corrosion resistance of the composition tantalum coating-nitinol was studied in different corrosion media.