Nickel-Titanium Lab Report

In this study, nickel-titanium surfaces are coated with phenolic thin films of tannic acid and pyrogallol with the purpose of studying their corrosion resistance in physiological environments. Three tests are performed: the open circuit potential test, potentiodynamic polarisation and potentiostatic electrochemical impedance spectroscopy. Polarisation measurements are scrutinized in order to gain knowledge concerning the kinetics of the cathodic and anodic reactions, while the open circuit and impedance spectroscopy help to study the electrolyte-surficial interactions. It is found that coating nitinol with polyphenols reduces the metal, resulting in depletion of the native oxide layer and thus a decrease of corrosion resistance. Pyrogallol
Inside the living body, implanted metals are primarily exposed to highly oxygenated body fluids containing inorganic ions (chloride, carbonate, calcium, etc.), proteins, lipids, amino acids and cells that assemble around the implanted metal [1]. Biomaterials devices made of smart metals such as the equiatomic binary alloy, nickel-titanium (commonly known as nitinol), may corrode when in contact with body fluids that act as a conducting electrolyte solution favourable for electrochemical corrosion. Nitinol shape memory alloys typically are covered by a naturally formed thin adherent oxide layer of titanium known as a passive film. This film is very stable and nitinol alloys are resistant to many forms of corrosive attacks; however, this passive film can be attacked by acidic solutions containing chloride [3]. In addition, the oxides formed on the nitinol surface always contain a certain fraction of nickel. Contrary to pure titanium and Ti4Al6V medical alloys, which easily repassivate after surface damage, the nitinol oxides have a lower self-healing ability in scratch tests, a lower resistance to localized corrosion [4, 5].
In previous studies, electrochemical tests have been used to evaluate the corrosion resistance of nitinol alloys in various physiological environments [6-16]. The correlation of the results from these studies in regards to the corrosion resistance
All these measurements were performed on a Gamry potentiostat/galvanostat Zero-resistance Ammeter Reference 3000 (Gamry Instruments Inc., Warminster, PA), and data were analysed with Gamry Echem 7.03 software (Gamry Instruments Inc., Warminster, PA) and EIS data with Zview 3.4d ( Scribner associates, Inc., NC). An OCP was continuously monitored for 24 h, and the potential was recorded as a function of measuring time every 0.5 s prior potentiodynamic polarisation measurements performed in the potential range from -1000 to 1600 m V with a scanning rate of 0.1667 m V s-1. The electrode was immersed in the test solution for 60 minutes prior each test in order to stabilise the system temperature [22]. EIS tests were carried out at OCP with 10 mV of signal amplitude and a frequency interval from 0.01 Hz to 50 kHz. EIS measurements were performed subsequently after 1, 3, 6, 12 and 24 h prior each