Nitinol has become an important material for treating patients through minimally invasive surgery due to its ability to recover to its original shape after being mechanically strained or heated. The corrosion behavior of nitinol surface is an area of general interest due to its direct contact with different body fluids containing salinity and a variety of different pH values. The aim of the project here is to elucidate the corrosion behavior of oesophageal nitinol stents when exposed to a range of acidic (HCl-containing) electrolytes. A key focus of this study was to assess the effect of environment and passivation treatment on the corrosion response. The effect of low to high concentrations of chloride and changes in pH were assessed, simulating nitinol stents exposed within the oesophagus system. Electrochemical polarisation testing followed by assessment of the surface condition by optical microscopy and electron microscopy was carried out. The presence of different redox reactions was probed using pure nickel, pure titanium and compared to nitinol surfaces. Assessment of the breakdown potential (EBD) and the corrosion potential (Ecorr) were used to evaluate the corrosion response. This investigation showed that pure titanium has superior corrosion resistance followed by nitinol and pure nickel. The corrosion response in low to neutral pH environment containing chloride ions showed pure titanium and nitinol exhibit better corrosion resistance than pure nickel, with effective passivation and minimal active dissolution. When the chloride (HCl) concentration is increasing lowering the pH of the electrolyte, an increased metallic dissolution with reduced pitting potentials and higher passive current densities exhibiting poor passive film stability were observed in all three investigated materials. Information of the corrosion behaviour in HCl solution obtained in this study will be used in a parallel research project to carry out an in-situ X-ray computed Tomography study, with the initial results reported in here. A range of passivation treatments for nitinol was explored, including the effect of citric acid (C6H8O7) and nitric acid (HNO3) pre-exposures. Subsequent electrochemical tests in HCl showed that citric acid pretreatment did not improve the passivity, however nitric acid exposure for at least 30 mins at RT showed excellent passivation characteristics with reduced passive current densities. Longer passivation exposures did not provide additional protection.
Corrosion Screening of Nitinol Surfaces
Durgappa, D. (Author). 1 Aug 2025
Student thesis: Master of Philosophy