The corrosion resistance of Nitinol alloy in simulated physiological solutions Part 2: The effect of surface treatment

Surface finishing of Nitinol for implantable medical devices: A review

Nitinol (NiTi), a nickel-titanium alloy, has been used for various cardiovascular, orthopedic, fracture fixation, and orthodontic devices. As with most other metallic biomaterials, the corrosion resistance and biocompatibility of NiTi are primarily determined by the properties of the surface oxide layer such as thickness, chemical composition, structure, uniformity, and stability. Currently, a number of finishing methods are used to improve the properties of surface oxide of NiTi with an ultimate goal to produce a defect-free, impurity-free, thin homogeneous oxide layer that is stable and composed of only titanium dioxide (TiO₂ ) with negligible amount of Ni species. This review discusses the effects of various surface finishing methods such as mechanical polishing, electropolishing, magnetoelectropolishing, heat treatments at different temperatures, passivation, chemical etching, boiling in water, hydrogen peroxide treatment, and sterilization techniques (steam autoclave, ethylene oxide, dry heat, peracetic acid, and plasma-based treatments) on the properties of a surface oxide layer and how it impacts the corrosion resistance of NiTi. Considering the findings of the literature review, a checklist has been provided to assist with choosing finishing/sterilization methods and relevant rationale and recommendations to consider when selecting a surface finishing process for NiTi used in implantable medical devices.

The role of surface oxide thickness and structure on the corrosion and nickel elution behavior of nitinol biomedical implants

Biocompatibility is an important factor for metallic medical device implants, and corrosion resistance of implantable alloys is one aspect of biocompatibility. Corrosion behavior of nitinol is strongly dependent upon the nature of the surface oxide, which forms during processing. The surface oxide is comprised of a mixture of titanium and nickel oxides, and subsequent thermal exposure (e.g., during shape setting) and surface removal (e.g., electropolishing, mechanical polishing, etching, etc.) influence its structure. Corrosion behavior is often assessed through testing methods such as cyclic potentiodynamic polarization (e.g., ASTM F2129) and nickel ion release. Studies have suggested that a correlation exists between oxide thickness and nickel ion release, with thicker oxides eluting more nickel. It is hypothesized that the composition of the surface oxide, and not only its thickness, influences the corrosion performance of nitinol. To investigate this, nitinol wire samples were processed to produce surface oxides with different structures both in terms of thickness and composition. These samples were tested per ASTM F2129 and nickel ion release testing.

Improvement of the Corrosion Resistance of Biomedical Zr-Ti Alloys Using a Thermal Oxidation Treatment

Binary Zr-Ti alloys spontaneously develop a tenacious and compact oxide layer when their fresh surface is exposed either to air or to aqueous environments. Electrochemical impedance spectroscopy (EIS) analysis of Zr-45Ti, Zr-25Ti, and Zr-5Ti exposed to simulated physiological solutions at 37 °C evidences the formation of a non-sealing bilayer oxide film that accounts for the corrosion resistance of the materials. Unfortunately, these oxide layers may undergo breakdown and stable pitting corrosion regimes at anodic potentials within the range of those experienced in the human body under stress and surgical conditions. Improved corrosion resistance has been achieved by prior treatment of these alloys using thermal oxidation in air. EIS was employed to measure the corrosion resistance of the Zr-Ti alloys in simulated physiological solutions of a wide pH range (namely 3 ≤ pH ≤ 8) at 37 °C, and the best results were obtained for the alloys pre-treated at 500 °C. The formation of the passivating oxide layers in simulated physiological solution was monitored in situ using scanning electrochemical microscopy (SECM), finding a transition from an electrochemically active surface, characteristic of the bare metal, to the heterogeneous formation of oxide layers behaving as insulating surfaces towards electron transfer reactions.

The effect of pH, fluoride and tribocorrosion on the surface properties of dental archwires

Nickel-titanium and stainless steel are the most commonly used alloys for orthodontic treatments. Even though both are known to be resistant to corrosion, there are circumstances that can lead to undesired situations, like localized types of corrosion attack, wear during sliding of an archwire though brackets and breakdowns due to iatrogenic causes. The aim of this research was to analyse the influence of environmental effects on the corrosion and tribocorrosion properties of NiTi and stainless steel dental alloys. The effects of pH and fluorides on the electrochemical properties were studied using the cyclic potentiodynamic technique. The migration of ions from the alloy into saliva during exposure to saliva with and without the presence of wear was analysed using ICP-MS analyses. Auger spectroscopy was used to study the formation of a passive oxide layer on different dental alloys. It was found that lowering the pH preferentially affects the corrosion susceptibility of NiTi alloys, whereas stainless steel dental archwires are prone to local types of corrosion. The NiTi alloy is not affected by smaller increases of fluoride ions up to 0.024M, while at 0.076M (simulating the use of toothpaste) the properties are affected. A leaching test during wear-assisted corrosion showed that the concentrations of Ni ions released into the saliva exceeded the limit value of 0.5μg/cm²/week. The oxide films on the NiTi and stainless steel alloys after the tribocorrosion experiment were thicker than those exposed to saliva only.