Nitinol: Tubing versus sputtered film – microcleanliness and corrosion behavior

2017

Inclusions appear to play a crucial role in the initiation of pitting on nitinol, but the reason remains unclear. Furthermore, it has not been established whether the type of inclusion is a central factor. In this study, potentiodynamic polarization together with scanning electron microscopy and energy dispersive X-ray spectroscopy were used to provide more insight into the initiation of pits on electropolished nitinol wire. Corrosion was limited to a single primary pit on each of the few wire samples that exhibited breakdown. The pit contained numerous Ti NiO inclusions, but secondary pits that developed within the primary pit provided evidence that these inclusions were the sites of pit initiation. Although several theories have been proposed to account for pit initiation at inclusions in mechanically polished and electropolished nitinol, titanium depletion in the adjacent alloy matrix appears to provide the most viable explanation. The key factor appears to be the size of the inclusion and therefore the extent of titanium depletion in the alloy matrix. The type of inclusion evidently plays a secondary role at most. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1605-1610, 2018.

Section: Introductionmentioning

confidence: 99%

Pit initiation on nitinol in simulated physiological solutions

2017

“…Despite the increased pitting susceptibility, E b – E corr remained relatively high with a value close to 0.7 V, suggesting that the EP nitinol used in this work would be unlikely to actually undergo pitting corrosion in gastric fluid under in vivo conditions. It should be recognized, however, that EP nitinol from other sources may differ in pitting resistance from the present samples, because the susceptibility to breakdown of EP nitinol is influenced by processing and material quality . A further point is that E b – E corr values for single wires are generally not indicative of the susceptibility to crevice corrosion—a consideration for implants involving multiple contacting wires (for example, braided wire)—because of the propensity of alloys to undergo crevice corrosion at lower potentials than those for pitting corrosion…”

Section: Discussionmentioning

confidence: 73%

“…It should be recognized, however, that EP nitinol from other sources may differ in pitting resistance from the present samples, because the susceptibility to breakdown of EP nitinol is influenced by processing and material quality. 28,44,45 A further point is that E b -E corr values for single wires are generally not indicative of the susceptibility to crevice corrosion-a consideration for implants involving multiple contacting wires (for example, braided wire)-because of the propensity of alloys to undergo crevice corrosion at lower potentials than those for pitting corrosion. 46 Electrochemical impedance spectroscopy The thickness of the oxide in the neutral solutions was not significantly affected by the difference in chloride concentration, with values of 5.0-5.3 nm.…”

Section: Cyclic Potentiodynamic Polarizationmentioning

confidence: 99%

The electrochemical behavior of nitinol in simulated gastric fluid

2016

Increased use is being made of nitinol for implants that are exposed to gastric fluid. However, few corrosion studies have involved nitinol in an appropriate acidified chloride solution. In this work, the electrochemical behavior of electropolished (EP) nitinol was examined in simulated gastric fluid, the corresponding neutral solution with the same concentration (0.6%) of NaCl, and 0.9% NaCl. Cyclic potentiodynamic polarization was used to evaluate the susceptibility to pitting corrosion, while electrochemical impedance spectroscopy was used to examine the passive oxide film. The potentiodynamic tests showed that the susceptibility of EP nitinol to pitting corrosion is affected by chloride concentration and pH. Acidification, in particular, resulted in the susceptibility being markedly higher in gastric fluid compared with that in the corresponding neutral NaCl solution. The impedance data could be fitted using a parallel resistance-capacitance (as a constant phase element) circuit associated with the oxide film. The thickness of the oxide was determined from the capacitive component and found to be little affected by chloride concentration. In contrast, acidification increased the solubility of the oxide enough to decrease the thickness of the film from 5.3 nm in 0.6% NaCl to 4.2 nm in gastric fluid. The resistivity of the oxide obtained from the resistance was affected by chloride concentration (0.7 × 10 and 1.7 × 10 Ω m in 0.9% and 0.6% NaCl, respectively) and particularly by pH (6.3 × 10 Ω m in gastric fluid). The resistivity values suggest that the oxide was more defective in the neutral solutions. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B:2394-2400, 2017.

“…The surface in the scratched area could in fact be quite different from that produced by a treatment such as electropolishing or passivation, particularly with regard to inclusions. Pitting appears to initiate at inclusions in EP nitinol and 316 stainless steel . Exposure to a higher concentration of possibly larger inclusions in the scratched area means that the value of E b obtained by the scratch method may not accurately reflect the pitting resistance of an alloy with a treated surface.…”

Section: Methodsmentioning

confidence: 99%

The use of electrochemical techniques to evaluate the corrosion performance of metallic biomedical materials and devices

2018

The corrosion performance of metallic biomedical materials and devices is commonly evaluated using electrochemical techniques. Although test standards involving such techniques have been released to address some forms of corrosion, a key issue is application of the results with regard to use of an implantable device in vivo. This review focuses on nitinol, 316L/LVM stainless steel, and Co-Cr alloys and is intended to provide some perspective on the significance of results from tests concerning general corrosion, localized corrosion, galvanic corrosion, and fretting corrosion of these alloys in simulated physiological solutions. It also examines the factors that could cause differences in the corrosion performance between in vitro and in vivo exposure, with the goal of providing some rationale for applying electrochemical characteristics obtained from the tests to predict the corrosion performance in vivo. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res B Part B: Appl Biomater, 2018.