Improvement in corrosion resistance of NiTi by anodization in acetic acid

NiTi was galvanostatically anodized in acetic acid to increase the oxide film thickness for improving corrosion resistance. The galvanostatic anodization behavior of NiTi in acetic acid differed markedly from that of Ti. In particular, the anode potential reached was much lower for NiTi, and thus only thin oxide films could be obtained. With a suitable choice of anodizing conditions, the anodic oxide film formed had a thickness of 20–25 nm, as determined by profilometric measurement. Atomic force microscopy (AFM) revealed that the surface roughness was increased after anodization. Analysis by X-ray photoelectron spectroscopy (XPS) showed a low Ni/Ti ratio of 0.04 at the anodic oxide surface versus a value of 0.30 for bare NiTi. Electrochemical impedance measurements of the anodized sample in Hanks' solution at 37 °C recorded a 9-fold increase in polarization resistance, and cyclic polarization tests also recorded a matching reduction in the passive current density. These observations indicate that anodization of NiTi can serve as a simple low-temperature method to enhance the corrosion resistance of NiTi when used as an implant material.     

Effect of ceramic conversion treatments on the surface damage and nickel ion release of NiTi alloys under fretting corrosion conditions

Recent researches have demonstrated that surface modification can improve the fretting wear resistance of NiTi alloys in air or enhance their aqueous corrosion resistance without fretting. However, little is known about the behaviour of surface engineered NiTi under fretting corrosion conditions. This is important for such body implants as orthodontic arch wires and orthopedic bone fixation devices because they need to withstand the combined attack of corrosion from body fluid and mechanical fretting. In this study, a NiTi alloy was ceramic conversion (CC) treated at 400 and 650 °C. The effect of the surface treatment on the fretting corrosion behaviour of NiTi alloy was investigated using fretting corrosion tests in the Ringer’s solution. The experimental results have shown that the CC treatment can convert the surface of NiTi into a TiO₂ layer, which can effectively improve the fretting corrosion resistance of NiTi alloy and significantly reduce Ni ion release into the Ringer’s solution. Detailed SEM observations revealed that the untreated samples were severely damaged by adhesion and delamination; the high temperature (HT) (650 °C/1 h) treated samples were damaged mainly by spallation and adhesion; and the low temperature (LT) (400 °C/50 h) treated samples were characterised by mild abrasion. Mild oxidation and corrosion were also observed for all three types of samples tested under fretting corrosion conditions.     

Effect of Ta2O5/TiO2 thin film on mechanical properties,corrosion and cell behavior of the NiTi alloy implanted with tantalum

The NiTi shape memory alloy has been modified by plasma immersion ion implantation (PIII) with Ta at different incident currents to improve the corrosion resistance and other surface and biological properties. The surface topography, chemical components, mechanical properties, corrosion resistance and cytocompatibility are investigated. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) revealed that Ta implantation led to the formation of compact Ta₂O₅/TiO₂ nano-film on the surface of the NiTi alloy. The results of Auger electron spectroscopy (AES) showed that Ni was suppressed in the superficial surface layer of the modified NiTi alloy samples. The results of nano-indentation illustrated a lower level of nano-hardness and Young's modulus after Ta implantation. Potentiodynamic anodic polarization curves showed that the corrosion resistance of NiTi alloys was enhanced by Ta implantation. Cells reached confluency and a double-layered structure had developed after cultured for three days. The NiTi alloy modified by a moderate incident current possesses a uniform and slippery surface morphology and the largest surface roughness, leading to the best corrosion resistance and the highest cell proliferation rate, respectively.     

On the electropolishing of NiTi braided stents – challenges and solutions

Nickel Titanium (NiTi) alloys possess special mechanical properties and good biocompatibility hence used as base material for the production of vascular stents. Normally, vascular stents are machined from NiTi tubes, using laser cutting processes. Braiding is a promising alternative for the machining of certain NiTi stents. However, a surface finish treatment, such as electropolishing of the braided stents, is still required in order to achieve a medical‐grade surface finish. The thermally‐grown oxide resulting from the shape‐setting heat treatment, following the braiding must be removed. Moreover, electropolishing is required to achieve optimum corrosion resistance. Therefore, the aim of this study is to find suitable parameters for the effective electropolishing of NiTi textile stents. Electropolishing of a device with such a complex geometry is challenging, hence a custom‐designed electrolytic cell was constructed and used in this study. We examined the stent surfaces before and after electropolishing, using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Potentiodynamic tests were performed in NaCl 0.9% solution for as‐received and electropolished samples. The results from the present study indicate an improvement in surface quality of the braided stents after electropolishing. Potentiodynamic tests revealed that electropolishing improves the corrosion resistance of the NiTi stents.     

Study of oxidized layer formed on aluminium alloy by plasma oxidation

This paper presents results of plasma oxidizing of aluminium alloy which was carried out in a low-frequency pulse excited plasma. The processing temperature was about 450 °C and the processing time was 3 h. The modified layer was characterized by X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. Atomic force microscopy and scanning electron microscopy observations indicated that no evident defects appeared on the surface of the oxidation layer. Immersion experiments in 3.5% NaCl solution were carried out to test the effect of plasma oxidation on the corrosion resistance of surface oxidized aluminium alloys. After immersion for 200 h, the severe corrosion occurred on the untreated sample surface while no evident corrosion was observed for the treated sample.     

Surface characteristics and electrochemical corrosion behavior of NiTi alloy coated with IrO2

The aim of this work is to investigate the surface characteristics and corrosion behavior of NiTi (50.6 at.% Ni) shape memory alloy coated by a ceramic-like and highly biocompatible material, iridium oxide (IrO₂). IrO₂ coatings were prepared by thermal decomposition of H₂IrCl₆ · 6H₂O precursor solution at the temperature of 300 °C, 400 °C and 500 °C, respectively. The surface morphology and microstructure of the coatings were investigated by scanning electron microscope (SEM) and glancing angle X-ray diffraction (GAXRD). X-ray photoelectron spectroscopy (XPS) was employed to determine the surface elemental composition. Corrosion resistance property of the coated samples was studied in a simulated body fluid at 37 ± 1 °C by electrochemical method. It was found that the morphology and microstructure of the coatings were closely related to the oxidizing temperatures. A relatively smooth, intact and amorphous coating was obtained when the H₂IrCl₆·6H₂O precursor solution (0.03 mol/L) was thermally decomposed at 300 °C for 0.5 h. Compared with the bare NiTi alloy, IrO₂ coated samples exhibited better corrosion resistance behavior to some extent.     

Alumina coating formed on medical NiTi alloy by micro-arc oxidation

Al₂O₃ coatings were prepared on NiTi alloy by micro-arc oxidation in an aluminate solution. Thin-film X-ray diffraction (TF-XRD) indicated that the coating consisted of only Al₂O₃ crystal phase. Energy dispersive X-ray spectrometer (EDS) showed that there was about 2.53 at.% Ni in the surface layer, which was greatly lower than that of NiTi substrate. Scanning electron microscopy (SEM) showed that the coating exhibited a typical porous surface and excellent adhesive interface between the coating and the substrate. Direct pull-off test showed that the coating had a mean coating-substrate bonding strength of 28 ± 2 MPa. The results of electrochemical impedance spectroscopy (EIS) study and potentiodynamic polarization test indicated that the corrosion resistance of the coated sample was increased by two orders of magnitude compared with uncoated sample.     

Determination of corrosion rate of orthodontic wires based on nickel‐titanium alloy in artificial saliva

The goal of this study was to determine corrosion behavior of three orthodontic wires based on nickel‐titanium alloy (NiTi) in artificial saliva at temperature of 37 °C as function of immersion time. Following orthodontic wires were used: uncoated (NiTi), rhodium coated (Rh NiTi) and nitrified (N NiTi) orthodontic wires. Corrosion of investigated orthodontic wires were monitored by measuring of Ni²⁺ and Ti⁴⁺ ions released in artificial saliva by inductively coupled plasma‐optical emission spectroscopy (ICP‐OES) after 3, 7, 14, 21 and 28 days of immersion. Obtained results indicate that corrosion reaction of the NiTi wires in artificial saliva follows the parabolic rate law. According to the obtained values of parabolic corrosion rate constants, corrosion susceptibility of orthodontic wires decreases in the following order: Rh NiTi wire (K_p = 2.48 μg²/cm⁴ h) > NiTi wire (K_p = 1.6 × 10^–3 μg²/cm⁴ h) > N NiTi wire (K_p = 6.0 × 10^–4 μg²/cm⁴ h). These results indicate that in comparison with uncoated NiTi wire, rhodium coating significantly increases corrosion susceptibility, while nitrification effectively suppresses the release of Ni²⁺ and Ti⁴⁺ ions.     

New plasma surface-treated memory alloys: Towards a new generation of “smart” orthopaedic materials

This paper describes the corrosion resistance, surface mechanical properties, cyto-compatibility, and in-vivo performance of plasma-treated and untreated NiTi samples. Nickel–titanium discs containing 50.8% Ni were treated by nitrogen and carbon plasma immersion ion implantation (PIII). After nitrogen plasma treatment, a layer of stable titanium nitride is formed on the NiTi surface. Titanium carbide is also found at the surface after carbon plasma implantation. Compared to the untreated samples, the corrosion resistances of the plasma PIII samples are better by a factor of five and the surface hardness and elastic modulus are better by a factor of two. The concentration of Ni leached into the simulated body fluids from the untreated samples is 30 ppm, whereas that from the plasma-treated PIII are undetectable. Although there is no significant difference in the ability of cells to grow on either surface, bone formation is found to be better on the nitrogen and carbon PIII sample surfaces at post-operation 2 weeks. All these improvements can be attributed to the formation of titanium nitride and titanium carbide on the surface.     

Characterization and corrosion behavior of NiTi–Ti2Ni–Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi–Ti₂Ni–Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni–Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni–Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Corrosion behavior of a laser-welded NiTi shape memory alloy

Corrosion behaviors of the laser-welded Ni–49.4 at.% Ti shape memory alloy and base metal in 0.9% NaCl solution were investigated by means of electrochemical techniques (the open circuit potential measurement, linear and potentiodynamic polarizations). The results showed that corrosion resistance of the laser-welded NiTi alloy is better than that of the base metal. Compared to the base metal, the laser-welded NiTi alloy exhibits higher open circuit potential, higher polarization resistance, a wider passive region and higher breakdown potential. The improvement of corrosion resistance of the laser-welded NiTi alloy is ascribed to a smoother, defect free surface and an absence of carbides.     

Characterization and corrosion behavior of NiTi-Ti2Ni-Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi-Ti₂Ni-Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni-Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni-Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Effect of substrate temperature on corrosion performance of nitrogen doped amorphous carbon thin films in NaCl solution

Nitrogen doped amorphous carbon (a-C:N) thin films were deposited on p-Si substrates by DC magnetron sputtering at varying substrate temperature from room temperature (RT) to 300 °C. The bonding structure, surface morphology and adhesion strength of the a-C:N films were investigated by using X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, atomic force microscopy (AFM) and micro-scratch testing. The corrosion behavior of the a-C:N films was evaluated by potentiodynamic polarization test in a 0.6 M NaCl solution. The results indicated that the corrosion resistance of the films depended on the sp³-bonded cross-link structure that was significantly affected by the substrate temperature.     

Corrosion characteristics of a selective laser melted Co–Cr dental alloy under physiological conditions

This study explores the corrosion behavior of a dental cobalt–chromium (Co–Cr) alloy fabricated by selective laser melting (SLM) under conditions simulating the oral environment. Specimens were fabricated using either SLM or traditional casting methods. Microstructure and surface characteristics were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and X-ray diffraction. In addition, sample corrosion characteristics were assessed using electrochemical impedance spectroscopy. Large microstructural differences were observed between SLM and traditional cast samples, with SLM exhibiting a more compact and homogeneous microstructure. The SLM samples also exhibited a slightly thicker surface oxide layer in comparison to traditional cast samples. In artificial saliva at pH 5, no differences were observed in the electrochemical corrosion properties or XPS surface characteristics of SLM and traditional cast specimens. However, in artificial saliva at pH 2.5, significant differences in electrochemical corrosion properties were observed, with SLM specimens exhibiting more corrosion resistance in comparison to traditional cast samples. The enhanced corrosion resistance of SLM samples in an acidic environment provides further support for their use in dental applications, where the oral environment can become temporarily acidic after meals.     

Electrochemical behavior of biocompatible AZ31 magnesium alloy in simulated body fluid

Dense oxidation coatings have been successfully developed on biocompatible AZ31 magnesium alloy, using microarc oxidation technique, to improve the corrosion resistance. Three different deposition voltages of 250, 300, and 350 V have been employed. The effect of voltage on the coating corrosion resistance has been evaluated through electrochemical experiments in a simulated body fluid (SBF) up to 7 days. Potentiodynamic polarization and electrochemical impedance spectroscopy scans were performed in the SBF solution, followed by optical microscopy surface inspection. The results indicate that the corrosion rates of the coatings are in the order of 250 < 300 < 350 V after immersion for 7 days, and the charge transfer resistance (R _ct) of the three samples is in the order of 250 > 300 > 350 V. Both the electrochemical tests and the surface inspection suggest that the 250 V coating has the highest corrosion resistance, with lowest corrosion current density, highest R _ct, and the best surface quality.     

Corrosion behavior of ion nitrided AISI 316L stainless steel

In the present work the corrosion susceptibility of ion nitrided AISI 316L stainless steel was investigated for two different nitriding times and compared with the corrosion susceptibility of the untreated material. Plasma nitriding for short times (30 min) produced the “S” phase or expanded austenite (γ_N), with a thickness of ∼ 5 μm and a micro-hardness of 1300-1400 HV_0.025 (6.5 times higher than the untreated material). Plasma nitriding for long times (6 h) resulted in the precipitation of iron and chromium nitrides.To evaluate the corrosion resistance of both untreated and nitrided samples, anodic potentiodynamic polarization curves and immersion tests were performed in 1 M NaCl at room temperature. It was found that the corrosion resistance depends on the nitriding time. Samples nitrided for half an hour developed a much better corrosion resistance - close to that observed in the untreated samples - than those nitrided for 6 h. Samples nitrided for half an hour showed high roughness probably due to the presence of sliding bands developed in the expanded austenite phase. These sliding bands provide appropriate sites for the developing of the corrosion process. This would explain the results obtained in the corrosion tests. Samples ion nitrided for 6 h showed a severe and massive surface damage due to corrosion.Ion nitriding of AISI 316L stainless steel for short periods of time (30 min in the present case) may be an interesting surface treatment process that efficiently improves the surface hardness of the steel with some reduction in its corrosion resistance.     

Thermal oxidation of CP-Ti: Evaluation of characteristics and corrosion resistance as a function of treatment time

Commercially pure titanium (CP-Ti) samples were subjected to thermal oxidation (TO) treatment at 650 °C for 8, 16, 24 and 48 h. The morphological features, structural characteristics, microhardness and corrosion resistance in Ringer's solution of thermally oxidized samples were compared with that of the untreated one, to ascertain the suitability of thermally oxidized sample as a bio-implant. The thickness, morphological features and phase constituents of the oxide film formed during thermal oxidation (TO) exhibit a strong dependence on the treatment time. Samples oxidized for 48 h lead to the formation of oxide grains along with a thick oxide film consisting of rutile and TiO phase. Samples oxidized for 24 h lead to the formation of oxide grains with thinner oxide layer at the grain boundary. Almost a 3 fold increase in hardness is observed for samples oxidized for 48 h compared to that of the untreated sample. Based on the corrosion protective ability, the untreated and thermally oxidized samples can be ranked as follows: {TO 48 h} > {TO 16 h} > {TO 8 h} ≈ {TO 24 h} > untreated. From corrosion protection point of view, TO for 48 h is a promising surface treatment and it can be a suitable alternative to the untreated CP-Ti as a bio-implant.     

Effect of Laser Printing Mode on Surface Topography,Microstructure, and Corrosion Property of Additive-Manufactured NiTi Alloy

Laser powder bed fusion (LPBF) is an emerging metal additive manufacturing method that can pave a pathway for manufacturing NiTi shape memory alloys (SMAs) with high performance. Considering the unique characterizations of LPBF process, the position and sequence of laser irradiation are different under different laser scanning modes, which will affect the performance of as-built samples. Herein, four different chessboard sizes are utilized to fabricate NiTi parts. The surface quality and relative density first increase and then decrease with the increasing chessboard size, obtaining the optimal surface roughness of 9.95 μm and relative density of 99.7%, respectively, at a chessboard size of 5 mm. As the chessboard size increases, the more pronounced precipitation of Ni₄Ti₃ with a higher quantity induces a strengthening effect, leading to a higher microhardness value of ≈290 HV_0.2 at a chessboard size of 9 mm. The electrochemical test shows a better corrosion resistance with a corrosion potential of 0.101 V and a corrosion current density of 1.670 × 10⁻⁵ A cm⁻² at a chessboard size of 5 mm. The corrosion mechanism is further revealed. This work emphasizes the importance of chessboard size as a reference for optimizing the process of additive-manufactured NiTi SMAs.     

The study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 steel under hot and cold wall conditions

This paper reports on a comparative study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 low alloy steel under modern hot wall condition and conventional cold wall condition. Plasma nitriding was carried out at 500 °C and 550 °C with a 25% N₂ + 75% H₂ gas mixture for 8 h. The wall temperature of the chamber in hot wall condition was set to 400 °C. The treated specimens were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness and surface roughness techniques. The wear test was performed by pin-on-disc method. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were also used to evaluate the corrosion resistance of the samples. The results demonstrated that in both nitriding conditions, wear and corrosion resistance of the treated samples decrease with increasing temperature from 500 °C to 550 °C. Moreover, nitriding under hot wall condition at the same temperature provided slightly better tribological and corrosion behavior in comparison with cold wall condition. In consequence, the lowest friction coefficient, and highest wear and corrosion resistance were found on the sample treated under hot wall condition at 500 °C, which had the maximum surface hardness and ε-Fe_2–3N phase.     

Formation of a nano-pattering NiTi surface with Ni-depleted superficial layer to promote corrosion resistance and endothelial cell-material interaction

Zirconium ion implantation was performed on NiTi alloy to suppress Ni ion release as well as to improve corrosion resistance and cell-material interaction. A thicker Ni-depleted nano-scale composite layer formed after Zr implantation and the corrosion resistance was evidently increased in aspects of increased E _br ? E _corr (difference between corrosion potential and breakdown potential) and decreased corrosion current density. 2.5/2 NiTi sample possessed the highest E _br ? E _corr, more than 500 mV higher than that of untreated NiTi, suggesting a significant improvement on pitting corrosion resistance. Ni ion release rate of Zr–NiTi was decreased due to the depletion of Ni in the superficial surface layer and the diffusion resistance effect of the ZrO₂/TiO₂ nano-film. Increased surface wettability induced by increased surface roughness was obtained after Zr implantation. Zr–NiTi samples were found to be favorable to endothelial cells (ECs) proliferation, especially after 5 and 7 days culture.