The corrosion resistance of laser-welded composite arch wire (CoAW) with Cu interlayer between NiTi shape memory alloy and stainless steel wire in artificial saliva with different acidities and loads was studied. It was found that both the solution pH and the stress had a significant influence on the corrosion behaviors of the CoAW samples. Decreasing the solution pH or increasing the loading stress caused the increase of Cu release and weight loss. The corroded morphology formed on the surfaces of the CoAW was the consequence under the combined effect of corrosion and stress.
NiTi shape memory (Nitinol) alloy and stainless steel (SS) are widely and successfully used as orthodontic wires, self-expanding cardiovascular and urological stents, and so forth [
Composite arch wire (CoAW) is an arch wire solder connection made by Nitinol and SS wires. CoAW combines the advantages of both materials by virtue of correcting malposed teeth during the teeth alignment stage and at the same time maintaining the stability of the antitooth. The application of CoAW could not only effectively reduce the suffering of patients and simplify therapeutic operation, but it can also reduce the number of subsequent visits. However, successful application of any advanced material depends not only on its inherent properties, but also on the development of joining technology for itself or other dissimilar materials [
The corrosion resistance of orthodontic wire is an important factor determining its biocompatibility [
As a biomaterial like arch wire, the resistance to general and localized corrosion, as well as the resistance of harmful metal ions, is prerequisite for in-oral applications [
Therefore, in this paper, the stress corrosion behaviors of the CoAW laser welded by Nitinol and SS wires with Cu interlayer in artificial saliva with different acidities and loading force were studied.
Ti-44.73 wt. % Ni SMA wire (purchased from Smart Co., Beijing), Fe-18Cr-8Ni stainless steel (Grikin Advanced Materials Co., Ltd.), and pure Cu foil were used as base metals in this investigation. The dimensions of the wires are 30 mm (length) × 0.64 mm (width) × 0.48 mm (thickness). The pure Cu interlayer is 0.2 mm of thickness. The base metal was ground using SiC papers of nos. 800, 1200, and 2000 grit to remove oxide layer and then ultrasonically degreased in acetone. The Nitinol and stainless steel wire were fixed on a self-constructed fixture by wire-to-wire butt with the pure Cu interlayer as shown in Figure
(a) Schematic diagram of laser welding; (b) device for applying 3-point bending forces to composite arch wire.
The common physiological solution of 0.9% NaCl was prepared as the chloride solution. The artificial saliva (AS) was a phosphate buffered saline solution of the composition shown in Table
Modified Fusayama artificial saliva used in this study.
Composition | mg/L |
---|---|
NaCl | 400 |
KCl | 400 |
CaCl2·2H2O | 795 |
NaH·PO4·H2O | 690 |
KSCN | 300 |
Na2S·9H2O | 5 |
Urea | 1000 |
The specimens were immersed in modified Fusayama artificial saliva with different acidities and maintained at 37°C for periods up to 28 days. A 3-point flexure fixture, made of ceramic sheet, was used to apply a continuous bending force that deflected the displacement of 1.0, 2.0, 3.0, 5.0 mm as shown in Figure
After 14 days of immersion, the solution was collected and substituted by the new solution, and after 28 days each retrieved sample was cleaned. Precision electronic balance (M2-P, Sartorius, Germany) was used to measure the weight changes after soaking in artificial saliva. The collected solutions were individually analyzed for copper using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 3300DV, Perkin Elmer, Boston, USA). The detection limit of ICP-OES used in this study was 0.01 ppm for copper ion.
All the electrochemical measurements were performed using a CHI 920C electrochemical workstation. The counter electrode was a rectangular platinum plate and the reference electrode was a saturated calomel electrode (SCE). Additionally, the test solutions were not deaerated before the electrochemical measurement. All test samples were mechanically ground using SiC papers up to no. 2000 grit to guarantee consistent surface roughness and then embedded in cold-curing epoxy resin, exposing a sample surface area of 20 × 0.64 mm2.
After an initial delay of 60 min to accomplish the equilibrium, the scanning rate was 5 mV/s, starting from-1 V/SCE to remove the surface film heterogeneity. The cyclic potentiodynamic tests were conducted between −1000 and +1500 mV. Each test contained 3 specimens.
The surface morphologies of composite orthodontic wires were observed using environmental scanning-electron microscopy (SEM, ZEISS EVO18, Germany) equipped with an energy dispersive spectrometer (EDS) analyzer (INCA-X-Max, UK).
Figure
(a) SEM surface morphologies of the welded composite arch wire; (b) EDS line analysis of the welded composite arch wire; ((c)–(g)) map EDS analysis of the welded composite arch wire.
The effects of chloride and pH on potentiodynamic polarization behaviors of CoAW are shown in Figure
The pitting potential (
Solution type |
|
|
|
---|---|---|---|
pH = 4.0 | 0.121 (0.010) | −0.628 (0.011) | 1.38 (0.014) |
pH = 6.75 | 0.260 (0.012) | −0.641 (0.014) | 1.02 (0.006) |
NaCl | 0.052 (0.008) | −0.534 (0.016) | 1.58 (0.011) |
Polarization curves for composite arch wires in different solutions.
In order to evaluate the repassivation ability of CoAW in the three categories of solutions, cyclic potentiodynamic polarization measurements were carried out. Figure
Representative cyclic polarization curves of composite arch wires in different solutions.
The SEM images of the CoAWs after the cyclic potentiodynamic polarization measurement are shown in Figure
SEM surface morphologies of potentiodynamic polarized composite arch wires: (a) pH 4.0; (b) pH 6.75; (c) 0.9% NaCl.
After soaking in AS at both pH 4.0 and 6.75 with different stresses, the typical surface morphologies of CoAWs are shown in Figure
SEM surface morphologies of unstressed and stressed composite arch wires immersed in artificial saliva with different pH: ((a)–(e)) pH = 4.0; ((f)–(j)) pH = 6.75.
(a) Weight loss and (b) copper release of the composite arch wires in artificial saliva with different pH values.
The biocompatibility of orthodontic arch wire is thought to depend mainly on the host reaction induced by the degradation of the material in bodily environment. The biologic response to metal is directly related to its corrosion performance, which is associated with the protective oxide film on the surface [
As indicated, different environments have important effect on the corrosion performance of CoAW. The standard and cyclic potentiodynamic tests were used to estimate the corrosion behaviors in different solutions. The results of standard potentiodynamic polarization test are shown in Figure
In Figure
Another important aspect of the biocompatibility behavior of CoAWs is their corrosion performance under stress condition in applications. Bending stress used in this study, unlike tensile stress, could cause homogenous deformation of orthodontic wires which would start at an initial point and propagate along the wires then allow body fluids and tissues to touch the wire surface under the damaged oxide layer. Rondelli and Vicentini found no effect of 4% strain on the localized corrosion behavior of Nitinol wires [
As shown in Figure
For clinical applications, the orthodontic arch wires have dynamic conditions rather than static conditions, which possibly damage the passive film. The open metal surface would react with the aggressive solution and release metal ions into the surrounding environment. We must understand the stress on corrosion behavior and ion release to ensure the stable surface properties of CoAW in clinical use and reexamine the loading conditions with respect to corrosion behavior. In clinical applications, orthodontic wires would be used for more than 1 month, and the continuous bending stress and acidity variation would affect the properties of the passive film of the composite wires. Therefore, the factor of stress and acidity should be considered on the corrosion behavior in the design and clinical use of CoAW and related-alloy wires. The clinical application and performance improvements of CoAW must refer to the results of pH and stress corrosion.
Under the different experimental conditions of this study, the following conclusions were drawn. Through the polarization test, the CoAW seems easily occurred with uniform corrosion in the artificial saliva solution. With the decrease of the pH, the corrosion resistance becomes worse. The applied bending stress would break the oxide film on the surface of the CoAW which leads to more Cu release and larger weight loss. The most serious corrosion occurred under the largest stress and lowest pH value. The factor of stress and acidity must be considered on the corrosion behavior in the design and clinical use of CoAW and related-alloy wires.