Corrosion resistance of a superelastic NiTi alloy coated with graphene–based coatings

Highlights

• A superelastic NiTi alloy was coated with graphene derivatives via dip–coating.

• Two graphene derivatives were used: GO and a composite of rGO + SEBS.

• The coatings were tested in terms of mechanical behavior and corrosion resistance.

• Both coatings depicted good mechanical resistance.

• However, rGO + SEBS coating presented the best corrosion protection capacity.

Abstract

The mechanical responses under superelastic loading cycles and the corrosion protection capacity of two graphene–based coatings applied to a NiTi alloy were evaluated. Graphene oxide (GO) and a composite of reduced graphene oxide (rGO) incorporated in a block copolymer of styrene–ethylene–butylene–styrene (SEBS) were deposited on the substrate via dip–coating. The morphology of the coatings was characterized by scanning electron microscopy (SEM) before and after uniaxial loading cycles up to 6%. Atomic force microscopy (AFM) was also used to evaluate the roughness of the coatings, which was expressed in terms of average roughness (Ra). The corrosion behavior was studied by potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis (MSA). SEM images suggested that both GO and rGO + SEBS create uniform coatings on the substrate. The measured Ra was 25 ± 3 nm for the bare NiTi, 24 ± 3 nm for the GO coated NiTi, and 8 ± 2 nm for the rGO + SEBS coated NiTi. Both coatings presented the capacity of following several superelastic cycles, maintaining their morphological integrity. PP and EIS showed that only the rGO + SEBS coating results in a significant improvement in terms of corrosion resistance. Furthermore, rGO + SEBS decreased the oxygen vacancies in the passive film when compared with the bare NiTi and GO coating.

Introduction

Near–equiatomic nickel–titanium (NiTi) alloys are well known for being a benchmark case among other smart materials due to their shape memory effect and superelasticity [1], [2]. These alloys also present excellent biocompatibility, being applied to several biomedical devices, such as expandable vascular stents, implants, orthodontic wires, and endodontic instruments [3], [4], [5], [6], [7]. The spontaneous formation of a passive titanium oxide layer provides good corrosion resistance for these alloys [8], but the presence of defects and irregularities on the surface may act as initial corrosion sites and stress concentrators, which can cause cracks and premature failures [9], [10]. Furthermore, some studies pointed out concerns over the nickel ion release and its systemic toxicity when NiTi devices are in contact with blood [11], [12], which is a major clinical problem. In the last decade, several surface modification techniques have been studied in order to modify the corrosion properties of the NiTi surface. Among them, the deposition of different titanium–based oxides [9], [13], [14], zirconia [10], [15], and hydroxyapatite [16] were reported as promising improvements in terms of corrosion resistance. However, these coatings are known for having a fragile mechanical behavior, which is an important concern when they are applied to an alloy that undergoes expressive reversible deformation [1].

Recently, graphene and its derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), have been applied to several metallic substrates as protective coatings against corrosion and wear [17], [18], [19]. Graphene is a single–atom sheet made of sp² carbon atoms distributed in a hexagonal honeycomb array [20]. Its thin–layered structure, combined with remarkably mechanical properties [21], thermal stability [22], and biocompatibility [23], makes this material an interesting option to tailor the surface of shape memory NiTi alloys. Chemical modification techniques, such as chemical vapor deposition (CVD), are among the most popular methods to produce graphene-based coatings, but the application of this technique to Ti-based substrates for biomedical use is still limited [24]. Zhang et al. [25] covered a NiTi alloy with graphene via chemical vapor deposition (CVD) and reported improvements in terms of biocompatibility and Ni²⁺ release prevention. Nevertheless, CVD is a high–temperature process that may exceed 1000 °C, jeopardizing structural features of the alloy and, consequently, damaging its shape memory effect. The deposition of GO on NiTi by electrophoretic deposition (EPD) was described by Zhang et al. [26] as an enhancement in terms of surface quality and corrosion protection over the CVD method. These findings are consistent with the recent results reported by Srimaneepong et al. [2], who studied the corrosion resistance and viability of human pulp fibroblasts of GO layers deposited on NiTi via EPD. On the other hand, the dip–coating technique has been proved to be another effective method to deposit graphene and its derivatives on metals under relatively low temperatures [27]. Dip-coating is a low-cost and easy method, presenting several advantages over other techniques, such as reproducibility, nonhazardous, and is suitable for large area deposition [28]. However, to the best of our knowledge, this process has not been yet investigated when applied to coat superelastic NiTi with graphene, GO, and rGO. Also, an investigation about the integrity of these coatings when the substrate is subjected to superelastic loading cycles is not cleared presented in the literature.

In the present work, we investigate the mechanical behavior and corrosion prevention capacity of two graphene–based materials [29] applied to the surface of a superelastic NiTi alloy by dip–coating: GO and rGO/SEBS block–copolymer nanocomposites. The use of SEBS reinforced with rGO is justified by the extensively reported biocompatibility of this elastomer [30], [31]. Uniaxial load cycles and scanning electron microscopy (SEM) were applied in order to evaluate the integrity of the coatings after some amount of superelastic work. The corrosion behavior was characterized by employing the following electrochemical techniques: potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis (MSA).