The addition of Si to the Ti–35Nb alloy and its effect on the corrosion resistance, when applied to biomedical materials
Introduction
The increase on the population’s life expectancy as well as the high number of victims from traffic accidents and degenerative diseases are leading to an increasingly demand for new orthopedic implant materials [1]. Around 70–80% of such implants are made out of metallic biomaterials that are an alternative to damaged bone structures’ rebuilding. They help improving patients’ quality of life [2], [3]. Such materials must meet a series of requirements related to biocompatibility, corrosion resistance, low elastic modulus values and mechanical resistance [1], [4], [5].
Amid the conventional biomaterials used in implants, there are the stainless steel 316 L, Co–Cr–Mo based alloys as well as titanium (Ti) and its alloys [1], [5]. However, throughout the years, titanium alloys have been highlighted, once – depending on their elements – they can show a better set of properties that are desirable for orthopedic applications [2], [6]. Nowadays, the most used Ti alloy is the Ti–6Al–4V. Although, studies report that the liberations of small amounts of alloy elements can cause cytotoxic effects to the human body and lead to adverse reactions when it presents elastic modulus incompatible to that from the bone. It might cause bad stress distribution between the implant and the bone, causing bone resorption and premature fails in the implant [7], [8], [9].
As a result, recent researches are focused on studying β titanium alloys that comprise biocompatible elements able to favor such alloys’ elastic modulus reduction in order to minimize bone resorption [3]. Based on such scenario niobium (Nb) and silicon (Si) addition to titanium alloys is becoming frequent. Nb presents high biocompatibility as well as high β-stabilizer effect, helping to reduce the alloy’s elastic modulus [10], [11]. More recently, Si has been added to Ti alloys as a way to improve their mechanical properties. Some studies found in the literature show the feasibility of adding Si to Ti–Nb alloys, once it enables the development of materials with reduced elastic modulus and high mechanical resistance [12], [13], [14], [15]. However, it is worth understanding that such metallic alloys also present high corrosion resistance.
One knows that metallic materials used for orthopedic applications are, in general, subjected to corrosive processes when implanted in human bodies because, once there, they get in touch with the physiological medium which presents a complex composition, comprising organic acids, amino acids, proteins and many inorganic ions such as chloride [4], [16]. These ions are highly aggressive and can react with the implanted material and jeopardize the performance of it, changing its mechanical properties and biocompatibility, thus decreasing the prosthesis’ lifetime.
Therefore, the behavior of these new materials, in terms of corrosion, still demands careful investigation, since they will be subjected to aggressive environs such as the human body. The current study aimed to investigate the influence of Si addition over Ti–35Nb alloy’s corrosion resistance, when placed in physiological medium. Polarization curves and electrochemical impedance spectroscopy were performed to measure such influence.
Section snippets
Experiment
Ti–Nb–xSi (x = 0; 0.15; 0.35; 0.55) (% in mass) alloys were prepared with a fusion of Ti, Nb and Si – commercially pure elements – it was done using a furnace arc with tungsten non consuming electrodes under argon atmosphere as well as a water-cooled copper hearth. The ingots were re-casted eight times in order to ensure the elements’ complete fusion. After fusion, the samples were subjected to compositional homogenization thermal treatment in a quartz tubular furnace under argon atmosphere at
Results and discussions
The anodic and cathodic polarization curves from Ti–35Nb–xSi (x = 0; 0.15; 0.35; 0.55) alloys are shown in Fig. 1. Based on such curves it is possible observing the behavior of these materials regarding their corrosion resistance. It is observed that the curves’ profile describes a typical behavior from metals that suffer passivation when a certain level of electric current is observed due to the formation of a protecting oxide film over the metals’ surface which inhibits the corrosion process’
Conclusions
The polarization assays enabled observing that there is a displacement tendency in the corrosion potential for more noble potential values in the Ti–35Nb–0.15Si and Ti–35Nb–0.35Si alloys and for the less noble ones in the Ti–35Nb–0.55Si alloy. It was also noticed that the Ti–35Nb–0.55Si alloy passivates itself in longer currents, indicating that the material faces a decrease in corrosion resistance when compared to others. Such behavior was assigned to the microstructure presented by the
Acknowledgment
Thanks to CAPES by the financial support.
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