Surface Coating Effect on Corrosion Resistance of Titanium Alloy Bone Implants by Anodizing Method

A bone implant is a medical device used to strengthen the existing bone structure or supports an injured bone structure. In this case, approximately 90% of implants in Indonesia imports. Therefore, given the rising demand for implants, the development of bone implants is a crucial debate issue. Based on the biomedical viewpoint, implant stability and the osseointegration process, which has the potential for rehabilitation, are the most important internal factors in the implantation of medical devices. Therefore, it is essential to create bone implants that have high-quality and effective when placed within the body (Dewi et al., 2020; Genisa et al., 2020; Izmin et al., 2020). Orthopedic implants are now made from various materials, including polymers, ceramics, metals, and composites. The majority of metals are bio tolerant, although titanium and its alloys have a bioinert nature under specific circumstances (Koju, Niraula and Fotovvati, 2022).
       Titanium Ti-6Al-4V ELI is a commonly used implant material in the medical field due to its mechanical and corrosion resistance properties  (Szymczyk-Ziolkowska et al., 2022; Jaafar et al., 2020; Atmani et al., 2018; Karambakhsh et al., 2011). This corrosion resistance property emerges due to the formation of a natural oxide layer, which mainly contains TiO₂ on the titanium surface when it contacts the air (Lestari et al., 2020). In addition, the Ti-6Al-4V ELI alloy material is the most commonly used material for implants in orthopedics (Kashyap, Rashid, and Khanna, 2022; Swain et al., 2021; Szymczyk-Ziolkowska et al., 2021; Gabor et al., 2020; Kiel-Jamrozik et al., 2015). Furthermore, to its advantages, this material has several weaknesses, including the thin natural oxide layer on titanium causing implant wornness, low bone bonding capacity, and incapability to prevent metal ions exposure due to implant degradation. Besides, titanium alloys also have poor osseointegration in long-term implantation (Lestari et al., 2020).
      Implant surface modification is necessary to obtain good implant properties and performance for the body. This surface modification is important to increase surface energy by providing surface roughness and chemical composition. This will increase tissue adhesion and implant integration as well as reduce bacterial reactions and inflammatory responses in the body (Rani and Jatolia, 2018). In this case, one of the methods that can be employed to improve implant performance is an anodizing method. Anodizing method is metal oxidation carried out in certain electrolytes by producing an electric field at the metal/electrolyte interface (Izmir and Ercan, 2019). This method is the easiest method to modify the thickness, composition, and morphology of the metal oxide layer. Moreover, the anodization process is used to restrict the infiltration process (Kiel-Jamrozik et al., 2015). A thin oxide layer grows on the implant’s surface during this process. Properties of the layer depend on the electrolyte, production method, oxidation time, and electric parameters of the process (Kiel-Jamrozik et al., 2015; Szewczenko et al., 2015; Ziebowicz, Ziebowicz and Baczkowski, 2015; Al-Mobarak and Al-Swayih, 2014; Indira, Mudali and Rajendran, 2013; Fadl-allah, Quahtany and El-Shenawy, 2013; Bhola et al., 2011; Szewczenko et al., 2010; Narayanan and Seshadri, 2007; Nagy et al., 2005; Van-Gils et al., 2004; Roessler et al., 2002).
       According to prior research, some inorganic ions, such as molybdate and metavanadate, can passivate titanium in solutions of sulfuric and hydrochloric acids (Mogoda, Ahmad, and Badawy, 2004). Anodizing has also been performed in phosphoric acid solutions under different conditions to improve the corrosion resistance of Ti–6Al–4V alloy in the simulated physiological environment such as (Karambakhsh, Afshar, and Malekinejad, 2012) conducted anodizing on the Ti-6Al-4V alloy in phosphoric acid electrolyte and evaluated its corrosion resistance in 3 different solutions (Ringer's solution, artificial saliva solution, and a mixture of Ringer's solution + H₂O₂). The research further resulted that the higher the voltages, the higher the corrosion resistance obtained due to the passivity produced by the anodizing process. Furthermore, Martinez, Flamini, and Saidman (2022) studied the impact of inhibitor anions on the alloy corroded in Ringer solution. In this case, galvanostatic anodization of Ti–6Al–4V alloy with inorganic inhibitors such as Na₂MoO₄, NaH₂PO₄, and NH₄VO₃ solutions produced colorful thin oxide layers. Their findings demonstrated that, regardless of the solution employed, compact, amorphous oxides free of pores or fractures may be produced. The alloy anodized in Na₂MoO₄ solution had the lowest corrosion current density. In addition, no signs of corrosion or fractures were seen (Martinez, Flamini and Saidman, 2022).
      (Tamilselvi, Raman and Rajendran, 2006) also evaluated the corrosion of Ti-6Al-4V ELI alloy using the electrochemical impedance spectroscopy method in a Simulated Body Fluid (SBF) solution. This solution was used to simulate the physiological conditions in the body. In addition, to develop the color chart of the Ti-6Al-4V alloy, anodizing was also carried out in trisodium phosphate electrolyte (Wadhwani et al., 2018).
     Research concerning the anodizing process of Ti-6Al-4V alloy has been numerously conducted; however, it is only limited to the use of corrosion resistance test solution, causing its inability to simulate the body's physiological condition. Therefore, the current research used SBF solution. The effect of various voltage on the implant color visual, the mass of the oxide layer formed, oxide film thickness, and implant corrosion resistance were investigated in this study. Moreover, the correlation between film thickness and its corrosion resistance was also elucidated. In addition, the trisodium phosphate electrolyte solution was chosen because it provides better corrosion resistance compared to the acidic electrolyte solution (Karambakhsh et al., 2015).