The Ti6Al4V alloy is one of the most frequently used Ti alloys for biomedical applications, such as orthopedic or dental implants due to its excellent mechanical properties, biocompatibility, and high corrosion resistance [1-4]. For the long-term use of metal implants made from Ti6Al4V, it is crucial to enhance osseointegration between the implant and human bone. To improve osseointegration, hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂), a bioceramic material with a chemical composition similar to the human skeletal system and low cytotoxicity, is used [5-7]. HAp, in its nanoscale powder form, has a high specific surface area, which enhances biocompatibility and bioactivity in the body. It forms an apatite layer similar to bone on the surface after implantation, improving the bond between the implant and the bone [8,9]. However, when exposed to a humid environment, its mechanical properties deteriorate, which presents a limitation for its use in low-load bearing applications [10]. While HAp has traditionally been synthesized artificially, some reports suggest that using natural HAp-containing eco-friendly biological waste, such as equine bone (EB), can aid in bone growth and improve osseointegration [11,12]. EB, in particular, offers the significant advantage of being free from foot-and-mouth disease compared to bovine or porcine bones. Moreover, the increasing number of slaughtered horses has resulted in a higher supply and lower costs, making it a promising material for biomaterial research.

It is known that the HAp coating process performed at high temperatures using conventional plasma spray methods results in phase decomposition and the formation of an amorphous phase, leading to low crystallinity. This problem accelerates the delamination rate of the coating, thereby shortening the lifespan of the implant [5,13]. To address this issue, powder metallurgy (PM) was used to add EB to Ti6Al4V, resulting in the production of a Ti6Al4V-EB composite powder.

To be used as a biomedical material, the ability to reproduce custom parts is essential. Therefore, the Ti6Al4V-EB composite was manufactured using selective laser melting (SLM), a process within the field of additive manufacturing (AM) that utilizes a composite powder. SLM selectively melts a laser beam at desired locations based on 3D designs, layering one layer at a time to reproduce even complex internal structures [14]. This advantage allows for the application of various materials and enables the efficient production of products by reducing costs and material waste, making it suitable for biomedical applications [15]. However, during SLM manufacturing, locally high thermal gradients can lead to the formation of a a'-Martensite phase (hexagonal closed-packed), resulting in high strength but low ductility, which poses challenges for biomedical use. To overcome this issue, heat treatment was performed after SLM. The heat treatment temperature was based on the β phase transition temperature, as the strength and ductility vary with the β phase content [14,16].

As the exposure time of metallic implants in the human body increases, the implant may degrade through ion release into surrounding tissues or organs via body fluids, potentially leading to reduced osseointegration and bone conductivity, and eventually cause the implant to peel off, which can have significant long-term health impacts [17]. Human body fluids are approximately pH 7.4, exhibiting slightly alkaline properties. Thus, metallic implants should have minimal ion release even after prolonged use and exhibit excellent corrosion resistance in body fluid environments. Currently, there is a lack of research on the surface corrosion behavior of metal implants with added EB in body fluid conditions [18-20]. Therefore, to investigate the corrosion behavior over time of Ti6Al4V-EB composites produced using SLM, a potentiodynamic polarization test was conducted using a 0.9% NaCl solution, similar to the human body fluid environment.

Ti6Al4V forms a very thin and dense oxide layer (TiO₂) on its surface, which can reform after damage by reacting with oxygen. This property contributes to the excellent corrosion resistance of Ti6Al4V [16]. Fig 2 shows the corrosion behavior in the potentiodynamic polarization test with the addition of EB and various heat treatment temperatures.

The results, including the corrosion current density (I_corr) and corrosion potential (E_corr), are presented in Table 1.

Ti6Al4V exhibits the most stable corrosion current density, although metastable pitting is observed.

However, with the addition of EB, the occurrence of metastable pitting decreases or is not observed. The Ti6Al4V-EB heat-treated at 1020 °C exhibited the highest corrosion current density at 1.74×10^-7 A/cm² and the lowest corrosion potential at -0.46 V_SCE. Despite these values, metastable pitting was not observed. This suggests that the addition of EB, uniformly distributed on the surface, reduces the specific surface area, thereby mitigating the rate of ion diffusion and providing higher corrosion resistance compared to Ti6Al4V [21-24].

The Ti6Al4V-EB heat-treated at 940 °C demonstrate the most stable corrosion behavior, with a corrosion current density of 5.68×10^-8 A/cm² and a corrosion potential of - 0.39 V_SCE. According to the report by Choubey, A, et al., Ti6Al4V manufactured using the SLM process showed improved corrosion resistance when there was a lower amount of metastable α' phase and a higher content of β phase[25]. In previous studies on the effect of heat treatment temperature on the β phase content, it was found that the content was 3.5% at 940 °C, 8.2% at 1020 °C, and only 1.2% without heat treatment [26].

Therefore, even though the Ti6Al4V-EB heat-treated at 1020 °C had the highest β phase content, the Ti6Al4V-EB heat-treated at 940 °C exhibited higher corrosion resistance. This is due to the formation of the metastable α' phase during the 1020 °C heat treatment, resulting in the presence of three phases: α, β, and α'. The formation of a galvanic cell driven by the potential difference is further promoted in this case, compared to only two phases, making the interface more sensitive to corrosion. As the interface becomes more sensitive to corrosion, it result in lower corrosion resistance [1,27-29].

Fig 3 presents the results of the potentiodynamic polarization tests conducted after 24 and 48 hours of immersion to observe the corrosion behavior of the Ti6Al4V-EB composite with different EB additions and heat treatment temperatures when inserted into the human body. The I_corr and E_corr values are provided in Tables 2, respectively.

The results showed similar trends for different immersion durations. For the as-built Ti6Al4V-EB and Ti6Al4V-EB heat-treated at 1020 °C, there was a significant increase and decrease in corrosion current density in the range of -0.3 to 0.8 V_SCE (24 hours) and above 1.0 V_SCE (48 hours immersion), indicating oxide layer breakdown and repassivation.

Furthermore, the as-built Ti6Al4V-EB after 24 hours of immersion and the Ti6Al4V after 48 hours exhibited a high frequency of metastable pitting, likely due to an unstable oxide layer formation on the surface[28,30]. After immersion, the Ti6Al4V-EB heat-treated at 1020 °C showed fluctuations in current density due to the potential difference caused by the formation of the metastable α' phase, as discussed in Fig 2. However, it returned to stable behavior due to re-passivation. Lastly, the Ti6Al4V-EB heat-treated at 940 °C showed the most stable corrosion behavior without metastable pitting, even after increased immersion time, and did not form a metastable α' phase that could create potential differences. This indicates superior corrosion resistance. Additionally, the hardness before and after corrosion can increase due to the oxide layer formed on the surface [31].

Fig 4 shows SEM images of the surface of samples, observed before and after immersion in simulated human body fluid, following the potentiodynamic polarization tests. Before immersion, the surface of the samples generally maintained a smooth appearance with almost no pores. However, after immersion, pores appeared, and both the number and size of these pores increased as the immersion time extended from 24 to 48 hours.

The size of the pores increased similarly, ranging from approximately double to up to six times the initial size, which can promote the occurrence of pitting corrosion [28,32]. Although Ti6Al4V exhibited the fewest pores, metastable pitting was observed, similar to samples with added EB, indicating that it reacts more sensitively to corrosion behavior.

Fig 5 and 6 show images of the oxide layer thickness analyzed by EDS mapping, depending on the immersion time. The elements identified through EDS mapping are Ti, Al, V, and O.

The regions with a high density of O indicate the distribution of the oxide layer, and it can be seen that the thickness of this layer increased by approximately 1.58 to 4.1 times with the addition of EB. Additionally, the thickness of the oxide layer increased to a maximum of 13.1 μm after 24 hours and up to 9.6 μm after 48 hours of heat treatment, with a uniformly formed oxide layer. The reduction in oxide layer thickness, despite the increased immersion time, suggests that the formed oxide layer dissolves with prolonged immersion, leading to a decrease in thickness [33]. The change in thickness due to EB addition and heat treatment can be explained by the oxidation reaction rate at high temperatures. As the diffusion rate of O atoms and Ti ions increases at high temperatures, the rate of oxide formation accelerates, resulting in reduced space for the oxides to disperse and a denser oxide layer filling the gaps, which slows down the oxidation rate and thickens the layer uniformly [34]. The addition of EB causes uniform distribution on the surface, forming a thicker and denser oxide layer compared to Ti6Al4V, which likely enhances corrosion resistance by blocking ion diffusion [22,26,32,35-36].

This study utilized powder metallurgy to manufacture Ti6Al4V-EB nanocomposites by adding 0.05 wt% EB to Ti6Al4V, followed by production via selective laser melting (SLM). To evaluate its corrosion behavior when implanted in the human body, a potentiodynamic polarization test was conducted in a 0.9% NaCl solution, and the results are as follows. Additionally, if these results are considered for the development of implant materials, it is expected that they could be applicable in clinical trials and potentially used as cost-effective graft materials;

1) During the potentiodynamic polarization test, metastable pitting was observed in Ti6Al4V; however, it was reduced or not observed with the addition of EB. This can be attributed to the uniform dispersion of EB on the surface, which alleviates the ion diffusion rate.

2) The corrosion behavior of Ti6Al4V-EB is influenced by the potential difference between the metastable a' phase and β phase, which promotes the formation of galvanic cells, making the interface sensitive to corrosion and resulting in lower corrosion resistance. Consequently, Ti6Al4V-EB heat-treated at 940 °C, which does not form the metastable a' phase and does not exhibit metastable pitting, showed the most stable corrosion behavior.

3) After performing a potentiodynamic polarization test and observing the surface through SEM, it was found that the initially smooth surface before immersion exhibited an increase in the number and size of pores by approximately 2 to 6 times as the immersion time increased. This indicates that the oxidized areas expanded with prolonged immersion time.

4) The thickness of the oxide layer was found to be the greatest after EB addition and heat treatment at 940 °C and 1020 °C. This is because the oxidation reaction rate increases at high temperatures, leading to the formation of a large number of oxides, which improves the thickness of the oxide layer as the particles are dispersed and densely fill the matrix. Additionally, the reduction in oxide layer thickness over time can be attributed to the dissolution of the oxide layer due to prolonged immersion, resulting in decreased density.