## 3 Results and Discussion
This surface morphology may be due to the competitive action of Cl\({}^{-}\) and NO\({}^{3-}\) ions, where the former tends to cause localized corrosion while the latter promotes uniform oxidation. Although this surface might not be as conducive to cell adhesion as the porous structures, it may exhibit good corrosion resistance. The 0.5 M HF/0.5 M HNO\({}_{3}\) mixed electrolyte treatment (Figure 9d) resulted in the most pronounced surface alteration, forming etch pits with significant depth and width. These pits present irregular shapes and interconnect, creating a complex three-dimensional network structure. This structure likely results from the interaction between the strong corrosive action of HF and the oxidizing effect of HNO\({}_{3}\). While this highly rough surface may provide an ideal environment for cell attachment and growth, it might also affect the material's mechanical properties. Comparing the results of the four mixed electrolyte treatments, we can observe that the electrolyte combination significantly influences the final surface morphology. The NaF/H\({}_{2}\)SO\({}_{4}\) and NaF/HNO\({}_{3}\) combinations produced relatively uniform porous structures, which may be more suitable for applications requiring large surface areas. The HCl/HNO\({}_{3}\) combination, resulting in a relatively smooth surface, might be more appropriate for scenarios where maintaining material integrity is crucial. The HF/HNO  
Figure 8: Raman spectra of Ti-6 Al-4V surfaces at **(a)** short wavenumber range (250–700 cm\({}^{-1}\)) and **(b)** longer wavenumber range (1000–2000 cm\({}^{-1}\)) under various treatment conditions. combination, producing a highly rough structure, may be most conducive to cell adhesion and osseointegration, but its potential impact on material strength should be considered in applications. Figure 10 presents the Raman spectra of the Ti-64 alloy after electrochemical corrosion treatments with various mixed acid electrolytes. These spectra reveal significant differences in the surface oxide layer structures, reflecting the unique effects of different electrolyte combinations on material surface modification. In the 250-700 cm\({}^{-1}\) range (Figure 10a), the spectral features produced by various treatment methods differ markedly, suggesting diversity in the surface oxide layer structures. The sample treated with 0.5 M HF/HNO\({}_{3}\) exhibits the strongest spectral response, particularly a broad peak at approximately 450 cm\({}^{-1}\), typically associated with the Eg mode of the anatase phase. The sample treated with 0.5 M NaF/H\({}_{2}\)SO\({}_{4}\) shows distinct peaks at about 520 cm\({}^{-1}\) and 640 cm\({}^{-1}\), potentially corresponding to the A1g and Eg modes of the rutile phase. In contrast, samples treated with 0.5 M HCl/HNO\({}_{3}\) and 0.5 M NaF/HNO\({}_{3}\) display weaker spectral features, indicating the possible formation of thinner or more inhomogeneous oxide layers. In the 1000-2000 cm\({}^{-1}\) range (Figure 10b), all samples exhibit broad features, but with varying intensities and shapes. Features in this range are typically related to the second-order scattering of TiO\({}_{2}\) or surface defects. The sample treated with 0.5 M HF/HNO\({}_{3}\) shows the strongest spectral response in this range, particularly a broad peak between 1300 and 1600 cm\({}^{-1}\), which may be associated with oxygen vacancies in non-stoichiometric TiO\({}_{2}\). Other mixed acid-treated samples also show similar trends but with lower intensities, indicating differences in surface defect structures. Comprehensive analysis suggests that the composition of mixed acids significantly influences the structure and composition of the oxide layer on the Ti-64 surface. The HF/HNO\({}_{3}\) mixture appears to induce the most pronounced surface oxidation, potentially forming a mixed structure of anatase and amorphous TiO\({}_{2}\). This structure may be beneficial for bioactivity, explaining the previously observed favorable cell response.