Annealing-Induced Modifications in the Electrochemical Response of Sputtered Bi₂O₃ Films on 316L Stainless Steel

The application of metallic thin films on 316L stainless steel (SS) bio-implants has expanded substantial consideration in the biomedical field owing to their potential to improve corrosion resistance, biocompatibility, and long-term durability in physiological environments. In the present study, Bi₂O₃ films were coated on 316L SS specimens using DC sputtering and thermally treated at different temperatures (200°C, 400°C, and 600°C) to investigate their structural, morphological, and electrochemical properties in simulated body fluid (SBF). Surface profilometry and scanning electron microscopic analysis revealed that annealing promotes grain growth and morphological densification, with higher temperatures increasing surface roughness due to enhanced diffusion and phase transformations. Energy dispersive X-ray analysis confirmed compositional stability post-annealing, while X-ray diffraction analysis demonstrated a progressive improvement in crystallinity and phase purity, with grain size increasing from 11.3 nm (as-deposited) to 24.8 nm (600°C-treated), confirming the evolution from amorphous/nanocrystalline to well-defined monoclinic α-Bi₂O₃. In vitro electrochemical studies revealed a strong correlation between annealing temperature and corrosion resistance, with 600°C-treated films (BO6T) exhibiting optimal performance by showing highest charge transfer resistance (R_ct = 7503.9 kΩ cm²), and lowest corrosion current density (0.0006 µA/cm²). These results highlight that thermal treatment enhances both structural integrity and electrochemical stability, making high-temperature annealed Bi₂O₃ films promising for biomedical applications requiring noble corrosion resistance.