Effect of pre-annealing temperature on the microstructure and corrosion behavior of Ni88.6-Cr11.4 alloy for biomedical applications

Abstract

Nickel–chromium (Ni–Cr) alloys offer high strength, wear resistance, shape-memory effect, and broad clinical applications. This study evaluates the effect of pre-annealing on their electrochemical corrosion behavior. Samples were annealed at 500 °C and 700 °C and compared with a non-annealed reference. Microstructure and composition were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), while corrosion behavior was examined by open-circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). Annealing at 500 °C and 700 °C resulted in lattice expansion (from 3.551 Å to 3.561 Å) and a reduction in crystallite size (from 9.40 nm to 8.10 nm), accompanied by chemical inhomogeneity leading to degradation of the passive oxide layer. These changes accelerated corrosion: compared to the non-annealed alloy (0.0125 mm/year), the rate increased to 0.0356 mm/year at 500 °C and 0.313 mm/year at 700 °C. Concurrently, passive current density (I_pass) doubled from 20 to 40 \(\mu \)A cm⁻², while the Pitting potential (E_pirs) shifted from +0.290 mV to –0.287 mV, indicating weaker passivation. EIS confirmed declining charge transfer resistance with temperature. Post-corrosion surface analysis confirmed these findings: SEM revealed increased roughness and defects, while EDX detected reduced oxygen content, consistent with thinning of the protective oxide film after annealing. Thus, high-temperature pre-annealing, therefore, markedly degrades corrosion resistance, underscoring the need for optimized heat treatment in dental applications.