Corrosion Risk Analysis of CoCrMo Alloy as a Function of Microstructure: Biomedical Applications

CoCrMo alloys, widely used in orthopedic implants and various biomedical applications, exhibit excellent corrosion resistance and mechanical properties. However, specific microstructures, such as banded structures, or conditions like exposure to infectious environments can lead to localized corrosion and metal ion release. This localized corrosion raises concerns about potential adverse effects, despite the alloy’s overall corrosion resistance. While the electrochemical nature of this alloy is well studied, the microstructure’s effect needs further research. Therefore, this study aims to evaluate CoCrMo alloys’ corrosion behavior in two microstructures: homogeneous (transverse) and banded (longitudinal) under simulated normal, infectious, and inflammatory conditions. CoCrMo disks were prepared and polished following the standard metallographic protocol for a surface finish of <50 nm. The homogeneous CoCrMo rod was cut perpendicular to the axis, while the banded CoCrMo rod was cut parallel to the axis. Bovine calf serum (30 g/L) with a pH of 7.6 was used as the electrolyte to simulate normal physiological conditions present at the joint. To simulate infectious conditions, lipopolysaccharide (LPS) (15 µg/L) was added to BCS, while 0.5 mM hydrogen peroxide was added to BCS to simulate inflammation. The electrochemical test followed the ASTM G61 standard using a three-electrode system and a standard electrochemical protocol. Using Tafel’s estimation, the corrosion potential and corrosion current were determined. Electrochemical impedance spectroscopy (EIS) data was utilized to generate Bode and Nyquist plots and construct an equivalent electric circuit to determine polarization resistance (Rp) and double-layer capacitance (Cdl). The corroded surfaces were characterized by white light profilometry, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Our study showed that the CoCrMo specimens with homogeneous microstructures exhibited increased corrosion resistance compared to banded microstructures in all three conditions. The EIS data supported this observation, revealing higher Rp and lower capacitance for the homogeneous structure. SEM observations revealed larger pitting in the banded microstructure compared to homogeneous. The banded microstructure, with increased grain boundary exposure, heightens the risk of intergranular and galvanic corrosion. Further exploration is needed to understand microstructural mechanisms and develop strategies to inhibit increased corrosion risk. Our investigation emphasizes the vital role of material composition and configuration in microstructural and corrosion behavior.