ICMCTF 2022 Session D1-2-MoA: Surface Coatings and Surface Modifications in Biological Environments II

Ti6Al4V is the most used alloy in orthopedic implants such as total hip, knee and shoulder replacements. Implants are conventionally made by cast or wrought alloys, but additively manufactured (AM) implants are increasingly used. Although Ti6Al4V is known for its great corrosion behavior, there are increasing reports of corrosion and fretting-corrosion related implant failures. Currently, it is unknown how alloy microstructure impacts the electrochemical behavior of Ti6Al4V, and its implication on in-vivo corrosion. Therefore, we tested six frequently occurring microstructure types occurring in Ti6Al4V implant components. Our hypothesis was that, despite identical chemical composition, differences in microstructural features can dictate the corrosion behavior of implant alloys. This study included three types of wrought alloys, one cast alloy, and two types of AM alloys: wrought alloys with A) fine equiaxed grains, B) coarse equiaxed grains and C) bimodal grain; D) lamellar dendritic (cast alloys); AM alloys with E) lath-type grains and F) needle-like grains. While A-E exhibited varying degrees of β phase within an α matrix, F exhibited a α’ martensitic structure. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were performed within simulated joint fluid (30 g/L protein) at pH 7.6 and 37°C to determine the corrosion behavior. We observed differences in corrosion current (icorr), polarization resistance (Rp) and capacitance (Q), but not in the corrosion potential (Ecorr). The needle-like group had the inferior corrosion behavior attributed to the metastable nature α’ and the presence of built defects (local crevice corrosion), followed by equiaxed coarse and lath-type groups attributed to the galvanic coupling between α and β phase, specifically when a difference in Ti and V content of >10%occurred between both phases. Therefore, the microstructure does influence the corrosion behavior of Ti6Al4V implants, however the distribution of alloying elements also played a role.

Ti6Al4V is a commonly used alloy for biomedical applications as it has excellent corrosion properties, which are mainly attributed to the oxide layer on the surface. Nevertheless, the early failure of total hip replacements has happened on Ti6Al4V alloys due to its poor tribocorrosion behavior. Previously, we have proved that carbide-derived carbon (CDC) can provide superb protection to Ti6Al4V from tribocorrosive damages¹. However, the basic corrosion behavior of CDC stills remains unknown. In this work, experiments were conducted to investigate CDC’s corrosion behaviors in comparison with the substrate alloy (Ti6Al4V).

Two groups of experiments were designed to evaluate CDC’s corrosion performances: (1) Ti6Al4V as the control group, and (2) the CDC. Each group was repeated three times (N=3) to confirm the reproducibility. For Group (1), Ti6Al4V discs (11 mm dia x 7 mm) were ground and polished to a mirror finish. In Group (2), CDC was fabricated by the electrolysis method². Furthermore, a three-electrode corrosion chamber was employed, where the tested sample was used as the working electrode, a graphite rod as the counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. With an aim at the application for hip implants, bovine calf serum (BCS) was selected as the electrolyte, with the temperature maintained at 37˚C. Finally, the electrochemical protocol for the Ti6Al4V was set as open-circuit potential 1 (OCP1) for system stabilization – potentiostatic (PS) for surface cleaning – OCP2 (stabilization) – electrochemical impedance spectroscopy (EIS) – Cyclic polarization. Same protocol without PS was followed for Group (2) as the CDC layer was coated on the surface. After the corrosion testing, JEOL JSM-IT500HR SEM with Oxford AZtec EDS and Bruker-Nano Contour GT-K Optical Profilometer were utilized to examine the sample surface.

The geometric sample area exposed to the solution is 0.1256 cm², which was used to calculate the current density and impedance for Ti6Al4V. However, instead of having a smooth surface like the polished Ti6Al4V, the prepared CDC has a rough and porous structure with a large surface area. Therefore, we estimated our CDC area exposed to the solution based on the Brunauer-Emmett-Teller (BET) surface area of Ti-CDC reported by Huang et al.³. Consequently, according to the potential dynamic and EIS results, the CDC shows higher corrosion resistance than Ti6Al4V. However, the actual surface area of our CDC products is still needed, which will be achieved in the upcoming studies.

1. Cheng, K Surf. Coat. Technol.2020
2. Sun, Y Surf. Coat. Technol.2021
3. Huang, P Science.2015