The Electrochemical Corrosion Behavior of AISI 316L, Co 28Cr-6Mo, and Ti-6Al-4V Alloys in Presence of Bovine Serum Albumin in Phosphate Buffered Saline Solutions

Metallic implants form a passive layer on the surface which significantly reduces their corrosion rate. However, the presence of dissolved oxygen, chloride, phosphate, inorganic and organic molecules, etc in the body fluid influences the corrosion behaviour of bio-implants. One of the organic molecules in the body fluid is protein that has various concentrations. In this study, the influence of protein concentration on the electrochemical behaviour of AISI 316L, wrought Co-28Cr-6Mo and Ti-6Al-4V was investigated.

 Open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS)  and linear polarization resistance were conducted in aerated solutions of phosphate buffered saline (PBS) at various concentrations of Bovine Serum Albumin (BSA) at 37°C. The experimental results of OCP measurements indicated that Ti–6Al–4V requires longer time to reach a steady-state potential compared to 316L and Co-28Cr-6Mo alloys. The cathodic polarization branch of 316L samples shifted to a lower potential value when BSA was added to the solution. This can be explained by formation of the absorbed film which restricted the oxygen diffusion from the surface by decreasing the number of active sites. The cathodic and anodic branches of Co-28Cr-6Mo alloy moved to lower potentials when the BSA concentration was higher than 0.4 g L^-1. The current density of Ti-6Al-4V alloy in the passive region decreased when the BSA concentration increased from 0.2 to 4 g L^-1. A breakdown potential was not detected for Ti-6Al-4V up to 1 V vs. Ag/AgCl.

 The impedance of 316L, Co-28Cr-6Mo, and Ti-6Al-4V alloys can be characterized by large semicircle capacitive loops with one time constant. Therefore, a simple Randle equivalent circuit (R_S (R_p Q)) was used to obtain electrical equivalent circuit parameters. The obtained polarization resistance from the impedance data along with Tafel extrapolation and linear polarization resistance techniques were used to calculate the corrosion rates of bio-implants at various BSA concentrations in PBS solution. Ti-6Al-4V alloy has the lowest corrosion rates compared to 316L and Co-28Cr-6Mo. The differences of the corrosion rates obtained by three methods were less than a decade. In addition, the presence of BSA did not make a significant change in the corrosion rate of the metallic samples.

 For future work, the electrochemical behaviour of these metallic samples in de-aerated conditions will be investigated using the same techniques. Immersion testing will be used to detect ion release from the samples in different BSA concentrations. The composition of the oxide layer of the samples at different potentials will be investigated by the X-ray photoelectron spectroscopy (XPS) to study the contribution of BSA to oxide layer formation.