Corrosion behavior and biocompatibility of additively manufactured 316L stainless steel in a physiological environment: the effect of citrate ions

In order to mitigate potential implant failures, it is essential to determine the corrosion behavior of biomaterials in a realistic physiological environment. In order to simulate the real oxidative nature of human body fluid, this research considers the effects of a complexing agent while determining the corrosion behavior of 316L stainless steel (SS) that has been fabricated by Selective Laser Melting (SLM) process. The results show that the complexing agent, i.e. the citrate ion, in Phosphate Buffer Saline (PBS) solution strongly affects the passivation behavior of 316L SS by complex species formation. However, due to a rapid solidification process, the microstructural properties of the additively manufactured metal are not similar to that of the conventionally manufactured counterpart. The microstructure of the SLM 316L SS contains refined sub-grains within each coarse grain and the formation of micro-inclusions i.e. MnS is restricted. The SLM 316L SS had better pitting resistance and passive film stability. E_corr for the SLM 316L SS was consistently higher and the breakdown potential, E_bd, was more than three times higher compared to the wrought counterpart as determined by cyclic potentiodynamic polarization. Moreover, the SLM sample had a wider passive region and higher charge transfer resistance (R_t) (approximately 1.5 to 2.5 times) as determined by cyclic voltammetry and electrochemical impedance spectroscopy, respectively. In addition, the attachment and proliferation tendency of MC3T3-E1 pre-osteoblast cells were studied to evaluate biocompatibility. The SLM part had better cell proliferation. To summarize, in a physiological environment, the SLM 316L SS outperformed the conventional wrought 316L SS in terms of corrosion resistance and biocompatibility.