An Electrochemical and Tribocorrosion Study on Austenitic High Nitrogen Steel in Orthopedic Applications

Introduction: Arthritis or rheumatism is the leading cause of disability. Majority of the patients, who goes through this disability, finds their solution by doing Total Hip Replacement (THR) surgery. More than 285,000 hip replacements are performed in the U.S. each year. It is estimated that by 2030 there will be an increment of 175% in the THR surgeries rising the numbers to 0.5 Million. Thus, these numbers portray the importance of hip replacement surgeries in the near future. Recently, there is a growing concern on the metal-on-metal (MoM) hip implants among the orthopedic clinicians and researchers. This is partially due to wear and corrosion behavior of the metals used for such implants, particularly their synergistic interactions lead to early failure and release of the metal ions to the host body. Majority of the hip implants are made of cobalt-chromium-molybdenum alloy (CoCrMo alloy), Ti6Al4V (Ti alloy) and Stainless Steel (316L). In the body, these implant metals are exposed to extremely complex and variable conditions, which can lead to degradation of the material and subsequent adverse biological reactions that has led to rise of many diseases and infections in the human body. We hypothesized that Austenitic High Nitrogen Steel (AHNS) will have better electrochemical and tribocorrosion behavior than the other commonly used implant metals CoCrMo, Ti6Al4V and stainless steel (SS). AHNS will prove that it has strong wear and corrosion properties. Hence this study has two objectives: 1. To study the electrochemical characteristics of AHNS and compare with other commonly used implant alloys, CoCrMo, Ti6Al4V and stainless steel (SS). 2. To study the tribocorrosion behavior under tribocorrosion behavior under Potentiodynamic, Free Potential and Potentiostatic conditions in stimulated biological environment in order to verify the improvement in corrosion kinetics (from electrochemical test) under combined exposure to wear and corrosive joint environment of AHNS with CoCrMo alloy. Methods: The samples (n=3) of AHNS, CoCrMo alloy, Ti6Al4V alloy, stainless steel (control) were used. Electrochemical testing: The electrochemical tests were conducted on samples with specially made corrosion cell and a Gamry made potentiostat (G700) was used and test protocol was based on the ASTM standard G61. Tribocorrosion: Tribocorrosion test assist to study corrosive behavior of the metal samples under combined exposure to wear and corrosion. In this study, the pins of AHNS, CoCrMo alloy and Ti6Al4V were undergone tribocorrosion tests- potentiodynamic, free potential and potentiostatic mode. The electrolyte was the same type used in corrosion test (BCS). Results and Discussion: The findings of this investigation validate our hypothesis and suggest that AHNS shows superior corrosion behavior compared to other traditional alloys in particular to CoCrMo. Electrochemical results exhibited the enhanced corrosion kinetics leading to a stable passive layer formation and better corrosion resistance of AHNS compared to CoCrMo alloy and Ti alloy. Passivation and repassivation kinetics under anodic conditions, demonstrate the satisfied tends in the overall corrosion behavior. AHNS portrayed more noble potential characteristics than other alloys. Compared to CoCrMo alloy, AHNS showed low corrosion current; however it is higher than Ti alloy showing the qualitative ranking of the stability of passive films. Under tribocorrosion exposure, AHNS, exhibit an exceptional behavior of minimal shift of the potentiodynamic curve to the high current region. Compare to other alloys, AHNS exhibits better tribocorrosion behavior under potentiostatic, free potential and potentiodynamic modes. AHNS displayed more stable current conditions during mechanical stimulation as compared to CoCrMo. This is partly due to AHNS having superior corrosion kinetics compared to other alloys. The surface of AHNS shows better scar properties compared to CoCrMo, and thus this is due to low wear loss during tribocorrosion study. AHNS key factor is its adherent supply of nitrogen which helps in preserving the characteristics of the metal alloy and reduces the wear debris in the corrosion kinetics. The addition of nitrogen has been reported to considerably increase the passive film stability and resistance toward corrosion. By considering the toxicity of metals for clinical study, AHNS study shows that it had less Chromium compared to CoCrMo, since CoCrMo is known for high amount of chromium in it. The AHNS displayed high levels of Iron (Fe) and Manganese (Mn) compared to traditional alloys which are essential for strength and ductility of an implant. In general, AHNS portrayed superior electrochemical and tribocorrosion behavior compared to other traditional alloys. The Nickel-free AHNS could be considered as a potential implant metal for orthopedic applications. Its superior corrosion and wear characteristics could assist in solving current concerns of the metal based orthopedic implants. AHNS has also proven to be a biocompatible metal and has shown superior properties for hard tissue implants. Current study will be extended to identify any other limitation of AHNS before considering an alternative implant metal, particularly in hip prosthesis application.