Corrosion behaviour of medical CoCr alloy after nitrogen plasma immersion ion implantation

Surface treatment of medical CoCr alloys L605 by nitrogen plasma immersion ion implantation (PIII) leads to the formation of a hard and wear resistant surface layer, consisting of nitrogen in solid solution. However, a detailed investigation of the corrosion properties by potentiodynamic polarization and electrochemical impedance spectroscopy shows that even at processing temperatures of 350 °C, where no CrN precipitates are observed, no complete passivation of the surface by formation of a protective Cr2O3 layer is possible leading to enhanced corrosion rates further increasing with increasing PIII processing temperature. It is postulated that the enhanced affinity of chromium for nitrogen leads to a reduced mobility inside the alloy, thus prohibiting a timely surface passivation. Nevertheless, a surface modification where a moderate decrease in corrosion resistance coupled with a significant reduction in generated wear particles should be feasible for biomedical applications.