Effect of Mo content on the passivation and localized corrosion behavior of laser engineered net shaped (LENS) Co-Cr-Mo alloys in a simulated physiological solution

In this work, the influence of varying the Mo content in a biomedical grade Co-28.2Cr-5.5Mo (CCM) alloy on the microstructure and electrochemical behavior was evaluated. Four samples were manufactured using laser engineered net shaping (LENS) – a control CCM alloy with no added Mo and three samples with added 6, 12 and 18 wt% Mo. Scanning electron microscopy (SEM) images revealed that increasing the Mo content increased the cell size with the +12% and +18% Mo samples showing interconnected and thicker cell boundaries. Electron microprobe maps indicated an enrichment of Cr and Mo in the cell boundaries and triple junctions in case of all the samples with the +18% Mo sample showing a segregation of these elements within the cell boundaries as well. Upon correlating this micro-segregation with results from the electrochemical experiments, it was concluded that the +6% Mo sample showed the most compact, least defective passive film while the +18% Mo showed the most defective, heterogeneous passive film. Using SEM and scanning Kelvin probe force microscopy (SKPFM) on the corroded surfaces after cyclic polarization, the formation of micro-galvanic cells was identified. The regions depleted of Cr/Mo (cell interiors, regions around carbides, etc.) were found to be more anodic and most susceptible to localized attack once the passive film broke down. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) results were used to confirm that a Cr/Mo ratio of 2.45 was most optimal for synergistic effect of Cr and Mo resulting in the highest corrosion resistance and passive film stability.